THE ALTERATION ZONING MODEL OF PORPHYRY COPPER DEPOSIT IN COLLISIONAL OROGEN: CASE STUDIES OF PORPHYRY COPPER DEPOSITS IN GANGDISE BELT, XIZANG(TIBET)
THE ALTERATION ZONING MODEL OF PORPHYRY COPPER DEPOSIT IN COLLISIONAL OROGEN: CASE STUDIES OF PORPHYRY COPPER DEPOSITS IN GANGDISE BELT, XIZANG(TIBET)
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作者：
Meng Xiang-jin
岛弧和陆缘弧环境下斑岩铜矿的蚀变分带模式已为人们所认识，但碰撞造山带中斑岩铜矿的蚀变分带特征尚不清楚。为此，本文选取了喜马拉雅-青藏造山带内的驱龙、冲江、亭宫三个典型斑岩铜矿，对其蚀变分带进行了系统研究。喜马拉雅-青藏造山带碰撞后伸展作用形成的冈底斯中新世斑岩铜矿具有相似的蚀变分带特征。斑岩铜矿带三个区的蚀变填图表明，根据蚀变矿物组合可以区分三个带。从内向外依次为钾硅酸盐蚀变带、石英绢云母蚀变带和青岩蚀变带。最内侧的蚀变带（钾硅酸盐）以钾长石+次生黑云母+石英±硬石膏蚀变为特征，与石英绢云母蚀变带关系密切，并被其包围。在钾硅酸盐带中，黄铜矿-辉铜矿组合与石英脉群、细脉或石英-硬石膏脉石组合或石英-硬石膏脉伴生。石英-绢云母蚀变带叠加在钾硅酸盐蚀变之上，蚀变矿物组合为石英+绢云母+钾长石。该带是原始岩石成分与相关石英细脉的细粒置换，通常聚集在钾硅酸盐带中。在石英绢云母带中，硫化物组合由黄铁矿、黄铜矿、辉铜矿、辉铜矿和少量方铅矿、闪锌矿组成。在周围的火山岩序列中存在一个以大量方解石、绿帘石和方解石为特征的青岩蚀变带。受构造控制的泥化蚀变局部呈斑块状，叠加在其它蚀变带之上。粘土蚀变带的主要矿物有高岭石、石英、绢云母、黄铁矿、黄铜矿、铜蓝、辉铜矿和孔雀石。冈底斯斑岩铜矿带中，矿体多产于钾硅酸盐蚀变带中。驱龙、崇江地区主要蚀变带的蚀变体成分反映了蚀变过程中元素的得失。钾硅酸盐蚀变带、石英绢云母蚀变带和青玉蚀变带蚀变岩的稀土元素配分模式与新鲜岩石的稀土元素配分模式高度一致。蚀变岩的稀土元素由内钾硅酸盐带向外青岩蚀变带发生系统的变化。与成矿有关的元素迁移是斑岩岩浆流体与岩石之间不断反应的结果，并形成蚀变分带。斑岩铜矿的蚀变分布和成矿作用均受岩浆流体作用的控制。根据冈底斯带斑岩铜矿泥化蚀变不发育、缺乏表生富集带的事实，推断区域性隆升在同矿化期或成矿后期均未发生。斑岩型铜存款的成矿特征与形成存款的构造环境密切相关。
The alteration-zoning model of porphyry copper deposits in island-arc and continental margin arc settings has already been known, but the features of the alteration zoning of porphyry copper deposits in collisional orogen are not clear. So three typical porphyry copper deposits e.g. Qulong, Chongjiang and Tinggong, in Himalayan-Tibetan orogen, are chosen for systematically studying their alteration zoning. The Gangdese Miocene porphyry copper deposits generated during post-collisional extension in the Himalayan-Tibetan orogen have the similar features of alteration zoning. The alteration mapping in three districts in the porphyry copper belt shows that the three zones can be distinguished according to the altered mineral assemblages. The concentric alteration zones from the inner outward are successively the K-silicate zone, the quartz-sericite zone, and the propylitic zone. The innermost alteration zone (K-silicate) is characterized by K feldspar + secondary biotite + quartz ± anhydrite alteration and is closely related to and surrounded by the quartz-sericite alteration zone. In K-silicate zone, the chalcopyrite-molybdenite assemblage is associated with quartz vein swarms and veinlet or with gangue assemblage of quartz and anhydrite or quartz-anhydrite vein. The quartz-sericite alteration zone with altered mineral assemblage of quartz +sericite+ K feldspar is superimposed on the K-silicate alteration. The zone occurs as fine-grained replacements of original rock constituents with associated quartz veinlets, which usually cluster in the K-silicate zone. In quartz-sericite zone, the sulfide assemblage is composed of pyrite, chalcopyrite, molybdenite, bornite, and minor galenite and sphalerite. A propylitic alteration zone, characterized by large amounts of chlorite, epidote, and calcite, is present in the surrounding volcanic sequence. The argillic alteration controlled by structure locally occurs as patches, overprinted the other alteration zone. The main minerals in argillic alteration zone are kaolinite, quartz, sericite, pyrite, chalcopyrite, covellite, chalcocite, and malachite. In the Gangdese porphyry copper belt, the orebodies mostly occur in the K-silicate alteration zones. The altered bulk compositions of main alteration zones in Qulong and Chongjiang districts indicate the gains and losses of elements in alteration processes. The chondrite-normalized REE patterns of the altered rocks from the K-silicate zone, quartz-sericite zone and propylitic alteration zone, respectively, are highly coincident with those of the fresh rocks. The REE of the altered rocks systematically changed from inner K-silicate zone to outer propylitic alteration zone. The elemental mass transferring associated with mineralization resulted from the continuous reaction between the rocks and the fluids originated from the porphyry magmas, and resulted in the alteration zoning. The distribution of alteration and the mineralization of porphyry copper deposits both are controlled by the magmatic fluid process. It is inferred that the significant regional uplift did not take place at synmineralization time or post-mineralization time based on the facts of the weakly-developed argillic alteration and the lack of supergene-enriched zone in the porphyry copper deposits in the Gangdise belt. The mineralization characteristics of porphyry copper deposit closely related with the tectonic setting in which the deposit occurred.