DISCOVERY OF GOLD BENEATH A BARREN OPALINE CAP AT PAINTED HILLS, NW NEVADA, AND THE ASSOCIATION OF GOLD WITH MOLYBDENUM IN A LOW-SULFIDATION EPITHERMAL SYSTEM

A low-sulfidation, volcanic-hosted epithermal system of probable Middle Miocene age (15 Ma) is exposed at Painted Hills, 17 miles SW of Denio in NW Nevada. Host rocks consist of bimodal gently-dipping basalt overlain by felsic volcaniclastics (Tt) and an upper volcaniclastic sandstone (Ts) intruded by a silicified, felsic exogenous dome, dated previously at 14 Ma. Tt is extensively argillized and contains anomalous As, Hg and Sb but no Au. Overlying Ts is replaced by stratiform opal and contains a stockwork of N-trending steep opal veins up to 100m wide; veins are very weakly anomalous in As, Sb and Hg. A N-trending, steeply E-dipping silicified (chalcedonic) fault exposed 500m S of and on strike with the opal stockwork has E-side-down offset and contains more strongly elevated As, Hg and Sb. The alteration occurs for 1.6km, 300m inboard of and parallel to the modern range front fault. The only indication of Au prior to drilling in 2007 was rare float of chalcedonic, pyritic silicified Tt from below the opal veins excavated by Hg mining in the 1970's. Gold reaches 107 ppb with high Hg (>100 ppm), As (76 ppm), Sb (59 ppm) and Mo (156 ppm). Although there is no sinter, the opal and argillic alteration indicate that exposures represent the upper parts of an epithermal system. The exploration model, based on other districts, envisioned Au increasing below the opal zone coincident with increasing T, coarser silica and proximity to a boiling level, where high-grade Au-Ag-Se would be expected. Four core holes intersected multiple zones of pyritic silicification typically 25-40m thick with sulfidic chalcedonic vein stockworks. Silicification occurs across a width of at least 200m and is blind beneath the barren sandstone cap and the far narrower opal and chalcedonic structures. Au increases within 100m below surface in silicified and veined intervals directly below exposed opaline/chalcedonic structures. Subsurface Au values are low (330 ppb max) but widespread. Gold correlates strongest with Mo. The presence of vein sediment, absence of bladed calcite/quartz and absence of sugary quartz indicate that any Au-Ag rich boiling level remains deeper or lateral to the area drilled. The strong Au:Mo correlation is similar to other productive low-sulfidation Au deposits such as the similar-aged, high-grade Sleeper vein deposit 53 miles to the SE.