内蒙古狼山—渣尔泰山中元古代SEDEX型矿带火山活动与成矿的关系

Relationship between Mineraizing Process and Synsedimentary Submarine Volcanic Activity in the Langshan—Zhaertai Mesoproterozoic SEDEX Belt,Inner Mongolia

狼山—渣尔泰山中元古代SEDEX型成矿带与世界典型SEDEX型矿床有重要差别,成矿期间火山活动明显,在霍各乞、东升庙、炭窑口矿床唯一含矿岩组中各有岩类不同的变质海相火山岩夹层。依据:①基性火山岩的Sm-Nd等时线年龄为1491～1824 Ma(稍老或接近各矿床的铅模式年龄),ε_(Nd)(t)=3.48～6.4、轻稀土富集、重稀土亏损,其成岩物质来自地幔或下地壳;②一些矿石也是轻稀土富集、重稀土亏损、与火山岩夹层的稀土球粒陨石标准化曲线具有相似性;③各种硫化物的铅同位素都有一些分布在地幔和下地壳铅演化曲线附近;④部分黄铁矿的Co/Ni》1;⑤在有些矿石中还新见到火山岩块与岩屑、凝灰岩块和晶屑;⑥各矿床的Zn、Pb、Cu、Fe硫化物矿体都是在火山岩夹层的上覆地层中大量产出,甲生盘矿床缺少火山岩(只有凝灰岩)夹层时,没有铜矿体,而霍各乞、东升庙、炭窑口矿床含矿岩组中有火山岩夹层时,便都有铜矿体,可以推测同沉积期内间歇性的小规模海底火山喷发与成矿关系密切:它既直接提供了一部分成矿物质(与火山岩夹层同源),又导致长期存在地热异常,极有利于海底热水对流系统的形成与循环,使深部富含Zn、Pb、Cu、Fe的成矿流体不断地顺同生断裂上升至海盆喷流沉积成矿。这也证明元古宙SEDEX型矿床成矿作用的多样性和复杂性。

The Langshan — Zhaertai Mesoproterozoic SEDEX metallogenic belt located in the western part of the northern margin of the North China Platform comprises the Huogeqi, Dongshengmiao, Tanyaokou Jiashengpan deposits and some occurrences, which are different from the middle Proterozoic typical SEDEX deposits in the world in the fact that the meta-basic volcanic rocks in Huogeqi, the meta-double-volcanic rocks in Dongshengmiao and potassic spilite with blastoporphyritic and blastic-glomeroporphyritic texture in Tanyaokou deposits have been picked out from the only ore-bearing second formation of the Langshan Group in recent years. The metallogenic process of these deposits are closely relevant to the synsedimentary volcanic activities based on the following facts:① The Sm-Nd isochron age of the basic volcanic rocks varies from 1491 Ma to 1824 Ma (more than or close to the model age of the lead isotope of all sulphide minerals)in these deposits, withε_(Nd)(t) =(3. 48～6. 4) ± 0. 8,and the REE composition of them is enriched in light REEs and depleted in heavy REEs, which indicate these volcanic rocks, formed in the pull-apart basins along the north margin of the North China platform during the Mesoproterozoic, were derived from the mantle or lower crust. ②The REE composition of some Pb-Zn-Py-ores is also enriched in light REEs and depleted in heavy REEs. The chondrite-normalized REE patterns of them are similar to the patterns for the basic-volcanc rocks. ③ In the lead isotope composition diagram of Doe and Zartman (1979), most of 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.④The Cobalt content (ppm) of some pyrites is much higher than the nickel content of them (Co/Ni = 11.91～12. 19). ⑤ Some volcanic blocks and debrises have been picked out from some pyrite and pyrrhotite ores, which show the source of ores directly derived from volcanic eruption and the variety and complexity of the metallogenic process.⑥All Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying the metamorphic volcanic rocks in the only ore-bearing second Formation. In the Jiashengpan deposit lacking synsedimentary volcanic rocks in the host succession only Pb and Zn ores occur without Cu ore, but the Dongshengmiao, Tanyaokou and Huogeqi deposits with synsedimentary volcanic rocks in the host succession contain Cu ore, indicating relatively high ore-forming temperatures, besides Pb and Zn ores. The synsedimentary volcanic eruption directly supplied some ore-forming metals, and resulted in secular geothermal anomaly which is favorable for the circulation of the submarine convective hydrothermal system, resulting in the precipitation of deep mineralizing fluids exhaling into anoxide basins along the synsedimentary fault system in the Langshan-Zhaertai rift.