澜沧江火山岩带官房铜矿区矿床地球化学与成矿模式

1、澜沧江火山岩带官房铜矿区矿床地球化学与成矿模式        澜沧江火山岩带是“三江”构造岩浆带中一个重要的地质构造单元,也是十分重要的铜、铅、锌、银、金、钨、锡、钼多金属成矿带,资源潜力巨大。笔者在省院省校合作项目的支持下,以“官房铜矿地质地球化学与成矿模式”为题,开展了博士论文工作。首先对澜沧江火山岩带云县段岩浆岩的形成演化,弧火山岩的成矿机理及成矿规律进行了研究,接着重点探索了官房铜矿的矿床地质和地球化学特征及控矿因素,总结了关键找矿标志,建立了成矿模式。持续四年的矿床研究成果,及时地指导了云县江天公司的勘查

2、与开发工作,使官房铜矿点,发展成为了一个中大型铜银矿床和中型矿山,取得了良好的经济与社会效益,实现了理论研究成果及时转化为现实生产力的目标。滇西云县官房铜矿定位于“三江”特提斯构造域澜沧江火山弧内,历经洋壳俯冲造山、碰撞造山、造山后伸展、走滑转换造山几个构造演化阶段;多期次构造岩浆活动和澜沧江深大断裂及与之相交的东西向隐伏断裂和次级构造,控制了官房铜矿田的产出。官房铜矿是近年来澜沧江火山岩带南段取得较大突破的铜矿床之一。矿床产在三叠系弧火山岩中,含矿围岩为晚三叠世小定西组中基性火山岩,为高钾钙碱性钾玄岩系列,岩石地球化学具明显的活动大陆边缘弧火山岩的特征;微量元素和Sr、Nd同位素特征既表现出

3、壳源岩石的特点,又具有幔源岩石的特征,显示其源区具有壳幔混源性质,这种源区的形成是特提斯澜沧江洋板块向东俯冲消减作用的结果。与弧火山岩毗邻的云县花岗岩体具有略贫硅、富钾、过铝、钙碱性“S”型花岗岩特征,地球化学特征显示成岩物质主要为壳源;锆石颗粒U-Pb同位素测年结果表明岩体为多期次复式岩体,主体形成于华力西晚期-印支期,系洋壳板块俯冲消减后兰坪-思茅陆块与保山陆块开始碰撞形成的同碰撞花岗岩;侵位于弧火山岩中的燕山早期老毛村花岗岩株(Rb-Sr等时线年龄为169Ma±5Ma),具高硅、富钾、过铝、钙碱性“S”型花岗岩特征,在地球化学特征上,与中三叠世忙怀组“碰撞型”高钾流纹质火山岩十

4、分相似,均为碰撞后陆壳重融的产物,反映了区内岩浆活动由碰撞前、同碰撞至碰撞后的演化特征;岩株富含成矿物质与挥发分,与官房铜矿关系密切。区内如此强烈的岩浆活动为成矿作用提供了动力源和部分物源。官房铜矿矿体严格受火山机构与三叠纪火山岩相控制,主要产于火山喷发旋回顶部的紫红色杏仁状玄武岩、安山玄武质角砾岩的断裂破碎带,呈似层状、透镜状、囊状、陡倾脉状产出,展现出似层状为楼,陡倾脉状为梯的“楼-梯”分布格局。矿体在平面和剖面上的蚀变矿化分带特征明显,主要蚀变类型有硅化、黄铁绢英岩化、碳酸盐化、绿泥石化、绿帘石化、绢云母化和沸石化等,其中以硅化和黄铁绢英岩化与成矿关系最为密切。矿石矿物类型较为复杂,以一

5、套中低温热液矿物组合为特征,铜矿物中均含银,含金量微;银在矿石中的分布有较明显的选择性,多以类质同象的方式赋存在含铜硫化矿物中,而在方铅矿中不含银,这种现象是官房铜矿有别于其它金属矿床之处,反映了弧火山-岩浆侵入成矿的热液演化特征。矿床地球化学研究表明,官房铜矿床赋矿地层不足以提供全部成矿物质,成矿物质绝大部分来自于深部岩浆和热水对流循环过程中的陆壳基底及岩浆途经的围岩。含矿流体介质水为以大气降水为补给的岩浆水,属低中盐度的H_2O-NaCl-CO_2体系,从早期到晚期由酸性向弱碱性演化,矿床形成于一个相对开放的环境。矿床成因研究结果显示,深大断裂、岩浆通道有利于深部物质上侵;火山喷发后期才真

6、正涉及到成矿事件,有火山喷发期后火山热液,次火山岩浆侵入及燕山期岩浆侵入所带来的部分岩浆热液,大部分为加热的地下水溶液;成矿物质为深部物质及热水所经过的地层浸取出来的物质。因此,确定矿床成因类型为与火山-岩浆侵入有关的热液型铜多金属矿床。区内自上而下明显增强的硅化和扩大的黄铁绢英岩化晕圈,昭示了隐伏的中酸性岩株或次火山岩体的存在。由此建立了官房铜矿床成矿模式,同时总结出了构造、岩性、露头、矿化蚀变、地球物理和地球化学异常及遥感等6项关键找矿标志,肯定了研究区的找矿前景。推测深部可见有斑岩型和接触交代型铜矿Langcangjiang volcanic belt is one of the mos

7、t important tectonic units in Sanjiang tectonomagmatic belt (the three rivers: Jinsha, Langcang, and Nujiang River), and it is also a very important metallogenic belt of polymetals as copper, lead, zinc, silver, gold, tungsten, tin, molybdenum, and so on, with a great resource potential.For over fou

8、r years, with the support of the college cooperative project and taking "Geochemistry and Metallogenic Model of Guanfang Copper Deposit" as topic, the author has conducted the studies about the formation and evolution of the magmatic rock, the ore-forming mechanism and metallogenic regular

9、ity of arc volcanic rocks. Furthermore, the author has focused on its geology, geochemical characteristics and ore-controlling factors of Guanfang Copper Deposit, summed up key prospecting signs, and set up a metallogenic model. These researching results have provided good guidance for the explorati

10、on and development work of Jiangtian Company in Yunxian, and have brought good economic and social benefits.Guanfang Copper deposit is located in Langcangjiang volcanic rock arc in the Sanjiang Tethyan structural domains in the west Yunnan, which has experienced several orogenic stages including sub

11、duction of oceanic crust, collision extension of the post-collision, strike-slip transfer and it is controlled by multi-phase tectonic and magmatic activity and Lancangjiang deep fault and the intersection with the east-west hidden fault and secondary structures.Guanfang Copper Deposit is one of pro

12、ductive copper deposits in south segment of Langcangjiang. Produced within arc volcanic rocks of Triassic, its wallrock is the neutral-basic volcanic rock of Xiaodingxi Formation, Later Triassic epoch, a series of potassium-high calc-alkaline to shoshonite. The geochemical result shows the arc volca

13、nic rock of active continental margin characteristics, and the microelements, isotope of Sr and Nd indicates that it is crustal mixing by mantled, which shows that the source zone is mixed, the resulted of eastward subduction of the Tethyan Lancangjiang plate. While its neighbour Yunxian granite mas

14、s is a calc-alkaline S-type of weakly poor silicon, potassium-rich and peraluminous. The geochemical result shows that the crust source characteristics of ore-forming material, and the U-Pb isotope dating of zircon grain indicates that it is a multistage compound massif, of which the main body come

15、into being at late stage of Variscan to Indo-Chinese epoch . It is the product of continent collision after the subduction of oceanic crust, formed at primary tectogenesis stage (between Lanping-simao plate and Baoshan plate) in this region. Laomaocun granite stock which intruded in the arc volcanic

16、-hosted during the early Yanshan movement (Rb-Sr isochron age is 169Ma±5Ma) is also a calc-alkali S-type of high silicon, potassium-rich and peraluminous, which is very similar to those of collisive and K-high liparite arc volcanic rock of Manghuai Formation, Middle Triassic Epoch. Both of thes

17、e are continental crust re-melting product after the collision and it reflects the evolution of the region characteristic of magmatic activity which has undergone form pre-collision period to the collision period to post-collision period. It is closely related with Guanfang Copper deposit that the s

18、tock rich in minerals and volatile. In this region, such intensive magmatic activity has supplied enough power sources and some ore-forming material for the mineralization.Controlled by volcanic mechanism and Triassic volcanic suite, the ore bodies mostly developed within the faulted and shattered z

19、one consisted of amaranth amygdaloidal basalt and andesitic-basaltic breccias. They assumed in bedded, lentoid, pocketed and steeply dipping forms, and a "storey-ladder" distributive structure. In plane and cross-sectional view, the mineralized alteration zonation is very obvious of the ma

20、in alteration style including silicification, pyrite-sericite-quartz alteration, carbonation, chloritization, epidotization, sericitization, and zeolitization, and so on, on which the silicification and pyrite-sericite-quartz alteration have close relationship with the mineralization. The ore minera

21、l style is relatively complex, characterised by a suit of middle-low temperature hydrothermal mineral combination, with silver and weak gold content being in copper minerals. The silver element would exist in copper-bearing sulphide ore by isomorphous type rather than galena ore, which made Guanfang

22、 Copper Deposit differ from other metallic deposits, and also reflected that the hydrothermal evolution feature of ore-forming by volcantic arc magma intrusion in this area.The geochemical studies indicated that most of the ore-forming materials come from continental basement and enclosing rock wher

23、e hypomagma-thermal convection happened and the magmatic fluid passed. The medium water of ore-bearing fluid is magmatic water recharged from the precipitation; it is a H_2O-NaCl-CO_2 system of low-middle salinity evoluted from an acidic to weakly basic. It is proved that the deposits formed in a ki

24、nd of relatively open environment.The metallogenic studies show that deep fault, magmatic passageways are beneficial to the upward intrusion of deep material. The ore-forming materials including volcanic hydrothermal solution of post-effusive period, some magmatic hydrothermal fluid brought along the intrusion of subvolcanic magma and Yanshanian magma and most of heated groundwater solution could test that the ore-forming event roots in the later period of volcanic eruption. Therefore, it is concluded that Guanfang Copper Deposit is a hydrothermal Cu-polymetallic deposit related with volcano-