Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland
Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland
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DOI：
10.1016/j.oregeorev.2016.08.031
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发表时间：
2017
影响因子：
3.3
通讯作者：
S. Graham;D. Holwell;I. McDonald;Gawen R. T. Jenkin;N. J. Hill;A. Boyce;J. Smith;C. Sangster
中科院分区：
文献类型：
--
作者：
S. Graham;D. Holwell;I. McDonald;Gawen R. T. Jenkin;N. J. Hill;A. Boyce;J. Smith;C. Sangster
岩浆硫化物矿床通常产于超镁铁质-镁铁质系统中，然而，矿化作用可以发生在更多的中碱性岩浆中。Sron Garbh是侵位于苏格兰廷德鲁姆地区达拉迪安变质沉积物中的斜长闪长岩侵入岩，在斜长岩带部分有岩浆铜镍铂金硫化物矿化。因此，它是碱性岩石赋存的岩浆硫化物矿化的一个例子，也是不列颠群岛已知的最重要的矿化贴面侵入体。侵入体的形状不规则，有一个闪长岩边缘，由角闪石堆积物组成，被归类为辉石岩。侵入体的中央部分由未矿化但含黄铁矿的闪长岩组成。闪长岩和闪长岩具有相似的微量元素地球化学特征，表明闪长岩是闪长岩的一种分异产物，属于苏格兰加里多尼德岩系的高Ba-Sr侵入岩。矿化以浸染的原生黄铜矿-黄铁矿-PGM组合和气泡状黄铁矿-黄铜矿组合的形式存在，并具有显著的富钴黄铁矿。两个组合均含有少量的镁橄榄石和镍钴硫化物。矿化为富铜、富PPGE、富Au和贫IPGE，铂族矿物组合以Pd矿物为主，但块状岩石的铂/钯比值约为0.8。对硫化物的激光烧蚀分析表明，黄铁矿和镍钴硫化物是铂的主要宿主，铂在黄铁矿中以固溶体形式存在，浓度高达22ppm。所有贱金属和贵金属之间的良好相关性表明，尽管有一些次生黄铁矿和PGM的证据，但金属的热液再活动性很小。硫同位素数据表明，岩浆硫化物组合中含有地壳S。其来源不太可能是当地的石英岩，但深海中存在着富含S的达拉迪亚沉积物。Sron Garbh岩浆Cu-Ni-PGE-Au矿化的形成可归因于碰撞后板片的脱落，使含水的低程度部分熔融发生，产生了通过地壳上升的富Cu-PPGE-Au熔体，同化了达拉迪安沉积物中的地壳S。在苏格兰加里多尼德地区的贴面侵入岩中发现大量富含PGE的硫化物，这表明该地区具有某些特征，使其比世界上其他碱性省更具远景，这可能与后加里东期板片剥落事件有关。我们认为，先前存在的岩浆硫化物或“参考”地幔在低程度部分熔体中的不一致熔融可以产生具有特征的分馏、含铜PPGE-Au半金属、含水、碱性熔体，如果它们经历硫化物饱和，则有可能产生碱性岩浆硫化物矿床。
Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or ‘refertilised’ mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.