FE-OXIDE-CU (AU) DEPOSITS OF THE YERINGTON DISTRICT, NEVADA: LINKS TO DISTRICT-SCALE SODIC-CALCIC ALTERATION AND PORPHYRY CU DEPOSITS

Fe-Cu-Au occurrences in the Yerington district share many features (Na-Ca alteration, zoning, element suites) with other mafic to felsic Jurassic systems in SW North America. Fe-Cu(Au) ores occur in the Yerington batholith and its contact metasomatic aureole. Magmatic-hydrothermal fluids caused K-silicate, sericitic, and advanced argillic alteration with abundant Cu-Fe sulfides associated with porphyry dikes that represent the end-stage of batholith evolution. Synchronous Na-Ca alteration in the batholith was produced by brines derived externally from the evaporite-bearing Jurassic sedimentary section.

Small Fe deposits formed in batholith/volcanic cover within 3 km of the Jurassic paleosurface. Mag - apatite with Act ± Qz ± Cp in veins/replacements lie structurally above large volumes (>1 km³) of Na-Ca alteration (oligoclase - Act - titanite) near porphyry dikes at Blue Hill & MacArthur (Easter pros.). At Blue Hill, Fe-oxide deposits lie between deeper Na-Ca alteration and shallower Chl - Ep, K-feldspar - Chl, Chl - Ser , and tourmaline - albite alteration. We infer that Fe, K, and P was leached during Na-Ca alteration by heating brines, and transported to shallow sites of cooling and deposition.

Other lodes (Blue Jay, Buckskin Mine, Anaconda pros.) consist of Cp - Au (Cu ~ 1 wt.%; Au ~ 0.01-10 ppm) with Mag - Hem - Qz - Py - Chl, and wallrock alteration to Chl - Ep -Hem - feldspar ± Act -bearing assemblages. They locally post-date porphyry-related advanced argillic alteration.

Large Mag - Cp deposits in the contact aureole post-date early calc-silicate hornfels (1-3 vol.% Po - Py) and form as skarn breccia/vein fillings and carbonate replacements (Lyon: >350 MT @ ~31 wt.% Fe, 0.61% Cu, ~0.05 ppm Au; Cyprus internal rept.). Ad - Cp - Py is early and may be followed by Mag (after Hem), Act, Di, Chl or Calcite. Most Cp - Py - Mag ± Po is relatively late. Fe-Cu skarns differ from Yerington Fe-oxide-poor Cu skarns, characterized by coarse-grained Ad - Di and association with sulfide-bearing porphyries. The Fe-Cu skarns may have formed at lower temperature than Ad stability where Na-Ca fluids exited the batholith at 1-2 km depth.

Abbr: actinolite (Act); andradite (Ad); chalcopyrite (Cp); chlorite (Chl); diopside (Di); epidote (Ep); specular hematite (Hem); magnetite (Mag); pyrite (Py); pyrrhotite (Po); quartz (Qz); sericite (Ser).