Genesis and mechanisms of metal enrichment in the Baimazhai Ni-Cu-(PGE) deposit, Ailaoshan Orogenic Belt, SW China

Yiguan Lu, C. Lesher, Li‐Qiang Yang, M. Leybourne, Wen-yan He
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Abstract

The ∼259 Ma Baimazhai Ni-Cu-(platinum-group element) deposit is located in the Ailaoshan-Red River fault zone on the southwest margin of the Yangtze Plate in the Jinping area of southeastern Yunnan Province. The intrusion is lenticular (∼530 m long × 190 m wide × 24–64 m thick) and concentrically zoned (margin to core) from gabbro through pyroxenite to peridotite. It contains ∼50 kt of Ni-Cu-(platinum-group element) mineralization, concentrically zoned (margin to core) from disseminated through net-textured to massive sulfides with an average grade of 1.03 wt.% Ni, 0.81 wt.% Cu, and 0.02∼0.69 ppm Pd+Pt. The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, violarite, galena, and cobaltite. The mineralization is enriched in Ni-Cu-Co relative to the platinum-group elements and the host rocks are enriched in highly incompatible lithophile elements relative to moderately incompatible lithophile elements with high Th/Yb and intermediate Nb/Yb ratios. These host rocks, and those at most other Ni-Cu-platinum-group element deposits in the Emeishan Large Igneous Province, have high γOs and intermediate εNd values, indicating that they crystallized from a magma derived from a subduction-modified pyroxenite mantle source and modified by crustal contamination. The initial concentrations of metals in the primary magma are estimated to have been on the order of 200 ppm Ni and 100 ppm Cu, but only 0.4 ppb Pd, 0.2 ppb Pt, 0.005 ppb Rh, 0.02 ppb Ru, and 0.01 ppb Ir. The δ34S values of ores and separated sulfides range from 5.8‰ to 8.6‰, between the ∼10‰ value of sulfides in the metasedimentary country rocks and the 0 ± 0.5‰ value expected for magmas derived from MORB-type mantle, or the –2.5 ± 0.3‰ value expected for subduction-modified mantle, consistent with equilibration at magma:sulfide mass ratios (R factors) of 100–1000. Variations in Ir100 and Pd100 (metals in 100% sulfide) are consistent with 40–60% fractional crystallization of monosulfide solid solution to form Ni-Co-intermediate platinum-group element (Ru, Os, Ir)-rich massive ores and Cu-palladium/platinum-group elements (Pt, Pd, Rh)-Au-rich residual sulfide liquids. This process is also recorded by magnetite: Type I (early magmatic), type II (late magmatic), and type III (secondary) magnetites exhibit progressively lower Cr-Ti-V concentrations. The platinum-group element contents in base-metal minerals are low, and only pentlandite, violarite, and cobaltite contain detectable concentrations of Pd, Rh, and Ru. There is abundant textural evidence for metamorphic-hydrothermal alteration of sulfides in the Baimazhai intrusion, with secondary violarite, chalcopyrite, and pentlandite being enriched (Ag, Sb, Au, Pb) or depleted (Sn) in more mobile chalcophile elements. The different tectonic and petrogenetic settings of the Baimazhai and other deposits in China highlight the potential of Ni-Cu-platinum-group element deposits to occur in subduction or post-subduction settings and demonstrate that the key controls are magma flux and access to crustal S. Exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic Ni-Cu deposits.
哀牢山造山带百马寨镍铜矿床金属富集成因及机制
侵入体呈透镜状(长~ 530 m ×宽190 m ×厚24 ~ 64 m),从辉长岩到辉石岩再到橄榄岩呈同心带状(边缘到岩心)。它含有~ 50kt的Ni-Cu(铂族元素)矿化,从浸染到网状织构到块状硫化物呈同心带(边缘到岩心),平均品位为1.03 wt.% Ni, 0.81 wt.% Cu和0.02 ~ 0.69 ppm Pd+Pt。硫化物组合包括磁黄铁矿、黄铜矿和镍黄铁矿,还有少量磁铁矿、紫辉石、方铅矿和钴矿。成矿主要富集Ni-Cu-Co族元素,相对于铂族元素,寄主岩石富集高不相容亲石元素,相对于中等不相容亲石元素,具有高Th/Yb和中等Nb/Yb比值。这些主机的岩石和其他Ni-Cu-platinum-group最多元素在峨眉山大火成岩省的存款,有很高的γOs和中间值εNd值,表明他们从岩浆结晶来自subduction-modified辉岩地幔源和修改地壳污染。据估计,原始岩浆中金属的初始浓度约为200ppm的Ni和100ppm的Cu,但只有0.4 ppb的Pd, 0.2 ppb的Pt, 0.005 ppb的Rh, 0.02 ppb的Ru和0.01 ppb的Ir。矿石和分离硫化物的δ34S值在5.8‰~ 8.6‰之间,介于变质沉积岩中硫化物的~ 10‰和morb型地幔岩浆的0±0.5‰之间,或俯冲变质地幔的-2.5±0.3‰之间,与100-1000岩浆与硫化物质量比(R因子)的平衡一致。Ir100和Pd100(100%硫化物中的金属)的变化与40-60%的单硫化固溶体分离结晶形成富ni - co -中间铂族元素(Ru, Os, Ir)块状矿石和富cu -钯/铂族元素(Pt, Pd, Rh)-富au的残余硫化物液体相一致。这一过程也记录在磁铁矿中:ⅰ型(早期岩浆)、ⅱ型(晚期岩浆)和ⅲ型(次生)磁铁矿中,Cr-Ti-V浓度逐渐降低。贱金属矿物中的铂族元素含量较低,只有镍黄铁矿、紫黄铁矿和钴矿含有可检测浓度的Pd、Rh和Ru。白马寨岩体中硫化物变质—热液蚀变的结构证据丰富,次生紫铜矿、黄铜矿和镍黄铁矿富集(Ag、Sb、Au、Pb)或贫(Sn)更具流动性的亲铜元素。白马寨和中国其他矿床的不同构造和成岩环境突出了镍铜铂族元素矿床在俯冲或后俯冲环境下的潜力,并表明岩浆通量和地壳s通道是控制镍铜铂族元素矿床的关键因素。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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