AN ANALYSIS OF MAGNETIC SUSCEPTIBILITY AT THE MIOCENE BASALT-HOSTED FIRE CREEK EPITHERMAL GOLD DEPOSIT, LANDER COUNTY, NEVADA

The Fire Creek deposit is a narrow vein, low sulfidation epithermal Au±Ag deposit in Lander County, Nevada. Gold mineralization is hosted in Miocene basalts and mafic tuffs and is structurally controlled by a network of faults and dikes. Magnetic data on the property include a ~4,200 acre ground-magnetic survey and magnetic susceptibility data collected from 177 diamond core holes comprising 235,309 ft. of core. The primary goals of this study are to identify magnetic survey anomalies, differentiate basalt flows from intrusions, constrain the geologic model, improve geologic log quality, and identify alteration zones that may be related to ore formation.

Unaltered basalt typically has a relatively high magnetic susceptibility and individual basalt packages possess distinct magnetic susceptibility signatures. Low magnetic susceptibility anomalies are associated with tuff units and altered basalts. Alteration in basalt units is typically constrained to an envelope around fluid conduits (i.e. faults and/or intrusive contacts) that grades outward from variable silicification directly proximal to ore to argillic and propylitic assemblages.

Preliminary results indicate strong subhorizontal and subvertical controls on magnetic susceptibility. Comparison with the Fire Creek 3D geologic model and geologic logs show that subhorizontal features delineate known volcanic stratigraphy and identify basalt flows not previously recognized during logging. Subvertical anomalies appear to track ore-stage faults. These anomalies and interpretations are consistent with the ground-magnetic survey.