胶东大尹格庄金矿碳酸盐矿物的特征、物源及其在金成矿过程中的作用

胶东金矿的成因研究一直是矿床学研究的热点, 其中金的物质来源与活化、沉淀机制是最受关注的问题之一。大尹格庄金矿位于胶西北地区的招平断裂带中段, 是区内典型的超大型破碎蚀变岩型金矿, 以存在大量碳酸盐脉而有别于区内其他金矿床, 这些热液成因的碳酸盐脉对该矿区Au成矿具有重要作用, 因而是破解胶东巨量Au物质来源和成矿机制的良好对象。本研究通过细致的岩相学观察, 发现大尹格庄金矿中的碳酸盐矿物主要包括菱铁矿和方解石, 以脉状、浸染状存在于黄铁绢英岩型矿石中, 其中部分菱铁矿与黄铁矿等金属硫化物共生, 其他菱铁矿则与石英共生, 方解石则呈浸染状和脉状两种形式产出, 菱铁矿和方解石均为热液型碳酸盐矿物。扫描电子显微镜研究表明, 在菱铁矿和方解石中均发现有金矿物的存在。Au含量分析结果表明, 菱铁矿和方解石是大尹格庄金矿的重要载金矿物, 形成于成矿作用的主要阶段。碳酸盐矿物原位微区稀土元素数据也显示出热液型碳酸盐矿物的特征; 同位素地球化学数据显示大尹格庄金矿中碳酸盐矿物与胶东地区前寒武纪变质基底中的大理岩明显不同。C-O同位素分析显示, 大尹格庄金矿中菱铁矿的δ¹³C_PDB为-5.12‰~-4.82‰, δ¹⁸O_SMOW为12.13‰~12.80‰, 具有明显的幔源特征; 而方解石δ¹³C_PDB为-0.64‰, δ¹⁸O_SMOW为6.21‰, 显示出幔源流体在上升过程中受到了一定程度的围岩的影响。金矿中菱铁矿显示轻稀土相对亏损, 重稀土相对富集, 有轻微的负Eu异常, (⁸⁷Sr/⁸⁶Sr)_i为0.709842~0.710087, ε_Nd(t)为-18.5~-17.1;方解石的(⁸⁷Sr/⁸⁶Sr)_i为0.710861, ε_Nd(t)为-12.7, 指示形成碳酸盐矿物的CO₂流体可能来自于幔源的基性岩浆。结合胶东以及整个华北克拉通中生代地质演化, 本文认为胶东巨量金可能来自于富集的岩石圈地幔, 成矿物质随这些幔源基性岩浆的活动沿断层等壳内软弱面向上运移, 而CO₂流体在这个过程中起到了促进作用, 并随含金流体最终运移至地壳浅部发生金成矿作用。

Characteristics,origin and role of the carbonate minerals in gold mineralization of Dayingezhuang gold deposit,Jiaodong area

The genetic studies of gold deposits in Jiaodong area have always been hot topics, among which the source of gold and its re-activation and precipitation mechanism are the most concerned issues. Dayingezhuang gold deposit is located in the middle section of Zhaoping fault zone in northwestern Jiaodong, which is a typical disseminated and stockwork-style gold deposit in the region. It is distinguished from other gold deposits in Jiaodong area by the presence of a large number of carbonate veins or veinlets which play an important role for gold mineralization in this deposit, and thus it is a good target to track the material source and mineralization mechanism of Au. Through detailed petrographic observations, this study found the carbonate minerals in Dayingezhuang gold deposit mainly include siderite and calcite, which exist in the pyrite sericite ore in veined and disseminated forms. Some siderites have obvious symbiotic relationships with metal sulfides such as pyrite, while others are associated with quartz. Calcites occur in disseminated and veined forms, and can be divided into two types according to their mineral associations. Both of the siderite and calcite are hydrothermal carbonate minerals. Scanning electron microscopy studies show that gold minerals exist in both siderite and calcite. Combined with Au content, it is indicated that the siderite and calcite are also the important gold-bearing minerals in Dayingezhuang gold deposit, they are formed in the main stage of mineralization. The rare earth elements data obtained by in situ analysis (LA-ICP-MS) of carbonate minerals also show that they have characteristics of hydrothermal origin, and the changes of Eu abnormalities indicate that calcite may be more strongly affected by terrigenous materials than siderite. Isotopic geochemical data show that the carbonate minerals in Dayingezhuang gold deposit are obviously different from those in the Precambrian metamorphic basement in Jiaodong area. The C-O isotopic analysis shows that the δ13CPDB and δ18OSMOW of siderites in Dayingezhuang gold deposit are within the ranges of -5.12‰~-4.82‰ and 12.13‰~12.80‰, respectively, showing mantle source characteristics. However, the δ13CPDB and δ18OSMOW of calcites are -0.64‰ and 6.21‰, which means that calcites in the deposit are affected by country rock to some extent. Siderite in the gold deposit relatively shows loss of light rare earth elements and enrichment of heavy rare earth elements, with a slight negative Eu anomaly, and its (87Sr/86Sr)i and εNd(t) values are 0.709842~0.710087 and -18.5~-17.1, respectively. The measured (87Sr/86Sr)i and εNd(t) of calcite are 0.710861 and -12.7, indicating that the carbonate minerals might be originated from a basic magma of mantle source. Combined with the Mesozoic geological evolution of Jiaodong area and the whole North China Craton, we suggest that giant gold in Jiaodong area might be sourced from the enriched lithospheric mantle, and CO2 fluid played a promoting role during the process for the ore-forming materials migrated upward along the weak surface such as faults in the crust with basic magma which eventually migrated to the shallow crust with gold-bearing fluid to produce gold mineralization.