东准噶尔蒙西斑岩型铜钼矿床地质地球化学特征

Geological and geochemical characteristics and geological implications of the Mengxi porphyry copper-molybdenum deposit in East Junggar region

蒙西斑岩型铜钼矿床位于古亚洲洋成矿域准噶尔—南蒙古成矿带北塔山—琼河坝铜、镍、金、铁成矿区,是东噶尔琼河坝矿集区有新突破的斑岩铜钼矿床之一。笔者从蒙西斑岩型铜钼矿床形成的地质背景、矿床地质特征和地球化学方面入手,分析认为含矿斑岩主要为一套中晚志留世的中酸性岩石,围岩为中-上奥陶统荒草坡群(O2-3hcq),是早古生代的斑岩铜矿,而不是前人认为的晚古生代斑岩铜矿床。成矿期次主要分为黄铁矿-石英、黄铜矿-黄铁矿-石英和碳酸岩-石英3个阶段。矿石含70%多黄铁矿,15%的黄铜矿,还有少量磁铁矿和辉钼矿。次生矿物有孔雀石、褐铁矿和蓝铜矿。矿石中矿物组合显示中-低温特征。元素地球化学特征表明,蒙西铜钼矿床的岩石属高铝高钠钙碱性花岗斑岩,大离子亲石元素Sr、K、Rb、Ba、Th等相对富集,高场强元素Nb、Ta、Hf相对亏损。(Yb+Nb)-Rb和Y-Nb判别图解表明岩石形成于火山弧。(R1-R2)构造图解表明为同碰撞期花岗岩,由此可以初步确认该地区主要侵入岩形成于同碰撞壳源参与的火山弧构造环境。所以蒙西斑岩型铜钼矿床的成矿构造属性为同碰撞期大陆岛弧,说明了东准噶尔为早古生代存在构造-成矿作用。

The Mengxi porphyry copper-molybdenum deposit is located in the Beitashan-Qiongheba copper,nickel,gold and iron ore district of the Junggar-Southern Mongolia metallogenic belt within the Paleo-Asian oceanic metallogenic domain.Based on an analysis of the geological background as well as geological and geochemical features of this region,the authors hold that the ore-bearing porphyry consists mainly of MiddleLate Silurian intermediate-acid rocks,with the wall rocks being Middle-Upper Ordovician Huangcaopo Group（O2-3hcq）,and the ore deposit is an Early Paleozoic porphyry copper deposit instead of a Late Paleozoic porphyry copper deposit considered by previous researchers.The ore-forming process can be divided into three stages,i.e.,the pyrite-quartz stage,the chalcopyrite-pyrite-quartz stage,and the carbonate rock-quartz stage.Metallic mineral ores include more than 70% pyrite,15% chalcopyrite,and a small amount of magnetite and molybdenite.Secondary minerals are malachite,limonite,and blue copper ore.The mineral assemblages are mainly of mediumlow temperature.Element geochemical features show that rocks of the copper-molybdenum deposit are Al-high,Na-high calc-alkaline granite relatively enriched in ion lithophile elements Sr,K,Rb,Ba,Th and depleted in high-field strength elements Nb,Ta,Hf,Ti.The discrimination diagrams of（Yb ＋ Nb）-Rb and Y-Nb show that the rocks were formed in a volcanic arc.The（R1-R2） structural diagram suggests syn-collisional granite.It is thus concluded that the main intrusive rocks in the region resulted from the participation of volcanic arc crustal collision in the tectonic setting,and that the structure in the Mengxi porphyry copper-molybdenum deposit is a syn-collisional continental island arc.