碰撞造山型斑岩铜矿蚀变分带模式--以西藏冈底斯斑岩铜矿带为例

岛弧环境斑岩铜矿蚀变分带模式已为人们所熟知 ,但碰撞造山环境的斑岩铜矿蚀变分带特征尚不清楚。对此 ,文中以西藏冈底斯斑岩铜矿带为例 ,选择驱龙、冲江、厅宫 3个典型斑岩铜矿 ,对其蚀变系统进行了系统研究。依据蚀变矿物组合可分为 3个蚀变带 ,呈环带状分布。从中心向外依次为钾硅酸盐化带、石英绢云母化带、青磐岩化带。泥化带不太发育 ,通常叠加在其它蚀变带之上。钾硅酸盐化带主要蚀变矿物为钾长石、黑云母、石英、硬石膏 ,伴有大量的黄铜矿与辉钼矿 ,是成矿物质的主要堆积区。石英绢云母化带与钾硅酸盐化带渐变过渡或叠加其上 ,是次于钾硅酸盐化带的储矿部位。蚀变矿物组合为绢云母 +石英 +钾长石 ,金属硫化物有黄铁矿、黄铜矿、辉钼矿、斑铜矿 ,少量的方铅矿、闪锌矿。主要的辉钼矿以石英 +辉钼矿脉的形式出现于本矿带。青磐岩化在斑岩体内不发育 ,矿化极微弱。蚀变岩石组分分析表明 ,岩石蚀变及其分带是岩浆流体 /岩石反应时K ,Na ,Ca ,Mg等组分迁移的结果 ,矿化伴随着蚀变发生。钾硅酸盐化带、石英绢云母化带和青磐岩化带的蚀变岩石与未 (弱 )蚀变斑岩具有一致的稀土配分模式 ,REE含量有规律地变化 ,说明蚀变岩石均经历了源于岩浆的流体的交代 ,不同的蚀变形成于岩浆流体演化的不同阶段。蚀?

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 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.