胶东金矿成矿物质来源:来自与金成矿有关地质单元金含量的约束

Sources of Ore-forming Materials in the Jiaodong Gold Deposits:Constraints on Gold Content from Geological Units Related to Gold Mineralization

胶东是世界第三大金矿集区,金矿找矿和成矿理论研究不断取得新突破,但对矿床成因和成矿物质来源的认识尚存在较大分歧。为了科学合理地解释金矿床相对于各类围岩高达万倍的Au富集,通过系统采集胶东地区未遭受矿化蚀变的晚中生代花岗岩和早前寒武纪变质岩样品,对其进行了Au元素含量测试。结果表明:97件花岗岩Au含量平均值为0.31×10^(-9),其中玲珑型、郭家岭型和伟德山型花岗岩的Au含量平均值分别为0.25×10^(-9)、0.28×10^(-9)和0.35×10^(-9),397件早前寒武纪变质岩的Au含量平均值为1.05×10^(-9),总体与地壳中Au元素丰度和华北板块中Au含量平均值接近或偏低,但早前寒武纪变质岩中Au含量是晚中生代花岗岩中的3.39倍。结合前人研究结果认为,胶东地区不存在Au异常富集的源区或矿源岩(层),巨量金的成矿物质源自古老变质基底在晚中生代的部分熔融,在下地壳基底变质岩部分熔融形成花岗质岩浆的过程中,在超高温环境中约70%以上的金以类似“熔炼”的方式析出,并产生了贫金花岗岩。成矿前与岩浆活动相关的流体有利于Au元素的迁移和富集;成矿期及其后花岗岩的岩石地球化学成分发生了显著变化,影响了流体的化学平衡,为金的沉淀提供了有利条件。

Jiaodong is recognized as the third largest concentration of gold deposits globally.Recent advancements have been achieved in the exploration of gold deposits and the associated metallogenic theories.However,significant gaps remain in the understanding of the genesis of these deposits and the sources of mineralizing materials.A critical question that persists is how to scientifically and coherently elucidate the enrichment of gold(Au)in deposits that exceed the concentrations found in the surrounding rocks.In this study,we systematically collected and analyzed samples of Late Mesozoic granite and Early Cambrian metamorphic rocks from Jiaodong that have not undergone mineralized alteration to determine their gold(Au)content.The average Au content of 97 granite samples was found to be 0.31×10^(-9).Specifically,the average Au contents for Linglong-type,Guojialing-type,and Weideshan-type granites were 0.25×10^(-9),0.28×10^(-9),and 0.35×10^(-9),respectively.Additionally,the average Au content of 397 samples of Early Precambrian metamorphic rocks was determined to be 1.05×10^(-9).The abundance of gold in all geological units in Jiaodong is comparable to or lower than that in the Earth’s crust and the average gold content in the North China Plate.However,the gold content in early Precambrian metamorphic rocks is 3.39 times higher than in late Mesozoic granite.Based on previous studies,it is inferred that there is no source area or ore-source rock(layer)with anomalous gold enrichment in Jiaodong.Instead,the substantial gold mineralization is attributed to the partial melting of the ancient metamorphic basement during the Late Mesozoic.During the formation of granitic magma via partial melting of basement metamorphic rocks in the lower crust,over 70%of the gold(Au)precipitates in a comparable“melting”process with in the ultra-high temperature environment,resulting in the production of Au-poor granites.The fluids associated with the magmatic activity of pre-mineralization Linglong and Guojialing-type granites facilitated the migration and enrichment of Au.The substantial alterations in the geochemical com-position of Weideshan and Laoshan-type granites during and subsequent to the mineralization period disrupted the chemical equilibrium of the fluids,thereby creating favorable conditions for the precipitation of gold(Au).