Diatreme-hosted fluorite mineralization in S-Namibia – A tale of cryogenic brine formation and fluid mixing below an unconformity in the context of Pangea rifting

Abstract

The Garub fluorite deposit in south Namibia is hosted by a Neoproterozoic tuffisite diatreme which is cut by a hydrothermal vein with base metal mineralization. Petrographic observations indicate a single hydrothermal event producing a paragenesis of fluorite-baryte with base metal sulfides, quartz and carbonates. Microthermometry data from fluid inclusions show that hydrothermal fluorite and baryte were precipitated from a high-salinity (16.9–22.7 wt% NaCl + CaCl2), moderate-temperature brine (180–210 °C). This brine formed by mixing between a warmer, Ba- and F-rich endmember likely representing a deep-seated basement brine (chemically equilibrated with Namaqua-Natal-Metamorphic Complex basement rocks) and a cooler, SO4- and Ca-rich endmember that likely equilibrated with Nama Group limestone. Chlorine/Br mass ratios between 152 and 612 indicate the mixing of a fluid with a halite dissolution brine signature (endmember 1) and an equally saline brine derived from Nama Group limestone (endmember 2). Rubidium/Cesium (Rb/Cs) ratios between 2.2 and 20.5 provide evidence for significant fluid interaction with clay minerals. Since clay minerals are abundant in the tuffisite of the Garub diatreme, these Rb/Cs ratios indicate that fluids migrated along the diatreme-gneiss contact along a zone of weakness. Late-stage quartz and calcite likely reflect cooling and induced a shutdown of the hydrothermal system. The sulfide phases in this hydrothermal vein are strongly depleted in trace elements compared to other hydrothermal vein districts, which is interpreted as an inherited signature of the fluid source.There is no record of evaporites in the sedimentary rocks of the Nama Group. Based on the fluid chemistry of the mixed fluid, which clearly indicates halite dissolution and water-rock interaction with basement rocks, it is concluded that the basement brine fluid endmember is likely generated through cryogenic brine formation during the large scale Dwyka glaciation event. As a farfield consequence of Pangea breakup fluid pathways were established that acted as host structures for the Garub fluorite deposit. This could be the first evidence for unconformity-related hydrothermal fluorite deposit formation in the context of Dwyka cryogenic brine formation in southern Africa.