Contribution of Precambrian basements to the Mesozoic ore-fluid system: An illustration using the Majiayao gold deposit, Jiaodong, China

Majiayao deposit represents a significant example hosted in the Archean high-grade metamorphic terrane. The.mineralization is characterized by auriferous quartz-sulfide veins together with notable Fe-bearing carbonates.(ankerite and siderite), iron oxides and barite. In this study, the comprehensive in-situ analyses of vein-hosted.hydrothermal minerals (such as: xenotime U-Pb dating, ankerite Rb-Sr isotopes and monazite Sm-Nd isotopes).and sulfur isotopic analyses of gold-bearing sulfides are used to constrain the timing of mineralization and to.better understand the ore genesis. The syn-ore hydrothermal xenotime yielded a U-Pb age of 120.0 ± 1.2 Ma.marking timing of gold mineralization, which is much younger than wallrocks’ latest metamorphism at 1950 to.1850 Ma. The relatively higher and variable initial 87Sr/86Sr ratios (0.715809–0.718753) of ore-related ankerite.may be leached from ore-hosting Archean metamorphic rocks and/or regional Neoproterozoic Penglai Group.low-grade metasedimentary rocks via extensive fluid-rock interactions. This inference is also supported by the.newly reported near-zero δ34S values (–1.2 to 1.4‰) of pyrite from altered wallrocks, which are significantly.comparable to the surrounding Archean plagioclase amphibolite. The monazite has relatively homogenous εNd.(t=120Ma) values ranging from –19.3 to –17.6, which are much higher than the Nd isotope compositions of the orehosting.Archean high-grade metamorphic rocks. Combining the widespread mafic dykes and previous relevant.geochemical studies, it is proposed that the ore-forming fluids were probably derived from deeply mantle-related.contents, followed by intense interaction with Archean basements at the shallow crustal level. In addition, the.progressive decrease of δ34S values of pyrite and more appearances of iron oxides and barite from early to late.mineralization stages, are ascribed to gradual fluid oxidation by extensive carbonatization of Fe-rich Archean.wallrocks.

马家窑矿床是太古宙高级变质岩区的一个重要实例。矿化特征为含金石英 - 硫化物脉以及显著的含铁碳酸盐（铁白云石和菱铁矿）、铁氧化物和重晶石。在本研究中，对脉石热液矿物进行了综合原位分析（如：磷钇矿U - Pb定年、铁白云石Rb - Sr同位素和独居石Sm - Nd同位素）以及含金硫化物的硫同位素分析，以确定矿化时间并更好地理解矿石成因。与成矿同期的热液磷钇矿得出的U - Pb年龄为120.0 ± 1.2 Ma，标志着金矿化的时间，这比围岩在1950至1850 Ma的最新变质作用要年轻得多。与矿石相关的铁白云石相对较高且变化的初始87Sr/86Sr比值（0.715809 - 0.718753）可能是通过广泛的流体 - 岩石相互作用从赋矿的太古宙变质岩和/或区域新元古代蓬莱群低级变质沉积岩中淋滤出来的。这一推断也得到了新报道的蚀变围岩中黄铁矿近零的δ34S值（-1.2‰至1.4‰）的支持，该值与周围的太古宙斜长角闪岩显著可比。独居石具有相对均一的εNd（t = 120 Ma）值，范围从 - 19.3至 - 17.6，远高于赋矿的太古宙高级变质岩的Nd同位素组成。结合广泛分布的镁铁质岩脉和先前相关的地球化学研究，提出成矿流体可能源自深部地幔相关物质，随后在浅地壳水平与太古宙基底发生强烈相互作用。此外，从矿化早期到晚期，黄铁矿的δ34S值逐渐降低，铁氧化物和重晶石出现得更多，这归因于富铁的太古宙围岩广泛碳酸化导致的流体逐渐氧化。