Abstract

The Candelaria mining district in central western Nevada preserves a record of tectonism, magmatism, and mineralization related to early Mesozoic development of the cordilleran magmatic arc. A deformed basement of Ordovician chert-phyllite (Palmetto complex) and its thin parautochthonous cover of Permo- Triassic marine clastic rocks (Diablo and Candelaria Formations) are structurally overlain, along the Pickhandle- Golconda thrust system, by ophiolitic tectonic melange forming the base of the Golconda allochthon. The total assemblage is cut by three overlapping swarms of silicic dikes comprising, from the oldest to the youngest: fine-grained, sparsely porphyritic calc-alkaline rhyolites; medium-grained, strongly porphyritic, high K calc-alkaline dacites; fine-grained calc-alkaline rhyolites. U-Pb zircon dating gives ages of 222 + or - 2 Ma for the older rhyolites, 192 + or - 3 Ma for the porphyries, and 196 + or - 24 Ma for the younger rhyolites. These swarms are products of the Triassic-Jurassic cordilleran magmatic arc, which developed under a long-lived, broadly extensional to transtensional tectonic regime, to generate a major intra-arc graben system characterized by voluminous caldera-related ignimbrites. The dike swarms possibly mark the site of an eroded caldera. The dacite porphyries are genetically associated with important sediment-hosted disseminated silver (-gold-base metals) mineralization, which occurs in thick tabular zones along the Pickhandle thrust and in a subparallel footwall structure--the lower Candelaria shear--which cuts reactive carbonaceous marly shales at the base of the Candelaria Formation. Mineralization occurs where the porphyries intrude the Pickhandle thrust and the lower Candelaria shear. The 192 + or - 3 Ma date for the dacites defines Candalaria as the oldest disseminated precious metal deposit in the Great Basin. There are similarities in tectonic setting, magmatic association, metal composition, and possibly, age between Candelaria and the Darwin-type Pb-Ag-Zn vein replacement deposits of southeastern California. Differences in host-rock composition and host-rock amenability to structural preparation may account for grade-tonnage differences between Candelaria (large, low grade; hosted by thin-bedded marly shales) and the Darwin-type deposits (small, high grade; hosted by more massive carbonate rocks). K-Ar ages for sericites from the hydrothermal alteration envelope at Candelaria range from 91 to 132 Ma (mid to Late Cretaceous). The discrepancy between these ages and those of the silicic dikes is attributed to regional-scale reheating and resetting of K-Ar isotopic systematics, related to development of the younger, crosscutting Jurassic-Cretaceous cordilleran magmatic arc. This arc generated the Sierra Nevada batholithic belt, which is represented locally by several small calc-alkaline granite porphyry stocks, dated at 84 to 97 Ma (K-Ar biotite), exposed within a few kilometers of Candelaria.

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