By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
In 1860 placers were discovered in the Blewett (Peshastin) district, in what is now Chelan County; at that time this area was part of Kittitas County. The first lode discoveries were made in the same district in 1874.
The largest gold-producing district of the State is the Chelan Lake whose output from 1938 through 1956 boosted Chelan County to the lead in production for the State. This district, however, was inactive from 1957 to 1959. Other important gold districts are the Wenatchee and Entiat.
Production of the county from 1903 through 1959 was 666,198 ounces of lode and byproduct gold and 581 ounces of placer gold. Production data for the years 1952-55 were combined with those of Ferry and King Counties (U.S. Bureau of Mines, 1952-55); consequently, separate figures for Chelan County are not available. Production data before 1903 also are not available, but Weaver (1911, p. 71) estimated the value of placer and lode gold from the Blewett district between 1870 and 1901 at $li/2 million (about 72,000 ounces).
The Blewett (Peshastin) district is in south-central Chelan County, at lat 47Â°25' N. and long 120Â°40' W.
Rich gold placers were discovered on Peshastin Creek in 1860 and were worked intermittently for several years. In 1874 vein deposits were found in Culver Gulch. For many years only the oxidized parts of the veins were mined, but in the late 1890's extensive development revealed new and rich ore bodies and for a while the district prospered. After 1910, however, there was only small-scale activity. Total production from 1870 to 1959 was approximately 850,900 ounces.
The oldest rocks in the area are of Carboniferous age and consist of the "contact" schist, the Peshastin Formation, composed of black slates and quartzites, and the Hawkins Formation, composed of a series of breccias, tuffs, and flows (Weaver, 1911, p. 26-56). Intruding these formations are large masses of peridotite, extensively altered to serpentine. Small irregular masses and stringers of Mesozoic granodiorite intrude the peridotite. Unconformably overlying the older rocks are sandstone, conglomerate, and shale that compose the Swauk Formation of Eocene age. Cutting all these rocks are diabase dikes and small masses of gabbro.
The ore bodies occupy fissure veins in the serpentine. Vein material consists of quartz, calcite, and talc. Native gold and iron oxide occur in the oxidized zone and arsenopyrite and pyrite occur at depth. Gold placers along Peshastin and Negro Creeks are of two types: older channel and bench gravels and younger gravels along the present stream courses (Weaver, 1911, p. 82, 83).
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