青藏高原碰撞造山带Pb-Zn-Ag-Cu矿床新类型：成矿基本特征与构造控矿模型

Thrust-controlled,sediments-hosted Pb-Zn-Ag-Cu deposits in eastern and northern margins of Tibetan orogenic belt:Geological features and tectonic model

地处青藏高原东、北缘的兰坪、玉树及沱沱河地区，广泛发育包括金顶超大型矿床在内的大量新生代Pb、Zn、Cu多金属矿床。这些矿床均产于该高原东缘晚碰撞构造转换环境，主体赋存于第三纪前陆盆地内部，以沉积岩容矿，与岩浆活动无关，受逆冲推覆构造系统控制，显著区别于世界已知的各类以沉积岩容矿的贱金属矿床。研究表明，伴随印度-亚洲大陆碰撞造山而产生一系列逆冲断裂系，将前陆盆地侧缘的中生代地层切割成叠置的构造岩片，并推覆叠置于盆地沉积地层之上，形成单冲式或对冲式逆冲推覆构造系统，并控制了Pb-Zn-Ag-Cu矿床的形成与发育。根据逆冲推覆构造控矿式样和矿化特征，可以识别出4种矿床式：①产于逆冲推覆构造系统前锋带“构造穹隆＋岩性圈闭”内的金顶式Zn-Pb矿床；②受控于前锋带冲起构造的河西-三山式Pb-Zn-Ag-Cu矿床；③产于主逆冲断裂带派生-的次级断层或平移断层内的富隆厂式Ag-Cu或Cu矿床；④产于主逆冲断裂上盘灰岩层间破碎带内的东莫扎抓式Pb—Zn矿床。这些矿床的矿体多受不同级次的断裂控制，多孔砂岩、白云岩化灰岩及构造破碎带是有利矿化部位。多数矿体显示开放空间充填成矿特点，少数显示层控性，属后生成矿。金属矿物组合主要为低Fe闪锌矿＋方铅矿＋黄铁矿组合及低温Cu硫化物（黝铜矿系列为主）＋Ag硫化物（辉银矿、黝银矿、汞银矿）＋方铅矿±闪锌矿组合，脉石矿物组合主要为方解石＋重晶石±萤石±白云石±天青石，局部见沥青。成矿流体以盐水体系为主，盐度w（NaCleq）变化于1％～28．0％之间，成矿温度较低，通常在80～190℃，显示盆地卤水±大气降水的特点。逆冲推覆构造系统对矿床的控制主要体现在：其深部拆离滑脱带可能是流体流长距离侧向迁移的优选通道，主逆冲断裂是成矿流体垂向运移和向上排泄的主要途径，浅部各类样式的逆冲构造是流体汇聚的主要场所。成矿物质以盆地沉积岩贡献为主，部分可能来自幔源岩石。矿床金属组合可能与成矿流体迁移一汇聚过程中流经岩石的性质有关：矿区发育灰岩建造时，出现zn_Pb（rZn多于Pb）矿化；若发育碎屑岩建造，尤其是红层，则出现Cu-Ag（-Pb）矿化。因此，笔者将这种逆冲推覆构造控制的新类型矿床称之为造山型Pb-Zn-Ag-Cu矿床，其成矿模式可表述为：伴随着印度一亚洲大陆持续碰撞，青藏高原东、北缘中生代构造岩片向盆地中央推覆并置，形成单冲式或对冲式逆冲推覆构造系统，流体从造山带沿拆离滑脱带长距离向前陆盆地方向运移，运移过程中淋滤围岩的金属物质，通过主逆冲断裂垂向沟通，进入浅部各式逆冲构造部位从而形成不同样式的矿床。经综合分析，提出了青藏高原东、北缘受逆冲推覆构造控制的贱金属矿床的勘查要素。

Numberous of Pb-Zn-Ag-Cu ore deposits with significant economic values occur in Lanping basin and Yushu and Tuotuohe areas on the eastern and northern margins of Tibetan orogenic belt. Being hosted by sediments and having no obvious affinity to igneous ac- tivities, these deposits occur in continent-eontinent （Indo-Asian） collisional orogen and are controlled by thrust-nappe structures, thus obviously different from the well-known sediments-hosted base metal deposits in the world. They were formed in Tertiary foreland basins developed from Paleo-Tethyan rifting basins and Mesozoic depression basins. During mineralization, thrust-nappe structures juxtaposed Mesozoic strata over Tertiary strata through a series of parallel reverse/thrust faults in the basins. The deposits are strictly controlled by thrust-nappe structures. There exist four types of ore deposits： the Jinding-type deposits is formed in a thrusting-related dome structure combined with lithologic trap, the Hexi-Sanshan-type deposit is controlled by a pop up structure, the Fulongchang- type deposits are located in thrusting-related second-order faults within elastics, and the Dongmozhazhua-type deposits are hosted by second-order faults within limestone of the hanging-walls of major thrust faults. Their ore bodies are controlled by faults, where sandstone, dolomitized limestone and structural cracking zones are favorable loci of ore concentration. Ores were mainly derived from openspace filling and replacement, resembling epigenetic mineralization. Metal sulfides are present as low-Fe sphalerite ＋ galena ＋ pyrite or Cu-sulfides （tetrahedrite, bornite, chalcopyrite, chalcocite） ＋ Ag-bearingsulfides （argentite, freibergite, Ag-tetrahedrite） ＋ galena ＋ sphalerite. Gangue minerals are calcite ＋ fluorite ＋ barite ＋ dolomite ＋ celestine ＋ quartz, with rare bitumen. Fluid inclusions contain mainly salt aqueous solutions and locally and subordinately CO2 and hydrocarbons, with homogenization temperatures predominately of 80-190℃ and salinities of 1 wt% -28 wt% NaC1 eq, indicative of basinal brines ± meteoric water as the source of oreforming fluids. Studies show that the thrust-nappe structures provided channelways for lateral and vertical migration of ore-forming fluids as well as open space for ore accumulation. Isotope data suggest that ore-forming materials were mainly derived from basinal sedimentary rocks with or without the contribution of mantle-derived rocks. Metal composition and abundance of a deposit depend probably on types of rocks that the ore-forming fluids encountered during their migration and accumulation. In the ore district, the existence of limestone generally corresponds to Zn-Pb （Zn 〉 Pb） mineralization, whereas the occurrence of clasties （especially for red bed） corresponds to Cu-Ag （-Pb） mineralization. A preliminary metallogenic model for the Pb-Zn-Ag-Cu deposits can be described as follows： India-Asian collision led to the lifting of the orogenic belt relative to the Tertiary foreland basins in the eastern and northern margins of the Tibetan plateau, and strong compression resulted in the thrusting of these basins. Consequently, fluids generated from the orogen migrated laterally along gently-dipping detachment faults of the thrust systems towards the basins and evolved into ore- forming fluids through interaction with country rocks. The ore-forming fluids migrated vertically along major thrust faults and finally accumulated in the open spaces induced by thrust-nappe structures at shallow levels to form ore deposits.