Location: This lode prospect is on the same low knoll, locally called Arsenic Hill, along the west side of Rock Creek as the Rock Creek bench placer (NM210). Cathcart (1922, p. 245) described the location as opposite the mouth of Sophie Gulch. It is 3,400 feet due east of the Snake River road and about 1.6 miles south-southwest of Mount Byrnteson. Rock Creek is an east tributary to Snake River with headwaters on the south flank of Mount Byrnteson. The map location is midway between the 250- and 300-foot contours, just inside the north boundary of section 23, T. 10 S., R. 34 W., Kateel River Meridian. The prospect was included in locality 43 of Cobb (1972 [MF 463], 1978 [OFR 78-93]) and is locality 15 of Hummel (1962 [MF 247]).
Geology: Cathcart (1922, p. 245) reported free gold in vuggy crystalline quartz in quartz-muscovite-graphite schist at this locality, which he described as a hydraulic pit (Reinisch) on a bench opposite Sophie Gulch. In recent years this bench has been called Arsenic Hill. The name apparently was applied in the 1980s by geologist R.V. Bailey to abundant exposures rich in arsenopyrite and its oxidation products, hematite, limonite, and scorodite. Arsenic Hill is the low rounded ridge opposite the mouth of Sophie Gulch on the west side of Rock Creek. The crest of the ridge is at an elevation of about 280 feet. It is underlain by moderately resistant quartz-mica schist. Northeast-striking sheeted quartz veins are as much as one-half foot thick. The veins are spaced about 3 to 5 feet apart near the top of the ridge. Southwest of the crest of Arsenic Hill, the quartz-mica schist appears to be much more altered and only locally crops out. This area was explored by Placer Dome trench 87-1, which cuts northwest across the ridge, and by Kennecott Exploration Company trench 94-8, which trends northeasterly subparallel to the sheeted vein structure. The Kennecott trench cuts across fold- and possibly fault-controlled mineral zones in schist that mainly contain arsenopyrite, which has been extensively oxidized to scorodite, hematite, and limonite. Sheeted veins cut across the northwest-aligned arsenopyrite-rich zones. Albite is locally abundant in the northwest-aligned structures. Sheeted veins exposed near the southwest end of trench 94-8 are deformed, apparently by recent slump and creep. The Arsenic Hill deposit explored by Placer Dome and Kennecott Exploration Company contains little gold. The quartz-mica schist of Arsenic Hill is cut off on the southeast by the Arsenic Hill fault, a structure that crosses the ridge at an elevation of about 230 feet. Where exposed in trenches and subcrops, the fault locally contains graphitic zones with crystalline quartz, apparently similar to the gold-bearing rocks originally described by Cathcart (1922). Rocks southeast of the fault are relatively soft, graphitic calc-mica schist and resistant graphitic quartz schist. This graphitic schist is bounded to the northwest by the Arsenic Hill fault and to the northeast by the Sophie Gulch fault.
Workings: Placer gold was produced from the elevated bench deposit on the south end of the ridge (NM210); badly sloughed pits and shallow trenches reflect exploration before about 1920. The area has been explored, especially by trenching, sequentially since the 1980,s by R.V. Bailey, Placer Dome, and Kennecott Exploration Company.
Age: Mid-Cretaceous; veins cross cut regionally metamorphosed schist; see NM207. The earliest albite- and arsenopyrite-rich replacement deposits at Arsenic Hill are along ductile folds in the host schist. Sheeted, non-deformed quartz veins cut the deposits along the ductile folds.
Alteration: Albitization; sulfidization (especially introduction of arsenopyrite); and oxidation of arsenopyrite.

Commodities (Major) - Au; (Minor) - Ag
Development Status: Undetermined
Deposit Model: Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a).

Select Mineral List Type

Standard Detailed Gallery Strunz Chemical Elements

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

ⓘ Silver

Mineral List

8 valid minerals.