滇西北休瓦促钨钼矿区复式岩体地质及其成矿特征——来自年代学、氧逸度和地球化学的约束

休瓦促钨钼矿床位于义敦岛弧南段的香格里拉北部,目前已达中-大型。该区发育由两期岩体(晚三叠世的黑云母花岗岩和晚白垩世的斑状二长花岗岩)叠加而成的复式岩体,而该矿区钨钼矿形成于82~86Ma。然而,两期岩体间关系,以及晚三叠世岩体对矿床的形成有怎样的贡献,目前未有报道。笔者通过对两期岩体形成时间及其地球化学特征的研究,从成岩年代学、岩浆氧逸度等方面探究上述问题,并揭示两期岩体的成矿特征,为找矿实践提供一定的参考依据。研究表明,黑云母花岗岩结晶年龄为211.7±2.6Ma,SiO_2偏高(69.48%~73.73%),属高钾的钾玄质系列,偏铝质,富集轻稀土元素和大离子亲石元素而亏损重稀土元素以及高场强元素Nb、Sr、Ti等,有微弱的负Eu异常,岩浆氧逸度相对较高(fO_2=-19.4~-9.1,平均-13.7);斑状二长花岗岩结晶年龄为76.8±3.8Ma,且有一颗锆石核部的年龄值为219±2.6Ma,该岩体与前者有相似的地球化学特征,SiO_2高(67.35%~75.65%),属偏铝质的钾玄质系列,ΣREE较高,高于黑云母花岗岩,铕负异常明显大于前者,岩浆氧逸度相对较低(fO_2=-30.4~-18.2,平均-23.4)。据此,结合休瓦促矿区断层发育情况,提出矿区内两期岩体以近南北向F_4为界呈断层接触关系,北西向走滑断层(F_1-F_3)为控矿构造。本文认为,晚期斑状二长花岗岩对早期黑云母花岗岩具有一定的继承性关系,黑云母花岗岩来源于晚三叠世甘孜-理塘洋向西俯冲环境下地壳的部分熔融,岩浆富水、氧逸度高,利于形成Cu-Au矿床,且在找矿实践中得到部分验证;斑状二长花岗岩来源于加厚下地壳的部分熔融,岩浆贫水、氧逸度低于黑云母花岗岩,与W-Mo矿床相关。即,晚白垩世岩浆热液沿矿区断裂-裂隙系统运移,继承和发展早期岩浆活动,形成脉状细晶岩和W-Mo矿床。

Geological and mineralized characteristics of the composite complex in Xiuwacu W-Mo mining district, NW Yunnan, China:Constraints by geochronology, oxygen fugacity and geochemistry

The Xiuwacu W-Mo deposit is located at the north of Shangri-La area, southern part of Yidun arc and its reserve has reached to medium-large size ore deposit. This is a composite complex that is composed of two-period intrusions (biotite granite of Late Triassic and porphyry monzogranite of Late Cretaceous) in Xiuwacu district, while the W-Mo deposit was formed during 82~86Ma. However, there are no explicit reports about the relationship between these two intrusions of different ages and whether the Late Triassic intrusion had contributed to the W-Mo metallogenesis. So, the authors try to answer these questions by geochronology, oxygen fugacity and other ways, after the research of their forming dating and geochemistry characteristics of these two intrusions, wishing to reveal their metallogenic characteristics and offer some theoretical support for the prospecting practice. Research shows that the crystalline age of biotite granite is 211.7±2.6Ma with high SiO₂(69.48%~73.73%), belonging to shoshonitic series, metalumnious, enrichment of LREE and LILE, depleted in HREE and HFSE of Nb, Sr, Ti, with weak negative Eu abnormal, high magmatic oxygen fugacity (f_O2=-19.4~-9.1, average -13.7); While the crystalline age of porphyry monzogranite is 76.8±3.8Ma with the age in a core of the zircon grains as 219±2.6Ma, and the rock has the similar geochemical characteristics with the former, high SiO₂ (67.35%~75.65%), belonging to shoshonitic series, metalumnious, but the rock has higher REE (higher than the biotite granite), significant negative Eu anomaly (greater than the biotite granite), relatively lower magmatic oxygen fugacity (f_O2=-30.4~-18.2, average -23.4). Based on the above, combining with the widely distributed of faults in Xiuwacu district, two periods intrusions are contacted with each other along F₄ with nearly SN strike, and the NW strike wrench faults (F₁-F₃) are ore control structures. This paper argues that there was an inheritance relation between the later porphyry monzogranite and the former biotite granite. The biotite granite was rooted in fraction melting of the lower crust just over the Ganzi-Litang Ocean westward subduction zone in Late Triassic, thus the magma was enriched with water and had high oxygen fugacity, which was effectually to form Cu-Au deposits, and this has been partially proved in practice. While the porphyry monzogranite was rooted in fraction melting of the thickened lower continental crust, as the magma had less water and lower oxygen fugacity, it has the potential to form W-Mo deposits. In a word, as the magmatic hydrothermal fluid was migrating along the fault-crack system in this area in Late Cretaceous, it inherited and developed the former intrusion and finally formed the aplite veinlets and the Xiuwacu W-Mo deposit in this area.