岩浆热液出溶和演化对斑岩成矿系统金属成矿的制约

EXSOLUTION AND EVOLUTION OF MAGMATIC HYDROTHERMAL FLUIDS AND THEIR CONSTRAINTS ON THE PORPHYRY ORE-FORMING SYSTEM

岩浆热液过渡阶段对于与岩浆热液有关矿床的形成非常重要。以往的研究多侧重于岩浆结晶阶段和低于固相线的热液阶段过程和演化 ,但对于流体从熔体出溶到熔体最后固结过程的理解却很有限。基于流体包裹体冷热台研究、单个流体和熔体包裹体原位无损成分分析技术 ,并结合挥发份和成矿元素在共存相间分配的实验和质量平衡计算模拟 ,岩浆热液出溶和演化对金属成矿制约的研究取得了很大进展。文中从岩浆中挥发份的出溶和演化、成矿元素在岩浆热液过渡体系各相之间的分配、斑岩矿床成矿流体及与金属成矿的关系、浅成热液矿床成矿流体及与金属成矿的关系几个方面进行了阐述。研究表明 :( 1)岩浆熔体不仅含有足够的挥发性组分 ,而且出溶的挥发份能够被圈闭在流体包裹体中而成为岩浆出溶热液的实物证据。 ( 2 )挥发份和成矿元素不仅在岩浆熔体和出溶的溶液间分配 ,还将在熔体与盐水溶液、熔体与气相以及盐水溶液与气相间进行分配。Cu在岩浆蒸气中比在共存的熔体中要富集数百倍 ,而Cu ,As,Au(可能作为HS配合物 )则偏向于分配进入与液体相共存的蒸气相中。 ( 3 )成矿元素在熔体 /溶液间的分配系数受控于熔体中初始水含量与饱和水含量之比值和岩浆熔体与共存出溶水溶液的w(Cl) /w(H2 O)和w(F) /w(Cl)比值。 ( 4 )

Magmatic-hydrothermal transitional processes are of critical importance for the formation of intrusion-centered hydrothermal ore deposits. Although some aspects of the early magmatic stage and the subsolidus hydrothermal stage are relatively well understood, much less is known about the transition stage, i.e., the interval between the initiation of fluid exsolution from a crystallizing melt and the final solidification of magma. The studies on the exsolution and evolution of magmatic hydrothermal fluids and their constraints on metallic ore formation, by means of microthermometry, in-situ nondestructive quantitative analyses of individual fluid and melt inclusions, experiments on partitioning of volatiles and ore metals among different phases and mass transfer modeling, have made enormous progresses over the last ten years. In this paper, some advances of knowledge in this field are described, including the exsolution and evolution of magmatic hydrothermal fluids and their constraints on metallic ore formation, the partitioning of volatiles and ore-forming metals among different phases, and the relationships between ore-forming fluids and metallic ore formation in porphyry copper and high sulfidation epithermal deposits etc. Some conclusions can be drawn as follows:(1) Magmatic melt contains abundant volatiles enough to form hydrothermal solution and the volatiles exsolved from magmatic melt can be trapped in fluid inclusions, being the direct evidence of exsolving hydrothermal solution from magmatic melt. (2) The volatiles, trace metals and other elements may partition among crystallizing melt, saline aqueous solution, gas phases and minerals. The latest researches show that copper is several hundred times more concentrated in magmatic vapor than in coexisting magmatic melt and that Cu, As and Au (probably as HS complexes) preferentially partition into the vapor phase in a coexisting liquid brine. (3) The partitioning coefficients of ore metals among melt, fluid and gas are controlled by the ratio of initial water in the melt to the water saturation level, w(Cl)/w(H 2O) and w(F)/w(Cl) ratios in magmatic melt and exsolving aqueous fluid. (4) The porphyry copper deposits have a distinct assemblage of fluid inclusions, characterized by halite-bearing inclusions, vapor-rich inclusions, and the common occurrence of chalcopyrite daughter minerals in inclusions. The ore-forming fluid and ore metals are mainly derived from magmatic melt.(5) Epithermal deposits share certain similarities in the evolution of ore-forming fluids with porphyry copper deposits, which indicates a genetic link between these two types of ore deposits. Finally, a great deal of inspiration and provocation on studies of ore deposits related magmatic hydrothermal fluids in our country has been gained from the above-mentioned.