一种酸淋滤成因的含钼石英脉——来自小秦岭北矿带大湖-秦南矿区的证据

大湖-秦南钼矿床位于小秦岭金矿田北矿带,属于东秦岭钼钨成矿带小秦岭-外方山成矿亚带。其矿床类型为含钼石英脉型,与小秦岭北矿带含金石英脉空间上伴生。根据野外地质观察、PIMA测试等结果,对大湖-秦南含钼石英脉的产出、矿石结构构造、蚀变特征进行了研究。含钼石英脉为奶白色,易碎,发育蜂窝状、团块状、角砾状构造。角砾成分为太华群片麻岩角砾、细脉状浸染状辉钼矿化的中生代花岗斑岩角砾等。蚀变有钾化、硅化、绢云母化、高岭土化以及硬石膏化等。根据野外地质特征,蚀变可分为早期钾化+硅化+绢云母化+黄铁矿化+辉钼矿化,晚期为硅化+高岭土化+硬石膏化等。含钼石英脉中的围岩角砾或多或少地发育高岭土化。辉钼矿化主要充填于石英脉的裂隙和蜂窝中。根据包裹体显微测温数据推测含钼石英脉的成矿温度为134～463℃。含钼石英脉中的δ18 O石英值为10.7‰～11.5‰,δDH2O为-96‰～-110‰;这一特征反映了既有岩浆水,也有变质水和天水的参入,高岭土δ18 O为0.1‰～2.3‰,反映了热液成因。黄铁矿δ34S值为-5.778‰～-7.841‰,以富含轻硫为特征,反映了其形成时有生物硫参入。在此基础上,通过区域地质分析,讨论了高岭土化的成因,进而认为大湖-秦南含钼石英脉很可能是与秦岭造山带在245～211Ma全面陆陆碰撞过程有关的花岗斑岩侵位形成的斑岩型钼矿体,在后期金矿形成时,在酸性热液淋滤下进一步富集。

Mo-Bearing Quartz Vein by Acid Leaching: Evidences from Dahu-Qinnan Mine in Northern Belt of Xiaoqinling Gold Province,Central China

The Dahu-Qinnan molybdenum deposit is located in northern belt of the Xiaoqinling gold province and metallogenically belongs to the Xiaoqinling-Waifangshan sub-zone of the eastern Qinling Mo-W metallogenic belt,Central China.Spatially associated with Au-bearing quartz veins,the molybdenite mineralization in the area is characterized by Mo-bearing quartz veins.The occurrences,textures and alterations of Mo-bearing quartz veins in Dahu-Qinnan area have been studied based on the field observation and PIMA instrument analysis.These Mo-bearing quartz veins are friable with cream color,and are of vuggy,cloddy,breccias structure.The breccias in these Mo-bearing quartz veins are mostly gneiss of the Taihua Group and porphyritic granite dyke of Mesozoic with stockwork and disseminated molybdenite mineralization.These breccias are replaced more or less by argillic minerals.Alteration associated with molybdenite-bearing quartz veins is mainly potassium alteration(only in the breccias),alteration to sericite,sillicification(residual silica),alteration to kaolinite,smectite and anhydrite.Based on field evidences,alteration in the area can be divided into 1) Potassium+sericite+sillicification+pyrite+molybdenite in early stage and 2) sillicification(residual silica)+kaolinization+anhydrite in later stage.Molybdenite and other sulfide minerals occur as the infillings in fissures and pore space in molybdenite-bearing quartz veins.The ore-forming temperatures of Mo-bearing quartz veins are from 134℃ to 463℃.The δDH2O is from-96‰ to-110‰,and δ18Oquartz is from 10.7‰ to 11.5‰ respectively,revealing a mixing processes of magmatic,metamorphic fluids and meteoritic water.δ18O value of kaolinite is from 0.1‰ to 2.3‰,also suggesting a hydrothermal origin.δ34Spyrite is from-5.778‰ to-7.841‰,indicating the involvement of organism in metallogeny.Based on above studies and combined with the geological setting of the deposit,it is suggested that the molybdenite-bearing quartz veins in the area are likely to be porphyry molybdenum orebodies associated with porphyry granite dyke formed probably at ca.245-211 Ma during the continental-continental collision of the Qinling orogeny.They were leached and enriched by high sulfidation hydrothermal fluids during later gold-forming epoch.