岩浆流体在热液矿床形成中的作用

THE ROLE OF MAGMATIC FLUID IN THE FORMATION OF HYDROTHERMAL ORE DEPOSITS

岩浆流体在浅部分离为岩浆卤水和蒸汽相 ,CO2 、SO2 的加入将增加不混溶区间。Ag ,Zn ,Pb ,Sn等在高盐度卤水中呈氯化络合物的形式搬运 ,Cu、Au呈I价态的二硫化络合物的形式在富硫的蒸汽相中搬运。岩浆流体与大气水混合的稀释和热效应 ,是导致Sn元素沉淀的主要机制 ,流体混合需要长期稳定的抽送系统 :( 1)对流体界面混合 ;( 2 )两组裂隙处相遇混合。斑岩Cu矿床早期以岩浆流体为主导 ,晚期大气水普遍存在。反应性强、富含金属的岩浆流体从侵入体往外运移并且与主岩反应 ,形成带状分布的蚀变矿物组合。高硫化浅成热液矿床的早期以流体对主岩的广泛淋滤为特征 ,流体呈酸性和氧化性。密度差使得低盐度液体与深处高盐度卤水在空间上分离。低硫化浅成热液矿床的成矿流体呈低盐度、中性pH值和处于还原性、静水压力条件 ,流体沸腾是成矿卸载的主要机制。富Au型矿床与低盐度富气相流体有关 ,富Ag型矿床与较高盐度的流体有关。在热液系统的寿命中 ,导致矿化的流体活动仅在短暂的时期内存在。热液系统之间在岩浆标志上的变异是由于岩浆流体的间歇性贡献或缺失造成的。

Within the shallow part of the crust, the magmatic fluid is separated into a magmatic brine and a vapor. Addition of SO 2 , CO 2 would result in fluid saturation in magma and increase the fluid immiscibility. Most metals such as Ag, Zn, Pb, Sn are carried in the brine by stable chloro-complexes, whereas Cu and Au are probably held in the S-enriched vapor phase by some stable bisulfide complexes. Tin precipitation is driven by mixing of hot magmatic brine with cooler meteoric water as a result of the dilution and thermal effects. For the two fluids always mix at the same place for an extended period of time, a stable plumbing system is required, which may be attained either by (1) mixing at the interface of two overlying convection cells or by (2) mixing of magmatic fluid ascending in one vein with meteoric water penetrating from a cross-cutting vein. Porphyry Cu mineralization is dominated by magmatic fluid in the early stage, although late meteoric water is not only commonly present but perhaps critical in enhancing porphyry metal concentration to ore grade. Zoned alteration assemblages are formed as the reactive and metal-bearing magmatic fluid moves away from the intrusion, cools and reacts with the country rock. The early stage of the high-sulfidation epithermal ore deposits is characterized by extensive leaching of the host rocks by an acidic and oxidative fluid. The spatial separation of low-salinity liquid associated with the high-sulfidation ore with the underlying higher-salinity liquid at depth is attributed to their density contrast. Low-sulfidation epithermal ore deposits are characterized by a low-salinity, near-neutral and reducing fluid with pressures controlled by hydrostatic conditions. Fluid boiling may probably be the main mechanism for ore precipitation. The Au-rich low-sulfidation epithermal ore deposit is related to the low-salinity vapor-enriched-fluid, whereas the Ag-rich one to the higher-salinity fluid. The fluids responsible for mineralization may be present only for short periods during the lifetime of the hydrothermal system, possibly at the time of individual tectonic or hydraulic fracturing events. The variability in magmatic signatures between geothermal systems may be caused by intermittent contributions (or lack thereof) of magmatic fluid.