Discovery of Gold Mineralization Beneath a Barren Opaline Silica Cap at Painted Hills, Northwestern Nevada, and the Association of Gold with Molybdenum in Low-Sulfidation Epithermal Systems

摘要

A low-sulfidation, volcanic-hosted, epithermal system of probable middle Miocene age (~15 Ma) is exposed in the Painted Hills in northwestern Nevada. Host rocks consist of gently dipping basalt, known only from drilling, and successively overlying felsic volcaniclastic rocks and volcaniclastic sandstone. A body of silicified felsite in the southern part of the area is interpreted as a pyroclastic vent, dated previously at 14 Ma. Alteration and mineralization occur along the eastern range front of the McGee Mountains. Argillized felsic tuffs and volcaniclastics contain weakly anomalous As, Hg and Sb but no Au or Mo. The overlying sandstone contains stratiform opal and a 100-m-wide stockwork of north-trending opal veins, which are similarly only weakly anomalous in As, Sb and Hg. A northerly-trending, silicified fault 500 m south of and on strike with the opal stockwork is 55 m below the opal veins, is chalcedonic and contains strongly elevated As, Hg and Sb. The silicification and argillic alteration occur along a 1.6 km north-northeast trend inboard of and parallel to the modern 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 1970s; gold reaches 107 ppb with strongly-anomalous Hg, As, Sb and Mo. The opal veins, 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 with 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. Drilling intersected multiple zones of silicification, typically 25-40 m thick, with local stockworks of sulfidic chalcedony veins. The silicification occurs across a width of at least 200 m. Gold values increase 100 m below surface within silicified and veined intervals, although gold reaches only 330 ppb Au over 0.75 m. Gold correlates strongly with molybdenum, which reaches 1560 ppm Mo over 0.6 m. 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 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. Although the gold intersected is clearly subeconomic, the exploration model was validated, and much of the system remains unexplored. The strong correlation of gold with molybdenum is similar to the middle Miocene, low-sulfidation, volcanic-hosted, bonanza-grade Sleeper gold deposit 85 km to the southeast. Elevated Mo with a strong correlation with gold also occurs in a low-sulfidation gold system in the South Hills of western Nevada that is also likely middle Miocene. The Mo could indicate a link with associated felsic magmatism.