Publication Date

8-2016

Date of Final Oral Examination (Defense)

4-6-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geoscience

Department

Geosciences

Supervisory Committee Chair

Mark D. Schmitz, Ph.D.

Supervisory Committee Member

Paul R. Donaldson, Ph.D.

Supervisory Committee Member

Clyde J. Northrup, Ph.D.

Abstract

The Eocene aged Trans-Challis Fault System of central Idaho provides the tectonic and magmatic framework for a series of Au-Ag and Cu-Mo ore deposits. From its northernmost extension near Butte, Montana to its southwestern terminus in the Boise Basin of south-central Idaho the Trans-Challis Fault System is associated with some of the richest precious metal deposits found in Idaho. However, the southernmost tip of the Trans-Challis Fault System, composed of the Horseshoe Bend and Pearl mining districts, remains understudied, receiving little economic or academic attention. As a result, how the Pearl to Horseshoe Bend mining districts fit within the established framework of the Trans-Challis Fault System and associated mineralization is poorly characterized. Significantly, no high-resolution mapping or modern geochemical and geophysical techniques have been applied to areas within these historically productive mining districts. This study employs detailed bedrock mapping, high-precision U/Pb geochronology, high-resolution soil geochemistry, ground-based magnetic anomaly mapping, and electrical resistivity and induced polarization geophysical imaging to characterize spatial patterns to create a model for structurally controlled mineralization within the Horseshoe Bend Mining District.

Integration of these datasets with knowledge gained from other studies along the Trans-Challis Fault System has led to the characterization of the structural framework hosting mineralization near Horseshoe Bend, Idaho. Geologic mapping reveals NE-SW and E-W trending dike swarms and associated en echelon mineralized vein systems oriented sub-parallel to the NE trend of the Trans-Challis Fault System. U/Pb ages on zircon grains within the dikes date emplacement during the late Early Eocene to the Early Oligocene. Surficial geochemistry surveys reveal east-west oriented, en echelon, zones of anomalously high gold concentrations with subordinate north-south oriented arms. Magnetic anomaly mapping reveals lineaments of sharp magnetic gradients spatially correlated with mapped dike patterns, as well as zones of magnetic lows spatially correlated with surface geochemical gold concentration anomalies. Electrical resistivity and induced polarization subsurface imaging techniques outline a series of east-west oriented, northeast stepping, conductivity, chargeability, and metal factor highs that correlate with a similarly oriented magnetic anomaly over the survey area, and en echelon mineralized vein systems mapped in adjacent bedrock.

The Early Oligocene age of the andesite dike phase reported to follow mineralization either extends the duration, or changes the timing, of the mineralizing events associated with this section of the Trans-Challis Fault System. Mapping, geochemical and geophysical data strongly suggest the controlling factor in mineralization location and geometry is the underlying structural framework of the system. Based on these geometries and orientations, a dextral Riedel shear array oriented 070° is proposed to adequately model the structural architecture controlling mineralization within the Horseshoe Bend Mining District.

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