Gejiu District - Malage, Songshujiao, Gaosong, Laochang, Kafang
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The Gejiu tin-polymetallic ore district is located several tens of kilometres southeast of Gejiu City, Yunnan Province, SW China. The district comprises at least five significant Sn-Cu-Pb-Zn polymetallic deposits/mining centres, from north to south over a 25 km interval, namely Malage, Songshujiao, Gaosong, Laochang and Kafang. Malage and Kafang are ~6 km ENE and 20 km SSE of Gejiu City respectively.
(#Location - Laochang: 23° 17' 27"N, 103° 12' 4"E).
The five deposits, together, contain ~300 Mt of Sn ores at an average grade of 1% Sn, plus another 300 Mt of Cu ores averaging 2% Cu, and 400 Mt
of Pb–Zn ores with an average grade of 7% Pb+Zn (Cheng et al., 2013).
Gejiu is one of the largest and oldest tin mining districts in the world, having been worked intermittently for >2 millenia, since the 202 BC to 220 AD Han Dynasty.
The ~1600 km2 Gejiu tin-polymetallic district is situated on the western margin of the Cathaysia Block of South China, adjacent to the Yangtze Craton in the north and the Sanjiang Fold Belt in the west. To the north, it is bounded by the Mile-Shizong Fault, to the west by the regional Ailaoshan-Honghe strike-slip fault, and to the south by the North Vietnam Block. The district has been part of a tectonic sag depression for much of its geological history, with well-preserved Cambrian to Quaternary rock successions. Late Triassic to Cretaceous strata are preferentially exposed near and at surface as a result of uplift associated with Yanshanian (Mesozoic) tectonic event. The bulk of the outcrop in the Gejiu area is of the >3000 m thick Middle Triassic Gejiu Formation carbonates, and the 1800 to 2800 m thick Middle Triassic Falang Formation (Qin and Li, 2008). The latter is predominantly composed of fine-grained clastic sedimentary and carbonate rocks with interlayered mafic lavas.
The district is dislocated by numerous NNE-, NE and north-south faults. The north-south Gejiu fault is the dominant structure in the district, dividing it into eastern and western sectors, with the major deposits in the eastern of these blocks. A prominent igneous body in this area, the ~300 km2 Gejiu batholith, is composed of Mesozoic gabbro, with mafic microgranular enclaves, porphyritic biotite granite, equigranular biotite granite, syenites and mafic dykes. This batholith is predominantly in the western sector, which also contains mafic to intermediate and alkaline rocks. Lesser granitoids are distributed throughout the region. Granites in the vicinity of mineralisation have undergone intense greisen, albite and K feldspar alteration, with or without skarns in carbonate wallrocks.
The Gejiu granitic batholith is one of the largest intrusions in the western Cathaysia Block intruding Mid-Triassic sedimentary rocks. It comprises six phases, as follows:
• Phase 1- porphyritic biotite granite with 3 to 5 cm euhedral K feldspar phenocrysts. It is composed of ~40 vol.% plagioclase, 15 to 20 vol.% quartz, 10 to 25 vol.% K feldspar and 10 to 20 vol.% biotite, with accessory magnetite, titanite, apatite, zircon and allanite.
• Phase 2 - fine-grained porphyritic biotite granite with variable phenocrysts. It is composed of ~40 vol.% K feldspar, ~25 vol.% plagioclase, ~25 vol.% quartz and 5 to 10 vol.% biotite, with the same accessory minerals as for Phase 1.
• Phase 3 - porphyritic biotite granite with medium-grained euhedral to subhedral K feldspar phenocrysts, best developed in the Malage, Songshujiao and Laochang ore deposits. Phenocryst generally range from 1 to 3 cm, sometimes as much as 6 cm. It comprises ~25 vol.% plagioclase, 10 to 20 vol.% K feldspar, ~30 vol.% quartz and ~10 vol.% biotite, with accessory magnetite, zircon, apatite, titanite, fluorite, allanite and tourmaline.
• Phase 4 - a coarse to medium-grained equigranular biotite granite, mostly localised in the Laochang ore deposit. It is composed of >30 vol.% quartz, ~40 vol.% K feldspar, ~20 vol.% plagioclase and ~5 vol.% biotite, with accessory zircon, magnetite, monazite, apatite and minor titanite.
• Phase 5 - medium to fine-grained leucogranite in the Xinshan area of the Kafang deposit. It is fine-grained and equigranular, containing little or no biotite, with ~40 vol.% quartz, ~35 vol.% K feldspar and ~30 vol.% plagioclase, and accessory zircon, apatite and monazite.
• Phase 6 - occurs as fine-grained equigranular granitic dyke swarms and small stocks around the granite margins, especially the Baishachong and Shenxianshui stocks. Major minerals are ~60 to ~70 vol.% quartz and ~ 25 vol.% K feldspar with minor plagioclase, muscovite, zircon.
The Gejiu granite batholith formed between ~85 and 77 Ma, and is transitional from metaluminous to weakly peraluminous and has high-K and is alkali-rich (Cheng and Mao, 2010). Geochemical and Sr-Nd-Hf isotopic data suggest the parent Gejiu granitic magma underwent high degrees of fractional crystallisation following its formation due to the partial melting of Mesoproterozoic crust with minor input from mantle-derived magmas (Cheng and Mao, 2010). The geochemical data indicate that degrees of fractional crystallisation are distinctive from each other and that the later phases tend to be more evolved. Mineralisation is spatially related to highly evolved granitic phases (Cheng et al., 2013).
The five main deposits may be summarised as follows:
This deposit is located in the northern-most part of the Gejiu district, over an area of ~40 km2. It is predominantly underlain by the Mid-Triassic Gejiu Formation, which comprises up to >1000 m of limestone, dolomitic limestone and dolomite. The major ore-controlling structures are the NW-trending Masong anticline and its associated faults (Sun et al., 1987). Faults are considered to be crucial to ore fluid migration and ore genesis (Jiang et al., 1997). The Late Cretaceous 81 ±4.9 Ma equigranular granite in the north and 90.4 ±6.3 Ma porphyritic granite in the south, are the main igneous rocks in this area, which whilst having different petrological and geochemical compositions, are believed to be derived from the same magma chamber, with different degrees of fractionation (Cheng and Mao, 2010; Lu, 1987).
The deposit is Sn–Cu-dominant, with varying amounts of accompanying Pb, Zn, W, Mo and Be. Two ore styles are recognised in this deposit.
i). skarn in the contact areas between porphyritic granite and carbonate wallrocks, with Sn and Cu grades of from 0.02% to 0.05% and 0.3% to 1.56%, respectively. Ore minerals include pyrrhotite, chalcopyrite, pyrite, beryl, bismuth, molybdenite, scheelite, sphalerite, galena, cassiterite and arsenopyrite, commonly occurring as disseminations and/or as veins in skarn. Orebodies are generally lenticular, centred on the porphyritic granite, and are characterised by a well defined metal zoning.
ii). stratabound ore in some layers of the various country rock sedimentary rocks. This style is commonly distal to granite, with an average tin grade of 2.39%. Tin is the dominant metallic element, and is generally associated with varying Cu, Pb, Zn, In and Bi, with a quartz gangue.
The shape of the Malage orebodies is quite complicated, and includes stratabound, vein and tabular. Alteration is extensive, with skarn being the major alteration type, comprising garnet, pyroxene, tremolite, actinolite, wollastonite, epidote and phlogopite. Other alteration products include alkali metasomatism, both potassic and sodic, quartz-muscovite greisen, and sericite and chlorite alteration of the intrusive rocks.
Songshujiao is located to the SE and NE of the Malage and Gaosong deposits respectively. The Mid-Triassic Gejiu Formation in this deposit is composed of dolomite, interbedded dolomitic limestone and limestone, whilst the main structures are the NNE-trending Wuzishan anticline and the NW aligned Baishachong fault. Mineralisation is centred on granitic cupolas and well developed strike-slip fault zones that provide critical conduits for granitic magma-derived hydrothermal fluids as well as space for the precipitation of ore minerals. Three fault sets are evident within the deposit, trending trend NW, NE and east-west. Fault zones are usually ~3 m wide, infilled with small amounts of oxidized ores. The principal igneous rocks are a porphyritic biotite granite. Skarn is commonly developed along the contact between granite and carbonate wallrocks.
Two mineralisation types are recognised:
i). Skarn Sn–Cu mineralisation, predominantly developed along the contact between porphyritic granite and Gejiu Formation carbonates, occurring as lenses, strata-bound bodies or as veins, generally distributed around granitic cupolas. The orebodies generally vary from ~5 to 50, and locally up to ~100 m in thickness, and are from 100 to 500 m long. The skarn assemblage includes pyroxene, scapolite and garnet, with ore minerals pyrrhotite, chalcopyrite, arsenopyrite, marmatite, pyrite, magnetite, cassiterite, scheelite and molybdenite. The deposit contains Sn, Cu, Zn and W with minor In, Bi and Ag. Skarn type Ore accompanied by skarn alteration constitutes ~66% of the Sn and ~80% of the Cu resources in the Songshujiao deposit.
ii). Stratabound Sn-Pb mineralisation, which is predominantly hosted by interbedded limestone and dolomite, distal to the granite contact. These ores account for ~60 to 70% of the total Pb resources in the deposit, and are relatively complex, commonly controlled by fault, fold and/or fault slip structures. The stratabound orebodies have strike lengths of is 20 to 50 m, with a few as long as >100 m. Their thickness is commonly >20 m. Most are weathered and contain limonite.
At the Gaosong deposit, the Gejiu Formation has been divided into three units, the: i). Bainidong Unit, a light grey to grey limestone, with minor banded and lenticular calcareous dolomite; ii). Malage Unit, consisting primarily of dark gray to gray limey dolomite, with minor dolomitic limestone; iii). Kafang Unit, mainly grey to light grey limestone and limey dolomite interlayers. This unit is the main host to Gaosong mineralisation.
The major ore controlling structures are the 8 km long, NE trending, Lutangba fault and its associated subsidiary faults which cut the Kafang Unit. Quartz–muscovite/greisen alteration, muscovite, sericite and skarn minerals are intensively developed on the margins of the granitic cupola and in the contact zone between granite and carbonate. Two ore types are recognised:
i). Skarn sulphide mineralisation as disseminated, veinlet and massive ores along the contact between granite and limestone and/or dolomite. The principal skarn minerals includes pyroxene, garnet and scapolite, with sulphides being associated with a retrograde skarn assemblage of actinolite, tremolite and chlorite. The principal ore minerals are arsenopyrite, pyrrhotite, chalcopyrite and marmatite, with lesser cassiterite, pyrite, scheelite, native bismuth, molybdenite and magnetite. Tin grades average ~0.5 to 1%, with maxima of up to 3%.
ii). Weathered mineralisation which has complex outlines, and include stratabound, irregularly banded, lenticular and veinlet. These occur in different layers and levels within the host sequence, generally 3 to 5 and locally up to 8 to 9 layers, and are controlled by strike-slip fault zones and fractures. These orebodies can be tens to hundreds of metres long and 100 to 200 m wide. Ores are composed of limonite, hematite, goethite and clay minerals, with minor cassiterite, marmatite, anglesite, pyrolusite and malachite, in a gangue that includes primarily scorodite, siderite, calcite, quartz, phlogopite, fluorite, tourmaline, chlorite, garnet, tremolite, pyroxene, with minor plagioclase, jarosite and kaolinite. The average Sn grade is ~2%.
This polymetallic deposit is located between the Gaosong deposit in the north and the Kafang deposit to the south and is the most important in the Gejiu district, containing ~50% of the known tin resources. The deposit is bounded to the north by the Beiyinshan fault and the Laoxiongdong fault in the south. Mineralisation is associated with six east-west trending faults that are interpreted to have provided conduits for the ore fluids and sited of deposition. Another six NE- and/or NW-trending faults dislocate the deposit. Equigranular and porphyritic granites are the major igneous rocks in this area, intruding the Gejiu Formation, the stratigraphy of which is similar to that described above.
The main ore styles are: i). Skarn mineralisation, which includes skarn-sulphide Sn and Cu ores, with minor associated Bi, In and Ga. These skarns account for ~60% of all the Sn resources in the deposit. Most ores occur around granitic cupolas. The principal metallic minerals are pyrrhotite, arsenopyrite, chalcopyrite, pyrite, marmatite, cassiterite and scheelite, with a gangue assemblage that includes pyroxene, garnet, plagioclase, fluorite, phlogopite, quartz and chlorite. Minor amounts of weathered ore occurs as limonite, goethite, malachite and plumbojarosite, with associated sericite, phlogopite, muscovite, quartz and calcite.
ii). Stratabound mineralisation, occurs in zones of interlayered limestone and dolomite, and accounts for ~25% of the total Sn resources in the deposit. These ores have been strongly weathered to an assemblage of cassiterite, hematite, limonite, goethite,malachite, scorodite, conichalcite, anglesite, cerusite, plumbojarosite and wadite, with minor arsenopyrite, pyrite, chalcopyrite, sphalerite, galena and marmatite.
iii). Carbonate-hosted vein mineralisation, comprising tourmaline-quartz veins, tourmaline-K feldspar-skarn veins, tourmaline-skarn-cassiterite veins, and tourmaline-phlogopite veins hosted by Gejiu Formation carbonates. Lithology, stratigraphy and structure of the folded and faulted host limestone and dolomite, and the proximity of the nearby granite influence both the scale of mineralisation and vein distribution (Cheng et al., 2012). The ore zone has an approximately rhombic shape, bounded by the Huangnidong, Aotoushan, Meiyuchong and Longshupo faults in the east, west, north and south respectively. Granite occurs 200 to 1000 m below the surface, elongated in a NE-SW direction beneath the ore veins. The veins vary from several cm to >200 m in length and from several mm to ~1 m in thickness. Tin is the dominant metal extracted from vein orebodies with average grades of 0.42% Sn, 0.13% Be, and 0.11% WO3.
This is the southernmost deposit to the east of the Gejiu fault in Gejiu district. In this area there are 60 to 100 m thick basaltic lava flows intercalated within the Gejiu Formation carbonate beds and the sequence is intruded by fine-grained equigranular Xinshan biotite granite. Three types of ore are recognised, namely:
i). Skarn Cu-Sn-Mo-W-Au-Bi polymetallic mineralisation occurring along the contact between Xinshan granite and carbonate wallrocks. Ore minerals include magnetite, chalcopyrite, pyrrhotite and cassiterite, with minor pyrite, scheelite, native bismuth, native gold, sphalerite, galena, molybdenite, wolframite and arsenopyrite. The skarn alteration assemblage includes garnet, pyroxene, epidote, actinolite, tremolite, sericite, chlorite, calcite and quartz.
ii). Stratabound Cu-dominant mineralisation which is hosted in the basaltic lavas forming roughly sheet-like or tabular orebodies that are ~200 to 400 m long and up to 10 m in width. The ore assemblage is primarily chalcopyrite, pyrite, arsenopyrite, molybdenite and pyrrhotite with a gangue that include actinolite, phlogopite and tremolite, with minor pyroxene, calcite and fluorite.
iii). Stratabound Cu-Sn-Pb-Zn polymetallic mineralisation hosted by limestone and/or dolomitic limestone layers up to 2 km from the granitic pluton. Metallic minerals include pyrrhotite, cassiterite and pyrite, while the gangue assemblage includes variable amounts of quartz, tourmaline, tremolite and fluorite. The orebody-host rock contacts are sharp, although localised ore veins emanating from the main orebodies, cut the host rock. There is only minor alteration around these orebodies, although some banded garnet-pyroxene-wollastonite-calcite skarns follow bedding of the immediate host rocks along strike and parallel to the orebodies. However, these are better developed closer to ore.
In each of the deposits, mineralisation typically occurs as an extensive hydrothermal system centered on a shallow Late Cretaceous granitoid cupola. Metal zoning is well developed both vertically and horizontally over the entire district, from W+Be+Bi±Mo±Sn ores internal to granite intrusions, to Sn+Cu-dominated ores along intrusion margins passing out to more distal Pb+Zn deposits in the surrounding host carbonate (Cheng et al., 2013).
Mica samples from all of the mineralisation styles from the five deposits described above yielded ages range from 77.4±0.6 Ma to 95.3±0.7 Ma. These are similar to the existing zircon U-Pb age of the granitic intrusions (77.4±2.5 to 85.8±0.6), indicating a genetic relationship between the mineralisation and the intrusions.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2013.
Record last updated: 5/4/2020
This description is a summary from published sources, the chief of which are listed below.
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Cheng, Y., Mao, J., Chang, Z. 2013 - The origin of the world class tin-polymetallic deposits in the Gejiu district, SW China: Constraints from metal zoning characteristics and 40Ar-39Ar geochronology: in Ore Geology Reviews v.53, pp. 50-62.|
Jiang Z, Oliver N H S, Barr T D, Power W L, Ord A 1997 - Numerical modeling of fault-controlled fluid flow in the genesis of Tin deposits of the Malage ore field, Gejiu mining district, China: in Econ. Geol. v92 pp 228-247|
Mao, J., Cheng, Y., Chen, M. and Pirajno, F., 2013 - Major types and time-space distribution of Mesozoic ore deposits in South China and their geodynamic settings: in Mineralium Deposita v.48, pp. 267-294.|
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