滇东北富乐铅锌矿床微量元素和S-Pb同位素地球化学研究

Trace elemental and S-Pb isotopic geochemistry of the Fule Pb-Zn deposit,NE Yunnan Province

富乐铅锌矿床位于我国西南川滇黔铅锌多金属矿集区的东南部,矿体呈层状、透镜状赋存于中二叠统阳新组白云岩中,受层间破碎带控制。该矿床铅锌金属资源量超过50万吨,铅锌平均品位大于15. 6%。碳酸盐岩围岩溶蚀、重结晶和热液角砾岩化是该矿床普遍发育的热液蚀变类型,是酸性热液流体与碳酸盐岩围岩化学反应的结果。热液白云岩在成矿期前、成矿期和成矿期后都可形成,晚期形成的热液白云岩往往会部分替换早期形成的白云石,热液沿裂隙充填过程中,会引起围岩重结晶作用,形成明显的蚀变晕。矿石中主要硫化物包括闪锌矿、方铅矿和黄铁矿,伴有少量的黄铜矿和黝铜矿,白云石和方解石是主要脉石矿物。矿石的主要构造有致密块状、浸染状、脉状和角砾状。本次工作在富乐矿床厘定出三种颜色闪锌矿,即黑色、红色和棕色。LA-ICPMS研究表明,三种颜色闪锌矿中Cd、Cu、Ga和Ge等元素不同程度富集,而Fe、Mn和In等元素有不同程度亏损。在LA-ICPMS时间分辨率剖面图中,上述元素均呈水平直线出现,与Zn和S等主要元素的含量曲线平行,表明它们可能以类质同象形式赋存于闪锌矿中。而Sb、Pb和Ag等元素在LA-ICPMS时间分辨率剖面图中,呈较大波动趋势,暗示这些元素可能以微细粒包体存在,这与显微观测发现闪锌矿中有方铅矿微小矿物颗粒相吻合。本研究初步认为富乐闪锌矿颜色可能是Ni、Cu、Tl、Ga、Hg、Fe和Cr等多种元素共同引起的,其中Ni、Cu和Ga使闪锌矿呈紫色,Cu使闪锌矿呈红色,Ga使闪锌矿呈黄色。三种颜色闪锌矿样品的硫同位素组成变化较小,其δ34S值变化范围为12. 2‰~14. 6‰,具有富集34S特征,与二叠系海相硫酸盐的δ34S值相似,暗示热化学硫酸盐还原作用(TSR)可能是该矿床HS^-或S^2-离子形成的主要机制。不同颜色闪锌矿的207Pb/204Pb、206Pb/204Pb和208Pb/204Pb值分别为15. 604~15. 737、18. 570~18. 732和38. 532~38. 667。大部分闪锌矿的Pb同位素组成与基底昆阳群浅变质岩石的Pb同位素组成相似,少量闪锌矿具有与峨眉山玄武岩和赋矿沉积岩相似的Pb同位素组成,表明富乐铅锌矿床的成矿金属主要来源于基底岩石,并可能受到沉积岩和玄武岩的影响。综上所述,本文认为富乐铅锌矿床是一个形成于挤压背景下、受层间构造控制的高品位、富分散元素后生碳酸盐岩容矿型铅锌矿床。

The Fule Pb-Zn deposit is located in the southeastern Sichuan-Yunnan-Guizhou Pb-Zn polymetallic metallogenic province,South China. The ore bodies occur as layers and lenses in dolostones of the Middle Permian Yangxin Formation,and are controlled by interlayer fracture zones. The Pb + Zn metal reserves are about 0. 5 million tones with an averaged Pb + Zn grade of more than 15. 6%. The dissolution, recrystallization and hydrothermal brecciation of host carbonate rocks within and peripheral to mineralized rocks are common in the deposit. These effects may develop in combination with silicification and dolomitization and constitute the major form of wall rock alteration in the deposit. These are the results of the chemical reaction between acid hydrothermal fluids and wall rocks. Hydrothermal dolomite can be formed pre-,syn-and post-ore stages and the late-formed hydrothermal dolomite often has partially replaced the early-formed hydrothermal dolomite. When hydrothermal fluids filled along the fissures and obvious alteration halo can be formed on both sides. The sulfides in the primary ores are mainly sphalerite,galena and pyrite,associated with a small amount of chalcopyrite and tetrahedrite. Dolomite and calcite are the main gangue minerals. The main ore structures include massive,disseminated,vein and breccia. Three colors of black,red and brown sphalerites are identified in the Fule deposit. LAICPMS study shows that the elements of Cd,Cu,Ga and Ge are enriched in sphalerite,while the contents of Fe,In and Mn are depleted. In the LA-ICPMS time-resolved profiles,the above-mentioned elements show a level-shaped,consistent with the content changes of S and Zn,indicating that these elements may occur in sphalerite in the form of isomorphous substitution. However,the elements Sb,Pb and Ag fluctuate greatly in LA-ICPMS time-resolved profiles,suggesting that those elements may occur as fine-grained mineral inclusions,which is consistent with the microscopic observation of galena particles in sphalerite. Systematic study shows that the color of Fule sphalerite may be caused by several elements( such as Ni,Cu,Tl,Ga,Hg,Fe and Cr) or element association.Purple sphalerite is attributable to the substitution of Ni,Cu and Ga for zinc in the sphalerites structure,red sphalerite is enriched in Cu and yellow sphalerite has high Ga content. The sulfur isotopic compositions of the three colors of sphalerites vary from 12. 2‰ to14. 6‰. These are similar to the sulfur isotopic compositions of sulfate in Permian sedimentary rocks,suggesting that δ34S values are enrichment in sulfides and the resources of HS- or S2- in the hydrothermal fluids were originated mainly from evaporated sulfate by thermochemical sulfate reduction( TSR).207Pb/204Pb,206Pb/204Pb and208Pb/204Pb values of sphalerite are ranging of 15. 604 ~15. 737,18. 570 ~ 18. 732 and 38. 532 ~ 38. 667,respectively. The Pb isotopic ratios of most sphalerites are plotted into the field of the basement metamorphic rocks of the Proterozoic Kunyang Group,with a few plotted into the fields of Emeishan basalts and orehosting sedimentary rocks. This shows that the metallogenic metals of the Fule Pb-Zn deposit are mainly derived from the basements,with a certain influence from sediments and basalts. In summary,the Fule Pb-Zn deposit is a carbonate-hosted,high-grade,dispersed elements-rich epigenetic Pb-Zn deposit. The Pb-Zn mineralization occurred within interlayer structures under the background of tectonic compression.