A modified genetic model for the Huangshandong magmatic sulfide deposit in the Central Asian Orogenic Belt, Xinjiang, western China
A modified genetic model for the Huangshandong magmatic sulfide deposit in the Central Asian Orogenic Belt, Xinjiang, western China
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DOI：
10.1007/s00126-014-0524-5
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发表时间：
2014
影响因子：
4.8
通讯作者：
Ya-Jing Mao;K. Qin;Chusi Li;Dongmei Tang
中科院分区：
文献类型：
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作者：
Ya-Jing Mao;K. Qin;Chusi Li;Dongmei Tang
新疆北方黄山东镍铜存款矿床是中亚造山带迄今发现的最大岩浆硫化物存款矿床。主侵入体是一个274 Ma的复合镁铁质-超镁铁质侵入体，由四个独立的侵入单元组成：一个大型层状辉长岩序列（I期）、一个片状超镁铁质岩体（II期）、一个岩墙状辉长岩体（III期）和一个不规则超镁铁质单元（IV期）。重要的硫化物矿化存在于最后三个侵入体单元中，主要为浸染状和网状结构的硫化物（磁黄铁矿、镍黄铁矿和黄铜矿）。黄山东镁铁质-超镁铁质侵入岩具有橄榄石低钙、全岩Nb、Ta负异常等弧形地球化学特征。这与二叠纪东天山的后俯冲背景一起，表明源地幔在石炭纪之前被板片衍生流体修改过。幔源岩浆在新形成的弧壳下部的一个阶段性岩浆房中形成。到达黄山洞的第一批岩浆中镍的分馏和贫化最严重，结晶出富铁贫镍的橄榄石（Fo 67，<300 ppm Ni）。第二批岩浆更原始，结晶出更原始的橄榄石（Fo 81 -84）。第三批岩浆也被高度分馏和贫镍，结晶富铁和贫镍橄榄石（Fo 72，~600 ppm Ni）。最后一批岩浆再次变得更加原始，结晶出最原始的橄榄石（Fo 81 -86）。黄山东超镁铁岩中橄榄石原生晶中圆形硫化物包裹体的出现，表明橄榄石结晶过程中存在不混溶的硫化物液滴。辉长岩脉中浸染状硫化物矿石的镍含量主要在5 ~ 8wt%之间，这一含量太高，不可能是由岩脉的母岩浆产生的。在岩脉（第三阶段）和超镁铁岩片（第二阶段）的浸染状硫化物矿石的镍，铜和铂族元素（PGE）的tenors是非常相似的。这些观察结果，加上岩浆侵位的顺序，表明硫化物液体截留在岩浆的堤坝形成的深度由以前的脉冲更原始的岩浆。黄山东岩体中最原始二辉橄榄岩的母岩浆含MgO约10wt%。模拟结果表明，黄山东二辉橄榄岩母岩浆中硫化物的饱和度可能是分离结晶作用的结果。黄山东存款的浸染状硫化物矿石中相对于Ni和Cu的显著PGE亏损可能是由于硫化物保留在源地幔中。
The Huangshandong Ni–Cu deposit is the largest magmatic sulfide deposit discovered to date in the Central Asian Orogenic Belt in northern Xinjiang, western China. The host intrusion is a 274-Ma composite mafic–ultramafic intrusion consisting of four separate intrusive units: a large layered gabbroic sequence (phase I), a sheet-like ultramafic body (phase II), a dyke-like gabbronorite body (phase III), and an irregular ultramafic unit (phase IV). Important sulfide mineralization is present in the last three intrusive units, predominantly as disseminated and net-textured sulfides (pyrrhotite, pentlandite, and chalcopyrite). The Huangshandong mafic–ultramafic intrusive rocks are characterized by arc-like geochemical signatures such as low Ca content in olivine and negative Nb–Ta anomalies in whole rocks. This, together with a post-subduction setting for the East Tianshan in the Permian, suggests that the source mantle was modified previously by slab-derived fluids in the Carboniferous. The mantle-derived magma was ponded in a staging chamber in the lower part of the newly formed arc crust. The first batch of magma to arrive at Huangshandong was most fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo67, <300 ppm Ni). The second batch of magma was more primitive, crystallizing more primitive olivine (Fo81–84). The third batch of magma was also highly fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo72, ~600 ppm Ni). The final batch of magma became more primitive again, crystallizing the most primitive olivine (Fo81–86). The occurrence of rounded sulfide inclusions in olivine primocrysts in the Huangshandong ultramafic rocks indicates that immiscible sulfide liquid droplets were present during olivine crystallization. The Ni tenors of disseminated sulfide ores in the gabbronorite dyke vary mainly between 5 and 8 wt%, which are too high to have been produced by the parental magma of the dyke. The Ni, Cu, and platinum-group elements (PGE) tenors of disseminated sulfide ores in the dyke (phase III) and the ultramafic sheet (phase II) are remarkably similar. These observations, together with the sequence of magma emplacement, suggest that the sulfide liquids entrapped in the magma of the dyke formed at depth by a previous pulse of more primitive magma. The estimated parental magma for the most primitive lherzolites in the Huangshandong intrusion contains 10 wt% MgO. Modeling shows that sulfide saturation in the parental magma of the Huangshandong lherzolites could have resulted from fractional crystallization. Significant PGE depletions relative to Ni and Cu in the disseminated sulfide ores of the Huangshandong deposit may be due to sulfide retention in the source mantle.