胶东乳山金矿床成矿过程:周期性压力波动诱发的流体不混溶

位于胶东东部苏鲁地体内的乳山金矿曾是我国单脉金储量最大的矿床,其主矿脉为一具有复杂内部结构的富金石英脉,形成于包含周期性流体活动的增量增长过程。该矿床成矿流体演化、精细成矿过程和金沉淀机制仍缺乏有效制约。本研究在详细结构构造观察基础上,在代表单次成矿流体活动的同一石英层内识别出分别代表三个连续成矿阶段的三类黄铁矿,开展激光剥蚀-电感耦合等离子质谱原位微量元素测试。结果显示不同阶段黄铁矿微量元素成分基本一致,Co、Ni、As等元素因成矿流体间歇性压力波动而周期性地以不同含量进入黄铁矿,形成这些元素的韵律成分环带。Au等其他微量元素在不同阶段黄铁矿内均匀分布,其分布行为受压力波动影响较小。间歇性压力波动和由此引发的周期性流体不混溶使乳白色粗粒石英和黄铁矿、烟灰色中细粒他形石英和黄铁矿依次大规模沉淀,金银碲化物、银金矿、自然金和方铅矿、闪锌矿、黄铜矿等硫化物随后在愈加富Au、Ag、Te、Pb、Zn和Cu等的流体中近于同时沉淀。在此过程中成矿流体虽整体表现为还原性,但其还原性随着压力波动而不断递减氧化性持续增加;流体碲逸度早期保持稳定,后期则大幅上升。金以可见金形式充填先成黄铁矿裂隙或沿黄铁矿边缘分布,周期性流体压力波动引发的间歇性流体不混溶导致H_2S、CO_2和CH_4等气体大规模逸出,金硫络合物失稳分解,金被吸附至黄铁矿内水力致裂形成的裂隙面发生沉淀。排除了先成黄铁矿内不可见金再活化为可见金的可能性,认为周期性流体压力波动引起的流体不混溶是引发乳山金矿床可见金高效沉淀的关键机制。

Ore-forming processes of the Rushan gold deposit, Jiaodong: Fluid immiscibility induced by episodic fluid pressure fluctuations

The Rushan gold deposit, located in the Sulu terrane in eastern Jiaodong Peninsula, once reserved the largest gold resource within a single quartz-gold vein in China. High grade auriferous quartz ores, characterized by complex internal textures, were incrementally deposited in processes involving episodic fluid activities. Ore-forming fluid evolution, ore-forming processes and corresponding gold precipitation mechanism, however, remain unclear. Three types of pyrite representing three consecutive mineralization stages are identified based upon field investigation and microscopic observation. In-situ LA-ICP-MS trace element analyses conducted on three pyrite populations from one single quartz layer that was precipitated in one single fluid event suggest that composition for each specific trace element in all the three types of pyrite is broadly the same. Co, Ni and As episodically substituted Fe and S in growing pyrite in variable amounts, attributed to an oscillatory precipitation environment induced by episodic fluid press fluctuations. The rest of the trace elements, such as Au, are uniformly distributed in the pyrite grains without being significantly perturbed by fluid pressure fluctuations. Episodic fluid pressure fluctuation and its resulting fluid immiscibility triggered the massive precipitation of coarse-grained milky-white quartz and pyrite firstly, which was succeeded by medium-fine smoky-gray quartz and pyrite occurring either as infillings in veinlets or disseminated particles. Lately, Au-Ag telluride, electrum, native gold and sulfide minerals including galena, sphalerite and chalcopyrite were precipitated from a progressively Au-Ag-Te-Pb-Zn-Cu concentrated fluid. The ore-forming fluids were generally reducing during the above process, but with a progressively increasing oxidizing capability due to the ongoing fluid pressure fluctuations. Tellurium fugacity stayed stable in earlier time but was dramatically elevated when the Au-Ag telluride was precipitated. Gold is visible in the microfractures of pyrite grains or along their boundaries. Large amount of H₂S, CO₂ and CH₄ escaped from the fluids because of the fluid pressure fluctuations and their resulting episodic fluid immiscibility, the gold bisulfide in which gold was transported was thus destabilized and decomposed, and gold minerals were subsequently chemisorbed onto the microfractures of pyrite. It is the episodic fluid immiscibility induced by fluid pressure fluctuations that plays a crucial role for gold deposition in the Rushan gold deposit, not the mechanism in which invisible gold in gold-rich pyrite is remobilized from the pyrite and re-concentrated as visible gold.