江西武山铜矿床海底喷流与岩浆热液叠加成矿作用: 控矿地质条件、矿石结构构造与矿床地球化学制约

长江中下游成矿带存在一套产于泥盆系五通组砂岩和石炭系黄龙组白云质灰岩层间的层状含铜硫化物矿体,对其成因存在很大争议。本文以产出典型层状矿体的武山铜矿为解剖重点,结合区域控矿地质要素、矿石结构构造特征及矿石中黄铁矿的稀土元素地球化学,提出层状矿体是海底喷流同生沉积与岩浆热液叠加成矿作用的产物。对武山铜矿层状矿体中的胶黄铁矿和黄铁矿、矽卡岩矿体中黄铁矿和脉状矿体中黄铁矿进行的稀土元素含量分析发现,从层状矿体胶黄铁矿、层状矿体黄铁矿、到矽卡岩和脉状矿体黄铁矿,稀土总量和稀土配分曲线显示递变规律,即层状矿体胶黄铁矿具有较低的稀土总量和轻重稀土分异不明显的较平坦型配分曲线;而矽卡岩和脉状矿体黄铁矿具有较高的稀土总量和轻重稀土分异较明显的右倾型配分曲线。层状矿体黄铁矿的稀土特征则介于两者之间,反映了岩浆热液的叠加作用。根据矿物组合共生关系及矿石结构构造的研究,可将武山铜矿黄铁矿分为3个期次:I期为微球粒、草莓状、条带状、纹层状沉积型黄铁矿; II期为半自形、自形粒状和港湾状黄铁矿,可见与长英质斑晶、岩屑或晶屑凝灰岩伴生或共生, 说明黄铁矿形成与同沉积期火山凝灰岩的密切关系。III期为块状、粗晶状、碎裂状黄铁矿。黄铜矿的形成晚于I、II期黄铁矿,成微粒状、脉状交错穿插或包裹早期球粒状、粒状黄铁矿及长英质矿物。对新发现的灰泥丘构造的详细研究表明,武山铜矿中含矿的灰泥丘与武山外围乌石街出露的不含矿的灰泥丘具有不同的特征,其中前者具有封闭的孔洞系统,而后者为开放的孔洞系统。总之,武山铜矿控矿地质条件、矿石结构构造及不同类型矿石黄铁矿的稀土元素证据表明矿床存在两期成矿事件,即海西期海底喷流同生沉积成矿期和燕山期岩浆热液叠加成矿期。

Submarine hydrothermal exhalation with superimposed magmatic-hydrothermal mineralization in the Wushan copper deposit, Jiangxi Province: Constraints from geology, ore texture and ore deposit geochemistry

A stratiform copper-bearing massive sulfide deposit occurs within the Devonian sandstone and Carboniferous dolomitic limestone strata in the Middle-Lower Yangtze River Mineralization Belt. The genesis of this bedded ore has long been debated. In this paper, we take Wushan deposit as an example, because this deposit occurs a well-defined bedded sulfide ore. A combined evidence of geology, ore structure and texture, and pyrite rare earth element geochemical characteristics indicate that the bedded sulfide ore deposit was a product of syn-sedimentary hydrothermal exhalative and superimposed magmatic-hydrothermal ore-forming processes. The rare earth element study shows that the colloform pyrite has the lowest REE contents and a flat REE curve in the chondrite-normalized REE pattern diagram, whereas the pyrite from skarn and vein-type ores show the highest REE contents and a steep REE curve with LREE enrichment and HREE depletion. The pyrite from the bedded ores show a REE characteristic in between the colloform pyrite and the skarn/vein pyrite, indicating a superimposed hydrothermal event on this pyrite in the bedded ores. Three stages of pyrite formation have been recognized: Stage I of laminated, collofom, and framboidal pyrite, stage II of subhedral and euhedral pyrite, locally associated with felsic rock fragments and volcanic crystals, indicating a close relationship with syn-sedimentary volcanic rocks, and stage III of massive, coarse-grained and broken pyrite. The chalcopyrite was formed later than the stage I/II pyrite, and may enclose the framboidal pyrite or crosscut the pyrite. Two types of carbonate mound structures are identified, one is ore-related carbonate mound which show a closed micro-framework structure, another is ore-barren carbonate mound which show an open micro-framework structure. In summary, this study supports a two stage mineralization model for the Wushan copper deposit, namely, the first stage of syn-sedimentary hydrothermal exhalative stage and the second stage of magmatic-hydrothermal ore-forming stage.