The Kalatongke magmatic Ni–Cu deposits in the Central Asian Orogenic Belt, NW China: product of slab window magmatism?

Abstract

The Permian Kalatongke Ni–Cu deposits in the Central Asian Orogenic Belt are among the most important Ni–Cu deposits in northern Xinjiang, western China. The deposits are hosted by three small mafic intrusions comprising mainly norite and diorite. Its tectonic context, petrogenesis, and ore genesis have been highly contested. In this paper, we present a new model involving slab window magmatism for the Kalatongke intrusions. The origin of the associated sulfide ores is explained in the context of this new model. Minor amounts of olivine in the intrusions have Fo contents varying between 71 and 81.5 mol%, which are similar to the predicted values for olivine crystallizing from coeval basalts in the region. Analytic modeling based on major element concentrations suggests that the parental magma of the Kalatongke intrusions and the coeval basalts represent fractionated liquids produced by ∼15% of olivine crystallization from a primary magma, itself produced by 7–8% partial melting of depleted mantle peridotite. Positive e Nd values (+4 to +10) and significant negative Nb anomalies for both intrusive and extrusive rocks can be explained by the mixing of magma derived from depleted mantle with 6–18% of a partial melt derived from the lower part of a juvenile arc crust with a composition similar to coeval A-type granites in the region, plus up to 10% contamination with the upper continental crust. Our model suggests that a slab window was created due to slab break-off during a transition from oceanic subduction to arc–arc or arc–continent collision in the region in the Early Permian. Decompression melting in the upwelling oceanic asthenosphere produced the primary magma. When this magma ascended to pond in the lower parts of a juvenile arc crust, it underwent olivine crystallization and at the same time triggered partial melting of the arc crust. Mixing between these two magmas followed by contamination with the upper crust after the magma ascended to higher crustal levels formed the parental magma of the Kalatongke intrusions. The parental magma of the Kalatongke intrusions was saturated with sulfide upon arrival primarily due to olivine fractional crystallization and selective assimilation of crustal sulfur. Sulfide mineralization in the Kalatongke intrusions can be explained by accumulation of immiscible sulfide droplets by flow differentiation, gravitational settling, and downward percolation which operated in different parts of the intrusions. Platinum-group element (PGE) depletion in the bulk sulfide ores of the Kalatongke deposits was due to depletion in the parental magma which in turn was likely due to depletion in the primary magma. PGE depletion in the primary magma can be explained by a relatively low degree of partial melting of the mantle and retention of coexisting sulfide liquid in the mantle.