Structural analysis and ore controls in the Coeur D'Alene mining district, Idaho

Abstract

Beltian rocks in the Coeur d' Alene district of Idaho, lying within the Lewis and Clark line, display a complex history of deformation and metamorphic fabrics. The earliest fabric is a near-bedding-parallel schistosity (assigned to Dl). Upright folds of northerly trend lie north of the district; comparable folds partly overturned trend WNW within the district, subparallel to the Osburn fault. The folds are assigned to D2a.Metamorphic cleavage (assigned to D2b) trends generally WNW but cuts across the folds in various ways. It shows major differences in two principal areas. In a three-mile-wide zone subparallel to and immediately north of the Osburn fault, cleavage is axially symmetrical about an approximately 40°-NWplunging stretching lineation. Outside that zone, the cleavage is dominantly planar, dips steeply to the SSW, and contains a strong dip-line stretching lineation.Semi-brittle faults and veins (assigned respectively to D2c and D2d) cut the cleavage and are in part axially symmetrical about a steep, SSW-plunging axis, paralleled by ore-shoot axes and slickenlines. Veins are concentrated in several zones or mineral belts symmetrically arrayed across the three-mile-wide zone of cleavage. Post-ore normal faults are axially symmetrical about a near-vertical axis. Brittle faulting in and near the Osburn fault zone cuts all the other structures.

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

“…The Belt Supergroup south of the Coeur d'Alene district experienced greenschistto amphibolite-facies metamorphism that has been attributed to either Precambrian tectonism of uncertain origin (Reid and Greenwood, 1968;Reid et al, 1981, Hyndmann et al 1988 or more commonly to emplacement of the Cretaceous Idaho Batholith (e.g., Hietanen, 1984;Grover et al,1992;Lang and Rice, 1985a,b,c). Recent Lu/Hf garnet dating has demonstrated that much of the metamorphism to the south is in fact Proterozoic in age (Zirakparvar et al, 2010;Nesheim et al, 2012).…”

“…Recent Lu/Hf garnet dating has demonstrated that much of the metamorphism to the south is in fact Proterozoic in age (Zirakparvar et al, 2010;Nesheim et al, 2012). Several models for the origin of Coeur d'Alene mineralization suggest that fluids generated during low-grade greenschist facies metamorphism transported the metals to the sites of deposition (Reid and Hayden, 1993;Constantopoulos, 1994).…”