New insights into the genesis of IOCG deposits: From a case study of the Yinachang deposit in SW China

The Yinachang Fe-Cu-Au-U-REE deposit is hosted by the lower part of the Paleoproterozoic Kunyang Group in the central Yunnan Province of SW China in an area known to contain iron-oxide Cu-Au mineralisation. The deposit includes: (i) the sulphide assemblage chalcopyrite – pyrite – magnetite – cobaltite – molybdenite – cassiterite; (ii) Rare Earth Element assemblage bastnaesite – parasite – fergusonite – xenotime – monazite; (iii) the uranyl hydroxide mineral vandendriesscheite; (iv) gold commonly hosted by ankerite; and (v) the gangue mineral assemblage apatite – biotite – fluorspar – quartz – calcite. The ore is commonly massive, disseminated, banded, or in calcite veins. The banded mineralisation contains high concentration of Au, Co, Mo, U, Fe, Cu and REE, and the vein-type mineralisation has abundant REE minerals of bastnaesite, parisite, fergusonite and xenotime. Monazite exists as inclusions in apatite or as discrete grains on the surface of magnetite, and the uranium is associated with chalcopyrite. The mineralising process can be divided into an initial Na-alteration followed by the ore-forming assemblages of Fe-REE and Cu-Au-U-REE mineralisation. The average δ¹⁸O_H2O values in the quartz and calcite change are between 3.5 and 10.7‰, with δD_V-SMOW values ranging between −98.2 and −47.7‰. The calcite, siderite and ankerite have δ¹³C isotope composition ranges from –14 to +1‰ and δ¹⁸O compositions of +6.9 to +22‰. The H–O–C isotope systematics indicate that the mineralising fluids have mixed magmatic and metamorphic sources, whereas the carbon is sourced from marine carbonate, and the low temperature alternation of polyphase mantle material. In situ δ³⁴S sulphide value ranges between 1.1 and 11.9‰, and the bulk δ³⁴S value ranges between −4.7 and 10.5‰. In situ Pb isotopes of sulphide samples have ²⁰⁶Pb/²⁰⁴Pb values of 18.58–18.76, ²⁰⁷Pb/²⁰⁴Pb values of 15.51–15.66 and ²⁰⁸Pb/²⁰⁴Pb values of 37.47–38.06. These values plot in the upper crust Pb-evolution curve, indicative of a complex source of Pb in the deposit. The Pb is interpreted to have a deep-seated magmatic source with a contribution from the crust. It is also proposed that the REE-mineralised fluids are derived from a late stage of alkaline magma differentiation. The ⁴⁰Ar-³⁹Ar biotite date indicates that the Precambrian rocks in the Kangdian region were regionally metamorphosed at ca. 900 Ma. The Yinachang Fe-Cu-Au-U-REE deposit is interpreted as an IOCG deposit based on geological and isotopic studies. The banded ore and vein type ore are the main exploration targets for uranium, gold and REE. This study helps us better understand IOCG deposits in China.