By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Grant County covers much of the southwestern part of the Blue Mountain region of northeastern Oregon where pre-Tertiary gold-bearing rocks are exposed.
The geology of the county, as summarized by the Oregon Department of Geology and Mineral Industries (1941, p. 14-15), includes rocks that range in age from Paleozoic to Tertiary. The oldest rocks are greenstone, argillite, and chert containing small lenses of Permian limestone. Several tens of thousands of feet of Upper Triassic and Jurassic rocks, mostly tuffaceous sandstone, shale, and some limestone, are exposed in the southwest part of the county. The Paleozoic rocks were intruded by peridotite, gabbro, and diorite, and all, including the Mesozoic rocks, were intruded by granodiorite. Tertiary rocks cover much of the county. These are the Clarno and John Day Formations (tuff, lava, and agglomerate of Eocene age), the Columbia River Basalt (Miocene age), Mascall Formation (tuffs that overlie the Columbia River Basalt), and Rattlesnake Formation (tuffs and lavas of Pliocene age).
Soon after the initial discoveries of placer gold at Griffin Gulch in 1861, discoveries were made at Sumpter and Canyon Creek, and by 1864 nearly all the mining districts of the Blue Mountains area were known (Lindgren, 1901, p. 563-564). The important gold-producing districts in Grant County were the Canyon Creek, Granite, Greenhorn (partly in Baker County), North Fork, Quartzburg, and Susanville.
From 1880 to 1899, Grant County produced $3,022,564 (about 146,000 ounces) in gold (Lindgren, 1901, p. 573). From 1904 through 1957 it produced 77,840 ounces of lode gold, 226,835 ounces of placer gold, and 19,967 ounces undifferentiated as to source. Approximate total gold production through 1959 was 470,600 ounces.
CANYON CREEK DISTRICT
The Canyon Creek district, which is between lat 44°11' and 44°30' N. and long 118°45' and 119°33' W. and includes most of the drainage area of the John Day River in Grant County, is noted chiefly for its gold placers along the river. Lindgren (1901, p. 717) estimated that the early production from these placers was about $15 million in gold (about 725,000 ounces). Hydraulic operations were conducted in the 1880's, and dredges were in operation sporadically after 1900.
Recorded gold production for the district from 1904 through 1959 was 899 ounces from lode mines, 91,711 ounces from placers, and 504 ounces from undifferentiated sources. Total production, including Lindgren's estimate of early production, was about 818,000 ounces.
Quartz veinlets containing rich pockets of native gold occur in carbonatized gabbro, serpentine, and greenstone in the vicinity of Little Canyon Mountain. Although these deposits were spectacular, they were small, and their production was not sustained.
The Granite district is between lat 44° 45' and 44°54' N. and long 118°18' and 118°33' W., in eastern Grant County.
As early as 1862 placer gold was mined from the gravels of Granite Creek, Clear Creek, and Bull Run; in 1874 lode mining became commercially important when the Monumental and La Belleview mines, the most productive lode mines in the district, were discovered. Much of the early placer mining was done by the Chinese, who at one time outnumbered the Americans (Lindgren, 1901, p. 686). From World War II through 1959 the district was virtually idle with the exception of the Buffalo mine which supplied nearly all the lode gold mined in eastern Oregon during that period (Koch, 1959, P.I).
Bedrock in the Granite district consists predominantly of a group of bedded metasedimentary rocks that were originally shales and small amounts of limestone, sandstone, and interbedded lava flows (Koch, 1959, p. 3-5). In the eastern part of the district a large granodiorite intrusion cuts the metasedimentary rocks. The layered rocks were thrown into east-trending isoclinal folds. Unconformably overlying these rocks are less severely folded and somewhat faulted Tertiary auriferous gravels, andesite tuff-breccias, and lava flows (Koch, 1959, p. 3-5).
The lode deposits are in veins in the pre-Tertiary metasedimentary rocks near or in the granodiorite. The veins are of two types: (1) open-space fillings characterized by quartz and calcite gangue and fragments of unreplaced wallrock, and (2) dike material, gouge, crushed country rock, and sparse quartz and calcite. In veins of the first type, the major metallic constituents are pyrite, chalcopyrite, tetrahedrite, arsenopyrite, sphalerite, and galena. In veins of the second type, pyrite is predominant, and in many veins it is the sole metallic mineral (Koch, 1959, p. 9-11).
Koch (1959, p. 38) estimated the total lode production of the Granite district to be $1,800,000, most of which was in gold and in small amounts of silver. This would represent, conservatively, about 75,000 ounces of gold. Recorded lode production for the district from 1904 through 1959 was 37,250 ounces. Placers yielded $1,033,000 in gold through 1914 (Oregon Dept. Geology and Mineral Resources, 1941, p. 40). Recorded placer production from 1904 through 1959 was 34,080 ounces and total gold production for the district was about 160,000 ounces.GREENHORN DISTRICT
The Greenhorn district straddles the Baker-Grant County line. The western part of the district, in Grant County between lat 44°33' and 44°45' N. and long 118°18' and 118°43' W., will be discussed here.
The mines in the Greenhorn district reached their peak of productivity between 1895 and 1910 (Allen, 1951, p. 398). From 1910 until 1942 there was only sporadic activity, and from 1942 through 1959, almost none. Only fragmentary records of early production are available. A total of the estimates of early production of individual mines is $346,000 (about 16,800 ounces) in gold (Oregon Department of Geology and Mineral Industries, 1941, p. 66-97). Recorded production from 1932 through 1959 was 4,829 ounces from lode mines, 4,612 ounces from placers, and 425 ounces undifferentiated as to source.
The following summary of the geology of the district is from T. P. Thayer (written commun., 1962). Paleozoic meta-argillites and greenstones were intruded by lower Mesozoic ultramafic rocks, by diorite, and by related dikes of probable mid-Cretaceous age. These rocks are folded and faulted. Tertiary flows and tuffs overlap the older rocks on the south and east. The ore deposits are mostly fissure fillings within or near dikes that were emplaced in fractures in pre-Tertiary rocks. Quartz, pyrite, arsenopyrite, pyrrhotite, sphalerite, chalcopyrite, and tetrahedrite were deposited during the first of two stages of mineralization. A second stage included the preceding minerals and galena, calcite, specular hematite, and gold.
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