LARAMIDE DEXTRAL MOVEMENT ON THE COLORADO MINERAL BELT INTERPRETED FROM STRUCTURAL ANALYSIS OF VEINS IN THE IDAHO SPRINGS MINING DISTRICT

The Idaho Springs mining district forms the central portion of a structurally controlled hydrothermal base and precious metal vein system in the Front Range portion of the northeast-trending Colorado mineral belt. Approximately 1 M ounces of gold and 55 M ounces of silver were mined between 1860-1914. Three new Ar-Ar plateau ages on sericite indicate the veins formed during the Laramide between 66.6±1.0 – 61.9±1.3 Ma. Mineralization is mostly in quartz-sulfide veins in faults and extension fractures. The extent and orientation of mineralized veins is strongly controlled by Precambrian structures. The mineralized vein system is mostly within the two-mile wide northeast-trending Precambrian Idaho Springs-Ralston shear zone (IRSZ). Although veins formed mostly in conjugate strike-slip faults, right lateral veins dominate (average strike 061°) and formed parallel to Precambrian foliation in the IRSZ (average orientation 060° 74NW). Veins widen where they strike across the host rock foliation and approach the maximum compressive stress orientation. Southeast of the southeast boundary of the IRSZ the regional strike of Precambrian large-scale folds and foliation is northwest, or nearly perpendicular to similar structures to the northwest, and economic veins are not present.

Kinematic indicators (slickenlines, fault offsets, en echelon vein patterns and vein thickness variations with strike change) were compiled from interpretations of published mine maps and from underground mapping in the Bald Eagle mine. Fault-slip analysis was applied to all applicable veins to model the orientation of s1 at the time of their formation (average trend is 076° ± 12°). This orientation is nearly parallel to proposed s1 orientations of the Laramide orogeny from previous workers in the Front Range, particularly Erslev and Selvig (1997; s1=072°), who conducted fault slip analysis in Mesozoic sedimentary rocks near the projection of the IRSZ shear zone to the northeast. That the orientations are nearly identical suggests that the Laramide stress was consistently oriented in the northern Front Range. Also the IRSZ was properly oriented for dextral reactivation at this time.