T297 Economic Geology of the Extensional Terrains

Wednesday, 28 September 2016
1:30 PM - 5:30 PM
Room 301

PRELIMINARY CHARACTERIZATION OF GOLD OCCURRENCE IN THE FIRE CREEK EPITHERMAL DEPOSIT, NORTH CENTRAL NEVADA

Paper number: T297-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

The Fire Creek Mine, Lander County, Nevada, is exploiting epithermal Au-Ag rich veins associated with extensional tectonics and mafic magmatism of the Middle Miocene Northern Nevada Rift. Fire Creek contains proven + probable reserves of 240,000 tons at a grade of 1.311 opt Au and 0.972 opt Ag. Most mineralization occurs in high angle veins emplaced in a package of basalt and basaltic andesite flows. Decimeter to meter scale veins are often associated with high angle faults and mafic dikes trending between N15°W and N-S. Individual vein composition can vary from coarse, bladed calcite with subsidiary quartz (±adularia) to quartz dominated with lesser carbonate (±adularia). Host rocks adjacent to veins and faults are typically propylitized to argilitized and in some instances silicified. Overall, the unusual geologic setting resembles that of the Mule Canyon low-sulfidation epithermal deposit approximately 12 km to the north.

Gold at Fire Creek occurs in four types and associations: 60-70% of gold occurs in discrete, up to cm-thick, bands on or near the wall rock contact, very fine grained gold occurs throughout the veins and in the wall rock, crystalline gold can occur in open spaces or vugs, and gold is hosted in fault gouge and clay zones of uncertain origin. Some gold is associated with sulfide minerals.

Gold in the Vonnie Vein was characterized by reflected light microscopy and SEM-EDS. Vein gold crystallized near the wall rock contact is skeletal dendritic electrum with individual dendrites up to 2 mm in length. The Au:Ag at.% ratio of dendritic crystals varies systematically from Au48 near the contact with the wall rock to Au41 at the dendrite tips approximately 1 cm into the vein. Isolated electrum grains (10-40 micrometers diameter) in the wall rock or in the vein near the wall rock contact have higher Au:Ag ratios, averaging Au56.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-285912

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