Geologic description
The country rocks in the area of this occurrence are pelitic paragneiss, subordinate quartzofeldspathic paragneiss, and minor amphibolite and tactite (Berg and others, 1977, p. 123-124; Berg and others, 1988). These rocks, accompanied by small intrusions of pegmatite and diorite, form a large roof pendant in Tertiary or Cretaceous foliated granodiorite and quartz diorite of the Coast Range batholith. The metamorphic rocks probably represent marine argillaceous, clastic, and minor mafic volcanic strata that underwent high-grade regional metamorphism in Cretaceous or Tertiary time. The premetamorphic age of the strata is uncertain, but they probably are mainly Paleozoic and may be as old as Precambrian (Berg and others, 1988, p. 26; Gehrels and others, 1990; Crawford and others, 2000). The occurrence is a band of iron-stained paragneiss 450-900 feet wide and about 3 miles long (Berg and others, 1977, p. 123-125). It consists of small quartz veins and stringers that both parallel and crosscut the schistosity of the enclosing metamorphic rocks, which locally are intruded by small pegmatite dikes and sills. Berg and others (1977) reported anomalous amounts of molybdenum in rock (table 3, no. 16) and stream-sediment (figs. 9, 12) geochemical samples collected in this area. Eleven follow-up chip samples collected by the U.S. Bureau of Mines contained up to 110 ppm Cu, 15 ppm Pb, 60 ppm Zn, 1.5 ppm Ag, 100 ppm Mo, and, in one sample, a trace of W (Berg and others, 1977, area M-1, p. 123-125). In addition, several concordant lenses of chocolate-brown-weathering amphibolite up to about 2 feet wide and 15 feet long contain a small amount of pyrrhotite and a trace of chalcopyrite. |
