Sulfur sources for ultramafic-hosted sulfide mineralization in the Tianzuo hydrothermal field, 63.5° E, South-west Indian Ridge: Insights from sulfur and carbon isotopes,Geological Journal

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Sulfur sources for ultramafic-hosted sulfide mineralization in the Tianzuo hydrothermal field, 63.5° E, South-west Indian Ridge: Insights from sulfur and carbon isotopes
Geological Journal ( IF 1.4 ) Pub Date : 2022-12-10 , DOI: 10.1002/gj.4661
Jia Wang ₁ , Teng Ding ₁ , Tingting Tan ₂ , Chunhui Tao _{2,

3,

4} , Jin Liang ₃ , Bin Wu ₅

Affiliation

To date, discovered ultramafic-hosted seafloor massive sulfide (UM–SMS) deposits are rare, remain poorly understood in terms of their genesis, and typically form along slow- and ultraslow-spreading mid-oceanic ridges. The Tianzuo hydrothermal field (THF) is the only UM–SMS deposit identified so far along the ultraslow-spreading South-west Indian Ridge (SWIR). This study presents new detailed mineralogical and S–C isotopic data for serpentinized peridotite, gabbro, serpentinite, and sulfide ore samples from the THF and to provide new insights into the processes that formed the deposit. Chromium-reduced sulfur (CRS) and sulfate–sulfur (SS) in samples of serpentinized peridotite have positive δ³⁴S values (up to 10.22‰ and 20.67‰, respectively). The CRS formed through thermochemical reduction of seawater sulfate during reactions between seawater and ultramafic rocks, whereas the SS most likely occurred as dissolved sulfate phases in seawater occupying the pore space of serpentinized peridotite after exposure at the seafloor. The serpentinite samples contain SS with δ³⁴S values of 19.21‰–20.9‰, indicating infiltration by seawater that led to the deposition of seawater sulfate. The sulfide ores have acid-volatile sulfur (AVS), CRS, and SS δ³⁴S values of 10.99‰–12.07‰, 2.34‰–10.98‰, and −4.13‰ to 2.76‰, respectively, indicating that the AVS and CRS within the samples are dominated by sulfur reduced from seawater sulfate (e.g., anhydrite) and leached from the surrounding wall rocks, although the re-oxidization of microbial-reduced sulfur most likely generated the sulfate with low and negative δ³⁴S values. The total carbon and total organic carbon δ¹³C values of the THF samples are negative and range from −21.99‰ to −0.34‰ and −26.7‰ to −21.02‰, respectively, further suggesting that the samples record microbial activity. Gabbro samples have strongly negative AVS (−10.69‰) and CRS (−7.2‰) δ³⁴S values, indicating that microbial-reduced sulfur derived from shallow levels in the crust was incorporated into the hydrothermal system that circulated through the deeper-seated gabbroic units. Our results suggest that the ultramafic and gabbroic rocks, seawater, and microbial activity all provided sulfur to the THF, with well-developed fractures and local magma supply probably being the key factors controlling the formation of this field and similar UM–SMS deposits elsewhere.

中文翻译：

西南印度洋中脊 63.5° E 天座热液田超镁铁质硫化物矿化的硫源：来自硫和碳同位素的见解

迄今为止，已发现的超镁铁质海底块状硫化物 (UM-SMS) 矿床很少见，对其成因仍知之甚少，并且通常沿着缓慢和超缓慢扩张的洋中脊形成。天座热液田 (THF) 是迄今为止沿着超缓慢扩张的西南印度洋岭 (SWIR) 发现的唯一 UM-SMS 矿床。本研究提供了来自 THF 的蛇纹石化橄榄岩、辉长岩、蛇纹岩和硫化物矿石样品的新的详细矿物学和 S-C 同位素数据，并提供了对矿床形成过程的新见解。蛇纹石化橄榄岩样品中的铬还原硫 (CRS) 和硫酸盐-硫 (SS) 具有正 δ ³⁴S 值（分别高达 10.22‰ 和 20.67‰）。CRS 是海水与超镁铁质岩石反应过程中海水硫酸盐的热化学还原形成的，而 SS 很可能是海水中溶解的硫酸盐相，在暴露于海底后占据蛇纹石化橄榄岩的孔隙空间。蛇纹岩样品含有SS，δ ³⁴ S值为19.21‰~20.9‰，表明海水渗透导致海水硫酸盐沉积。硫化矿有酸挥发硫（AVS）、CRS、SS δ ³⁴S 值分别为 10.99‰–12.07‰、2.34‰–10.98‰ 和 −4.13‰ 至 2.76‰，表明样品中的 AVS 和 CRS 主要是从海水硫酸盐（例如硬石膏）中还原的硫和从周围的围岩，尽管微生物还原的硫的再氧化最有可能产生具有低和负 δ ³⁴ S 值的硫酸盐。THF样品的总碳和总有机碳δ ¹³ C值为负值，分别为-21.99‰至-0.34‰和-26.7‰至-21.02‰，进一步表明样品记录了微生物活动。辉长岩样品具有强负 AVS (−10.69‰) 和 CRS (−7.2‰) δ ³⁴S 值，表明来自地壳浅层的微生物减少的硫被纳入热液系统，该热液系统循环通过更深层的辉长岩单元。我们的研究结果表明，超基性和辉长岩、海水和微生物活动都为 THF 提供了硫，发育良好的裂缝和当地岩浆供应可能是控制该油田和其他地方类似 UM-SMS 矿床形成的关键因素。

更新日期：2022-12-10

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