柴达木盆地西北缘牛鼻子梁镁铁-超镁铁质岩体年代学及其地质意义

Ziron U-Pb geochronology of Niubiziliang mafic-ultramafic intrusion on the northwest margin of Qaidam Basin,Qinghai

柴达木盆地西北缘新发现的牛鼻子梁铜镍矿床位于阿尔金南缘断裂和柴北缘断裂交汇部位。矿区内包含3个镁铁-超镁铁岩体,其中Ⅱ、Ⅲ号岩体中可见铜、镍硫化物矿化,区内岩体岩相分带明显,由南往北可分为橄榄岩相、辉石岩相和辉长岩相。橄榄岩相岩石包含角闪二辉橄榄岩、角闪橄榄岩、二辉橄榄岩、斜长二辉橄榄岩,辉石岩相岩石包含橄榄二辉岩、二辉岩。镍、铜矿化与橄榄岩相岩石关系密切。本文利用LA-ICP-MS锆石U-Pb同位素定年法测得Ⅰ号岩体闪长岩形成年龄为(388.0±2.8)Ma,Ⅱ号矿化岩体二辉橄榄岩形成年龄为(402.2±2.8)Ma,Ⅲ号矿化岩体斜长二辉橄榄岩形成年龄为(402.8±2.6)Ma。属早泥盆世,表明牛鼻子梁岩体形成于造山后陆内拉张环境,为晚古生代早期幔源岩浆活动的产物。牛鼻子梁铜镍矿床是中国除了夏日哈木矿床外又一形成于早泥盆世新的铜镍矿化类型,具有较好的镍铜硫化物矿床成矿潜力。

The newly-discovered Niubiziliang Ni-Cu deposit is located in the intersection part of the Altum southern fault zone and the Qaidam northern fault zone, Northwest China. There are three mafic- ultramafic intrusions, of which No. Ⅱ and No. Ⅲintrusions are Ni- Cu sulfide- bearing intrusions. The intrusions consisted of, from the south to the north, peridotite facies,pyroxenite facies and gabbro facies. The peridotite facies contains hornblende lherzolite, hornblende peridotite, lherzolite andplagioclase lherzolite, the pyroxenite facies contains olivine websterite and websterite, and the peridotite facies consists of Ni- Cu bearing rocks. High-precision LA-ICP-MS zircon U-Pb dating yielded the concordant ages of 388.0±2.8 Ma, 402.2±2.8 Ma and402.8±2.6 Ma for diorites of No.Ⅰ intrusion, lherzolites of No.Ⅱ ore-bearing intrusion and plagioclase lherzolite of No. Ⅲ orebearing intrusion, respectively. Regional tectonic evolution background indicates that the Niubiziliang rocks were formed in a postorogenic extension environment in early Devonian and that the magma originated from mantle material. The Niubiziliang maficultramafic intrusions are new type rocks containing nickel mineralization, and they have good potential for Ni- Cu sulfide exploration.