Relationship of grade, tonnage, and basement lithology in volcanic-hosted epithermal precious-and base-metal quartz-adularia-type districts

Abstract

Examination of grades, tonnages, and basement rocks for 88 epithermal precious- and base-metal quartz-adularia-type districts in North, Central, and South America, and Japan reveals that the type of basement rock below the mineralized veins is useful for predicting grade and size of deposits. Epithermal districts overlying basement with salt and evaporites, or rocks with trapped sea water, have a median tonnage (production and reserves) of 1.4 million metric tons and median grades of 1.5g/t Au, 130g/t Ag, 2.5%Pb, 1.7%Zn, and 0.16%Cu. Districts overlying sedimentary basements have a median tonnage of 0.77 million metric tons and median grades of 7.5g/t Au, 110g/t Ag, <0.025%Zn, <0.005%Cu, and <0.001%Pb. Districts overlying igneous basements have a median tonnage of 0.3 million metric tons and median grades of 5.9g/t Au, 38g/t Ag, <0.25%Zn, <0.002%Cu, and <0.003%Pb. This investigation suggests that basement rocks affect vein components in epithermal precious- and base-metal quartz-adularia-type deposits.