Although California has been one of the most productive regions in North America for diamonds, the region does not fit any classical exploration model for diamondiferous kimberlite or lamproite.
The state has produced many diamonds, including the fourth-largest (32.99 carats) in North America. Geological evidence supports the view that the diamonds originated from more than one nearby source.
Diamonds were first discovered in northern California in 1849, near Placerville, as a byproduct of placer gold mining. In 1852, diamonds were discovered in the Cherokee hydraulic gold placer mine, 9 miles north of Oroville.
Most Californian diamonds have been found in the Sierra Nevada and the Klamath Mountains, downstream from serpentinized ophiolite complexes and melanges. Fewer than 1% have been found in beach sands in northwestern California and southwestern Oregon.
In total, more than 600 diamonds (both gem and industrial stones) have been documented from placer operations in California. Undoubtedly, hundreds, and most likely thousands, of diamonds were rejected by the historic dredges and hydraulic mining operations and reported to the tailings.
Current diamond genesis models predict that commercial diamond deposits will be confined to: kimberlite pipes emplaced in stable Archean cratons; lamproites that intrude accreted Proterozoic belts along the margins of the cratons; and placer deposits derived from these source terranes. California is an unstable accreted terrane constructed of slices of Phanerozoic oceanic and reworked crustal material, and therefore does not fit any current exploration model. Yet California has been the third most productive terrane for diamonds in the U.S. — after Murfreesburo, Arkansas, where more than 90,000 diamonds have been mined from lamproite, and the Colorado-Wyoming state line district, where more than 112,000 diamonds have been mined from kimberlite.
The close association of the placer diamonds with serpentinized ophiolite and melange suggest possible preservation of diamonds in an obducted mantle slab. The presence of a mantle slab is supported by the occurrence of pyrope garnet and chromian diopside in some diamond placers, and chromian-diopside-bearing pyroxenite found in place, in northern California. Possibly, diamondiferous eclogite, garnet pyroxenite, or garnet serpentinite contributed the diamonds to the placers.
— W.D. Hausel is senior economic geologist with the Geological Survey of Wyoming.
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