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In 2013, a diamond drill program tested an extensive advanced argillic alteration lithocap within the Hu’u project on eastern Sumbawa Island, Indonesia. A very large and blind copper-gold deposit (Onto) was discovered, in which copper occurs largely as disseminated covellite with pyrite, and as pyrite-covellite veinlets in a tabular block measuring at least 1.5 × 1 km, with a vertical thickness of ≥1 km. Copper and gold are spatially related with a series of coalesced porphyry stocks that intrude a polymictic diatreme breccia capped by a sequence of intramaar laminated siltstones, volcaniclastic and pyroclastic rocks, and overlain by andesite flows and domes. The porphyry intrusions were emplaced at shallow depth (≤1.3 km), with A-B–type quartz veinlet stockworks developed over a vertical interval of 300 to 400 m between ~100 and 500 m below sea level (bsl), 600 to 1,000 m below the present surface, which is at 400 to 600 m above sea level. In the area drilled at Onto, the diatreme breccia, all porphyry intrusions and, to a lesser extent, the surrounding older andesite sequence have all been overprinted by intense subhorizontal advanced argillic alteration, zoned downward from illite-smectite, quartz-dickite to quartz-alunite and quartz-pyrophyllite ± diaspore alteration. The alteration package includes two particularly well-developed zones of residual quartz with vuggy texture in subhorizontal zones at shallow depth, the upper one is still porous but the lower horizon, ~100 m thick, is largely silicified and is located at or near the top of the quartz-alunite alteration. Mineralization starts below the lowermost silicic horizon with more than 90% of the current resource in quartz-pyrophyllite-alunite and quartz-alunite alteration. Mineralization is dominated by a high-sulfidation assemblage of covellite-pyrite ± native sulfur largely in open-space fillings and replacements, but also as discrete pyrite-covellite and covellite only veins down to at least 1 km. Although the greatest amount of copper occurs as paragenetically late covellite deposited during formation of the advanced argillic alteration, approximately 60% of resource at 0.3% Cu cutoff still occurs within the porphyry stocks, indicating the porphyry stocks are a fundamental control on mineralization. There is considerable remobilization and dispersion of copper and, to a lesser extent, gold into the surrounding pre-mineral breccia and the late intermineral intrusions from the two earliest porphyry phases, resulting in quite consistent copper and gold grades throughout the currently delineated mineral resource. The very high sulfidation state of the mineralization is thought to be a consequence of the metal-bearing ore fluids cooling in the advanced argillic-altered host rocks in the absence of a rock buffer. Early chalcopyrite-bornite ± pyrite mineralization with potassic ± chloritic and sericitic alteration is only preserved on the margins of the system and more rarely at depth in a few holes 600 m bsl (~1,100 m below surface) but makes up only a small proportion (~8%) of the current resource. The Onto system is exceptionally young and formed rapidly in the middle Pleistocene and is not significantly eroded. A U-Pb zircon age for the andesite that caps the volcanosedimentary host rocks provides a maximum age of 0.838 ± 0.039 Ma, with a slightly younger porphyry zircon crystallization age of 0.688 ± 0.053 Ma. Re-Os dating of molybdenite that is associated with both the quartz vein stockwork and high-sulfidation assemblage copper mineralization shows overlap between 0.44 ± 0.02 and 0.35 ± 0.0011 Ma. 40Ar/39Ar ages for alunite within the advanced argillic alteration block ranges from 0.98 ± 0.22 to 0.284 ± 0.080 Ma, and alunite closely associated with covellite spans a period from 0.537 ± 0.064 to 0.038 ± 0.018 Ma.

The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn– rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300° ± 40°C from fluids of salinity higher than that of seawater (3.5–6.8 wt % NaCl equiv). According to laser ablation-inductively coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23–227 ppm) rather than in the primary sulfides ( 1,000 ppm) than all other sulfides (1 are more consistent with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various proportions of seawater and basalt or peridotite at 350°C shows that mineral assemblages broadly similar to those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious metals. A magmatic input is suggested by the occurrence of plagiogranites and tonalites dredged on sea floor in the Semenov area, which could be a potential source of Au-rich magmatic fluids, and by mineralogical and geochemical similarities with magma-related, low- to intermediate-sulfidation epithermal systems, namely high Au and Ag grades, high Au/(Cu + Zn + Pb) and Au/Ag ratios, and presence of Ag, Bi, and Te minerals. The likely crucial role of silicic melts in producing high Au and Ag grades suggests that exploration for precious metal-rich, volcanic-hosted massive sulfide deposits should be primarily directed to sites in which evolved igneous rocks occur on sea floor. Both in modern and ancient mafic-hosted deposits, zones characterized by abundant deposition of silica could be good clues to the presence of significant gold.

La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies. Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitic alteration with epidote, chlorite, calcite, specularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40 Ar/ 39 Ar geochronology on hydrothermal muscovite (i.e., sericite) to ~3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40 Ar/ 39 Ar ages constrain epithermal mineralization to between ~2.6 and ~1.6 Ma. Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor-rich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from ~143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite δ 34 S ranges from −16.9 to −11.3‰ at La Mascota and −8.3 to −6.1‰ at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on δ 18 O and δ D data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water.

A subseafloor replacement-style barite and sulfide occurrence was drilled in shallow waters at the Palinuro volcanic complex, the northernmost Aeolian arc volcano in the Tyrrhenian Sea, Italy. Using a lander-type drilling device, 11 successful drill holes yielded a total of 13.5 m of core from a sediment-filled depression located at a water depth of 630 to 650 m. The longest continuous drill core recovered consists of 4.84 m of massive to semimassive barite and sulfides with abundant late, native sulfur overprint. Seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active, although black smoker activity does not occur on the seafloor. The recovered drill core shows that the subseafloor deposit is zoned with depth. The top of the mineralized zone is comprised of a variably silicified vuggy barite-sulfide facies that shows notable polymetallic metal enrichment, while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades. Metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time. The barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis. During the main stage of the mineralization, the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite, trace famatinite, and rare cinnabar. Lower temperature precipitates formed during the main stage of mineralization include sphalerite, galena, pyrite, opal-A, and barite, which are associated with traces of Pb-Sb-As sulfosalts such as bournonite-seligmannite, or semseyite. A distinct mineral assemblage of fine-grained anhedral enargite, hypogene covellite, chalcopyrite, and galena is commonly associated with colloform sphalerite, galena, and pyrite as a late phase of this main stage. Colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence. The deposit is interpreted to have formed from fluids having an intermediate-sulfidation state, although excursions to high- and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite. Laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition (δ34S = −39 to +3‰), whereas a more narrow range is observed in the massive pyrite at depth (δ34S = −10 to 0‰). Similar variations were also documented for the late native sulfur overprint. Overall, the negative sulfur isotope ratios at depth, the intermediate- to very high sulfidation conditions during mineralization, and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth. Biological processes are interpreted to have played a major role during late stages of ore formation. The combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite, the intermediate- to high-sulfidation characteristics, and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment. The barite and sulfide occurrence at Palinuro shares many characteristics with porphyry-related base metal veins and intermediate-sulfidation epithermal deposits, suggesting that metallogenic processes associated with arc-related magmatic-hydrothermal systems are not restricted to the subaerial environment.

At Colquijirca, central Peru, a Miocene diatreme-dome complex is associated in space and time with several large epithermal polymetallic (Cu-Zn-Pb-Au-Ag) deposits of the Cordilleran class. Of these deposits, Smelter and Colquijirca, located in the northern sector of the district, are part of a continuously mineralized northsouth corridor that extends for nearly 4 km outward from the diatreme-dome complex and to depths of 1 km below surface. This corridor is zoned from Cu-(Au) ores in its inner parts (Smelter deposit) to peripheral Zn- Pb-(Ag) ores (Colquijirca deposit). The Smelter-Colquijirca corridor has undergone minor erosion, providing a good example of a nearly intact paleo-epithermal system of the Cordilleran class. This description of the hypogene mineralogical patterns of the Smelter-Colquijirca corridor leads to the proposal that they are the result of superimposition in time and space of three main stages. During an early quartzpyrite stage, in which basically no economic ore deposition occurred, carbonate rocks surrounding the Marcapunta diatreme-dome complex were replaced by quartz and pyrite. This was followed by the main ore stage, which was largely superimposed on the quartz-pyrite replacements and produced zonation of ore minerals and metals along much of the Smelter-Colquijirca corridor. The zoning from Cu ores to Zn-Pb ores is complex and comprises a number of distinct and well-defined zones that display abrupt or gradual interfaces between zones. From internal to external parts, these zones consist mainly of the following mineral associations and assemblages: (1) enargite ± (luzonite, pyrite, colusite, tennantite, goldfieldite, ferberite, gold-silver tellurides, bismuthinite, gold, alunite, zunyite, kaolinite, dickite, smectite, illite, sericite, quartz); (2) enargite ± (pyrite, quartz, bismuthinite, alunite, dickite, kaolinite, smectite, illite, sericite); (3) bornite ± (pyrite, quartz, alunite, dickite, kaolinite, barite); (4) tennantite, barite ± (dickite, kaolinite, chalcopyrite, Bi- and/or Ag-bearing minerals); (5) chalcopyrite, sphalerite, galena ± (pyrite, quartz, dickite, kaolinite, barite); (6) sphalerite, galena, pyrite ± (hematite, kaolinite, siderite, magnetite, marcasite); (7) Zn-bearing carbonate zone; and (8) a barren outer zone consisting mainly of calcite. The crosscutting relationships and zoning patterns indicate that during the main ore stage, the inner Cu zones progressively overprinted the Zn-Pb zones. Evidence for the subsequent contraction of the mineralization front is also recognized. A mainly fault-controlled late ore stage was the last recognized episode of mineralization in the Smelter-Colquijirca corridor, related to the deposition of chalcocite, digenite, and covellite as veinlets cutting and replacing the different Cu-bearing zones of the main ore stage; it is not economically important. The different mineral associations and assemblages provide insights into the chemistry of the hydrothermal fluids, particularly that of the main ore stage. The acidity and oxidation state of the main ore-stage fluids varied considerably prior to significant interaction with the host rock. This is best observed in the enargite-bearing zones in which most acidic fluids formed alunite-zunyite-enargite−bearing assemblages and less acidic fluids formed smectite-illite-enargite and muscovite-enargite−bearing assemblages. The common kaolinite-dickite-enargite assemblage was related to a fluid with intermediate character between these two end-member styles. The significant fluctuations in the acidity of the ore fluids in the central parts of the mineralizing system are interpreted to reflect mixing between variable amounts of acidic and oxidized magmatic vapor-derived fluids and less acidic low to moderate saline ore-forming fluids of magmatic origin.

Small, strata-bound copper sulfide deposits are found associated with bitumen in Lower Cretaceous basaltic andesite flows of the Pabellon Formation near Copiapo, north Chile. The physical characteristics of the bitumen and hydrocarbon biomarkers provide clues to the genesis, evolution, and possible role during copper mineralization. Biomarker analysis reveals a predominantly bacterial origin, with minor contributions from phytoplankton and higher plants, which were deposited in a shallow marine environment. An expulsion temperature between 64° and 87°C was determined, typical for low maturity petroleum. This petroleum was altered during migration by mixing with brines, increasing its viscosity by aromatization and sulfurization. At the moment of the accumulation, the temperature of the brine was under 150°C. The viscous oil filled the primary and secondary porosity of the basaltic andesite, which acted as an oil trap. After the oil accumulation, a hotter, hydrothermal influx resulted in thermal alteration of the petroleum and a further decrease of light alkane concentrations. High sulfur concentrations in the hydrothermal fluids led to further aromatization of the organic compounds, generating highly alkylated benzonaptho- and dinaphthothiophenes. Copper in the sulfur-rich solutions was reduced, triggering the precipitation of bornite, chalcopyrite, digenite, chalcocite, and covellite through thermochemical sulfate reduction. The bitumen interacted with metals by virtue of its inherent reducing characteristics (activated carbon) and was itself oxidized to form a pyrobitumen residue.

The Basin Lake copper-gold prospect lies in western Tasmania’s Mount Read Volcanics and is hosted in a series of calc-alkaline andesites, quartz-feldspar porphyries, mudstones, carbonates, and sandstones between the Tyndall Group and the Central Volcanic Complex. Alteration at the Basin Lake prospect occurs over a strike length of 1.4 km and includes thin, strata-bound pyrophyllite-quartz-paragonite-kaolinite-pyrite-alunite alteration zones, up to 12 m wide and containing up to 50 wt percent pyrophyllite, with local fluorite veining. These zones grade out to paragonite-muscovite-kaolinite-quartz-pyrite and muscovite-carbonate-chlorite alteration zones. Extensive propylitic alteration (chlorite-carbonate-epidote) affects most other rocks outside these zones. Mineralization consists of thin strata-bound zones of massive and vein pyrite, tennantite, and chalcopyrite, with trace covellite and galena, hosted mainly within an intensely silicified core of the pyrophyllite-quartzsericite alteration zone. Pyrite has δ34S values of –1.4 to +6.9 per mil, although marginal vein pyrite in the propylitic zone has δ34S values around 12.4 per mil. Large silicified glacial erratic boulders at surface contain massive and vein pyrite, enargite, and tennantite, with minor barite, and trace covellite, stannoidite, and mawsonite. Pyrite and enargite have δ34S values of 1.7 to 6.8 per mil; barite has δ34S values around 35.2 per mil with 87Sr/86Sr around 0.7108. The alteration and mineralization at the Basin Lake prospect is similar to that associated with high-sulfidation copper-gold systems formed by acidic, relatively oxidized fluids. A new geochemical vector, here termed the “advanced argillic alteration index” [AAAI = 100 (SiO2)/(SiO2 + 10MgO + 10CaO + 10Na2O)], has been devised to help quantify the intensity of alteration. The values of the AAAI at Basin Lake are similar to those of several high-sulfidation epithermal deposits. The low sulfide δ34S values are similar to those at other sulfide occurrences in the Mount Read Volcanics that have previously been considered to be barren, are lower than those of nearby volcanic-hosted massive sulfide deposits, and may indicate a magmatic fluid component. However, the δ34S and 87Sr/86Sr values of Basin Lake barite at the assumed highest exposed level of the system and higher δ34S values in pyrite from marginal veins are similar to those of Cambrian volcanic-hosted massive sulfide systems, indicating the involvement of reduced seawater sulfate at these locations. Calcite carbon and oxygen isotope values, silicate oxygen isotope values, and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic, magmatic fluids were likely neutralized by seawater. This occurrence strengthens the case for prospecting the Mount Read Volcanics and other similar submarine volcanic belts for copper-gold and gold-only deposits that formed by the actions of hyperacid oxidized fluids.

The textures and mineralogical assemblages of high-grade ore samples from the low-sulfidation Au-Ag epithermal deposits of Pongkor (2 Ma; West Java, Indonesia) and Nazareno (10 Ma; Cordillera Shila, southern Peru) were studied to investigate processes related to post-depositional alteration of primary mineral parageneses. Both deposits are characterized by a late primary high-grade base and precious metal stage composed of chalcopyrite-sphalerite-galena-pyrite–polybasite-pearceite–tennantite-tetrahedrite–electrum. Minerals of the polybasite-pearceite series, the main Ag carriers, were found to be transformed into acanthite. Acanthite was also found locally in association with native silver, as a replacement product of base metal sulfides, as veinlets, alternating rhythmically with supergene iron oxyhydroxides replacing pyrite, as well as associated with barite, iron oxyhydroxides and covellite as boxwork fillings. Primary electrum grains with a composition of about Au0.6Ag0.4 are partially replaced by Au-Ag sulfide, locally identified as uytenbogaardtite (Ag3AuS2), and/or by acanthite, with both of the replacing minerals commonly associated with vermicular electrum of a greater fineness (up to Au0.8Ag0.2). The occurrence of the replacement textures near the present-day surface and the associated secondary minerals, strongly suggests a supergene origin for the Au and Ag sulfide phases. The comparison of Pongkor and Nazareno also suggests that supergene remobilization of Au and Ag is geochemically significant and can occur in widely differing morphological and climatic contexts.

The Escondida porphyry copper deposit, located in northern Chile, was one of the two largest copper producers of the world in the 1990s. The hydrothermal evolution of this deposit is associated with the emplacement of a late Eocene-Oligocene quartz monzonitic to granodioritic intrusive stock complex composed of at least three intrusive phases and hosted by Paleocene andesite. This Paleocene andesite is underlain by Mesozoic and Paleozoic sedimentary and volcanic rocks, which are characteristic of the basement of the Precordillera de Domeyko province in northern Chile. We propose that the intrusive stock associated with the mineralization at Escondida was emplaced in a tensional gash formed between sinistral strike-slip faults of the regional Domeyko fault system. The complete evolution of the porphyry system is characterized by overprinting of pervasive and vein-associated alteration-mineralization styles grouped in three main hydrothermal stages. The early stage includes a zone of pervasive biotitization of andesite and development of a silicification shell around the intrusive complex, propylitic alteration around the biotitic zone, and a vein-associated orthoclase-quartz ± anhydrite-biotite alteration that mark the end of stage one. This early alteration contains magnetite, chalcopyrite, and bornite, with less than 0.5 vol percent of sulfides and a hypogene copper grade of

Chuquicamata, in northern Chile, is the world’s greatest copper orebody. It was controlled, from the initial intrusions (probably at 36–33 Ma) through mineralization (last major hydrothermal event at 31 Ma) to postmineral brecciation and offset, by the West fault system. East porphyry, West porphyry, Banco porphyry, and Fine Texture porphyry make up the Chuqui Porphyry Complex. East porphyry, the dominant host rock, has a coarse, hypidiomorphic-granular texture. Intrusive contacts between most porphyries have not been found, but early ductile deformation, subsequent pervasive cataclastic deformation, and faulting affects all of the rocks and makes recognition of intrusive contacts very difficult. Potassic alteration affects all porphyries, comprises partial K feldspar and albite replacement of plagioclase, and more widespread biotite replacement of hornblende, with igneous texture largely preserved. It is accompanied by granular quartz and quartz-K feldspar veinlets, which contain only trace disseminated chalcopyrite ± bornite remains from this early stage. Fine-grained quartz-K feldspar alteration, with destruction of biotite and apparently following albitization of plagioclase, accompanies strongest cataclastic deformation and destruction of igneous texture. A band of quartz-K feldspar alteration, up to 200 m wide and 1,500 m long, lies along the southward extension of Banco porphyry dikes and is the locus of the bornite-digenite center of the sulfide zoning pattern. This passes east through chalcopyrite-bornite to chalcopyrite-pyrite as sulfide abundance fades out. Sulfides in quartz-K feldspar alteration are abundant only where there is intense crackle brecciation. Propylitic alteration is superimposed on biotitic alteration at the eastern edge of the deposit, but there is no pyritic fringe. Westward, this zoning is interrupted by the superposition of pyritic main-stage veins with pervasive quartz-sericite. Veins of quartz-molybdenite, up to 5 m wide and cutting all porphyries, were emplaced between the early and the main stages. These veins and early-stage quartz veins are commonly segmented and sheared, with fine recrystallization of quartz that eliminates all original fluid inclusions. It is not clear whether quartz-K feldspar alteration was formed later or earlier than quartz-molybdenite veins. Main-stage veins were focused along a structural zone adjacent to the West fault. This stage is distinctly younger than early-stage mineralization, although it occupies many of the same structures and may involve massive remobilization of earlier mineralization. It may represent a more brittle and much shallower environment, which followed significant erosion of the upper parts of the early mineralization system. Main-stage veins with quartz, pyrite, chalcopyrite, and bornite were formed during dextral shear of the West fault system. The last mineralization of the main stage was enargite, digenite, covellite, pyrite, and minor coarse sphalerite, along with sericite, and locally alunite but only local traces of pyrophyllite and dickite. Some northwest enargite veins were apparently opened after the sense of shear on the West fault system changed to sinistral. Vein and veinlet filling faults and fault-related shatter zones contain the overwhelming proportion of copper at Chuquicamata in all alteration zones and assemblages, including pyrite-free early-stage assemblages. Practically all of these fractures have been opened and mineralized more than once. A still poorly understood late stage formed digenite with relatively coarse grained covellite from deep in the sericitic zone and flaring upward and outward under what became the supergene chalcocite enrichment blanket. The presence of associated anhydrite, typical also of earlier stages but largely leached or hydrated to gypsum by later supergene action, proves this is not supergene covellite, but it is otherwise very difficult to distinguish from supergene covellite. Rims of sphalerite on primary sulfides, almost invariably with inner rims of coarse-grained covellite and/or digenite, occur below the chalcocite blanket from which Zn has been leached. The sphalerite rims are interpreted by most as supergene, but the close association with apparently late hydrothermal covellite-digenite and their absence in all other porphyry copper deposits suggests they too may be hydrothermal. A partially preserved leached capping and oxide copper ore, replacing an upper chalcocite blanket, overlie a high-grade supergene chalcocite body that extends up to 800 m in depth in the zone of fault brecciation and pervasive main-stage sericitic alteration. Some leached copper moved laterally to form exotic copper oxides and silicate in adjacent gravels. Continued movement on the West fault produced a wide zone of brecciation and major displacements of mineralized rock. Net sinistral displacement of about 35 km is indicated by regional mapping, but the details of how much of each stage of mineralization was displaced and how far on which splits of the fault are not well understood. The uniqueness of Chuquicamata is due to its intimate and complex relationship with active regional faulting and to superposition of at least two distinct periods of mineralization.

In the Kupferschiefer deposits of Poland, Cu, Ag, Pb, Zn, Fe, noble metals, and other transition elements were concentrated by a process of catalytic oxidation and desulfurization of organic matter. They occur as several well-defined horizons that are zoned from the bottom upward: noble metals (southwest perimeter of orebodies), Cu, Pb, Zn, and pyrite.The noble metal content in black shale clay-organic matrices reaches 1,900 ppm Au, 1,900 ppm Pd, and 600 ppm Pt. Gold valence determinations suggest that Au occurs in two forms: native microinclusions with 0 valence containing silver, and Ag-free microinclusions with gold valence +1 probably bound to bitumens. The noble metals content in the organic matrix of thucholite reaches 2,528 ppm Au, 5,000 ppm Pd, and 1,770 ppm Pt. Pt is probably bound to tetrapyrrole rings, with Ni and V as vanadyl.The vertical distribution of metals is controlled by redox interfaces. On the reduced side of the interface, up to 0.5 m thick, the following minerals occur: a continuous Ag-Au series, several Pd arsenides and sulfide-arsenides, Bi sulfides, tennantite, Co-Ni arsenides, castaingite CuMo 2 S 5 , thucholite, covellite, and digenite. On the oxidized side of the interface, up to 1 m thick, the following minerals occur: gold of high fineness, hematite, PdBi sobolevskite, clausthalite, native Pb, plumbian gold, native palladium, graphite, and relicts of organic matter. The oxidized portion of the redox interface, containing lower noble metal contents, occupies a much larger area than the reduced counterpart with its higher pay metals values.

Minerals of the Cu-Fe-S system from the Lubin and Rudna mines in the Polish Kupferschiefer display textures and compositional trends consistent with copper depletion. The compositions, optical properties, textures, and distribution of Cu-S minerals indicate progressive alteration from chalcocite to covellite in the sequence chalcocite-djurleite-anilite-geerite-spionkopite-yarrowite-covellite. Bornite compositions plot on the Cu-Fe-S phase diagram along a line between ideal bornite (Cu 5 FeS 4 ) and Idaite (Cu 3 FeS 4 ). The range in bornite compositions extends from ideal bornite to Cu (sub 3.5) FeS 4 . Copper-poor bornites within the composition range Cu (sub 4.5) FeS 4 to Cu (sub 3.5) FeS 4 texturally appear to have partially decomposed to covellite and chalcopyrite. Both of these mineralogical trends have been previously reported from zones of supergene alteration in other copper deposits, where they result from copper leaching. Grains observed to be of uniform anilite composition indicate that the alteration occurred at low temperatures, probably less than 40 degrees C. In the Lubin mine samples, the most copper-depleted minerals, yarrowite, covellite, copper-poor bornite, and chalcopyrite occur in association with veins of gypsum, anhydrite, and atacamite. This association probably results from the interaction of saline mine waters with the sulfides. In the Rudna mine samples, copper leaching is confined to distinct impermeable horizons, most notably at the top of the Weissliegendes and in the Kupferschiefer at the contact between the Cu and Pb zones. On both horizons, the mineralogical change passing upward corresponds to increasing copper depletion. Considering the impermeability of the horizons and the low temperature of anilite formation, the horizon-specific copper depletion probably occurred during early diagenesis...

The strata-bound copper-silver-antimony deposit at Copper Hill, New Mexico, developed in a distinctive structural trap, here termed a "brittle-ductile trap," during regional retrograde metamorphism of Early Proterozoic metasedimentary rocks. The deposit is localized near the contact between massive orthoquartzite (Ortega Formation) and the overlying schists (Rinconada Formation). Peak regional metamorphism at 4 kbars and 500 degrees C occurred near the end of a regional shortening event that formed the upright, west-plunging Copper Hill anticline. At Copper Hill, late metamorphic, vertical, north-south-striking faults and fractures cut the Ortega Formation quartzite and terminate against folded Rinconada Formation schists. Synmetamorphic quartz veins, several centimeters to 1 m thick, fill many of the fractures and terminate against the overlying schists. At the quartzite-schist contact, many of the veins merge into mushroom-shaped bodies that project along the contact. Fracture-controlled and disseminated Cu-Ag-Sb minerals occur within the quartz veins and in quartzite near the veins. The mineral assemblage includes malachite, chrysocolla, stibiconite, cuprite, and minor chalcocite and covellite, but some of these are late-stage alteration-oxidation minerals derived from an originally sulfide- or oxide-dominated suite. A syn- to late metamorphic age for the primary deposit is indicated by: (1) the crosscutting nature of the mineralized veins, (2) local replacements of kyanite and staurolite by economic minerals, and (3) evidence for postmineralization annealing of the veins and quartzite. During retrograde metamorphism, the quartz veins and massive quartzite behaved brittlely whereas the overlying schists deformed ductilely. SiO 2 -bearing and then Cu-Ag-Sb-bearing metamorphic fluids migrated through the fractured quartzite and ponded below the folded impermeable schists. Although the Copper Hill deposit is not currently economically viable, it presents a model for retrograde metamorphic mineralization in rocks with varying mechanical properties that may be important in other regions.

Mississippi Valley-type ore samples from two mines in the Viburnum trend of southeast Missouri have been examined with the SHRIMP ion microprobe to determine the extent of their microscopic sulfur isotope variations. Approximately 140 SHRIMP delta 34 S values were obtained. Microscopic delta 34 S variations within and among the intergrown Fe-Cu-Pb-Zn sulfides are very large (-10 to +25%) and indicate an extremely complex history of mineral deposition. Textures record the general paragenesis: Fe sulfide, galena and sphalerite, chalcopyrite and bornite, and covellite. Early FeS 2 (delta 34 S = -10 to + 10%) is generally much more depleted in 34 S than later galena or chalcopyrite (delta 34 S = 0-25%). Both early FeS 2 and late chalcopyrite growth sequences exhibit large zonations in delta 34 S, typically from light to heavy values over a 15 per mil range. Isotopic zoning in galena crystals is comparatively negligible, but variations among different galena crystals encompass a large total range in delta 34 S values (0-25%) that is similar in magnitude to the total ranges observed in early FeS 2 and late chalcopyrite. Where later sulifides crosscut and replace earlier sulfides, the SHRIMP data indicate that there has been negligible local recycling of S from earlier sulfides in the ore-forming zone; each generation of metal appears to have brought in its own isotopically distinct and evolving batch of sulfur. During successive deposition of each metal generation, the observed delta 34 S variations require mixing among two or more isotopically distinct distal source fluids feeding already reduced sulfur to the ore-forming zone, and/or the existence of in situ sulfur isotope fractionation mechanisms, producing reduced sulfur whose isotopic composition varied progressively with time in the ore-forming zone.

The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of 95 wt percent SiO 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO 2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite+ or -pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines. Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization. Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (delta 18 O = 7+ or -2 per thousand, delta D = -25+ or -5 per thousand, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a delta 18 O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the delta 18 O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCl equiv (some inclusions contain daughter minerals of halite and sulfur). The T h values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees C at Akeshi to about 200 degrees C at Iwato and 230 degrees C at Kasuga. The presence of vapor-rich inclusions, some with T hv similar to T hl , indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200 degrees to 300 degrees C, AuCl 2 (super -) complexing may dominate over HAu(SH) 2 at salinities above about 2 wt percent NaCl. Several conditions are conducive for high sulfidation mineralization to occur: (1) a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid, (2) the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich, (3) the gas-rich (HCl and SO 2 +H 2 S) vapor ascends to the surface with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and (4) the dense, metal-bearing fluid also ascends into this leached zoned and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water. If the saline liquid is not released from its source adjacent to the magma, due to lack of fracturing, or if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached barren rock which is characteristic of many eroded volcanic terranes.

The Salt Chuck intrusion, Alaska, is a Paleozoic, Alaskan-type, ultramafic-mafic rock complex that contains a Cu-Au-Pd-Ag deposit from which about 0.3 million metric tons of ore at an estimated grade of 0.95 percent Cu, 1 ppm Au, 2 ppm Pd, and 6 ppm Ag were recovered. Near the mine, layered gabbroic rocks are intruded by a massive clinopyroxenitic unit with a few thin magnetite layers. Clinopyroxenite contains cumulus clinopyroxene, magnetite (with ilmenite lamellae), and apatite; postcumulus minerals are plagioclase, magnesiohastingsite, and phlogopite-eastonite. Alteration and metamorphism have produced assemblages of epidote, actinolite, chlorite, sericite, titanite, and calcite, especially in contact with sulfides. Most of the sulfide ore occurs in clinopyroxenite near its contact with layered gabbro; sulfides commonly consist of chalcopyrite, bornite, digenite, chalcocite, and covellite. The sulfides are disseminated in clinopyroxenite, but some are in pyrite-pyrrhotite veins, epidote veinlets, and calcite-rich veins.Platinum-group minerals (PGM) are predominantly kotulskite (PdTe) as single grains or with sperrylite (PtAs 2 ), palladium antimony minerals, and gold; the PGM occur mainly free in epidote, at sulfide grain boundaries, and in decreasing quantities in bornitc, chalcopyrite, digenite, chalcocite, and covellite. Kotulskite is variably intergrown with or rimmed and successively altered by temagamite (Pd 3 HgTe 3 ), sopcheite (Pd 3 Ag 4 Te 4 ), and hessitc (Ag 2 Te).The high Cu and precious metal content is interpreted to have resulted when a calcium-bearing fluid remobilized elements from primary magmatic sulfide assemblages. This fluid was either deuteric, or external fluid that had become enriched in Cl by reaction with magmatic amphibole, mica, and apatite (remnants of which have Cl contents in excess of 0.1 wt %). The fluid migrated along or to the clinopyroxenite-gabbro contact where it reacted with the primary igneous mineral assemblage, replacing magmatic sulfides by more Cu-rich assemblages and precipitating platinum-group minerals. This process continued to very low temperature where digenite, chalcocite, and covellite were stable and mercury- and silver-bearing PGM partly replaced earlier kotulskite.

The Skouries porphyry copper deposit is located in the eastern Chalkidiki peninsula, northern Greece. Along with several other porphyry copper occurrences, this deposit establishes the northeastern part of the Servomacedonian zone as a porphyry copper region. Amphibolite, mica schist, and two-mica gneiss are the predominant rock types exposed in the Skouries area. These rocks have been intruded by the Skouries porphyry stock, which is of subalkaline composition and is Tertiary in age (18 Ma). The Skouries porphyry intrusion and the surrounding country rocks are strongly fractured and intensely altered by hydrothermal fluids. In addition to intense silicification, a prominent alteration type is potassic, whereas phyllic and propylitic types are less common.Mineralization mainly includes chalcopyrite, pyrite, bornite, chalcocite, and magnetite in the form of veins, stockworks, and disseminations. Native gold and electrum commonly are present as small inclusions in chalcopyrite. Although lateral zoning is not defined clearly, a vertical zoning in terms of both metal mineralogy and overall metal content is apparent. A zone of oxidation (azurite-malachite) covers the upper part of the ore and is underlain by a zone of reduced rock rich in covellite and chalcocite, which in turn is underlain by the most important ore type, including a chalcopyrite-pyrite-bornite-magnetite assemblage.Both weakly and intensively altered porphyry samples from the deposit were analyzed for major oxides, trace elements, and all platinum-group elements (PGE). The stock exhibits high levels of Cu, Au, Ag, B, Bi, Co, Se, and Te. Barium and Sr contents are also high while Rb is low, resulting thereby in high K/Rb and low Rb/Sr ratios. Palladium concentrations range from less than 2 to 480 ppb. Ruthenium content ranges from 8 to 41 ppb and Pt from 2 to 10 ppb. Osmium, Ir, and Rh content is below detection limit. The average Au/Ag ratio is 0.87, which is consistent with the range of several worldwide porphyry copper deposits.On the basis of factor analysis of selected geochemical data, a strong positive interelement correlation (r > or = 0.83) between Au, Ag, Pd, nd Te coupled with their association with silica appears to support a correlation of these precious metals with silicification rather than potassic-type alteration. Such a relationship may suggest similar behavior of these metals and a common origin. A positive correlation of Cu with iron (r = 0.86), which is consistent with the association of chalcopyrite with magnetite, and the low correlation of Te and Pd with Cu may suggest that these metals and, to a lesser extent, copper have been redistributed during a late stage of evolution of the Skouries porphyry system. Furthermore, the close association of Te with precious metals, combined with the relatively low temperature for telluride minerals stability, may indicate that at least partly precious metal tellurides have been formed in a relatively shallow and less hot environment.

Concentration of minor and trace elements in host mineral phases has been estimated using a new approach which avoids difficulties in the direct analysis of complexly intergrown ores using existing techniques, such as neutron activation of grain concentrates of only limited purity, or the relatively high detection limit of the electron microprobe. In contrast, the new method utilizes a general relationship between whole-rock composition, i.e., gold or silver assays, mineral abundance, and mineral composition. Assuming limited compositional variation over relatively short distances in individual drill hole intervals, least squares regression provides a best fit of mineral composition to systems of linear mass balance equations relating whole-rock assays to mineral abundance. Matrix algebraic methods using singular value decomposition have been applied to over 1,000 composite samples in the Butte District of Montana. Maximum estimated precious metal contents in sulfides within the zoning patterns described in parts per million are: chalcocite-digenite, 926 ppm Ag, 5 ppm Au; bornite, 1,075 ppm Ag, 3 ppm Au; chalcopyrite, 211 ppm Ag, 2 ppm Au; pyrite, 9 ppm Ag, 0.3 ppm Au. Locally, and with significantly greater uncertainties, covellite appears to contain up to 10,000 ppm Ag (1 wt %), and enargite-tennantite contains up to 1,700 ppm Ag.--Modified journal abstract.

Metasomatic effects have been thermodynamically computed for magma-derived sulfur gases, which may contaminate convecting meteoric fluids in the monzonite K-silicate protore, and are compared with natural assemblages in the Butte District of Montana. The advanced argillic assemblage with covellite is produced in hypogene environments at 300 degrees C by sulfur metasomatism of protore only for a limited range of net sulfur valences, essentially -1 to +2, placing limiting constraints on the possible ratios of SO 2 and H 2 S gases. A second important factor is the relative reaction rates of protore and contaminating sulfur gases. Contamination by sulfur gases is a third factor of importance. A final factor is that a minimum negative net volume of reaction causes, by wall-rock dissolution and fracture enlargement, the high permeability necessary for circulation of enormous quantities of hydrothermal fluids and vein growth characteristic of this extreme alteration facies.--Modified journal abstract.

Marcopper porphyry copper deposit is located in the Philippines near the center of the island of Marinduque, 180 km south of Manila. The island9s tectonic history is dominated by northwesterly trending elements of the Philippine mobile belt. Mineralization is related to differentiated intermediate intrusives which occupy the core of a northwesterly trending regional anticline. The deposit occurs on the western margin of a middle Miocene quartz diorite stock with the majority of the copper mineralization within a granodiorite porphyry phase. Eocene and Oligocene metasediment and meta-volcanic wall rocks are locally brecciated and mineralized.The orebody contains 102,000,000 tons averaging 0.58 percent copper. It is 1,400 m long, up to 600 m wide, and has been drilled over a vertical extent of 450 m.Leaching, oxidation, and secondary enrichment have taken place within the top 30 to 90 meters. Primary mineralization is largely vein and fracture filling with some dissemination. Chalcopyrite, pyrite, and magnetite are ubiquitous together with minor amounts of bornite, covellite, molybdenite, gold, and silver.An irregular alteration/mineralization zoning accompanies the primary orebody. A central core of the highest grade copper mineralization is accompanied by potassic alteration and heavy quartz stockwork locally intensified to complete silica flooding. Phyllic followed by propylitic alteration zones surround the potassic zone. Irregular argillic alteration zones occur between the inner propylitic and inner zones. The plus 0.25 percent copper contour approximately follows the phyllic-propylitic alteration zone boundary.

Fletcherite, Cu(Ni, Co) 2 S 4 , a new thiospinel mineral, occurs in copper-rich pods in the Fletcher mine of the Viburnum Trend (New Lead Belt) of southeast Missouri. Fletcherite occurs as euhedral to subhedral crystals 5 to 200 mu m in diameter, disseminated in pods of bornite, chalcopyrite, and digenite. Other associated minerals include covellite, pyrite, vaesite, galena, and tetrahedrite. Reflected light examination reveals that fletcherite takes a good polish, is creamy white, has a moderate reflectivity, is isotropic, and has a polishing hardness between chalcopyrite and pyrite. Micro-indentation hardness measurements yield VHN values of 368 to 464 kg/mm 2 . Measured reflectivities, R% (at specific wavelengths) are: 36.4 (420 nm), 40.9 (460 nm), 42.5 (500 nm), 43.4 (546 nm), 45.3 (589 nm), 43.8 (600 nm).The compositional range observed among 39 individual analysis is Cu (sub 1.13) Ni (sub 1.04) Co (sub 0.84) Fe (sub 0.06) S (sub 4.00) to Cu (sub 0.67) Ni (sub 2.11) Co (sub 0.61) Fe (sub 0.01) S (sub 4.00) . The Space Group is Fd3m, Z = 8, a o = 9.520Aa. The name is for the Fletcher mine of the St. Joe Minerals Corporation in which the type material was found.

The Summitville Au-Cu-Ag deposit occurs within a porphyritic quartz latite, primarily in a zone of "vuggy silica" alteration characterized by leaching of all major elements except silica and iron. Vuggy silica is best developed between an elevation of 3,700 and 3,500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz-alunite alteration. The vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3,400 m. These deep vuggy silica zones are enclosed by quartz-kaolinite alteration with little or no alunite.Sulfide mineralization occurs primarily within the vuggy silica. The sulfide assemblage changes with decreasing elevation from covellite + luzonite + or - native gold to chalcopyrite + tennantite. Sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite. Gold also occurs in a near-surface barite + goethite + jarosite assemblage that crosscuts the vuggy silica.Consideration of alunite, kaolinitc, and pyrite stability relations and of aluminum speciation at 250 degrees C indicates that a pH of 2 or below and a log f (sub O 2 ) of -31 + or - 1 (at 250 degrees C) were necessary to produce vuggy silica alteration. The chalcopyrite + tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions. The covellite + luzonite + or - gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization. Barite + gold mineralization is interpreted as a near-surface assemblage produced under highly acidic, oxidizing conditions.These textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid-leaching event. The change from extreme acid-leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic-vapor-dominated to a liquid-dominated system, leading to more reducing and less acidic conditions during sulfide deposition.

The geology, general mineralogy, and chemistry of a massive, mixed-sulfide deposit on Vanua Levu, Fiji, are described. The deposit is located in a sequence of Miocene-Pliocene rhyodacites, andesites, and basalts, with associated volcaniclastic sediments. It is believed that it was formed during a period of waning volcanism by fumarolic activity near the sea bed, and it shows resemblances to the kuroko ores of Japan.The common primary sulfides are pyrite, chalcopyrite, sphalerite, and galena with lesser amounts of enargite, tennantite, bornite, and idaite. Lead-zinc sulfides are concentrated toward the top of the deposit and copper sulfides toward the bottom. Covellite occurs in a zone of supergene enrichment below a well-developed gossan. Ore minerals were deposited by a mixture of open-space precipitation and replacement, and colloform textures are common. There is some evidence that Cu, Pb, and Zn have a direct magmatic origin. Ore deposition was accompanied by silicification, argillization (kaolinite and montmorillonite), introduction of gypsum, and barite.

Theoretical modeling of the sequences and amounts of Ag, Co, Cu, Fe, Mn, Pb, and Zn precipitated from 25 degrees C, 3-molal chloride waters saturated with dolomite or calcite, quartz, talc, gypsum, and initially, hematite or goethite was performed assuming parameters of constant chloride molality, pH, and concentrations of major dissolved species. Precipitation sequences were modeled by progressively increasing H 2 S fugacities from a minimum defined by the cuprite-native copper join to a maximum defined by the rhodochrosite-alabandite join in the respective waters. The initial contents of Ag, Co, Cu, Fe, Mn, Pb, and Zn in the waters were defined by source and transport constraints outlined in Haynes and Bloom (1987). Successive modeling runs were made at pH values of 7.6, 6.6, and 6.1, with pH controlled by the CO 2 -calcite (or dolomite) buffer in all waters. The choice of parameters, as well as their variation during the runs, was constrained by a hypothesis that most sulfide deposition in stratiform copper deposits hosted by low-energy sediments occurs within 50 cm of the sediment-water interface during bacterial sulfate reduction (Haynes, 1986a). Initial water compositions approximated continental, closed-basin waters derived from basaltic and granitic provenances, respectively, so that the likely total compositional range of probable source rocks for stratiform copper deposits was represented in the modeling.A generalized precipitation sequence is native silver-chalcocite-bornite-galena-linnaeite-sphalerite-cattierite-pyrite. Native copper appears prior to chalcocite at a pH above 6.6 if the initial copper concentrations in the waters are 800 ppm or at a pH above 7.2 if copper concentrations are 100 ppm. Linnaeite does not precipitate at pH values less than 6.3. Precipitation of CoCO 3 and rhodochrosite occurs in basalt-derived waters upon acidification from an initial pH of 7.6. Chalcocite, galena, linnaeite, sphalerite, and alabandite are the most abundant phases precipitated, and all, with the exception of alabandite, precipitate at H 2 S fugacities above the hematite-pyrite join. The amounts of bornite, covellite, and pyrite precipitated are small and chalcopyrite precipitation does not occur.Precipitation of all lead, cobalt, and zinc, as sulfides, requires H 2 S fugacities above 10 (super -12) bars at a pH of 7.6 or above 10 (super -9) bars at a pH of 6.6. Chalcocite is the only sulfide precipitated if H 2 S fugacities do not exceed 10 (super -14) bars at a pH of 7.6 or 10 (super -12) bars at a pH of 6.6.Modeling predicts a sulfide zonation sequence and metal ratios similar to those observed in stratiform copper deposits hosted by low-energy sediments. Modeling demonstrates that H 2 S fugacity in host rocks exerts an important control on metal ratios. If H 2 S fugacities do not exceed 10 (super -14) bars, deposits will consist of copper and silver, with no associated lead, zinc, and cobalt; if H 2 S fugacities exceed 10 (super -12) bars, deposits will have associated trace or minor lead, minor or significant cobalt, and minor or major zinc, depending on the source rock and transport constraints detailed in Haynes and Bloom (1987).Modeling suggests that chalcocite precipitation resulting from pyrite replacement is unlikely because the amount of copper removed from the water by such a process will be less than 2 ppm. However, precipitation of hematite or copper-iron sulfides will accompany pyrite replacement if iron saturation limits are locally exceeded. Local precipitation of hematite or copper-iron sulfides as a result of pyrite replacement is consistent with the occurrence of hematite-chalcocite intergrowths at White Pine and the occurrence of associated leucoxenebornite in the Kamoto Principal deposit.Modeling fails to predict the small cobalt contents in stratiform copper deposits with associated lead and zinc, and it fails to predict the occurrence of abundant chalcopyrite and bornite in most large stratiform copper deposits hosted by low-energy sediments. Small cobalt contents of deposits such as those within the German Kupferschiefer are probably caused by limited cobalt availability in source rocks during the metal-leaching episode. Relatively large amounts of bornite and chalcopyrite possibly originated by a postburial alteration of chalcocite-pyrite composite grains deposited during the bacterial sulfate reduction episode. Evidence for such postburial alteration is currently not diagnostic and clearly requires further research.Modeling was performed using parameters constrained by the hypothesis previously described by Haynes (1986a). Because predicted zonal sequences and predicted metal ratios, with the explicable exceptions of chalcopyrite, most bornite, and cobalt, are similar to those observed in stratiform copper deposits, the hypothesis could be a realistic description of the cause and timing of native copper, native silver, chalcocite, some bornite, galena, sphalerite, linnaeite, and cattierite precipitation in such deposits.

The Jardin arsenian Cu-Ag deposit of the Copiapo mining district, northern Chile, is in many respects representative of the volcanic-associated, disseminated, cupriferous manto mineralization characteristic of this segment of the central Andean orogen. Containing ca. 1 to 2 million metric tons of ore with a grade of 1.75 percent Cu and 140 g/metric tons Ag, the crudely stratiform deposit is hosted by the upper, brecciated and unwelded part of a approximately 75-m-thick, continental rhyolitic ignimbrite and by an overlying approximately 2-m succession of tuffaceous lacustrine sedimentary rocks. The ignimbrite represents the local basal unit of the Paleocene Hornitos Formation. The reduced, ore-hosting, sedimentary horizons were deposited in an ephemeral saline pond and comprise coarse and fine breccias, tuffaceous sandstones and silt-stones, and calcareous-carbonaceous shales containing thin coaly seams and accumulations of plant debris.The greater part of the mineralization occurs as disseminations, discontinuous veinlets, thin concordant lenses and, most strikingly, cylindrical bodies, 1 to 8 mm in diameter, oriented perpendicular to bedding. Chalcocite, bornite, and tennantite (ca. 1-4.3% Ag) are the major ore minerals and are associated with minor sphalerite, digenite, covellite, chalcopyrite, wit-tichenite, native silver, acanthite, stromeyerite, mckinstryite, jalpaite, and the ruby silvers. Pyrite and marcasite are subordinate constituents of the ore and widely display evidence of replacement by the Cu and Ag minerals. Mineralization in both the ash flow and sediments is associated with moderate argillic and carbonate alteration. The hanging wall of the orebody lies within a red fanglomerate, encroachment of which terminated the accumulation of the lacustrine strata.The sulfidic pipes commonly display concentric mineral zonation suggestive of the successive, in part open-space, precipitation of pyrite and the Cu-Ag minerals. Textural evidence suggests that at least the introduction of pyrite into these bodies occurred prior to the deposition of the immediately overlying strata. The pipes, both individually and in aggregate, do not resemble rhizoliths or dewatering structures. More probable origins are as Scolithus sp. lebensspuren or as gregarious hydrothermal conduits directly reflecting the upward penetration of ore-forming fluids. Sulfide-free pipes are extremely rare, and we favor the latter interpretation.The Jardin mineralization displays features characteristic of several ore deposit types, particularly of the red-bed and epithermal clans. Although the temporal relationships of Cu-Ag mineral deposition and lacustrine sedimentation are uncertain and direct evidence of the conditions of mineralization unavailable, our preferred genetic model involves the convective flow of metal-bearing ground waters through the permeable upper zone of the ignimbrite into the mantling lacustrine strata, during both the fumarolic stage of the tuff and the deposition of the sediments. If this be the case, the emplacement of the disseminated mineralization may have taken place in less than a century; the temporal constraints would be reduced if the Cu-Ag minerals formed largely through replacement of early diagenetic pyrite. The hydrothermal fluids are inferred to have been Cl rich, low temperature, neutral to weakly alkaline, and with an f (sub O 2 ) exceeding the hematite-magnetite buffer; they probably derived their metals and arsenic from nearby, weathered, Upper Cretaceous base and precious metal deposits exposed by the sub-Hornitos Formation erosion surface. Ore deposition occurred in response to reduction, neutralization and moderate cooling on contact with the roof zone of the ignimbrite and the carbon- and sulfur-rich sediments; there is no evidence for sulfide replacement of organic detritus.The Jardin manto may perhaps best be interpreted as an unusual, Ag-rich, red-bed copper deposit nucleated in part by an ash flow cooling on the floor of an intermontane basin. However, the Paleocene epoch in the Copiapo district saw the development of numerous Ag-Cu-As epithermal vein systems, and the deposit could also represent an analogous center in which ore deposition was controlled by a sequence of permeable reduced sediments rather than by faults.

Stoichiometric reaction coefficients of minerals and aqueous species in overall heterogeneous chemical reactions have been lithologically determined expressing intense late-stage modification of early low-grade copper-molybdenum mineralization during the growth of the large vein systems at Butte, Montana. Application of the principles of irreversible thermodynamics and conservation of mass in open chemical systems has provided the basis of a rock analysis method offering a quantitative deduction of the reaction chemistry, mass transfer, and magnitude of progressive late Main Stage hydrothermal alteration of the pre-existing mineralized wall rock. The critical information is mineral mass data derived from a systematic analysis of diamond drill core and crosscut samples from the western edge of the zone of superposition of hydrothermal events. The stoichiometric coefficient of each mineral is determined from the slope of its molar variation with Main Stage reaction progress variable, xi , related directly to the number of moles of an index mineral used for reference. Relict chalcopyrite has been chosen in this regard as it was formed during the pre-Main Stage hydrothermal event in the area studied and has undergone progressive destruction during Main Stage activity. Stoichiometric coefficients of aqueous species are determined by charge balance and conservation of mass within the Stoichiometric equations. By a conceptual removal of all severe, modifying effects caused by the large and relatively high grade Main Stage veins controlling wide and intense alteration envelopes, the nature of the early pre-Main Stage hydrothermal events have been reconstructed.The early high-temperature (600 degrees -700 degrees C) mineralization consisting of fracture-controlled molybdenite and disseminated pyrite and chalcopyrite is found to be distinctly zoned in its unmodified state at the elevation of this study, approximately 3,400 feet below the surface. The early zonation in plan, from outside toward the center of the pattern, is a peripheral pyritic zone with approximately 5 weight percent pyrite, an oxide zone consisting of 1 to 2 percent magnetite plus hematite with a copper grade contributed by early chalcopyrite of 0 to 0.2 percent, and an axially symmetric maximum early chalcopyrite zone with over 0.5 percent copper surrounding a lower grade chalcopyrite interior zone with approximately 0.2 percent copper. Molybdenum grades increase continuously from essentially zero at the outer edge of the maximum early chalcopyrite zone to over 0.05 percent inside the interior low-grade early copper zone. No pre-Main Stage bornite has been recognized.Superposition of late, relatively low temperature (200 degrees -350 degrees C), continuous high-grade Main Stage veins and ubiquitously related alteration envelopes upon the early disseminated sulfides and associated potassium silicate alteration has resulted in profound mineralogic effects. The earliest Main Stage modification of mineralized wall rock involves very little effect on the pre-existing pyrite and chalcopyrite, but plagioclase is converted in an outer argillic facies first to montmorillonite which in turn is replaced by kaolinite. Magnetite is partially oxidized to hematite during the early clay-forming event.Following these incipient, Main Stage effects, more intense effects are monitored continuously by the reaction progress variable and include continuous chalcopyrite destruction and hypogene leaching of early disseminated chalcopyrite from the pre-Main Stage assemblage in the near-vein sericitic facies of alteration. Molybdenite is apparently unaffected by strong sericitization. During the early part of intense Main Stage events, total copper decreases in the rock. As the reaction continues, Main Stage fluids eventually reach saturation with respect to bornite, which precipitates in vein structures. Finally chalcocite, digenite, covellite, and enargite form in narrow high-grade veins resulting in a net copper addition or hypogene enrichment. Throughout the entire span of Main Stage reaction, biotite, hematite, magnetite, orthoclase, and kaolinite after montmorillonite after plagioclase are progressively altered to an assemblage of sulfides, sericite, and quartz. Sphalerite of pre-Main Stage or early Main Stage age is slightly leached during Main Stage events.Interpretation of the Main Stage reaction chemistry in terms of calculated phase equilibria at 250 degrees C implies a strong pre-Main Stage mineralized wall-rock buffering effect on the hydrothermal fluids during the early stages of Main Stage reaction. With the progressive destruction of the early potassium silicate assemblage of chalcopyrite, magnetite, biotite, and orthoclase, oxidation effects proceeded, with a reduction in a (sub Fe (super +2) ) /a 2 (sub H (super +) ) by three orders of magnitude, essentially at constant a (sub Cu (super +) ) /a (sub H (super +) ) . First, saturation with respect to pyrite and bornite was reached, and ultimately high-grade chalcocite, covellite, enargite assemblages formed during the most intense sericitization of the wall rock at values of xi greater than 0.5. The presence of early pre-Main Stage chalcopyrite is interpreted to be of primary importance in buffering the initial Main Stage fluid composition at a relatively high value of log a (sub Cu (super +) ) /a (sub H (super +) ) near -4.5 causing subsequent hypogene sulfide saturation in the high-grade veins. Although the approach taken is admittedly macroscopic in nature in that the physical details of diffusion and infiltration are not considered, it has been possible to analyze exceedingly complex rock masses and to understand the coupling of mineralization and alteration, revealing the importance of element recycling to the vein-forming process. It is possible that much of the element content, i.e., base metals and sulfur, of high-grade Main Stage Butte veins originated as disseminated sulfides in the early hydrothermal protore mass which underwent a progressive hypogene redistribution and enrichment during a late-stage fracture-fluid circulation event possibly related to cooling porphyry intrusives that were emplaced after and geometrically offset from the earlier pre-Main Stage intrusives and hydrothermal events.

The Number One orebody is the largest and most sulfide-rich ore zone (at least 200,000 tons containing 8.4 wt % Cu) at the Ruby Creek copper deposit, in the southwestern Brooks Range of Alaska. Pyrite and copper-bearing sulfide minerals are concentrated within the matrix of a dolostone breccia body, which is enclosed by phyllite and calcic marble of Middle to Late Devonian age. The Number One orebody has three mineralogical zones that grade into each other: (1) an outer zone, widest toward the hanging wall, containing mostly pyrite with minor amounts of chalcopyrite and traces of carrollitc and sphalerite; (2) an intermediate zone containing major chalcopyrite and pyrite, minor tennantite-tetrahedrite, bornitc, carrollitc, and sphalerite, and traces of galena; and (3) a core zone containing major bornitc, chalcopyrite, pyrite, and chalcocite, minor carrollitc, digenite, and sphalerite, and traces of galena, covellite, and the germanium-bearing sulfides renierite and germanitc. Small clots of anthracitelike organic material (anthraxolite) are found throughout the ore. Much of the pyrite is fine grained and was deposited before the other sulfides, being increasingly replaced by Cu-bearing sulfides from the outer zone to the core. Some of this pyrite recrystallized into coarser grains having cobaltiferous rims, and these grains were generally not replaced. The sulfide minerals are commonly pseudomorphous after lath-shaped crystals within the dolomite clasts; no unreplaced examples were found, though the crystals closely resemble those of marcasite. The abundant fine-grained, porous pyrite also may represent replacement of marcasite. The origin and timing of brecciation and ore deposition remain unknown, though dolomitization and ore deposition appear to have occurred in preexisting carbonate breccia. Close mineralogical and geologic similarities are noted with the Kipushi Cu-Zn-Pb deposit in Zaire and with several other carbonate-hosted copper-rich organic-bearing deposits. The common concentration of the rare metals Co, Ge, and Ga in these structurally and mineralogically complex ores should increase their economic attractiveness.

Phase relations in the system Cu-Co-S have been studied using sealed silica capsule methods between 400 o and 900oC. The high-temperature portion of the system is dominated by the coexistence of the chalcocite-digenite solid solutionvith cobalt sulfides (COS2, COl_xS, Co4S: ) . AS temperature is decreased to 880o___ 5oC, the dominant ternary phase, the carrollite-linnaeite solid solution. appears with a composition of Cu0.aCo2.aS 4. The copper-rich compositional limit is extended on cooling, reaching a maximum (CuCo2S4) at ,500oC. Below 507oC covellite is stable and coexists in the sulfur-rich portion of the system with copper-bearing cattlerite. Below 500oC the major changes involve a change in stable assemblages (at ,450oC), in which the high-tempera- ture pair Co + Cu2S is replaced by the pair Cu +Co0S s. the disappearance of Co_xS (

The oxidation effects of potassium-silicate protores from heated meteoric water have been thermodynamically computed and confirm previously described copper vein-forming mechanisms involving hypogene leaching and enrichment of chalcopyrite-bearing protore (Brimhall, 1979). Intense copper leaching of protore chalcopyrite is shown to be related to a state of peroxidation in which the only stable minerals are quartz, pyrite, and muscovite following the complete destruction of the wall-rock oxygen fugacity buffer assemblage including hematite dissolution. Because modification of the protore continues to higher fluid/rock ratios, copper is extracted from the fluid phase and again is fixed in the rock as sulfides, first as bornite and ultimately as chalcocite-covellite associated with the advanced argillic alteration assemblage. This end-stage assemblage should occur only where extremely high fluid/rock ratios are reached in zones of unusually high permeability near the source of the heat responsible for the fluid motion.The results of the calculations are interpreted to mean that certain intervals of fluid/rock ratio are characterized by net base metal leaching and other intervals by metal fixation. Attainment of only intermediate fluid/rock ratios may result in the major amount of the copper in a protore being remobilized into solution and leaving the system through ground-water circulation. In contrast, in highly evolved systems where the fluid/rock ratio has increased to extremely high values, the presence of an advanced argillic alteration assemblage with chalcocite and covellite strongly suggests the presence within a district of a partially oxidized, potassium-silicate, disseminated sulfide protore, including a strongly leached and sericitized quartz-pyrite protore relict.Near-surface vein mineralization may conceivably be related to primary or unoxidized protore at depth in two distinct ways, depending upon the origin of the intrusives and the fracture systems responsible for circulation of oxidizing meteoric water. First, late intrusives which are cogenetic with the magmatic heat source of the protore may remobilize ore-forming elements upon deep hypogene oxidation of pyritic disseminated sulfides. Second, much later, totally unrelated intrusive activity and deep crustal faulting, perhaps related to entirely different petrotectonic regimes, may provide access for meteoric water to penetrate much older protores, resulting in the formation of near-surface ore deposits by element recycling.It is therefore suggested that near-surface hydrothermal base and precious metal deposits be explored to considerable depths for possible low-grade protore potential.

Rich copper mineralization has taken place intermittently along the 150-km Singhbhum copper belt, Bihar, India, the highest concentration being noted in a 50-km section from Badia to Turamdih. Derivatives of basic volcanics host the orebodies in most places; in a few others "soda granites" that formed metasomatically principally from the basic volcanics contain the ores. Orebodies are peneconcordant with their host rocks.Chalcopyrite is the dominant ore mineral at most places, only locally yielding the position to pyrite. Other minerals present are pyrrhotite, pentlandite, millerite, magnetite, ilmenite, molybdenite, tetradymite, wehrlite, cubanite, sphalerite, galena, skutterudite, bismuthinite, mackinawite, valleriite, uraninite and rutile. Supergene phases include marcasite, pyrite, violarite, cuprite, native copper, chalcocite, covellite. Quartz, chlorite, biotite, and sericite in varying proportions constitute the gangue.The ores have been subjected to dynamothermal-dynamic metamorphism. Effects of thermal metamorphism are noted locally. Textural features such as polygonization, equiangular triple-junction points in polycrystalline aggregates, annealing twins, poikiloblastic growths, etc., in the ores may be attributed to dynamothermal metamorphism. This metamorphic event affecting the ores is correlated with the retrogressive metamorphism along the belt. Later deformation crushed and bent some of the ore minerals and produced deformation twins in others.Late dolerite dikes, from less than 2 m to 7 m thick, thermally affected the ores at Badia-Mosaboni Mines, producing exsolved lamellae of chalcopyrite + pyrite in a talnakhitic matrix close to the intrusives. In addition, valleriite mackinawite, and an unknown mineral developed, and chalcopyrite diffused out from the original grains. Ores collected 27 cm from the contact of a dike 7 m thick do not show lamellae of chalcopyrite, but pyrite has exsolved and chalcopyrite diffused out. A sample collected 1 m from the same contact shows only diffusion of chalcopyrite. Ilmenite in the sulfide ores close to the intrusive contact lost iron along grain boundaries.

Strata-bound silver-bearing copper sulfides occur in two thin green siltstone units, a thin phosphatic green siltstone bed and a stromatolitic oolitic carbonate bed, of the Spokane Formation near Wolf Creek, Montana. Red mudstone and fine-grained sandstone sequences containing sedimentary structures indicative of shallow-water deposition with periodic emergence surround these mineralized zones. The 10- to 30-cm-thick carbonate bed, the lowest and most intensely mineralized unit, crops out discontinuously over 7.5 km. It varies from an ooid-peloid packstone to an ooid-peloid-bearing calcareous siltstone. Chalcocite family minerals, covellite and bornite + or - chalcopyrite, pyrite, and galena occur in the eastern part of the area. Chalcopyrite and pyrite + or - chalcocite family minerals, covellite, and galena occur in the western part.Sulfur isotope values of the sulfides are compatible with bacterial production from seawater sulfate. Oxygen and carbon isotope values of the oolitic limestone are slightly lighter than those obtained for 1 b.y.-old marine limestone elsewhere but are within the range of Ravalli Group and Middle Belt carbonate rocks of the Belt Supergroup.Element analyses show that Cu, Ag, Mo, Pb, and Cd in the mineralized units are enriched over average country-rock values from the enclosing section. Ni and Fe are more abundant in the country rock and Co values are similar in both rock types. Elemental values of the wall-rock samples are very similar regardless of stratigraphic position, color, or grain size.Textures in the oolitic limestone indicate mineralization occurred during diagenesis. Following deposition of calcareous clastics, dolomite, gypsum, and halite formed in evaporitic early diagenesis. Pseudomorphic replacement of these minerals by quartz and albite followed. Pyrite formed prior to silicification and was partially to totally replaced by copper sulfides and galena as waters containing copper and lead apparently moved upward through the sequence. Radial calcite and microspar formed both during and after mineralization, Finally, greenschist facies metamorphism altered the rocks, causing partial to almost complete destruction of the original fabrics and recrystallization of the sulfides in some areas.

The phase equilibria of the copper-sulfur system were investigated using electrochemical cells from 0 degrees to 250 degrees C over the composition range Cu/S = 0.95-2.10. Vapor-saturated aqueous cupric sulfate and cuprous chloride solutions were used as the electrolytes. The two blaubleibender covellites, low digenite, tetragonal phase, and protodjurleite were found to be metastable. The Cu (sub 1.9) S phase was demonstrated not to be a real phase in the system.The stable invariant points (Cu/S in parentheses) were as follows:Covellite (1.000 + or - 0.001) --> high digenite (1.732 + or - 0.005) + liquid sulfur, T = 507 degrees + or - 2 degrees C;high chalcocite (2.000 + or - 0.004) + Cu --> high digenite (2.000 + or - 0.004) + Cu, T = 435 degrees + or - 8 degrees C;low chalcocite (2.000 + or - 0.002) + Cu --> high chalcocite (2.000 + or - 0.002) + Cu, T = 103.5 degrees + or - 0.5 degrees C;djurleite (1.942 + or - 0.002) --> high digenite (1.834 + or - 0.002) + high chalcocite (1.988 + or - 0.005), T = 93 degrees + or - 2 degrees C;low chalcocite (1.993 + or - 0.002) --> djurleite (1.960 + or - 0.002) + high chalcocite (1.990 + or - 0.005), T = 90 degrees + or - 2 degrees C;anilite (1.750 + or - 0.003) --> covellite (1.000 + or - 0.001) + high digenite (1.765 + or - 0.002), T = 75 degrees + or - 3 degrees C; andhigh digenite (1.805 + or - 0.002) --> anilite (1.750 + or - 0.003) + djurleite (1.934 + or - 0.002), T = 72 degrees + or - 3 degrees C.The standard free energies of formation determined for each stable and metastable phase as a function of temperature are as follows:Covellite: 273.15 to 388.36K, delta G o f = -12,726 - 0.54T + or - 50 cal/mole and 388.36 to 717.73K, delta G o f = -13,548 + 1.587T + or - 50 cal/mole;blaubleibender covellite I (1.1): 273.15 to 423.15K, delta G o f = -13,463 - 0.45T + or - 100 cal/mole;blaubleibender covellite II (1.4): 273.15 to 423.15K, delta G o f = -14,675 - 2.36T + or - 100 cal/mole;anilite (1.750): 273.15 to 348.15K, delta G o f = -18,169 - 2.02T + or - 70 cal/mole;low digenite (1.765): 273.15 to 348.15K, delta G o f = -18,366 - 1.17T + or - 100 cal/mole;djurleite (1.934): 273.15 to 345.15K, delta G o f = -19,063 - 3.28T + or - 100 cal/mole;djurleite (1.965): 273.15 to 363.15K, delta G o f = -19,178 - 3.53T + or - 100 cal/mole;low chalcocite (2.000): 273.15 to 376.65K, delta G o f = -19,226 - 4.112T + or - 120 cal/mole; andhigh chalcocite (2.000): 376.65 to 708.15K, delta G o f = -18,386 - 6.01T - 0.000941T 2 + or - 300 cal/mole;where the numbers in parentheses are the Cu/S ratio of the phase and sulfur is the phase stable at the respective temperature and 1 atmosphere pressure.

Copper sulfides and copper-iron sulfides have partly replaced or are closely associated with carbonaceous fossil plant material, mainly logs, in the Agua Zarca Sandstone Member of the Chinle Formation of Triassic age. Small amounts of native silver occur with the sulfides. Malachite, chrysocolla, and azurite fill the interstices and fractures in the clastic host rocks, forming irregular halos around the mineralized fossil plant material.Primary sulfide minerals, chalcocite, covellite, bornite, chalcopyrite, and pyrite, were probably deposited by ground-water solutions moving through paleochannel complexes in Triassic time shortly after deposition of the host rock; mineralization occurred where there was a favorable reducing environment provided principally by the carbonaceous material. The ultimate source of the copper may have been older deposits in the Precambrian rocks of the Uncompahgre highland in northern New Mexico and Colorado. The sulfide minerals were partly oxidized later, and copper in the form of malachite, chrysocolla, and azurite was disseminated in the adjacent host rock. Major faults post-date deposition of the sulfide minerals and can be expected to offset some of the mineralized paleochannels.Large-scale deposition of the sulfide minerals occurred at fossil logjams within paleochannels. Orientations of cross-bedding and fossil logs can be used to determine the trends of the paleochannels. These trends should be followed in subsurface exploration.

Oil field brines commonly contain ppm quantities of Zn, Pb, and H 2 S but negligible quantities of Cu. Theoretical analysis of the water-rock interactions expected during migration through evaporitic and red-bed sequences suggests that these reducing oil field brines can become transformed into far more oxidized fluids capable of transporting Cu, Pb, Zn, and sulfate and of forming sediment-hosted Cu-rich deposits. Calculation of the chemical mass transfer accompanying reaction of a typical reducing brine with anhydrite, hematite, and quartz, followed by the assemblage quartz, K-feldspar, muscovite, hematite, chlorite, and cuprite, reveals that the aqueous phase becomes greatly undersaturated with respect to sphalerite, galena, chalcopyrite, bornite, pyrite, and pyrrhotite as it achieves log f (sub O 2 ) values well above the magnetite-hematite buffer consistent with saturation with respect to anhydrite, hematite, and chalcocite and ore-forming concentrations of copper and sulfate. Consequently, evaporites are not only possible sources of salinity and (oxidized) sulfur for evolving basinal brines but may be crucial to the development of the f (sub O 2 ) conditions required to transport Cu in ore-forming concentrations. Reaction of the oxidized brine with organic matter (approximated by graphite) and diagenetic pyrite results in precipitation of the zonation sequence chalcocite, bornite, chalcopyrite, and pyrite with variable amounts of hematite, covellite, sphalerite, and galena, depending on the relative rates of reaction of the graphite and pyrite.

A systematic x-ray study of the solid phases of the system Cu 2 S-CuS has been made. A special camera was designed to make x-ray photographs of powder samples at elevated temperatures. This camera allows the specimen to remain heated continuously, and the heat treatment can be controlled as in a furnace. The resulting phase diagram shows that the system contains three compounds and four phases. The compounds are:chalcocite, ideally Cu 2 Sdigenite, ideally Cu 9 S 5 covellite, CuS.Chalcocite undergoes transformation, and no cubic high temperature phase appears up to at least 300 degrees C. The revised inversion scheme is:above 105 degrees C., hexagonal basic structurebelow 105 degrees C., orthorhombic superstructure.The superstructure phase is capable of dissolving up to eight atomic per cent CuS, while the basic structure can dissolve only two atomic per cent CuS. Below approximately 78 degrees C. digenite has the ideal composition Cu 9 S 5 , but above this temperature it takes increasing amounts of either Cu 2 S or CuS into its composition. Evidently digenite has been regarded as the phase of chalcocite stable above 91 degrees C. Certain regions of the phase diagram of this system may be applied to problems of geologic thermometry, providing that proper criteria are recognized.

The Bonanza King mine lies on the east slope of the Humboldt Range, and in an area of folded and faulted Triassic volcanics.The ore body is a sulphide quartz vein carrying gold and silver, associated with fractures in a diabasic dike.Primary minerals are quartz, pyrite, tetrahedrite (argentiferous), galena, sphalerite, chalcopyrite and native gold. Secondary enrichment appears to have yielded covellite, chalcocite, stephanite, and argentite. After deposition of most of the quartz and sulphides, reopening of the fracture permitted the deposition of additional quartz, together with tourmaline and some more sulphides. The solutions are thought to have come from a neighboring granodiorite stock, and the deposits are regarded as mesothermal or possibly hypothermal.

The ores of this deposit show an unusual amount of oxidation and secondary enrichment believed to have taken place during pre-Cambrian time. The ore minerals approximately in order of abundance are: chalcocite, cuprite, malachite, azurite, native copper, chrysocolla, pyrite, chalcopyrite, black oxide, covellite, and bornite. Primary sulphides are pyrite, chalcopyrite, and presumably bornite. The secondary sulphide ore is almost entirely chalcocite, but a little bornite and covellite are found. Malachite commonly replaced chalcocite directly but also replaced cuprite and occurs as nodular and boulder-like masses as well as in streaks in kaolinite. Azurite formed with malachite apparently in much the same sequence except that it was not found replacing sulphides. Cuprite directly replaces sulphides and is commonly replaced by malachite. Chrysocolla occurs mainly with malachite, and native copper also formed at places during oxidation.No peculiarities or changes were noted that might be attributed to the pre-Cambrian age of the oxidation. The minerals, their sequence and textural relations seem entirely normal.

New X-ray and microscope data on phases in the range Cu (sub 1.73) to Cu 2 S, at temperatures up to 700 degrees C, cover the critical part of the system Cu-S. Phases stable at low temperature include digenite (to 76 degrees C for Cu (sub 1.765) S, 83 degrees C for Cu (sub 1.840) S), djurleite (to about 93 degrees C), and orthorhombic chalcocite (to 93 degrees C for Cu (sub 1.993) S, 103.5 degrees for Cu 2 S). Within the range Cu (sub 1.73) S to Cu 2 S, high-digenite solid solution is the only stable phase at 507 degrees C (upper stability limit of covellite). Hexagonal chalcocite inverts to high digenite (Cu 2 S) at about 435 degrees C. Natural stable assemblages agree with experimental data. The lower stability limit of high digenite has qualified usefulness as a geothermometer.

A pyrometasomatic magnetite-hematite deposit in sedimentary rocks of Permian age near the margin of the Lone Mountain stock in Lincoln County, N. Mex., contains between 0.015 percent and 0.031 percent uranium. The deposit is composed mainly of magnetite with lesser amounts of hematite, hydrated iron oxides, pyrite, leuchtenbergite(?), gypsum, chalcopyrite, metatorbernite, torbernite(?), covellite, sphalerite(?), quartz, marcasite, and an unidentified uranium-bearing mineral; it is surrounded by an aureole of recrystallized limestone, gypsum, epidote, and actinolite with lesser amounts of specularite, phlogopite, fluorite, pyrite, and chalcopyrite. Autoradiographs and polished section studies suggest that most of the uranium is dispersed in the iron oxide minerals. The association of iron and uranium may be related to the melting points of the elements. The deposit probably formed at a rather low temperature by self-oxidation of a ferrous hydroxide hydrosol contained in a mildly alkaline solution.

The dominant facies of Cu mineralization at White Pine, Michigan, is characterized by a distinct relationship of Cu to sedimentary features and by restriction to beds at the base of the Precambrian Nonesuch Shale formation. A second minor facies of mineralization, however, is fracture-controlled. The fracture-controlled mineralization is of 2 types: 1) Fracture-filled veins. The distribution of fractures in the White Pine mine is controlled by the dominant White Pine fault. These fractures were mineralized with calcite, quartz and chalcocite during or shortly after faulting. Native Cu, native Ag, blue chalcocite, covellite, bornite, chalcopyrite, and pyrite also occur in certain veins and probably formed by alteration of the earlier chalcocite. 2) Halos. Halos are chalcocite-bearing zones in wall rock adjacent to the veins and symmetrical with respect to the vein. Halos are mineralogically and chemically distinct from the adjacent wall rock in that native Cu is absent, and the average Cu content is lower. The amount of chalcocite varies systematically across the halos, decreasing away from the veins and becoming sparse in the outer parts of the halos. The over-all deficiency of Cu in the halos is apparently compensated by the occurrence of chalcocite in the veins; halos are confined to chalcocite-bearing veins, and the width of each halo appears to be proportional to the amount of chalcocite in the central vein. The fracture-controlled mineralization is interpreted to have originated from low temperature (less than 105 degrees C.), hypogene, solutions. These solutions deposited calcite and quartz in the fractures. The distribution of Cu in the veins and halos is inferred to be due to mobilization of Cu by solutions diffusing into the walls. The process involved is: 1) oxidation of metallic Cu along a diffusion front, 2) migration of Cu ions toward the vein, and 3) deposition of chalcocite in the veins and adjacent wall rock. There is no evidence for the introduction of Cu into the fractures from outside the cupriferous sequence.

The thermodynamic relations between partial pressures and molecular or ionic activities of aqueous solutions containing S are presented graphically as functions of the 5 variables: T, P, pH, Po 2 , and (Sigma S) (the total activity of aqueous S species). These diagrams provide a quantitative basis for correlation of the composition of aqueous solutions to the fields of stability of minerals up to 250 degrees C. The correlation is based on the principle that at equilibrium, partial pressures fixed at any one temperature by a univariant assemblage of minerals in an anhydrous system are not affected by the addition of H 2 O if the assemblage is unchanged. The application of this principle to the anhydrous Fe-S-O system demonstrates that up to 250 degrees C. the common assemblage pyrite, pyrrhotite, and magnetite may be deposited at equilibrium only by alkaline ore solutions. This assemblage has been deposited in nature and is stable in the dry system over a temperature range from 675 degrees to below 250 degrees C. The association of sphalerite with pyrrhotite, pyrite, digenite, covellite, and barite, but not with free S, limits thermodynamically the aqueous conditions during deposition. At 250 degrees C. this ore solution must be neutral to weakly alkaline with HS (super -) as the predominant S-containing, aqueous ion; Ps 2 is usually between 10 (super -8.5) and 10 (super -14.7) atm. and Po 2 between 10 (super -36) and 10 (super -50) atm. Therefore, the solubility of sphalerite must be sufficient to account for the transport required for ore formation at 250 degrees C. under the conditions where pH=7.5, Po 2 =10 (super -40) and (Sigma S)=1.0 m.

In the chilled marginal gabbro of the Skaergaard intrusion are small specks of chalcopyrite and bornite; the amount of sulphur is .005 percent. This is taken to be representative of the amount in the 300 km 3 of original Skaergaard magma.In the layered rocks, up to the latest ferrogabbros, sulphides occur in small composite units mostly about 0.4 mm across and forming about .02 percent by weight of the rock. The sulphides are bornite, digenite and chalcopyrite, with some covellite replacing the digenite. In melanocratic bands consisting largely of the heavier crystal fractions, copper sulphides are ten times as abundant as in average rock. It is concluded that immiscible droplets of dominantly copper sulphide liquid formed in the silicate liquid at this stage and could accumulate, as a result of gravity, along with the heavy, primary precipitate silicate and oxide minerals.In the fayalite ferrogabbros at the top of the layered series droplet-shaped patches of pyrrhotite (mostly altered to marcasite) with a little chalcopyrite but no bornite make up about 2 percent of the rock. At this stage in the fractionation an immiscible sulphide liquid, rich in ferrous sulphide, had formed. In the latest rock fraction, the acid granophyre, pyrrhotite or pyrite with a little chalcopyrite is patchily present up to 4 percent by weight of the rock.The composition of the latest pyrrhotite has been determined and the composition of earlier immiscible copper sulphide and iron sulphide liquids has been indirectly estimated. The copper-rich sulphide liquid apparently began separating from the silicate magma when the amounts of copper and sulphur in the magma were about .02 and .01 percent respectively. This is considered to represent the solubility of the copper sulphide in the ordinary basic magma at the time of formation of the hypersthene olivine gabbros. With the changing composition resulting from further fractionation the solubility of the copper sulphide in silicate magma increased until the amounts at saturation were .05% Cu and .03% S. The iron sulphide matte separated later when the sulphur content of the silicate magma had risen to about .06 percent and the ferrous iron was about 15 percent. The partition of Cu, Ni and Co between silicate and sulphide liquids is roughly determined at various stages of the fractionation.The Skaergaard sulphides are low in nickel because, by the time an immiscible iron sulphide liquid separated, nickel had been reduced to very low amounts in the magma as a result of its abundant entry into early olivine and pyroxene. Nickel does not enter the copper-rich sulphide liquid so readily as the later iron-rich sulphide liquid. Cobalt is strongly concentrated in the copper-rich liquid and moderately in the iron-rich. The amounts of palladium, indium and gold have also been determined in selected rocks, from which it appears that at low concentrations the ease of entry of these elements with the partial exception of gold into the sulphide liquid is not markedly greater than into the silicate.It is suggested that nickeliferous iron-rich sulphides such as the disseminated ores of the Bushveld and Insizwa which separated at an early stage in the crystallization of the magma were formed because there was a relatively high initial sulphur content.

A study of a suite of 250 polished surfaces of ores from the Mount Isa mine, Queensland, Australia, reveals, at high magnifications, unusual and interesting replacement structures. The ore minerals are remarkably stratified in sheared and altered slate of pre-Cambrian age. The common minerals are pyrite, sphalerite, galena, pyrrhotite, arsenopyrite, marcasite, quartz, and carbonate in various combinations. Several uncommon minerals are present. Five varieties of pyrite are recognized, of which three were deposited before crushing and shearing of the host rock; the differences are probably due to varying proportions of FeS. Marcasite is derived from pyrrhotite. Oxidation and supergene enrichment processes and products are discussed. The mineralization sequences are: (I) early deposition of pyrite in the slate; (2) crushing and faulting of the pyritic slates and deposition of sulphides of iron, zinc, copper, and lead; (3) oxidation and supergene sulphide enrichment yielding chalcocite, covellite, anglesite, cerussite, jasper, goethite, lepidocrocite. A syngenetic origin for the ore is excluded and an origin by metasomatic replacement is proposed.

Some of the relationships that might be used in determining whether the ores are syngenetic or epigenetic are discussed. Some experiments were run to determine the amount of comminution of silicate rocks and sulfide ores in a stream environment. The possible equations that might produce covellite, chalcopyrite and bornite are cited.

The polishing of many sections of chalcocite, digenite, and covellite on the lead lap has shown that physical changes can take place which may affect the interpretation of textures, and that special care should be exercised in polishing this group of minerals. The changes are probably attributable to two causes: (1) a mechanical dragging and smearing of these minerals into each other, and (2) some type of surficial solid solution which is induced under the conditions encountered during polishing. The effects may be avoided by careful polishing on a cloth lap, but even with the cloth lap too much pressure must not be used. Most of the physical changes would probably be noticed during a microscopical examination, but "blue chalcolite" which might easily escape detection can be formed.

The electrochemical cell AgAgIAg (sub 2+x) S,S 2 (vapor), described by Wagner (1953), has been modified to measure sulfur fugacities buffered by mineral assemblages sealed in pyrex capsules. The cell was calibrated against the f (sub S 2 ) fixed by S (liquid) and Ag + Ag (sub 2+x) S. The extent of solid solution between Ag (sub 2+x) S and AgI was determined at 410 degrees C + or - 10 degrees C. The sulfur fugacities buffered by bornite + pyrite + chalcopyrite, covellite + pyrite + idaite, pyrite + bornite + idaite, pyrite + pyrrhotite, nickel monosulfide + vaesite, and covellite + high digenite were measured over temperatures ranging up to 445 degrees C. Except for the last buffer, the results are in good to fair agreement with published data.

The solubility of covellite (CuS) was measured in aqueous sulfide solutions under varying conditions: up to 4.95 molal HS (super -) ; pH 3.5 to 13.5; 20 degrees to 200 degrees C; and 2 to 83 atm total pressure. The solubility of covellite is a function of the concentration of HS (super -) , H 2 S pressure, and temperature. Three important copper-bisulfide complexes are formed, the stability of which depends on pH and temperature. The following reactions represent the formation of the complexes predominating in the pH ranges given, at the temperatures, pH9s, and ionic strength (mu ) indicated:At 25 degrees C: pH 2 S(g) + HS (super -) = Cu(HS) 3 (super -) K = 10 (super -4.40+ or -0.20) (mu = 0)K9 = 10 (super -4.25+ or -0.20) (mu = 1.0)pH > 7.3 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -3.67+ or -0.20) (mu = 0)K9 = 10 (super -3.67+ or -0.20) (mu = 1.0)At 200 degrees C: pH 2 S(g) + 2HS (super -) = Cu(HS) 4 (super 2-) K = 10 (super -2.30+ or -0.3) (mu = 0)K9 = 10 (super -3.60+ or -0.3) (mu = 1.0)pH > 6.6 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -2.13+ or -0.20) (mu = 0)K9 = 10 (super -2.12+ or -0.20) (mu = 0)The boundary between the complexes Cu(HS) 3 (super -) and CuS(HS) 3 (super 3-) is dependent on the total sulfur concentration (Sigma S = HS (super -) + H 2 S) of the solution and ionic strength. The pH limits given here are for mu = 1.00. The pH boundary at 200 degrees C is relative to a neutral pH of 5.6. There is no evidence for the existence of CuS-H 2 S complexes in the very low pH region ( approximately 3), or CuS-S (super 2-) complexes in the high pH region (10-13.5). Solubilities calculated using the above data compare well with those observed in these experiments.If it is assumed that a solubility of at least 10 ppm Cu is necessary for significant ore transport, bisulfide complexes are potential ore carriers at 200 degrees C and a total sulfur concentration of 0.25 molal, or greater, at neutral to weakly alkaline pH9s. However, complexes between Cu and either NH 3 or Cl (super -) in bisulfide solutions are not geologically important, at least at low temperatures.Covellite transported as bisulfide complexes would be deposited by any mechanism that lowers the stability of these complexes, such as a decrease in temperature or pH, oxidation, or dilution. The conditions of covellite deposition obtained in the laboratory experiments agree well with those deduced from natural occurrences, although additional solubility data are needed for the more common copper sulfides digenite and chalcocite.

Low temperature lead-zinc-silver mineralization of Tertiary age characterizes the Palomas Mining District in the western part of Sierra County, in southwestern New Mexico. The ore occurs in fissure veins and replacement bodies in Paleozoic limestones and dolomitic limestones. Structural control of ore deposition has been exerted by fractures or shaly horizons. The principal hypogene minerals are galena, sphalerite, chalcopyrite, and argentite. Associated minerals are pyrite, polybasite, and tetrahedrite. The gangue consists mainly of talc, with associated calcite, quartz, barite, and clay minerals. Supergene minerals include covellite, native silver, limonite, carbonates of lead, zinc, and copper, silver halides, and a little vanadinite, descloizite, and pyromorphite.Talc, which occurs both as vein filling and in wall-rock alteration, is thought to have been formed by the action of silica-bearing hypogene solutions. Little transportation appears to have taken place during the early stages of oxidation, as evidenced by the intimate association of hypogene and supergene silver minerals. The mineralization is believed to have been brought about by alkaline hydrothermal solutions whose origin is in doubt, but may be connected with the sources of Tertiary volcanic activity in the area. An unusual feature of the deposits is the occurrence of talc as the chief gangue mineral.