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Longtou |
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Guangxi, China |
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Mn
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High-grade Lower Carboniferous Longtou carbonate-hosted manganese deposits are found ~25 km south of the city of Hechi, Guangxi Province, on the southwestern South China Block and ~570 km WNW of Guangzhou. Similar mineralisation occurs over a wide surrounding area of at least 100 km radius.
The Longtou Mn deposit is the largest and most economically significant Carboniferous Mn deposit in the South China Block (Hou and Xue, 1996; Yan et al., 2017) and has been mined for over 60 years. The annual production of Mn-carbonate ores since 1950 has been ~0.1 Mt. Remaining ore reserves are greater than 13.5 Mt of Mn carbonate (Ru et al., 1992). The average grade is 18.56 wt.% Mn.
The southwestern South China block underwent extension and the development of a passive continental margin during the Devonian to Carboniferous (Liu et al., 2002; Gu et al., 2012; Du et al., 2013; Deng et al., 2017, 2019; Chen et al., 2018; Wang et al., 2020), leading to a palaeogeographic pattern of isolated carbonate platforms surrounded by deep basins in the Late Devonian (Bureau of Geology and Mineral Resources of Guangxi Zhuang Autonomous Region, 1985; Yang et al., 2012; Du et al., 2013). By the early Carboniferous, the southwestern South China block was located near the equator (Metcalfe, 2013). As such, it remained a passive continental margin but with less isolated platforms (Ma et al., 2009). Extension persisted into the late Permian but ceased when oceanic subduction began in the Triassic (Bureau of Geology and Mineral Resources of Guangxi Zhuang Autonomous Region, 1985; Du et al., 2009, 2013; Ma et al., 2009; Yang et al., 2012).
The Longtou deposit was formed in a Lower Carboniferous extensional basin. The open carbonate platforms of the South China Block were surrounded by basinal facies that transitioned to bathyal sea facies to the south and connected to a branch of the Paleo-Tethys Ocean (Chen et al., 2018). The facies distribution suggest that in the Longtou region the platform was a gently sloping mixed clastic-carbonate shelf, with Mn mineralisation being mainly developed along the shelf margin (Ru et al., 1992).
Primary manganese mineralisation occurs at the top of the Lower Carboniferous ~359 Ma Baping Formation, which has been metamorphosed to greenschist facies (Ru et al., 1992). This ~130 m thick succession is composed of carbonate rocks with minor chert and siltstone that are laterally correlative throughout the southwestern South China Block (Ru et al., 1992; Hou and Xue, 1996). Stratiform Mn horizons are interbedded with bioclastic limestone (Zhang, 1995; Yan et al., 2017) and are generally composed of parallel-laminated rhodochrosite (MnCO3), Mn-calcite with minor alabandite (MnS), and amalgamated rhodochrosite, Mn calcite, and braunite (Mn2+Mn 3+6[SiO4)O8]. The Baping Formation conformably overlies the Luzhai Formation, an ~40 m thick succession of siltstone and chert. Its base is a conformable contact with the Upper Devonian Wuzhishan Formation, which is composed of limestone and chert (Chen et al., 2018; Wang et al., 2020).
Vertical lithofacies indicate that the Luzhai and Baping Formations represent deposition through one relative sea-level cycle that is ~170 m thick and comprises siltstone, chert, carbonate rocks and Mn mineralised horizons. The base of the sequence lies conformably on the carbonate rocks of the Wuzhishan Formation, taken to indicate the area was located beyond the influence of subaerial exposure during lowstand. The lowstand tract is ~40 m thick and comprises current-rippled siltstone (facies 1 of the Luzhai Formation) and parallel-bedded chert (facies 2 of the Luzhai Formation) that accumulated on the middle shelf. These facies are commonly interbedded and composed of variable quartz and Fe silicates influenced by the input of terrigenous silts. Sedimentary rocks of the transgressive system tract are ~130 m thick and composed of middle shelf parallel-bedded crinoid packstone to grainstone (facies 3 of the Baping Formation) and distal shelf lime mudstone (facies 4 of the Baping Formation). Commencement of transgression is marked by the first appearance of facies 4 lime mudstone overlying facies 1 siltstone, recording a sharp increase in sea level. The sediments of facies 3 and 4 are composed of heterozoan carbonate rocks with the absence of silts, except where facies 1 is interbedded with facies 4 at the base of the Baping Formation, representing parasequences of higher-frequency sea-level fluctuations. Interbedded facies 3 and 4 throughout the transgressive system tract indicates numerous parasequences in an overall trend of deepening water (retrogradation). Mn carbonate lime mudstone (facies 5), Mn carbonate and braunite in crinoid packstone to grainstone (facies 6), and local (laterally discontinuous) Mn calcite-coated grainrich packstone to grainstone (facies 7) indicate maximum flooding, where the stacking of Mn ore horizons indicate stratigraphic condensation and diminished sedimentation (Föllmi, 2016). During maximum flooding, the 'carbonate factory' was shut down and replaced by the precipitation of authigenic Mn minerals on the middle (facies 6 and 7) to distal shelf (facies 5). The maximum flooding zone is unconformably overlain by limestone breccia, indicating that the highstand system tract is missing. It has been suggested this might be the result of the influence of erosion (Read and Grover, 1977) that happened locally in the early Carboniferous. Chen et al. (2022) conclude from their study of this area that the Palaeozoic deep ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Mn- and Fe-rich deposits in areas such as the South China Block platforms.
This summary is drawn from Chen et al., 2022, cited below, which should be consulted for a detailed description of the individual lithofacies and paragenesis.
The most recent source geological information used to prepare this decription was dated: 2022.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Chen, F., Pufahl, P.K., Wang, Q., Matheson, E.J., Shabaga, B.M., Zhang, Q., Zeng, Y., Le, X., Ruan, D. and Zhao, Y., 2022 - A New Model for the Genesis of Carboniferous Mn Ores, Longtou Deposit, South China Block: in Econ. Geol. v.117, pp. 107-125.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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