By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
In south-central Snohomish County gold was produced from several districts in the western part of the Cascade Range. These districts - Monte Cristo, Index, Silverton, Silver Creek and Sultan - are more or less contiguous and occupy an area about 10 miles wide and 20 miles long extending from about the center of the county south to the King County boundary.
The Monte Cristo and Silverton districts produced most of the gold in the county. Patty (1921, p. 282) estimated that the combined early production of all metals, but mostly gold, from these districts was worth $7 million. From 1903 through 1956, production was 9,595 ounces of lode gold and 535 ounces of placer gold.
MONTE CRISTO DISTRICT
The Monte Cristo district, on the west slope of the Cascade Range, is about 40 miles east of Everett, along the Everett and Monte Cristo Railroad.
Overlying an older granitic terrane is a thick sequence of arkose beds that Spurr (1901, p. 791) correlated with the Swauk Sandstone of Eocene age. Andesite of Miocene age overlies the arkose, and several masses of tonalite, of later Miocene age, cut the andesite and produced considerable contact metamorphism. Basaltic dikes cut both the arkose and andesite. Pleistocene volcanic activity was fairly intense in this area. Flat-lying andesite, tuff, and breccia of Pleistocene age unconformably overlie all older rocks.
Spurr (1901, p. 804-805) believed that ore deposition began in late Pliocene or early Pleistocene time and continued to the Recent. The most important deposits are mineralized joints within or near the tonalite masses. Ore minerals are galena, sphalerite, chalcopyrite, pyrite, and arsenopyrite. An upper zone, which is nearest the surface, contains galena, sphalerite, and chalcopyrite and is richer in gold and silver than ore at depth (Spurr, 1901, p. 864).
There is no record of activity in the Monte Cristo district from 1918 through 1959, and the only production data for the district are estimates of early combined production from the Monte Cristo and Silverton districts of $7 million in gold (Patty, 1921, p. 282). The Monte Cristo district probably produced roughly half of this amount.
The Silverton district is about 12 miles northwest of Monte Cristo along the Stilaguamish River.
Copper-gold-silver ore was discovered in this district in 1891 (Thyng, in Landes and others, 1902, p. 66). The district became dormant about 1910 and remained inactive through 1959.
The dominant bedrock in the district is granite overlain by sedimentary rocks (Thyng, in Landes and others, 1902, p. 64). The ore bodies occur in fissure veins at the contacts of granite and diorite bodies. Chalcopyrite, the chief metal-bearing mineral, contains the gold. Argentiferous galena is abundant, as are pyrite, arsenopyrite, pyrrhotite, and sphalerite. Pyrargyrite occurs in small quantities (Thyng, in Landes and others, 1902, p. 65).
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