【作者】 张遵遵；

【导师】 李泽琴； 王奖臻；

【作者基本信息】 成都理工大学 ， 地球化学， 2011， 硕士

【摘要】 矽卡岩型矿床是西藏冈底斯地区重要的矿床类型之一,是目前西藏铁、铜、铅、锌、银矿的最主要来源。西藏蒙亚啊铅锌矿床位于南冈底斯-念青唐古拉成矿带,念青唐古拉（弧背断隆）成矿亚带,是本区较为重要和典型的一个矽卡岩型矿床。本文在矿床地质特征研究的基础上,进行了矿床地球化学综合研究,探讨了成矿物质来源,提出蒙亚啊矽卡岩型铅锌矿床为层控矽卡岩型,并探讨了该矿床的成矿机制。地层常-微量元素地球化学研究表明,矿区来姑组、洛巴堆组地层在沉积成岩时受到不同程度的热水沉积作用的影响。地层微量元素地球化学研究表明矿区来姑组岩石的沉积环境为含氧环境;洛巴堆组灰岩的沉积环境为贫氧环境。来姑组与洛巴堆组岩石微量元素的较大差异应该是由于所处的构造环境及沉积物源不同引起的,本次通过地层的元素地球化学研究所得出的认识,佐证了来姑组沉积环境为大陆边缘裂谷环境,洛巴堆组为岛弧裂谷环境的观点。通过地层的成矿元素研究表明,本区石炭-二叠纪地层是区内重要的矿源层,来姑组岩石的Cu、Pb、Zn等元素的含量总体上高于洛巴堆组,并且明显高于大陆地壳丰度,更有可能成为本区的矿源层。岩体微量元素、稀土元素地球化学研究表明,矿区石英斑岩与矿区外围西南约4公里处的岩体具有相似的微量元素及稀土元素特征,推测两者是同源的,具有一定的演化关系,源岩可能为上地壳贫粘土的变质砂岩,具有明显的岛弧一同碰撞岩浆的特点。另外通过矿区石英斑岩与冈底斯地区斑岩体的对比研究,显示出与主碰撞期斑岩具有相似的地球化学特征,而与后碰撞伸展阶段的斑岩体有着较大的差别,推测本矿区的石英斑岩是印-亚大陆碰撞阶段,由于地壳缩短加压升温引起的上地壳物质部分熔融作用的结果。通过矿区蚀变围岩的化学成分研究,表明来姑组砂岩、灰岩在热液作用过程中主要带出了Al₂O₃、K₂O、SiO₂等物质成分,带入了CaO、Fe₂O₃、MgO等;板岩在矿化过程中,带入的是CaO、SiO₂等,带出的是Al₂O₃、Fe₂O₃、MgO、Na₂O等,这与近矿砂岩、灰岩广泛发育碳酸盐化、矽卡岩化,矿体附近板岩多发生碳酸盐化和硅化的地质事实一致;根据矿体附近蚀变的围岩与远矿围岩的对比研究,表明围岩在热液作用后带出了Cu、Pb、Zn等成矿元素。以上表明了本矿床与热液交代作用关系密切。矿石金属硫化物稀土元素研究表明矿石和来姑组地层岩石具有相近的稀土元素配分模式,均具有轻稀土富集,重稀土亏损,为相对平缓的右倾型,同时具有明显的Eu负异常和Ce无异常或较弱的正异常,表明其在成矿物质来源上具有某种亲缘关系。矿石硫、铅同位素,碳氧同位素研究表明,矿区地层为蒙亚啊矿床的形成提供了一定的成矿物质,成矿物质来源具岩浆和地层双重作用、两种来源特点。根据矿石中Pb、Zn没有相关性,赋矿围岩在热液交代中Zn元素相对亏损明显,而岩体的成矿元素与大陆地壳相比Pb元素富集趋势明显,推测成矿物质Pb可能主要来自岩体,Zn可能更多的来源于地层。综合矿床成矿地质特征,提出本矿床为层控矽卡岩型铅锌矿床,认为是在热水沉积的基础上,在喜山期发生的印-亚板块碰撞作用产生地壳物质熔融形成广泛的岩浆作用的环境下,通过含有一定成矿物质的岩浆期后热液长期的顺层交代、淋滤地层成矿物质形成富矿流体,在特定的岩性组合条件下沉淀成矿。更多还原

【Abstract】 As one of the most important types of ore deposit in Gangdese area of Tibetan, skarn-related ore deposits are currently the main source of iron, copper, lead, zinc and silver production in Tibetan. The Mengya’a lead and zinc deposit in Tibet, which is located in Nyainqentanglha metallogenic sub belt of south Gangdise-Nyainqentanglha metallogenic belt（retro-arc depressions and uplifts） , is a significant and typical skarn ore deposit in this area.Based on ore deposit geology, by studying geochemistry of the ore deposit comprehensively and discussing the origin of ore-forming substances, this thesis suggests that Mengya’a lead and zinc deposit is of stratabound skarn type. Finally, the genesis of the ore deposit has been specified.Geochemical studies on major and minor elements in strata demonstrate that Laigu Formation and Luobadui Formation have been influenced differently by hydrothermal sedimentation during their sedimentation and diagenesis. Meanwhile, geochemical studies on minor elements in strata suggest rocks from Laigu Formation were deposited under aerobic conditions, In sharp contrast, rocks from Luobadui Formation were deposited under anaerobic conditions. Diversity in contents of minor elements in rocks from Laigu Formation and Luobadui Formation were resulted from different tectonics and origins of sediments. Conclusions made from element geochemical researches correspond with idea that sedimentary environments of Laigu and Luobadui Formations are rifted continental margin and rifted arc respectively. Studies on ore-forming elements in strata manifest that Cu> Pb and Zn contents of Laigu Formation generally higher than those of Luobadui Formation and obviously surpass the abundance of continental crust, which makes Laigu Formation possible to be the potential source bed of this area.Geochemical studies on minor elements and REE （Rare Earth Elements） show that, characteristics of minor elements and REE of quartz porphyry in mining area closely resemble those of rock masses 4 kilometers southwest to the mining area, from which it could be inferred that the 2 rock masses share a same source and have a relationship considering the evolution process. The source rocks possibly are clay-depleted metasandstones from upper crust, which has obvious island arc -syn-collisional natures. Additionally, comparative research on quartz porphyry in mining area and porphyry masses in Gangdise areas demonstrates the geochemistry of the 2 sorts of porphyry are similar to that of porphyries of main collision stage but different to that of post collision extending stage. So it could be inferred that quartz porphyries in this area were resulted from partial melting of materials from upper crust induced by increasing temperature and pressure that caused by shrinking crust.Study on chemical composition of altered wall rocks in mining area suggests that substances like Al₂O₃ K₂O SiO₂ emigrated from sandstones and limestones during hydrothermal process, meanwhile CaO -, Fe₂C>3 and MgO migrated in . However, considering plates, Al₂O₃ Fe₂O₃> MgO and Na₂O emigrated out and CaCK SiC>2 migrated in , which is consistent with the geology that carbonatization and skarnization are present in sandstones and limestones adjacent to ore bodies, in addition, carbonatization and silicification usually exist in plates close to ore bodies. According to comparative study on altered wall rocks adjacent to ore bodies and wall rocks away, hydrothermal fluids leached ore-forming elements as Cu> Pb and Zn out of wall rocks. Above has demonstrated that the ore deposit is strata bound and entwined with hydrothermal metasomatism.Researches on REE in metal sulphides manifest that, REE distribution patterns of ores and rocks from Laigu Formation are quite similar, which represents being rich in light REE and depleted in heavy REE. The curve is gently right-dec lined. Furthermore, obvious Eu negative anomalies and Ce invisible or weak positive anomalies, which suggest that with respect to the origin of ore-forming substances, ores and rocks from Laigu Formation have a genetic relationship with each other somewhat.Study on lead, sulphur, carbon and oxygen isotopes shows that, strata provided a certain amount of ore-forming substances for Mengya’a ore deposit. The source of ore-forming substances has undergone dual effects by magma and strata, and has characteristics of the both. According to the facts that no correlations exist between Pb and Zn in ores, ore-bearing wall rocks are relatively more depleted in Zn during hydrothermal metasomatism. However, comparing to continental crust, rock masses are more enriched in Pb, from which it could be inferred that Pb in ore-forming substances is probably sourced from rock masses. As for zn, it is possibly derived from strata.Considering geology and mineralization as a whole, the ore deposit is one of strata bound skarn lead and zinc deposits. Beginning with hydrothermal sedimentation, in such environment with extensive magmatism resulted from melting of substances of crust caused by collision of Indian plate and Asian Plate in Himalayan, the mineralization was formed in certain lithologic associations by ore- bearing fluid that resulted from long term metasomatism along the beds and leachinj ore-forming substances by magma hydrothermal fluids with certain ore-forminj substances.更多还原