滇西宝兴厂斑岩铜钼金矿床成矿流体特征

Characteristics of ore-forming fluid of the Baoxingchang Cu-Mo-Au deposit western Yunnan China

宝兴厂斑岩铜钼金矿床是三江成矿带上与富碱斑岩有关的典型斑岩型矿床,产出于金沙江-哀牢山深大断裂带中部东侧。宝兴厂矿床铜、钼、金、铁等各类型矿化皆有发育,具有复杂的岩浆活动及热液演化。矿区岩浆岩主要为喜马拉雅期富碱复式岩体,包括正长斑岩、石英二长斑岩、花岗斑岩和斑状花岗岩等,具有多期次侵入特征。铜钼矿体主要分布于花岗斑岩和斑状花岗岩内部,铁金矿体主要分布于岩体内外接触带上,矿体呈脉状、透镜状或似层状。热液蚀变由内向外分带显著,依次为钾硅酸盐化（黑云母化）、绢云母化、青磐岩化（绿泥石-绿帘石化）,局部黏土化。本文通过系统的野外观测、详细的岩芯编录以及全面的岩相学观察,依据矿物共生组合、矿化热液脉体穿切关系及蚀变特征,将宝兴厂矿床内主要矿化脉体分为3类：A脉、B脉及D脉。通过对3类脉体内石英中流体包裹体的显微测温工作和成矿流体物理化学条件计算,剖析了成矿流体演化特征,探讨了成矿作用过程与成因机理。A脉与钾长石化和黑云母化蚀变关系密切,多为不规则脉状,宽约1~5mm,矿物组合一般为石英±钾长石±黑云母±少量黄铜矿±少量黄铁矿。石英多呈他形细粒,少量黄铁矿、黄铜矿沿石英颗粒边界呈浸染状产出。脉体中常含有黑云母、钾长石,两侧常见钾长石蚀变晕。A脉中一般没有矿化。B脉宽约15~30mm,矿物组合一般为：石英±辉钼矿±黄铜矿±黄铁矿。靠近脉壁的石英多为他形细粒,向中心转变为长柱状垂直于脉壁对称生长。硫化物呈线状分布于脉体的中心或边缘。B脉一般没有蚀变,偶见少量的绿帘石化-绿泥石化。D脉与绿泥石化-绢云母化关系密切,脉体规则连续,脉体宽度变化范围大,为1~30mm。矿物组合一般为石英±绿泥石±黄铁矿±少量黄铜矿。石英数量较少,多呈半自形-他形粗粒,相对于B脉黄铁矿含量明显增多,黄铜矿含量减少,呈浸染状分布,脉体中钾长石、黑云母常蚀变为绢云母和绿泥石,脉体两侧常具有绿泥石-绢云母蚀变晕。A脉形成于成矿早阶段斑岩尚未固结时,其流体包裹体以含子晶（NaCl子晶为主）多相包裹体和富气相包裹体组合为特点,均一温度为364~550℃,盐度分别集中在45.64%~52.89%NaCleqv（含子晶多相包裹体）和3.3%~16.34%NaCleqv（气液两相包裹体）两个区间内,该阶段流体显示出沸腾、不混溶及发生相分离特征。根据A脉中5个含石盐子晶的包裹体压力估算图,得出宝兴厂矿床A脉中LVH相包裹体被捕获时的最低压力为50~145MPa,按地压梯度27MPa/km换算,A脉形成的深度最少1.8~5.4km。B脉形成于成矿主阶段,石英中发育含子晶多相包裹体（NaCl子晶）和富气相包裹体,均一温度为210~410℃,盐度集中在34.24%~52.04%NaCleqv和5.23%~13.99%NaCleqv两个区间内,该阶段成矿流体发生减压沸腾作用,使得Cu、Mo、Au大量沉淀,根据NaCl-H2O体系P-T相图压力估算,B脉的形成压力大约为15~48MPa,形成深度为0.56~1.78km。D脉形成于成矿晚阶段,石英以发育大量富液相包裹体为特征,均一温度为223~303℃,盐度集中在3.53%~11.71%NaCleqv范围内,该阶段成矿流体以中-低温、低盐度的岩浆热液与大气降水的混合流体为主,流体压力也降低到15MPa,形成深度不超过0.56km。宝兴厂矿床热液流体演化总体趋势为：由早阶段的高温、中-高盐度的岩浆热液向成矿晚阶段中-低温、低盐度的岩浆热液＋大气降水混合流体转变。

The Baoxingchang Cu-Mo-Au deposit, tectonically occurring in the east-central part of the Jinshajiang-Ailaoshan suture zone, is one of the typical alkali-rich porphyry deposits in the Sanjiang metallogenic belt. The Cu, Mo, Au and Fe are lhe principal ore-metals developed commonly in the Baoxingchang deposit. Igneous rocks are mainly comprised of Himalayan alkali-rich complex, including syenitic porphyry, quartz monzonite porphyry, and porphyritic granite. Most Cu-Mo orebodies occur within the granitic rocks, while the Fe-Au orebodies distribute along the interface between the granitic rocks and sedimentary rocks, and also these orebodies occur mainly as vein-, lens- and stratoid-shape. Based on the field observation, hydrothermal alteration can be divided into four zones, zoning from proximal to distal as potassium-silicification （biotitization） zone, locally clayization zone, sericitization zone and propylitization zone. Three main types of hydrothermal veins have been identified： A-type, B-type and D-type veins based on its mineral assemblages, cutting relationship and alteration features. A-type veins with width of 1 ~ 5mm, occurred as irregular shape, are characterized by biotitization, K-feldsparization, and weak mineralization. Quartz, K-feldspar, biotite, less chalcopyrite and pyrite are the major mineral assemblages in this type of vein. Besides, K-feldspar alteration halo is commonly observed on both sides of A-type veins. B-type veins with a width of 15 -30mm, having cut through the A-type veins, have a straight contact with the wall rocks and no alteration halos were observed beside them. Mineral assemblages are mainly quartz, molybdenite, chalcopyrite and pyrite. Quartz developing close to the edge of veins are fined-grained and anhedral, and elongated and symmetrically grow vertically to vein-wall in the center of veins, while the sulfides are hnearly distributed in the center or edge of veins. D-type veins varying from 1 ~ 30mm in width and cutting through the earlier ore veins, are characterized by developing chlorite-sericite alteration halos on both sides of veins. Quartz, chlorite, pyrite and less chalcopyrite are the main minerals occurred in the veins. D-type veins mainly occurred in the wall rocks closing to the contact zone, with their boundaries more regular and continuous. The quartz in the veins are mostly subhedral or anhedral coarse-grained, with a low quantity, moreover, increasing amount of pyrite and reduced chalcopyrite are disseminated distributed within the veins, and also K-feldspar and biotite were usually altered into sericite and chlorite. In this paper, we conducted systematic micro-thermometric study on fluid inclusions of quartz from individual type of vein. Based on the outcomes, we concluded that the A-type veins were formed at the early mineralization stage while the porphyry was not consolidated yet. The classification of the fluid inclusion at this stage contains daughter mineral （NaC1 crystal）-bearing multiphase inclusions and vapor-rich water inclusions. Ore-forming fluids have the homogenization temperature of 364 ~ 550~C with the salinity ranging from 45.64% to 52. 89% NaCleqv （daughter mineral-bearing mnltiphase inclusions） and 3.3% to 16. 34% NaCleqv （vapor-rich water inclusions）. These signatures indicate that the fluids processes of boiling and immiscibility occurred in this mineralization stage. Based on the pressure estimation of five NaC1 crystal-bearing inclusions from the A-type veins, the calculated capture-pressure of daughter minerals-bearing multiphase inclusions are 50 ~ 145 MPa at least, corresponding to the forming depth of 1.8 ~ 5.4km using the geo-pressure gradient of 27MPa/km. B-type veins were formed during the main mineralization stage. Daughter mineral-bearing inclusions and vapor-rich water inclusions are well developed in the quartz. These inclusions have a homogenization temperature of 210 -410~C and salinities of 34. 24% ~ 52. 04% NaCleqv and 5.23% ~ 13.99% NaCleqv, respectively. Decompression and boiling of ore-forming fluids caused the precipitation of Cu, Mo and Au at this stage. Based on the P-T phase diagram in the NaC1-H20 system, the estimated forming pressure of B-type veins is 15 ~ 48MPa, with a corresponding depth of 0. 56 - 1.78km. D-type veins were formed at the late mineralization stage. Liquid-rich water inclusions are characteristically developed in the quartz. These inclusions have a homogenization temperature of 223 - 303℃, and salinity varying from 3.53% to 11.71% NaCleqv. All of those features show that the ore fluids have gone through the mixing of medium-low temperature, low-salinity magmatic hydrothermal fluids and the meteoric fluids. The ore-forming pressure at this stage has decreased to 15MPa, and the forming depth is lower than 0. 56kin accordingly. The entire evolving trend of the hydrothermal fluids of the Baoxingchang deposit is： early-stage high-temperature, high-medium-salinity magmatic fluids transforming to the mixing of magmatic fluids and meteoric fluids of medium-low temperature and low salinity at the late stage.