To paraphrase Jeff Hedenquist from his 2016 report on the Unga Project:
“The main body of Orange Mountain...consists of residual quartz.... The original lithology here is difficult to discern, given the strong silicic alteration (silicification) of the rock after the leaching event that produced the residual quartz. Locally there are textures that indicate brecciation...prior to the strong silicification. Subsequently the silicic alteration was cut by veins of massive cryptocrystalline quartz... with open-space fill of barite in places. To the west and NW of the principal silicic body there are relatively thin lithocap horizons... with vuggy texture...; these horizons appear to be lithic tuff, although the texture may also in places be due to post-silicic brecciation and silicification.... These tuff horizons dip to the SW to WSW on the west side of Orange Mountain. To the east and SE of the main silicic body, the lithology appears to dip to the SE, beneath reportedly fresh basaltic andesite....
In summary, residual quartz ... alteration of tuffaceous horizons at Orange Mountain occurs over a central area of ~1 x 1 km, with more extensive silicic ribs plus quartz-alunite and alunite-clay alteration to the WSW (Fig. 7a). The hypogene advanced argillic alteration is characteristic of magmatic vapor condensates related to a shallow intrusion. A syn-hydrothermal polymict fragmental unit with juvenile clasts <1 km from the main silicic body indicates syn-hydrothermal magmatism and eruption, with a crater-lake setting indicated by the laminated water-lain sediments. Following this alteration, including in distal locations, there were cross-cutting quartz veins in this area at the surface, as well as to the NE and SW along structural trends toward the Shumagin and Aquila vein systems.
Examples of residual quartz are shown below, with typical vuggy texture in a polymictic fragmental volcanic (left), and nearly completely replaced with later quartz (right).”
Residual Quartz, Orange Mountain (from Hedenquist, 2016)
Battle Mountain Gold Corp drilled three diamond coring holes in 1983 and intersected dekametric intervals of trace gold in silicified volcanics containing abundant pyrite and with mercury to >5 g Hg/t, but low arsenic, silver, copper, lead, and zinc.
Core from Orange Mountain drill hole OM-3, showing silicic alteration and brecciation, the latter cemented by iron oxides (original pyrite that was supergene oxidized, likely along structures related to the brecciation); 199.5' and 222'; 0.34 and 0.08 g/t Au, respectively, with 62 and 252 ppm Cu. Samples from 244' to 266.5' are unoxidized, with 265.9' and 266.5' showing a metallic black sulfide (tennantite-tetrahedrite or enargite?) from an interval with >2 wt% Cu, >1000 ppm As and 1700 ppm Sb, plus 45 ppm Bi and 420 ppm Ba; Au is 20 ppb, with 1 ppm Ag - the other two unoxidized silicic core samples contain 15 and 25 ppb Au.
Silicified volcanics from skeleton cores from the Battle Mountain Gold 1983 drilling campaign