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Laramide reverse faults in southeastern Arizona commonly are obscured by midto late Cenozoic extension and subsequent cover, resulting in debate about their configuration and origin. A new mid-Cenozoic paleogeologic map depicts the structural configuration before extension, and new structural reconstructions characterize Laramide shortening in terms of structural style, magnitude, evolution, and timing. Reverse faults restore to moderate to high angles, are associated with fault-propagation folds, and involve significant basement and thus constitute thick-skinned deformation. The paleogeologic map suggests several major basement-cored block uplifts, many of which are newly identified. The largest uplifts may measure 150 km along strike, similar to those in the classic Laramide province of Wyoming and Colorado. Estimated shortening across the central study area is 14% or 23 km, whereas it is only 5% (9 km) to the north and 11% (12 km) to the south. Shortening by this mechanism is inadequate to explain previous estimates of crustal thickening in the region (~50-60 km). Therefore, magmatic underplating, lower-crustal flow, or underplating of trench sediments and lithospheric material also may have contributed to thickening. Shortening largely occurred from 86 Ma to 64 Ma and possibly as late as 53 Ma, with initiation being younger to the northeast or north. Integration with data from southwestern New Mexico implies complex geometry for the subducting flat slab. Finally, reverse faults generally do not appear to have reactivated older faults, as previously suggested, primarily because reverse faults have associated fault-propagation folds in rocks that predate supposed reactivated structures. [ABSTRACT FROM AUTHOR]

Precise [sup.40]Ar/[sup.39]Ar ages on pretectonic, syntectonic, and posttectonic rhyolites from the Robinson mining district, which is within a highly extended domain in east-central Nevada, indicate that extreme extension (~400%) occurred in fewer than 900 k.y. during the Eocene. The duration of extension is tightly delimited by an Eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units. The rhyolites represent at least five eruptive episodes, including the preextensional and synextensional rhyolite of White Hill emplaced between 37.6 and 37.4 Ma, and the postextensional rhyolite of Garnet Hill emplaced at 36.7 Ma, although some of it is perhaps as old as 37.1 Ma. Thus, the duration of extreme extension at Robinson was between 0.9 and 0.3 m.y., comparable to or shorter than other highly extended areas in western North America. The extensional domain in east-central Nevada is a composite feature, produced by localized, episodic, rapid extension that began in early Eocene time and continued to at least middle Miocene time. Keywords: Ar/Ar, geochronology, extension, accommodation zones, rhyolite, tilting.

This study integrates a cross-sectional restoration of normal faults and porphyry- related hydrothermal alteration zones with six new U-Pb zircon dates to constrain the ages of mineralization and large-magnitude Cenozoic extension at Spruce Mountain, northeastern Nevada (USA). Paleozoic sedimentary rocks and sparse rhyolitic intrusive rocks host a porphyry molybdenum deposit dated as ca. 38 Ma that contains associated skarn, carbonate replacements, fissure veins, and jasperoid. At least six crosscutting sets of normal faults with variable dip directions (east, west, and north) and angles (<15° to >60°) are identified at Spruce Mountain, reflecting overprinting phases of extension. Based on the restored cross section, normal faulting at Spruce Mountain resulted in ~6.9 km (120%) of total extension and ~35° of net eastward tilting. The first four fault sets, late Eocene (ca. 38 Ma) or older, collectively accommodate most of the total extension (~5.4 km). Later faults probably were active in the Miocene and Quaternary. All restored faults had initial dips of 45° or greater, and the restored preextensional structure of Spruce Mountain consists of west-vergent folds and gentle westward dips of Paleozoic rocks. Spruce Mountain is classified as a rhyolitic porphyry Mo or Climax-type deposit, with extensive skarn. Eocene rhyolite porphyry dikes associated with porphyry Mo mineralization locally intrude the early generations of normal faults at Spruce Mountain. However, many normal faults also postdate molybdenum mineralization and have partially dismembered the system. Cross-sectional restoration of the postmineralization faulting suggests that the original deposit had an ~6 km long by ~3 km deep elliptical pattern with carbonate replacement deposits surrounding skarn. [ABSTRACT FROM AUTHOR]

This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40Ar/39Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40Ar/39Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73-68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bomite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argilhc alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61-59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59-58 Ma) enargite-chaleocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61-59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from >10 m.y. to less than the geochronologic precision currently achievable. [ABSTRACT FROM AUTHOR]

The root zone of a porphyry system is a specific region beneath a porphyry orebody that was a site of focused fluid flow, as evidenced by abundant quartz veins, widespread wall-rock alteration, or porphyry dikes merging downward into a porphyritic granite cupola. These zones constitute an important source region of ore fluids and other components, and in certain geologic terrains the characteristics of root zones may point to previously undiscovered deposits. The root zones of four Laramide porphyry copper systems in Arizona recently have been characterized at a reconnaissance level: the Miami Inspiration system associated with the Schultze Granite, the Sierrita-Esperanza system associated with the Ruby Star Granodiorite, the Ray system associated with the Granite Mountain pluton, and the Kelvin-Riverside system associated with the Tea Cup pluton. The two well-studied root zones related to the Jurassic Yerington batholith in Nevada, and associated with the Yerington mine and the Ann-Mason deposit, provide a basis of comparison. All six systems occur in areas with unusually large exposures in both lateral and vertical paleodirections, locally to paleodepths of > 10 km, because of postore extensional faulting and associated tilting. No two systems are 'alike, but many share the presence of the following hydrothermal characteristics: quartz veins and potassic alteration, sodic-calcic and sodic alteration, calcic alteration, and relatively coarse grained muscovite-quartz (greisen). Quartz veins and potassic 'alteration are focused centrally, directly above related cupolas; sodic-calcic and sodic alteration, calcic alteration, and evidence for leaching of silica are observed on the deep flanks of certain systems; and greisen occurs directly beneath ore within and beneath coeval cupolas in many systems. Certain systems exhibit evidence of multiple cycles of release of magmatic fluid followed by incursion of saline ground waters, which are analogous to the biological cycle of exhale- inhale, respectively. The characteristics of the root zones provide important constraints on the exsolution and transport of the magmatic aqueous phase that leads to ore formation, the variable incursion of external fluids into the hydrothermal system, and the degassing of magmatic volatiles that may not be related directly to porphyry ore formation. The most robust conclusions are drawn from the localities that offer the greatest quality of exposure and degree of continuity (including compelling structural reconstructions) between the roots and the ore deposit, from the studies that identify timelines linking processes in the roots with those in the mineral deposit, and from systems in which the deposit itself is well characterized. [ABSTRACT FROM AUTHOR]

Fluorine-rich hydrothermal mineral assemblages developed at Henderson, Colorado, as 12 rhyolitic stocks of Oligocene age were emplaced, crystallized, and cooled inside a much larger and slightly older, rhyolitic intrusion, forming a Climax- type molybdenum deposit. The stocks are grouped into three intrusive centers, and the mineral assemblages, including ore minerals, are grouped according to their temperatures of formation as either high-, moderately high, moderate-, or low-temperature assemblages. The latter three groups are termed lower temperature assemblages, which are subdivided into two suites on the basis of position: above intrusive centers and on the flanks of the Seriate center. Integration of the mineralogy, sequence, and spatial positions of mineral assemblages defines the geochemical evolution of hydrothermal fluids, places qualitative constraints on mass transfer, and provides insight into the development of the geochemical halos that are used in exploration for porphyry orebodies. A reconnaissance fluid inclusion study targeted the lower temperature assemblages, and a geologic assessment of results indicates that we were unable to identify fluid inclusions from several assemblages, After pooling meaningful results, the ranges of temperatures of formation for each group of lower temperature assemblages are moderately high, 600° to 460°C: moderate, 530° to 310°C; and low, 390° to 200°C, with significant differences in salinity between the two suites of assemblages, The suite formed above intrusive centers was deposited by more saline fluids (salinity commonly 28-65 wt % NaCl ± KCl equiv) than the suite formed on the flanks of the Seriate center (mostly <29 wt % NaC1 + KCl equiv). The lower temperature assemblages described here do not overlap in space with higher level propylitic alteration, but fluid inclusions in one sample of propylitized rock indicate temperatures of 320° to... [ABSTRACT FROM AUTHOR]

At Henderson, Colorado, 12 rhyolitic stocks of Oligocene age crystallized in three intrusive centers whose tops are 1 km below the surface of Red Mountain: Henderson (oldest), Seriate, and Vasquez (deepest and youngest). A variety of fluorine-rich hydrothermal mineral assemblages are grouped according to their temperatures of formation, either high-, moderately high, moderate-, or low-temperature (the latter three groups are termed lower temperature assemblages). The groupings were based on relative age relationships in areas free of cyclical events, and absolute temperature estimates based on fluid inclusion study later were made for many mineral assemblages, as detailed in a companion paper. Numerous crosscutting contacts of individual stocks combined with limits of associated high-temperature assemblages can be interpreted as spatially extensive time lines, thereby enabling temporal correlation of spatially separated events. High-temperature silicic (quartz-fluorite) and intense potassic (quartz-K-feldspar-molybdenite) assemblages developed in numerous cycles, each corresponding to emplacement of a stock and deposition of molybdenite. Molybdenite occurs without pyrite mostly in the high-temperature assemblages. In contrast, lower temperature assemblages broadly envelop intrusive centers, rather than single stocks. Some of the moderately high temperature, less intense potassic assemblages (including mottled K-feldspar-quartz and magnetite-K-feldspar) also formed during multiple cycles. Moderate-temperature sericitic assemblages (including topaz with pyrite or magnetite and sericite with pyrite or magnetite) only locally exhibit evidence for multiple cycles, and low-temperature intermediate argillic assemblages (pyrite-day and several assemblages containing base metal sulfides, F-bearing, Mn-rich garnet, and rhodochrosite) were deposited in only a single event. These lower temperature assemblages terminate 700 m below the present surface... [ABSTRACT FROM AUTHOR]

The article presents a citation of geologist Marco T. Einaudi for the R. A. F. Penrose Gold Medal for 2008, the highest award of the Society of Economic Geologists (SEG) Awards. His brief personal, educational and career background is provided. He is said to have stimulated the establishment of the Yerington district in Nevada as a classic locale for economic geology. It adds that Einaudi is famous scientifically for his basic contributions to the origin of skarns, porphyry copper deposits and base-metal lode deposits.