安徽高家塝钨钼矿床花岗闪长质侵入岩岩浆起源和演化及其对成矿能力的约束

安徽高家塝钨钼矿床位于江南过渡带,为一大型斑岩-矽卡岩型矿床,矿体赋存于小型花岗闪长斑岩体及其内外接触带中,紧邻的大型花岗闪长岩体中未见矿化。为查明制约两者成矿能力差异的原因,本文从岩石学、锆石U-Pb年代学、黑云母矿物化学、岩石地球化学等方面分别对矿区两个花岗闪长质侵入岩体开展了系统的对比研究。结果表明,花岗闪长斑岩成岩年龄为145. 1±2. 1Ma～144. 9±2. 2Ma,花岗闪长岩为142. 5±1. 8Ma～141. 8±1. 6Ma,前者侵位结晶稍早于后者。两者具有近于一致的主量、微量、稀土和Sr-Nd同位素组成特征,显示矿区两个花岗闪长质侵入岩体是由同一岩浆活动先后侵位到相近空间所形成,其原始岩浆具有相同的壳幔混合来源,即上涌的幔源玄武质岩浆与由其底侵引起挤压加厚的扬子下地壳部分熔融岩浆的混合,与长江中下游成矿带铜陵矿集区中酸性侵入岩不同的是,岩浆在上升过程中或滞留于浅位岩浆房中时明显地同化混染了扬子上地壳物质。然而,起源相同的花岗闪长质岩浆历经演化并先后侵位结晶时,其岩浆特征和结晶条件发生了显著变化,表现为:花岗闪长斑岩结晶时继承大量元古代锆石,花岗闪长岩则较少见有继承锆石,综合两者岩体特征和侵位结晶条件,显示前者岩浆熔体规模小、岩浆温度低、冷却结晶较快,岩体形成于富含F、Cl和相对还原的环境;而后者岩浆熔体规模巨大,岩浆温度相对较高,冷却结晶慢,岩体形成于贫F、Cl和相对氧化的环境。这在一定程度上影响了矿区两个花岗闪长质侵入岩体的成矿能力,演化早期偏还原性的花岗闪长斑岩岩浆以及其中较高的F、Cl含量更有利于钨富集于岩浆期后热液流体中,进而形成大型钨(钼)矿床。此外,相较于大型花岗闪长岩体而言,浅成侵位的小型花岗闪长斑岩体具有更为发育的裂隙系统以及受围岩更大影响而发生强烈矽卡岩化,也为矿质富集和沉淀提供了有利条件。本文研究为皖南地区钨(钼)矿床的找矿勘探及成矿模式的建立提供了新依据。

The magmatic origin and evolution of granodioritic rocks and their constraints on mineralization in Gaojiabang W-Mo deposit

The Gaojiabang W-Mo deposit, located in the Qingyang County, Anhui Province, is a large porphyry-skarn deposit in the north of Jiangnan Transitional Belt. The small-scale granodiorite-porphyry apophyse outcropped in the deposit has a genetic link with W-Mo mineralization, while the large-scale granodiorite pluton, which is adjacent to granodiorite-porphyry apophyse, has not been found to have any genetic link with mineralization. To illuminate this issue, we compared petrology, zircon U-Pb chronology, biotite mineral chemistry and petrogeochemistry characteristics of the above two granodioritic bodies, respectively. The results indicate that the granodiorite-porphyry was formed slightly earlier than granodiorite, the former was formed in 145.1±2.1Ma~144.9±2.2Ma, the latter was formed in 142.5±1.8Ma~141.8±1.6Ma. The major elements, trace elements, rare earth elements and Sr-Nd isotopic compositions of the two granodiorite bodies are nearly consistent, indicating that they were derived from the same magmatism and successively emplaced into the adjacent area. The primitive magma of the two granodiorite bodies is a mixture of crust and mantle, namely the upwelling mantle basaltic magma mixed with the partially molten magma of the lower crust of the Yangtze Plate. Comparing to the Tongling area, the mineralization belt in the Middle-Lower Reaches of the Yangtze River, the magma of the intrusions in this deposit assimilated with more upper crust matter of the Yangtze Plate. However, the magma characteristics and crystallization conditions of the two granodioritic bodies had changed dramatically during the evolution of magma since a large amount of Proterozoic zircons were inherited from granodiorite-porphyry during crystallization, while less commonly from granodiorite. After comparing their rock mass characteristics, emplacement and crystallization conditions, we find that the granodiorite-porphyry was formed in a reduced environment with enriched F and Cl, the magmatic melt of this intrusion is characterized by small-scale, low temperature and fast cooling crystallization; while the granodiorite was formed in an environment with poor F, Cl and relative oxidized, the magmatic melt of which is in huge scale, the magmatic temperature is relatively higher, and the cooling crystallization is slow. These characteristics and the magma evolution and crystallization conditions may indicate the influence on the metallogenic ability:the reduced magma and its higher content of F and Cl are more conductive to tungsten enrichment in the post-magmatic hydrothermal fluid, thus forming large tungsten (molybdenum) deposit. Moreover, compared with the large-scale granodiorite pluton, the fracture systems of small-scale granodiorite-porphyry body are more extensive, and stronger skarn alteration occurred between granodiorite-porphyry and surrounding rocks, which also provides favorable conditions for mineral enrichment and precipitation.