Geologic description
The Moonshine Mine was first staked in 1905 and was worked intermittently from 1906 to 1924 (Wright and Wright, 1908; Herreid and others, 1978; Roppel, 1991; Maas and others, 1991, 1995). The initial workings were a 90-foot shaft, a glory hole, and a 200-foot adit. Sometime between between 1921 and 1924, a 1,600-foot tunnel was driven to undercut the ore body, but it apparently was misaligned and did not cut mineralization. There were also several surface trenches. There may have been a small shipment of ore in 1907 and one or more small shipments in 1916. The rocks in the vicinity of the Moonshine Mine are marble and greenschist of the Wales Group of Late Proterozoic and Cambrian age; they generally strike east-west and dip steeply north (Wright and Wright, 1908; Herreid and others, 1978). The metamorphic rocks are cut by numerous diabase dikes, 1 to 6 feet thick. The deposit is a well-defined, sulfide-bearing quartz vein, 2 to 4 feet thick, that strikes about N65W and has a near-vertical dip (Wright and Wright, 1908; Herreid and others 1978; Maas and others, 1991, 1995). The vein is along a shear zone that separates marble from greenschist; most workers believe that the sulfides replaced the carbonate in the shear zone. The vein contains scattered masses and pods of sulfides. The gangue is calcite and quartz, and the sulfides are mainly galena and sphalerite, with minor pyrite and chalcopyrite. The galena contains microscopic blebs of polybasite and covellite. The shear zone is marked by wallrock dolomitization and by quartz veinlets that extend into the wallrock. Much of the galena float at the workings is encrusted with a thick rind of cerussite and anglesite (D.J. Grybeck, unpublished field notes, 1985). The underground workings in the ore zone are largely inaccessible, but high-grade samples from surface dumps have often been assayed. Maas and others (1991), for example, collected samples of 'high-grade' from the glory hole that contained up to 4,668 parts per million (ppm) silver, 74.74 percent lead, and 3.51 percent zinc. Selected samples of galena collected by Donald Grybeck (unpublished data, 1985) contained 10 to 1,000 ppm silver, 200 to 500 ppm copper, 500 to 1,000 ppm antimony, 1,000 ppm to more than 1 percent zinc, major lead, and less than 0.05 ppm gold. |
