内蒙古狼山—渣尔泰山中元古代被动陆缘热水喷流成矿特征

THE CHARACTERISTICS OF HYDROTHERMAL EXHALATIVE MINERALIZATION OF THE LANGSHAN-ZHAERTAI BELT, INNER MONGOLIA, CHINA

狼山—渣尔泰山中元古代被动陆缘产有东升庙、炭窑口、霍各乞和甲生盘等热水喷流沉积矿床 ,与世界中元古代的SEDEX型矿集区有许多相似之处 :①矿床的产出受华北古陆北缘裂陷槽内三级断陷盆地控制 ;②各大矿床都具有鲜明的层控特征 ,所有矿体总体呈层产在中元古界的白云石大理岩、碳质千枚岩 (或片岩 )中 ;③矿石具有细纹层状、条带状构造 ,喷流沉积成矿特征十分明显 ;④成矿过程中伴有明显的同生断裂活动 ,它在一定程度上控制了矿体的空间分布及其组合 ;但不同矿床同生断裂活动的强度、时限、规模都不同 ,从而导致不同矿床在相同含矿岩组中矿体产出的先后顺序不同和大量层间砾岩与同生角砾状矿石的形成 ;⑤厚大Zn ,Pb ,Cu复合矿体具有明显的分带性 ,自下至上 ,Cu/ (Zn +Pb +Cu)比值由高→低 ;⑥重晶石层发育 ,多与黄铁矿层互层状产出 ,也有与闪锌矿层互层 ,但与世界典型SEDEX型矿床又有重要差别。成矿期间火山活动明显 ,在霍各乞、东升庙、炭窑口矿床惟一容矿的狼山群第二岩组中先后发现了具有变余斑状或聚斑状结构、变余杏仁构造的基性火山岩、钠质“双峰式”火山岩和钾质“双峰式”海相火山岩及凝灰岩夹层。结合①各种硫化物的铅同位素主要分布在地幔和下地壳铅演化曲线附近 ;

The Langshan-Zha'ertaishan hydrothermal exhalative belt in the passive continental margin consists of Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulphide deposits. Most of their ore-forming characreristics are much similar to that of the Mesoproterozoic SEDEX-type provinces all over the world. They are as follows: (1) All deposits of this type in the belt occur in the third-order fault-basins in Langshan—Zha'ertaishan aulacogen along the northern margin of the North China Platform. (2) These deposits with all their orebodies hosted in the Midproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have apparent stratabound nature. (3) Ores display laminated and banded structures, showing the clear depositional features. (4) There are some evidences for synsedimentary faulting, which to a certain extent account for the temporal and spatial distribution and the size of the orebodies in all deposits, and the formation of interbedded conglomerates and ore-breccias. (5) The big and thick Pb+Zn+Cu orebodies show lateral and vertical zonation of sulfides; the Cu/( Pb+Zn+Cu) ratio decreases gradually from the bottom top-wards. (6) Barite is interbedded with pyrites and sometimes with sphalerites. However, some characteristics. such as high Co/Ni ratio in the pyrites, the volcanic activities synchronous with metallogenesis, et al., of the Langshan-(Zha'ertaishan) metallogenic belt, are different from that of the typical SEDEX deposits of the world. The meta-basic volcanic rock in Huogeqi, the sodic “bimodal” volcanic rocks in Dongshengmiao and the potassic “bimodal” volcanic rocks with blastoporphyritic and blasto-glomeroporphyritic textures as well as blasto-amygdaloidal structure in Tanyaokou deposits have been discovered in the only ore-bearing second formation of the Langshan Group in the past decade. It is concluded that the metallogeny of some deposits hosted in the Langshan Group is closely relevant to the synsedimentary volcanic activities based on the following facts: (1) Most of the lead isotopes in sphalerite, galena, pyrite, pyrrhotite and chalcopyrite are plotted on both sides of the line for the mantle or between the lines for the mantle and lower crust in the lead isotope composition diagram of Doe and Zartman (1979). (2) The cobalt content of some pyrites is much higher than the nickel content (Co/Ni=11.91～12.19). (3) Some volcanic blocks and debris have been found from some pyritic and pyrrhotitic ores. (4) All the Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying the metamorphic volcanic rocks. (5) In the Jiashengpan deposit that lacks synsedimentary volcanic rocks in the host succession, only Pb and Zn ores occur without Cu ore, but in the Dongshengmiao, Tanyaokou and Huogeqi deposits with synsedimentary volcanic rocks in the host succession Cu ores occur. The process of synsedimentary volcanic eruption directly supplied some ore-forming materials, and resulted in secular geothermal anomaly favorable for the circulation of the submarine convective hydrothermal system, which account for the precipitation of deep mineralizing fluids exhaling into anoxidic basins along the synsedimentary fault system in the Langshan-Zhaertai rift. The Dongshengmiao, Tanyaokou, Huogeqi deposits hosted in Langshan Group appear to be a transitional type of mineral deposits between SEDEX and VMS-types but with a bias towards SEDEX, while the Jiashengpan deposit hosted in Zha'ertai Group is of a typical SEDEX one. These evidences, together with other new discoveries of Mesoproterozoic volcanic rocks and the features of metallogeny in the neighboring Bayan Obo deposit suggest the diversity, complexity and uniqueness of the middle Proterozoic Langshan-Zha'ertai-Bayan Obo ore belt.