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Meixian Orefield - Dingjiashan, Fengyan |
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Fujian, China |
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Zn Pb Ag Cu Mo
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Super Porphyry Cu and Au
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The Meixian Zn-Pb polymetallic orefield is located in southern-central Fujian Province in south-eastern China, ~200 km north of Xiamen and 100 km west of Fuzhou. The orefield contains a number of Zn-Pb-(Ag) deposits of varying size, the largest of which are Dingjiashan and Fengyan, as described below. Others include Jingjikeng, Guandou, Xiadu, Zaitou and Xiekeng.
(#Location: Fengyan - 26° 15' 42"N, 118° 17' 17"E; Dingjiashan - 26° 14' 11"N, 118° 13' 17"E).
Regional Setting
The orefield lies within the Southeastern Coastal Metallogenic Belt where Mesozoic volcanic rocks are extensively exposed, with widespread associated W-Sn-Cu-Mo-Pb-Zn-Au-Ag polymetallic mineralisation. In addition, it includes many Zn-Pb polymetallic deposits with subordinate Cu-Au-Mo-Ag. The metallogenic belt also contains stratabound Zn-Pb(-Ag) deposits, such as the Dingjiashan, Fengyan, Bawaiyang, Shuiji, Shanhou, Wangdihou, Longgang, Xiashan, Xiyuan and Daixi hosted by Proterozoic metamorphic basement rocks in basement windows surrounded by Jurassic volcanic cover. The latter suite of deposits have been variously interpreted to represent i). volcanogenic hosted massive sulphide (VHMS) deposits related to Proterozoic volcanic rocks that have been metamorphosed and deformed by Palaeozoic and Late Mesozoic tectonic events (Xu et al., 1996, Ye et al., 1999, Wu et al., 2004, Wu et al., 2019, Di et al., 2006, Feng et al., 2008, Zhang et al., 2009); or ii). skarn altered replacement deposits related to Late Mesozoic granites (Zhang et al., 2011, Mao et al., 2018, Mao et al., 2021). Both interpretations acknowledge the metallogenic contributions by the Late Mesozoic granites.
The generally NNE-SSW aligned Southeastern Coastal Metallogenic Belt is located along the south-eastern margin of the Cathaysia Block. The latter was accreted to the Yangtze Craton to its NW, separated by the Qinhang Suture, to form the greater South China Block at ~1.1 to 0.9 Ga (Chen et al., 1991; Mao et al., 2021a). The NE-trending Zhenghe-Dapu crustal scale transform fault separates the Southeastern Coastal Metallogenic Belt from the Proterozoic rocks of the Cathaysia block (Liu et al., 2018; Mao et al., 2021).
In central Fujian, the regional stratigraphy comprises a Proterozoic basement window, surrounded by Palaeozoic to Lower Mesozoic sedimentary and Upper Mesozoic continental clastic and volcanic sequences. The basement comprises:
Palaeoproterozoic Mayuan Group - an amphibolite-facies metamorphic suite, the of which protoliths may have been mainly carbonaceous sandy mudstone and minor carbonate and intermediate-mafic volcanic rocks (Feng et al., 2008; Wu et al., 2019);
Meso- to Neoproterozoic Mamianshan Group - upper greenschist to lower-amphibolite facies volcano-sedimentary rocks, including granulite and schist, intercalated with minor magnetite-bearing diopsidic quartzite and (dolomitic) marble (Feng et al., 2008). This group is subdivided into the lower Longbeixi Formation, which is a significant host to Zn-Pb mineralisation, and the overlying Daling Formation.
Palaeozoic cover sequences are locally exposed to the SW, whilst Mesozoic successions are mainly found in the northern and eastern sections of the region. The NE-SW trending Zhenghe-Dapu fault and associated secondary structures are interpreted to have influenced the regional distribution of granites and Zn-Pb mineralisation (Gao et al., 1999; Zheng et al., 2000; Ni et al., 2018).
This part of central Fujian has experienced a complex magmatic evolution from the Proterozoic to the Mesozoic, with the Early Palaeozoic to Late Mesozoic magmatism interpreted to be closely linked to polymetallic mineralisation (Gao et al., 1999; Di et al., 2006; Mao et al., 2013; Mao et al., 2018; Wu et al., 2019).
Early Palaeozoic ~470 to 380 Ma granitoids are predominantly composed of two-mica syenogranite and monzogranite, which have been closely linked to Nb-Ta-Sn-Be-Li mineralisation (e.g., Gao et al., 1999; Zheng et al., 2000), and are accompanied by minor serpentinised ultramafic rocks, quartz diorite and granodiorite of similar age (Gao, 2007).
Carboniferous to Triassic ~350 to 200 Ma magmatism is only weakly developed and includes metaluminous to peraluminous and shoshonitic to high-K calc-alkaline diorite to syenite (Gao et al., 1999, Gao, 2007; Wu et al., 2019). Many of these granitoids host weak W-Cu-Pb-Zn-Au mineralisation (Gao et al., 1999; Di et al., 2006).
Middle Jurassic to Early Cretaceous peraluminous high-K and calc-alkaline diorite to granite are the most widely distributed granitoids (Feng et al., 2008; Sun et al., 2014; Xiao et al., 2020; Mao et al., 2021), many of which are interpreted to be genetically linked to regional Cu-Pb-Zn-Ag mineralisation (Mao et al., 2013; Mao et al., 2021). In addition to these intrusive rocks, Late Mesozoic subvolcanic rocks were emplaced along regional faults or within rift basins (Wu et al., 2004; Wu et al., 2019; Di et al., 2006).
Dingjiashan
The Dingjiashan Zn-Pb-Ag deposit is located 2 to 3 km west of the town of Meixian in the south-western section of the Meixian orefield, where the locally exposed rocks predominantly comprise Meso- to Neoproterozoic and Upper Jurassic sequences. The former include the Longbeixi Formation, which is composed of marble and quartz-mica schist, and the overlying Daling Formation mica schist, phyllite and meta-quartz sandstone to quartzite, intercalated with thin-bedded marble. The Longbeixi Formation is the principal host to mineralisation, and occurs in the eastern part of the deposit area. The Upper Jurassic sequence is in unconformable to faulted contact with the Proterozoic sequence, and is largely found in the central and western part of the area. It is mainly represented by the Changlin Formation, which is composed of thin-bedded greyish-green argillaceous siltstone, thick quartz sandstone, sandy conglomerate, andesite and andesitic-basalt.
The principal structure in the deposit area is the NE-trending Meixian anticline which has a core of the Longbeixi Formation, flanked by limbs of the Daling Formation. Four fault sets are distinguished, trending NE, NW, near east-west and near north-south. The NE trend is dominant, but is cut by the other three sets of faults.
The unconformity between the Proterozoic and Jurassic sequences is a key control of mineralisation, with Zn-Pb sulphides locally occurring in rocks on both sides of the horizon. Hydrothermal titanite which coexists with sphalerite yielded a 207Pb-corrected weighted mean 207Pb/238U age of 144 ±2 Ma in the Early Cretaceous (Xing et al., 2021). The are virtually no intrusive rocks are exposed at Dingjiashan, except for a few NNW-trending diorite dykes that are seen to locally cut both the orebodies and marble country rocks. However, several granitic porphyry dykes and quartz porphyry stocks are exposed at the periphery of the deposit area, with the former being LA-ICP-MS zircon U-Pb dated to ~148.9 to 152.0 Ma (Sun et al., 2014).
The Dingjiashan deposit comprises 17 lensoid orebodies of various sizes and grades, which are mainly enclosed by skarn alteration and localised along interlayer fractures between marble and quartz-mica schist. Individual orebodies are relatively continuous, among which those designated III1, III2, â…ˇ and I3 are the largest. In all the 17 orebodies, Pb as galena tends to be richest in the upper levels, whilst Cu-Fe as pyrrhotite and chalcopyrite tends to be enriched at lower levels.
The Dingjiashan alteration/mineralisation has been divided into four paragenetic stages:
Stage I, prograde skarn - which mainly comprises pyroxene with subordinate garnet and rhodonite, which commonly crosscut, enclose or replace the marble of the Longbeixi Formation.
Stage II, retrograde alteration - which is characterised by abundant epidote, quartz and magnetite; minor actinolite, tremolite and biotite; trace chlorite, ilvaite and hematite. This assemblage replaced/crosscuts garnet and pyroxene. The principal stage II metallic mineral is magnetite, which is mainly disseminated, but is locally massive. This magnetite occurs as fracture-infill in pyroxene, coexists with retrograde minerals (e.g., epidote and biotite), and is filled by sulphide, all of which indicates the magnetite formed after pyroxene but before sulphides.
Stage III, sulphide mineralisation - the main Zn-Pb-Ag mineralisation stage, which comprises various sulphide minerals, including abundant sphalerite, galena and pyrrhotite, and minor chalcopyrite and pyrite. These sulphides generally replace the marble of the Longbeixi Formation, the garnet, pyroxene and magnetite, as well as epidote, and chlorite. Silver mainly occurs within galena with concentrations ranging widely from a few tens of g/t to almost 500 g/t (ECMEDBNM, 2012, Xing et al., 2021), although a small amount is found as native silver (Chen, 2000).
Stage IV, quartz-calcite - represented by quartz–calcite veins (with minor pyrite and fluorite) that cut both the skarn and Zn-Pb-Ag mineralisation.
The Dingjiashan deposit is estimated to contain a proven reserve of 5.35 Mt @ 1.14% Pb, 4.07% Zn, plus >200 t of contained Ag @ 32.82 g/t Ag, as well as economic Cu, Cd and In (Xing et al., 2021).
Fengyan
The Fengyan Zn-Pb (Ag) deposit is located in the northeastern corner of the Meixian orefield, ~3 km NE of the town of Meixian. As at Dingjiashan, the exposed stratigraphy iss mainly of the Proterozoic Longbeixi and Daling Formations and the Jurassic Lishan Formation, with the Longbeixi Formation being the principal host to mineralisation. The latter is mainly found in the central and western part of the deposit area, whilst the Lishan Formation is largely to the east and comprises greyish-black argillaceous siltstone, carbonaceous shale, and fine-grained quartz sandstone.
NE and near north-south trending faults are the dominant structures. Exposed igneous rocks are predominantly red biotite granite to the northwest and NE-trending, 50 to 500 m wide pink granitic porphyry dykes that are up to 4.5 km long. The biotite granite, which locally shows strong potassic alteration, contains 20% quartz, 45 to 50% K feldspar, 20 TO 25% plagioclase and 5 to 10% biotite. The granite porphyry dykes contain 20 to 35% phenocrysts of quartz and K feldspar in a 65 to 80 vol.% fine-grained groundmass of quartz, K feldspar and biotite (Xing et al., 2022). The latter have yielded LA-ICP-MS zircon U-Pb ages of 158 to 155 Ma (Chen et al., 2018, Xiao et al., 2020). However, Xing et al. (2022) report LA-ICP-MS zircon U-Pb dating of the granite porphyry yielded a weighted mean 206Pb/238U age of 145.0 ±1.0 Ma, whilst hydrothermal titanite grains in the stratabound Pb-Zn mineralisation, intergrown with epidote and galena, yielded a weighted mean 207Pb-corrected 206Pb/238U age of 147.3 ±4.4 Ma.
Deep drilling has intersected biotite moyite (a quartz-rich biotite granite) and granite porphyry intrusions with associated Mo mineralisation beneath the current Fengyan mine (Chen et al., 2018, Xiao et al., 2020). The porphyry Mo mineralisation is hosted by the moyite and granite porphyry, or fills fracture zones of the Longbeixi Formation chlorite (-epidote) quartz schist. Metallic minerals include abundant molybdenite and subordinate pyrite, pyrrhotite, magnetite and sphalerite with minor scheelite and chalcopyrite. The non-metallic gangue mainly includes quartz, epidote and chlorite, with minor K feldspar, diopside, actinolite, tremolite and calcite (Xiao et al., 2020). Molybdenite occurs as veins or disseminations, and commonly coexists with quartz. Tonnage and grade figures for the Mo orebodies is still confidential in 2022 (Xing et al., 2022). Molybdenite from the concealed Mo orebody has a Re-Os model age of ~145.3 to 145.5 Ma (Xing et al., 2022).
Tens of stratabound Zn-Pb mineralised lenses of varying sizes have been identified at Fengyan, mostly trending NE and dipping at 8 to 30°NW. These lenses are mainly hosted within pyroxene skarn alteration with minor garnet, chlorite and rhodonite. The Longbeixi Formation quartz-mica schist is also locally mineralised along layer boundaries. Compared to Dingjiashan, the mineralised lenses at Fengyan are less continuous, but have higher Zn grades (ECMEDBNM, 2013). Chen et al. (2018) reported a vertical sulphide zonation similar to that at Dingjiashan, in which the galena content is higher in the upper part of the lense, whilst the sphalerite and chalcopyrite contents are higher at lower levels. The Zn/Pb ratio at Fengyan gradually increases from 1.59 to 58.43 with depth (Chen et al., 2018). The Mo mineralisation at depth below the Zn-Pb orebodies is located near the intrusive contact between the moyite and the Longbeixi Formation (Xiao et al., 2020). These data are taken to imply the Fengyan stratabound Zn-Pb mineralisation is the distal zone of porphyry style mineralisation.
The Fengyan alteration/mineralisation has also been divided into four paragenetic stages:
Stage I, prograde skarn - which is characterised by widely developed pyroxene, garnet and local rhodonite. This asseblage commonly encloses/crosscuts/replaces the host carbonate rocks.
Stage II, retrograde alteration - which is characterised by the magnetite-hematite mineralisation, with associated retrograde epidote, chlorite, actinolite, ilvaite, quartz and calcite that replaces/crosscuts garnet and pyroxene. Magnetite grains generally occur in the massive, disseminated and banded mineralisation, mainly as anhedral and local subhedral aggregates.
Stage III, sulphide mineralisation - the main Zn-Pb-Ag mineralising stage, characterised by various sulphides associated with quartz, comprising abundant sphalerite, minor galena, chalcopyrite and pyrite, and trace pyrrhotite. Silver is mainly found within galena, with a minor amount occurring as very fine-grained native silver (ECMEDBNM, 2019, Xing et al., 2022). Sulphide mineralisation commonly overprints marble, prograde garnet, pyroxene and rhodonite, and the retrograde altered magnetite, epidote and chlorite.
Stage IV, quartz-calcite - which forms a series of base metal sulphide-barren calcite-quartz veins locally with trace epidote and pyrite, which crosscut both the wallrocks and Zn-Pb mineralisation.
The Fengyan deposit is estimated to contain a proven reserve of 6.06 Mt @ 1.18% Pb, 6.24% Zn, as well as economic Ag, Cu, Cd, Co and In (ECMEDBNM, 2019).
The information in this summary is largely drawn from Xing et al. (2022)
The most recent source geological information used to prepare this decription was dated: 2022.
Record last updated: 21/10/2022
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.
Fengyan Dingjiashan
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Selected References:
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Pan, Y. and Peng, Z., 2016 - 3D quantitative prediction of Dingjiashan deposit based on morphological analysis of geological interfaces: in Journal of Theoretical and Applied Information Technology, v.88, pp. 350-357.
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Xing, B., Mao, J., Liu, H., Xiao, X., Jia, F., Li, H., Guo, S., Li, H. and Huang, W., 2022 - Porphyry Mo and distal Zn-Pb mineral system: An example of the Fengyan Zn-Pb-Mo deposit in central Fujian, SE China: in Ore Geology Reviews v.144, 18p. doi.org/10.1016/j.oregeorev.2022.104821.
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Xing, B., Mao, J., Xiao, X., Liu, C., Guo, S., Li, H., Huang, W. and Lai, C., 2022 - Metallogenic discrimination by sphalerite trace element geochemistry: An example from the Fengyan Zn-Pb deposit in central Fujian, SE China: in Ore Geology Reviews v.141, 16p. doi.org/10.1016/j.oregeorev.2021.104651.
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Xing, B., Mao, J., Xiao, X., Liu, H., Jia, F., Wang, S., Guo, S., Huang, W., Li, H. and Pirajno, F., 2021 - Genesis of the Dingjiashan Pb-Zn-Ag deposit, central Fujian region, SE China: Constraints from geological, mineralogical, geochronological and sulfur isotope data: in Ore Geology Reviews v.139, Part A, doi.org/10.1016/j.oregeorev.2021.104446.
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Xing, B., Mao, J., Xiao, X., Liu, H., Jia, F., Wang, S., Huang, W. and Li, H., 2021 - Genetic discrimination of the Dingjiashan Pb-Zn deposit, SE China, based on sphalerite chemistry: in Ore Geology Reviews v.135, doi.org/10.1016/j.oregeorev.2021.104212.
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Xing, B., Mao, J., Xiao, X., Liu, H., Yu, L., Li, H., Guo, S., Li, H. and Huang, W., 2022 - Genesis of the Dingjiashan and Fengyan Zn-Pb polymetallic deposits in central Fujian, SE China: Evidence from magnetite geochemistry: in Ore Geology Reviews v.144, 19p. doi.org/10.1016/j.oregeorev.2022.104851.
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