江西大湖塘钨矿田平苗矿区含矿花岗岩矿物学特征及对成矿的指示意义

赣西北大湖塘钨矿田位于江南造山带东段，是世界级超大型W-Cu-Mo多金属矿田，矿化类型以细脉浸染型矿化为主，成矿作用与燕山期高分异花岗岩密切相关。本文通过对平苗矿区与成矿关系密切的燕山期花岗岩中黑云母和斜长石等矿物进行系统的原位主量元素和微量元素分析，探究岩浆的氧逸度、岩浆系统的深部动力学特征和详细结晶过程，并对成矿作用的指示意义进行探讨。研究表明，似斑状二云母花岗岩从岩浆结晶早期到晚期，一直保持较低的氧逸度（NNO-QFM），可能与岩浆源区中更多的富泥质沉积岩有关，这种富钨的泥质岩源区和还原性岩浆环境更有利于钨矿的形成。燕山期花岗岩中斜长石的钙长石（An）和CaO含量均远低于晋宁期黑云母花岗闪长岩，很难为白钨矿化提供足够的钙，而黑云母花岗闪长岩由于体积巨大、钙含量高，很可能为区内大规模的白钨矿化贡献了大量的钙元素。斜长石原位分析显示，An和Al₂O₃含量之间存在显著的正相关性，而与FeO含量之间无明显的正相关性，FeO随着An含量的增加基本保持稳定，斜长石中Sr和Ba含量之间也无显著的负相关性，表明该区岩浆房为化学成分相对封闭的岩浆系统，岩浆演化过程中只有热量混合和/或减压作用，没有发生明显的镁铁质岩浆注入与混合。因此，钨、铜、硫等成矿元素应主要来自岩浆源区双桥山群的富泥质变质沉积岩和变质玄武岩的贡献，而非由外来基性岩浆的补给提供。该区岩浆岩形成于华南板块由挤压向伸展的转换期，挤压环境有利于在地壳浅部形成长期稳定的、规模较大的高分异岩浆房，促进成矿元素高度富集和大规模岩浆热液的形成，导致该区发生大规模的钨铜矿化。

Mineralogical characteristics and its metallogenic implications of ore-bearing granites in the Pingmiao W-Cu deposit, Dahutang tungsten ore field, South China

The giant Dahutang W-Cu-Mo polymetallic ore field in the Northwest Jiangxi, situated in the eastern section of Jiangnan Orogen, is one of the largest tungsten deposits in the world. It is dominated by veinlet-disseminated type of orebodies whose mineralization is closely associated with the highly evolved Yanshanian granites. In this paper, we present systematic in-situ major- and trace-element analysis across plagioclase and biotite from the ore-related granites in the Pingmiao deposit to elucidate the oxygen fugacity of the magma, the dynamics of their magmatic system and the detailed processes of magma crystallization as well as their metallogenic implications. The study shows that the coarse-grained porphyritic two-mica granite remained at low oxygen fugacities (NNO-QFM) from the early to late stages of fractional crystallization, which may be related to more argillaceous sedimentary rocks in the magmatic source. The tungsten-rich argillaceous magma source and the reduced magmatic environment are more favorable for the tungsten mineralization. The Yanshanian granites are not likely to provide sufficient calcium for such large tonnage of scheelite mineralization because of the low CaO contents. The calcium needed for the scheelite mineralization may be mainly provided by the Neoproterozoic biotitic granodiorite due to its large volume and calcium contents in this field. The plagioclase crystals analyzed show a strong positive correlation between anorthite and Al₂O₃, while no correlation exists between anorthite and FeO since the latter mainly remained constant with the increase of anorthite. In addition, there is no significant negative correlation between the contents of Sr and Ba in plagioclase. These studies indicate that the magmas chambers experienced chemically closed evolution affected by only thermal mixing and/or decompression, without chemical mixing with mafic magma from mantle. The ore-forming elements (W, Cu, S, etc.) should be mainly derived from the magma sources of Shuangqiaoshan Group (argillaceous metamorphic sedimentary rocks and basic volcanic rocks) instead of the mafic magma recharging. The magma granites were formed during the transformation period from compressional regime to extensional regime. The stress environment facilitated the formation of long-term stable, large-volume, highly evolved felsic magma chambers in the shallow crust, which accelerated the enrichment of ore-forming elements and the formation of large volumes of magmatic hydrothermal fluids, and induced the formation of massive tungsten and copper mineralization in the district.