西天山吐拉苏盆地安山岩中发现岩石包体及地质找矿意义

Rock Enclaves Discovered in Andesite in the Tulasu Basin, Western Tianshan, and Its Geological Prospecting Significance

新疆西天山斑岩型铜矿找矿前景良好,但面临攻坚;吐拉苏盆地是西天山浅成低温热液型金矿集区,晚古生代吐拉苏盆地是北天山洋向南向伊犁板块之下俯冲形成的"西南太平洋型"岛弧环境,在金矿深边部寻找斑岩型铜金矿值得关注。笔者最近在岛弧安山岩中发现硫化物矿化二长斑岩、细晶闪长岩等岩石包体。岩石包体发现于塔乌尔别克金矿区,多呈浑圆体或次棱角状,随机分布于安山岩中,与围岩界线截然,常见安山岩围绕岩石包体形成冷凝边。二长斑岩包体显示似斑状结构,斑晶由斜长石和钾长石组成;细晶闪长岩主要由斜长石和角闪石组成,细晶结构,块状构造;寄主安山岩为斑状结构,斑晶为斜长石和少量角闪石及单斜辉石,基质由短柱状斜长石及少量单斜辉石组成,玻基交织结构。二长斑岩包体、细晶闪长岩包体和寄主安山岩三者具有极为相似的微量元素和稀土元素组成,均富集大离子亲石元素、相对亏损Nb、Ta等高场强元素。这些岩石学和地球化学特点反映岩石包体是岛弧火山喷发过程中从下覆地质体捕掳成因,指示安山岩层之下存在同源斑岩体并可能已发生硫化物矿化。综合考虑吐拉苏盆地铜矿化点、酸性热液蚀变、高硫型和低硫型浅成低温热液金矿的组合关系,认为盆地内发育斑岩-浅成低温热液金铜成矿系统,塔乌尔别克是斑岩型铜金矿找矿重要靶区。

Western Tianshan, Xinjiang, is endowed with good exploration prospecting for porphyry-type copper deposit hut also faces great challenge. The Tulasu basin is an ore concentration area of the epithermal gold deposits in western Tianshan. During late Mesozoic, the basin was a southwestern Pacific-type island-arc environment resulting from southward subduction of northern Tianshan Ocean to the Yili plate. Special concern should be paid over porphyry-type Cu-Au deposit at depth or areas adjacent to those epithermal gold deposits during gold exploration. Some sulfide mineralized rock enclaves （monzonite porphyry and fine-crystalline diorite） were recently discovered in the andesite in the Tawurbieke gold district, Tulasu basin. The enclaves occurring generally as round or subangular are randomly distributed within andesite, which is distinguished from country rock, with a chilled margin formed around enclaves. The monzonite porphyry enclave exhibits porphyaceous texture, and the phenocrysts consisit of plagioelase and K- feldspar. The fine-crystalline diorite enclave is composed mainly of plagioclase and hornblende with aplitie texture and massive structure. The host andesite shows porphyry texture, and the phenocrysts eonsisit of plagioclase, minor hornblende and clinopyroxene~ the groundplasms are composed of short column plagioclase and minor clinopyroxene with hyalopilitic texture. The monzonite porphyry enclave, the fine- crystalline diorite enclave and the host andesite shows similar trace elements and the rare-earth-element compositions, such as a pronounced enrichment of large ion lithophile elements, depletion of high field strength elements. The petrological and geochemical features suggests that the rock enclaves were captured from the underlying body by the eruption process of island-arc magma, and the cognate porphyry body underlying the andesite strata have been mineralized. Based on copper mineralization, acid hydrothermal alterations, the high-sulfidation and the low-sulfidation epithermal gold deposits in the basin, it is believed that the porphyry-epithermal Cu-Au mettallogenic system might occur in the Tulasu basin, western Tianshan, and the Tawuerbieke may be an important target for porphyry Cu-Au exploration.