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Dongchuan - Luoxue, Lanniping, Yinmin, Tangdan |
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Yunnan, China |
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Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Dongchuan sediment hosted copper deposits, including the Luoxue, Lanniping, Yinmin and Tangdan, are located ~120 km NE of Kunming, Yunnan Province, southern China.
The South China craton in which these deposits are located, is bounded by the Tibetan Plateau to the west, and is separated from the Indochina block to the south by the Red River Fault. It comprises the Yangtze block to the northwest, which hosts the sedimentary rock-hosted strata-bound copper deposits, and the Cathaysia block to the southeast, which was probably amalgamated at ~0.83 Ga (Wang et al., 2007; Zhou et al., 2009; Zhao et al., 2011).
Palaeo- and Mesoproterozoic sequences of the Yangtze block, including the Dahongshan, Dongchuan, and Kunyang Groups, are widely distributed in the Kangdian region of southwest China (Wu et al., 1990; Greentree et al., 2006; Greentree and Li, 2008; Zhao et al., 2010). The area containing these sequences is bounded by the Luzhijiang Fault to the west and cut by a series of subordinate NNE-trending structures.
According to Zientek et al., 2015) this sequence was deposited in the Kunyang Aulacogen, an intracratonic rift that was developed on the western margin of the Yangtze Craton between ~2 to 1.9 Ga (Hua, 1991; Lifang, 2002; Zhao et al., 2004, 2010), and was associated with the breakup of the Nuna-Columbia supercontinent. The early stage of the rift is characterized by voluminous volcanic eruptions, which are represented by alkaline mafic rocks in the Palaeoproterozoic Dahongshan Group of Yunnan and the pre-Palaeoproterozoic Hekou Formation of southwestern Sichuan (Hua, 1991). Sedimentary rocks were deposited into elongated grabens associated with the aulacogen (Hua, 1991; Zhao et al., 2010). The sedimentary sequence begins with terrestrial red beds that are overlain by lagoonal to tidal flat evaporite-bearing dolomitic rocks, then marine black shale and carbonate rocks. These rocks were subsequently affected by the Late Proterozoic Jinning Orogeny, the Palaeozoic Panxi Rift event, and the Tertiary Himalayan Orogeny (Hua, 1991).
The oldest rocks in the region belong to the Dahongshan Group, which consists of an amphibolite-facies volcano-sedimentary sequence hosting the Dahongshan iron oxide-copper-gold (IOCG) deposits (Zhao and Zhou, 2011). A volcanic unit within this group has been dated at 1675±8 Ma (SHRIMP zircon U-Pb age; Greentree and Li, 2008). The sequence includes:
Hekou Formation, >1250 m thick - spilitic and keratophyric volcanics and associated sediments hosting significant Cu-Fe deposits and mineralisation. Capped by an unconformity.
Lower Formations, >7525 m thick - terriginous and ferruginous sandstones and slates intercalated with limestones and tuffs.
The Dongchuan Group includes the Yinmin, Luoxue, E'touchang and Luzhijiang Formations, all of which contain copper mineralised zones (Sun et al., 1991; Gong et al., 1996). The contact between the Dahongshan and Dongchuan Groups is not exposed, and the two may either be lateral equivalents (Zhao et al., 2010) or the Dongchuan Group may overlie the Dahongshan Group (Wu et al., 1990). Rocks of the Palaeo- to Mesoproterozoic Dongchuan Group are distributed discontinuously in a narrow <35 km wide and >390 km long, north-south belt in the Sichuan and Yunnan Provinces of south central China. The rocks are weakly deformed and were subjected to lowest greenschist grade metamorphism (Wu et al., 1990). The sequence includes:
Yinmin Formation, 20 to 1300 m thick - composed of alternating purple slate and dolomitic sandstone to slate with conglomerate at the base, comprising the oldest continental red-bed sequence in the Yangtze block (Wu et al., 1990; Hua, 1993). This unit carries 130 to 300 ppm Cu throughout.
Luoxue Formation, 40 to 500 m thick - Stromatolitic dolomite, argillo-arenaceous dolomite intercalated with thin bedded slate. This unit hosts the main orebodies at Dongchuan. It is interpreted as being deposited in an inter tidal environment, and is yellowish white to pink to grey-blue in colour. Copper mineralisation is concentrated within argillaceous and arenaceous banded dolomites which are intercalated with carbonaceous seams or in black carbonaceous and argillaceous dolomites. These rocks are laminated with the copper being concentrated within the sandy laminae. The copper bearing sulphides show vertical zoning: a chalcocite zone at the top, followed by a bornite zone, then a chalcopyrite zone and a pyrite zone at the bottom. The ore also contains tetrahedrite.
E'touchang (or Heishan) Formation, 860 to 1600 m thick - contains shales and siltstones, now largely slates, and rare tuff beds, which pass upward into mixed less metamorphosed siltstone, dolostone, and mudstone. Locally, the basal part of the formation is a highly carbonaceous lower black slate with intercalated dolomite and tuff; middle grey-green to yellow silty banded slate intercalated with thin bedded dolomite; upper blue-grey thin bedded limestone and black slate. Minor copper mineralisation is found in carbonatic facies.
Luzlujiang Formation, 1150 to 2030 m thick - lower carbonaceous, argillaceous, tuffaceous dolomite and tuff; upper siliceous banded dolomite and limestone with stromatolites. This unit hosts major mineralisation at Shishan in Yunnan. An unconformity terminates this unit.
Detrital zircons from the Yinmin Formation have ages of ~3.5 to ~1.78 Ga (U-Pb; Zhao et al., 2010). Tuffaceous rocks
in the Yinmin and E'touchang Formations give ages of 1742±13 and 1503±17 Ma, respectively (zircon U-Pb; Sun et al., 2009; Zhao et al., 2010), indicating the Dongchuan Group was deposited during the latest Palaeoproterozoic to early Mesoproterozoic (~1.7 to ~1.5 Ga). The long age span suggests significant disconformities may exist within the Dongchuan Group. Tuff from the overlying Kunyang Group have yielded ages of 1032±9 and 995±15 Ma (SHRIMP zircon U-Pb; Greentree et al., 2006; Zhang et al., 2007), suggesting that this sequence unconformably overlies the Dongchuan Group sedimentary rocks.
The Kunyang Group includes the following:
Upper Formations, >3500 m thick - terrigenous and ferruginous sandstone and slate intercalated with limestone and andesite with a conglomerate at the base. An unconformity terminates this unit also.
Breccia bodies, known as the Yinmin Breccia, cut the Yinmin, Luoxue, E'touchang and parts of Luzhijiang Formations of the Dongchuan Group (Wu et al., 2007). These breccia masses range from a few metres to several kilometres in diameter, and are both clast and matrix supported, containing rounded to angular clasts of Yinmin Formation siltstones and sandstones and less commonly the Luoxue and Eítouchang Formations, and rare mafic igneous rock that range in size from <1 cm to a few or tens of metres in diameter. The clasts are set in a fine-grained matrix of dolomite, ankerite, albite, quartz, potassium feldspar and biotite. Contacts with wall rocks are gradational over cms from crackle-brecciated wall rock to polylithologic breccia. The Yinmin Breccia is megascopically similar to breccias in the Katanga Province of the Democratic Republic of Congo. Low-angle faults occur at the base of the Yinmin Formation.
The majority of the widespread sedimentary rock-hosted stratabound copper deposits of the Kangdian/Dongchuan region occur in dolostones of the Luoxue Formation immediately above the contact with the Yinmin Formation (Sun et al., 1991; Gong et al., 1996). Individual deposits frequently persist for hundreds of metres to km along this lithologic interface, with the largest distributed over a strike length of ~30 km. These include the Luoxue Mine that contains ~0.5 Mt of contained copper. Immediately to its north and essentially part of the same orebody is the Yinmin Mine with >0.21 Mt of contained copper. The Tangdan deposit contains ~120 Mt of 1.1% Cu (Gong et al., 1996).
The host Luoxue Formation, is weakly mineralised throughout the entire district, while the actual deposits are spatially associated with the Yinmin Breccia and many are cut by mafic intrusions. The Dongchuan copper deposits contain both stratabound and discordant styles of mineralisation. Zones of stratabound mineralisation contain copper sulphides as disseminations and veinlets hosted within dolostone of the lower Luoxue Formation and less commonly in black shales of the lowermost E'touchang Formation. Zones of discordant mineralisation contain copper sulphide-bearing veins occupying fractures, joints and faults, either concordant to, or crosscutting, both the Luoxue and E'touchang Formations.
Although relatively high grade (1 to 10% Cu), discordant mineralised zones in the Dongchuan district account for only ~3% of the total copper reserves (Gong et al., 1996). None of the Dongchuan copper deposits have significant by-product metals such as Co, Ag, Pb or Zn. Some (e.g., the Yinmin and Lanniping deposits) contain small bodies of massive magnetite/hematite in the underlying Yinmin Formation or Yinmin Breccia (Sun et al., 1991; Gong et al., 1996). It is unclear whether the iron-oxide bodies associated with the Yinmin Breccia, some of which are cupriferous, are genetically related to the copper deposits in the overlying rocks, although the breccias may represent zones of intense mineralising fluid flow through the Yinmin Formation. The majority of the overlying stratabound copper and iron deposits are hosted by the Dongchuan Group occur in the cores of tight to faulted anticlines and in diapiric blocks.
Tangdan Deposit
Tangdan is the largest strata-bound copper deposit in the Kangdian region, hosted by a sequence that includes the Luoxue and Yinmin Formations, which are in fault contact with, or overlain by, black shales of the E'touchang Formation. A gabbroic intrusion dated at 1047±3 Ma (zircon U-Pb; Zhao, 2010) and 1059 Ma (K-Ar; Gong et al., 1996) cuts the E'touchang and Luoxue Formations immediately to the northwest of the deposit. These Dongchuan Group rocks are unconformably overlain by Sinian (late Neoproterozoic) dolostones of the Dengying Formation to the southeast of the mine.
The orebodies at Tangdan are mostly hosted in dolostones of the lower Luoxue Formation but locally extend throughout the Luoxue Formation and into black shales of the lowermost E'touchang Formation. Minor sulphides occur in dolostones of the uppermost portion of the Yinmin Formation. Both stratabound and discordant mineralisation occurs in all stratigraphic positions. Sulphides occur as disseminations (generally localised along dolostone laminae, along stylolites, and throughout the carbonaceous slates), veinlets and to a lesser extent, stockworks. The ore minerals include chalcopyrite, bornite and chalcocite with minor pyrite, digenite and covellite. Sulphides are commonly intergrown with fine- to medium-grained replacive quartz.
Siliciclastic and tuffaceous rocks of the Yinmin Formation are regionally altered throughout the Kangdian area, producing a variety of secondary minerals including quartz, dolomite, ferroan dolomite, calcite, albite, K feldspar, sericite/muscovite, biotite, chlorite, hematite, and minor apatite and tourmaline. The matrix of the Yinmin Breccia was particularly susceptible to alteration, converted to a fine- to coarse-grained assemblage of albite and ferroan dolomite
with minor apatite. Ferroan dolomite replaces earlier dolomite, while albite is rimmed and locally replaced by potassium feldspar, which in turn may be replaced by muscovite-chlorite. Quartz grains have been extensively overgrown by diagenetic-hydrothermal quartz. The Yinmin Formation and Yinmin Breccia were subjected to early sodic alteration followed by a potassic (dominantly K feldspar with lesser biotite) alteration event.
The basal Luoxue Formation at Tangdan contains a thin, <3 m sequence of pale- to medium-grey argillaceous dolostones. The argillaceous component of these dolostones, together with blocky diagenetic minerals, were pervasively replaced by a mixture of K feldspar, magnesian chlorite and quartz, frequently with very fine grains of chalcopyrite and microscopic 1 to 5 µm disseminated quartz. Locally the basal Luoxue Formation dolostones have a slight reddish color due to the presence of minor hematite along microfractures. Dolostones within this unit are ferroan, with a pale grey to white colour, and are commonly well-mineralised, and cut and replaced by zones of K feldspar indicating potassic alteration postdated ferroan dolomite alteration. Argillaceous beds within these dolostones have been altered to an assemblage of fine-grained K feldspar and muscovite or chlorite and muscovite.
The basal E'touchang Formation displays variable potassic alteration and silicification. Weakly veined, sulphide mineralized black shale contains significant quartz intergrown with sulphides, suggesting hydrothermal silicification. Heavily veined slate has been altered to a K feldspar-quartz-muscovite rock by intense potassic alteration.
While sulphides occur from the upper Yinmin Formation through to the lower E'touchang Formation at Tangdan, they are largely concentrated in weakly argillaceous dolostones of the lower and upper Luoxue Formation and within black shales of the basal E'touchang Formation. Diagenetic pyrite is common in the basal E'touchang Formation where it forms disseminated euhedral to subhedral crystals. Pyrite is commonly partially replaced by copper sulphides. Chalcopyrite is the dominant sulphide, and bornite, where present is commonly intergrown with chalcopyrite. The bulk of the chalcocite at Tangdan has been interpreted as supergene in origin, although hypogene chalcocite has been reported from deep levels of the mine by Gong et al. (1996).
Copper sulphides do not appear to be spatially associated with zones of either sodic or potassic alteration. Both chalcopyrite and bornite are typically intergrown with disseminated replacive quartz, indicating they were precipitated during silicification. A large proportion of the sulphides in both the Luoxue and E'touchang Formations occurs as micrometer-sized grains disseminated in silicified rocks, and are virtually invisible and difficult to discern in thin section. Chalcopyrite and bornite are also found in late quartz-sulphide-(dolomite-calcite-ankerite-siderite-muscovite-chlorite) veins that crosscut silicified rock. Barren quartz veins indicate silicification continued after sulfide precipitation.
The Tangdan deposit has been subjected to weathering and a supergene assemblage of chalcocite, digenite, covellite and malachite developed. Digenite commonly replaces bornite and is in turn replaced by chalcocite. Supergene alteration occurs along high-angle faults and fractures and is recognized to depths of several tens of metres.
The cumulative tonnage in the Dongchuan district (Gong et al., 1996) is 391 Mt @ 1% Cu; including,
Luoxue Mine - 50 Mt @ 1% Cu;
Yinmin Mine - 21 Mt @ 1% Cu, which is immediately to the north of Luoxue;
Tangdan deposit - ~120 Mt of 1.1% Cu.
The most recent source geological information used to prepare this decription was dated: 2012.
Record last updated: 30/6/1012
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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Fan, P.-F., 1984 - Geologic setting of selected Copper deposits of China: in Econ. Geol. v.79, pp. 1785-1795.
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Ruan Huichu, Hua Renmin, Cox D P 1991 - Copper deposition by fluid mixing in deformed strata adjacent to a salt diapir, Dongchuan area, Yunnan Province, China: in Econ. Geol. v86 pp 1539-1545
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Zhao X-F, Zhou M-F, Hitzman M W, Li J-W, Bennett M, Meighan C and Anderson E, 2012 - Late Paleoproterozoic to Early Mesoproterozoic Tangdan Sedimentary Rock-Hosted Strata-bound Copper Deposit, Yunnan Province, Southwest China : in Econ. Geol. v.107 pp. 357-375
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Zhao, X.-F., Zhou, M.-F., Li, J.-W. and Qi, L., 2013 - Late Paleoproterozoic sedimentary rock-hosted stratiform copper deposits in South China: their possible link to the supercontinent cycle: in Mineralium Deposita v.48, pp. 129-136.
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