甘肃小西弓金矿床成矿物质来源和含矿流体运移轨迹同位素示踪

ORE-FLUID EVOLUTION OF THE XIAOXIGONG GOLD DEPOSIT HOSTED IN PROTEROZOIC METAMORPHIC ROCKS, GANSU PROVINCE, AS DEDUCED FROM SULFUR, OXYGEN, HYDROGEN AND LEAD ISOTOPIC EVIDENCE

小西弓矿床是北山地区最重要的金矿床。金矿化在中元古代糜棱岩化绿片岩和长英片岩中呈脉状或蚀变岩团块产出,并且同海西期花岗岩类侵入岩体具密切时空联系。详细的硫、氧、氢和铅同位素研究结果表明,金矿床成矿物质和含矿流体主要来自海西期花岗岩类侵入岩、中元古代变质岩地层与大气降水。晚古生代时期,哈萨克斯坦—北山与塔里木板块沿柳园—大奇山深大断裂发生碰撞与地体拼贴,其中挤压与伸展转换期的构造变形作用不仅致使前海西期岩体(层)发生深熔,形成花岗质熔浆,而且诱发含矿流体向低温与低压构造破碎带运移,进而形成小西弓金矿床。

The Xiaoxigong gold deposit is located in the convergence area of the Kazakhstanian-Beishan and Tarim paleo-plates, and is one of the most important gold deposits in northwest China. Gold mineralization occurred within the Middle Proterozoic metamorphic rocks as quartz veins and altered rock blocks, and is spatially and temporally related to the Hercynian granitoid intrusions. Oxygen and hydrogen isotope data on quartz separates from the gold-bearing quartz veins, metamorphic rocks and granitoid rocks revealed that the ore fluid for the deposit was derived mainly from a mixed source. Lead isotope data on sulfides, K-feldspar and whole-rock samples from the Xiaoxigong area constitute two mixing lines cutting the Pb-growth curves of upper crust, orogene, lower crust and mantle. Both geological evidence and isotope data showed that lead, gold and other metallic elements of the Xiaoxigong deposit came mainly from Hercynian granitoid intrusions that were probably derived from re-melting of preexisting supracrustal rocks due to the collision between the Kazakhstanian-Beishan and Tarim paleo-plates during the Paleozoic orogeny. Studies also showed that the gold-bearing veins were precipitated from magma-dominated fluids at the early ore-forming stage. The emplacement and crystallization of Early Hercynian granite magma may produce the ore fluids, whereas simultaneous tectonism facilitated the migration of the fluids. These early-formed gold veins may be superimposed by an additional hydrothermal event (evolved meteoric water) at the late ore-forming stage, which was also related to the Hercynian granitoid intrusions. As a result, the tonnage and ore grade of these gold veins are greatly increased.