Chapter 13 Iron Oxide Copper-Gold Deposits in China: A Review and Perspectives on Ore Genesis

Abstract

Abstract Iron oxide copper-gold (IOCG) deposits are hydrothermal deposits characterized by abundant low-Ti Fe oxides, economic Cu-Au grades, structurally controlled orebodies, widespread presulfide alkaline alteration, and commonly significant volumes of breccia. They may also have elevated U, Co, Ag, and rare earth elements (REEs), and may have no clear spatial association with igneous intrusions. The IOCG sensu stricto deposits are rare in China, but the Proterozoic Fe-Cu-(Au-REE) deposits in the Kangdian region, southwestern China, have been confirmed to be of this type. Other related hydrothermal deposits, including what we could classify as Fe-Cu-Au skarn and iron oxide apatite deposits along the middle and lower Yangtze River belt, carbonatite-related Fe-REE-Nb deposits (i.e., Bayan Obo), and volcanic rock-hosted Fe-(Cu-Au) deposits in eastern Tian Shan and Altay, have been previously mentioned as Chinese IOCG deposits in some of the literature but show some important differences from IOCG deposit types. They are hence not included as IOCG deposits in this review. The Kangdian IOCG deposits are hosted in the 1.74 to 1.68 Ga metavolcanic-metasedimentary rock sequences of the Dahongshan, Hekou, and Dongchuan groups. The ore-hosting strata are fluvial to intertidal facies sedimentary-volcanic successions in a late Paleoproterozoic rift-related basin of the western Yangtze block. They comprise basal conglomerates and sandstones with minor tuffaceous and mafic volcanic rocks, grading upward to interbedded carbonate. The orebodies are generally stratabound and/or structurally controlled. They are spatially associated with 1.69 to 1.65 Ga diabase intrusions and hydrothermal breccia bodies of various sizes. The paragenetic sequence of the deposits generally includes pre-ore Na-(Ca) alteration (stage I) dominated by albite (and local amphibole); Fe-(REE) mineralization (stage II) with magnetite, siderite, and subsidiary REE-enriched apatite; and Cu-(Au-REE) mineralization (stage III) with chalcopyrite, ankerite, biotite, K-feldspar, sericite, chlorite, and local bornite and light REE minerals. Geochronological studies have shown that the Kangdian IOCG deposits formed during multiple mineralization/hydrothermal events. The most important of these was temporally and spatially associated with emplacement of 1.66 to 1.65 rift-related diabase intrusions. Another important event was related to 1.08 to 1.0 Ga rift-related magmatism in the region, but was mainly present in the deposits in the northern part of Kangdian belt. Neoproterozoic magmatic-metamorphic events (0.85–0.83 Ga) were widespread in the Kangdian region and were important for remobilization of Cu and REEs, and for upgrading the preexisting orebodies. Other hydrothermal events at 1.45 and 1.3 Ga are recorded locally and appear not to be of economic importance. Fluid inclusion, stable isotope, and radiogenic isotope studies have shown that the stage I and II ore-forming fluids were dominantly magmatic in origin, possibly derived from deep-seated magma chambers. However, nonmagmatic fluids from various sources in the shallow crust (e.g., basinal brine, meteoric water) were involved to various degrees in the formation of Cu-(Au) ores during stage III. Ore metals were largely derived from deep-seated magma chambers. Fluid-wall rock interactions and fluid mixing were important mechanisms for the precipitation of Cu sulfides. The Kangdian Fe-Cu-(Au) deposits are notably rich in REEs, and the formation of economic REE ores has a complex remobilization history. The REEs were mainly remobilized from apatite in earlier Fe oxide ores and/or country rocks and precipitated as monazite and REE-bearing carbonate minerals at a later stage of the same mineralizing event or during metamorphic-hydrothermal events hundreds of millions of years later. The IOCG deposits in the Kangdian belt formed in an intracratonic rift setting at a time when underplating of mafic magmas induced large-scale fluid circulation and pervasive Na-(Ca) metasomatism in the volcanic-sedimentary rocks. Hydrothermal brecciation of the country rocks occurred at the tops of the igneous intrusions and/or along zones of weakness within the country rocks due to overpressure imposed by the ore-forming fluids. Magnetite and hematite precipitated early along the main fluid channels, whereas Cu sulfides are mainly hosted within structures in the country rocks where sulfide saturation is favored. Such an ore-forming mechanism may be widely applicable to other IOCG deposits worldwide.