西藏冈底斯南缘努日铜钨钼矿床地质特征与矽卡岩矿物学研究

摘要

西藏山南地区努日铜钨钼矿床位于冈底斯火山-岩浆弧构造带东段南缘,是新近探明的一个大型矽卡岩型铜钨钼矿床.矿区内出露有白垩系比马组和旦师庭组及大量晚白垩世和古近纪的侵入岩.矿区内的矽卡岩呈层状、似层状产在白垩系比马组地层中,矽卡岩矿物主要为石榴子石、辉石、硅灰石、角闪石、绿帘石、符山石等；金属矿物主要有黄铜矿、黄铁矿、辉钼矿、白钨矿、斑铜矿、黝铜矿等.电子探针分析结果表明,矽卡岩矿物中石榴子石主要以钙铁榴石和钙铝榴石为主,辉石主要为透辉石,角闪石属于镁角闪石-阳起石,帘石主要为绿帘石.矽卡岩类型在水平和垂向上具有较好的分带性,依次由石榴子石矽卡岩过渡到透辉石矽卡岩,再过渡到透辉石硅灰石矽卡岩,这种分带特征表现了流体交代作用的变化.矿化类型和矿化组合也具有一定的分带性,浅部以矽卡岩型钨矿化为主；随着深度的增加,逐渐过渡为脉状的铜矿体或铜钼矿体,在局部较深的钻孔中还有少量的斑岩型矿化,主要以铜矿化为主,伴有较弱的钼矿化.石榴子石组分在垂向和水平方向上均具有规律性的变化,由钙铁榴石占主体逐渐过渡为钙铝榴石占主体.成分剖面显示石榴子石的组分和化学成分随着环带的变化而变化,说明石榴子石是由一种脉动式流体形成的,可能是由流体化学成分的自身再平衡和生长过程中流体流量的改变而引起生长速率的改变共同实现的.通过含铁律比值(Kp)的计算,得出努日矿床形成于弱酸性、较强氧化状态.结合矽卡岩矿物分布和成分变化特征,推测努日矿区的矽卡岩可能是由深部侵入体分异出的热液沿着层间的破碎带或断裂,经过较远距离的运移,与地层中的碳酸盐岩发生交代作用而形成.渗透交代作用可能是形成矿区矽卡岩的主要原因,流体的温度和氧逸度变化对于形成不同的矽卡岩矿物具有重要作用.努日矿床的矽卡岩为浅部矽卡岩,可能存在统一的斑岩型-矽卡岩型成矿系统,深部具有较大的找矿潜力.%The Nuri Cu-W-Mo deposit is a newly explored and proved large-size deposit, which lies on the southern margin of eastern Gangdese volcanic-magma arc and belongs to the southern subzone of the Gangdise metallo-genic belt. There are some Late Cretaceous and Paleogene intrusive rocks exposed in the ore district. The strata in the ore district are mainly Cretaceous Bima Group and Danshiting Group. The skarn mainly occurs within the carbonate rocks and lithologic conversion boundaries, not in direct contact with the intrusions. The skarn minerals are mainly gamet, pyroxene, wollastonite, vesuvianite, epidote and actinolite. Metallic minerals mainly include chalcopyrite, pyrite, molybdenite, scheelite, bornite and tetrahedrite. Electron microprobe analyses show that the garnet comprises mainly grossular and andradite, with minor augite in skarn. The end member of pyroxene is dominated by diopside. The amphibole in the Nuri deposit is magnesiohornblende-actinolite belonging to ferroedenite. The end member of epidote group is dominated by epidote. In the horizontal and vertical direction, the skam type shows significant zoning from garnet skarn through diopside skarn to diopside-wollite skarn from south to north in horizon profile and from shallow to deep in vertical profile. The zoning reflects the change of metasomation. Meanwhile, the mineralization also has zoning character. In the shallow part, the mineralization is mainly skarn type with tungsten mineralization. With the increasing depth, the mineralization type changes to vein mineralization with copper or copper-molybdenum mineralization. Some porphyry-type mineralization exposed in some drilling boles, characterized by copper mineralization with a small quantity of molybdenum. The components of gamet vary in the space, gradually changing from the dominance of andradite to that of grossular. The garnet records complex zoning patterns, the end member and chemical composition changing with the changing zonation. The type of zoning pattern demonstrates that the fluid which formed garnet was episodic, probably resulting from the geochemical self-organization and changes in fluid in association with garnet growth rates. The Kp shows that the deposit was formed in a weakly acid and fairly strong oxidation state. Combined with the distribution and composition variation, the authors infer that the skarn of the Nuri deposit was formed by metasomatism between carbonate and ore-forming fluid. The fluid might have originated from the deeper intrusion and migrated a long distance along the shatter zone or faults in the strata. Infiltration was probably the primary factor for the formation of skarn in the deposit. The changes of temperature and oxygen fu-gacity must have played an important role in the formation of skarn minerals. The skarn exposed in the Nuri deposit is the superficial skarn, with the probable existence of uniform skarn-porphyry ore-forming system. Therefore, there exsits great potential in finding porphyry Cu ore bodies in the deeper part of the existing skarn type mineralization, and this conclusion is of important significance for further exploration in the Nuri deposit and adjacent areas in south Tibet.