Preliminary geologic map and sections of the northeastern part of the Seiad Quadrangle, Siskiyou County, California: Supplement 4 from "Geology and ore deposits of the northeast quarter of the Seiad quadrangle, California" (Thesis)

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This paper describes the results of a field and laboratory study of the geology and ore deposits (dominantly the chromite properties) of the northeast quarter of the Seiad quadrangle, California. The Seiad quadrangle is located in the central Klamath Mountains in a region of rugged ridges and deep canyons. The oldest rocks are schists overlain by arenites, argillites, marble and meta-volcanic rocks. They are intruded and metamorphosed by quartz-diorite, chromium-bearing peridotite, and granodiorite probably representing at least two phases of intrusion. Recent alluvium and remnants of old terrace gravels are found along the Klamath and Scott Rivers. No definite ages have been assigned to the rocks because of the lack of fossil evidence. The older rocks are probably pre-Mesozoic, while some of the terrace gravels may be of Pleistocene age. The laboratory study of tie chromite ores indicates that they are late magmatic (pneumotectic) ores whose structure is controlled by flowage, fracture-filling and replacement of the peridotite intrusions. The chromite deposits described in this report are in the Klamath Mountains of northern California. The oldest rocks in the area are mica, chlorite, and hornblende schists of pre Cambrian (?) age, which are overlain by a complex series of metamorphosed volcanic and sedimentary rocks of Paleozoic (?) age. Into all these rocks quartz diorite, peridotite, and granodiorite were successively intruded. Auriferous terrace gravels, possibly of Pleistocene age, Recent gravels, and alluvium partly fill some of the larger canyons. The chromite deposits, which occur in peridotite, range in size from a few tons to more than 100,000 tons. The ore of minable grade has an average chromite content of 35 percent. Reserves in the district, as estimated from known outcrops, are believed to be approximately 125,000 tons. At current prices of $20 to $25 a ton it is unlikely that any of the deposits can be profitably worked. Though the major part of the tonnage is of low grade (20 percent of chromite), the material could be concentrated to a 45- or 50-percent product. Hand sorting would yield small tonnages of shipping ore.

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