The formation of vein-type barite (± base metal, gold) deposits in northern Madagascar and its link with Mesozoic Pangean rifting.

Along the boundary between the Precambrian basement and the Permo-Mesozoic sedimentary basin of northernmost Madagascar, an extensive network of epigenetic quartz-barite veins can be found. The veins are extensional in nature and show a polyphase paragenesis. Some of the veins, especially in the northern part of the district, hold some minor Zn-Pb sulfides and erratically distributed, native gold mineralization. The latter has been exploited in the past and is currently subject to artisanal mining activity. Saline aqueous fluid inclusions in the various quartz and barite vein generations indicate involvement of a relatively cool (<100 °C), high-salinity (~24 eq. wt% NaCl) fluid and a hotter fluid (>160 °C) of lower salinity (~5 eq. wt% NaCl). Analyses of fluid inclusion leachates suggest that the high-salinity end member was a bittern brine, residual to seawater evaporation, whereas the low-salinity end member was a hydrothermal, metal-bearing fluid that had been sourced from the crystalline basement. Sporadic aqueous-carbonic fluid inclusions possibly represent involvement of a third fluid component of metamorphic or magmatic affinity that might have been responsible for the minor gold mineralization. The δ³⁴S composition of the barites averages 20.6 ± 2‰ V-CDT and is indicative for a seawater origin of sulfur. Sulfate was sourced either from evaporite minerals (e.g., gypsum) present in the Early Mesozoic sedimentary sequence or was originally concentrated in the bittern brine. While the timing of mineralization remains enigmatic due to the lack of reliable radiometric age determinations, the metallogenic constrains support vein formation during or shortly after the establishment of evaporitic conditions, likely around the Middle Jurassic, in the intracratonic rift basin that developed between the East African and Madagascar-Seychelles-India plates. [ABSTRACT FROM AUTHOR]