Geochemistry And Metallogenic Model Of Guanfang Copper Deposit In Lancangjiang Volcanic Belt, Yunnan, China

Langcangjiang volcanic belt is one of the most 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 four 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 regularity 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 exploration 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 subduction 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 productive 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 volcanic 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 mass 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 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-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 these 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 stock 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 zone 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 main 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 mineral 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 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 where 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 H₂O-NaCl-CO₂ system of low-middle salinity evoluted from an acidic to weakly basic. It is proved that the deposits formed in a kind 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-magma intrusion. Increased obviously from top to bottom of the silicification and broadened the pyrite-sericite-quartz alteration halo in this ore area shows the existence of hidden neutral-acid stock and subvolcanic rock mass. This paper set up the metallogenic model of Guanfang Copper Deposit and summarized some key ore-hunting evidence as structure, lithology, outcrop, mineralized alteration, geophysics and geochemical anomalies and remote sensing, et al. Furthermore, the author delineats the targets for exploration in studied region. At present the large area of pyrite-sericite-quartz alteration and silicification exposed by prospecting work could infer that large-scale porphyry and contact metasomatism copper ore is likely presented in the deep part of minefield.