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The Caixiashan carbonate hosted Zn-Pb deposit is located ~200 km SW of Hami city in the Xinjiang Uygur Autonomous Region of NW China.
The deposit is situated between the Central Asian Orogenic Belt and Tarim block, and hosted in the northern margin of the Central Tian Shan Terrane, which lies in the southern part of the Eastern Tianshan ranges, between the Junggar Basin in the north and Tarim Basin in the south.
The Eastern Tianshan was affected by the collision, from the Ordovician to the Carboniferous, of the Junggar and the Tarim blocks (Allen and Natal'in, 1995; Pirajno et al., 2008) and is usually divided into four parts from, north to south, namely the Dananhutousuquan, Kanggurtag-huangshan shear zone, Aqishan Yamansu volcanic belt and Central Tianshan Terrane. These divisions are separated by a series of east-west-trending faults, which are from north to south,the Kanggurtag, Yamansu, Aqikekuduke and Tuokexun fault zones.
The Caixiashan deposit is hosted in Precambrian platform carbonates of the late Mesoproterozoic Kawabulake Group. A Re/Os age of pyrite from the Kawabulake Group has been dated at 1019±70 Ma (Li et al., 2016), whilst a metamorphosed diorite cutting the group was dated at 1141±60 Ma (Xiu et al., 2002), and a granitic gneiss from the unit gave a metamorphic age (LA-ICP-MS U/Pb) of 941.9±4.9 Ma (Peng et al., 2012).
Other, smaller Pb Zn deposits (e.g., Jiyuan, Hongxishan, Hongyuan and Shaquanzi) are hosted within in the same rocks in the eastern part of the Central Tianshan Terrane.
The Kawabulake Group is well exposed in the Central Tianshan Terrane between the Aqikekuduke Fault to the north and the Tuokexun Fault to the SSW. It is stromatolitic (e.g., Conophyton cylindrical; Dong, 2005), and is unconformably overlain by the Lower Cambrian Huangshan Formation and is in fault contact with mid-Carboniferous Matoutan Formation in the deposit area. It is, in turn, conformably underlain by the Mesoproterozoic Xingxingxia Group in the Eastern Tianshan area (Cai et al., 2013; Xiu et al., 2002).
Li et al. (2002) suggest that during the Mesoproterozoic to Middle Neoproterozoic, a continental rift or passive continental margin existed on the northern margin of the Tarim Block, along the shoreline of the ancient (proto-Paleoasian Ocean basin, which formed prior to at least 890 Ma; Zheng et al., 2010). During the Palaeozoic, the Central Tianshan Terrane drifted northwards to the active southern margin of the Siberian Craton, resulting in the closure of the Paleoasian Ocean in the Late Carboniferous and Permian (Xiao et al., 2009). Subduction on both the northern and southern margins of the Central Tianshan Terrane no later than ~350 Ma, formed active arcs and back- and intra-arc systems (Xiao et al., 2009)
The host Kawabulake Group is composed of two lithologic successions. The basal succession comprises siltstones with chert interlayers, overlain by slate with siliceous interlayers and sandstone lenses, followed by sandstone and slate with quartz veins and capped by slate after a layer of siltstone. The upper succession comprises a basal package of sandstone containing a dolomitised marble interval, which hosts the Caixiashan deposit, overlain by a narrow carbonaceous shale within a siliceous rock layer, conformably succeeded by a massive sandstone layer, then a very thin layer of carbonaceous shale with siltstone lenses, capped by phyllite interbedded with siltstone and chert, with some small intercalations of carbon-bearing-shale, chert and andesite.
The bulk of the Caixiashan deposit occur as four high-grade ore zones (at a 0.5% Pb + Zn cut-off), numbered I to IV. Zone I strikes 078° and dips at 80°S, with an outcrop area that is between 6 and 210 m in width and 1000 m long. Zone II, which is 1500 m SE of Zone I strikes at 75° and dips at 70 to 75°S, exposed at the surface over a width of ~100 to 350 m and 1300 m in length. Zone III, which is ~500 m SW of Zone II, strikes at 78° and dips at 65 to 75°S, outcropping over a width of ~100 to 350 m, and strike length of ~500 m. Zone IV, which is 300 m south of the western half of Zone III, strikes at 90° and dips 70°S with an outcrop width of ~200 m and strike length of 900 m.
The orebodies of the Caixiashan deposit predominantly occur within dolomite to dolomitic-marble, or at the contact zone between dolomitic marble and the main carbonate unit of the Mesoproterozoic Kawabulake Group. They occur as massive layers, disseminations or veinlets distributed over broad intervals within the four ore zones, as listed above.
The Caixiashan Zn-Pb deposit is localised by at least three episodes of faulting, the earliest of which includes normal faults being slightly earlier than or coeval with the deposition of mineralisation, with a similar strike and dip (i.e., 70 to 80°S) to the bedding, and occur mostly toward the basin margin, where it is indicated they are synsedimentary, controlling the deposition of the Kawabulake Group.
The second episode comprises NE-trending normal faults which control the distribution of high grade ore in Zones I and IV. The third episode comprises faults that cross cut those of the second episode, which are restricted to the main deposit and usually cut both the ore body and host rock.
There is no no clear spatial relationship between magmatism in the mining area and the mineralisation, with many of the magmatic rocks having been emplaced between 350 and 320 Ma (Gao et al., 2006 ). In fact, numerous diorite dykes emplaced along fractures and crosscutting the orebodies, dilute and decreases the ore grade, and have generated variable degrees of tremolite and silica alteration in the wall rock.
During the Palaeozoic, the Mesoproterozoic carbonate rocks were locally metamorphosed to greenschist facies, whilst dynamic metamorphism having generated mylonites along faults.
Mineralisation and Alteration
Open-space filling of voids, brecciated zones and replacement of the host rocks is evident, with sedimentary fragments cemented by calcite, quartz and/or sulphides. The wall rock alteration assemblage includes dolomite, quartz and tremolite. Dolomitic alteration is the most significant, occurring in both footwall and hanging wall rocks, with a close association between sulphides and dolomitised marble, where the sulphides are commonly found as disseminations and veinlets within the dolomite. Dolomite is generally found as either fine-grained crystals in the rock matrix, or medium- to coarse-grained euhedral crystals.
The ore mineralogy of the main mineralising stage is relatively simple assemblage, with the principal ore mineral being sphalerite with lesser galena, with associated pyrite, pyrrhotite, arsenopyrite with trace chalcopyrite, kilbrickenite (Pb9Sb22S42), tetrahedrite (Cu12Sb4S13), boulangerite (Pb5Sb4S11), bournonite (CuPbSbS3) and pyrargyrite (Ag3SbS3; Liang et al., 2008). The gangue minerals are mainly dolomite, calcite and tremolite, minor quartz.
Each if the four ore zones is composed of a series of lens-like orebodies with widths that vary from a few metres to 100 m in width and down dip lengths of as much as 1200 m and are encapsulated within an envelope of dolomitisation that defines the individual zone with the dimensions detailed above.
The mineralisation paragenesis at Caixiashan can be divided into four stages (after Li et al., 2015):
•Diagenetic pyrite stage, characterised by pyrite, dolomite and calcite. Pyrite 1 occurs as massive accumulations hosted in limestone, and has in part been replaced by thin veins of sphalerite, or coexists with calcite and dolomite containing numerous fluid inclusions. In thin section, the massive pyrite typically has framboidal and colloform textures. The framboidal variety contains tiny pieces of anhedral pyrite, locally enclosed by fine-grained sphalerite. This pyrite locally exhibits pseudomorphic replacement of colloform pyrite by sphalerite in which the original texture is preserved.
•Pyrite-sphalerite-galena-carbonate stage, which has been divided into three sub-stages by Li et al. (2015), from early to late, i.e.,
i). Pyrite-alteration (pyrite 2), which commonly occurs as layers within the matrix which is composed of medium-grained recrystallised calcite and dolomite. The uniform layering of this pyrite 2 within the matrix is taken to suggest the pyrite was formed simultaneously with the recrystallised carbonates, distinct from pyrite 1. Pyrite 2 is also locally replaced by small amounts of sphalerite of the succeeding sub-stage as well as narrow galena veins. Some of these sphalerite/galena veins occur between the pyrite layers whereas others cut those layers. The host rock clasts are cemented by dolomite + sphalerite.
ii). Sphalerite-carbonate, which is characterised by abundant massive sphalerite that is co-precipitated with medium-grained subhedral to euhedral dolomite and calcite, followed by lesser disseminated sphalerite coexisting with calcite veins that locally crosscut the host limestone. This sphalerite also replaces pyrite 1. Some of the massive sphalerite of this sub-stage ore contains minor pyrrhotite as small grains with linear distribution showing possible exsolution textures, suggesting the pyrrhotite was probably co-precipitated with the massive sphalerite. This main stage sphalerite also locally coexists with arsenopyrite.
iii). galena-pyrite-carbonate, that is dominated by abundant fine-grained galena which occurs as small veins cutting the dolomitised marble host rock, or locally cutting the massive sphalerite ore of the preceding sub-stage. This galena commonly has sharp boundaries with, but locally replaces granular pyrite precipitated during this sub-stage (pyrite 3, which is commonly granular and coarse-grained, in contrast to the fine graine pyrite 1 and 2), suggesting that the galena probably precipitated slightly later than pyrite 3. The arsenopyrite included in the previous sub-stage is found in both the host rock and within pyrite 3, implying the arsenopyrite predates pyrite 3. Textures show that brecciated limestone is cemented by pyrite 3 or by calcite/dolomite. Galena is usually found as anhedral grains, veinlets in a carbonate matrix or crosscutting sphalerite. Some pyrrhotite is enclosed by massive galena, suggesting the pyrrhotite may have formed a little ahead of the galena, consistent with the observation of coexisting sphalerite and pyrrhotite. Meanwhile, abundant medium- to coarse-grained dolomite and calcite precipitated during precipitation of galena and pyrite 3. Trace Pb-bearing minerals such as kilbrickenite, tetrahedrite, boulangerite, bournonite and pyrargyrite are also present in this stage.
•Magmatic/metamorphic reworking, magmatic-hydrothermal reworking at Caixiashan is characterised by a post-mineralisation of assemblage of tremolite-quartz-calcite, with local sphalerite-quartz, and trace pyrrhotite and chalcopyrite, all of which overprint the main stage sphalerite and calcite/dolomite. The fine-grained calcite of this stage occurs as veins crosscutting the massive sphalerite as well as the calcite veins of the previous stage. Tremolite occurs as veins cutting sphalerite, and coexists with some subhedral coarse-grained calcite, although locally, tremolite veins cut the earlier calcite and sphalerite. Some fractures are filled with quartz that occurs as chalcedony or fine- to medium-grained, anhedral to subhedral crystals and locally coexists with euhedral tremolite (Li et al., 2015).
•Supergene stage is characterised by jarosite which is well developed along fractures and is found to depths of up to 17 m below the surface.
Current reserves are quoted at: 131 Mt at 3.95% Pb+Zn (Zn > Pb; cutoff by 0.5% Pb + Zn (Cao et al., 2013).
This summary is drawn from the references listed below.
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Li, D.-F., Chen, H.-Y., Hollings, P., Li, Z., Mi, M., Li, J., Fang, J., Wang, C.-M. and Lu, W.-J., 2016 - Re-Os pyrite geochronogy of Zn-Pb mineralization in the giant Caixiashan deposit, NW China: in Mineralium Deposita v.51 pp. 309-317|
Li, D.-F., Chen, H.-Y., Li, Z., Hollings, P., Chen, Y.-J., Lu, W.-J., Zheng, Y., Wang, C.-M., Fang, J., Chen, G. and Zhou, G., 2015 - Ore geology and fluid evolution of the giant Caixiashan carbonate-hosted Zn-Pb deposit in the Eastern Tianshan, NW China, : in Ore Geology Reviews v.72, pp. 355-372.|
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