System type: Volcanic-hosted, low-sulfidation epithermal gold
Target: High-grade veins and disseminated mineralization
The Painted Hills project is in northwestern Nevada, in Humboldt County, 17 miles southwest of the town of Denio. The project exposes the upper parts of a low-sulfidation, volcanic-hosted epithermal gold system of probable Middle Miocene age and lies along the regional northeast-trending Black Rock Structural Boundary (BRSB), at its intersection with a northwest-trending fault zone. The BRSB localizes the productive Hog Ranch deposits and the Mountain View gold system southwest of Painted Hills. These deposits are members of a productive class of gold-silver deposits that formed during the Middle Miocene (~15 Ma) in the northwestern Great Basin and commonly contain high-grade veins in addition to disseminated mineralization. Notable examples include the Sleeper, Midas (Ken Snyder), Fire Creek, Mule Canyon, Rawhide, Goldbanks, Hollister and Hog Ranch deposits. Like Painted Hills, most of these deposits lie along regional fault zones like the BRSB.
Host rocks at Painted Hills consist of a gently-dipping sequence of basalt flows (known only from drilling) and successively overlying felsic tuffs/volcaniclastics and volcaniclastic sandstone. These units underlie the Middle Miocene (~15 Ma) Cañon rhyolite exposed on the west side of the mineralized area. A silicified felsic pyroclastic vent (intrusive exogenous dome) in the southwest part of the project area has been dated at 14 Ma.
The felsic tuffs and volcaniclastics are extensively argillized and contain anomalous As, Hg and Sb but no Au or Mo. The overlying sandstone is notably unaltered but locally contains stratiform opal and a 100 m-wide stockwork of north-trending opal veins. The opal is weakly anomalous in As, Sb and Hg. A northerly-trending silicified fault is exposed 500 m south of and on strike with the opal stockwork but is 55 m below the elevation of the exposed opal veins. Silicification along this fault is distinctly chalcedonic and contains strongly elevated As, Hg and Sb. The silicified pyroclastic vent also contains elevated As, Sb and Hg. The silicification and argillic alteration occur along a 1.6 km north-northeast trend that is inboard of and parallel to the range-front fault, which is several hundred meters to the east.
The only indication of gold prior to drilling was rare float of chalcedonic, pyritic silicified tuff from below the opal vein stockwork excavated by small-scale mercury mining in the 1970’s; gold reaches 107 ppb with strongly-anomalous Hg, As, Sb and Mo. The opal veins and stratiform opal replacements, the argillic alteration and elevated Hg indicate that exposures represent the upper portions of an epithermal system. The discovery of at least anomalous gold prompted the first exploration drilling in 2007. The exploration model envisioned increasing gold at depth below the opal zone coincident with increasing crystallinity of silica and proximity to a deeper boiling level, where high-grade Au-Ag mineralization would be expected. The change with depth from opal to chalcedony containing anomalous gold supported this model, which has been proven in several districts in Nevada (e.g., Goldbanks and Hollister), where gold has been discovered beneath barren or Hg-bearing opal.
Four core holes totaling 1,852m (6076.5 ft) were angled to cross beneath the exposed vein and alteration trend; about 440m of strike was tested. All holes intersected multiple zones of silicification typically 25-40m thick with local stockworks of sulfidic chalcedony veins. The silicification occurs across a width of at least 200m. Gold values increase within 100m below surface within silicified and veined intervals, although gold reaches only 330 ppb over 0.75m. Gold correlates strongly with molybdenum, which reaches 1560 ppm Mo over 0.6m. Fine-grained blebs of molybdenum sulfide, interpreted as jordisite, occur within black, pyritic chalcedony veinlets. Gold is concentrated in these veinlets, with one “high-graded” separate yielding 1.5 ppm Au and 1.65% Mo. Mercury and Sb are enriched in shallower silicified zones, with Mo-As-Au enriched deeper. The strong correlation of gold with molybdenum is encouraging, as this also occurs at the Middle Miocene, bonanza-grade Sleeper vein deposit 85 km to the southeast.
The presence of vein sediment and the absence of bladed calcite/quartz, sugary quartz and adularia indicate that the potential gold-silver rich boiling level remains deeper or lateral to the small area drilled to date. Additional drilling is recommended to explore other portions of the mineralized trend, the range front and pediment to the east, and follow-up in the area drilled in 2007. The Sleeper deposit lies along a major range-front fault zone under alluvial cover, a similar setting to Painted Hills.