黔西北乐开铅锌矿床成矿物质来源及矿床成因:来自硫、铅同位素的证据

Sources of metallogenic materials and genesis of Lekai lead-zinc deposit in northwestern Guizhou Province:Evidence from S and Pb isotopes

赋存于泥盆系望城坡组白云岩中的乐开铅锌矿床位于扬子地块西南缘川滇黔接壤铅锌矿集区,主要发育似层状矿体,具有典型的“逆(逆断裂)导-张(张断裂)运-岩(受断裂影响的碳酸盐岩破碎空间被碳质黏土岩圈闭形成的有利岩性组合)储”构造控矿模式。矿石矿物主要为闪锌矿、方铅矿、黄铁矿,发育(网)脉状、角砾状、浸染状等构造与交代、充填、共边等结构,后生成矿特征明显。硫化物δ^(34)S值为11.1‰~18.1‰(均值约14.7‰),明显高于幔源岩浆硫的δ^(34)S值,与泥盆纪同期海水硫酸盐的δ^(34)S值相近,显示硫化物中的还原硫可能是赋矿地层中的高溶解度硫酸盐热化学反应(TSR)的产物。硫化物铅同位素^(206)Pb/^(204)Pb值为18.400‰~18.767‰(均值为18.565‰);^(207)Pb/^(204)Pb值为15.660‰~16.058‰(均值为15.791‰);^(208)Pb/^(204)Pb值为38.580‰~39.432‰(均值为39.059‰),变化范围相对较大。铅同位素的^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb图解与△γ-△β图解显示明显的壳源特征,同时暗示沉积岩石与基底岩石共同提供了成矿物质。综合矿床地质、硫化物硫、铅同位素特征,笔者认为乐开铅锌矿床的成矿过程为盆地流体循环萃取沉积岩石与基底岩石的金属元素后形成含矿流体,含矿流体被深大断裂导入上覆沉积地层的特殊构造部位(被碳质黏土岩圈闭的碳酸盐岩破碎空间)时,富含的热量导致沉积盖层中硫酸盐发生热化学反应(TSR),生成大量的S^(2-),与含矿流体中的Pb^(2+)、Zn^(2+)、Fe^(2+)等金属阳离子结合成矿。乐开铅锌矿床的地质地球化学特征与MVT型矿床类似,因此,乐开铅锌矿床属于MVT型铅锌矿床。成矿物质来源与矿床类型的确定有利于川滇黔接壤铅锌矿集区同类型铅锌矿床的勘查与开发。

The Lekai lead-zinc deposit,which is hosted in the dolomite of the Devonian Wangchengpo Formation,is located in the Sichuan-Yunnan-Guizhou lead-zinc metallogenic province(SYGMP)on the southwest margin of the Yangtze block.It mainly develops layered ore bodies,and has a typical fault-controlled style of‘reverse fault importing-tension fault transporting-lithologic assemblages stored ore fluids(the fracture space of carbonate rock affected by fault is trapped by carbonaceous clay rock to form favorable ore bearing lithologic association)’.Ore minerals are mainly sphalerite,galena and pyrite,with veined,brecciated,and disseminated structures,and the ore textures consist of metasomatic,co-dissolved and filling morphologies,with obvious epigenetic metallogenic characteristics.Theδ^(34)S values of single grain sulfide range from 11.1‰to 18.1‰(mean 14.7‰),which is obviously higher than that of mantle magma-derived sulfur,but close to that of Devonian seawater sulfate,revealed that S2−originated from high-solubility sulfate within the ore-hosting strata through thermochemical sulfide reduction(TSR)processes.The Pb isotope ^(206)Pb/^(204)Pb of single grain sulfide ranges from 18.400‰to 18.767‰(mean 18.565‰).The ^(207)Pb/^(204)Pb values ranged from 15.660‰to 16.058‰(mean 15.791‰).The ^(208)Pb/^(204)Pb values range from 38.580‰to 39.432‰(mean 39.059‰),with a relatively wide range.The ^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb and△γ-△βdiagrams of Pb isotopes obviously show crust source characteristics,and suggest that the sedimentary rocks and basement rocks both provided ore-forming materials.Based on the characteristics of deposit geology,structural ore-controlling,S and Pb isotopes of sulfide,this paper holds that the metallogenic process of the Lekai lead-zinc deposit is:the basin fluid extracted metal elements from sedimentary rocks and basement rocks and formed ore-bearing fluids;the ore-bearing fluid is diverted by deep faults into special structural parts of the overlying sedimentary strata(carbonate fracture space trapped by carbonaceous clay rocks);under the action of the heat of the ore-bearing fluid,the sulphates in the sedimentary strata underwent TSR,and a large amount of S^(2-)was generated,which combined with the metal cations such as Pb^(2+),Zn^(2+)and Fe^(2+)in the ore-bearing fluids resulting the formation of sulfide ores.The geological and geochemical characteristics of the Lekai lead-zinc deposit are similar to those of MVT deposits,thus we propose that the Lekai lead-zinc deposit is MVT deposit.The determination of ore-forming material source and deposit type is beneficial to the exploration and development of the same type of lead-zinc deposit in the SYGMP.