Potassic and sodic alteration accompanying gold mineralization in the Rattling Brook Deposit, western White Bay, Newfoundland Appalachians

Abstract

Gold mineralization is hosted by foliated, late Proterozoic granitoid rocks of the Rattling Brook area, western White Bay, Newfoundland. The mineralization is associated with sulfides and occurs in veins and fractures and in disseminated form. Associated hydrothermal alteration occurred in two stages. During stage 1 potassic alteration the host granodiorite was hydrothermally converted over large areas to a rock of granitic composition through the formation of microcline and sericite by pervasive reaction of plagioclase and mafic minerals with hydrothermal fluids. Stage 2 sodic alteration is restricted to fractures, veins, and vein selvages and overprints both stage 1 alteration and unaltered rock. It consists of microcrystalline to fine-grained albite with lesser amounts of ankerite, quartz, sericite, and siderite accompanied by pyrite, arsenopyrite, and gold. Altered rocks are enriched in K, Na, Au, As, S, and CO 2 , weakly enriched in W, Sb, and Br, and depleted in heavy rare earth elements. The gold mineralization and alteration extends into unconformably overlying Eocambrian to Cambrian sediments. The absence of deformation in the alteration and ore assemblages suggests that the mineralization postdated Acadian deformation in the area and thus is Late Silurian or younger. Both alteration episodes are considered to be part of the same hydrothermal event, which resulted in formation of the large low-grade deposit. CO 2 -rich fluids circulated through the crust along major structures, leached elements from a variety of rocks, and redeposited them in fractures and veins within overlying granitoids and sedimentary rocks. Stage 1 fluids initially permeated along grain boundaries and converted the host to a rock of granitic composition. Subsequent increases in the a (sub Na+) /a (sub K+) ratio and/or temperature were accompanied by fracturing, albitization, and deposition of sulfides and gold during stage 2.