The main rock-types are ultramafic to mafic, but granites, granophyres and felsites, containing economic tin deposits, fluorite and pegmatite minerals are also present (Cairncross and Dixon, 1995).
Several mineralized veins cut across gabbro, granophyres and felsites. The gangue minerals in the veins are quartz and siderite and the primary ore minerals were argentiferous galena, associated with zinc (sphalerite), copper (chalcopyrite) and minor antimony mineralization.
Dunn (1877) described the rock in which the cobalt ores occur as a very fine-grained
felsite. Beck (1907) described the country rock not as a quartzite, but as a quartz-feldspar rock with the feldspars largely altered to sericitic mica, however he was not against it being called an aplite.
Box 2, Victor, CO) has a large new supply of sylvanite and calaverite crystals and masses (to several mm) on typical
felsite matrix from the Cresson mine, Cripple Creek,Teller County.
In order of decreasing age they are (1) agmatite containing blocks of host rock in a coarse-grained igneous matrix, (2) felsite, rhyolite porphyry, and microgranite, (3) layered and massive gabbro, (4) medium- to coarse-grained amphibolebiotite granite, (5) heterogeneous mafic rocks with a generally dioritic to tonalitic matrix containing schliers and blocks ranging from layered gabbro, to monzonitic, syenitic and granitic varieties.
The upper margin of the granite consists of an almost continuous layer of fine-grained, pink, felsic rock varying from featureless felsite to microgranite with abundant granophyric intergrowth and drusy miarolitic cavities, to rhyolite porphyry that is commonly spherulitic.
However, several kilometres along strike to the northeast in the Island Falls quadrangle, a lens of conglomerate, basal to the Shin Brook Formation and as thick as 30 metres, consists of pebbles of felsite, quartzite and slate in a chlorite-rich sandstone matrix (Ekren and Frischknecht 1967).
1996) along with clasts of quartzite and felsite (Skinner 1974).
However, the provenance of numerous
felsite and granite pebbles in the conglomerate is less obvious.