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班公湖地区富镍硫化物超基性岩位于藏北班公湖-怒江缝合带西段,呈透镜状产出于侏罗系地层中,岩石已完全蚀变为蛇纹岩,恢复其原岩为方辉橄榄岩。地球化学研究表明:相对于原始地幔,该类超基性岩富w(MgO)(40.96%~42.82%),贫w(Al_2O_3)(0.19%~1.76%)、w(CaO)(0.11%~0.38%)和TiO_2(0.02%~0.06%);稀土元素总量低(0.238×10-6~2.044×10-6),远低于原始地幔值,稀土元素配分型式呈U型;强烈亏损高场强元素Y;岩石的锆石年龄为太古代(2 479 Ma±31 Ma)。以上特征表明其为残余地幔橄榄岩成因。富镍硫化物超基性岩中的镍硫化物呈似球状分布于蛇纹石中,主要为镍黄铁矿,偶见与镍黄铁矿共生的赫硫镍矿。电子探针分析结果表明,镍黄铁矿中w(Ni)为36.77%~42.62%,w(Fe)为18.83%~28.84%,w(S)为31.34%~38.3%。赫硫镍矿中w(Fe)为0.91%~13.36%,w(Ni)为60.5%~70.08%,w(S)为25.25%~27.23%。赫硫铁矿Fe含量高(除两件样品外均>2%mol),且与镍黄铁矿的接触边界平直。结合前人的实验资料,认为Fe-Ni硫化物形成于高温阶段(>300℃),为贫S的地幔Fe-Ni-S熔体捕虏体成因。
The nickel-rich sulphide ultramafic rocks in the Bangong Lake area are located in the western part of the Bangong Lake-Nujiang suture zone in northern Tibet, and are lenticularly produced in the Jurassic strata. The rock has been completely altered into serpentinite to restore its original Rock is a hterite. Geochemical studies show that the ultramafic rocks of this type have higher MgO (40.96% -42.82%), Al 2 O 3 (0.19% -1.76%) and w (CaO) (0.11% -0.38%) than the original mantle %) And TiO_2 (0.02% ~ 0.06%). The total amount of rare earth elements is low (0.238 × 10-6 ~ 2.044 × 10-6), which is much lower than the original mantle value. Strong element Y; zircons of the rocks were Archean (2 479 Ma ± 31 Ma). The above characteristics indicate that it is the cause of residual mantle peridotite. Nickel sulphides in nickel-rich sulphide ultramafic rocks are spherically distributed in serpentine, mainly pentlandite, and occasionally hesenickelite symbiotic with pentlandite. The results of electron probe analysis showed that the content of w (Ni) is 36.77% ~ 42.62%, w (Fe) is 18.83% ~ 28.84%, w (S) is 31.34% ~ 38.3% in the pentlandite. The content of w (Fe) in hectorite ore is 0.91% ~ 13.36%, w (Ni) is 60.5% ~ 70.08% and w (S) is 25.25% ~ 27.23%. Hef pyrite has high Fe content (> 2% mol except 2 samples), and the contact boundary with pentlandite is flat. Combined with the previous experimental data, it is considered that the formation of Fe-Ni sulfide at high temperature (> 300 ℃) is the origin of the Fe-Ni-S melt xenolith in the depleted mantle.