西藏甲玛铜多金属矿矿床地质特征及其矿床模型

Geological Features and Metallogenic Model of the Jiama Copper-Polymetallic Deposit in Tibet

甲玛铜多金属矿是西藏冈底斯中段东部取得找矿突破的超大型矿床, 探明夕卡岩型矿体铜钼铅锌金银均达大型以上规模, 初步探明角岩型矿体铜钼金属资源量也达大型以上规模。通过详细的矿体地质特征、岩浆岩岩石地球化学特征、成岩成矿年代学等方面的研究, 认为矿床类型属于典型的与斑岩有关的夕卡岩型-角岩型铜多金属矿。夕卡岩型矿体分布于晚侏罗世多底沟组与早白垩世林布宗组的层间扩容空间中, 角岩型矿体赋存在角岩中。矿床规模宏大, 具斑岩成矿系统的围岩蚀变和矿石特征, 识别出6种矿石类型、29种金属矿物和四期围岩蚀变。成矿元素的平面分带由浅部向深部由Pb＋Zn（Au＋Ag）→Pb＋Zn（Cu＋Au＋Ag）向Cu（Mo＋Au＋Ag）→Cu＋Mo（＋Au＋Ag）→Mo演化, 构成了一个完整的与岩浆作用有关的成矿元素分带、矿石矿物分带。含矿岩浆岩SiO2变化于59.58%~73.16%, 表现为富K2O, 过铝质, 低Mg, 并富F（平均0.08%）、Cl（平均0.02%）的特点, 为过铝质高钾钙碱性和钙碱性岩, 稀土元素总量变化在70.35×10-6~175.01×10-6之间, 平均为116.47×10-6, 高Sr、低Y和Yb, 具明显的正Sr异常和明显的Nb、Ta、Ti负异常, 大离子亲石元素Rb、Ba、Sr富集, 在Y-Sr/Y图解中投点于冈底斯含矿斑岩区, 具有高钾似埃达克质岩特点, 具C型埃达克岩特征。在岩浆演化过程中显示钙碱性岩系-高钾钙碱性岩系的演化趋势, 表现为闪长玢岩-花岗斑岩的岩石系列, 并显示一定的岩浆混合特征, 基性岩浆的混合有利于铜多金属矿成矿, 特别是伴生金、银的高含量与此有关。无矿斑岩脉或外围岩体的锆石LA-ICP-MS U-Pb年龄在16.27±0.31 Ma（MSWD=1.9）-15.99±0.34 Ma（MSWD=2.5）, 具有成矿前侵位的特征; 含矿斑岩脉的锆石SHRIMP年龄在14.2±0.2 Ma和14.1±0.3 Ma之间, 稍晚于主成矿期; 斑岩型矿石中的辉钼矿Re-Os同位素等时线年龄为14.78±0.33 Ma, 角岩型矿石中辉钼矿Re-Os同位素等时线年龄为14.67±0.19 Ma, 夕卡岩型矿石成矿时代也在15 Ma左右（14.5-15 Ma）, 主成矿期在中新世Langhian期。与辉钼矿的成矿年龄相比, 花岗斑岩的侵位年龄稍早, 而闪长玢岩相对较晚。由此, 建立了基于推覆滑覆构造控制矿体分布, 岩浆侵位后提供物质来源的矿床模型, 为区域找矿指明了方向。

The Jiama copper-polymetallic ore deposit is a superlarge ore deposit in the eastern part of central Gangdise. Its discovery is a breakthrough in the exploration work in that skarn-type Cu, Mo, Pb, Zn, Au and Ag and hornfels-type Cu-Mo in this ore deposit all reach large-size reserves. Geological features of the ore bodies, geochemistry of magmatic rocks and chronology of diagenesis and mineralization indicate that Jiama is a typical skarn-hornfels type copper polymetallic ore deposit related to porphyry. The skarn-type ore bodies are distributed in the expansion space between Late Jurassic Duodigou Formation and Early Cretaceous Linbuzong Formation, whereas the hornfels-type ore bodies occur in hornfels. There exist six ore types, twenty-nine metallic minerals and four periods of wall rock alteration in this ore deposit, characterized by wall rock alteration and ores of the porphyry metallogenic system. From top to bottom, metallogenic element zoning is in order of Pb＋Zn（Au＋Ag）→Pb＋Zn（Cu＋Au＋Ag）→Cu（Mo＋Au＋Ag）→Cu＋Mo（＋Au＋Ag）→Mo, constituting a complete metallogenic element zoning and ore mineral zoning related to magmatism. The ore-bearing magmatic rocks contain 59.58%~73.16% SiO2 and have peraluminous nature; in addition, they are enriched with K2O, F（0.08% on average） and Cl （0.02% on average）, but is poor in Mg. These data indicate that the ore-bearing magmatic rocks are peraluminous high-K calc-alkaline rocks and regular calc-alkaline rocks. The ore-bearing magmatic rocks also have a total of rare earth elements between 70.35×10^-6 and 175.01×10^-6 （116.47×10-6 on average）, higher Sr, lower Y and Yb, with an obvious positive Sr anomaly and evident negative anomalies of Nb, Ta and Ti, and also have relatively abundant Rb, Ba and Sr. In the diagram of Y-Sr/Y, these magmatic rocks belong to the ore-bearing porphyry rocks of Gangdise, somewhat with the features of high-K adakitic rocks, like C-type adakite. The magmatic evolution indicates the evolutionary trend of regular high-K calc-alkaline rocks characterized by the rock series of diorite porphyrite and granite porphyry and features of certain magma mixing. The basic magma mixing was in favor of mineralization of copper polymetallic ore deposit, especially the mineralization of associated high-content Au and Ag. The U-Pb ages of barren porphyry dykes or peripheral wall rocks dated by LA-ICP-MS of zircons are 16.27±0.31 Ma（MSWD=1.9）~15.99±0.34 Ma（MSWD=2.5） indicating that the intrusive activity took place before mineralization. The SHRIMP U-Pb ages of zircons from the ore-bearing porphyry dyke are 14.2±0.2 Ma and 14.1±0.3 Ma, implying that the intrusive activity was slightly later than major mineralization. Re-Os isotopic isochron age of molybdenite from porphyry-type ore is 14.78±0.33 Ma, that of hornfels-type ore is 14.67±0.19 Ma, and that of skarn-type ore is also about 15 Ma（14.5-15 Ma）. These data suggest that the major mineralization period should be Miocene Langhian Stage. Therefore, this study presents the ore-forming control based on the nappe-slipping structure, and also puts forward the model of the ore deposit which holds that magma provided mineral sources after the intrusion. Therefore, this study gives further guide for regional prospecting.