西藏邦铺斑岩矽卡岩矿床成矿流体特征——来自流体包裹体及C_H_O同位素的制约

Characteristics of ore_forming fluids in the Bangpu porphyry_skarn deposit:Evidence from fluid inclusions and stable isotope compositions

选取西藏冈底斯斑岩成矿带东段的邦铺矿床斑岩矿区2条勘探线上的11个钻孔,进行了详细的岩芯编录和矿物组合、脉体穿切关系研究,将该矿床内与斑岩成矿相关的脉体划分为A、B、D脉3种类型。通过对矽卡岩矿区的详细地表及平硐观察,发现了石榴子石、阳起石、绿帘石等一系列代表流体演化特征的矿物。邦铺矿床具有典型斑岩型矿床的蚀变分带特征,从中心向外依次表现为黑云母化-硅化-绢云母化-青磐岩化,泥化呈"补丁状"无规则分布在绢云母化和青磐岩化之上。矽卡岩化则以典型矽卡岩矿物的出现为特征。A脉中绝大多数包裹体均一温度为320~550℃,盐度主要集中在两个区间内,分别为17.0%~22.0%(气液两相包裹体)和30.8%~67.2%(含子晶包裹体);B脉中绝大多数包裹体均一温度为380~550℃,盐度主要集中在1.6%~10.1%、23.2%~24.5%(气液两相包裹体)和30.8%~67.2%(含子晶包裹体)3个区间内;D脉中绝大多数包裹体均一温度为213~450℃,盐度为7.3%~11.6%。流体包裹体研究表明,与斑岩成矿的相关流体具有从高温、高盐度向低温、低盐度演化的特征;形成A、B脉的流体发生了强烈的沸腾作用,由此导致的压力波动是Mo、Cu沉淀的主要原因。16件与斑岩成矿相关的石英δDV-SMOW=-107.1‰^-185.8‰,δ18OV-SMOW=9.5‰~14.5‰;15件与矽卡岩成矿相关的石榴子石、绿帘石、石英及方解石δDV-SMOW=-184.7‰^-126‰,δ18OV-SMOW=3.9‰~12.9‰;4件斑岩成矿后期的方解石δ18OV-SMOW=-1.6‰~10.4‰,δCV-PDB=-6.5‰^-3.4‰;6件与矽卡岩成矿相关的方解石δ18OV-SMOW=1.8‰~11.9‰,δCV-PDB=-5.1‰~4.6‰。C_H_O同位素分析数据表明,邦铺整个斑岩-矽卡岩成矿系统流体主要经历了岩浆脱水去气和大气降水加入这两大地质过程。

Bangpu in Tibet is a large but poorly studied Mo-rich （--0.089%）, and Cu-poor （--0.32%） por- phyry deposit formed in a post-collisional tectonic setting. The deposit is located in the Gangdise porphyry cop- per belt （GPCB）, and formed at the same time （- 15.32 Ma） as other deposits within the belt （12--18 Ma）, although it is located further to the north and has a different ore assemblage （Mo-Pb-Zn-Cu） compared with other porphyry deposits （Cu-Mo） in this belt. Two distinct mineralization events have been identified in the Bang- puhyry deposit, which are porp Mo-（Cu） and skarn Pb-Zn mineralization. According to detailed drilling catalog and study of mineral assemblages and vein intersection relationship in 11 drill holes, veins in the Bangpu deposit can be divided into three types, which are A, B, D veins respectively. Also, minerals such as garnet and epidote are also been detected which can help to invert fluid evolution process in the skarn mineralization period. Along the circular alteration zonation from a central potassic alteration zone through silicifieation and phyllic alteration zones to an outermost propyiitie alteration zone, argillic alteration is present as patches within the phyllic and propylitic alteration zones. Skarn alteration is identified by the presence of typical skarn minerals within the sur- rounding rocks. A-type veins contain fluid inclusions that generally homogenize between 320℃ and 550℃, and have w （NaCleq） values that are generally split between salinities of 17.0 % -- 22.0 % （gas-liquid two-phase in- clusions） and 30.8 % --67.2 % （crystal-bearing inclusions）; fluid inclusions within B-type veins generally ho- mogenize at temperatures between 380℃ and 550℃, and have w （NaCleq） values that are clustered within three distinct intervals, i.e., 1.6%- 10.1%, 23.2%-24.5% （gas-liquid two-phase inclusions）, and 30.g% N 67.2% （crystal-bearing inclusions）; fluid inclusions within D-type veins have homogenization temperatures and salinities of 213-450℃ and 7.3 % -11.6 %, respectively. Fluid inclusion microthermometry indicates that the ore-forming fluids related to the porphyry mineralization evolved from high-temperature and high-salinity fluids to low-temperature and low-salinity fluids. Intense boiling occurring in A, B veins indicates that pressure fluctu- ation contributed a lot to the deposition of Mo and Cu. 16 quartz samples related to porphyry mineralization have the following H-O isotope data： δ DV-SHOW= -185.8‰- - 107.1‰, 318 OV_SMOW = 9.5‰- 14.5‰, while δ DV-SHOW = - 184.7‰ -- - 126‰, δ18OV-SMOW = 3.9‰ -- 12.9‰ for garnet, epidote, quartz, and calcite related to skarn mineralization. δ18OV-SMOW - 1.6‰ -- 10.4%, δ CV-PDB = -- 6.5‰ -- -- 3.4‰ for calcite in por- phyry period and δ18OV-SMOW = 1.8‰--11.9‰, δ CV-PDB = --5.1‰--4.6‰ for calcite in skarn period. All the C-H-O isotope data demonstrate two processes of fluid evolution in the Bangpu porphyry-skarn deposit, i.e., dehydration-degassing process and atmospheric precipitation addition process.