胶西北三山岛伟晶岩型脉状钼矿化成因及对胶东钼成矿的指示意义

The genesis of pegmatite-type molybdenum mineralization in Sanshandao,and their implications for molybdenum deposit in Jiaodong,East China

胶东是我国重要的金矿集中区,同时也是重要的铜钼多金属成矿区。胶东中东部地区分布有一些不同规模的钼矿床,但是在胶西北地区,尚没有钼矿化的报道。随着深部探矿工作的不断推进,近来在胶西北三山岛金矿区,已发现含辉钼矿化的伟晶岩发育于胶东群和玲珑岩体内。伟晶岩主要由钾长石、石英、白云母、辉钼矿、萤石、黄铁矿组成,此外还含有少量的斜长石、黄铜矿、闪锌矿和赤铁矿等矿物。辉钼矿Re-Os模式年龄为149.9~151.3Ma,加权平均值为150.0±1.0Ma,Re-Os等时线年龄为149.7±1.5Ma,表明伟晶岩形成于约150Ma。钾长石中还发育有大量的矿物包体,包括黄铁矿、方铅矿、赤铁矿、金红石、氧化铬、重晶石、萤石、天青石、碘化锑以及一些含Fe、Mn、Mg、Ca、Sb的碳酸盐矿物,表明原始流体富含金属元素,具备形成金属矿床的潜力。石英中流体包裹体主要有两种类型:纯CO2包裹体和CO2-H2O包裹体,并且一些包裹体中存在少量的CH4、N2等组分。显微测温结果显示,纯CO2包裹体初熔温度的分布范围为-57.3^-56.6℃,部分均一温度的分布范围为18.5~25.1℃,包裹体的密度为0.71~0.79g/cm3。CO2-H2O包裹体的初熔温度为-57.4^-56.6℃,部分均一温度为9.4~31.1℃,笼合物消失温度为4.1~8.3℃,均一温度分布范围为279~350℃,其对应的盐度为3.33%~10.33%NaC leqv,包裹体的密度为0.74~1.00g/cm3。显微测温结果表明流体为富含挥发分、中低盐度的H2O-CO2-NaC l型热液。石英中流体包裹体的δD值的范围为-69.6‰^-56.3‰,δ18O值的分布范围为3.2‰~5.6‰,黄铁矿的δ34S值的范围为4.1‰~5.0‰,表明伟晶岩的形成与岩浆热液关系密切。钾长石的Pb同位素分析结果:206Pb/204Pb=16.7212~17.0287,207Pb/204Pb=15.4093~15.5139,208Pb/204Pb=37.4811~37.9003,其特征与玲珑花岗岩相似,而与胶东群变质岩和中生代脉岩的Pb同位素组成不同。辉钼矿Re含量变化范围为4.66×10-6~29.20×10-6,平均15.49×10-6。Re含量介于壳源与壳幔混源之间,但更偏向壳源,表明成矿物质主要来自于地壳。综合以上的研究成果,本文认为伟晶岩形成于玲珑花岗岩岩浆期后热液,其中辉钼矿化的出现对胶东钼矿化具有重要的指示意义。

Jiaodong is not only an important gold deposits concentration area,but also a crucial copper-molybdenum poly-metal metallogenic area. Molybdenum deposits of different scales are located in the central and eastern Jiaodong,whereas molybdenum mineralization has not been found in the northwestern Jiaodong by far. As deep gold exploitation proceeding in recent years,pegmatites hosted in Neoarchean Jiaodong Group and Late Jurassic Linglong granitoids have been found in the Sanshandao goldfield,northwestern Jiaodong. They are mainly composed of K-feldspar,quartz,muscovite,molybdenite,fluorite,and pyrite,with minor plagioclase,chalcopyrite,sphalerite,and hematite. The Re-Os dating of five molybdenite samples yields a Re-Os model age ranging from 149. 9Ma to 151. 3Ma,with an average value of 150. 0 ± 1. 0Ma and an isochron age of 149. 7 ± 1. 5Ma,suggesting the pegmatites formed at ca. 150 Ma. Mineral inclusions identified by energy dispersive spectrometer（ EDS） are well preserved in K-feldspar phenocrysts,including pyrite, galena, hematite, rutile, chromium hemitrioxide, barite, fluorite, celestite, antimony iodide, and carbonate containing Fe,Mn,Mg,Ca,and Sb. These mineral inclusions indicate that the primary fluids are enriched in metallic elements,which have remarkable potential to produce metallic deposit. Fluid inclusions in the quartz are divided into two types,including pure CO2 fluid inclusions and CO2-H2 O fluid inclusions. A small quantity of CH4 and N2was detected in some fluid inclusions.Microthermometric studies show that final melting temperature of solid CO2,homogenization temperature and bulk density of the pure CO2 fluid inclusions range from- 57. 3 to- 56. 6℃,18. 5 to 25. 1℃,and 0. 71 to 0. 79 g / cm^3,respectively. For the CO2-H2 O fluid inclusions,final melting temperature of solid CO2,partial homogenization temperature,total homogenization temperature,bulk density and salinity range from- 57. 4 to- 56. 6℃,9. 4 to 31. 1℃,279 to 350℃,0. 74 to 1. 00 g / cm^3,and 3. 33% - 10. 33% Na Cleqv,respectively. The microthermometric data suggests that the fluid belongs to H2O-CO2-Na Cl system,which is characterized by lowmiddle salinity and richness in volatiles. δD and δ18O of the fluid inclusions in quartz vary from- 69. 6‰ to- 56. 3‰ and 3. 2‰ to5. 6‰,respectively. δ34S of pyrite ranges from 4. 1‰ to 5. 0‰. The analysis results of H-O-S stable isotopes support that the formation of the pegmatites was closely related to magmatic water. Pb isotopic data of the K-feldspar are as follows：206Pb /204 Pb =16. 7212 - 17. 0287,207 Pb /204 Pb = 15. 4093 - 15. 5139,208 Pb /204 Pb = 37. 4811 - 37. 9003,which is similar to the Linglong granitoid,but different from the Jiaodong Group metamorphic rocks and the Mesozoic dike rocks. The contents of Re in molybdenite change from4. 66 × 10^- 6to 29. 20 × 10^- 6,with an average value of 15. 49 × 10^- 6. The values are between the mantle source and the crust source,but closer to the crust source,implying metallogenic material might come from the crust. On the basis of the above achievements,we hold that pegmatite formed from post magmatic hydrothermal solution after the formation of the Linglong granitoid. The occurrence of molybdenum mineralization in this region has significant implications for molybdenum deposits prospecting and scientific research in Jiaodong.