DO INHERITED BASEMENT DEFORMATION ZONES CONTROL THE LOCATION OF ONE OF COLORADO'S MOST PRODUCTIVE REGIONS OF EPITHERMAL, POLYMETALLIC VEINS?

The role of Precambrian structural inheritance in the localization of Phanerozoic mineral deposits has been a question of long-standing interest in economic geology. However, it is rare that large data sets are available for detailed analysis of multiple structural elements in basement rocks that might lead to permeability enhancement during subsequent mineral deposit formation over broad regions. The central Colorado Rocky Mountain Front Range (CCRMFR) is an exception. An intermittent, but long history of mining and geologic mapping has generated exceptional surface and subsurface structural and ore deposit-related data.

Interpretations of CCRMFR data has ingrained in the literature the idea that localization of epithermal veins in this region was controlled by crustal weaknesses, such as the Proterozoic Idaho Springs-Ralston Ductile Shear Zone (ISRZ). However, recent compilation of structural and mineral deposit data from existing 1:24,000 geologic maps, reports, and new data from outcrop in the CCRMFR results in several observations: 1) There is little correlation between the locations of inferred mineral deposit-related plutons and the ISRZ or major brittle faults; 2) Mapped structures suggest that myriad directions of potential structural permeability existed and that metalliferous hydrothermal fluids may have flowed in many directions at any given time during evolution of the Colorado Mineral Belt; 3) Ore-bearing veins and small displacement fault-veins that carried these fluids, show steep dips and either preferential ENE trends that are well correlated with model paleostress directions for the Laramide orogeny, or radial trends around Late Cretaceous to Tertiary igneous intrusions; and 4) There are only minor differences in orientation and intensity of potential structures that may have controlled permeability from within the ISRZ compared with similar structures outside the ISRZ. These observations suggest that Proterozoic inheritance in the CCRMFR is not the primary control of mineral deposit location. Rather, responsible processes likely include a) proximity to shallowly emplaced plutons and b) competition between varying magnitudes and orientations of shallow regional horizontal principal stresses, overburden load, and local stress perturbations related to pluton emplacement.