Geologic description
The Spruce Hen mine is along the western contact of a large body of Cretaceous, porphyritic granite. Limestone has been replaced by scheelite, fluorite, garnet, and other typical contact-metamorphic minerals (Byers, 1957). Concentrations of scheelite were found in zones as much as 6 inches wide in blocks of fine-grained, altered igneous rock (Byers, 1957). Berg and Cobb (1967, p. 220) reported that scheelite deposits were found in tactite, silicified limestone, granite, and pegmatitic dikes and in small quartz veins in schist. In 1951, a sample of tungsten ore was submitted for testing at the metallurgical division of the Bureau of Mines (Saunders, 1958). A petrographic examination of the sample revealed it to be a contact metamorphic rock; it consists predominantly of idocrase and pyroxene, with some quartz, fluorite, garnet, wollastonite, and small amounts of scheelite, chlorite and epidote. Traces of limonite and calcite were present. This study showed that most of the scheelite is unlocked by grinding the ore to a minus 65 mesh, but grinding to minus 100 mesh was required to effect maximum liberation. By 1918, five lode prospects were being explored by trenches (Mertie, 1917). One lode was 3 to 4 feet wide, made up of schist and metamorphosed basic rock and averaged 1-2 percent scheelite with no gold. A similar lode was 4 feet wide and trended N. 33 E., with a 45 NW dip. Development work from 1916 to 1918 consisted of two shafts and many pits and trenches (Byers, 1957). An inclined shaft reportedly was sunk 70 feet on a 3-foot-thick, northwest-dipping ore body. In 1943, a pit and a trench exposed a badly weathered lode about 3 feet thick. Samples from this trench averaged 0.44percent tungsten tri-oxide (WO3). In 1957, one ton of ore was milled from this prospect (Saunders, 1958). |
