The chromite deposits described in this report are in northcentral Siskiyou County, Calif. They are in two long tabular masses of peridotite which lie end to end and trend northnorthwest across the valley of the Klamath River. The Seiad Creek-Red Butte mass extends 10 miles north of the river and the Hamburg-McGuffy Creek mass 8 miles south. The outcrops of both are from 1 to 2 miles wide. The rocks that bound the peridotite on the east are thoroughly reconstructed schist and those bordering it to the west are less-metamorphosed volcanic and sedimentary rocks. The schists have been considered to be pre-Cambrian age, whereas the rocks to the west are probably of early Triassic age.
Strictly speaking, the chromite-bearing masses are complex bodies of ultramafic rock consisting mainly of dunite but also containing saxonite, lherzolite, pyroxeni,te and hornblendite (listed in order of abundance). Only the dunite contains chromite ore.
The tabular peridotite masses lie parallel to planar structures in the older rocks, but in their internal make-up no large- scale structural pattern has been recognized. The most con- spicuous internal structure is the linear arrangement of the chromite-rich schlieren in the ore-bearing parts of the peri- dotite. A typical ore body may include parts made up of evenly disseminated chromite; the richest and largest bodies, however, are made up of concentrated, closely spaced chromite schlieren of different size, shape, and richness. Some schlieren contain 80 to 95 percent of chromite, but most of them contain 25 percent or less, and the ore must be concentrated to yield a marketable product.
Chromite is composed of chromic oxide, alumina, ferric oxide, ferrous oxide, and magnesia in different proportions. Hence the ~bromic-oxide content of pure chromites differ. It is consistently high for the chromite from this area as it ranges from 55 to 58 percent. The iron content is likewise consistently high, ranging from 14 to 21 percent.
The total reserves of indicated and inferred ore as of May 1945 were 274,500 short tons, which would average 8 percent Cr2O3. The Seiad Creek mine (Mountain View group) contained the largest deposit; reserves amounted to 212,000 short tons. The McGuffy Creek mine contained 50,000 short tons of indicated and inferred ore. Four other deposits, the Emma Bell, Kangaroo, Mountain, Fairview, and Ladd, contain a few thousand tons of ore each.
