印度尼西亚中爪哇陆乌洛变质杂岩中砂金的主要来源

IF 1.1 4区 地球科学 Q3 GEOLOGY Resource Geology Pub Date : 2022-01-01 DOI:10.1111/rge.12300
Renaldi Suhendra, R. Takahashi, A. Imai, Hinako Sato, N. Setiawan, A. Agangi
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引用次数: 0

摘要

中爪哇Luk Ulo变质杂岩是白垩纪俯冲和增生的产物,包含多种类型的原岩。该区原生矿化主要有两种类型,即(1)海底玄武岩-块状硫化物矿化和(2)低品位变质岩-脉状矿化。这些原生矿化的后期侵蚀形成了沿河流的砂金矿床。然而,来源从未被确定。在矿物学、矿物化学、全岩地球化学和硫同位素分析的基础上,对研究区砂金的来源、原生成矿特征和构造演化进行了研究。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,矿石和寄主岩均保留了前变质结构和矿物。该VMS型矿化特征包括:(1)黄铁矿切割寄主岩的壳状石英脉;(2)局部硅化分带为层间绿泥石/蒙脱石-绿泥石-湖沸石-方解石-绿帘石蚀变;(3)黄铁矿为主,含少量砷黄铁矿、黄铜矿和黄铁矿;(5)硫同位素特征,δ34S中值为+3.1‰,表明硫来自岩浆源或玄武岩,生物源硫贡献较小。另一方面,低品位变质岩-矿脉型成矿被确定为造山型金矿化,成矿矿脉形成于变质高峰后。其特征为:(1)黄铁矿-砷黄铁矿矿石中含有少量的毒砂、方铅矿、四面体、黄铜矿和闪锌矿,(2)石英-伊立石-石墨蚀变组合,(3)矿化矿脉与变质主岩的叶理交叉切割,(4)黄铁矿锑含量高(高达1.7 wt%),金红石锑含量高(高达160 ppm),(5)相对较高的成矿温度(423±9℃,根据毒砂和石墨地温计计算)。(6)矿石中位δ34S为−9.8‰的再活化-沉积硫特征。若干证据表明,砂金很可能来源于周围地区造山带型金矿的侵蚀作用。这一证据包括含金矿石存在低品位变长岩和类变长岩,具有造山带型金矿化特征,而VMS型矿石含金贫乏。该地区洋中脊和增生带相关矿化的出现反映了白垩纪洋陆地壳块体的俯冲和融合。巽他弧白垩系基底岩中金矿化的发现,表明拓宽金矿找矿对象的重要性,不仅要包括年轻的火山岩,而且要包括相对古老的变质基底岩。
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Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia
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.
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来源期刊
Resource Geology
Resource Geology 地学-地质学
CiteScore
2.30
自引率
14.30%
发文量
18
审稿时长
12 months
期刊介绍: Resource Geology is an international journal focusing on economic geology, geochemistry and environmental geology. Its purpose is to contribute to the promotion of earth sciences related to metallic and non-metallic mineral deposits mainly in Asia, Oceania and the Circum-Pacific region, although other parts of the world are also considered. Launched in 1998 by the Society for Resource Geology, the journal is published quarterly in English, making it more accessible to the international geological community. The journal publishes high quality papers of interest to those engaged in research and exploration of mineral deposits.
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