Exploration Techniques at the Thorn Ag-Au-Cu High-Sulfidation Epithermal Prospect, Northern British Columbia, Canada

Abstract

The Thorn prospect is an El Indio-style high-sulfidation epithermal prospect in a little-explored belt of Late Cretaceous subaerial volcanics and subvolcanic intrusions in northwestern British Columbia, Canada. More than 30 massive sulfide (pyrite-enargite-tennantite/tetrahedrite) veins, quartz-sulfide (quartz-pyrite-enargite-tennantite/tetrahedrite) veins and quartz-vuggy silica-alunite veins/breccias fill ENE fractures and faults over an area of 1,600 × 1,900 m. They are hosted within a Late Cretaceous feldspar-quartz-biotite granodioritic porphyry stock and flanked by alteration envelopes a few meters to tens of meters wide: an inner envelope of intense sericite-clay-pyrite and an outer one of weak clay-sericite-chlorite. Where several veins are close together, the sericite-clay-pyrite alteration envelopes coalesce into zones up to 150 m wide. The most successful exploration techniques have been prospecting, silt and soil geochemistry, airborne magnetic and electromagnetic surveys and diamond drilling. Silt and soil geochemical anomalies for Au, Ag, As, Cu, Pb, Sb and Zn mark the known veins and indicate prospective areas where veins have not yet been discovered. Magnetics and resistivity outline the gross property-wide structure and lithologies. Most of the sericite-clay-pyrite alteration and known veins are outlined by the <264 ohm-m resistivity contour, which covers an area of 800 × 2,300 m. Weak EM conductors are thought to represent undiscovered massive sulfide and quartz-sulfide veins, and their flanking sericite-clay-pyrite alteration.