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Oxygen, hydrogen, sulfur, and carbon isotopes have been analyzed in the Pea Ridge magnetite-apatite deposit, the largest historic producer among the known iron deposits in the southeast Missouri portion of the 1.5 to 1.3 Ga eastern granite-rhyolite province. The data were collected to investigate the sources of ore fluids, conditions of ore formation, and provenance of sulfur, and to improve the general understanding of the copper, gold, and rare earth element potential of iron deposits regionally. The δ 18 O values of Pea Ridge magnetite are 1.9 to 4.0‰, consistent with a model in which some magnetite crystallized from a melt and other magnetite—perhaps the majority—precipitated from an aqueous fluid of magmatic origin. The δ 18 O values of quartz, apatite, actinolite, K-feldspar, sulfates, and calcite are significantly higher, enough so as to indicate growth or equilibration under cooler conditions than magnetite and/or in the presence of a fluid that was not entirely magmatic. A variety of observations, including stable isotope observations, implicate a second fluid that may ultimately have been meteoric in origin and may have been modified by isotopic exchange with rocks or by evaporation during storage in lakes. Sulfur isotope analyses of sulfides from Pea Ridge and seven other mineral deposits in the region reveal two distinct populations that average 3 and 13‰. Two sulfur sources are implied. One was probably igneous melts or rocks belonging to the mafic- to intermediate-composition volcanic suite that is present at or near most of the iron deposits; the other was either melts or volcanic rocks that had degassed very extensively, or else volcanic lakes that had trapped rising magmatic gases. The higher δ 34 S values correspond to deposits or prospects where copper is noteworthy—the Central Dome portion of the Boss deposit, the Bourbon deposit, and the Vilander prospective area. The correspondence suggests that (1) sulfur either limited the deposition of copper or was cotransported with copper, and (2) sulfur isotope analysis may be useful in evaluating southeast Missouri iron deposits for copper and possibly for gold.

The Caspiche porphyry gold-copper deposit, part of the Maricunga gold-silver-copper belt of northern Chile, was discovered in 2007 beneath postmineral cover by the third company to explore the property over a 21-year period. This company, Exeter Resource Corporation, has announced a proven and probable mineral reserve of 1,091 million tonnes (Mt) averaging 0.55 g/t Au, all but 124 Mt of which also contain 0.23% Cu, for a total of 19.3 Moz of contained gold and 2.1 Mt of copper. The deposit was formed in the latest Oligocene (~25 Ma) during the first of two volcanic and corresponding metallogenic epochs that define the Maricunga belt. The gold-copper mineralization is centered on a composite diorite to quartz diorite porphyry stock, within which five outward-younging phases are routinely distinguished. The centrally located, early diorite porphyry (phase 1) hosts the highest-grade ore, averaging ~1 g/t Au and 0.4% Cu. The subsequent porphyry phases are quartz dioritic in composition and characterized by progressively lower gold and copper tenors. Stock emplacement was both pre- and postdated by the generation of large-volume, andesite-dominated breccias, with tuffaceous matrices, which are believed to be shallow portions of diatremes. The deposit is characterized by a central gold-copper zone and partially overlapped but noneconomic molybdenum halo. The gold-copper mineralization in the lower half of the deposit accompanies quartz ± magnetite-veined, potassic-altered rocks, whereas the shallower mineralization occurs within quartz-kaolinite–dominated, advanced argillic alteration. Upper parts of the advanced argillic zone are characterized by siliceous ledges, some auriferous, composed of vuggy residual quartz and/or silicified rock. The chalcopyrite-pyrite mineralization in the potassic zone was partially transformed to high sulfidation-state sulfides and sulfosalts during the advanced argillic overprint, although the underlying chalcopyrite-bornite assemblage was mainly too deep to be affected. The deposit terminates downward in a sulfide-deficient, potassic-calcic zone defined by K-feldspar, actinolite, and magnetite, which formed at the expense of biotite. A relatively minor, shallowly inclined zone of intermediate sulfidation epithermal gold-zinc mineralization, comprising narrow veinlets and disseminations, abuts the late-mineral diatreme contact. Supergene sulfide oxidation throughout the deposit is relatively shallow, and chalcocite enrichment extremely minor. The Caspiche deposit is thought to have been emplaced at relatively shallow paleodepths, within the southern, flat-bottomed part of the premineral diatreme vent. Earliest porphyry system development, probably north of the present deposit, appears to have been aborted by diatreme formation. Much of the gold and copper in the Caspiche deposit was introduced during the potassic alteration stage, with the highly telescoped, advanced argillic overprint being responsible for only minor redistribution of the two metals, and the addition of arsenic. The late-mineral diatreme was emplaced west of the Caspiche deposit, and caused destruction of only its uppermost peripheral parts. The late-mineral diatreme was both pre- and postdated by advanced argillic alteration. Finally, the intermediate sulfidation epithermal gold-zinc zone was localized by the enhanced permeability provided by intense fracturing along the underside of the upward-flared, late-mineral diatreme contact.

The Hasancelebi deposit in eastern Turkey, with proven reserves of 95 million metric tons, is a copper-gold-bearing iron oxide deposit. It is chiefly hosted by rocks that underwent widespread sodic-calcic and potassic styles of alteration. In the Hasancelebi district, pervasive Na-Ca and K-Fe alteration types are overprinted by sericitization and by late alteration that occur in multiple, overprinting systems. The age of hydrothermal alteration and mineralization (ca. between 74-68 Ma) overlaps with the age of alkaline magmatism in the Hasancelebi district. Crystallization and cooling of alkaline magmatism is associated with hydrothermal features that spanned the duration of the district’s igneous history. The 40Ar/39Ar and U-Pb geochronology of magmatism indicate that the hydrothermal system was synchronous with diabase and syenite and/or microsyenite porphyry intrusions and consisted of several discrete phases of Na-Ca and K-Fe alteration. The oldest alteration (phase 1) formed at ca 74.4 to 74.3 Ma; it is spatially and temporally associated with the emplacement of diabase dikes and contains scapolite and phlogopite. The next younger alteration (phase 2) formed at about 71.3 Ma and is spatially and temporally associated with syenite porphyry and microsyenite porphyry intrusions. It consists of scapolite, garnet, pyroxene, and actinolite that are superimposed on the first alteration phase. The next alteration assemblage (phase 3) formed at about 68.6 Ma and consists of phlogopite 2 and magnetite mineralization. Still later alteration (phase 4) overprints phases 1, 2, and 3 and consists of sericite-quartz, chalcopyrite, hematite, calcite, fluorite, and barite. Oxygen ( δ 18O) and hydrogen ( δ D) isotope analyses were carried out on phlogopite, sericite, barite, calcite, fluorite, and quartz representing the mineral assemblage formed during phases 1 and 4 of the hydrothermal alteration. The calculated δ 18O (H2O) and δ D (H2O) composition of the fluids that formed preore K-Fe alteration and postore alteration of phase 4 range between 15.1 to 8.5 and −124 to −85.6 per mil, respectively, and partially overlap the range for traditional magmatic waters. Oxygen and H isotope compositions from alteration minerals highlight two major points: Na-Ca and K-Fe alteration assemblages are likely derived from high-temperature primary magmatic fluids, and late sericitization is likely derived from fluids exsolved from cooling alkaline plutons during hydrothermal fracturing and consequent degassing at moderate temperatures. The initial ɛ Ndi values of the alteration zones range from −2.02 to −2.08, close to the calculated ɛ Nd value of associated syenite porphyry at the time of alteration ( ɛ Nd (68.64Ma; −1.9). Nd-Sr isotope compositions combined with 40Ar-39Ar and U-Pb geochronology of the associated alteration suggest that the fluids were sourced from coeval diabase dikes and syenite porphyry and/or microsyenite porphyry intrusions. The rare earth element characteristics, 40Ar-39Ar and U-Pb geochronology of alteration and fresh magmatic rocks, δ 18O, δ D, Nd, and Sr isotope systematics together suggest that the fluids responsible for the high- to moderate-temperature assemblages are largely derived from the coeval alkaline igneous rocks (diabase and microsyenite porphyry dikes) that intrudes the sedimentary-volcanosedimentary basin.

Alteration veins containing Cl-poor actinolite and actinolitic hornblende are a common feature throughout the Archean-aged gneisses and granitoids along the northern margin of the Sudbury Igneous Complex, Ontario, Canada. Crosscutting relations and structural measurements show that the veins postdate the Sudbury impact event but predate the emplacement of footwall-style Cu-Ni-PGE (platinum-group element) sulfide ores. The veins are four to five times more abundant in brecciated, mineralized outcrop than in unbrecciated, barren outcrop. Analyses of vein-hosted amphibole by electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS) show elevated concentrations of Ni (up to ~100 times background; increasing from ~100 ppm to ~1.0 wt %), Cu (up to ~4 times background) and Sn (up to ~2.5 times background) in proximity to sulfide veins. Copper and Sn contents in amphibole are only anomalous within 10 to 20 m of mineralization and require quantification by LA-ICP-MS. However, anomalous Ni concentrations in vein-hosted amphibole may be detected with a high level of confidence solely by EMP up to ~700 m from mineralization in rocks that contain no visible sulfides in hand sample or thin section and show no bulk rock Ni or Cu anomalies. The relationship between the Ni content in amphibole (in ppm) and mineralization proximity (distance “D” in meters) is given by the following equation: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[[Ni]_{amp}\ =\ 5100\ (D)^{{-}0.45}.\] \end{document} The data indicate wide-scale Ni mobility in hydrothermal solutions in the Sudbury footwall. The formation of Ni-rich amphibole in the fracture networks, by either the growth of primary Ni-rich amphibole or metasomatic Ni enrichment of preexisting (Ni-poor) amphibole, was associated with circulation of a sodium- and halogen-rich fluid between ~390o and 550oC. Nickel may have been released into the footwall environment by extensive hydrothermal alteration of Ni-rich contact-style sulfide ores that lie in the hanging wall of the Cu-rich footwall-style ores. In the footwall rocks, pre-existing fracture networks (now amphibole-bearing veins) acted as common structural pathways, first for the circulation of early Ni-rich fluids associated with leaching in the contact region and the onset of footwall mineralization, and second for the emplacement of Cu-rich footwall-style sulfide veins. Other metal concentrations in amphibole (Pb, Zn, Co) show no relationship to mineralization proximity. These metals may have been derived locally from country rocks during alteration reactions that promoted amphibole growth in the fractures. Identification of fracture-hosted amphibole with anomalous Ni contents in thin section, or Ni-Cu-Sn anomalous amphibole grains liberated from surface-weathered rocks hosting sulfide veins may serve as useful proximity indicators for hidden footwall-style sulfide deposits at Sudbury.

The Empire Cu-Zn skarn deposit is unusual because of the proximal position of Zn mineralization, abundance of endoskarn, and the extremely vermicular texture of quartz phenocrysts in the related intrusive rocks. Cu-Zn skarn occurs at the contact between Upper Mississippian White Knob limestone and the granite porphyry phase of the Mackay Stock which consists, from early to late, of quartz monzodiorite, granophyre, granite porphyry, porphyritic granite, and many dikes. The late phases have high F and also extremely vermicular quartz phenocrysts. Endoskarn is more abundant than exoskarn. The earliest alteration of the intrusive rocks consists of disseminated diopsidic pyroxene (Di64Hd36 to Di88Hd12), actinolite, and titanite. This assemblage was cut by early scapolite (Me18 to Me35, mostly Me18-26) and/or green pyroxene (Di14Hd80 to Di20Hd77) veinlets, with or without wollastonite halos. These early veins were then cut by main-stage endoskarn veins that typically have a garnet + minor pyroxene inner zone, a wollastonite and/or pyroxene ± Ca-rich plagioclase (An56 to An89) envelope, and a halo containing disseminated, fine-grained alteration minerals of the same assemblage as the envelope. Some veins contain only the envelope assemblage and are interpreted to represent the alteration front. The inner zone locally contains vesuvianite. Where many veins intersect, endoskarn is massive. Pyroxene is zoned around fluid conduits; the distal pyroxene is Fe rich (hedenbergitic) whereas the proximal pyroxene is Fe poor (diopsidic). The garnet changes in the opposite way, being Fe poor-Al rich (grossularitic) in locations distal to the fluid conduits, and Fe rich (andraditic) in proximal locations. In contrast, in the exoskarn, all pyroxene is diopsidic and garnet is andraditic. Weak, retrograde alteration composed of quartz + calcite + chlorite with minor fluorite, talc, and epidote overprinted both endoskarn and exoskarn. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskarn. Sphalerite precipitated together with chalcopyrite in proximal locations and is associated with retrograde alteration. Other ore minerals include minor molybdenite, bornite, pyrite, galena, arsenopyrite, native Au, as well as supergene minerals such as chrysocolla, malachite, azurite, native Cu, and limonite. Fluid xenoliths from pyroxene in early endoskarn veinlets homogenize at >600°C. Massive endoskarn and exoskarn replacing limestone inclusions in granite porphyry formed at 500° to >700°C, whereas the highest temperature inclusions, >700°C, occur in narrow garnet + minor pyroxene veins. Fluid inclusions in exoskarn replacing wall rock have homogenization temperatures of 500° to 650°C. Retrograde alteration and Cu-Zn mineralization occurred at 250° to 300°C. Fluid inclusions in prograde minerals contain daughter minerals, whereas fluid inclusions in retrograde minerals do not, indicating a decrease in salinity with time. Late-stage fluids have low eutectic temperatures, indicating the possible presence of KCl, NaCl, FeCl2, CaCl2, MgCl2, K2CO3, and/or Na2CO3. Formation of the unusually abundant endoskarn, the proximal position of Zn mineralization, and the extremely vermicular texture of quartz phenocrysts are all believed to have been promoted by the high F content of the magmatic fluid. These features may serve as exploration indicators of associated high F mineralization such as buried porphyry Mo deposits.

The Bjorkdal quartz vein-hosted gold deposit is located ~25 km northwest of Skellefte in northern Sweden, within a Paleoproterozoic volcanosedimentary sequence at the margin of a quartz-monzodiorite granitoid. Northeast-trending (030°– 050°) quartz veins from the eastern open pit within the Bjorkdal deposit contain quartz, scheelite, tourmaline, calcite, and sulfides, with visible gold. Vein quartz shows undulatory extinction and sutured margins or is polycrystalline in form, features which suggest postcrystallization deformation. Coarse scheelite crystals (>5 mm) within the quartz veins are crosscut by thin veins of quartz, calcite, sulfides, and gold. The calcite in these crosscutting fractures is variably replaced by biotite or actinolite. Tourmaline from the quartz veins has low total REE contents (

The Lac des Iles Pd mine, with reserves of 88 million metric tons (Mt) containing 1.51 g/t Pd, is hosted by the 2.69 Ga Lac des Iles intrusive complex in the southern Wabigoon subprovince of the Superior province of Canada. The known economic concentration of Pd occurs in the Mine Block intrusion, the central body of the intrusive complex, where gabbroic rocks range from leucogabbro to pyroxenite and show complicated textures, such as breccias, magma mingling, and pods and veins of pegmatite. The ore is characterized by low concentrations of sulfide (typically less than 3 vol %) and exceptionally high Pd contents (Pd/Pt ~10, Pd/Au ~13). Detailed mapping of the southern Roby and Twilight zones shows that early leucocratic rocks are barren and that the bulk of Pd was introduced by late melanocratic magmas. The average concentration of Pd in the melanocratic rocks, excluding the High-Grade zone, is estimated to be ~4 ppm. The concentrations of sulfur correlate positively with those of base metals, platinum group elements (PGE), and Au. Furthermore, sulfide grains commonly show exsolution textures. The evidence suggests a magmatic origin of the PGE mineralization where the PGE were concentrated in an immiscible sulfide melt in the parental magma. Bulk chemical compositions suggest that all mafic igneous rocks in the mineralized zones, except for late clinopyroxenite, are cogenetic. The hypothetical parental magmas have high MgO and low (~15× chondrite), unfractionated rare earth elements (REE) with (Ce/Yb)chondrite 35% of Pd in the deposit) on the eastern margin of the Roby zone have much higher concentrations of Pd than any other rocks and do not show correlations between sulfur and precious and base metals. Furthermore, the rocks are intensely and pervasively altered to actinolite, talc, anthophyllite, hornblende, chlorite, sericite, calcite, and quartz. These observations suggest subsolidus enrichment of Pd and mobility of S. The lack of apparent fluid pathways within the High-Grade zone and the distribution of the zone are consistent with the enrichment of Pd at high temperatures by fluids that originated from the mafic magmas. The textures of the Lac des Iles deposit are similar to those of contact-type PGE deposits, but there are fundamental differences between the two. The Lac des Iles deposit is not localized near the contact between the host intrusion and the country rocks and evidence of the assimilation of the host rocks is lacking. Instead, the mineralization at Lac des Iles has many features in common with layered intrusion-hosted deposits, in which pulses of primitive magma introduced the PGE. Unlike the quiescent magma chambers of most layered deposits, the magmas at Lac des Iles were intruded energetically, forming breccias and magma-mingling textures.

Interaction of base metal sulfides (pyrrhotite, pentlandite, and chalcopyrite) with aqueous fluids in the UG2 chromitite layer and the Merensky reef in the western Bushveld Complex has produced four common replacements: (1) replacement of sulfides and orthopyroxene by actinolite or tremolite, (2) replacement of sulfides and plagioclase by epidote, (3) replacement of sulfides by calcite, and (4) replacement of sulfides by magnetite. The first three replacements are directly related to the presence of sulfides in the rocks and occur at the grain scale; silicate minerals that are not in direct contact with sulfides are usually not affected. The replacement of sulfides by magnetite accompanies regional serpentinization superimposed on the Union section. Platinum group minerals, including Ru, Pt, and Pd sulfides, Pt and Pd tellurides, and Pd arsenides, occur in replacement aureoles around sulfide aggregates as well as within the aggregates, suggesting that the platinum group minerals are more resistant to hydrothermal alteration than the associated base metal sulfides. Redistribution of chalcopyrite, at least at the millimeter scale, variable losses of S and Ni, and perhaps decoupling of Pt and Pd at the centimeter scale took place during secondary hydrothermal alteration. These effects should be considered before models of primary mineralization processes can be meaningfully applied.

Ridgeway is a high-grade gold-copper porphyry deposit (54 Mt at 2.5 g/t Au and 0.77% Cu), related to an alkalic intrusive complex of monzonitic composition. The deposit occurs within the Cadia district of New South Wales, Australia, which consists of a cluster of four Late Ordovician gold-copper porphyry deposits and two iron-copper-gold skarn deposits with a combined gold resource in excess of 574 metric tonnes (t). The deposits lie on a 7-km-long, northwest-oriented corridor of alteration and mineralization, transverse to the axis of the postulated volcanic arc. Alteration and mineralization at Ridgeway are zoned around a vertically attenuated intrusive complex of monzodioritic to quartz monzonitic composition. Distinct styles of veining and alteration are related to different intrusive phases of the monzonite complex, with the intensity of alteration and grade of mineralization decreasing from early- to late-mineral intrusions. Early-mineral intrusions are associated with intense actinolite- magnetite-biotite (calc-potassic) alteration and up to four stages of high-grade quartz-magnetite-sulfide veining. Bornite is the most abundant sulfide formed during early-stage alteration and correlates well with gold. Moderate- to weak-intensity orthoclase-biotite plus or minus magnetite (potassic) alteration accompanies the inter- and late-mineral intrusions, this alteration being associated with chalcopyrite- and pyrite-rich quartz-orthoclase veins. Propylitic and sodic (albite-pyrite) alteration assemblages are peripheral to, and locally overprint, the potassic alteration. Phyllic alteration is restricted to the margins of late-stage faults. The fluid inclusion assemblage comprises one and two salt-bearing brine inclusions, in addition to aqueous liquid-vapor inclusions of low to moderate salinity. No low-density, vapor-rich inclusions are present, indicating that the fluids from which the quartz veins precipitated did not enter the liquid-vapor field of the H2O-NaCl system. The brine inclusions undergo final homogenization to liquid via halite dissolution. This phenomenon, in addition to the absence of low-density vapor inclusions, suggests that the mineralizing fluids at Ridgeway were nonboiling hypersaline brines that exsolved directly from the crystallizing magma. The presence of mineralized aplitic vein dikes and comb quartz layering are interpreted to indicate that the early and transitional stages of mineralization at Ridgeway formed at the transition between magmatic and hydrothermal conditions.

The Pena Colorada iron skarn is located in the southern part of the Guerrero terrane. It contains 150 million metric tons (Mt) with a grade of 36 percent magnetite. The deposit occurs at the contact of a 68 Ma equigranular diorite with mid-Cretaceous volcano-sedimentary rocks of the Tepalcatepec Formation. Volcanic units within this formation have tholeiitic affinity and REE patterns that are compatible with a primitive arc setting. The Pena Colorada diorite and associated voluminous aplitic phases are part of a Late Cretaceous calc-alkaline continental arc that subsequently intruded this basin. Northeast-vergent deformation affected the region in the Late Cretaceous. North-south and east-west reverse, and northwest and northeast strike-slip faults localize the mineralization. The alteration halo around the diorite intrusion is 500 m wide. Within an inner 200-m halo, the volcano-sedimentary section was affected by an early metamorphic event, with discontinuous bands of pyroxene (Di 68 Hd 29 Jo 3 ) hornfels and garnet (Gr 61 Am 36 Ad 3 ) disseminations. Metasomatic pyroxene, garnet, and plagioclase cut the early metamorphic event preferentially replacing carbonate and volcanic units up to and including a 35-m-thick marl located 200 m above the intrusion. Metasomatic pyroxene and garnet have average compositions of Di 77 Hd 21 Jo 2 and Ad 76 Gr 20 Am 2 Sp 1 Py 1 , respectively. Pyroxene compositions reach Di 94 Hd 5 Jo 1 , whereas garnet exhibits intragrain compositional zoning from Ad 49 Gr 46 in the core to Ad 100 Gr 0 in the rim. Albitization of preexisting protoliths modified igneous plagioclase compositions from Ab 67 An 32 Or 1 up to Ab 96 An 3 Or 1 . Albitization is followed by abundant fracture-controlled Fe-epidote and Cl-bearing chlorite (Cha 84 Cli 14 Pen 2 ) that extend from the marl unit outward some 300 m. This association is cut by a later assemblage of epidote-chlorite-prehnite that affects the entire 500-m width of the alteration halo and part of the intrusion. Chlorite (Cha 54 Cli 45 Pen 1 ) from this later event is richer in Mg than that from the previous association. Igneous hornblende in the intrusion is altered to actinolite. A potassic alteration event related to a hydrothermal breccia and aplitic dikes overprints the pre-existing calc-silicate associations. Alteration minerals in the breccia and the 200-m potassic halo around it, evolve from early K feldspar-(biotite)-quartz, to late jasperoid-fluorapatite-calcite veins. Mg-rich biotite (Phl 77 Ann 23 ) and REE-rich apatite contain F/Cl/OH ratios of 1/0/3 and 1/0/0, respectively. Massive magnetite (36‰)-specular hematite (7‰)-sulfide ( O, C, S, and H stable isotope determinations on minerals from the calc-silicate association are consistent with an igneous source for the initial fluids, which mixed outward with an increasing component of trapped seawater or evaporite. However, water in equilibrium with magnetite exhibits unusually enriched δ 18 O values relative to other coexisting mineral-derived fluids. A solution that equilibrated with limestone at relatively high temperature is required to explain these heavy values.

The metasediment-hosted distal scheelite skarns from the Riba de Alva mine, Barca de Alva-Escalhao region, northeastern Portugal, consist of three main levels of strata-bound, boudin-shaped, 0.5- to 4-m-thick bodies, having a total resource of 380,000 tons (t) with an average grade of 0.51 percent WO3. The boudinage of the Cambrian metasedimentary rocks of the Slate Greywacke Complex occurred early during the Hercynian orogeny, prior to the Granitic Complex intrusion and the skarn-forming process. The Granitic Complex corresponds to reduced, highly evolved, and metallogenically specialized S-type leucogranites, comparable to those commonly associated with Mo-poor W-F skarns. Four different skarn types were recognized: pyroxene skarn with Fe diopside± anorthite; amphibole-epidote skarns with actinolite + clinozoisite ± bytownite ± phlogopite; ore skarn with clinozoisite + scheelite + fluorite + calcite ± labradorite ± apatite ± Fe axinite ± armenite ± scapolite; and late-stage veins with quartz (or fluorite or calcite) ± clinozoisite ± andesine (or albite) ± adularia. Clinopyroxene ranges from Di80Hd20 to Di60Hd40. Amphibole compositions range from actinolite to Mg hornblende and are F rich (0.5–1.9 wt %). Clinozoisite, often zoned with Fe-poor cores, shows little compositional variation in the skarn types, having a low pistacitic component (5–13% Ps). Plagioclase compositions grade from anorthite in pyroxene skarn through andesine and albite in late-stage veins. Phlogopite, as amphibole, is F rich (2.45–4.70 wt %). An extremely pure Mo-free scheelite, the only ore mineral present, is restricted to the ore skarn type. The Riba de Alva skarns were formed, after intrusion of the Hercynian Granitic Complex, by selective infiltrative metasomatic alteration of argillaceous dolomitic limestone. An early stage of prograde alteration led to the formation of the pyroxene skarn, through reaction with an Si-rich and Fe-bearing fluid. Scheelite mineralization occurred during retrograde alteration, which was mainly fracture controlled and is characterized by a pervasive infiltration metasomatism with minor diffusion in the skarn walls. This alteration started with hydration reactions to form amphibole-epidote skarns, probably under lower XCO2 conditions. Ore skarn formation, with scheelite deposition, took place later during this retrograde stage, mainly along veins within the strata-bound boudins, as the result of a decrease in temperature and/or significant changes in the activity of some fluid components such as W, F, Na, Mn, P, and B, and particularly an increase in aCa. Monomineralic late-stage veins represent the waning stage of the skarn-forming process.

Tiwi is one of several large geothermal fields in the Philippines. In 1992, the drilling of Matalibong-25 provided nearly 1,650 m of continuous core from the geothermal reservoir. A maximum temperature of 275°C at 1,829 m and intense hydrothermal alteration were encountered in this well. Six stages of alteration and vein mineralization have been documented in the cored portion of the well. The earliest stage, which reflects the initial development of the system, is represented by the deposition of chalcedony and clays (stage 1). The transition to a high-temperature environment is marked by the appearance of sericite (stage 2) deposited by steam-heated waters. Stage 3 reflects episodic cycles of fluid upwelling and boiling followed by the incursion of cooler fluids. Veins deposited by boiling fluids are filled with quartz ± adularia ± epidote ± pyrite and base-metal sulfides, whereas heating of the recharging fluids led to the deposition of calcite and/or anhydrite. Maximum temperatures of fluid inclusions trapped in two quartz crystals deposited by the upwelling fluids range from 325° to 332°C. Temperatures varied widely during the recharge phase of these cycles. Fluid inclusions trapped in calcite and anhydrite suggest that mineral deposition occurred at temperatures ranging from 333°C to less than 270°C. Fluid-inclusion salinities indicate that seawater dominated below a depth of 1,600 m, whereas fresh waters dominated at shallower depths. Gaseous species trapped in these inclusions were released by crushing or thermal decrepitation and analyzed with a quadrupole mass spectrometer. CO2/CH4 and N2/Ar ratios indicate that the gaseous species trapped in calcite and anhydrite were derived primarily from meteoric and crustal sources. In contrast, gaseous species in quartz from Matalibong-25 and advanced argillic assemblages from other Tiwi wells have magmatic and crustal origins. Stage 4 represents a second major episode of sericite deposition. Subsequent mineralization consisted of wairakite ± epidote, followed by actinolite, and then calcite (stage 5). The presence of actinolite implies that temperatures exceeded 300°C at the end of this stage. Thermochemical modeling indicates that the modern (stage 6) fluids are again in equilibrium with sericite. 40Ar/39Ar spectrum dating of stage 3 adularia from three depths has been combined with the mineral parageneses and fluid-inclusion homogenization temperatures to constrain the thermal history of the geothermal system. Taken together, these data record the deposition of adularia at ~330°C between 314 and 279 ka, minor cooling followed by reheating to produce stage 5 actinolite at ~200 to 220 ka, incursion of marginal waters and cooling to 235°C by ~190 ka, a long period of quiescence to ~50 ka, and finally, development of the modern thermal regime at 10 to 50 ka in response to a recent subvolcanic intrusion.

Sir: We appreciate the effort and interesting contribution of Jami et al. (2007) on the origin of the Esfordi apatite-magnetite deposit. We have been working in the Bafq mining district of central Iran for almost eight years, resulting in three graduate theses on the mineralization of the area. Based on our field experiences and mineralogical and geochemical data, we wish to comment on the conclusion of Jami et al. (2007) that Esfordi is a typical Kiruna-type apatite-magnetite deposit. First, the deposit is composed of magnetite-apatite-hematite, rather than simply apatite-magnetite and therefore does not fit an orthomagmatic model for Fe mineralization. Furthermore, many authors (e.g., Force et al., 1991) consider Kiruna-type deposits as volcanic-sedimentary complexes, rather than as a simple, although extended, magmatic-hydrothermal process such as proposed by Jami (2005) and Jami et al. (2007). Second, the presence of actinolite, albite, chlorite, and tourmaline in brecciated and altered rhyolite (keratophyre) is consistent with a submarine vent complex and feeder zone of an exhalative submarine volcanic system. In this context, the Na metasomatism and albitic alteration could be due to interaction with seawater during submarine rhyolitic volcanism. Third, there are many structural and textural features—such as polymictic pebbly …

The Eloise deposit (3.1 million metric tons, Mt,5.5% Cu, 1.4 g/t Au and 16 g/t Ag) is hosted by Proterozoic rocks of the Soldiers Cap Group in the Eastern fold belt of the Mount Isa inlier. The deposit is characterized by very high grade, chalcopyrite-pyrrhotite-rich mineralization hosted by mafic silicate alteration. Alteration and mineralization lie within and adjacent to a major shear zone. The main metasomatism comprised three stages. Early pervasive albite alteration (stage I) was overprinted by stage II hornblende-biotite-quartz veins and alteration; stage III chalcopyrite, pyrrhotite, magnetite, pyrite, calcite, quartz, chlorite, and actinolite replaced and brecciated the earlier mafic silicates. The lodes occupy a narrow portion of a zoned, mineralized system that is at least 2 km in length. Magnetite-pyrite-rich mineralization occurs in the south, with chalcopyrite-pyrrhotite-rich mineralization in the central lode zone, and pyrrhotite-rich mineralization in the north. The system has a distinct chemistry, with enriched Cu, Au, Ag, Co, Ni, and Zn.Fluid inclusions in pre-, syn- and late- to postmineralization veins indicate an evolving fluid history. Primary inclusions and associated hornblende-biotite assemblages in premineralization veins indicate a high-temperature fluid (450 degrees -600 degrees C), and salinity estimates suggest it was an ultrasaline brine (32-68 wt % total salts). Fluid inclusions in stage III quartz in mineralized veins display a wide range of homogenization temperatures ( approximately 100 degrees -500 degrees C), and gangue assemblages (actinolite-chlorite-muscovite) suggest mineralization was a cooler event (200 degrees -450 degrees C). Inclusions associated with the mineralization have a lower salinity (30-47 wt % total salts) than do premineralization inclusions, and variable fluid inclusion types occur within the same clusters, which may reflect fluid mixing. The ultrasaline nature of the fluids and preliminary sulfur isotope results (0.4-2.0ppm) suggest the ore fluid was predominantly magmatic. Metals were carried as chlorocomplexes and H 2 S was the dominant sulfur species. Important factors in ore deposition were a decrease in temperature and salinity, and sulfidation of Fe-silicates. Eloise is interpreted to be a structurally controlled, distal, magmatic, hydrothermal vein deposit.

Evidence for the leaching of base metals by oxidized, saline brines has been recognized from dolerites of the Upper Tawallah Group, southern McArthur basin. In the Mallapunyah dome region, the Upper Tawallah Group consists of a series of shallow-level dolerite sills (Settlement Creek and Gold Creek Volcanics) that have intruded a sequence of fluvial, lacustrine and shallow marine hematitic sandstones and evaporitic siltstones and carbonates. Oxidized sandstones and evaporitic carbonates of the Lower McArthur Group overlie the Tawallah Group. Formation of the Mallapunyah dome occurred due to uplift at end-Tawallah Group times, marked by the emplacement of volcano-sedimentary breccias interpreted as debris flow deposits of the Gold Creek Volcanics.K-metasomatism has resulted in the replacement of dolerites of the Settlement Creek and Gold Creek Volcanics by potassic (orthoclase + quartz + or - sericite + or - hematite + or - dolomite + or - anatase + or - barite) and chlorite-orthoclase (chlorite-orthoclase-quartz + or - dolomite + or - sericite + or - actinolite + or - albite + or - anatase) mineral assemblages. Three varieties of potassic alteration have been recognized: intense, texturally destructive potassic alteration halos around quartz + or - hematite + or - dolomite + or - chlorite veins; intense, texturally destructive pervasive potassic alteration; and intense, pervasive, texturally destructive potassic alteration rinds ( 20 wt % NaCl equiv) Na-K-Ca-Mg-rich brines. The high salinities and the presence of hydrocarbons are consistent with brine derivation from the overlying evaporitic carbonates during diagenesis. Carbon-oxygen (carbonate) and oxygen (silicate) isotope analyses are consistent with the formation of dolomite veins and potassic alteration in the Settlement Creek Volcanics during interaction with low-moderate temperature ([asymp]100 degrees C) evolved meteoric waters + or - seawater (delta 18 O (sub (fluid)) [asymp] -1ppm; delta 13 C (sub (fluid)) [asymp] -7ppm), which descended through the overlying carbonate horizons, becoming progressively more saline and enriched in 18 O as they interacted with the partially lithified sediments.The fluids responsible for metal leaching and potassic alteration of the Tawallah Group dolerites are interpreted to have been low temperature, hematite-stable, saline brines with a high base metal transporting capacity due to their oxidized nature and low H 2 S contents. The metals acquired by these brines could have been transported significant distances through a suitable aquifer (e.g., well-sorted hematitic sandstone), and the resultant metalliferous brines were probably involved in the formation of at least some of the stratiform Pb-Zn, MVT, and breccia-hosted Cu deposits in the Upper Tawallah and McArthur Groups.

Proterozoic diabase of the Dripping Spring range occurs as sills in the Proterozoic Apache Group and the Troy Quartzite and as intrustive sheets in basement rocks. The aggregate thickness of the diabase sills and intrusive sheets averages about 450 m in the part of the range showing little mid-Tertiary extension. Laramide alteration is of two types, dominated by chlorite and actinolite, respectively, and formed mostly from clinopyroxene. Actinolite-dominated assemblages are higher in Na and Ca. Hydrothermal biotite is common in the central areas of both alteration types. Laramide alteration forms two distribution patterns: a subsequent pattern centered on Laramide intrusions and small porphyry deposits, characterized by actinolitic alteration, and a more extensive branching linear pattern that follows Laramide structures, centered on the larger Ray porphyry deposit, extending toward other Laramide districts and showing both alteration types. Alteration has apparently mobilized copper and other metals from diabase. The freshest diabase samples average about 120 ppm copper with little variation. In chloritic alteration, about 100 ppm of this copper is expelled in the most completely altered rocks. In actinolitic alteration, diabase may either gain or lose copper during alteration. Chloritic alteration constitutes roughly 70 percent of the diabase alteration in the study area, where alteration averages 41 percent complete. This implies liberation of about 9X10 6 tons (t) copper from diabase alteration, significantly less than the 16X10 6 t copper in Laramide mineral deposits of the superdistrict Ray, Superior. Chilito. Christmas. However, diabase alteration may have been a significant component of the supply of copper to the Laramide mineral districts of the area. Synmineral magmatic sources of copper are not documented in this area. The distribution of Proterozoic diabase coincides with the central part of the southeastern Arizona copper province, which may thus owe much copper availability to an unusual abundance of diabase. However, many unanswered questions remain about metal supply from altering diabase.

The Golden Giant mine is developed in the Archean Hemlo gold-molybdenum deposit hosted within the Hemlo-Heron Bay greenstone belt in northwestern Ontario, Canada. The ore occurs in steeply dipping tabular zones within metasedimentary rocks and within quartz-eye muscovite schists in association with microcline-mica-pyrite alteration, and a variety of other minerals.The Hemlo area has been intruded by quartz-feldspar porphyries (the Golden Giant porphyry and Sceptre porphyry). These porphyries are altered, mineralized, and highly deformed. The calc-alkalic granodioritic Cedar Creek stock and Cedar Lake pluton occur near the deposit. Feldspar porphyry sills and dikes generated from the stock and pluton cut the earlier porphyries, mineralized rocks, and many of the fabric elements. The Golden Giant rocks have been regionally metamorphosed to amphibolite grades. This early, M 1 metamorphisin (T = 500 degrees -600 degrees C, P = 5-6 kbars) probably accompanied the intrusion of the Cedar Lake pluton and produced mineral assemblages in metasedimentary rocks containing hornblende, kyanite, staurolite, garnet, and micas. A second, M 2 metamorphic event (T = 300 degrees - 400 degrees +C, P = 1-3 kbars) produced mineral assemblages containing actinolite, fibrolite, andalusite, calcite, garnet, epidote, chlorite, and muscovite. The Golden Giant mine rocks are strongly foliated, and commonly exhibit mylonitic and transposed fabrics, isoclinal folds, and boudinage in both mineralized and nonmineralized rocks.It is proposed that the primary Au-Mo mineralization occurred before the M 1 metamorphic event, possibly related to emplacement of the Golden Giant quartz porphyry in a premineralization structural zone. The mineralization was modified by the M 1 event, and severely deformed during mylonitization. Dikes from the Cedar Creek stock intruded the deposit sequence and overprinted the M 1 event with an M 2 metamorphism.

The Christy Ti-V-Nb deposit is hosted by the Arkansas Novaculite, adjacent to the carbonatite-alkaline Magnet Cove Complex. Mineralized rock contains up to 8 wt percent Ti, 2.68 wt percent V, and 0.124 wt percent Nb. Minor amounts of Li and Mo are present, and there is evidence for the preferential concentration of the middle to heavy rare earth elements (REE) in altered clay-rich dikes. Mineralized rock consists of (1) thermally recrystallized novaculite that is partly replaced by actinolite, sodic amphibole, taeniolite (KLiMg 2 Si 4 O 10 F 2 ), pyrite,and siderite, (2) brookite-bearing amphibole- and taeniolite-rich rock that contains siderite and pyrite, (3) quartz-brookite aggregates, and (4) goethite-rich rocks that form the oxidized zone of the deposit. Unaltered rock is scarce at the Christy deposit. Comparison of mineralized lithologies from the Christy deposit with those from two other areas of mineralization associated with the Magnet Cove Complex, the Magnet Cove Titanium Corporation deposit ("Rutile deposit") and the Hardy-Walsh prospect, provides mineralogical and chemical evidence for the source of the mineralizing fluids at the Christy deposit.Massive clay-rich (kaolinite + or - smectite) dikes are in contact with the mineralized zones of the Christy deposit. The presence of relict K feldspar and pyrite in the least altered dikes and certain characteristic trace elements (e.g., Ba, Rb) suggest that the Christy dikes are related to less pervasively altered rutile-mineralized, pyrite-bearing carbonate-feldspar dikes fromthe Rutile deposit. The feldspar-carbonate dikes from the Rutile deposit may be the products of intense alkali metasomatism of alkaline igneous silicate rocks that were emplaced prior to the intrusion of the Magnet Cove carbonatite.The Christy deposit formed through a series of complex processes. The initial phase of mineralization is directly related to the infiltration of novaculite by alkali-rich fluids that were probably derived from carbonatite magma. Introduction of the metasomatic fluids preceded emplacement of massive feldspar- and pyrite-bearing dikes that were subsequently altered to the clay-rich masses Titanium, V, Nb, and Li were introduced by the alkali-rich fluids at temperatures that were, as indicated by fluid inclusion data (Willis et al., 1991), as high as 600 degrees C. During the initial stage of mineralization, V was concentrated in aegirine and sodic amphibole, Li was concentrated in taeniolite, minor amounts of Ti were concentrated in aegirine, and pyrite formed. The replacement of novaculite by the aforementioned minerals yielded excess silica, which precipitated as quartz in veins and as clear overgrowths on recrystallized, inclusion-rich (partly replaced) quartz. Niobium- and V-bearing brookite precipitated with the quartz. Minerals formed during the first stage reacted with a second fluid at temperatures of 100 degrees to 300 degrees C, as indicated by fluid inclusion data (Willis et al., 1991), and V was then concentrated in smectite and goethite. The isotopic composition of siderite [delta 18 O SMOW = 23.8-25.0ppm, delta 13 C PDB = -7.1 to -8.9ppm suggests that the second fluid was a mixture of low-temperature metasomatic fluid and ground water. Vanadium was further concentrated in clay minerals (Breit et al., 1992), in goethite, and in vug minerals as low-temperature alteration proceeded. Brookite is not totally resistant to weathering and it is at least partly replaced by other oxides in the oxidized zone.

Located in the Eastern Cordillera of northwestern Argentina, the Aguilar mine and several smaller prospects are aligned north-south in a 30- by 5-km district. The Zn-Pb-Ag sulfide ores are hosted in intensely folded and faulted Lower Ordovician quartzites and hornfelses, near their contact with the Cretaceous Aguilar granite. The orebodies are stratigraphically conformable with the siliciclastic rocks and parallel to the contact metamorphic halo of the granite, in the pyroxene-hornblende and albite-epidote hornfels facies.The ores are associated with sediments formed in local depressions of a tectonically active, shallow-marine environment and consist of strata-bound and stratiform sulfide lenses and layers several hundreds of meters long and wide, and tens of meters thick. A distinctive ore stratigraphy is recognizable: disseminated and stockwork sulfides overlain by breccia-hosted sulfides and banded to massive sulfides. Lead-rich fissure veins and quartz veins cut these types of strata-bound sulfides; barite is abundant in one of the prospects. The ores are made up of fine- to medium-grained intergrowths of sulfides and sulfosalts in quartzite and calc-silicate gangue. The sulfides are intensely recrystallized and annealed in the Aguilar deposit, which is located in the pyroxene-hornblende metamorphic halo. In the Esperanza prospect, located in the lower grade albite-epidote halo, the sulfides are finer grained, less recrystallized, not annealed, and contain abundant framboidal pyrite and textures of soft-sediment deformation.The calc-silicate assemblage in the Aguilar deposit is characterized by subcalcic garnets with significant proportions of spessartine (Sp (sub 26-78) mole %), almandine (Al (sub 5-23) ), grossularite (Gr (sub 8-52) ), and andradite (Ad (sub 0-14) ). Pyroxenes are manganese rich and iron poor, with an endmember range of hedenbergite (Hd (sub 10-55) mole %), johannsenite (Jo (sub 15-35) ), and diopside (Di (sub 20-80) ). Late-stage skarn minerals include calcium-rich bustamite, subcalcic actinolite, chlorite, and vesuvianite with up to 20 wt percent rare earth elements.Sulfur isotope values range from 10.8 to 26.5 per mil for sulfides, and from 32.4 to 34.0 per mil for barite. The strongly positive delta 34 S values for sulfides and barite are consistent with fractionation of sulfide and sulfate from Lower Ordovician seawater. Lead isotope ratios in galenas from all Aguilar ores have means of 206 Pb/ 204 Pb = 18.04, 207 Pb/ 204 Pb = 15.64, and 208 Pb/ 204 Pb = 38.03. Potassium feldspar from the Cretaceous Aguilar granite is far more radiogenic, having ratios of 206 Pb/ 204 Pb = 19.28, 207 Pb/ 204 Pb = 15.67, and 208 Pb/ 204 Pb = 39.00. These isotope ratios suggest an early Paleozoic, crustal source for the lead in the ores, unrelated to the lead in the granite.The overall geologic setting and geometry of the Aguilar ores, the distinctive ore stratigraphy, the mineral composition and textures, the calc-silicate assemblage, and the stable and radiogenic isotope evidence suggest that the sulfides in the Aguilar district formed as exhalative accumulations in a Lower Ordovician, shallow-marine sedimentary basin. The present distribution of the ores and their metamorphic textures indicate overprinting by contact metamorphism during the emplacement of the Cretaceous Aguilar granite.

Hydrothermal fluid circulation in the Tayoltita geothermal system evolved from pervasive to fracture-controlled flow. The early, pervasive circulation produced the widespread propylitic alteration around the mine. In the lower levels of the system, this pervasive propylitization is dominated by chlorite, whereas in the upper levels epidote and albite are also major components of the alteration. Fluid temperatures associated with this stage ranged from >360 degrees C in the lower level of the mine (based on the alteration assemblage oligoclase + actinolite + chlorite + sphene) to 2.0 wt % NaCl equiv) in the earlier quartz associated with ore. The higher salinity inclusions (also present in the early, isolated quartz veinlets) are believed to represent early mixing of a saline (magmatic?) fluid component.

Fe-Zn-Cu sulfide mineralization in the Stone Hill district of the northern Alabama Piedmont is hosted by zones of felsic schist and garnet schist within the Ketchepedrakee Amphibolite. The felsic schist is characterized by quartz-pyrrhotite-biotite-muscovite-plagioclase-rutilesphalerite _ chalcopyrite _ pyrite _ hornblende _ epidote assemblages and the garnet schist is characterized by garnet-hornblende-quartz-biotite-pyrrhotite-chalcopyrite _ sphalerite _ pyrite _ chlorite _ actinolite _ calcite assemblages. Both schists are well banded and highly deformed. The felsic schist locally contains ellipsoidal, polycrystalline aggregates of quartzfeldspar-biotite that superficially resemble sedimentary clasts but clearly formed tectonically by boudinage of felsic bands. The following petrographic and geochemical data and field relationships suggest that both schists are hydrothermally altered basalt (the precursor of the Ketchepedrakee Amphibolite): (1) the felsic and garnet schist units are very localized lenses (

Monterrosas, Eliana, Ra[uacute]l, and Condestable are the most important copper mines in the central coast of Peru, between lat 12 degrees 309 and 14 degrees 309 S. Their related geologic setting, close age relation, and similar mineralogy and geochemistry argue in favor of their grouping together as amphibolitic Cu-Fe skarn deposits. In the Lower Cretaceous continental margin, 100 to 120 Ma, incipient rifting accompanied by batholithic gabbrodiorite intrusions at depth gave rise to an elongated trough in which submarine volcanic and sedimentary rocks accumulated. Skarn deposits developed in association with the intrusive activity are characterized by chalcopyrite concentrations with low Ti magnetite and/or pyrite in actinolite gangue; minor constituents are amphiboles of hastingsitic and tschermakitic varieties, apatite, pyrrhotite, and sodic scapolite. Copper ores are fracture controlled and hosted by Albian gabbrodiorite plutons at Monterrosas and Eliana. Volcanogenic strata-bound mantos, as well as disseminated sulfide stringers and minor veins, occur at Ra[uacute]l and Condestable in Hauterivian to Albian volcano-sedimentary rocks. Geochronologic K-Ar data identify a Lower Cretaceous episode of metallogenesis for Ra[uacute]l, Condestable, and Eliana. Geologically related iron deposits of Hierro Acari and Marcona are dated as Lower Cretaceous and Upper Jurassic, respectively.Petrography and major element chemistry of the gabbrodiorite suite at Monterrosas and Eliana indicate a low K tholeiitic character with calc-alkaline affinities and high Na 2 O/K 2 O ratios. Late magmatic and hydrothermal activity developed in the roof zones of the plutons and formed amphibole-magnetite-chalcopyrite deposits. Mineralization at Eliana involved introduction of Cu, Fe, and P along fractures and contacts to form ore shoots. The scapolite alteration halo is characterized by Si, Na, and Ca enrichment. The hydrothermal deposits of Ra[uacute]l formed mainly at subvolcanic levels with a dominant seawater component (Ripley and Ohmoto, 1977, 1979). Geologic correlation with Eliana and Monterrosas indicates that both magmatic and seawater components coincided to form amphibolitic Cu-Fe skarn deposits, which span the plutonic and volcanic environments.Lead isotope compositions of chalcopyrite, pyrite, and galena define a primitive trend for these skarn deposits compared to that of porphyry Cu deposits in southern Peru and Chile and Sn deposits in Bolivia. The isotopic contrast is interpreted as a result of rifting and extreme crustal thinning during the Lower Cretaceous along the continental margin of central and southern Peru. This tectonic and metallogenetic environment continued into northern and central Chile where amphibolitic Fe and Cu deposits of the same age range are known.

Progressive regional metamorphism of the Negaunee Iron Formation in the Marquette district from chlorite to sillimanite grade has resulted in the development of three metamorphic zones and two subzones. The diagnostic zone assemblages are, in order of increasing grade: 1, siderite + quartz + or - ankerite + or - minnesotaite + or - stilpnomelane + or - magnetite; 2, grunerite + or - quartz + or - ankerite + or - magnetite and grunerite + or - siderite + or - ankerite + or - magnetite; 3a, grunerite + or - garnet + or - quartz + or - magnetite and grunerite + or - Ca amphibole + or - quartz + or - calcite + or - magnetite; 3b, grunerite + or - olivine + or - clinopyroxene + or - quartz + or - calcite + or - magnetite and grunerite + or - Ca amphibole + or - olivine + or - magnetite. The grunerite, garnet, and Ca amphibole, and olivine and clinopyroxene isograds define the boundaries between the respective zones and subzones. Temperature estimates are approximately 300 degrees C for zone 1 and approximately 600 degrees C for subzone 3b. Pressure is estimated at between 200 to 300 MPa. Microprobe analyses of coexisting minerals indicate general attainment of exchange equilibrium and a strong localized influence of bulk composition on mineral chemistry.Theoretical analysis of phase relations on isobaric T-X (sub CO 2 ) diagrams leads to the following conclusions:1. Throughout zone 1, minnesotaitc-bearing assemblages are diagnostic of low X (sub CO 2 ) values, stilpnomelane-bearing assemblages represent low to medium values, and chlorite-bearing assemblages represent medium to high X (sub CO 2 ) values. Juxtaposition of these three assemblages at one locality indicates that fluid phase compositional gradients most likely existed.2. Throughout zone 3, actinolite-, hornblende-, and garnet-bearing assemblages reflect similar low to high X (sub CO 2 ) values. The continued juxtaposition of these various assemblages suggests the continued presence of fluid-phase compositional gradients into the highest grade metamorphic zones.3. Metamorphic reactions caused substantial, localized CO 2 enrichment in the coexisting fluid phase. A diversity of T-X (sub CO 2 ) paths was followed, typically differing on the scale of individual mesobands.

Apatite-rich iron deposits are interbedded with gneisses and intermediate to felsic calc-alkaline metavolcanics (450 m.y. old) which show some well-preserved porphyritic, spherulitic, and volcaniclastic textures. They are intruded by the heterogeneous Avnik granitoid and the homogeneous Yayla granite (350 m.y. old). Iron ores are banded, massive, or disseminated and are located in a gradational zone between the gneisses and better preserved parts of the metavolcanic rocks. Magnetite, fluorapatite, and actinolite are dominant minerals in the three types of deposits; accessory minerals are feldspar, quartz, mica, diopside, hornblende, crossite, and sphene. Evidence suggests that the apatite-rich iron deposits formed initially during volcanism, and it is concluded that they were immiscible liquids which had separated from strongly fractionated magmas. Similar rare earth element values in coexisting apatite and magnetite and in the associated metavolcanic rocks support this conclusion. The apatite-rich iron deposits were remobilized into stockworks containing large crystals of magnetite, apatite, and actinolite where these have been intruded by the Avnik granitoid.--Modified journal abstract.

In the Yerington district of western Nevada, large bodies of skarn formed in a limestone unit of Triassic age adjacent to a Jurassic batholith. This batholith is dominantly granodiorite but contains quartz monzonite stocks and dike swarms that host major porphyry copper deposits. The units enclosing the limestone, an andesitc tuff and a silty limestone, were also extensively metasomatized, as were outlying intrusions of granodiorite.Skarn formation is divided into an early skarnoid stage and a late metasomatic skarn stage. The skarnoid stage formed massive fine-grained granditc garnet in the andesitc tuff, limestone, and silty limestone, with subordinate amounts of pyroxene. The iron content of garnets increases significantly away from the granodiorite, whereas pyroxenes maintain a relatively constant diopsidic composition.Granodiorite apophyses, which occur in the Triassic rocks up to 0.5 km from the main batholith contact, are extensively altered to endoskarn. The main alteration type consists of massive, anhedral granditc garnet with a significant titanium component. The distribution of endoskarn suggests that proximity to limestone was a key factor in endoskarn formation and that one hydrothermal fluid altered both sedimentary and igneous rocks simultaneously.The second stage of alteration most dramatically affected the limestone, forming coarse-grained skarn. Two skarn types are recognized: (1) a magnesium-rich type in dolomitized marble, dominated by pyroxene, and (2) an iron-rich type in calcite marble, dominated by andradite. Age relations between the two are ambiguous, but constraints inferred from T-X (sub CO 2 ) stability relations suggest that magnesium-rich skarn formed first and at high temperatures, followed by iron-rich skarn. During a late stage, intermediate granditc garnet veined both skarn types, and actinolite (+ or - salite) formed locally at the andradite-marble contact.In detail, magnesium-rich skarn nearest the granodiorite is zoned from diopside through serpentine (replacing diopside) + calcite, then from clinohumite + calcite + dolomite to calcite + dolomite marble. Additional phases include monticellite, spinel, magnetite, ludwigitc, and szaibelyite. Farther from the batholith, clinohumite is absent, tremolite separates serpentine from pyroxene, and periclase occurs locally.Pyroxene and serpentine are enriched in iron with distance from the granodiorite, the pyroxene changing from nearly pure diopside to intermediate smite a kilometer away. This compositional variation is consistent with an increase in mu Fe /mu Mg , with time and with distance from the batholith. The presence of magnetite pseudomorphs after hematite and the variation in magnetite composition indicate that the oxidation state decreased with time near the granodiorite.Iron-rich skarn consists of andradite garnet, which on the outer skarn contact directly replaced calcite. Inclusions of wollastonite noted in garnet cores from one sample suggest that initially a wollastonite zone was locally present at the skarn margin. Veins of actinolite and magnetite cut garnet skarn; late actinolite at the garnet-marble contact, locally a site of sulfide deposition, suggests a reversal in the trend toward increasing mu Fe /mu Mg . At the Douglas Hill mine, apatite, quartz, and sulfides replaced andradite.Mineral assemblages and fluid inclusion data indicate that both magnesium- and iron-rich skarn formed at low X (sub CO 2 ) , generally less than 0.1. Temperatures of formation of magnesium-rich skarn are inferred to have been in the range 650 degrees to below 400 degrees C. Temperatures of formation of iron-rich skarn are not constrained by observed assemblages. Fluid inclusion data show that late apatite replaced andradite at temperatures between 120 degrees and 200 degrees C.

Tilting associated with extensional tectonics has exposed 1,800 m of continuous palcovertical relief through the Jurassic-age Yerington porphyry copper deposit. The rarely observed root zones of a porphyry system are characterized at Yerington by sodium-calcium metasomatism. The principal hydrothermal reactions involve the replacement of primary K-feldspar by oligoclase and of primary biotite by actinolite. Copper has been removed. Sodic-calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts. The reactions accompanying potassic alteration are the reverse of those accompanying sodic-calcic alteration. Plagioclase reacted to form K-feldspar; hornblende reacted to form biotite. The potassic and sodic-calcic alteration couplet principally accompanied each of two separate intrusive events: (1) Luhr Hill granite stock and associated porphyry dikes and (2) Walker River granite porphyry dikes. The younger Walker River intrusion was emplaced at slightly deeper levels than the older Luhr Hill complex. Vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement.At lower temperatures, late sodic alteration was superimposed on early sodic-calcic alteration at deeper levels in the system, whereas late quartz-sericite-pyrite alteration was superimposed on potassic alteration at higher levels. Sodic alteration is characterized by the replacement of K-feldspar by albite and of biotite by chlorite. Copper deposited during potassic alteration was remobilized. The transition to lower temperature alteration is associated only with the Walker River intrusion. The hydrothermal system associated with the emplacement of the Luhr Hill complex had not evolved to lower temperature assemblages prior to emplacement of the Walker River dikes.Phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic-calcic alteration were limited to the range of 360 degrees to 480 degrees C at pressures between 300 and 800 bars. Late sodic alteration took place at similar pressures but at temperatures less than 360 degrees C.Nearly simultaneous development of potassic and sodic-calcic alteration resulted from the flow of fluids away from intrusions at shallow levels (potassic alteration) and from the flow of fluids toward intrusions at deeper levels (sodic-calcic alteration). Because K/Na and Na/Ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature, cooling of high-temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic-calcic alteration.

Very large pyrite porphyroblasts are a striking feature of the metamorphosed, massive, pyrrhotite-rich sulfide ores at Ducktown, Tennessee. The crystals, up to 300 mm across, are commonly euhedral but exhibit varying degrees of elongation and postgrowth fracturing and granulation. The inclusion patterns within the various porphyroblasts indicate growth before, during, and after the development of schistosity in the silicate gangue. The sphalerite inclusions within single porphyroblasts show no systematic compositional variations from core to rim, partly because the larger porphyroblasts were formed by the aggregation of numerous growing pyrite grains and partly because fractures and chalcopyrite allowed many of the sphalerite inclusions to reequilibrate at lower temperatures and pressures. Sphalerite inclusions surrounding single growth centers, which were selected to avoid reequilibration problems, show that they grew at a nearly constant pressure of 6.8 + or - 0.8 kb. This information is used to relate the growth of the pyrite porphyroblasts to the pressure-temperature evolution of the deposit.Pyrrhotite, which forms the matrix for the porphyroblasts, ranges compositionally from 47.4 at. percent Fe in the matrix to 47.7 at. percent Fe immediately adjacent to the pyrite crystals. Chalcopyrite commonly occurs as fracture fillings and as pressure shadows adjacent to the pyrite porphyroblasts. Minor sphalerite, galena, and magnetite are present. Gangue minerals are dominantly quartz, calcite, sheet silicates, actinolite, and talc.

Mines Gaspe extracts copper and molybdenum from skarn and porphyry copper orebodies at Murdochville in the Gaspe Peninsula, Quebec. Both types of orebodies occur in the zoned Copper Brook aureole, which formed in calcareous Lower Devonian strata around the Copper Mountain plug, one of a group of small rhyodacite porphyry sills, dikes, and plugs of Upper Devonian to Mississippian age. Progressive metamorphism is marked by the alteration of diagenetic pyrite to pyrrhotite and the successive appearance of phlogopite, tremolite, diopside, and grossularitic garnet toward the plug. Diopside formation was accompanied by destruction of calcite, pyrrhotite, and carbonaceous material to form a bleached inner aureole. Similar metamorphic zones surround the nearby Porphyry Mountain plug and occur (without garnet) in the Porphyry Brook aureole. The three aureoles appear to coalesce some 1,200 to 1,500 m below surface.Crackle veinlets in the strata domed by eraplacement of the Copper Mountain plug were healed by quartz, feldspars, diopside, and grossularitic garnet, the same minerals that were formed in the metamorphosed wall rocks. Wollastonite partially replaced diopside in a ring around the plug. Iron metasomatism near the plug then replaced diopside with hedenbergitic pyroxene and replaced pale grossularitic garnet and wollastonite with darker andraditic garnet in veinlets and concretions. Massive skarn formed in some metamorphosed limestone units. The Porphyry Mountain plug also domed the surrounding sedimentary rocks. No crackle veinlets or wollastonite formed but andraditic garnet and hedenbergitic pyroxene were deposited in adjacent joints and bedding fractures. Metamorphism and metasomatism were followed, in some parts of the Copper Brook aureole, by formation of scapolite and a second generation of tremolite. Two major periods of mineralization followed.Early mineralization is characterized by pyrrhotite, minor chalcopyrite, and traces of sphalerite throughout the three aureoles. It is associated with alteration of Ca-Mg-Fe silicates to tremolite or actinolite, chlorite, and epidote or clinozoisite and with deposition of albite, calcite, and quartz. More intense mineralization formed replacement orebodies at Needle Mountain and Needle East, some distance from the Copper Mountain plug. These orebodies were cut by late 1 quartz veins containing chalcopyrite, pyrrhotite, fluorite, and traces of scheelite.Porphyry copper mineralization occurs as disseminations and in four sets of veins centered on the Copper Mountain plug. Late 2 veins, which contain quartz + anhydrite + K-feldspar with traces of chalcopyrite and magnetite, may be coeval with biotization and K-feldspar alteration in the plug. Late 3 veins, which account for most of the copper and molybdenum mineralization, contain quartz + or - anhydrite with chalcopyrite, pyrite, and molybdenite. They are variously associated with kaolinization, sericitization, and minor anhydrite alteration in the plug and adjacent intrusions and with formation of tremolite, epidote, chlorite, and minor anhydrite in the metasedimentary rocks. Late 4 veins contain calcite or dolomite, quartz + or - anhydrite, and pyrite, with traces of K-feldspar, chalcopyrite, sphalerite, and galena. They are associated with calcite + sericite + pyrite alteration of the plug and calcite, dolomite, and phlogopite alteration of the metasedimentary rocks. Late 3 and late 4 veins often occupy almost vertical fractures parallel to the local north-northwest joint trend. Late 5 veins contain calcite, quartz, laumontite, and apophyllite, with traces of K-feldspar, chlorite, molybdenite, chalcopyrite, and pyrite or pyrrhotite. Traces of scheelite occur in late 1, late 3, and late 5 veins and in quartz + garnet veins which predate all sulfide mineralization.A model is proposed whereby emplacement of a pluton generated a convective flow of water which metamorphosed the overlying strata to form the aureoles. The Copper Mountain and Porphyry Mountain plugs, apophyses of the pluton, intruded the metasedimentary rocks and superimposed further metamorphic and metasomatic effects on them. Early mineralizaotion, like the prograde metamorphism, is widespread throughout the aureoles (although very weak in most areas). Late mineralization, by contrast, is closely associated with the Copper Mountain plug. The succession of widespread and spatially restricted events suggests that the postulated pluton and the visible plugs alternately controlled the sequence of metamorphism, iron metasomatism, and early and late mineralization. The difference in zoning patterns between Mines Gaspe and other porphyry copper-skarn complexes was probably due to the fact that the Copper Mountain orebody formed in previously decarbonated wall rocks.

The only remaining gold producer of seven interconnected mines. The district has produced over 710,000 kg Au since 1913. The area is underlain by part of the Timiskaming Group. Three types of gold ore: (1) native Au in chloritic fault gouge or small quartz lenses, within a thrust-fault system, called break ore; (2) gold-bearing quartz veins in both hanging and footwalls of this fault system, termed vein ore; (3) several relatively wide zones of intensely fractured, bleached, silicified, and pyritized rock containing lenses and pods of quartz with native Au and telluride minerals, called breccia ore. The delta 18 O and delta D of rocks and mineral separates in these three ore types suggest that the ore was precipitated from hydrothermal fluids of delta 18 O = 7 to 9.6 per mil and delta D = -35 to -85 per mil, at 350 degrees to 460 degrees C. Initial hydrothermal regime was followed by downward penetration of oxidizing, sulfate-bearing fluids, delta 18 O = 0 to 2 per mil, delta D = -20 to -70 per mil and of probable marine and/or meteoric water origin, initially at temperatures of

Park Premier is a low-grade gold-rich porphyry copper system located about 3 km east of precious and base metal veins of the Park City district, Utah. The Park Premier stock consists of five small intrusions that were emplaced into coeval volcanic rocks at depths less than 1 km. Two of the intrusions are microaplitic granodiorite porphyries that developed strong hydrothermal alteration, stockwork quartz veins, and low-grade disseminated copper-gold mineralization. Early alteration of these intrusions included biotite replacement of hornblende, deposition of disseminated magnetite and actinolite, local sodic alteration of plagioclase, and formation of (1) quartz + magnetite + actinolite + or - Na plagioclase and (2) quartz + K feldspar + or - magnetite + or - chalcopyrite veins. Mineral compositions, fluid inclusion data, and inferred alteration reactions indicate that early alteration occurred at temperatures > or = 600 degrees C from hydrothermal fluids with unusually high salinities (> or = 82 wt % total salts) and low K/Na molar ratios ( or = 700 degrees C) and low pressure (< or = 200 bars). Quartz + pyrite + or - sericite veins and local alteration of high-temperature minerals to chlorite, sericite, calcite, and clay minerals occurred at much lower temperatures (< or = 270 degrees C) by the influx of dilute (0-3 equiv wt % NaCl), boiling meteoric(?) water.Large areas of the igneous wall rocks of the microaplitic porphyries are altered to a variety of hydrolytic assemblages, including albite + sericite, sericite, kaolinite-dickite, pyrophyllite, and alunite (all + quartz + or - pyrite). Albite + sericite, sericite, and local kaolinite-dickite alteration probably formed from chloride-dominated fluids derived from the microaplitic porphyries. Pyrophyllite and alunite alteration postdate all other types of alteration and formed along major northeast-trending fracture zones from fluids that were probably sulfate rich.The hydrothermal system at Park Premier contains several features that are unusual in other well-described porphyry copper systems but which may be common in gold-rich porphyry systems; they include: (1) early, extremely saline fluids that only migrated locally, (2) abundant magnetite, actinolite, and sodic plagioclase and relatively minor biotite, K feldspar, and sulfide minerals, (3) sparse quartz vein formation at temperatures < 500 degrees C, (4) little interaction of the microaplitic porphyries with dilute fluids until the intrusions cooled to temperatures < or = 270 degrees C, (5) early albitization of wall rocks adjacent to the microaplitic porphyries, and (6) relatively high Au contents and high Au/Cu ratios of the microaplitic porphyries. These differences probably arose primarily from the very shallow depths of emplacement, small size of the intrusions, and low(?) sulfur contents of the magmas.

Three distinctly different calc-silicate-ore suites in the Brooks Range, northern Alaska, show contrasting geologic and metallogenic associations. Both host rocks and plutonic rocks are of Devonian age and all lithologies were subjected to a major Late Jurassic-Early Cretaceous regional metamorphic event. One calc-silicate suite is associated with peraluminous, high initial 8?Sr/86Sr, presumably S-type granites and is characterized by anomalous Sn contents, stanniferous grandite and subcalcic garnets with crosscutting amphibole-clinozoisite, and a distinctive boron-rich mineral association. These skarns resemble the Sn-bearing skams of the Seward Peninsula, except that they have low cassiterite contents and show only limited greisen alteration. The noneconomic nature of the Brooks Range Sn skarns may be related to relatively deep levels of exposure. The second calc-silicate suite is associated with generally metaluminous, presumably /-type, quartz-sericite-pyrite-altered granite and granodiorite stocks and isolated fault slivers. This suite contains high Cu and Ag, variable Pb and Zn, and low Sn; is characterized by andraditic garnet, diopsidic pyroxene, and bomite-chalcopyrite, cut by epidote, actinolite, and pyrite veinlets; and generally resembles a continental margin, porphyry copper-related skarn. The third calc-silicate suite, the so-called "gnurgle gneiss," represents a regionally metamorphosed metalliferous chamical sediment and is not related to igneous-metasomatic processes. This suite is characterized by welldeveloped mineral foliation, generally low metal contents, lack of sulfide veining and retrograde alteration, and combination of metal ratios and calc-silicate mineral compositions not commonly observed in skarn deposits.

Electron probe major element analyses are presented for amphiboles and biotites from porphyry copper-generating and barren intrusions in the Western Highlands, Papua New Guinea.Amphibole crystals from mineralized intrusions are characteristically zoned from magnesio-hornblende cores, through actinolitic hornblende, to actinolite rims (Mg enrichment toward rims). The frequent lack of intermediate compositions might record a sudden change in crystallization conditions. Coexisting biotites display less well-marked but qualitatively similar compositional variations (increasing Mg/Mg + Sigma Fe and decreasing Ti toward rims).In contrast, amphiboles and biotites from barren intrusions display variations ranging from common Fe enrichment toward rims (a trend previously considered typical of calcalkaline rock suites) to rare Mg enrichment toward rims (a trend observed only in very felsic differentiates of barren suites).The range of ferromagnesian mineral compositional trends observed in mineralized and barren intrusions is attributed to a range of likely f (sub O 2 ) conditions: in barren intrusions, f (sub O 2 ) was initially low and possibly increased in final stages of crystallization, whereas, in intrusions yielding mineralization, f (sub O 2 ) was initially high and became increasingly higher during crystallization.

Contact metasomatic magnetite ores of the Cornwall type in Pennsylvania occur as replacements of carbonate rocks adjacent to sheets and dikes of Triassic diabase. In addition to large (10 8 tons) magnetite deposits at Cornwall and Morgantown (Grace mine), about 45 smaller occurrences are known in a zone 150 km long. Major gangue minerals include early diopside, phlogopite, and garnet followed by actinolite, ohiorite, talc, serpentine, pyrite, chalcopyrite, and pyrrhotite associated with abundant magnetite. Based on mineral assemblages, temperatures of deposition are inferred to be less than 500 degrees C for most of the magnetite.At Cornwall, 50 samples of magnetite have delta 18 O between 5.2 and 11.0 per mil (SMOW), with the highest values (mean (x) delta 18 O = 9.3ppm, standard deviation (Sigma ) = 0.66ppm, n = 19) adjacent to the diabase and the lowest values (x = 6.6ppm, Sigma = 0.69ppm, n = 14) near replaced marble. Thirty-five magnetite samples from 19 other Cornwall-type deposits fall within a similar range (x = 7.9ppm, Sigma = 1.97ppm). Silicates from the Cornwall deposit also show unusually high delta 18 O (13 actinolites, x = 12.4ppm, Sigma = 0.47ppm; 6 phlogopites, x = 12.3ppm, Sigma = 0.73ppm; 3 diopsides, x = 13.1ppm, Sigma = 0.19ppm). At Cornwall, 36 samples of pyrite and chalcopyrite have delta 34 S (CDT) of 6.3 to 19.9 per mil (x = 10.9ppm, Sigma = 2.91ppm).Calcites from the ore (delta 13 C = -5.9ppm (PDB), Sigma = 0.93ppm, delta 18 O = 17.5ppm, Sigma = 0.45ppm, n = 4) are distinctly depleted in both 18 O and 13 C relative to the unaltered host limestones (delta 13 C = -1.3ppm, Sigma = 0.44ppm, delta 18 O = 20.6ppm, Sigma = 0.68ppm, n = 15), which are similar to other Cambrian limestones in the literature. Marbles adjacent to ore are intermediate in 13 C and 18 O between carbonates in ore and unaltered limestones. Thermally metamorphosed marbles adjacent to diabase but away from ore are depleted only in 13 C relative to unaltered limestones.The magnetite, silicate, and carbonate minerals in the skarn-type ore in all 20 deposits are calculated to have formed in fluids with delta 18 O (sub H 2 O) of 13 to 16 per mil. The very high delta 18 O of the fluid is difficult to explain by conventional ideas on a magmatic origin for skarnforming fluids. The diabase, which has a normal igneous delta 18 O of 7.4 to 7.8 per mil, would have furnished fluids with delta 18 O of 7 to 10 per mil and therefore camlot be the direct source of the ore fluids.The preferred explanation of the 18 O-rich skarn is that an 18 O- and 34 S-enriched ore solution was formed by heating and circulation of meteoric, connate, or possibly magmatic waters in sedimentary rocks near the contact of the diabase. Exchange of 18 O with shales, argillaceous sandstones, or limestones at high temperatures created the 18 O-rich fluid. This fluid, upon encountering carbonate in the contact zone of the diabase, replaced it with magnetite and silicates. The relatively high Cu and Co contents of the Cornwall ore suggest that the ore fluid either contained a magmatic component or interacted with diabase to some extent. Zoning of 18 O away from the contact appears to result from decreasing temperature of deposition of the skarn farther from the contact.An alternative explanation is that the high delta 18 O values of magnetite and silicates were determined by the high delta 18 O values of the replaced carbonate in a diffusion-controlled process. In a diffusion-controlled process, the amount of water flowing through the replacement front might be negligible compared to the flux of Fe and other dissolved species. The presence of delta 18 O-rich alteration within the diabase and the lack of correlation of delta 18 O-rich of ore with delta 18 O of replaced carbonate indicate that this process is probably not the dominant one causing the high delta 18 O, but it might contribute to the range of values observed.

Equigranular monzonite of the Bingham stock, the main host for the Bingham porphyry copper deposit, was an augite-actinolite-biotite monzonite before alteration, the same composition as the largely unaltered and unmineralized Last Chance stock adjacent to the mine. Mineralogy and chemistry of the alteration is quantified in a 31-sample traverse which illustrates changes in rock composition that occur over a distance of 7,000 feet outward from the center of the orebody. Plagioclase becomes more soda rich, orthoclase becomes richer in potassium, and trioctahedral mica becomes richer in magnesium toward the center of the deposit. Plagioclase is progressively replaced by K-feldspar and augite is replaced by actinolite which is in turn replaced by phlogopite. Trends in whole-rock chemical composition are congruous with these mineral trends. Based on mineral abundance, two main zones are recognized in pervasive hydrothermal alteration, a central quartz-orthoclase-phlogopite zone corresponding to potassium silicate alteration, and an outer actinolite-chlorite-(epidote) propylitic zone.

El Romeral iron mine is in the coastal iron province of northern Chile, a north-trending belt of magnetite deposits about 30 km wide and 600 km long.Iron at El Romeral is mined from two orebodies in which magnetite is microscopically intergrown with actinolite. The Main orebody is a lenticular, steeply dipping magnetite-rich mass in a reentrant between two lobes of the Romeral diorite pluton. The North orebody consists of conformable pods of magnetite in actinolitized biotite schists with steeply dipping foliation. Both orebodies trend northerly, adjacent and subparallel to the eastern contact of the western lobe of the diorite pluton and within a north-trending network of anastomosing strike-slip faults.Ore-zone boundaries are gradational, and altered rocks within and around the ore-bodies contain magnetite, actinolite, plagioclase (An (sub 22-32) ), diopside, clinozoisite, sphene, chlorapatite, marialitic scapolite, tourmaline, chlorite, pyrite, calcite, micas, and clays.The schists, phyllites, and quartzites of El Romeral probably are late Paleozoic, whereas the andesite porphyry, diorite, and magnetite deposits are early Cretaceous in age.Emplacement and crystallization of the Romeral diorite pluton preceded ore deposition. During late stages of diorite crystallization, quartzite and andesite porphyry near diorite contacts were dioritized by metasomatic additions of plagioclase (An (sub 20-30) ), hornblende, and diopside. Iron in hematite-banded quartzite was mobilized and redeposited as hematite 80 to 145 m from the diorite contact. Cordierite porphyroblasts grew in phyllite and schist, and diopside and oligoclase partially replaced andesite porphyry between the east and west lobes of the diorite pluton.Magnetite ore deposition was hydrothermal and was accompanied by pervasive actinolitization, probably at temperatures mainly in the range 550 degrees to 475 degrees C. Left-lateral movement on the Romeral fault was concurrent with ore deposition and with emplacement of intramineral diorite aplite dikes into en echelon, northwest-trending, steeply dipping gash fractures, which opened in response to the left-lateral faulting.Ore deposition was followed by chloritization, by emplacement of a few minor granitic dikes, and by alteration of previously actinolitized diorite and phyllite to aplitic rocks consisting of secondary plagioclase (An (sub 8-25) ), quartz, microcline, and minor tourmaline. Aplitization was followed by emplacement of many biotitic diorite aplite dikes, by formation of late actinolite-magnetite-apatite veins, by locally intense argillization and martitization, by right-lateral movement on the late North-northeast fault, and by emplacement of the postore batholith east of El Romeral.

Barite, massive sulfide, and siliceous stockwork deposits of Kuroko type in the Lima region are associated with the Casma Group, a sequence of submarine volcanic rocks of Middle Cretaceous age. These deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill. Volcanic-hosted deposits occur in the entire region; sediment-hosted deposits are restricted to the eastern Casma volcanic facies, which intercalate with limestones and shales deposited on a shelf platform adjacent to the marginal basin. In most cases, mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz-serieite alteration; the latter have locally been dated at 116 to 106 m.y. by the K/Ar method. Strata-bound deposits of bedded barite, pyrite, sphalerite, and pyrrhotite overlie the feeder zones.The most important deposits of this kind are Leonila-Graciela and Juanita, with 4 million tons of produced barite and 2.5 million tons of production plus reserves of massive sulfide ore. They are located in a roof pendant of folded strata intruded by two plutons of the Coastal batholith. Contact metamorphism of hornblende-hornfels facies affects both ore deposits and wall rocks. K-Ar ages on hornblende-biotite pairs from the granitic rocks indicate that they were emplaced 82 and 65 m.y. ago. Whole-rock ages on postmetamorphic dikes vary between 31 and 39 m.y.P-T conditions for contact metamorphism of hornblende-hornfels facies at Leonila-Graciela are estimated at 2.1 to 2.6 kb and 300 degrees to 500 degrees C on the basis of sphalerite geobarometry, stratigraphic reconstruction, metamorphic mineralogy, and interpretation of discordant K/Ar age patterns. Mole percent FeS in sphalerites increases in a prograde sense from the actinolite zone at Juanita to the biotite-muscovite zone at Gracicla. In massive sulfide specimens it varies correspondingly from 15.4 + or - 0.2 to 17.6 + or - 0.7. Sphalerites from siliceous stockworks show the same trend with 14.7 + or - 0.4 mole percent FeS and 17.6 + or - 1.1 mole percent FeS. Metamorphic equilibration was reached only in the biotite-muscovite zone at Graciela. This is demonstrated by the homogeneity of high mole percent FeS values detected in sphalerite, which coexists in mutual contact with pyrite and hexagonal pyrrhotite.

Classic schist-hosted emerald deposits are generally attributed to interaction between invading pegmatitic magma or vapor phases with preexisting metasediments, metavolcanics, and/or ultrabasic rocks. Detailed studies of the Habachtal (Austria) and Leydsdorp (South Africa) emerald deposits indicate that the emerald formation was not due to a single-stage contact metamorphism at the border zone of magmatic intrusive bodies. On the contrary, the emeralds are found in metasomatic zones (blackwall zones) associated with ultramafic bodies formed by syn- to post-tectonic reactions during low-grade regional metamorphism. These reactions occur at the contact of preexisting beryl- and phenakite-bearing pegmatites and albitite pegmatoids with biotite-talc and actinolite schists (Leydsdorp), or at the contact of beryllium-rich garnet-mica schists and biotite-plagioclase-gneisses with serpentinites and talc-schists (Habachtal).Critical evidence for the regional metamorphic origin of emeralds includes the following points:1. Augen textures occur in the metagranitic and metapegmatitic country rocks.2. Polystage growth with simultaneous deformation is indicated by sigmoidal inclusion trails in the strongly zoned emeralds.3. No evidence of contact metamorphism is found in the emerald ores, nor in the immediate country rocks.4. Both the rare earth element fractionation pattern and the Yb/Ca vs. Tb/Ca ratios of the fluorites which coexist with the emeralds indicate the existence of an abundant nonpegmatitic fluid phase during the formation of the emeralds.The following reactions for the formation of emerald from preexisting phenakite are suggested by textural evidence:3 phenakite + 2 biotite = 2MgFe-beryl + 2K (super +) + 4Mg (super +2) 3 phenakite + 3 albite + 2Mg (super +2) = 2MgNa-beryl + Na (super +) + Al (super +3) .The high magnesium and chromium contents--mainly in the rims of the emeralds--reflect their formation in an increasingly Mg- and Cr-rich environment. Microthermometric studies of fluid inclusions in the emeralds of both deposits suggest that they formed from an H 2 O-CO 2 mixed fluid phase, with a salinity of 1 to 9 equiv wt percent NaCl and homogenization temperatures around 300 degrees C.Mass balance and geochemical analyses suggest that the transformation of serpentinites and Be-rich country rocks into blackwall zones liberates significant amounts of Be and enables the formation of emerald in the presence of Cr. Due to unfavorable growth conditions during regional metamorphism, the extent of emerald mineralization never exceeds that of pegmatites.

Gold-mineralized skarns occur near the village of Muara Sipongi in West Sumatra and were mined for gold prior to World War II. They are developed in limestones and andesitic volcanics of Permo-Triassic age into which Late Jurassic diorites and granodiorites have been intruded. The intrusions are of I-type affinity.The skarns range from andradite-diopside rocks to grossular-idocrase-wollastonite-diop-side rocks which formed at about 450 degrees to 650 degrees C. Later retrogressive alteration caused the formation of epidote, prehnite, pumpellyite, actinolite, chlorite, calcite, and quartz. These phases record temperatures down to less than 200 degrees C. Fluids during skarnification probably contained less than 5 mole percent CO 2 . During retrogressive hydration the skarns were mineralized locally with chalcopyrite, pyrite, magnetite, hematite, bornite, and gold, followed by sphalerite, arsenopyrite, marcasite, tetrahedrite, Co-Ni sulfarsenides, and Au-Ag tellurides.Chlorite-calcite retrogressive alteration of the skarns is related to quartz veining which is hosted by the volcanic members of the country rocks. These veins are enriched in Pb + Zn compared to the skarns. Gold occurs as inclusions within arsenopyrite. Fluid inclusion data indicate the vein-forming fluids to be weakly saline and to contain minor CO 2 but also to contain significant amounts of CH 4 and N 2 . Homogenization temperatures fall in the range 180 degrees to 240 degrees C, which together with the low CO 2 correlate with the conditions inferred for chlorite-calcite skarn alteration.The composition of native gold derived from skarn and associated veins is characterized by 5 to 35 at. percent Ag and up to 0.8 at. percent Cu. A suite of alluvial golds in the area have negligible Cu and 45 to 65 at. percent Ag and can be related to a nearby suite of Tertiary epithermal quartz veins.The bulk Au-Ag-Cu pattern and gold compositions of the Muara Sipongi mineralization are discussed and compared with other examples of gold-bearing skarns and Cu porphyries with which they are often associated. The source and controls on mobilization of the gold may be significantly influenced by regional faults in the area which form part of the 1,600-km-long Sumatra fault zone. Pervasive alteration of altrabasic and basic assemblages, which are associated with this fault system, is one mechanism for liberating Au and other metals that are subsequently channeled upward along faults to be deposited in a favorable environment. The gold was most probably transported by thio and/or carbonyl complexes.

Surface mapping, core logging, and compilation of underground mine maps in the Carr Fork area have led to the definition of the stratigraphic, structural, and time-space distribution of copper, copper-lead, and lead-zinc-silver ores in the western contact aureole of the Bingham stock. The succession of alteration-mineralization stages in sedimentary rocks is defined, and each stage tentatively is correlated with stages in the evolution of alteration-mineralization in the Bingham stock.Sedimentary rocks in the contact aureole are of Pennsylvanian age and consist of quartzite with lesser amounts of calcareous carbonaceous siltstone and limestone. Compressive deformation from the southeast during Mesozoic time formed east-west trending folds and bedding plane faults across the district; differential yielding resulted in over-turning of folds in the highly deformed Carr Fork area. Following the main period of folding, stress was relieved by northward thrusting; in the Carr Fork area a set of northwest-striking imbricated thrust or right-lateral strike-slip faults formed in the upper plate of the basal Midas thrust. A system of northeast-striking faults, which were later to serve as major ore solution conduits, originated largely as tension fissures parallel to the direction of maximum principal stress. Multiple intrusions of monzonite and quartz monzonite during mid-Tertiary time were controlled to a large degree by the zone of north-east faults and by steep bedding. An Early Stage of contact metasomatism produced: diopside in quartzite and in interbedded, thin silty limestone beds; wollastonite, with minor idocrase and garnet, in thick cherty limestone; and a trace amount of sulfides. The Early Stage is related at least in part to the emplacement of the quartz monzonite of the Bingham and Last Chance stocks and continued during the initial emplacement of the quartz monzonite porphyry. Actinolite alteration of diopside in quartzite and garnetization of wollastonite-bearing marble represent the beginning of Main Stage mineralization and are time equivalent with biotite-orthoclase alteration of igneous rocks. As the stock is approached, actinolite alteration in quartzite and hornfels exhibits textural changes from bedding streaks to orbicules and to envelopes on fractures; closer to the instrusion, actinolite is altered to biotite along fractures. Traces of pyrite accompany actinolite in the outermost zones of bedding streaks and orbicules. Chalcopyrite accompanies pyrite as the fracture filling in the envelope zones. Near the stock, molybdenite veinlets have primarily biotite envelopes. Crosscutting veinlet relations indicate that the biotite zone expanded from the stock onto the outer actinolite zone during a trend toward increasing chalcopyrite: pyrite and molybdenite: Cu-Fe-sulfide ratios. Late pyritic veinlets with actinolite envelopes represent the waning of Main Stage mineralization in quartzite.Two cherty limestone beds, 15 to 60 m thick and separated by 100 m of quartzite, contain the major copper-bearing skarns in the district. Main Stage alteration in limestone consists of andradite-diopside superimposed on Early Stage wollastonite. Actinolite-diopside-garnet-epidote endoskarn occurs in quartz monzonite adjacent to garnetized limestone. In wollastonite, sulfides are chalcopyrite and bornite, with minor galena and sphalerite. Chalcopyrite: pyrite ratios increase from the outer edge of the andradite-diopside zone toward the intrusive contact, where trace amounts of bornite reappear. Chalcopyrite: pyrite ratios also increase with depth. Massive magnetite replaced garnet adjacent to the stock at lower elevations, and hematite is absent. At higher elevations, the skarns have a lower iron-oxide content, hematite appears, and the hematite: magnetite ratio increases to unity near the surface. These relations suggest an increase in oxidation and sulfidation states toward the surface and away from the stock. The Main Stage of ore deposition in limestone skarns culminated in the partial destruction of andradite and diopside. Apparent reactions include: carbonation of andradite yielding calcite and quartz with hematite, magnetite, or siderite; and combined carbonation and sulfidation, yielding pyrrhotite, pyrite, and chalcopyrite in addition to calcite and quartz. In the more diopside-rich skarn of the outer zones, alteration assemblages accompanying sulfide deposition include andradite-actinolite or actinolite in addition to the products of sulfidation and carbonation. These reactions may have been largely the result of declining temperatures; it is unclear whether any new Cu, Fe, and S was introduced at this stage.A Late Stage of alteration produced pyrite, chlorite, montmorillonite, sericite, and talc from earlier calc-silicates and locally redistributed chalcopyrite. Lead-zinc and gold deposits, accompanied by arsenic-bearing minerals such as tennantite and arseno-pyrite, also belong to this time frame. The Late Stage is believed to be contemporaneous with sericite-pyrite alteration of intrusive rocks.The following metal zones and their maximum right-angle distances from the stock contact are recognized: (1) lead-zinc-silver northeast fissure ore (Pb/Cu = 30 to 10) in nonsilicated limestone, up to 1,200 m; (2) lead-copper northeast fissure ore (Pb/Cu = 5 to 0.2) in nonsilicated limestone, up to 900 m; (3) copper ore (Pb/Cu < or = 0.02) in garnet skarn, up to 450 m. These zones do not represent a contemporaneous zonal growth pattern but rather reflect a complex history of expansion and regression of zones dependent on rock compositions, permeabilities, and the evolution of the ore fluid.

The Frisco mine is near the city of Parral, in southern Chihuahua, Mexico. It is worked by San Francisco Mines of Mexico, Ltd., and produces daily 1,500 metric tons of ore with a grade of 0.5 gram (0.015 ounce) of gold per metric ton, 150 grams (5 ounces) of silver, 5 percent of lead, 8 percent of zinc, and 0.6 percent of copper.The ore comes from about 32 fissure veins, which occupy faults of small displacement in a calcareous shale formation of probable Cretaceous age. The veins are arranged in a complex pattern whose description and explanation is the principal aim of the paper. From their attitudes and physical characteristics, the veins can be grouped into four sets of mutually parallel veins, each set parallel to a different imaginary plane. These four sets make up two systems of veins, each composed of two sets of conjugate shears. The two systems are believed to have resulted from two stages of deformation--distinct in orientation of stress but not necessarily separated by any large amount of time.The vein matter was probably introduced by a hydrothermal fluid during the second stage of deformation, when the fractures of the second system were formed and those of the first system were reopened. The walls and breccia fragments within the veins were silicified and silicated, with the formation of quartz, diopside, actinolite, epidote, and ilvaite. During the same general epoch, but somewhat later at any one place, the sulfide minerals sphalerite, galena, chalcopyrite, pyrite, and arsenopyrite were deposited--in large part as replacements of altered shale fragments. In most places, the last minerals deposited were quartz, fluorite, and calcite. The mineralogy is like that of skarn deposits and like that of the veins at nearby Santa Barbara, which Lindgren included in his hypothermal class.

The Precambrian iron formation near Atlantic City, Wyoming, lies within a large southwestward-plunging syncline of metasedimentary and metavolcanic rocks. The ore is at least 2,400 million years old, and may have been deposited more than 2,680 million years ago, in which case it is roughly equivalent in age to the older iron formation deposits of the Canadian Shield. The geology of the Atlantic City deposit is similar to that of Precambrian iron formations of the Shield. Magnetite is the chief ore mineral at Atlantic City, and the gangue consists of quartz, actinolite and actinolitic hornblende.Chemical analyses of drill core material average in weight percent: total iron, 33.45; Fe 2 O 3 , 30.75; FeO, 15.49; Al 2 O 3 , 3.85; CaO, 1.31; MgO, 2.80; MnO, 0.09; Na 2 O, 1.22; K 2 O, 0.36; and SiO 2 , 44.13. The values are similar to published averages for Precambrian iron formations from the Canadian Shield.Comparisons of Ca/Mg, Ca/Na, Ca/K, Mg/Na, Mg/K and Na/K ratios from the Atlantic City deposit with various present-day geologic environments and with average crustal percentages has provided chemical information which may help in establishing a mode of origin for the Atlantic City iron formation. Analyses of fumarolic gases, of certain acidic volcanic waters, and of Dead Sea water samples are shown to be statistically similar to analyses of the Atlantic City iron formation. Close similarities in terms of these ratios with average crustal percentages also add significantly to an understanding of the possible genesis of the deposit.It is suggested that the iron formation at Atlantic City, Wyoming, may have been derived through subaerial weathering of moderately low-lying crustal rocks in an atmosphere low in oxygen, and that the leachate was then carried to and concentrated in a closed basin, where it may have received additional contributions from volcanic emanations. The question of whether the basin of deposition was marine or continental remains unanswered.

Deposits of monazite and Nb-bearing rutile in a belt more than 10 mi. long in northern Lemhi County, Idaho, are distinguished by their notable concentrations of such elements as Th, Ti, Nb, Ba, Ca, Fe, P, S, and the rare earths, and by their abundance of carbonates, chiefly calcite. These deposits occur as structurally controlled replacements of crystalline limestones, amphibolites, and porphyroblastic gneisses which form part of the complex metamorphic border zone of the Idaho batholith. Most of the deposits, which are small and tabular, show a sporadic and irregular distribution of minerals, the most abundant of which are actinolite, allanite, apatite, monazite, calcite, barite, ilmenorutile (Nb-bearing rutile), ilmenite, and magnetite. Other minerals, some of them sparse, include biotite, phlogopite, garnet, epidote, glaucophane, ancylite, bastnaesite, sphene, rutile, chlorite, pyrite, pyrrhotite, chalcopyrite, and quartz. Silicates, except sphene, were deposited first and were followed successively by phosphates, carbonates, sulfates, oxides, and sulfides. Most of the Th entered the allanite instead of the somewhat later monazite. The deposits possess all the compositional characteristics of the carbonatites, but differ from the type carbonatites in their seeming lack of association with alkalic igneous rocks. Although the deposits were regarded by others as replacements of marble beds and as products of metamorphic differentiation, the unique association and concentration of elements so characteristic of the carbonatites, coupled with the paragenetic development, suggests that they, like the carbonatites, are of magmatic rather than of metamorphic origin.

A copper deposit about 40 miles southwest of the Coeur d9Alene district, Idaho, shows a marked structural and mineralogical resemblance to the copper deposits of the Ducktown type. It occurs as a replacement along bedding plane fractures and fissures in quartzitic beds of the Belt series and its ore consists chiefly of pyrrhotite, chalcopyrite, and lesser cubanite in a silicate and carbonate gangue. Fracturing repeated several times during mineralization caused deposition in three distinct mineral stages, shown by the partial replacement of one group of minerals by later ones. Minerals deposited during the first stage include the silicates (diopside, tremolite, actinolite, biotite, epidote, tourmaline, microcline, and muscovite), and a little quartz; during the second stage, the carbonate (ferriferous dolomite); and during the third stage, the sulphides (pyrrhotite, chalcopyrite, arsenopyrite, and cubanite), and a little quartz and calcite. Except for its mode of occurrence, the deposit possesses most of the characteristic features of a pyrometasomatic deposit but is, nevertheless, a hypothermal deposit formed probably under exceptionally intense temperature conditions.

"Galena is uncommon in pyrometasomatic deposits but does occur in Precambrian sulfide deposits of this type in central Sweden, in what were almost pure carbonate rocks. Associated minerals are in one deposit andradite and fluorite, but mostly magnesium-fluosilicates with tremolite or actinolite. Two contrasting deposits are associated with the same granite body; one is dominantly galena and other quite minor sulfides, the other is dominantly copper sulfides and galena is essentially absent. Thus, strong fractionation of sulfides must have occurred."

Igneous rocks in the Carr Fork area include the Bingham stock (granite followed by granite porphyry, gradational granite porphyry, Type A porphyry, and quartz-latite porphyry), the Last Chance stock (same age as granite of Bingham stock), and Carr Fork dikes (three types correlative with porphyries of Bingham stock). A zonal pattern of hornblende-augite alteration products and of magnetite and sulfide minerals is related to distance from the center of the Bingham orebody. The sequence of alteration minerals, inward, is calcite, chlorite, quartz, actinolite, sericite, biotite, and orthoclase, and the sequence of metallic minerals is magnetite, pyrite, chalcopyrite, and bornite.

The orebodies of Minas Carlota are steeply dipping pyrite lenses arranged en echelon in isoclinally folded limestones. The lenses occur at faulted contacts with serpentinite or in tensional fold areas.Mineralization was in the approximate order: (1) sphene, picotite; (2) actinolite, albite; (3) quartz, calcite, chlorite; (4) anomalous quartz, rutile, sericite; (5) fine-grained pyrite; (6) coarse pyrite; (7) chalco-pyrite; (8) sphalerite; (9) galena. (1) represents a magmatic or contact-metamorphic phase, (2) dynamic metamorphism, (3) a phase immediately preceding pyritization, while (4) overlapped with (5) and (6). Early pyrite (5) originated by replacement of limestone along graphitic laminae concurrently with rock folding, at a suggested temperature of 600 degrees C. Later pyrite (6) formed at lower temperatures as a space-filling. (7), (8) and (9) comprise less than 10 percent of the ore. Post-ore deformation is limited to marginal ore and represents differential adjustment to late faulting.Chemical studies suggest that low gold, cobalt and nickel values are contained in the fine-grained pyrite.Of pyrite crystals the cube is the only euhedral form in gangue (?), whereas in the late pyrite (6) cubes and cubes modified by a possible octahedral zone are present. A search of the literature suggests that: i) the cube is the commonest form of pyrite, ii) the cube is dominant in veins containing sericite, chlorite and carbonate, iii) the cube where associated with anomalous fringing quartz has grown contemporaneously with rotation and rock movement, iv) the pyritohedron develops at higher temperatures than the cube, v) pyrite octahedrons from meosthermal deposits invariably contain slight manganese values.The ores are regarded as hydrothermal. Their occurrence adjacent to serpentinite is considered an indication of structural control rather than of contact metamorphism. Crystallization of the upper portion of the magma may have led to sulfide enrichment at depth. Renewed earth movement forced the ore fluids upwards along the flanks of the intrusions.Serpentinization preceded pyritization. A decrease in volume on serpentinization created tensional areas in the limestone which were infiltrated by the ascending ore fluids. Mineralization and serpentinization are believed closely related in time and may be post-Jurassic, pre-Maestrichtian.The Carlota orebodies are parallel to regional structure. An investigation into the geotectonic implications of such orebodies is long overdue.