Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia
Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia
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DOI：
10.1111/rge.12300
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发表时间：
2022-01
期刊：
影响因子：
1.4
通讯作者：
Renaldi Suhendra;R. Takahashi;A. Imai;Hinako Sato;N. Setiawan;A. Agangi
中科院分区：
文献类型：
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作者：
Renaldi Suhendra;R. Takahashi;A. Imai;Hinako Sato;N. Setiawan;A. Agangi
爪哇中部Luk Ulo变质杂岩是白垩纪俯冲和吸积作用的产物，含有多种类型的原岩。该区已识别出两种原生矿化类型，即(1)以海底玄武岩为主的块状硫化物矿化和(2)以浅变质岩为主的脉状矿化。后来，这些类型的原生矿化的侵蚀形成了沿河的砂金矿床。然而，消息来源一直没有确定。因此，本研究以矿物学、矿物化学、全岩地球化学和硫同位素分析为基础，旨在了解研究区砂金的来源、原生矿化特征和构造演化。在海底玄武岩和少量深海沉积岩中发现了火山成因块状硫化物(VMS)型成矿作用，矿石和围岩都保存了变质前的结构和矿物。这种VMS型矿化的特征包括：(1)以黄铁矿切割容矿岩石的碎屑状石英脉；(2)局部硅化分带性至层间chlorite/smectite‐chlorite‐laumontite‐calcite‐epidote蚀变；(3)以黄铁矿为主的矿石，含少量砷黄铁矿、黄铜矿和黄铁矿；(4)未变质的围岩和矿石；(5)硫同位素特征，中位数δ34S+3.1‰，表明硫来自岩浆来源和/或来自玄武岩中的硫，生物硫的贡献很小。低级变质岩脉矿化为造山型金矿化，矿化矿脉形成于变质作用高峰后。它的特征是(1)黄铁矿-砷系黄铁矿矿石中含有少量毒砂、方铅矿、方铅矿、方铜矿、黄铜矿和闪锌矿，(2)石英-伊利石-石墨蚀变组合，(3)贯穿变质主岩面理的矿化脉，(4)高Sb含量的黄铁矿(高达1.7wt%)和金红石(高达160ppm)，(5)较高的成矿温度(423±9℃，根据毒砂和石墨地温计算)和(6)矿石的重熔沉积硫特征，δ34S中值为−9.8‰。几条证据表明，砂金很可能来自周围地区造山型金矿的侵蚀。这一证据包括赋存于低品位变质岩和类变质花岗岩中的含金矿石的存在，具有造山型金矿化的特征，而VMS类型的矿石则不含金。大洋中脊-吸积带矿化的产出反映了白垩纪洋陆地块的俯冲和融合作用。舜达弧白垩纪基底岩中赋存金矿化的发现表明，拓宽金矿找矿靶区的重要性，不仅包括年轻的火山岩，而且包括相对古老的变质基底岩。
The Luk Ulo Metamorphic Complex, Central Java is a product of the Cretaceous subduction and accretion, and includes diverse types of protoliths. Two‐types of primary mineralization have been recognized in this area, namely, (1) seafloor basalt‐hosted massive sulfide mineralization and (2) low‐grade metamorphic rocks‐hosted vein type mineralization. Later erosion of these types of primary mineralization formed placer gold deposits along rivers. However, the source has never been identified. Thus, this study aims at understanding the source of placer gold, the characteristics of the primary mineralization, and the tectonic evolution of the study area on the basis of mineralogy, mineral chemistry, whole‐rock geochemistry, and sulfur isotope analyses. Volcanogenic massive sulfide (VMS)‐type mineralization was identified in the seafloor basalt and few deep‐sea sedimentary rocks, and both the ores and host rocks preserved pre‐metamorphic textures and minerals. The characteristics of this VMS‐type mineralization include (1) crustiform quartz veins with pyrite cutting the host rocks, (2) zonation of local silicification to interlayered chlorite/smectite‐chlorite‐laumontite‐calcite‐epidote alteration from central to outer zone, (3) pyrite‐dominated ores with minor amounts of arsenian pyrite, chalcopyrite, and marcasite, (4) unmetamorphosed host rocks and ores, and (5) sulfur isotope signature with a median δ34S of +3.1‰ suggesting sulfur derived from magmatic source and/or sulfur extracted from basaltic rocks with a small contribution of biogenic sulfur. On the other hand, low‐grade metamorphic rocks‐hosted vein type mineralization was identified as orogenic‐type gold mineralization, and the mineralized veins formed after the peak of metamorphism. It is characterized by (1) pyrite‐arsenian pyrite ores with minor amounts of arsenopyrite, galena, tetrahedrite, chalcopyrite, and sphalerite, (2) quartz‐illite‐graphite alteration assemblage, (3) mineralized veins cross‐cutting the foliation of metamorphic host rocks, (4) high antimony contents of pyrite (up to 1.7 wt%) and rutile (up to 160 ppm), (5) relatively high ore‐forming temperature (423 ± 9°C, calculated from arsenopyrite and graphite geothermometers), and (6) remobilized‐sedimentary sulfur signature of the ores with a median δ34S of −9.8‰. Several lines of evidence suggest that placer gold was likely derived from the erosion of orogenic‐type gold ores in the surrounding areas. This evidence includes the presence of gold‐bearing ores hosted by low‐grade metapelites and metagranitoid with characteristics of orogenic‐type gold mineralization, whereas the VMS‐type ores are barren in gold. The occurrence of the mid‐oceanic ridge‐ and accretion zone‐related mineralization in this area reflects the subduction and amalgamation of oceanic and continental crustal blocks during the Cretaceous period. Discovery of gold mineralization hosted in the Cretaceous basement rocks of the Sunda arc indicates the importance to broaden the gold exploration targets to include not only young volcanic rocks, but also relatively old metamorphic basement rocks.