Abstract

The western margin of the conterminous United States, covering roughly 300 000 mi2 (777 000 km2), is an agglomeration of tectonostratigraphic terranes accreted to the North American craton mainly during Mesozoic time. The terranes represent a number of fundamental crustal types: oceanic crust, island-arc crust, mélange, various combinations of the preceding three, batholithic, miogeoclinal, and platform.The distribution patterns of types of mineral deposits show that miogeoclinal terranes of the craton are characterized by replacement and vein-type lead–zinc–silver, skarn tungsten deposits, molybdenum, and tin, whereas accreted terranes contain all the known volcanogenic massive sulfide deposits, all chromite and chert-associated manganese, and all the large gold quartz-vein deposits, except Goldfield, Nevada. Carlin-type disseminated fine-grained gold deposits occur mostly in windows of Paleozoic miogeoclinal rocks in Nevada, but the only known fine-grained gold deposit in California is in very youthful volcanic rocks overlying oceanic-crust terrane. Large bedded-type barite deposits, although in the same general area and showing the same general trend as fine disseminated gold deposits in Nevada, are in allochthonous oceanic terrane. Mercury and antimony are dominantly in accreted terranes but antimony, in particular, also forms important deposits in cratonal rocks. Most of the large iron deposits are in the craton but a few are in accreted island-arc terranes.

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