滇西北中甸普朗斑岩铜矿床地球化学与成矿机理初探

A tentative study of ore geochemistry and ore-forming mechanism of Pulang porphyry copper deposit in Zhongdian, northwestern Yunnan

普朗斑岩铜矿是一个典型的与洋壳俯冲相关的大型乃至超大型矿床,通过对矿区代表性含矿斑岩与无/弱矿化斑岩的稀土元素特征对比、硫同位素示踪等方面的研究,表明矿石随Cu品位的增高,稀土总量(∑REE)和稀土分馏参数〔(LREE/HREE)、(La/Yb)N及(La/Yb)N、(Gd/Yb)N〕呈连续降低趋势,负铕异常一般显示为逐渐增强的趋势,但在发育有强烈的晚期钾长石脉的情况下,出现反弹减弱特征。硫化物(黄铜矿、黄铁矿和辉钼矿)的δ34SCDT变化范围为-2.23‰～3.75‰,基本呈塔式分布特征,主要为深源岩浆硫。

Located in Zhongdian volcanic arc in the southern part of the Yidun island arc, the Pulang porphyry copper deposit discovered in 1999 is a typical benioff subduction-related superlarge copper deposit. The porphyry bodies intruded into Triassic strata of clastic rocks with carbonates and volcanic rocks. Copper mineralization mainly occurs in quartz monzonite porphyry and quartz diorite porphyry, characterized by potassic alteration, quartz- sericite-pyrite alteration and propylytization. Ore minerals include mainly chalcopyrite, pyrite, molybdenite, pyrrhotite, subordinately magnetite, chalcocite, bornite, and locally malachite. Based on a comparison of REE characteristics between typical mineralized porphyry and barren or weakly mineralized porphyry as well as a S isotopic study, it is shown that, with the increase of Cu contents of the porphyries, both the total REE contents （∑REE） and the REE fractionation parameters [（LREE/HREE）, （La/Yb）N, （La/Yb）N, （Gd/Yb）N] decrease continuously, whereas the weak Eu anomalies on the REE distribution patterns of samples generally become stronger, with the exception of the weak Eu anomaly for the sample containing late stage K-feldspar coarseveins. The δ348CDT values of sulfides （pyrite, chalcopyrite and molybdenite） range from - 2.23‰ to 3. 75‰, and their normal distribution histogram indicates that sulfur was derived mainly from the magma. Combined with previous work on geological prospecting, petrology, geochronology and fluid inclusions, the authors comprehensively analyzed the geodynamic background for the metallogeny of the Zhongdian Cu-polymetallic belt and discussed preliminarily ore-forming mechanism of the Pulang porphyry copper deposit. It is believed that the low angle oblique subduction of the Ganzi-Litang ocean slab during the period from 235 Ma to 210 Ma strongly af- fected the Zhongdian area in such aspects as the formation of NW-trending fractures and folds and the subsequent island arc magmatic movement. A supercritical fluid rich in Cu, Fe, S, C1^-, HCO3^-, CO^2＋3, CO2, Na^＋, K^＋, Ca^2＋ components, which was formed through the evolution of the porphyritic magma derived from the partial melting of oceanic slab, reacted with the early emplaced porphyry and caused porphyry copper mineralization.