陕西镇安丘岭卡林型金矿金的赋存状态和富集机理

Occurrence and Enrichment Mechanism of Gold in the Qiuling Carlin-Type Gold Deposit,Zhen'an County,Shaanxi Province,China

丘岭金矿床是西秦岭地区重要的卡林型金矿之一,金矿化赋存于上泥盆统南阳山组和下石炭统袁家沟组地层中,容矿岩石的岩性为钙质粉砂岩、粉砂质页岩和泥质灰岩.金矿石中主要金属矿物为黄铁矿和毒砂,非金属矿物则以石英、方解石和绢云母为主.通过对矿石矿物黄铁矿和毒砂的扫描电镜-能谱分析、电子探针分析和激光剥蚀电感耦合等离子体质谱分析,对丘岭金矿床金的赋存形式和富集机理进行了较为详细的研究.结果表明,丘岭金矿床中金主要以次显微不可见金的形式存在,其次为显微可见金.次显微金包括:(1)固溶体金(Au+),主要存在于环带状细粒黄铁矿的含砷增生边区域和毒砂中,少量存在于环带状黄铁矿的核部不含砷区域;(2)纳米级自然金颗粒(Au0),存在于粗晶黄铁矿中.环带状细粒黄铁矿核部的次显微金可能主要以胶体吸附的形式存在,暗示容矿岩石在沉积成岩过程中有金的初步富集,而环带状黄铁矿幔部和毒砂中的Au则主要来源于成矿流体,以S和As的络合物形式搬运.显微可见金主要分布在细粒黄铁矿的晶体边缘和热液蚀变绢云母、石英及方解石中,粒径通常小于3～5μm,其形成可能与成矿流体中金的局部过饱和及成矿流体对细粒黄铁矿和毒砂中次显微金的活化和再次富集有关.

Located in the western Qinling orogen,the Qiuling gold deposit is a typical Carlin-type gold deposit.Gold mineralization is hosted in calcareous siltstones,silty shales,and argillaceous limestones of the Upper Devonian Nanyangshan Formation and the Lower Carboniferous Yuanjiagou Formation.Pyrite and arsenopyrite dominate the metallic minerals,whereas quartz,calcite,and sericite are the main non-metallic alteration minerals.An integration of optical microscopy,scanning electron microscopy equipped with energy dispersive system,electron microprobe,and laser ablation ICP-MS was used to study the occurrence,distribution,and enrichment mechanism of gold in the Qiuling gold deposit.The results indicate that gold occurs as submicroscopic and,less significantly,microscopic species.The submicroscopic gold is present as nanoparticles of native gold(Au0) in the coarse-grained pyrite and solid solution(Au+) in the arsenian rims of zoned pyrite and arsenopyrite,with lesser amount of gold nanoparticles in the As-deficient core of zoned pyrite.The sub-microscopic gold particles in the core of zoned pyrite were likely formed during the diagenesis of the host rocks through adsorption,while the structure-bounded Au(Au0 and Au+) in fine-grained zoned pyrite and arsenopyrite may have been transported as sulfur and arsenic complexes in ore-forming fluids of the main hydrothermal stage.Microscopic gold grains are mostly less than 3 to 5 μm in size and are contained in the marginal areas fine-grained pyrite and included in hydrothermal sericite,quartz,and calcite.They formed likely due to local saturation of gold in the ore fluids or as a result of leaching of the submicroscopic gold by late-stage fluids and subsequent re-precipitation.