Multiple intrusion and hydrothermal activity, eastern Breckenridge mining district, Summit County, Colorado

Abstract

The Breckenridge mining district is located in Summit County, Colorado, about 95 km west-southwest of Denver, Colorado. Mapping in the eastern end of the district has identified bodies of intrusive rhyodacite porphyry, intrusive breccia, and blow-out breccia in the vicinity of the old Wirepatch mine, an area mapped previously as exclusively quartz monzonite porphyry. The distribution of rock types, alteration patterns, and chronology of emplacement indicate that this portion of the district is part of an intrusive complex that vented periodically in middle Tertiary time. Propylitic alteration of rocks occurs near the margins of the area, and in the vicinity and to the east of the exposed intrusive rocks, the rocks have been affected by phyllic alteration. Seventy-four composite rock-chip samples were analyzed for lead, zinc, copper, and molybdenum. Eleven of the samples were analyzed also for gold. The amount of lead, zinc, and copper in the samples appears to be directly related to the degree of hydrothermal alteration of the rocks. Analyses of monzonite and quartz monzonite porphyry yield enrichment factors of 8.9 (lead), 4.3 (zinc), and 6.1 (copper) within the phyllic zone. Comparison of trace-metal distribution versus intrusive rock types indicates that the metals were added to the host rocks during or shortly following emplacement of the intrusions. Lead, zinc, and copper average 1,143, 1,191, and 259 ppm within samples of rhyodacite porphyry; 2,440, 365, and 278 ppm in samples of intrusive breccia; and 530, 978, and 381 ppm in samples of blow-out breccia. The highest mean molybdenum values are from the blow-out breccia (9.5 ppm), and the two samples bearing significant gold (0.46 and 0.47 ppm) came from the rhyodacite porphyry and the intrusive breccia. The presence of the intrusive rocks coupled with the pervasive hydrothermal alteration and the high trace-metal concentrations suggest that the area encompassing the Wirepatch mine may contain near-surface sulfide and precious metal deposits in addition to those already exploited. The possible existence at depth of additional intrusions suggests further that the area may contain porphyry-type molybdenum mineralization.