冈底斯西段鲁尔玛斑岩型铜(金)矿成矿流体性质及演化

Origin and Evolution of Ore-Forming Fluids in Luerma Porphyry Copper (Gold) Deposit from Western Gangdise

目前冈底斯成矿带报道的斑岩型矿床主要集中在东段,而鲁尔玛斑岩型铜(金)矿为冈底斯成矿带西段新发现的铜矿,具有钾硅酸盐化、绢英岩化、青磐岩等明显的斑岩型矿床蚀变特征.其热液脉体从早到晚化分为:钾硅酸盐化脉(A脉)、石英-金属硫化物脉(B脉)以及石英-绿帘石-碳酸盐化脉(D脉).对各阶段热液脉体的的流体包裹体进行了岩相学、显微测温、显微激光拉曼和H-O-C同位素等分析.发现A脉石英中流体包裹体的形成温度集中在390~460℃,盐度介于4.5%~21.6%NaCleqv和43.6%~59.6%NaCleqv两个区间;B脉石英中流体包裹体的形成温度集中在310~380℃,盐度介于3.6%~19.8%NaCleqv和6.0%~16.0%NaCleqv两个区间;D脉石英和方解石中流体包裹体的形成温度集中在200~320℃,盐度集中在0.4%~14.7%NaCleqv.拉曼分析表明,鲁尔玛铜(金)矿的流体包裹体含CO2、N2、CH4等气体及石盐子晶和多种金属硫化物和金属氧化物子晶.各热液脉体石英中流体包裹体的δDH2O,V-SMOW值的变化范围为-128‰^-110‰,δ18OH2O,V-SMOW值的变化范围为-9.09‰^-1.45‰,方解石的δ13CCal,V-PDB值的变化范围为-20.8‰^-19.8‰,δ18OCal,V-SMOW值的变化范围为-5.9‰^-4.9‰,展现出岩浆热液的特征,晚期还有大气降水的加入.研究结果显示,成矿流体属高温、高盐度、含CO2、N2、CH4等气体和Cu、Fe、Mo等金属元素的Ca+-Na+-Cl-H2O体系流体,具有典型的斑岩型铜矿床成矿流体的特征.成矿流体从深部封闭体系运移到浅部的开放体系,温压环境突变导致金属硫化物沉淀,形成A脉和B脉型矿化.随着成矿物质的大量析出,同时伴随着大气降水等因素的影响,流体温度、盐度迅速降低,产生D脉型矿化.

The current studies of the deposits in the Gangdese metallogenic belt is predominate in the eastern section, but the new discovered Luerma porphyry copper (gold) Deposit belongs to the western segment of Gangdise polymetallic metallogenic belt.The Luerma copper (gold) deposit developed typical porphyry deposits’hydrothermal alteration zones, which are divided as potassium-silicification zone, sericitization zone, clayization zone, and propylitization-propylitization zone from proximal to distal,respectively. Hornfels and malachite are also generally distributed in the mining area. Moreover, three main types of hydrothermal veins have been identified based on its mineral assemblages, cutting relationship and alteration features, which consist of the potassium-silicification vein (A vein), the medium-term quartz-polymetallic sulfides vein (B vein), and the epidote-carbonatation (Dvein), respectively. Petrographic, microthermometric, laser Raman microprobe and H -O -C results of fluid inclusions in different hydrothermal veins. Fluid inclusions of A vein’s homogenization temperatures, and salinities vary from 390~460 ℃, 4.5%~21.6% NaCleqv, 43.6%~59.6% NaCleqv, respectively;fluid inclusions of B vein’s homogenization temperatures, and salinities, vary from 310~380 ℃, 3.6%~19.8% NaCleqv, 6.0%~16.0% NaCleqv, respectively;fluid inclusions of D vein’s homogenization temperatures, and salinities, vary from 200~320 ℃, 0.4%~14.7% NaCleqv, 0.70~1.00 g/cm3, respectively.And the carbon, hydrogen, oxygen isotope test results reveals that the δDH2 O, V-SMOW values of fluid inclusions in quartz veins range from -128‰ to -100‰, and δ18O H2O, V-SMOW values of fluid inclusions in quartz veins range from -9.09 ‰ to -1.45‰, the δ13CCal, V-PDBvalues of calcite veins range from -20.8‰ to -19.9‰, and δ18OCal, V-SMOW values of calcite veins range 9.4‰ to 10.5‰, indicating a feature of magmatic hydrothermal, but may mixtured geothermal water in late stage. In brief, the ore-forming fluid of the Luerma copper (gold) deposit is a Ca+- Na+- Cl - H2O fluid system, with high contents of CO2, N2, and CH4, high homogenization temperature, high salinity and, low-moderate density, rich in metallic elements as Cu, Fe, and Mo et al., which characteristics similar to typical porphyry copper deposits. These studies suggest that, the luerma copper ore ore-forming fluid moved from the deep closed system to the shallow open system and broke through the critical state of decompressing boiling rapidly, which occurred phase separation resulting in the precipitation of metal sulfide, forming A vein and B vein type mineralization. Afterwards, as the heavy precipitation of minerals in ore bearing hydrothermal fluid, and the mixing of atmospheric precipitation, et al., the temperature and salinity of the fluid decreased rapidly, resulting in D vein mineralization.