Sulfide Ore Formation of the Kalatongke Ni-Cu Deposit as Illustrated by Sulfide Textures

Abstract

Abstract The Kalatongke magmatic Ni-Cu deposit features high Ni-Cu grades compared with other Ni-Cu deposits in the Central Asian orogenic belt. The sulfides, mainly hosted by olivine norite and gabbronorite, are characterized by high Cu/Ni ratios. There is wide variety of textural relationships in the mineralized rocks, including globular, sulfide matrix, emulsion, disseminated, net-textured, and semimassive to massive textures. Quantitative textural measurements reveal that more than 65 vol % of the total sulfide volume in disseminated ore (defined as containing 4–10 vol % sulfide) and more than 90 vol % of the total sulfide volume in net-textured ore (typically 16 vol % sulfide) are hosted in few interconnected networks with equivalent sphere diameters (ESDs) larger than 6 mm. This illustrates that sulfide coalescence is a critical process. The remaining sulfide blebs define two groups of log-linear particle size distribution (PSD), i.e., a finer group (ESD of 0.080–<0.529 mm) and a coarser group (ESD of 0.529–4.084 mm). The PSD of the finer group differs slightly among different types of mineralization. The origin of this group of sulfides is attributed to sulfide nucleation simultaneously with crystallization of the olivine-orthopyroxene-plagioclase phases from the ore-forming magma in the current magma chamber. The PSD of the coarser group from net-textured ores is parallel to that from the disseminated ore but has lower intercept values. Modeling results show that aggregation of ~40–70% sulfide blebs of different grain size from the disseminated ore into the networks can generate the coarser group sulfide PSD of net-textured ore. In addition, monosulfide solution (MSS, pyrrhotite component)-enriched sulfide globules are commonly located close to sulfide matrix ore breccias and emulsion-textured ores, rimming the net-textured and massive orebodies. These globules are different from the sulfide component that was in equilibrium with the mafic magma, suggesting they were formed by mechanical remobilization of a cumulus MSS-enriched component from a previously segregated and partially crystallized sulfide pool. These observations, combined with the ubiquitous chilled margin xenoliths in the high-grade ores, suggest that the Kalatongke deposit was the result of voluminous magma flow through the current location accompanied by sulfide reworking and percolation. All these textural characteristics could be explained by emplacement within a laterally propagating bladed dike. Moreover, the deformation recorded in the emulsion-textured and massive ores suggests the fault system remains active during magma solidification, driving downward migration of sulfide from the sulfide pool at magmatic temperature to form vein-type massive ore. We suggest that the Kalatongke deposit formed by magma pulses injecting into the current location and that the tectonic movement remains active during the solidification of the intrusion. The syntectonic emplacement model, which may be a common feature in the Central Asian orogenic belt Ni-Cu deposits, indicates that the fault systems beneath the intrusion are of great exploration interest for high-grade ores.