HYDROTHERMAL ALTERATION AND STABLE ISOTOPE CONSTRAINTS ON FORMATION OF THE MULE CANYON LOW-SULFIDATION AU-AG DEPOSIT, NORTH-CENTRAL NEVADA

The Mule Canyon AuÐAg deposit is along the northern Nevada rift, which formed during the early stages of bimodal magmatism and back-arc extension. It is hosted by mafic igneous rocks that formed an eruptive center between ~16.4 to 15.8 Ma. Hydrothermal alteration and Au-Ag ores formed at ~15.6 Ma during a hiatus in eruptive activity and were controlled by NNW- to N-striking high-angle fault and breccia zones.

Hydrothermal alteration is zoned outward from fluid conduits from silica-adularia, to adularia-smectite, smectite (argillic), and outer smectite-carbonate (propylitic) assemblages. Pyrite/marcasite (py) and arsenopyrite (asp) are abundant in all alteration types. Silica-adularia alteration was superimposed on argillic and propylitic alteration. Two main ore types are present: early disseminated and vesicle-filling Au-bearing arsenian py and asp overgrowths on py in argillically altered rocks, and later electrum and Ag-sulfides and selenides in stockwork and banded silica-adularia veins and breccias. High Au, Ag, As, Sb, and Se and low Cu, Pb, and Zn characterize both ore types.

Stable isotope data indicate that ore fluids consist dominantly of meteoric water that evolved by deep circulation through Paleozoic sedimentary rocks at low water-rock ratios (approx. 1). Calculated isotopic compositions of ore fluids are d¹⁸O=-3 to -7ä, dD=-120 to -124ä, d¹³C=-3 to -7ä, and d³⁴S=-3 to +8ä. Although some H₂S and CO₂ may have been derived from degassing Miocene magmas, and most H₂S and CO₂, ore metals and trace elements were obtained from Paleozoic rocks. Ore fluids were dilute, near neutral pH, reduced, H₂S rich, and CO₂ bearing. Ores formed at 100 to 200 m paleodepth. Temperatures in ore zones reached 230 to 265 deg. C at near lithostatic pressures, but most ore formed at <200 deg. C at near hydrostatic pressures. Sulfidation of wallrock Fe caused deposition of Au in As-rich overgrowths on py in early ores. Later boiling induced electrum and Ag-selenide precipitation in open spaces.

Mule Canyon is a mafic end-member of low-sulfidation epithermal Au-Ag deposits associated with Miocene bimodal basalt-rhyolite magmatism. It differs from low-sulfidation deposits associated with subduction-related calc-alkaline magmas in that little water, salt, and metals were derived from the bimodal magmas.