2D and 3D textures of sulfide ores from the Shitoukengde Ni-Cu sulfide deposit, East Kunlun Orogenic Belt, NW China: implications for the growth of orthopyroxene oikocrysts and formation of globular ores

Abstract

Orthopyroxene oikocrysts and globular ores are both observed in the Shitoukengde Ni-Cu sulfide deposit within the Eastern Kunlun Orogenic Belt in China. Through the utilization of microbeam X-ray fluorescence (micro-XRF) and electron probe micro-analyzer (EPMA) mapping techniques, complemented by 3D morphology analysis using high-resolution X-ray computed tomography (HRXCT), we conducted a comprehensive investigation into the 2D and 3D textures of orthopyroxene oikocrysts and globular ores in the Shitoukengde Ni-Cu sulfide deposit. The contents of orthopyroxene oikocrysts within the lherzolite gradually increases as it approaches the contact with coarse-grained orthopyroxenite. Both the orthopyroxene oikocrysts in the lherzolite and the cumulus orthopyroxene in the coarse-grained orthopyroxenite are centimeter-sized and contain corroded chadacrysts of olivine, exhibiting similar Cr-Al sector and oscillatory zoning. It indicates that the orthopyroxene oikocrysts rapidly grew in a dynamic and fluctuating magmatic environment, rather than in a static crystal mush. We propose that the orthopyroxene oikocrysts initially grew in a boundary layer between an olivine orthocumulate and an orthopyroxene-saturated magma. The orthopyroxene oikocrysts and olivine crystals were then entrained within a flowing magma and redeposited in their current location. Globular sulfides in the coarse-grained orthopyroxenite can reach sizes of up to one centimeter and are not accompanied by silicate caps. The particle size distribution (PSD) plots of the globular sulfides exhibit concave-up PSD curves, indicating that the larger sulfide droplets are likely formed through the coalescence of sulfide microdroplets. During postcumulus processes, the downward migration and coalescence of microdroplets within the interstitial framework of orthopyroxene cumulate lead to the formation of larger sulfide blebs. The coalesced sulfide blebs were then stranded in the pore spaces of the crystal mush due to the capillary effects, resulting in the formation of centimeter-sized globular sulfides. The morphology of coalesced sulfide droplets within orthopyroxene cumulate is influenced by the relative sizes of the sulfide blebs, pore bodies, and pore throats within the interstitial framework. This study proposed a cumulus origin for the orthopyroxene oikocrysts and highlights that the coarse-grained rocks facilitate the formation of the globular ores.