Falkenberg, Jan J., Keith, Manuel, Haase, Karsten M., Bach, Wolfgang, Klemd, Reiner, Strauss, Harald, Yeo, Isobel A., Rubin, Kenneth H., Storch, Bettina and Anderson, Melissa O. (2021) Effects of fluid boiling on Au and volatile element enrichment in submarine arc-related hydrothermal systems. Geochimica et Cosmochimica Acta, 307 . pp. 105-132. DOI 10.1016/j.gca.2021.05.047.

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Abstract

Highlights

• Decoupling of volatile element enrichment and magmatic volatile influx.
• Multiple sulfide generations with distinct trace element signatures.
• Boiling-induced pyrite precipitation revealed by textures and Tl/Pb, Sb/Pb and Bi/Pb ratios.
• Boiling-induced Au, electrum and Bi-telluride colloids lead to high Au grades.
• Metals sources: shallow upflow- (60–80%) and deep reaction (20–40%) zone.

Abstract

Shallow (<1500 mbsl) submarine arc-related hydrothermal systems can host base (Cu), precious (Au) and volatile elements (As, Se, Sb, Te, Tl) in significant quantities. Their wide application in the high-tech industry, but a potential eco-toxicological footprint gives them a strategic importance. However, the processes that concentrate these elements in submarine arc-related hydrothermal systems, compared to their mid-ocean ridge counterparts are still debated, and it is unclear whether boiling-related processes and/or the contribution of magmatic volatiles are key for their enrichment.

We present bulk sulfide-sulfate, isotope (S and Pb), and high-resolution microanalytical data of hydrothermal sulfides from the Niua South fore-arc volcano in north Tonga, where numerous black-smoker type sulfide-sulfate chimneys emit boiling fluids with temperatures (up to 325 °C) near the seawater boiling curve at ~1170 m water depth. Hence, this system represents an ideal natural laboratory to investigate the effect of fluid boiling on base, precious, and volatile element enrichment associated with hydrothermal seafloor mineralization. At Niua South, textural and chemical variations of multiple pyrite (framboidal, euhedral and massive), chalcopyrite (linings), and sphalerite (dendrites and linings) generations are indicative for sulfide precipitation from early low-temperature (~240 °C) fluids that underwent abundant mixing with ambient seawater (low Se/Tl and Co/Ni ratios in pyrite) and from later high-temperature (up to 325 °C) (high Se/Tl and Co/Ni ratios in pyrite). In addition, crustiform inclusion-rich pyrite that precipitated from high-temperature boiling fluids shows low Bi/Pb, Tl/Pb and Sb/Pb ratios due to volatile element loss (e.g., Tl and Sb) to the vapor phase compared to pyrite that formed during the low temperature stage. By contrast, late sphalerite (~280 °C) is enriched in elements with an affinity to Cl-complexes like Mn, Co, Ni, Ga, Cd, In, and Sn, and therefore precipitated from the corresponding Cl-rich liquid phase.

Gold occurs in solid-solution and as boiling-induced particles of native Au, electrum, and Au-rich Bi-tellurides in pyrite (up to 144 ppm Au), sphalerite (up to 60 ppm Au), and chalcopyrite (up to 37 ppm Au). These particles (<5–10 µm) probably formed during fluid boiling causing an extreme Au enrichment (>30 ppm) in the mature and late stage of chimney formation. Lead isotope data indicate that the hydrothermal fluids scavenged metals not only from the deeper basement in the reaction zone (20–40%), but also from young dacitic volcanic rocks near the seafloor in the upflow zone (60–80%). Sulfur isotope (δ34S = −0.3 to 4.4‰) and Se/S*106 values (<1500) of hydrothermal sulfides provide no evidence for a magmatic volatile influx and indicate that S, and most metals and semi-metals were likely leached from the host rocks. Hence, volatile (As, Se, Sb, Te, Tl), and precious (Au) element enrichments in arc-related submarine hydrothermal systems can be decoupled from magmatic volatiles and are instead a result of boiling-induced trace element fractionation – a hydrothermal enrichment process, which has been underestimated to date.

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