Mineralogy and major-element geochemistry of the lower Permian Greta Seam, Sydney Basin, Australia

Abstract

The mineralogy of the high-volatile bituminous coals and associated strata from the Greta seam, Sydney Basin, Australia, has been evaluated in this study. Although the seam is not immediately overlain by marine strata, percolation of marine water into the original peat bed is indicated by the petrological, mineralogical and geochemical characteristics, which resemble those of coals with marine roof strata. The upper and lower sections of the seam have contrasting mineralogy. Pyrite typically comprises 40 to 56 wt% of the mineral assemblage in the marine-influenced upper part of the seam section. The lower part contains much less pyrite (typically <5 wt%, organic-free basis), and also relatively abundant dawsonite (up to 14 wt%, organic-free basis). The minerals within most coal plies are largely of authigenic origin. These include pyrite, siderite, clay minerals (mainly kaolinite and Na-rich mixed-layer illite/smectite), and quartz, most of which have a relatively early, syngenetic origin. Minor Ti-bearing minerals, anatase or rutile, and phosphate minerals, fluorapatite and goyazite, were probably also formed during early diagenesis. Other minerals have features that indicate late-stage precipitation. These include abundant cleat- and fracture-filling dawsonite, which may be the result of reactions between earlier-precipitated kaolinite and Na₂CO₃- or NaHCO₃-bearing fluids. Minor albite may also be epigenetic, possibly precipitated from the same Ca–Al bearing fluids that formed the dawsonite. The most abundant detrital minerals in the Greta coals are quartz, poorly ordered kaolinite, illite and mixed-layer illite/smectite (I/S). These occur mainly in the floor, roof and other epiclastic horizons of the seam, reflecting periods of greater clastic influx into those parts of the original peat-forming environment. Detrital minerals are rare in the coals away from the epiclastic horizons, probably owing to almost complete sediment bypassing in the depositional system. Alternatively, any detrital minerals that were originally present may have been leached from the peat bed by diagenetic or post-diagenetic processes.

澳大利亚悉尼盆地Greta煤层的高挥发烟煤及其相关地层的矿物学特征在本研究得以评估。虽然煤层没有被海相地层直接上覆，岩石学、矿物学和地球化学特征显示海水渗透到原来的泥炭层，类似于具海相顶板岩层的煤的特征。含煤层上部和下部的矿物学特征反差大。黄铁矿通常占受海洋影响的上半部分煤层中的矿物组合的40至56 wt%。下部分所含黄铁矿少得多（通常< 5 wt％ ，不含有机物的基础上） ，而且片钠铝石相对丰富（高达14 wt％ ，不含有机物的基础上） 。大部分煤炭层内的矿物质在很大程度上是自生来源，包括黄铁矿、菱铁矿、粘土矿物（主要是高岭石和富钠混层伊利石/蒙脱石）和石英，其中大部分都是比较早期的同生来源。少数含钛矿物、锐钛矿或金红石和磷等矿物、氟磷灰石和磷锶铝石，很可能也形成于早期成岩作用。其他矿物具有的特点指示后期沉淀。这些特点包括大量的片钠铝石割理和裂缝充填，这可能是早期沉淀的高岭石和含碳酸钠或碳酸氢钠流体之间反应的结果。少数钠长石也可能是后生的，可能从形成片钠铝石的含钙-铝流体中沉淀而成。Greta煤炭中最丰富的碎屑矿物为石英、无序高岭石、伊利石和混层伊利石/蒙脱石（I / S ）。这些主要发现于煤层底面、顶面和其它表生碎屑层之内，反映了一段大量碎屑涌入原始泥炭形成环境的时期。碎屑矿物在远离表生碎屑层的煤炭中是罕见的。这可能是由于在沉积体系中沉积物几乎完全绕过。另外，任何原本存在的碎屑矿物可能已在成岩或后期成岩过程中从泥炭层中浸出。

KEY WORDS:

• mineral matter
• coal
• kaolinite
• dawsonite
• X-ray diffraction
• electron microscopy

ACKNOWLEDGEMENTS

The authors wish to thank CSIRO Energy Technology, and Peter Krempin of Austar Coal Mining Pty Ltd, for providing the channel and drill core samples, respectively, as well as other relevant data on which to base the investigation. Thanks are expressed to Irene Wainwright and Eugene White, of the Mark Wainwright Analytical Centre at UNSW, for assistance with the analytical program, and to Joanne Wilde of UNSW for preparation of polished and thin-sections. Thanks are also due to Owen Farrell and David Jacyna of CSIRO for preparation of the analytical samples. Thanks are also expressed to guest editor Adrian Hutton, reviewer James Hower and another anonymous reviewer for their constructive comments on the manuscript. This research was in part supported by the National Key Basic Research Program of China (no. 2014CB238904) and National Natural Science Foundation of China (no. 41302128).