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).
Emplacement conditions of a porphyritic felsite
dyke and timing of motion along the Coolin fault at Ben Levy, Co.
Ferguson (1946) and Ferguson and Weeks (1950) subdivided the slate series and felsite
into an older unit of andesite, chlorite schist, rhyolite, and felsite
, intruded by younger granite and alaskite, and overlain by Carboniferous sedimentary rocks of the Horton Group.
Dunn (1877) described the rock in which the cobalt ores occur as a very fine-grained felsite
The plutons, dykes, and sills of the Charlo plutonic suite consist of varied intermediate to felsic rock including quartz monzonite to monzogranite, quartz-plagioclase rhyolite porphyry, hornblende dacite porphyry, and felsite
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.
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).