The Dairi SEDEX Zn + Pb + Ag deposit (North Sumatra, Indonesia): Insights from mineralogy and sulfur isotope systematics

The Dairi Zn + Pb + Ag deposit is the only giant SEDEX deposit that has been discovered to date in Sumatra. Ore-gangue mineralogy and sulfur isotope studies have been carried out to elucidate the paragenetic sequence, deformation of ore, flow direction of ore forming fluid, and sources of sulfur in the Dairi deposit. Stratiform orebodies hosted by the Julu unit (interbedded dolomitic siltstone and carbonaceous shale; multilayers; accounting for greater than 90% of metal resources) consist of pyrite-rich massive, sphalerite-rich massive, bedded sulfide, galena-rich breccias, and vein ore types, whereas discordant orebodies hosted by the Jehe unit (massive and brecciated arenaceous dolostone) comprise vein and disseminated ore types. Fine-grained, subhedral-euhedral and framboidal pyrite (enriched in Co, Ni, Cu, As, Bi, Ag relative to hydrothermal pyrite) were precipitated in the diagenetic stage, followed by precipitation of pyrite (enriched in Zn, Pb, Mn, Tl relative to diagenetic pyrite), sphalerite, galena, chalcopyrite, pyrrhotite, acanthite, tetrahedrite, tennantite, pyrargyrite, boulangerite, freieslebenite, and diaphorite in the hydrothermal stage derived from the exhaled ore-forming fluid. Metamorphic pyrite (enriched in Co and depleted in Cu, Zn, Mn, Tl, Ag relative to the other pyrites) formed from pyrrhotite (depleted in Co relative to metamorphic pyrite) and soft sulfides were remobilized into pyrite fractures and veins during sub-greenschist metamorphism. Ore-forming fluid ascended through fractures in the Jehe unit (forming the discontinuous orebodies) and then it discharged and flowed from SE to NW on the seafloor (forming the stratiform orebodies). Reduced sulfur for diagenetic pyrite (δ³⁴S = -4.1‰ to + 11.6‰) was supplied by bacteriogenic reduction of Carboniferous seawater sulfate. Hydrothermal sulfur was considered as the source of sulfur for the Jehe-hosted orebodies (δ³⁴S = +3.5‰ to + 8.0‰). The heavier δ³⁴S of sulfides in the Julu-hosted orebodies (δ³⁴S = +6.4‰ to + 28.8‰) suggests large-scale bacteriogenic reduction of Carboniferous seawater sulfate in a restricted, anoxic basin. A bacteriogenic reduced sulfur reservoir, rather than hydrothermal sulfur, may be more plausible to account for the scale of the giant Dairi deposit.