与基性岩有关的层控钨锡:“S”型花岗岩钨锡矿的前身——以云南钨锡矿为例

Stratabound Tungsten and Tin Related to Basic Rocks: the Precursor of the S-type Granite-related W-Sn deposits—Examples from the W/Sn deposits in Yunnan Province

以云南几个典型钨锡矿带为例,结合国内外前人成果,研究对比了层控钨锡矿床与含钨锡的"S"型花岗岩之间的关系,发现:含钨锡的"S"花岗岩均产出于早期地层富集钨锡甚至形成独立的钨锡矿床的地区,如元古界的云南西盟—缅甸完冷锡矿带,寒武系都龙—白牛厂钨锡矿带,石炭系腾冲—梁河锡钨矿带,三叠系的个旧锡矿带,新近系的薅坝地锡矿带等;而在地层中没有明显钨锡富集的层位,即使有地壳重熔形成"S"型花岗岩也不会有钨锡矿床产出,如云南西北部的鲁甸花岗岩、加仁花岗岩等。根据对层控钨锡矿床的成矿年代、矿石结构构造、容矿围岩变质属性等方面的综合分析,证明所谓的"‘S’型花岗岩与W、Sn、Mo、Bi成矿关系密切"的认识,实际上是富集钨锡的早期层位在地壳加厚或受到构造热侵蚀时发生重熔,并使钨锡按照矿质沉淀的温度序列重新分配与富集的结果。因此,"S"型花岗岩并不具有含W、Sn、(Mo、Bi)的专属性,而富集W、Sn的地层重熔才是形成W、Sn矿床的关键。从众多的实例来看,层控钨锡矿常常与深源的(碱性)玄武岩关系密切,而不一定与"S"型花岗岩关系密切。只要有早期的层控钨锡矿存在,该矿层被任何一种火成岩穿切或吞噬,矿质都可能被迁移至合适的位置重新沉淀而成矿。因此深入分析和对比含钨锡的层位及其形成背景条件,是扩大钨锡矿资源量的关键;而层控钨锡矿与"S"型花岗岩接触带(特别是外接触带)及其相关的构造裂隙,很可能是矿质运移再分配并形成富钨锡矿的有利空间。层控钨锡矿卷入变质核杂岩,含矿层位可以被抬升至较浅部位,不仅利于开采,而且在中心部位的隐伏花岗岩体也可以形成富矿,是进一步找矿的方向。

Taking several W-Sn ore belts in Yunnan as examples and referring to the research results from China and abroad, the authors investigated the relationship between the W-Sn bearing stratabound W-Sn deposits and the W-Sn bearing S-type granites. The results show that all the W-Sn bearing granites occur in areas where the earlier strata are enriched in tungsten and/or tin, such as the Proterozoic tin （-zinc） belt from Ximeng of Yunnan to Wanleng of Myanmar, the Cambrian tungsten-tin belt in Dulong-Bainiuchang of southeast Yunnan, the Carboniferous tin-tungsten belt in Tengchong-Lianghe of west Yunnan, the Triassic tin belt in Gejiu of southeast Yunnan; and the Neogene tin belt in Haobadi of west Yunnan. In contrast, the S-type granites （such as the granites in Ludian and Jiaren of southwest Yunnan）, hosted in the strata without dominant enrichment of W and/or Sn, have no tungsten and/or tin. An analysis of dating results, structures and textures of the rocks and/or ores and the property of the metamorphism of the host rocks shows that the essence of “the S-type granites’ affinities with mineralization of W, Sn, Mo, Bi” resulted from the sequence of the remelting of the preexistent strata with enrichment of W, Sn, Mo and/or Bi and redistribution of the ore-forming materials including W and/or Sn in precipitation order of the metals controlled by temperatures, under the condition of crust thickening or thermal disturbance of deep magma. Therefore,the S-type granites do not necessarily constitute the prerequisite of bearing W, Sn, Mo, and Bi. Correspondingly, the strata enriched in W and /or Sn are the key to mineralization of W and Sn. Consequently, the through analysis of the W and Sn-bearing strata and the setting of the formation is the key to enlarging the resources of W and Sn. A number of case studies of W-Sn ore deposits show that the stratabound W/Sn ore deposits are closely related to the （alkalic） basalts in the depth rather than to the S-type granites. Once there exists the early W/Sn–bearing sequences that are cut or engulfed, the mineral substances can be transported to specific spaces to reprecipitate and from ore deposits. The contact zone, especially the outer contact zone, and the related structure fractures, are favorable spaces for formation of the W and Sn deposits as a result of migration and redistribution of the ore-forming material. When the stratabound W/Sn ore deposits are involved in a metamorphic core complex, the ore-bearing sequence can be uplifted to a shallow level convenient for exploitation, and there might also exist the ore shoot at the contact of the concealed granite in the center of the core complex.