滇西北中甸地区休瓦促岩浆热液型Mo-W矿床S、Pb同位素对成矿物质来源的约束

S-Pb isotopic geochemistry of Xiuwacu magmatic hydrothermal Mo-W deposit in Zhongdian area,NW Yunnan: Constrains on the sources of metal

滇西北中甸地区位于义敦岛弧南段,近年来勘探及研究工作发现区内燕山晚期发育有一期重要的Mo-Cu-（W）矿化,明显区别于义敦岛弧北段同期的Sn-Ag-Pb-Zn多金属矿化,也不同于同地区印支期大量发育的斑岩型Cu矿化。目前,中甸地区燕山晚期Mo-Cu-（W）矿化成矿物质来源研究相对较弱,尚不能很好地解释这些成矿作用主要成矿元素存在差别的原因。休瓦促矿床是中甸地区代表性的燕山晚期大型岩浆热液型Mo-W矿床,前人定年结果显示成矿年龄为~83Ma。矿区内发育有三阶段的晚白垩世花岗岩,岩性主要有黑云母花岗斑岩、二长花岗岩和碱长花岗岩;且碱长花岗岩体下方还发育有萤石-长石似伟晶岩脉。矿化类型主要为石英脉型、近石英脉蚀变花岗岩型和斑岩型Mo-W矿化;矿体主要产在岩体内部,受控于北北西向断裂构造。蚀变类型有钾化、云英岩化、绢云母化及硅化等。流体包裹体测温显示成矿流体为含CO2的中高温（146.6~550.0℃）,中低盐度（3.15%~12.51%NaC leqv）的H2O-NaC l热液,可能主要来自于岩浆期后热液。多种金属硫化物与燕山晚期花岗岩具有一致的初始Pb同位素组成（206Pb/204Pb=18.610~19.460,207Pb/204Pb=15.606~15.747,208Pb/204Pb=38.815~39.410）,显示其成矿物质可能主要源于壳源岩浆作用,S同位素特征（δ34SVCDT：2.07‰~4.33‰）也显示其来自于岩浆作用。通过对比休瓦促Mo-W矿化、义敦岛弧北段同期的Sn-Ag-Pb-Zn多金属矿化及中甸地区印支期斑岩型Cu矿化,这三种与岩浆有关的热液矿化的S、Pb同位素及岩石地球化学性质,发现这三种矿化的成矿岩浆与相应的成矿元素均具有较好的成矿专属性;并指示着在燕山晚期陆内环境下,中甸地区的Mo-Cu-（W）矿化成矿物质来源于加厚的中基性下地壳部分熔融而形成的I型花岗岩,义敦岛弧北段的Sn-Ag-Pb-Zn矿化则主要来源于中酸性变沉积岩地壳的部分熔融而形成的A型花岗岩;而中甸印支期斑岩型的Cu矿化则与幔源岩浆作用有着密切的关系。

A Late Cretaceous Mo-Cu-（ W） metallogenic belt was found in the Zhongdian area,southern portion of the Yidun Arc,which is not only distinguished from contemporary Sn-Ag-Pb-Zn mineralization in the northern Yidun Arc,but also different from the Late Triassic porphyry Cu mineralization in the same area. The reason why the metals of that above mineralization in the Yidun Arc are different remains unclear,as a lacking of researches on the sources of ore-forming materials of those Late Cretaceous deposits in the Zhongdian area. The Xiuwacu deposit is a large-scale Late Cretaceous Mo-W deposit in the Zhongdian area,with ages ranging from85 Ma to 83 Ma. Three stages of the Late Cretaceous granites developed in the area consist of biotite granitic porphyry,monzogranite,and alkali-feldspar leucogranite. In addition,fluorite-feldspar pegmatite vein occurred under the alkali-feldspar leucogranite. The ore bodies are mainly of the quartz-vein-type,greisen-type and porphyry type. They are hosted in hornblende biotite monzogranite and biotite monzogranite, and distributed extending NNW along regional faults. Hydrothermal alteration consists mainly of Kfeldspathization,greisenization, argillization, and silicification. Microthermometric data indicate a moderate-to-high temperature（ 146. 6 ~ 550. 0℃）,low-to-moderate salinity（ 3. 15% ~ 12. 51% Na Cleqv）,CO2-rich,aqueous ore-forming fluid,which were probably derived from postmagmatic hydrothermal solution. Both of the sulfide minerals and granites have similar Pb isotopic compositions（206Pb /204 Pb = 18. 610 ~ 19. 460,207 Pb /204 Pb = 15. 606 ~ 15. 747,208 Pb /204 Pb = 38. 815 ~ 39. 410） suggest that the metallogenic elements were probably derived from magmas. The S isotope of sulfide minerals（ δ34SVCDT： 2. 07‰ ~ 4. 33‰） also indicate a magma sources. Comparing the S-Pb isotopes and petrogeochemistry of Xiuwacu Mo-W mineralization with contemporary SnAg-Pb-Zn mineralization in the northern Yidun Arc and Late Triassic porphyry Cu mineralization in the Zhongdian area,it shows that each of those magma events has different characteristics and metallogenic specialization. In other words,partial melting of thickened moderate to mafic lower continental crust would generate fertile magmas for Mo-Cu-（ W） mineralization in the southern Yidun Arc;mixing of the mantle- and sediment-derived melts probably product favorable magmas for Sn-polymetallic mineralization in the northern Yidun Arc; and the Late Triassic porphyry Cu mineralization are genetically related to the mantle-derived magmas.