Geochemistry and U–Pb Geochronology of Garnet From the Somerset and Arab Gold Deposits, Vumba Greenstone Belt, Northeastern Botswana: Implications for the Timing of Gold Mineralization

ABSTRACT The Somerset and Arab gold deposits are hosted by metavolcanic and metasedimentary rocks of the Vumba Greenstone Belt (VGB) in northeastern Botswana. The Neoarchean VGB experienced several episodes of metamorphism, at conditions spanning from greenschist to granulite facies. The timing of gold mineralization with reference to metamorphism in the VGB is not well constrained. Gold‐bearing sulfides in the Somerset and Arab deposits occur associated with garnet, either as inclusions or fracture filling. Gold‐bearing sulfides also occur intergrown in textural equilibrium with garnets, thus suggesting that they grew simultaneously. In this study, we investigate the geochemistry and geochronology of different textural types of garnet as proxies for the timing and genesis of the associated gold mineralization in the Somerset and Arab gold deposits. Gold‐bearing sulfides at the Somerset and Arab deposits are pyrrhotite, arsenopyrite, pyrite, sphalerite, galena, and chalcopyrite. At the Somerset deposit, these gold‐bearing sulfides occur as dissemination and as garnet‐hosted inclusions in garnet‐biotite schist and garnet‐muscovite schist. At the Arab deposit, gold‐bearing sulfides occur as dissemination in garnet amphibolite and occasionally filling fractures, as well as inclusions in garnet. Mineral assemblages identified include amphibole‐garnet‐biotite‐quartz in the garnet amphibolite of the Arab deposit; garnet‐biotite‐quartz‐white mica‐amphibole in the garnet‐biotite schist; and muscovite‐quartz‐garnet ± amphibole in the garnet‐muscovite schist of the Somerset deposit. These mineral assemblages are indicative of amphibolite facies metamorphism. Garnet from the Somerset deposit can be classified into two types, based on the mode of occurrence, color, shape, and chemistry. Garnet 1, formed syn‐foliation, is brown, poikiloblastic, elongated, and is chemically classified as almandine. Garnet 2, formed post‐foliation, is colorless and euhedral, and is chemically classified as spessartine. On the other hand, only one textural and compositional type of garnet (garnet 3) occurs at the Arab deposit. This garnet is syn‐foliation, light brown, fractured, euhedral, and of almandine composition. Garnet 1 coexists with biotite, quartz, calcite, white mica, and pyrrhotite along the foliation of garnet‐biotite schist. Garnet 2 occurs in the lepidoblastic domains of the garnet‐muscovite schist and has pyrrhotite and zircon inclusions. Garnet 3 coexists with biotite and amphibole in the garnet amphibolite, and has inclusions of ilmenite, zircon, and pyrrhotite; pyrrhotite occasionally also occurs as fracture filling. Garnet 1 yielded a U–Pb Wetherill concordia lower intercept age of 2655 ± 58 Ma; garnet 2 yielded 2680 ± 113 Ma U–Pb Wetherill concordia lower intercept age, and garnet 3 yielded a 2679 ± 41 Ma Wetherill concordia lower intercept age. The crystallization ages of the three garnet types overlap and are consistent with the Limpopo Orogeny (∼2740–2620 Ma). Garnet‐biotite thermometry indicated temperature ranges of 548°C–577°C (average of 550°C, lower amphibolite facies) for garnet 1, and 599°C–682°C (average of 633°C, upper amphibolite facies) for garnet 3. This suggests that the mineralization event associated with the three garnet types spanned from peak metamorphism to post‐peak metamorphism.