Geologic description
A deposit at Goodluck Gulch, approximately located by Hummel (locality 4, 1962 [MF 247]) was relocated by Kennecott Exploration Company in 1995. It apparently was first reported by Moffit (1913, p. 131) who noted: 'A large amount of highly mineralized quartz is present in schist exposures south of Good Luck Gulch . . . Panning shows the presence of gold.' Bedrock was not visible when the prospect was visited by Cathcart (1922, p. 247-248), but he did find scheelite in thin-section examination of dump material. The location is important because Goodluck Gulch is the northernmost significant deposit in a belt of gold lodes east of Snake River that extends northerly from near Bonanza Hill (NM228) and the Anvil fault through Rock Creek (NM207). Surface expression of this deposit includes a large quartz-boulder felsenmeer on the Snake River plain adjacent to the deposit; white vein quartz boulders in the felsenmeer are as much as 3 feet across. Old prospect trenches are on the upland adjacent to the felsenmeer. The quartz vein system in bedrock probably also extends some distance into the Snake River flood plain because large blocks of quartz are visible in the Snake River gravels on a northwesterly projection of the deposit; these boulders are not present north of this projection. The deposit extends uphill nearly to the Snake River road (Hummel's 1962 location) but is best developed near Snake River. Relocation of the prospect was followed in 1995 by soil geochemical sampling, trenching, and drilling. The deposit consists of schist extensively replaced by arsenopyrite, schist replaced by tabular albite impregnated by arsenopyrite, and veinlets and extensive masses of quartz as much as several feet across. Most of the deposit is crudely stratabound, and some is folded. Drill core in GLC95-2 at a depth of about 140 feet cuts nearly isoclinal folds with saddle-reef type quartz-feldspar veins in small-scale folds and as much as 5 percent arsenopyrite disseminated in the folded schist. Pyrite is less abundant than arsenopyrite, and galena is sparsely disseminated in quartz. Near the surface, the sulfidized schist is mainly weathered to a reddish iron oxide, probably hematite. Late-stage mineralized quartz occupies northeast-striking fissures, the common direction for late sheeted veins in the area. Bedrock in the area is graphitic schist probably of early Paleozoic protolith age (Hummel, 1962 [MF 247]; Sainsbury, Hummel, and Hudson, 1972 [OFR 72-326]; Till and Dumoulin, 1994; Bundtzen and others, 1994). |
