Metallogenetic conditions, magma conduit and exploration potential of the Kalatongk Cu-Ni orefield in Northern Xinjiang

Abstract：The Kalatongk early-Permian Cu-Ni sulfide orefield, which consists of 13 small mafic intrusions and is located in the north of the central Asian orogenic belt （CAOB）, is the largest Cu-Ni ore deposit in Xinjiang. Some intrusions are unexposed, and the major economic Ni-Cu sulfide ores are found within four of these intrusions, i.e., Y1, Y2, Y3 and Y9 intrusions. What is the biggest crisis is the shortage of reserved resources of copper and nickel in the Kalatongk deposit after more than 30 years’ continuous mining and exploitation, with the deepest mining depth reaching 740 meters. The main host rock types are gabbro, norite, gabbro-norite and olvine-norite, with ores also occurring in the hornblende gabbro. However, the ultramafic rock, which is the main host rock in most of the Cu-Ni deposits in the world, has not been found in these intrusions. The deposits have significant magnetic anomalies (200 nT), gravity anomalies (0.29×10-5 m/s2), and induced polarizability anomalies, and are characterized by ore-bearing mafic rock, small-size intrusion clusters, full magmatic evolution, high copper content (Cu/Ni radios approximately 3∶2), high PGE content and widespread massive sulfide lens. The country rocks are highly deformed carbonaceous slate, schist and tuffaceous rocks, which hinder the conventional electrical exploration. Kalatongk mafic intrusions were derived from the upwelling of asthenosphere components, as shown by the study of petrology and geochemistry of the intrusions. Compared with the same-period magmatic Cu-Ni deposits in East Tianshan in the aspects of the petrographic types and percentages and the residual gravity anomalies, it is supposed that the Kalatongk intrusions are more likely to have a high content ultramafic rock, which could probably be found in the depth of mafic intrusions and has a much better ore-bearing potential. This inference has been confirmed by the discovery of ultramafic rock in the eastern part of Y2 intrusion 650-740 meters below the surface and in the western part of Y2 intrusion at the depth of 410-590 meters in 2013, through drilling in underground tunnel. This was for the first time that the fine-grained, intensely serpentinized and uralitized olivine pyroxenite and pyroxene peridotite were found in the ore district, which are thought to constitute the top of the buried ultramafic rock. In combination with the rock-controlling and ore-controlling structure, the pitch and dip direction of the intrusion clusters, the discovery and systematic observation of multiphase sulfide droplet structure and the researches on hornstone and baking of the country rocks, it seems more reasonable that the main magma conduit may be located between Y2 and Y3 intrusions instead of between Y1 and Y2 intrusions which was thought to be the main magma conduit in the past years. In consideration of the background of the Kalatongk orefield, the authors suggest that the utilization of the high-resolution shallow-level seismic survey, Controlled Source Audio-frequency Magnetotelluric methods (CSAMT) and borehole electromagnetic survey, combined with the conventional high-precision gravity, magnetic and induced polarization methods, can detect and predict the spatial location, shape and mineralization extent of the buried ore-bearing mafic-ultramafic bodies in the southern intrusion belt, and can then promote the deep prospecting work in the northern intrusion belt and the peripheral G21, G22 concealed mafic intrusions.