Relationships between deformation and mesothermal veins in the Sunshine Mine Area, Coeur d'Alene district, Idaho

Abstract

The Coeur d'Alene district in northern Idaho is a world class Pb-Ag mesothermal vein system that has produced about 360 million ounces of silver, lead, and zinc since the 1880s. Despite the long history of exploration and production, the district does not have a predictive model for exploration based on a sound understanding of structural controls on the silver ore deposits; this is certainly the case for the Sunshine Mine and surrounding area. Fault kinematic history in the district shows a regional scale faultsystem reactivated over time with dextral, sinistral, and dip-slip displacement. The fault system is superimposed on regional deformation fabrics that were examined for this study in the Sunshine Mine area. Cleavage sets observed in the Sunshine mine area, distinguished by orientation and superposition relationships, are consistent with the findings of Smith (2004) which defined cleavage sets referred to as S1, S2, and S3. Two additional deformation fabrics that appear spatially tied to fault zones formed between development of cleavages S2 and S3. The multiple cleavages, fault zones, and their intersections are interpreted to act as pathways for hydrothermal fluids associated with vein formation and silver ore deposition. Thin section kinematic analysis of vein and shear zone samples defined a dip-slip sense of shear associated with the Sterling vein. Electron Backscatter Diffraction (EBSD) analysis of vein and shear zone samples failed to define a lattice preferred crystallographic orientation that defined shear sense. Similarly, cathodoluminescence (CL) analysis of thin section texturesfailed to define a dominant shear sense and fault kinematics. Nevertheless, additional study using these techniques is warranted. Both field observation and thin section analysis demonstrate a direct relationship between shear zones, veins, and mineralization potential, clarifying the need for detailed fault maps for the Sunshine Mine area and Coeur d'Alene district. v

Isotope Discrimination of Source Waters, Flowpaths, and Travel Times at an Acid-Generating, Lead–Zinc–Silver Mine, Silver Valley, Idaho, USA

“…The Silver Valley is contained in the upper portion of the Coeur d'Alene River Basin (Figure 1), which is composed of quartzites, argillites, and siltites of the Mesoproterozoic Belt Supergroup that have been uplifted, folded, and fractured [9][10][11][12][13]. The Osburn Fault runs the length of the valley and is the origin of the NW-SE trending, strike-slip splay faults, such as the Alhambra, Cate, and Midland faults, that dip steeply southward through the mine site (Figure 4).…”

“…The upper workings of the Bunker Hill Mine (Figure 2) are located beneath Kellogg and Wardner peaks of the Coeur d'Alene Mountains and extend from Government Gulch across to the Milo Creek watershed (Figure 4). This upper portion of the mine contains complex fold and fault systems [9,12,13], and the area has been heavily mined since the 1880s. The combination of steeply dipping bedding planes, extensive mine workings, thousands of unplugged drillholes, and highly fractured and faulted stratigraphy has resulted in a complex system for the transport of infiltrating water derived from snowmelt in the surrounding mountains [14,16,19].…”