山东七宝山隐爆角砾岩型Cu-Au矿床流体包裹体及成矿流体演化特征

Fluid inclusions of the Qibaoshan Cu-Au deposit,Shandong Province,China

山东七宝山隐爆角砾岩型Cu-Au矿床位于郯庐断裂带东侧,矿体主要赋存在七宝山杂岩体中。通过显微学观察,发现在深部蚀变斑岩的重结晶石英斑晶中发现了含子晶包裹体（Type 1型）,同时伴生有气相（Type 2型）和富气相水溶液（Type 3型）包裹体。浅部矿坑石英脉中的流体包裹体主要为气液两相包裹体,并对较为发育的石英晶柱进行了详细的阴极发光特征和显微学观察研究。从其根部到末端可以划分为三个期次,其中第一期次石英环带中主要发育富液相包裹体（Type4型）,另外还发育少量的负晶形气液两相包裹体（Type 5型）,第二期石英环带中主要发育Type 5型包裹体,第三期石英环带中主要发育形态各异的气液两相包裹体（Type 6型）。显微测温结果显示,Type 1型包裹体均表现为子晶熔化均一特征,均一温度介于375-450℃之间,计算获得的盐度为43.8%-52.2%NaCleqv;Type 3型包裹体表现为临界均一特征,均一温度介于347-420℃之间,估算盐度为0.8%-7.1%NaCleqv;Type 4,5和6型包裹体均均一至液相,均一温度分别为221-327℃、199-379℃以及109-193℃,相应的盐度为2.4%-7.8%NaCleqv、10.2%-16.8%NaCleqv以及0.3%-4.0%NaCleqv。热力学计算获得Type 1型包裹体均一压力为623.46-1111.82bar,平均855.70bar。Type 3型包裹体均一压力范围为139.18-362.47bar,平均为250.70bar。由此可以认为,尽管富气相包裹体和含子晶包裹体共存,并且具备相近的均一温度范围,但是压力相差极为明显,所以这种共存特征并不能表征流体沸腾作用。根据前人实验研究结果,本文认为Type 2和3型包裹体代表了斑岩体结晶早期由于上覆地层隐爆而导致的压力降低条件下分异出来的流体,由于角砾岩筒后续固结引起压力增加,由Type 1型含子晶包裹体所代表的高盐度流体逐渐从成矿岩体中分异出来。当这两种流体沿裂隙通道向上运动时,被同期石英包裹体捕获而共存。这部分流体代表了深部斑岩成矿系统的流体。根据H-O-S同位素研究,Type 4和5包裹体主要来自岩浆流体,并且在后期经历了大气降水的混合,形成了Type 6型流体。这部分流体则代表了浅成热液系统的流体。由于Type 3和4型包裹体均为岩浆成因流体,两者盐度范围一致（5.0%-7.0%NaCleqv）,并且随着温度的下降,密度演化趋势明显,从0.4101增加至0.8816g/cm^3。本文认为代表斑岩系统的深部富气相成矿流体在向上逃逸过程中由于压力扰动而在超临界区域内发生相态收缩,随着温度下降形成了富液相成矿流体,转变为浅成热液系统的流体,在此过程中不会发生流体沸腾。由于Cu和Au易于被HS-络合而在气相中运移,并且液相流体之间的混合能力较强,所以当含金的富气相流体收缩成富液相流体时,可以与浅部的天然降水或者富Fe流体发生任意比例的混合,导致了Cu和Au的卸载成矿,这应该是七宝山Cu-Au矿化主要的成矿机制,但是不排除由于温度和压力的下降而引发的金属矿物沉淀成矿。

The Qibaoshan deposit is a large Cu-Au deposit located in southwest of Shandong Province,China. Three types of fluid inclusions are recognized in recrystallized quartz phenocrysts of the deep located altered porphyry： Type 1,halite-bearing inclusions;Type 2,pure vapor inclusions; and Type 3,vapor-rich inclusions. They commonly show coexistence in samples. Two types of fluid inclusions in quartz veins with different ore minerals representing different hydrothermal stages,and in quartz crystal column collected from the shallow open pit are identified in this study： Type 4,liquid-rich inclusions with vapour volume fraction of 5% - 8%; Type 5,aqueous inclusions with vapour volume fraction of 10% - 20%,which can be subdivided into 5a and 5b according to their differential occurrences. Microthermometric analyses indicate that Type 1 inclusions homogenized via halite dissolution at temperatures of 375 to450℃ with calculated salinities of 43. 8% to 52. 2% Na Cleqv; Type 3 inclusions displayed a critical homogenization mode based on the comparison between homogenization pressures and critical pressures at the same homogenization temperatures（ 347 to 420℃） with calculated salinities of 0. 8% to 7. 1% Na Cleqv; Type 4,5a and 5b inclusions homogenized to liquid phase at temperatures of 221 to327℃,199 to 379℃ and 109 to 193℃,respectively,with calculated salinities of 2. 4% to 7. 8% Na Cleqv,10. 2% to 16. 8%Na Cleqv,and 0. 3% to 4. 0% Na Cleqv,respectively. Type 2 inclusions were not analyzed in this study due to high vapor volume fraction（ 〉80%）,which makes observation of homogenization behaviors difficult. The coexistence of Type 1,2 and 3 inclusions is not caused by fluid immiscibility,because the homogenization pressures（ Ph） of Type 1 inclusions（ Ph = 623. 46 to 1111. 81bar） are much higher than those of Type 3 inclusions（ Ph = 139. 18 to 362. 47bar）. According to previous experimental researches,fluids represented by Type 2 and 3 inclusions are believed to be directly exsolved from the melts in early crystallization stage at low to intermediate pressure,which hosted abundant gold bisulfides,while the brine represented by Type 1 inclusions exsolved later at high pressure caused by crystallization and system resealing following brecciation,which enriched the Fe-chlorine complex. A mixing process is proposed in this study to interpret the coexisting phases. According to the H-O-S isotopic studies,hydrothermal fluids represented by Type 4 and 5a inclusions in epithermal system are of magmatic origin,which would be mixed by meteoric water in late stage generating a low salinity fluids represented by Type 5b inclusions. Based on the fact that,both Type 3 and 4 inclusions fluids are magmatic origin,sharing a same salinity range（ 5% - 7% Na Cleqv）,and display a gradually increasing density from 0. 4101 to0. 8816 g / cm^3 with temperature decreases from ca. 450℃ to ca. 250℃,we suppose that Type 4 inclusions-representing fluid is produced by contraction of Type 3 inclusions-representing fluids at elevated pressures,reflecting the hydrothermal transition from porphyry to epithermal system. The vapour contracting liquid fluids have physical ability to mix with any other aqueous ones including the brine represented by Type 1 inclusions in cryptoexplosion structure,which can consequently consume the HS-in brine causing the gold precipitation,and form the epithermal mineralization in this district. Besides,the meteoric water influx may also play a role in gold precipitation in Qibaoshan epithermal deposit.