Formation of weathering-derived magnesite deposits in the New England Orogen, New South Wales, Australia: Implications from mineralogy, geochemistry and genesis of the Attunga magnesite deposit
Formation of weathering-derived magnesite deposits in the New England Orogen, New South Wales, Australia: Implications from mineralogy, geochemistry and genesis of the Attunga magnesite deposit
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澳大利亚新南威尔士州新英格兰造山带风化菱镁矿矿床的形成：阿通加菱镁矿矿床的矿物学、地球化学和成因的影响
DOI：
10.1007/s00126-012-0440-5
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发表时间：
2013
影响因子：
4.8
通讯作者：
B. Dlugogorski
中科院分区：
文献类型：
--
作者：
H. Oskierski;J. Bailey;E. Kennedy;G. Jacobsen;P. Ashley;B. Dlugogorski
澳大利亚新英格兰造山带中由风化作用形成的结核状、隐晶质菱镁矿矿床是高纯度菱镁矿的重要来源。常见的结构特征及相关的同位素指纹表明，阿通加超镁铁岩中的风化菱镁矿矿床与昆瓦拉拉沉积物中的风化菱镁矿矿床之间存在密切的成因联系，而硅质碳酸盐岩蚀变和罕见的热液菱镁矿脉矿床则反映了不同的形成条件。土壤环境中碳酸盐的局部风化使稳定同位素组成向风化菱镁矿所特有的低δ13C和高δ18O方向偏移，而与侵入体有关的流体不会显著改变受影响碳酸盐的同位素组成。在阿通加，菱镁矿由不规则的结核状矿脉和矿体组成，填充在风化蛇纹岩寄主岩的断层和裂隙中，在普遍的蛇纹岩蚀变带中还有软粉状菱镁矿。阿通加的高品位菱镁矿可能会受到无定形二氧化硅和蛇纹石残余物的污染，但不像其热液等效矿床——皮埃蒙特菱镁矿矿床或大蛇纹岩带中其他广泛分布的硅质碳酸盐岩矿床那样含有白云石或含铁菱镁矿。较高的δ18O值与大气降水的表生成因相符，而较低的δ13C表明C3光合植物是阿通加菱镁矿碳的主要来源。我们推断，像阿通加菱镁矿矿床这样赋存于超镁铁岩中的由风化作用形成的结核状菱镁矿矿床是通过一个两步过程形成的，包括前期菱镁矿矿床的深成作用形成以及大气降水的完全表生叠加改造，大气降水通过土壤渗透获取碳。
Nodular, cryptocrystalline, weathering-derived magnesite deposits in the New England Orogen, Australia, provide a significant source of high-purity magnesite. Common textural features and related isotopic fingerprints indicate a close genetic relationship between weathering-derived magnesite deposits hosted by ultramafic rocks at Attunga and by sediments at Kunwarara while silica-carbonate rock alteration and rare hydrothermal magnesite vein deposits reflect contrasting conditions of formation. Localised weathering of carbonates in a soil environment shifts stable isotopic composition towards low δ13C and high δ18O typical for weathering-derived magnesites while intrusion-related fluids do not significantly change the isotopic composition of affected carbonates. At Attunga, magnesite consists of irregular, nodular veins and masses filling faults and cracks in the weathered serpentinite host rock as well as soft powdery magnesite in pervasive serpentinite alteration zones. The high-grade magnesite at Attunga can be contaminated by amorphous silica and serpentine relicts but does not contain dolomite or ferroan magnesite as observed for its hydrothermal equivalent, the Piedmont magnesite deposit, or other widespread deposits of silica-carbonate rock in the Great Serpentinite Belt. Heavy δ18O values are compatible with a supergene formation from meteoric waters while low δ13C suggests C3-photosynthetic plants as the predominant source of carbon for the Attunga magnesites. We infer that weathering-derived, nodular magnesite deposits hosted in ultramafic rocks like the Attunga magnesite deposit have formed in a two-step process involving the hypogene formation of a pre-cursor magnesite deposit and complete supergene overprinting by meteoric waters that acquired carbon from percolation through soil.