1313 / 2019-08-09 10:59:07

内蒙浩尧尔忽洞金矿床成矿时代厘定和多期成矿作用研究

13、矿产与资源 > 专题3：斑岩成矿系统——缅怀世界著名矿床学家Jeremy Richards

The Haoyaoerhudong gold deposit is hosted in the Mesoproterozoic carbonaceous and pyritic slate, phyllite, and schist that was folded into a tight syncline at the north margin of the North China Craton. The gold mineralization is characterized by abundant sulfides veins and pyrite-quartz veins. Detailed paragenetic studies of the host rocks and mineralization have defined five stages of pyrites. The earliest forms of pyrite, termed as Py1, is widely developed in the host rocks and occur as fine-grained dispersed pyrite in black carbonaceous slate and medium- to coarse-grained disseminated pyrite in pyrite-rich layers, which are interpreted as a sedimentary or diagenetic origin. Py2 with garnet and quartz inclusions and Py3 occurred as pyrite veins are intercepted to be formed through the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism. Py4 from the pyrite-quartz veins associated with free gold, contain inclusions of native gold, electrum, maldonite, galena, and chalcopyrite, and is considered as a product during post-peak metamorphism. While, Py5 with typical remobilized features is thought to be a product of melting of the former pyrites (Py1 to Py4) triggered by the large-scale Hercynian magmatism, which is supported by the solid evidence of the age of monazites (341 ± 6.6 Ma) associated with Py4.
Laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) trace element analyses of the various pyrite types show that the sedimentary/diagenetic Py1 contains relative low gold and high arsenic content, varying from 0.02 to 5.7 ppm and 450 to 1570 ppm, respectively, indicating a euxinic sedimentary environment. Py2 and Py3 from the bedding pyrite veins contain progressively high contents of invisible gold (0.08 - 1.9 ppm) and lowest As (0.8 -48 ppm). Some visible gold grains associated with Py3 are detected. Py4 from the pyrite-quartz veins contain higher invisible gold (0.11 - 630 ppm) and other trace elements. Abundant native gold, electrum, and maldonite occurred as inclusions within Py4 and monazite, veinlets along the fractures of garnet, and irregular shapes associated with quartz. Little and no gold is detected in Py5.
In suit secondary ion mass spectrometry (SIMS) sulfur isotope analyses of various pyrite, chalcopyrite, pyrrhotite, and arsenopyrite indicate that the sedimentary/diagenetic Py1 yield δ34S values in a broad range of -37.4 to 16.2 ‰ but mostly fall a range of 12 to 16 ‰, which indicate a sedimentary origin. Pyrrhotite, arsenopyrite, and chalcopyrite 1 (Ccp1) have similar δ34S values with Py1, suggesting pyrrhotite veins and associated other sulfides were sourced from the dissolution of Py1. Later generations of sulfides, including Py2 to Py5, and Ccp2 to Ccp3, yield δ34S values in a narrow range of 9.5 to 12.7 ‰, which relatively depleted δ34S values to the former sulfides presumably indicate a mixed sulfur source with a broad range of δ34S values.
The paragenetic, textural, chemical, and isotopic data suggest three distinct gold-introducing episodes at Haoyaoerhudong. Gold and arsenic were clearly initially concentrated in organic muds on the sea floor by desorption from clays, and enriched along the structures of diagenetic arsenic-rich pyrite (Py1). Subsequently, accompanying metamorphism, deformation, and shear zone activity, gold was liberated from the dissolution of diagenetic pyrites to form the bedding-parallel pyrite veins. Finally, with increasing metamorphic temperature, pyrite transformed into pyrrhotite, during which gold was released into the metamorphic fluids to become concentrated as native gold, electrum, and maldonite within pyrite and along structures of garnet and boudinaged bedding-parallel pyrite-quart veins. These multistage ore-forming processes are similar to the Sukhoi Log, Taldybulak Levoberezhny, and Sawayaerdun gold deposits, indicating that other black shale and turbidite-hosted gold deposits in the Central Asian Orogenic Belt may form by these similar processes.