Deciphering fluid origins in the Paleozoic Laoshankou Fe-Cu-Au deposit, East Junggar:Constraints from noble gases and halogens

• 作者: Pei Liang Huayong Chen Liandang Zhao Chao Wu Yuling Xie Chun-Kit Lai
• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2021,（5）
• 所属期刊栏目: Research Paper
• 页码范围: 69-80
• 页数: 12页
• 语种: 英文

摘要

To constrain the ore-fluid source(s) of the Laoshankou Fe-Cu-Au deposit (Junggar orogen, NW China), we ana-lyzed the fluid inclusion (FI) noble gas (Ar, Kr and Xe) and halogen (Cl, Br and I) compositions in the hydrother-mal epidote and quartz. Four hypogene alteration/mineralization stages, including (I) pre-ore Ca-silicate, (II) early-ore amphibole-epidote-magnetite, (III) late-ore pyrite-chalcopyrite, and (IV) post-ore hydrothermal vein-ing, have been identified at Laoshankou. Stage Ⅱ FIs have salinity of 15.7 wt.％(NaCl eq.), I/Cl molar ratios of 75 × 10?6–135 × 10?6, and Br/Cl molar ratios of 1.4 × 10?3–2.1 × 10?3. The moderately-high seawater-corrected Br*/I ratios (0.5–1.5) and low 40ArE/Cl slope (～10?5) indicate the presence of sedimentary marine pore fluid, which was modified by seawater reacting with the Beitashan Fm. volcanic rocks. Stage Ⅲ fluid is more saline than their stage Ⅱ and IV counterparts, reaching up to 23.3 wt.％(NaCl+CaCl2 eq.) close to halite sat-uration (～26 wt.％). The fluid has I/Cl ratios of 75 × 10?6–90 × 10?6 and Br/Cl ratios of 1.5 × 10?3–1.8 × 10?3. Considering the increasing 40ArE/Cl trend toward bittern brine and the higher 36Ar content than air-saturated water (ASW), a bittern fluid source is inferred from seawater evaporation, which was modified by interaction with organic-rich marine sedimentary rocks. Stage IV FIs have lower temperature (110–228 °C) and Br/Cl (0.90 × 10?3–1.2 × 10?3), but higher 36Ar content than ASW, indicative of dissolved evaporite or halite input. Considering also the lowδDfluid (?114‰to?144‰) andδ18Ofluid (2.1‰–3.5‰) values, meteoric water (with minor dissolved evaporites) likely dominated the stage IV fluid. The evaporites may have formed through continuous evaporation of the stage Ⅲ surface-derived bittern. Involvement of non-magmatic fluids and different ore-fluid origins in stages Ⅱ and Ⅲ suggest that the ore-forming process was different from a typical magmatic-hydrothermal fluid-dominated skarn mineralization, which was previously proposed for Laoshankou. Our noble gas and halogen study at Laoshankou provide new insights on the fluid sources for the Paleozoic Fe?Cu (?Au) deposits in the Central Asian Orogenic Belt (CAOB), and our non-magmatic fluid source interpretation is consis-tent with the basin inversion setting for the mineralization.