Abstract 0.0 Copper Plaza in Bagdad, Arizona. Proceed east on Arizona Highway 96 toward Hillside, Arizona. 0.4 Turnoff to the right to the Bruce mine. Outcrops ahead, on the left, and for the next one-half mile are Hillside Mica Schist intruded by Lawler Peak Granite. 1.0 Roadcut in Hillside Mica Schist intruded by Early Proterozoic foliated granodiorite. 1.2 Small creek from below the Cowboy mine enters Bridle Creek on the right. Outcrops are of dark metabasalt of the Bridle Formation (Anderson and others, 1955). Light-colored rocks are intrusive and extrusive metarhyolite. Roadcuts 0.4 miles ahead in light felsic metavolcanic rocks on the right, dark mafic metavolcanic rocks and Hillside Mica Schist on the left. 1.8 Eastern margin of of metavolcanic belt. Roadcuts in Hillside Mica Schist intruded by aplite-pegmatite. 2.3 Large roadcut contains Hillside Mica Schist intruded by numerous sills and dikes of aplite-pegmatite. 3.4 Tertiary fanglomerate in roadcut. Large dike of aplite-pegmatite projects across the highway. Outcrops ahead about one-quarter mile on the left are light aplite-pegmatite intruded by tan-weathering, ocher-stained, strongly jointed Lawler Peak Granite. On the right are outcrops of strongly jointed Lawler Peak Granite. 4.3 Junction of Arizona State Highways 96 and 97. Arizona State Highway 91 turns off to the right and connects with U.S. Highway 93 to Wickenburg and Phoenix. We continue along Arizona Highway 96 straight ahead through the roadcuts of Lawler Peak Granite. Road narrows through these roadcuts; please drive carefully. Most of the roadcuts are in Lawler Peak Granite, but some aplite-pegmatite

Abstract Porphyry molybdenum deposits are spatially, temporally, and genetically associated with porphyritic intrusions of quartz monzonite to high silica, alkali-rich granite composition. Most molybdenum is in quartz-molybdenite veinlets that are part of an intrusion-centered stockwork of veinlets. Associated minerals are pyrite and fluorite. Recoverable to geochemically anomalous tungsten, tin, copper, lead, and zinc commonly occur marginally and/or peripherally to the molybdenum ore. Stockwork deposits associated with intrusions of high silica, alkali-rich rhyolite, and granite porphyry are herein recognized as a distinct class, referred to as “Climax type.” These deposits generally are dome shaped, with each deposit centered on an intrusive cupola, such that the molybdenite zone mimics the shape of, and commonly straddles, the igneous contact. The Climax (Ceresco, Upper, and Lower orebodies), Red Mountain (Urad and Henderson orebodies), and Mount Emmons-Redwell deposits are composite Climax-type systems that formed by multiple pulses of intrusion and mineralization. Host rocks of Climax-type intrusions typically are warped, attenuated, domed, and fractured. Steeply dipping radial and concentric dikes, veins, faults, and joints indicate vertical orientation of the maximum principal stress during forceful emplacement of magmatic cupolas. Sparse inclusions of host rocks near contacts indicate magmatic stoping. Discontinuous stockwork veinlets resulted from forces generated by hydrothermal fluids that evolved from the magmas. Gently outward-dipping concentric veins and faults probably formed during cooling and contraction of intrusive cupolas. Climax-type rocks are silica rich, aluminous, calcium poor, and alkali rich, with K 2 O > Na 2 O. Essential minerals are quartz, potassic feldspar, and albite. Accessory minerals include fluorine-bearing biotite, fluorite, fluorine-rich topaz, spessartine, zircon, ilmenorutile, rutile, columbite, brannerite, uraninite, thorite, monazite, fluocerite, apatite, xenotime, aeschynite, and euxenite. Numerous textural features in Climax-type intrusions suggest that ore-forming fluids ex-solved directly from crystallizing magmas. Rhythmic quartz layers in porphyry indicate high water pressure and episodic build-up and release of volatiles. Replacement of albite pheno-crysts by nearly pure orthoclase in a groundmass containing albite suggests the presence of a separate hydrothermal fluid before formation of the groundmass. Zones of micrographic textures indicate areas of hydrothermal fluid accumulation prior to overpressure relief and release of the fluid. Aplitic groundmass textures suggest pressure quenching. Veins near igneous contacts commonly have both igneous and hydrothermal characteristics. Fluid inclusions from the Henderson mine that contain as much as 65 wt percent NaCl suggest that molybdenite mineralization formed at temperatures above 500°C, probably between 500° and 650°C. Consideration of phase equilibria in fluid inclusions indicates overpressures 150 to 250 bars greater than lithostatic pressure during mineralization. These overpressures probably caused the stockwork fracturing. Hydrothermal alteration is best recorded at Red Mountain. Five major pervasive rock alteration zones include the potassium feldspar zone, quartz-sericite-pyrite zone, upper and lower argillic zones, and the propylitic zone. Five additional zones of less areal extent include the vein silica zone, pervasive silica zone, magnetite-topaz zone, greisen zone, and garnet zone. Strontium and lead isotope data and trace element concentrations in Climax-type systems suggest that Climax-type igneous rocks represent extreme differentiates of parent magmas which formed by fractional partial melting of mafic- to intermediate-composition materials in the lithospheric mantle and lower crust, and that upper crustal rocks were not significantly involved in the generation or evolution of Climax-type magmas and ore leads. Age determinations, structural observations, and plate tectonic reasoning suggest that the Climax-type magmas of Colorado formed during a relatively atectonic transition, after subduction-related calc-alkaline igneous activity but before extension-related normal faulting and without or before known local basaltic volcanism. The high concentrations of silica, alkalis, and Rb in Climax-type rocks, coupled with the low concentrations of Ca, Sr, and Ba, suggest fractional crystallization of plagioclase and potassic feldspar. This, together with gravitational crystal-liquid separation, probably was the dominant differentiation mechanism in the deep crustal environment. The high inferred concentrations of water, F, Mo, W, Sr, U, Th, and Nb in Climax-type magmas suggest upward enrichment of these constituents by convection-driven thermogravitational diffusion (Hildreth, 1979), a process which probably became dominant as magma columns traversed steepening thermal gradients in progressively shallower environments. At depths between 2,000 and 10,000 ft, particularly volatile and molybdenum-enriched magma cupolas forcefully expelled ore-forming fluids. This caused stockwork fracturing of host rocks and pressure quenching of aplitic cupolas. Fracture filling by quartz, molybdenite, pyrite, fluorite, topaz, and/or huebnerite formed the orebodies.

Abstract Production from the Mercur mine was completed in March of 1997. From September 1982 through March 1997, over 2 million ounces of gold were produced from 22 million tons of ore. Gold production prior to 1982 is estimated to be approximately 1.5 million ounces. The gold deposits of the Mercur mine are situated in the southwest Oquirrh Mountains, approximately 30 miles south of Tooele, Utah. The gold deposits of Mercur lie along the eastern limb of the NNW-trending Ophir anticline. The anticline is one of a series of NNW-trending folds that have an asymmetrical shape due to folding of upper plate rocks during Sevier-age thrusting. The Ophir anticline plunges both to the north and south, forming a dome. The Mercur, Sacramento, Marion, and Golden Gate pits were the main deposits developed at Mercur. Mineralization developed along east-to northeast-striking faults and the north- to northwest-striking Mercur fault. Dilation and extension are seen along the east- to northeast-striking faults. It is postulated that the east-to northeast-striking faults represent extensional features that began with the doming of the Ophir anticline. The north- to northeast-striking Mercur fault is inferred to be related to the regional folding. Gold mineralization at Mercur occurs as sediment-hosted, disseminated (Carlin-type) deposits. Mineralization is hosted in a 200- to 240-ft sequence of thin-bedded, fossiliferous limestones and siltstones that form the top of the otherwise micritic Topliff Member of the Mississippian Great Blue Formation. This sequence of rocks, known locally as the Mercur Mine Series, is conformably overlain by the Long Trail Shale Member, which acted as an impermeable cap to the deposits. Intense oxidation, decalcification, argillization, and silicification are evident. Many parts of the deposits remain unoxidized. Carbon flooding is also present. Fracturing and decalcification were critical in preparation of the host rocks, increasing permeability and allowing access for mineralizing fluids. Calcite and quartz veinlets are common in the deposits, along with sulfides which include pyrite, realgar, and orpiment. Jasperoid is present in many areas as a conformable mass along the base of, and extending up into, the lower half of the Mercur Mine Series. The Porphyry Peak granodiorite (36.7 Ma) crops out north of Mercur as a series of NNW-striking dikes. The Eagle Hill rhyolite (aplite; 31.6 Ma) forms a combination sill and dike complex that bounds the Sacramento deposit on the south and west sides. Several hydrothermal breccias have been encountered at Mercur, the largest of which, the Thickener Breccia, accounted for over 90,000 ounces of gold production. The breccia is a heterolithic assemblage of rocks belonging to the Mercur Mine Series as well as to units above and below the Mercur Mine Series. Also present in the breccia are clasts of the two igneous rock types present in the district. Gold mineralization at Mercur is interpreted to have taken place after the emplacement of the Eagle Hill rhyolite. The presence of igneous clasts in the mineralized Thickener Breccia and the presence of rhyolite in a mineralized collapsed flower structure in the Sacramento pit suggest possible post-intrusion age mineralization.

Abstract Skarn deposits are one of the more abundant ore types in the earth’s crust and form in rocks of almost all ages. Skarn is a relatively simple rock type defined by a mineralogy usually dominated by calcsilicate minerals such as garnet and pyroxene. Although the majority of skarns are found in lithologies containing at least some limestone, they can form in almost any rock type during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. Although most are found adjacent to plutons, they also can occur along faults and major shear zones, in shallow geothermal systems, on the sea floor, and at lower crustal depths in deeply buried metamorphic terranes. Thus, neither a pluton nor limestone is necessarily required to form skarn. Most skarn deposits are zoned, and the general pattern is proximal garnet, distal pyroxene, and minerals like wollastonite, vesuvianite, or massive sulfides and/or oxides near the marble front. Recognition of distal alteration features such as bleaching, fluid escape structures, and isotopic halos can be critically important in exploration. Because most economic skarn deposits are related to magmatism, details of igneous petrogenesis and tectonic setting form a framework for exploration and classification. For the seven major skarn types (Fe, Au, Cu, Zn, W, Mo, and Sn) a general correlation exists among igneous major and trace element composition and skarn type. Plutons associated with Fe and Au skarns contain significantly more MgO and less K 2 O or SiO 2 , Au and Sn skarn plutons are more reduced, and Cu, Zn, and Mo skarn plutons are more oxidized than average skarn plutons. In terms of geochemical evolution, there is a fairly linear array from relatively primitive calcic Fe skarn plutons through Au, Cu, Zn, to W, Mo, to relatively evolved Sn skarn plutons. Calcic Fe skarn plutons are metaluminous, high in compatible elements such as Ni, V, and Sc, and have Rb/Sr <1. Relative to Fe skarn plutons, Cu skarn plutons have higher Si, K, Ba, Sr, La, and Fe 3+ /Fe 2+ , and contain significantly less Mg, Sc, Ni, Cr, and V. Au skarn plutons are similar to Fe skarn plutons in their metaluminous nature and their Si, Mg, Cr, and Sc contents and are similar to Cu skarn plutons in their Ni, V, and Y content. In general, plutons associated with Sn, Mo, and Wskarns have a much stronger crustal signature than do plutons associated with other skarn types. The seven major skarn classes have some distinctive differences. Iron skarns are mined for their magnetite content although minor amounts of Cu, Co, Ni, and Au may be present. Endoskarn may exceed exoskarn, including widespread albite, orthoclase, and/or scapolite that sometimes are of regional extent. Gold skarns span a range of geologic environments but most deposits are relatively reduced, are mined solely for their gold content, and have a distinctive Au-Bi-Te-As ± Co geochemical association. Most are associated with reduced, mafic plutons and dike/sill complexes with distal/early biotite ± K-feldspar hornfels aureoles; Fe-rich pyroxene is a characteristic mineral. Copper skarns are associated with I-type, magnetite series, calc-alkaline, porphyritic plutons, many of which have cogenetic volcanic rocks, stockwork veining, brittle fracturing and brecciation, and intense hydrothermal alteration; features collectively indicative of a relatively shallow environment of formation. Copper skarn mineralogy is dominated by andraditic garnet. Most Zn skarns occur distal to associated igneous rocks and are mined predominantly for Zn, although Cu, Pb, and Ag can be economically important. They can be distinguished from other skarn types by their Mn-and Fe-rich mineralogy. Most Zn skarn districts grade outward from skarn-rich mineralization to skarn-poor ores, veins, and massive sulfide bodies, which may contain few if any skarn minerals. Tungsten skarns are associated with coarse-grained, generally unaltered, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles; subcalcic garnet is a characteristic mineral. Molybdenum skarns are associated with leucocratic granites and commonly contain accessory W, Cu, Zn, Pb, Bi, Sn, and U; some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be economic. Hedenbergitic pyroxene is the most common calc-silicate mineral in Mo skarns and wollastonite, amphibole, and fluorite may be unusually abundant. Tin skarns are almost exclusively associated with high silica granites, typically generated by partial melting of continental crust, and have a characteristic suite of trace elements (Sn, F, B, Be, Li, W, Mo, and Rb). Many Sn skarn deposits develop a greisen alteration stage, characterized by high fluorine activities and the presence of minerals like fluorite, topaz, tourmaline, and muscovite, which is superimposed upon plutons, early skarn, and unaltered carbonate rocks. Most large skarn deposits record a transition from early/distal metamorphism resulting in hornfels, reaction skarn, and skarnoid, to later/proximal metasomatism resulting in relatively coarse grained ore-bearing skarn. Fluid inclusion and isotopic evidence indicates early metasomatism is caused by high-temperature (≥500°C), high-salinity (>50 wt % total salts) fluids of magmatic origin, which typically are enriched in Si, K, Na, Al, Fe, and Mg, and depleted in Ca, 18 O, and CO 2 relative to protoliths. Documented daughter minerals in fluid inclusions in skarn minerals include NaCl, KCl, CaCl 2 , FeCl 2 , CaCO 3 , CaF 2 , C, NaAlCO 3 (OH) 2 , Fe 2 O 3 , Fe 3 O 4 , AsFeS, CuFeS 2 , and ZnS. In addition, melt inclusions have been found in some skarn minerals. These contain complex daughter mineral assemblages and evidence for both chloride and carbonate melts at the time of skarn formation. Analyses of skarn-hosted inclusion fluids, both liquid and vapor, show high concentrations of K, Ca, Mn, Fe, Cl, Br, Cu, Pb, Zn, and As. With time, skarn metasomatic alteration evolves to lower temperature (≤400°C), generally hydrous, and sulfide-rich assemblages, termed retrograde alteration. Such retrograde alteration usually is accompanied by brecciation and is caused by influx of cooler, lower salinity (≤20 wt % total salts) fluids that are still magmatic in origin. One important difference between prograde and retrograde alteration is thought to be the timing and extent of phase separation of hydrothermal fluids.