粤东北嵩溪银锑矿床地质地球化学及成矿模式

【作者】 肖振宇；

【导师】 岳书仓；

【作者基本信息】 合肥工业大学 ， 矿床学， 2001， 博士

【摘要】 粤东北嵩溪银锑矿床位于华南加里东造山带南缘永（定）—梅（县）古生代坳陷带内，寨岗上断陷火山盆地北缘。三叠纪至早侏罗世为裂谷环境，中侏罗世处于挤压构造背景，晚侏罗世开始再次受到拉张，形成陆相火山盆地，并伴有大规模的中-酸性火山-侵入活动。嵩溪银锑矿床形成于晚侏罗世晚期至早白垩世，与这一时期的岩浆活动有关。 矿床主要容矿围岩为下侏罗统金鸡组黑色含炭沉积碎屑岩系，下伏一层玄武质熔岩。在区域上，该层位地层中的成矿元素含量与沉积岩中的平均含量相近偏低，而矿区地层中的成矿元素含量很高，表明矿化作用大大升高了成矿元素在矿区围岩中的含量，嵩溪银锑矿床不大可能是海底火山喷流-沉积或喷流沉积-改造矿床，而具有浅成低温热液矿床的成矿元素分布特征。嵩溪银锑矿床矿化作用主要受断裂构造控制，矿体主要是沿着断裂构造充填交代而成，主要容矿构造为北东向断裂，其次为东西向断裂。由于北东向断裂在矿区背斜的北东翼与地层产状相近，因而以往被认为是层状和似层状矿体，实际上矿体与地层呈低角度斜交。在背斜南西翼，北东向矿体则明显与地层高角度斜交，并可进入中侏罗统漳平群地层，更进一步证明了断裂构造对矿体的控制作用。此外，矿区Ag和Sb异常波集中心沿着背斜轴部分布，亦证明了矿化作用明显受构造控制。矿体中发育大量的脉状、细脉状、网脉状和梳状等充填交代构造，由矿体向围岩常可见脉状→网脉状→细脉浸染状→蚀变围岩的渐变过渡现象，这些构造特征以及热液破碎角砾岩的出现，为嵩溪银锑矿床的浅成热液成因提供了有力证据。 在嵩溪矿区，纹层状、草莓状和结核状构造为沉积成因黄铁矿所特有。与主成矿期热液成因黄铁矿相比，这类黄铁矿中的强不相容元素Co和Ni含量高（与同沉积期海底基性火山活动有关），但成矿元素却低两个数量级。成矿期后细脉状黄铁矿则继承了沉积成因黄铁矿的微量元素特征。这表明主成矿期的成矿物质不是来自容矿地层，而成矿期后细脉状黄铁矿则可能是围岩中沉积成因黄铁矿被活化、经过近距离迁移后再沉淀的产物。此外，纹层状和结核状黄铁矿的Ni/Co小于1（分别为0.36和0.57），具有沉积成因黄铁矿的微量元素地球化学特征；而主成矿期黄铁矿的Ni/Co则大于1，是嵩溪银锑矿床浅成热液成因的重要证据。 矿石矿物中除了硫化物外，还有一组特征的含硫盐矿物：银黝铜矿、含银辉锑矿、黝锑银矿等。矿石组分以富银、锑、砷为特征。成矿元素在剖面上和平面上具有分带现象，剖面上，上部富Sb、As；中部富Ag、Au、Pb、Zn；下部富Mo和Cu。围岩蚀变包括硅化、黄铁矿化、碳酸盐化、绢云母化及青磐岩化。蚀变作用具分带现象，从矿体中心向外依次为硅化黄铁矿化、绢云母化、碳酸盐化和青磐岩化。在上部还出现由于酸性淋滤作用产生的硅质帽。 嵩溪银锑矿床中的流体包裹体数量多、个体大，以气液包裹体和纯液相包裹体为主，还见含CO₂、NaCl子晶的多相包裹体。流体包裹体研究和矿物共生分析证明，成矿流体在迁移过程中发生过沸腾，成矿溶液为Na⁺—Ca²⁺—∑S^2-—HCO₃^-型，主阶段成矿温度在 130C～280oC；成矿溶液的盐度为 2．23～19．4’t％NaCI 当量，平均 9．4。t％NaCI当量；成矿溶液nH值分布在4．66～5．65之间，随成矿温度降低略有升高；成矿体系硫逸度和氧逸度随成矿温度降低而降低，但在Fe－S－0相图中均处在黄铁矿稳定域。 矿床中的热液成因黄铁矿、闪锌矿和毒砂的6“S＝4．0 9’hx9’hx4．2％，呈塔式分布，5吨集中在 0附近，表明其成矿体系具有均一化硫的同位素组成，而辉锑矿的 6一S变化范围较大（0．2％－－11．3％），除一个样品外均为负值，这是硫同位素分馏作用的结果。矿床中的碳具有岩浆源碳与有机碳混合的碳同位素组成，其中的有机碳可能来源于容矿围岩中有机质热演化过程中的去碳作用（轻烃气体的形成）。根据氢、氧同位素研究结果，嵩溪银锑矿床的上升成矿流体是由岩浆热液与循环地下水热液混合而成，这种混合成因含矿热液上升到地热系统上部与低矿化度、低8‘勺的低温地下水发生了不均匀混合，这种混合作用导致金属矿物沉淀，但成矿物质是由上升成矿流体带来的。铅同位素研究也表明，嵩溪银锑矿床成矿物质既不是来自容矿含碳沉积地层，也不是来自早株罗世玄武岩岩浆体系，而是由来自深部物质与地壳物质的混合，与成矿流体来源的推论一致。 本论文还重点对有机质与成矿的关系进行了研究，结果表明，容矿围岩中的有机碳含量变化范围大（0刀3％y．0％），但与成矿元素 Ag和 Sb的含量之间没有明显的相关关系。因此，有机碳没有直接参与金属元素的沉淀作用，但金鸡组有机质热成熟过程中释放出来的气体组分可能影叼成矿流体系统的物理化学条件，从而导致金属元素沉淀。沥青、次石墨和干酪根是构成矿区黑色岩系的主要有机质，有机质镜质体反射率在 1．75 W3二0％之间，经历了高成熟至过成熟演化，表明矿区围岩受到了后期热事件影?更多还原

【Abstract】 The Songxi Ag-Sb deposit, which is located in the northwest of Guangdong province of China, is formed in the northen margin of Zaigangshang volcanic fault basin developed in the Yongding-Meixian Paleozoic geotectogene. The extension affecting this region in late Tertiary and early Jurassic period was followed by a regional compression of middle Jurassic. During late Jurassic, this region was again in an extensional environment which resulted in the formation of Mesozoic volcanic basins and large scale volcano-intrusive complexes of acidic composition. Songxi Ag-Sb deposit formed in the end of Jurassic and the beginning of Cretaceous, and was genetically related to the late Jurassic magmatism. The ore of Songxi Ag-Sb deposit is developed in the Jinji formation of early Jurassic period and is hosted by a suite of basaltic volcano-sedimentary rocks under which there is a sheet of basalt. The concentrations of metal elements in Jinji formation outside the mining area is similar to or lower than the averages in sedimentary rocks all over the world. This fact challenges the argument that the metals in Songxi Ag-Sb deposit is mainly from Jinji formation and is similar to the elemental distribution around an epithernial deposit where ore forming metals are leached by convecting solution. The ore is emplaced via metasomatism and filling mainly along the faults that trend dominately in northeast direction with a few of EW-trending. Because the occurrence of the NE-trending faults is close to that of the strata on the NE limb of the anticline developed in the mining area, the ore veins have previously been regarded to be stratabound. On the SW limb of the anticline, however, the veins filling along the NE-trending faults clearly cut the strata and can intrude the middle Jurassic Zhangping group, providing strong evidences that the mineralization in Songxi Ag-Sb deposit is developed as ore forming fluid flow in fractures. In addition, the enrichment center of Ag and Sb mineralization halo distributes along the axial of the anticline, also confirming the importance of structure in controlling the mineralization. The ore is characterized by veinlet structure, net-veined structure and comb structure, and the gradual outward transformation of ore structure from veined via net-veined and veinlet-disseminated to disseminated is common. These characteristics in ore fabrics and the occurrence of cryptoexplosive breccia are strong evidences that Songxi Ag-Sb deposit is a hydrothermal deposit. In addition to sulfide minerals, Songxi Ag-Sb deposit is characterized by a group of sulfosalts that are rich in Ag, Sb and As. Ore zoning has been recognized both vertically and horizontally. The upper part is Ag-Sb mineralized zone; the middle part is Ag mineralized zone with a little degree of Au, Pb and Zn enrichment, and minor enrichment of Cu and Mo occur in the lower part. The wall rock alteration includes silicification, pyritization, sericitization, carbonation, with siliciflcation and pyritization at the mineralization center and carbonation to the outmost. On top of the ore deposit, there is a silicated cover that formed during the acidic leaching. Framboidal pyrite with lamellar structure is unique of sedimentary pyrite in Songxi. This kind of pyrite is genetically related to the basaltic volcanism in early Jurassic period and, as compared with the pyrite in the ore-veins, is rich in highly incompatible elements such as Co and Ni, and poor in Ag and Sb. This fact demonstrates that the Jinji formation can not be the major source of the ore-forming metals of Songxi Ag-Sb deposit. In addition, N更多还原