Geochemical and geophysical signatures at three precious metals exploration targets in Nevada

Abstract

Integrated geochemical and geophysical exploration techniques were applied to two prospects and one operating gold mine in Nevada. The prospects are associated with recent or active hot springs systems; the mine is in a volcanic hosted epithermal system. Total field magnetic and Very Low Frequency/ElectroMagnetic (VLF/EM) surveys were conducted in conjunction with geochemical sampling at these properties. Magnetic lows were found which coincided with positive VLF/EM conductors and with anomalous geochemical signatures at each property. Magnetic lows result from oxidation of syngenetic magnetite to epigenitic hematite and limonite by hydrothermal fluids. EM conductors result from argillic and sulfide minerals deposited by hydrothermal fluids along faults and fractures. Areas where geochemical and geophysical anomalies overlap are considered prospective. Prospect A is a sediment hosted target which is concealed beneath pediment gravels. Hot springs sinter deposits attracted attention to the prospect. Rock chip and soils sampling indicated gold in the system. Geophysical signatures suggested presence of alteration along faults. Limited drilling and geochemical analysis of cuttings supported this hypothesis. Weak alteration and low precious metals values led to dropping this prospect. Prospect B is a volcanic hosted target concealed beneath thin pediment gravels. Scattered outcrops of sinter that yielded up to 150 ppb Au attracted exploration attention. The prospect is located within a Known Geothermal Resource Area. Geophysical anomalies overlapped a +1000 ppb Hg soils anomaly. A bifurcating VLF/EM conductor was present in the overlap area. Geochemical analysis of drill cuttings suggested the presence of an undefined epithermal target. The Flowery Gold Mine is a volcanic-hosted, Comstock Type, epithermal deposit. Mineralization occurred at and near the contact of an overlying volcanic sequence with an underlying shallow intrusive. Rock chip sampling defined several outcropping ore grade structures at the property. VLF/EM surveys showed the presence of large conductive bodies downdip of outcropping ore grade structures. The VLF/EM conductors result from the presence of pyrite rich altered rock in the subsurface. Drilling has confirmed the presence of ore grade material associated with the conductors. Combined geochemical sampling and geophysical surveying can be cost effective in prospecting for epithermal deposits in certain terranes in the Basin and Range Province.