By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gold was first discovered in eastern Oregon in 1861 in Griffin Gulch in the Baker district, Baker County. The town of Auburn was soon established as the first settlement and base for exploration. By about 1870 the richest placers were exhausted, but quartz lodes were discovered and developed, although slowly, and by 1900 were substantially productive in the Cracker Creek, Cornucopia, and Sumpter districts. As placer production decreased, Auburn declined, and Baker became the most important town in the county.
Production data for Baker County before 1880 were not found. From 1880 to 1899, the county produced $8,958,073 (about 434,850 ounces) in gold (Lindgren, 1901, p. 573). From 1904 through 1957, it produced 747,548 ounces of lode gold, 402,490 ounces of placer gold, and 11,626 ounces unidentified as to source. Total recorded gold production through 1959 was about 1,596,500 ounces; from 1954 through 1959 only a few hundred ounces was produced.
Placer mining revived after 1912, and after World War II it was more productive than lode mining. Most of the county's gold production in recent years was from the Sumpter district placer mines, which were closed in 1955.
Lode deposits of Baker County generally are fissure veins that are related to intrusions of granitic, dioritic, and gabbroic rocks (Lindgren, 1901, p. 614). The deposits most commonly are found near contacts of these intrusive rocks with sedimentary or metasedimentary rocks.
Production in the Baker district has been chiefly from the placers in Griffin Gulch but this was in the early years and was unrecorded. After 1900 more than half of the gold produced in the district came from lode mines. Production of gold from 1906 through 1959 was 19,825 ounces from lode mines, 10,890 ounces from placers, and 5,437 ounces undifferentiated, a total of 36,152 ounces.
The oldest rocks of the district are greenstone, phyllite, quartz schist, and limestone composing the Burnt River Schist of probable pre-Carboniferous age (Gilluly, 1937, p. 9-13) and the Elkhorn Ridge Argillite, composed of argillite, tuff, lava, chert, and greenstone, of Permian and Triassic age (Bostwick and Koch, 1962). An unconformity separates these rocks from the superjacent Tertiary andesite and basalt flows. The pre-Tertiary rocks are thrown into strong folds that strike west, but the Tertiary rocks are only gently warped (Gilluly, 1937, p. 8).
The lode deposits are fissure and replacement veins in the pre-Tertiary rocks (Gilluly, 1937, p. 92). Gold, pyrite, chalcopyrite, sphalerite, and locally stibnite and galena occur in a gangue of quartz, sericite, carbonate, and a little clay and scheelite.
Placers have been worked in nearly all the gulches on the south end of Elkhorn Ridge, on Marble Creek, and on Salmon Creek. The most important placers were in Blue Canyon near Auburn, where some of the early discoveries were made.
CONNOR CREEK DISTRICT
The Connor Creek district is along the west drainage of the Snake River between lat 44°21' and 44°44' N. and long 117°03 and 117°18', W.
Placer mining began in this district in the 1860's along Connor Creek, and in 1871 lode gold was discovered at Connor Creek mine. After an estimated maximum production of $2 million in gold (Lindgren, 1901, p. 757), the mine was closed in 1910 and was reopened only briefly in 1915-18 (Gilluly and others, 1933, p. 50). Small amounts of placer gold were produced from the district until 1942. From that time through 1959 there was virtually no production. The district produced about 97,000 ounces of lode gold and about 6,100 ounces of placer gold through 1959.
The following summary of the geology of the district is from Gilluly, Reed, and Park (1933, p. 50). The country rock is dominantly black carbonaceous slate and quartz phyllite and contains small amounts of greenstone, chlorite schist, and limestone. These rocks are of possible Triassic and Jurassic age. The beds dip steeply to the northwest and strike N. 20°-45° E. Granitic rocks have intruded the metasediments west of the district. The gold deposits are in northwest-trending quartz veins that dip steeply southwest. Free gold occurs in the Connor mine with, some argentite and pyrite.
The Cornucopia district, between lat 44°57' and 45°05' N. and long 117°00' and 117° 15' W., reported very little activity until 1880-85 (Lindgren, 1901, p. 742). Its gold production to 1903 was valued at $1,008,000 (Oregon Department of Geology and Mineral Industries, 1939, p. 25). Production was fairly steady from 1903 through 1941, but it was only a few ounces from 1942 through 1959. Recorded production from 1907 through 1959 was 255,698 ounces of lode gold, 2,441 ounces of placer gold, and 5,800 ounces undifferentiated as to origin.
The oldest rocks in the area are metavolcanics and elastics of the Clover Creek Greenstone, of Permian age (Ross, 1938, p. 21). Other metasedimentary rocks that overlie the Clover Creek Greenstone have obscure stratigraphic relations with one another and are classed as Carboniferous and Triassic. Overlying the Paleozoic rocks are the Martin Bridge Formation and a thick section of younger sediments, all of Late Triassic age. At the close of the Jurassic the rocks were folded and metamorphosed, and in mid-Cretaceous time a granodiorite batholith intruded the series (T. P. Thayer, written commun., 1962). During the closing stages of this igneous activity, the veins were formed, uplift and dissection followed, then the basalt flows of the Columbia River Basalt were poured out on this erosion surface.The veins occupy shear zones in both the metamorphic and granitic rocks. The larger veins strike N. 40° E. and dip 40° W.; many are offset en echelon. Vein minerals consist of pyrite, sphalerite, chalcopyrite, galena, tetrahedrite, tellurides, and native gold, with quartz as gangue (Goodspeed, 1941, p. 185). Successive stages of microbrecciation and turbid quartz are not noticeable features of these veins.
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