Platinum Deposits in the United States
Platinum deposits have been proved to exist in the United States over a wide area in placer deposits associated with gold, but the proportion of the platinum metals to the gold is usually small. Platinum is obtained from sands in streams rising in the belt of serpentine rocks in central California, and from the serpentine areas in the Siskiyou and Trinity counties in the north-west of the State, which continue north-east into Curry, Josephine and Jackson counties in south-western Oregon. The sources of the platinum obtained along the foot-hills of the Sierra Nevada were old stream channels on the western slopes of the mountains, which are now buried beneath lava several hundred feet deep. These deep leads are also mined by drifting for their gold and platinum contents, and in a few cases a fair amount of the metals is extracted from them.
Some platinum was obtained by hydraulic mining, but this was carried on with difficulty, owing to the prevailing scarcity of water. Platinum occurs in black sands found on the Pacific coast in the counties of Coos, Curry and Josephine, Oregon and Del Norte, California. Formerly these beach deposits were rich in platinum, but at the present small quantities only are obtainable after stormy weather. This area was examined by the United States Bureau of Mines, but the results were inexact. In California most of the output of platinum was produced by dredging for gold in the Butte, Calaveras and Stanislaus counties. In Trinity county mining was in progress on the Trinity River, about 6 km below Junction City. The output from this source was stated to be 1,950 grams. Platinum, with gold and osmiridium, was also obtained on the Yuba River, about 12 miles east of Marysville in Yuba County. Some alluvial platinum was recovered at the Bean Hill Gold Mine, situated 20 km south-east of Placerville. Californian metal contains from 25 to 45%, indium. Its origin is believed to be the serpentine- and olivine-bearing rocks of the Sierra Nevada and other ranges.
In Oregon, in addition to the platinum obtained from the beach deposits near Bullards and Marshfield, it occurs in placer deposits, rich in chromite, in south-west Oregon, the principal output being derived from the Waldo district. Platinum also exists in small quantities in streams in the neighborhood of the Blue Mountains, eastern Oregon, where were installed miners on a small scale. Other platiniferous localities in eastern Oregon are the Granite and Canyon districts, and Spanish Gulch in Wheeler County.
In Colorado, platinum is known to exist in the black sands from Clear Creek. Its presence has also been reported in the gold gravels of the Iron Hill placer at Como, where it occurs mechanically combined with magnetite. Another occurrence is in a vein located near Villa Grove. An assay of material from this vein, taken at a depth of 500 m, showed the following values, 30-40 g/t gold; 25-30 g/t platinum, 150-200 g/t silver and 1-3% copper. In Nevada, the occurrence of platinum was reported in Clark County and was noticed by the United States Geological Survey to be in association with copper, nickel and cobalt ores from the Key West and Great Eastern Mines, near Bunkerville. The ore bodies are contained in pegmatites and basic intrusions, which carry pyrrhotite and chalcopyrite, the platinum content in the ore averaging about 6 g/t. Platinum, with palladium, was discovered at the Boss Gold Mine, situated 10 miles west of Goodsprings, in Clark County. The mine was originally worked for copper, and later for its copper and gold contents. The country rock consists of limestone of middle Carboniferous age, intruded by sills of quartz monzonite porphyry, the ore bodies occurring in a fault zone in the limestone. The copper ores comprise mainly chrysocolla and malachite, and contain traces only of platinum. The gold ore occurs in a fine-grained siliceous matrix, containing a bismuth-bearing variety of plumbo-jarosite (a hydrous sulphate of iron and lead). The rare metals are present in the free state, being apparently alloys of gold, platinum and palladium.