Trace Elements in Chromite as Indicators of the Origin of the Giant Podiform Chromite Deposit at Kempirsai, Kazakhstan

Abstract

This study presents a complete trace element data set of chromite from the world’s largest podiform chromite deposit at Kempirsai, Kazakhstan, together with platinum group element (PGE) and Re-Os isotope compositions, to explore the potential of trace elements in the study of podiform chromite deposits. Chromite orebodies in the Karaagash area of the Kempirsai Main ore field are hosted by dunites in the mantle sequence of the ophiolite. Dunites slightly distal to the orebodies are composed of olivine (forsterite content = 91.5–93.4) and minor euhedral chromite with variable Cr# (0.73–0.83). These dunites have primitive mantle-normalized Pt-Pd–poor PGE patterns and subchondritic 187Os/188Os ratios (0.12174–0.12220). Negative correlations between Cr# and incompatible trace elements (Sc and Ti) of the chromite are consistent with melt-rock reaction, whereas positive correlations between Cr# and compatible trace elements (V and Ga) reflect the role of fractional crystallization. It is therefore proposed that chromite in the distal dunites was crystallized from magmas after reaction with mantle peridotites. In comparison, chromite grains in the ores and adjacent dunites exhibit relatively uniform Cr# numbers (0.81–0.86) and higher 187Os/188Os values (0.12319–0.12414). Both compatible and incompatible trace elements of chromite exhibit negative correlations with Cr#, which are best explained by mixing between the modified magma after melt-rock reaction and a later injected magma from a more depleted source. After the ore formation, subsolidus reequilibration between olivine and chromite reduced Mg# and modified divalent trace elements of chromite in the order of Zn > Co = Mn > Ni but did not affect distribution of nondivalent trace elements, except V and Fe3+. The decoupling of redox-sensitive and redox-insensitive trace elements suggests that the redox states have been modified during subsolidus reequilibration. The existing iron equilibrium between olivine and chromite records high fO2 values (quartz-fayalite-magnetite buffer [∆QFM] 0.8–2.4), whereas correction of subsolidus reequilibration results in a decrease of the fO2 values toward the QFM buffer (∆QFM 0.1–0.6), indicating that increase in fO2 was not a critical factor in the ore formation. Principal component analysis and factor analysis were further carried out to identify differences in trace elements between high-Cr and high-Al podiform chromite deposits worldwide. High-Cr chromite deposits have variable divalent trace element compositions and show complex distributions of nondivalent trace elements. The former reflects longer and slower cooling histories of the high-Cr chromite deposits with respect to the high-Al chromite deposits, whereas the latter indicates more variable magma compositions and complex processes during the formation of the high-Cr chromite deposits.