Geology of silver mineralisation at Candelaria, Nevada, USA
Candelaria, situated in central western Nevada, along the western margin of the Great Basin, is a large and predominantly low grade, epigenetic disseminated- and vein-type Ag deposit, of Early Cretaceous age. It represents the eroded, deeply oxidised and fault-disrupted root of extensive stratiform quartz-dolomite stockworked and sericite-dolomite-altered zones of medium temperature pyrite-dominated Ag(-Pb-Zn-Sb-As±Cu±Au) sulphide-sulphosalt mineralisation, which is hosted by receptive sedimentary and igneous rocks within structurally favourable zones in a district-scale tectonic pinchout, and which is genetically associated with Cordilleran granodiorite porphyry hypabyssal magmatism (diking), of high K calc-alkaline affinity. The mineralisation occurs along and directly beneath the Pickhandle allochthon, a serpentinite-sheathed volcanic-sedimentary tectonic méange which forms a local 'sole' plate to the regionally extensive Golconda allochthon, which was emplaced onto the edge of continental North America during the Early Triassic Sonoma orogeny. Mineralisation occurred where an irregularity in the Pickhandle thrust plane, caused by thickening of the méange, effected locally deeper truncation of the parautochthonous foreland sequence in its footwall - chiefly marine sediments of the Lower Triassic Candelaria Formation - against the deformed cherts of the Ordovician basement (Palmetto complex), to form a structural trap. Within this trap, mineralisation is hosted mainly by carbonaceous, carbonate- and phosphate-rich (and trace metal-rich) black shales at the base of the Candelaria Formation and by dolomite-quartz-altered serpentinites at the base of the Pickhandle allochthon. Stable isotope data (O, H, S) point to a predominantly magmatic source for the hydrothermal fluids and ore sulphur, a source most likely to be the parent pluton to the granodiorite porphyry dikes. More ore metals were also of igneous origin (mass balance calculations rule out Candelaria member 1 as the chief metal source).