Albitization in the Antimony Line, Murchison Greenstone Belt (Kaapvaal Craton) : a geochemical and geochronological investigation

Abstract : The 3.09-2.97 Ga Murchison Greenstone Belt is one of several Archean volcano-sedimentary belts within the Kaapvaal Craton in southern Africa. Among the diverse ore deposits found within the belt, a set of Sb-(±Au) deposits are located in a major quartz-carbonate altered brittle-ductile structure known as the Antimony Line. The Antimony Line is thus clearly related to hydrothermal fluid circulation. In this study, we focus on albitites that run along the Antimony Line. Petrological and geochemical investigations indicate that albitization developed at the expense of a granodioritic protoliths under high fluid/rock ratios and also that Sb enrichment was concomitant with albitization. Oxygen isotopes on albitites point to a crustal origin for the hydrothermal fluid responsible for the albitization process. Geochronology on zircon and hydrothermal monazite identifies a 2.97-2.92 Ga magmatic crystallization event, disconnected from a ca 2.8 Ga hydrothermal alteration, and a potentially younger event around 2.0 Ga. These data highlight a likely magmatic-related primary enrichment at 2.97-2.92 Ga, followed by a secondary metamorphicrelated mobilization event around 2.8 Ga leading to a Sb-enrichment in the albitites and a last episode of alteration around 2.0 Ga.