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Prograde and retrograde metasomatic reactions in mineralised magnesium-silicate skarn in the Cu-Au Ertsberg East Skarn System, Ertsberg, Papua Province, Indonesia
Ore Geology Reviews ( IF 3.3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.oregeorev.2020.103697
Melanie J. Sieber , Frank J. Brink , Clyde Leys , Penelope L. King , Richard W. Henley

Abstract The 2.7–2.9 Ma Ertsberg East Skarn System (EESS) is a world-class Cu-Au skarn that formed within and adjacent to an intrusion within a paleodepth of 0.5 km and >2.5 km. Its economic mineralisation developed by sustained reaction of magmatic fluid with contact metamorphosed siliciclastic and carbonate rocks at the margin of the adjacent Ertsberg quartz monzodiorite intrusion. Based on high-resolution mineral mapping, chemical analysis and thermodynamic calculations, the multistage formation processes of the exoskarn components of the EESS are examined in the context of changing pressure, temperature, fluid composition and fluid phase. We show that contact metamorphism of dolomitic sediments occurred at 51 ± 5 MPa, between 700 °C and 800 °C and in the presence of a H2O-CO2-fluid containing ~10 to ~70 mol% CO2. This prograde metamorphism formed a forsterite + diopside + calcite + phlogopite + spinel assemblage. Such forsterite-dominated skarns account for ~55 vol% of the EESS exoskarns. Rare pargasite (previously unrecognized in this deposit) formed locally in the metamorphosed carbonate sequence where the protolith was composed of supratidal evaporites with dolomitic carbonate and interlayered calc-silicate rocks. The subsequent flux of a lower pressure magmatic gas containing SO2(g) caused sulphate metasomatism. This high temperature gas alteration of the metamorphic assemblage also caused skarn Cu-Fe-sulphide mineralisation. The influx of a SO2 gas through fracture permeability occurred at a temperature between ~600 and 700 °C and caused calcite to be replaced by anhydrite, with the coupled release of H2S(g). This in-situ release of H2S(g) scavenged trace Cu from the gas phase to deposit Cu-Fe-sulphides, which make the economic value of the distinct. We demonstrate that the formation of metal sulphides within forsterite skarns of the Ertsberg East Skarn System required a minimum flux of ~1,050 Mt SO2(g) and show that volcanic degassing may have occurred over a time span of ~3,900 years. As the system waned, the ambient fluid resulted in partial retrograde serpentinization of olivine and diopside without carbonation, and at temperatures sufficiently high to preserve anhydrite.

中文翻译:

印度尼西亚巴布亚省 Ertsberg 的 Cu-Au Ertsberg 东矽卡岩系统中矿化硅酸镁矽卡岩的顺行和逆行交代反应

摘要 2.7-2.9 Ma Ertsberg East Skarn System (EESS) 是世界级的 Cu-Au 矽卡岩,形成于 0.5 km 和 >2.5 km 古深度内的侵入体内部和附近。其经济成矿是由岩浆流体与相邻的 Ertsberg 石英二闪长岩侵入体边缘的接触变质硅质碎屑岩和碳酸盐岩持续反应而形成的。基于高分辨率矿物测绘、化学分析和热力学计算,在压力、温度、流体成分和流体相变化的背景下,研究了 EESS 外岩石成分的多阶段形成过程。我们表明,白云岩沉积物的接触变质作用发生在 51 ± 5 MPa、700 °C 到 800 °C 之间,并且存在含有约 10 到约 70 mol% CO2 的 H2O-CO2 流体。这种顺变质作用形成镁橄榄石 + 透辉石 + 方解石 + 金云母 + 尖晶石组合。这种以镁橄榄石为主的矽卡岩占 EESS 外矽卡岩的约 55 vol%。在变质碳酸盐层序中局部形成了罕见的帕加石(以前在该矿床中未被发现),其中原岩由潮上蒸发岩和白云质碳酸盐岩和夹层钙硅酸盐岩组成。含有 SO2(g) 的低压岩浆气的后续流动引起了硫酸盐交代作用。变质组合的这种高温气体蚀变也导致矽卡岩 Cu-Fe-硫化物矿化。SO2 气体通过裂缝渗透率的流入发生在约 600 至 700 °C 的温度下,导致方解石被硬石膏取代,同时释放 H2S(g)。这种 H2S(g) 的原位释放从气相中清除了痕量 Cu 以沉积 Cu-Fe-硫化物,这使得该产品具有独特的经济价值。我们证明,在 Ertsberg East Skarn System 镁橄榄石矽卡岩中形成金属硫化物需要 ~1,050 Mt SO2(g) 的最小通量,并表明火山脱气可能发生在 ~3,900 年的时间跨度内。随着系统的衰落,环境流体导致橄榄石和透辉石部分逆行蛇纹石化而没有碳化,并且温度足够高以保存硬石膏。050 Mt SO2(g) 并表明火山脱气可能发生在约 3,900 年的时间跨度内。随着系统的衰落,环境流体导致橄榄石和透辉石部分逆行蛇纹石化而没有碳化,并且温度足够高以保存硬石膏。050 Mt SO2(g) 并表明火山脱气可能发生在约 3,900 年的时间跨度内。随着系统的衰落,环境流体导致橄榄石和透辉石部分逆行蛇纹石化而没有碳化,并且温度足够高以保存硬石膏。
更新日期:2020-10-01
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