Geology, Alteration, and Mineralization of the Elder Creek Porphyry System, Battle Mountain, Nevada

摘要

Surface exposures at Battle Mountain, Nevada, of the Elder Creek porphyry system yield a nearly horizontal cross section through a deeply exposed Cu-Au porphyry system that is part of widespread Late Eocene to Early Oligocene magmatism in north-central Nevada. Detailed field mapping (1:5,000 scale) of lithology, structure, hydrothermal alteration, veins, and associated sulfides combined with the petro-graphic study of 95 thin sections and microprobe analyses, has led to a greater understanding of this magmatic-hydrothermal system in both time and space. Crosscutting relationships between alteration types and veins permit hydrothermal alteration to be divided into six main categories, including potassic and sericitic alteration derived from magmatic fluids, and sodic-calcic, potassic-calcic, calcic, and propylitic alteration that may be derived mainly from non-magmatic fluids. Mineralization associated with these alteration types, in conjunction with published fluid inclusion studies, were used to place constraints on the temperatures of formation and the oxidation and sulfidation state of this system. The Elder Creek area is underlain by clastic rocks of the Cambrian Harmony Formation forming the upper plate of the Dewitt Thrust, a splay of the Roberts Mountain Thrust of the Devonian-Mississippian Antler Orogeny. These strata are intruded by Devonian diabase dikes and four Eocene (~40-41 Ma) intrusive units, including a two-phase composite stock composed of a crowded granodiorite porphyry (40.3±1.0 Ma), a quartz-eye granodiorite porphyry (40.7±0.6 Ma), a set of hornblende granodiorite dikes (40.3±0.9 Ma), and late quartz monzonite dikes (40.6±0.6 Ma and 40.7±0.5 Ma) which crosscut alteration and mineralization. The Eocene intrusions contain accessory magnetite, titanite, apatite, zircon, and, in places, allanite; magmatic sulfides have not been recognized. The presence of titanite + magnetite and relatively Mg-rich biotite indicate that the magmas were relatively oxidized. Potassic, sericitic, sodic-calcic, potassic-calcic, calcic, and propylitic alteration types form distinct zones within and around the composite pluton and were defined based upon observed mineral assemblages. Alteration in siliciclastic rocks and igneous rocks includes: potassic alteration (Qz+Bio+Kspar±Rut with Po+Asp+Cpy+Py), sericitic alteration (Qz+Ser+Py with varying amounts of Chl+Asp+Cpy), sodic-calcic alteration (Na-plag+Act+Chl+Tit+Ep+Ap associated with the removal of metals from the host rocks), potassic-calcic alteration (Kspar+Act+Ep+Tit+Ap), calcic alteration (Ca-plag+Act+Dio+Qz along with disseminated Po+Cpy + pentlandite + gersdorffite + minor sulfides and Ni-arsenides) and propylitic alteration (Chl+Ser+Py±Act derived from more dilute fluids). Elder Creek is interpreted to be deeply exposed based on the abundance of sodic-calcic alteration, the scarcity of sulfides, and the occurrence of this system in two of the deeper thrust plates at Battle Mountain. Exhumation reflects a combination of erosion and partial structural dismemberment, the latter caused by three generations of NW- and NE-trending normal faults. Elder Creek resembles other porphyry related Cu-Au systems at Battle Mountain, in that all have similar ages, sulfidation states, oxidation states, types of alteration, and igneous rocks. Based alteration and structure, Elder Creek is the deepest exposed, Copper Canyon represents a mid-level exposure, and Buffalo Valley and Copper Basin represent the highest levels of a porphyry system in the Battle Mountain mining district.