安徽铜陵冬瓜山大型铜矿:海底喷流-沉积与矽卡岩化叠加复合成矿过程

摘要

The Dongguashan deposit in the Shizishan district, eastern China, is a large mineralized copper system within the Middle-Lower Yangtze metallogenic belt, which occurs in an intra-continental environment, and underwent a prolonged geologic history from Late Paleozoic continental rifting, through Middle Triassic continent-continent collision, to Jurassic-Cretaceous intra-continental deformation and magmatism. The main orebody occurs as a stratabound tabular system at the boundary between Upper Carboniferous carbonates and an underlying Upper Devonian sandstone sequence. It mainly consists of massive Cu sulfide lenses, stratiform Cu skarns, laminated Cu pyrite-serpentine sheet, and small anhydrite lenses. The laminated Cu pyrite-serpentine sheet is generally underlain by stratabound stringer Cu ore lenses in the Devonian footwall sandstone, whereas massive Cu sulfide lenses and overlying anhydrite layers (lenses)comprise three rhythmic sulfide-sulfate units in the main orebody. The stratiform Cu skarns commonly overlie the massive Cu sulfide lenses and are associated with sills that intruded along anhydrite layers in the host sequence, whereas small satellite magnetite orebodies commonly surround the steeply plunging quartz monzodioritic stocks (～ 137Ma). There are two main styles of alteration: subconcordant quartz-sericite alteration in the sandstone footwall; and prograde and retrograde skarn alteration, with associated Cu-Fe mineralization, related to intrusion of quartz monzodioritic stocks. The first style of alteration generally envelopes the stringer Cu ore lenses, and probably reflects diffusive discharge of submarine hydrothermal fluids during the Late Carboniferous. The second alteration style is expressed as ① magnesian skarn in the laminated Cu pyrite-serpentine sheet, and ② calcareous skarn widely occurring as stratiform Cu skarn bodies. The prograde mineral assemblages (mainly garnet and diopside) of both skarns were commonly metasomatized by tremolite, actinolite, chlorite, epidote, calcite and quartz during retrograde skarn alteration. Eighty sulfide samples from the deposit yielded a range of δ34S values from 0.5‰ to 8. 8‰,whereas associated anhydrites yielded δ34 S values varying between 14. 8‰ and 20. 5‰, implying two separated sources for hydrothermal sulfur at Dongguashan. There is a striking correlation between texture and δ34 S values of pyrites and anhydrites. The laminated, fine-grained anhydrites have a heavi er δ34 S value (20. 5‰) than those of coarse-grained anhydrites (14. 8‰), suggesting a source derived from the bacteriogenic reduction of Late Carboniferous seawater sulfate. Colloform and fine-grained pyrites have a relatively low δ34S (1. 3‰～5. 5‰), whereas crystalline pyrites exhibit an increase in δ34S (av. 4. 4‰→5.3‰→5.7‰) with increasing grain size. Sulfide textures, fluid inclusions, and the spatial distribution of alteration zones and Cu orebodies indicate that the Dongguashan deposit is a two-stage overprinting mineralized Cu system, in which the early-formed massive Cu sulfides and overlying sulfate caps, that was produced by Late Carboniferous sedimentary-exhalative processes, were overprinted by Late Jurassic skarn Cu mineralization.%冬瓜山铜矿是长江中下游成矿带铜陵矿集区内的一个大型铜矿.该矿床被惯称为矽卡岩型矿床,但具明显的层控特征.主矿体呈层状、似层状,赋存于上石炭统碳酸盐建造与下伏上泥盆统砂岩建造的接触带附近,主要由一系列层控块状含铜硫化物透镜体、含铜矽卡岩透镜体、磁铁矿矿囊以及层状含铜黄铁矿-蛇纹石矿席组成.主矿体的底部为根植于上泥盆统砂岩建造内的层控脉状-网脉状矿,下部为层状含铜黄铁矿+蛇纹石矿席,主体为块状含铜硫化物透镜体与上覆硬石膏层构成的三个硫化物-硫酸盐韵律单元.该"双层结构"特征总体类似于VMS和SEDEX型矿床,但矿石结构构造表明,这些硫化物-硫酸盐沉积均不同程度地遭受了后期热液-变质作用叠加改造.层控含铜矽卡岩通常发育于块状含铜硫化物透镜体之上,小型磁铁矿矿体或矿囊通常围绕陡倾的闪长岩株零星分布.这两类矿化均与燕山期岩浆活动(约137Ma)和石英二长闪长岩株侵位密切相关.矿区可见两种蚀变作用:石英-绢云母蚀变与矽卡岩蚀变.前者在下盘砂岩中形成似整合蚀变带,包裹层控网脉状矿,其成因可解释为晚石炭世热液流体在海底之下砂岩透水层内发生侧向迁移、弥散排泄及金属卸载;后者由燕山期石英二长闪长岩侵位引起,形成两类矽卡岩,其中,镁质矽卡岩发育在层状含铜黄铁矿-蛇纹石矿席,钙质矽卡岩形成层控含铜矽卡岩透镜体.冬瓜山矿床80件硫化物祥品的硫同位素δ34S组成介于0.5‰～8.8‰之间,伴生的硬石膏δ34S介于14.8‰～20.5‰之间,暗示两者的热液S有不同的来源:弱变质的细粒层纹状硬石膏,硫同位素δ34S为20.5‰,表明S来源于晚石炭世海水硫酸盐;弱变质的胶状细粒黄铁矿,δ34S变化于1.3‰～5.5‰之间,反映了热液S来自于海水硫酸盐的生物还原;热变质的晶质黄铁矿,硫同位素组成随结晶程度和颗粒大小的增加而升高(平均δ34S:4.4‰→5.3‰→5.7‰),暗示热液S可能部分来自矽卡岩-岩浆热液系统.冬瓜山矿床的矿化结构、矿体形态、结构构造、流体包裹体特征以及热液蚀变带与铜矿体的空间分布表明,冬瓜山铜矿是两期成矿叠加复合的产物:晚石炭世海底喷流-沉积作用,形成块状含铜硫化物矿化体,晚侏罗世岩浆侵位诱发矽卡岩-热液系统,叠加改造早期块状含铜硫化物和硫酸盐,形成以矽卡岩铜矿为主体的叠合型矿床.