Metamorphosed Pb–Zn–(Ag) ores of the Keketale VMS deposit, NW China: Evidence from ore textures, fluid inclusions, geochronology and pyrite compositions.

The Keketale Pb–Zn deposit is located in the Devonian volcanic-sedimentary Maizi basin of the Altay orogenic belt. The mineralization at Keketale is hosted in marbles and deformed volcanic tuffs and biotite–garnet–chlorite schists, folded into a series of overturned synclines formed in multiple deformation events. Keketale contains economic amounts of Pb (0.89Mt @ 1.51wt.%), Zn (1.94Mt @ 3.16wt.%) and Ag (650t @ 40g/t). Detailed petrographic studies have defined two main generations of sulfide development. The banded pyrite of the early Stage A is commonly stratiform, with minor galena, sphalerite and chalcopyrite. Stage B is characterized by a large amount of polymetallic sulfides including pyrrhotite, chalcopyrite, sphalerite and galena, with minor pyrite hosted in quartz veins. Three types of fluid inclusions (FIs), including mixed carbonic-aqueous (C-type), pure carbonic (PC-type) and aqueous (W-type), have been recognized in quartz of stage B. The C-type FIs have homogenization temperatures of 150–326°C and salinities of 0.2–16.6wt.% NaCl equivalent. The PC-type FIs are dominated by CO2 with minor CH4 and N2 and have initial ice-melting temperatures of −57.5 to −56.7°C, CO2 homogenization temperatures of 11–14.1°C. The W-type primary FIs were completely homogenized at temperatures of 124–359°C with salinities of 5.0–14.6wt.% NaCl equivalent. Such CO2-rich fluid inclusions are consistent with those discovered in orogenic-type deposits in the Altay area and elsewhere. Muscovite separates from the polymetallic quartz veinlets of stage B yield a well-defined 40Ar/39Ar isotopic plateau age of 259.33±2.56Ma, with an isochron age of 259.62±2.65Ma. This age is coeval with the closure of the Paleo-Asia Ocean and reactivation of the Ertix Fault system. LA-ICP-MS analyses of two generations of pyrite indicate that the banded pyrite of stage A is relatively depleted in metallic elements and contains low contents of Cu (0.39ppm), Ag (0.20ppm), Au (below the detection limits), Pb (17.43ppm) and Zn (14.38ppm); whereas the pyrite in quartz–polymetallic sulfide veinlets of the stage B is relatively rich in metallic elements, e.g., Cu (2.56ppm), Ag (3.07ppm), Au (0.01ppm), Pb (1047ppm) and Zn (1136ppm). The trace amounts of Cu, Pb, Zn, Au and Ag are interpreted to have been initially locked in the lattice of type-A pyrite, and then liberated and precipitated as micromineral inclusions with type-B pyrite during subsequent metamorphism and deformation. Two key factors are considered vital to the formation of economic ores of the Keketale Pb–Zn deposit, namely the original Devonian banded pyrite formed in a VMS system and subsequent Permian deformation and metamorphic processes that liberated Cu, Pb, Zn, Au and Ag from the lattice of type-A pyrite to form galena, sphalerite and chalcopyrite with minor muscovite in quartz veinlets. The model provides a new interpretation of VMS Pb–Zn deposit occurring in back-arc basin environments followed by collision, and new insights into the unique regional Fe–Cu–Pb–Zn–Au mineralization in the Altay orogenic belt. [ABSTRACT FROM AUTHOR]