新疆东准噶尔石炭纪坝西铜矿斑岩成因及对找矿的启示

Petrogenesis of porphyries in Baxi copper deposit in East Junggar,NW China and its enlightenment to mineral prospecting

新疆坝西铜矿位于东准噶尔琼河坝矿集区,是新发现的斑岩型铜矿。主要赋矿围岩为石英闪长岩、花岗闪长岩和石英二长闪长岩。锆石U-Pb测年显示,花岗闪长岩和石英闪长岩均形成于337 Ma,属早石炭世晚期。花岗闪长岩和石英闪长岩都具有高硅、中低钾,属钙碱性系列,富钙、富铝,为准铝质—弱过铝质。富集轻稀土及大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具有弱的负Eu异常,表现出弧花岗岩特征和某些埃达克岩特性。结合区域研究,初步认为形成于东准噶尔岩浆活动相对“宁静期”的坝西含矿石英闪长岩和花岗闪长岩,应与早期俯冲的弧物质的部分熔融有关,并发生了岩浆混合作用,且伴随有一定程度的结晶分异,同时也形成斑岩型铜矿,此时应处于从俯冲增生到后造山伸展的转换阶段。琼河坝地区除志留纪—泥盆纪发育大量矿产外,石炭纪也是一个重要的成矿期,值得关注。本次研究对北疆及邻区的晚古生代后碰撞斑岩矿产的找矿勘查工作具有重要的指示意义。

The recently discovered Baxi porphyry copper deposit is located in the Qiongheba ore-concentration area in East Junggar,Xinjiang.The ore-bearing porphyry are mainly composed of quartz diorite,quartz monzodiorite and granodiorite.Zircon U-Pb dating yielded two similar late Early Carboniferous ages of 337 Ma for quartz diorite and granodiorite.Quartz diorites and granodiorites are high SiO_2,CaO,Al_2O_3,and low K_(2O) contents,belonging the calc-alkaline,aluminum-weakly peraluminous.They are rich in LREE and LILE,such as Nb,Ta and Ti with weak negative Eu anomaly and high Sr and low Y contents,showing arc granitic characters with some Adakite futures.Combined with the regional research,we suggest the diorites and granodiorites in the Baxi porphyry copper deposit formed by partial melting of former subduction arc with magmatic mixing and following crystallization differentiation during the tectonic transition period from subduction to post-orogenic setting in Late Paleozoic.Porphyry copper deposits also formed in this period.In other words,except the Silurian-Devonian large-scale metallogenic stage,Carboniferous is also an important metallogenic stage in the Qiongheba area.It indicates that we should more attention on Late Paleozoic post-collision porphyry deposits in mineral exploration in northern Xinjiang and adjacent areas.