Geology of the Wood and East Calhoun mines, Central City District, Gilpin County, Colorado

Trace Elements Investigations 175
This report concerns work done on behalf of the Division of Raw Materials of the U.S. Atomic Energy Commission



The Wood-East Calhoun mine area is underlain by complexly folded Precambrian gneiss and pegmatite. The major fold in the area is an anticline that trends about N. 60° E. The Precambrian rocks are intruded by bostonite porphyry dikes of Tertiary age. All the rocks are cut by east- to northeast - trending faults that have been filled by precious metal-sulfide veins which have been worked chiefly for gold. The Wood vein occurs in an east-trending fault; the Calhoun vein occurs in a northeast-trending fault. Much of the uranium production of the Central City district has come from the Wood vein on Quartz Hill.

The veins consist chiefly of quartz; pyrite is the chief metallic mineral and chalcopyrite is next in abundance. Sphalerite, galena, tetrahedrite-tennantite, and pitchblende are locally present. Deposition began with alteration-stage quartz and pyrite followed in order by pitchblend, light-yellow pyrite, massive quartz, yellow pyrite, shalerite, comb quartz, chalcopyrite, tetrahedrite-tennantite, galena, chalcopyrite, pyrite, and gray to light-brown fine-grained quartz.

The veins of the Central City district are zoned, with quartz-pyrite veins near the center and galena-sphalerite veins on the periphery. The known pitchblende bodies are in the transition between these, but paragenetically, the pitchblende is earlier than all other metallic minerals.

A trace element study of the ore indicates an association of zirconium and molybdenum with uranium, of bismuth, antimony, and arsenic with copper, and of cadmium with zinc.

The pitchblende and other ore minerals are concentrated in ore shoots. The shoots are in open spaces controlled by the competency of the wall rocks, the presence of a prevailing direction of weakness in the rocks, and changes in strike and dip of the vein.

The pitchblende is thought to be a local constituent of the quartz-pyrite ores and to owe its origin to residual solutions from the quartz bostonite magma.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Geology of the Wood and East Calhoun mines, Central City District, Gilpin County, Colorado
Series title Trace Elements Investigations
Series number 175
DOI 10.3133/tei175
Year Published 1955
Language English
Publisher U.S. Geological Survey
Description Report: 59 p.; 3 Plates: 34.78 x 24.11 inches and smaller
Country United States
State Colorado
County Gilpin County
Other Geospatial Wood Mines;East Calhoun Mines;Central City District
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