Abstract

Previous stratigraphic assignments and structural interpretations of a small exposure of Paleozoic rocks surrounded by extrusive rocks of the Tertiary Challis volcanics in the vicinity of Bruno Mine along upper Bruno Creek in Custer County, Idaho, were erroneous. It is now known that these Paleozoic rocks belong to two dissimilar lithologic sequences separated by a major thrust fault. Since the older rocks at this locality are exposed by erosion of an overlying thrust plate of regional extent, the study area is named the Bruno Mine window, and the fault is called the Bruno Creek thrust.

The two distinctive Paleozoic sequences in the Bruno Mine window consist of: (1) a Premississippian quartzite and carbonate succession composed of a minimum of 20-30 feet (6-9 meters) of massive, pure quartzite overlain by approximately 600 feet (182 meters) of metamorphosed dolomitic limestone, sandy and cherty dolomite, with a few interbeds of quartzite near the base; and (2) the Mississippian Copper Basin Formation consisting of a thick, structurally complex assemblage of carbonaceous argillite and limestone with some interbeds of shale, siltstone, quartzite, and chert. The Premississippian quartzite and carbonate accumulated in relatively shallow water, while the Copper Basin rocks represent turbidites and hemipelagic deposits.

The quartzite and carbonate sequence is lithologically similar to the Middle Ordovician Kinnikinic quartzite and the upper Middle to Upper Ordovician Saturday Mountain Formation, respectively. If this comparison is valid, the rocks below the Bruno Creek thrust are Ordovician in age.

The Copper Basin Formation in this area is part of an allochthonous plate of regional extent that was thrust northeastward over the Ordovician(?) rocks during the Sevier orogeny in Late Jurassic to Early Cretaceous time. After thrusting, the Paleozoic rocks were intruded and metamorphosed age. Post intrusive erosion created the window and exposed the stock. Subsequently, the area was covered by extrusive rocks of the Tertiary Challis Volcanics.

Mineralizing solutions in the Bruno Mine area apparently emanated from the granitic intrusive during the Late Cretaceous. These fluids moved upward to the impervious argillite of the Mississippian Copper Basin Formation at the base of the Bruno Creek thrust, where metallic ions of lead and silver were concentrated. This resulted in mineralization of high areas in the relatively reactive Ordovician(?) carbonate unit immediately below the fault. Documentation of structural entrapment of ore-forming solutions in this region should prove of value in future exploration efforts.