Abstract

The Jurassic Yerington batholith, western Nevada, is a composite pluton that contains several centers of porphyry copper mineralization and is exposed in structural cross section at paleodepths ranging from 0 to 8 km. Within these exposures the McLeod Hill quartz monzodiorite, Bear quartz monzonite, and Luhr Hill granite form successive intrusions that are in turn volumetrically smaller ( approximately 75, 19, and 6 vol %, respectively), more deeply emplaced (tops at <1, 1.5, and 2.5-5 km), and more silica rich ( approximately 60, 66, and 68 wt % SiO 2 ). Strontium isotope ( 87 Sr/ 86 Sr initial = 0.7040) and oxygen isotope (delta 18 O approximately 6.8 ppm) values and major and trace element compositions indicate that the earliest (parent) magmas were high K andesites, probably derived by fractionation of basalt combined with lesser assimilation of crust, which possibly consisted of igneous arc rocks. Most high K andesitic magma crystallized as quartz monzodiorite, but some differentiated to quartz monzonite and granite. Major and trace element variations and the Sr isotope composition are consistent with a model in which granite was derived from quartz monzodiorite by fractionation of approximately 40 wt percent crystals in proportions similar to the mineralogy of cumulate gabbro. Differentiation occurred by downward and inward crystallization interrupted by two events of upward intrusion of, first, Bear quartz monzonite, and then, Luhr Hill granite. Luhr Hill granite locally grades into granite porphyry dikes that are temporally and spatially associated with porphyry copper mineralization immediately above the apices of granite cupolas in the batholith.Phase petrology and mineral compositions indicate an oxidation trend associated with evolution of magmatic aqueous fluids during both differentiation of the batholith and crystallization and subsolidus cooling of individual intrusions. During differentiation, the water content of magmas increased from <3 wt percent in the first-emplaced quartz monzodiorite magmas, which contained augite and biotite, to approximately 4 to 5 wt percent in hornblende- and biotite-bearing quartz monzonite and granite. The water content, Cu content ( approximately 65 ppm), and oxidation state prior to crystallization are typical of hydrous and oxidized calc-alkaline magmas. These water contents require separation of approximately 3 wt percent water prior to solidification, estimated at 675 degrees to 750 degrees C at 2 to 1 kb pressure (7- to 3.5-km paleodepth). Oxygen fugacity (f (sub O 2 ) ) increased slightly during differentiation, as reflected by decreased abundance of ilmenite and by the decreased atomic ratio of Fe/(Fe + Mg) of some Ca amphibole from approximately 0.40 to approximately 0.30. However, during crystallization and subsolidus cooling, f (sub O 2 ) increased sharply by approximately 2 to 3 log units from a proposed ilmenite-sphene buffer at approximately 800 degrees C to the hematite-magnetite buffer at approximately 500 degrees C. The highly oxidized conditions are also reflected in the low Fe/(Fe + Mg) of all mafic silicates: biotite (0.35-0.41), Ca amphibole (0.27-0.40), and augite (0.20-0.24). The Cl/F ratio of apatite decreases during differentiation from 0.08 to 0.01, probably reflecting partitioning of Cl into an aqueous fluid. The compositions of alkali feldspar and Fe-Ti oxides, the halogen content of biotite and Ca amphibole, the Ti content of most biotites, and the Al IV content of Ca amphibole all appear to reflect subsolidus conditions, compatible with the hypothesis of evolution of abundant aqueous fluids.Granite porphyry dikes associated with Cu mineralization originated at a approximately 3 to 6-km depth. When they were emplaced, they contained 50 percent phenocrysts (K-feldspar-quartz-plagioclase-hornblende-biotite) and therefore were water saturated and near their solidus temperature ( approximately 700 degrees C). They have aplitic groundmass textures caused by pressure quenching during rapid upward emplacement and attendant exsolution of aqueous fluids. High-salinity fluid inclusions in quartz phenocrysts and Cu-bearing quartz veins suggest that mineralization was caused by high-salinity aqueous fluids that separated from the granite porphyry dikes as they were emplaced. The dikes grade downward in texture into the >65-km 3 Luhr Hill granite. The low Cu content ( approximately 10 ppm) of the granite would allow extraction of approximately 50 ppm Cu from it during crystallization. These data suggest an orthomagmatic model in which both high- and low-density saline aqueous fluids formed during exsolution of water from the crystallizing Luhr Hill granite; and from which salts, Cu, Fe, and S strongly partitioned into the high-density "ore" fluid. Low-density fluids may have risen from the magma as a vapor plume. Mineralization occurred when fluid overpressures caused fracturing of wall rock and upward intrusion of granite porphyry dikes and high-density saline ore fluids. Both the f (sub O 2 ) path during subsolidus cooling and the deposition of most S as chalcopyrite-pyrite in ore suggest that reduction of magmatic S to sulfide may have been a mechanism for oxidation of magmatic ferromagnesian silicate and Fe-Ti oxide minerals.

First Page Preview

First page PDF preview
You do not currently have access to this article.