Rare Earth Element Fluorocarbonate Minerals from the Olympic Dam Cu-U-Au-Ag Deposit, South Australia
Rare Earth Element Fluorocarbonate Minerals from the Olympic Dam Cu-U-Au-Ag Deposit, South Australia
复制标题
南澳大利亚奥林匹克大坝铜-铀-金-银矿床中的稀土元素氟碳酸盐矿物
DOI：
10.3390/min7100202
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发表时间：
2017
期刊：
影响因子：
2.5
通讯作者：
Vadim Kamenetsky
中科院分区：
文献类型：
--
作者：
Danielle S. Schmandt;N. Cook;C. Ciobanu;K. Ehrig;B. Wade;Sarah E. Gilbert;Vadim Kamenetsky
奥林匹克大坝是位于南澳大利亚高勒克拉通的世界级角砾岩氧化铁铜金铀矿床。它含有高浓度的稀土元素 (REE)，这些稀土元素以氟碳铈矿、合晶石、萤石、独居石和磷钇矿的形式出现。这是第一项关注奥林匹克大坝最丰富的稀土矿物、氟碳铈矿和次级合质岩的矿物学和成分的研究。样品组延伸到整个矿床，代表不同的硫化物矿化类型（黄铜矿-斑铜矿和斑铜矿-辉铜矿）和各种类型的角砾岩，包括以花岗岩、岩脉和赤铁矿碎屑为主的角砾岩。稀土氟碳酸盐（氟碳铈矿和合质矿）通常以细粒（<50 μm）的形式散布在铜铁硫化物和脉石矿物中，也存在于角砾岩基质中。它们还局部集中在矿床各处富含稀土矿物的宏观区域内。这种粗粒度样本构成了本研究的主要目标。氟碳铈矿可分为三种一般结构组：基质（进一步分为浸染型、细粒型和粗短型）、不规则型（硫化物伴生型）和碎屑置换型。纹理主要由特定位置和主要矿物组合决定，形态和粒度通常由相关矿物（赤铁矿、硫化物）控制。主要元素浓度数据揭示了 REE 氟碳酸盐之间有限的成分变化；全部都是 Ce 主导的。 REE 氟碳酸盐之间细微的成分差异定义了从相对富集 La 相到富集 (Ce + Nd) 相的光谱。花岗岩衍生的热液可能是稀土元素氟碳酸盐中 F 以及一些 CO2 的来源，这也可能是由相关的镁铁质-超镁铁质岩浆作用造成的。然而，Cl-配体运输 REE 是最有可能的情况。短粗氟碳铈矿和合长岩可能较早形成，与花岗岩从长石中释放出钙的热液蚀变同时发生。其他类别的氟碳铈矿，特别是那些与硫化物共存并到达奥林匹克大坝 IOCG 矿化顶部（辉铜矿 + 斑铜矿带）的矿体相对较年轻。这种解释与不同类别的稀土元素模式的微妙变化是一致的。整个矿床中氟碳铈矿和萤石的常见组合是典型的赤铁矿角砾岩，可以在 T 约 300 °C 的中性、微酸性流体（绢云母稳定性）中沉积。
Olympic Dam is a world-class breccia-hosted iron-oxide copper-gold-uranium ore deposit located in the Gawler Craton, South Australia. It contains elevated concentrations of rare earth elements (REE) which occur as the REE minerals bastnasite, synchysite, florencite, monazite, and xenotime. This is the first study to focus on the mineralogy and composition of the most abundant REE mineral at Olympic Dam, bastnasite, and subordinate synchysite. The sample suite extends across the deposit and represents different sulfide mineralization styles (chalcopyrite-bornite and bornite-chalcocite) and breccias of various types, ranging from those dominated by clasts of granite, dykes, and hematite. The REE-fluorocarbonates (bastnasite and synchysite) typically occur as fine-grained (<50 μm) disseminations in Cu-Fe-sulfides and gangue minerals, and also within breccia matrix. They are also locally concentrated within macroscopic REE-mineral-rich pockets at various locations across the deposit. Such coarse-grained samples formed the primary target of this study. Three general textural groups of bastnasite are recognized: matrix (further divided into disseminated, fine-grained, and stubby types), irregular (sulfide-associated), and clast replacement. Textures are largely driven by the specific location and prevailing mineral assemblage, with morphology and grain size often controlled by the associated minerals (hematite, sulfides). Major element concentration data reveal limited compositional variation among the REE-fluorocarbonates; all are Ce-dominant. Subtle compositional differences among REE-fluorocarbonates define a spectrum from relatively La-enriched to (Ce + Nd)-enriched phases. Granite-derived hydrothermal fluids were the likely source of F in the REE-fluorocarbonates, as well as some of the CO2, which may also have been contributed by associated mafic-ultramafic magmatism. However, transport of REE by Cl-ligands is the most likely scenario. Stubby bastnasite and synchysite may have formed earlier, coincident with hydrothermal alteration of granite releasing Ca from feldspars. Other categories of bastnasite, notably those co-existing with sulfides, and reaching the top of the IOCG mineralization at Olympic Dam (chalcocite + bornite zone) are relatively younger. Such an interpretation is concordant with subtle changes in the REE patterns for the different categories. The common association of bastnasite and fluorite throughout the deposit is typical of the hematite breccias and can be deposited from neutral, slightly acidic fluids (sericite stability) at T ≈ 300 °C.