Magnetite Geochemistry of the Jinchuan Ni-Cu-PGE Deposit, NW China: Implication for Its Ore-Forming Processes

Abstract

The Jinchuan Ni-Cu-PGE deposit is the single largest magmatic Ni-sulfide deposit in the world, with three different hypotheses on its ore-forming processes (e.g., in-situ sulfide segregation of sulfide-bearing magma, deep segregation with multiple injections of magma, and hydrothermal superimposition) mainly based on study of whole-rock geochemistry and isotopes (e.g., S-Sr-Nd-Hf). In this study, we mainly concentrated on magnetite textural and geochemical characteristics from different sulfide ores to clarify the genetic types and geochemical difference of the Jinchuan magnetite, and to explore a new credible ore-forming process by magnetite formation process when combined with detailed deposit geology. Three types of magnetite from massive and disseminated sulfide ores were observed by different textural analysis, and they were shown to have different genetic types (mainly in geochemistry) and trace elemental features. Type I magnetite is subhedral to anhedral from massive Ni- (or Fe-) and Cu-rich sulfide ores, with apparent magmatic origin, whereas Type II (dendritic or laminar crystals) and III magnetite (granular crystals as disseminated structures) from disseminated Cu-rich sulfide ores may have precipitated from late stage of melts evolved from a primitive Fe-rich and sulfide-bearing system with magmatic origin, but their geochemistry being typical of hydrothermal magnetite, videlicet, depletions of Ti (< 20 ppm), Al (< 51 ppm), Zr (0.01–0.57 ppm), Hf (0.03–0.06 ppm), Nb (0.01–0.14 ppm), and Ta (0.01–0.21 ppm). Such different types of magnetite can be clearly distinguished from concentrations and ratios of their trace elements, such as Ti, V, Co, Ni, Zn, Zr, Sn, Ga, and Ni/Cr. Those different types of Jinchuan magnetite crystallized from (evolved) sulfide-bearing systems and their geochemistries in trace elements are controlled mainly by evolution of ore-related systems and geochemical parameters (e.g., T and fO2), with the former playing a predominant role. Combining the previous literature with this study, we propose that the Jinchuan deposit formed by multiple pluses of sulfide-bearing magma during fractional crystallization, with the emplacing of more fractionated and sulfide-bearing magma during sulfide segregation playing a predominant role. During this multiple emplacement and evolving of sulfide-bearing systems, Type I magmatic magnetite crystallized from primitive and evolved Fe-rich MSS (monosulfide solid solution), while Type II and III magnetite crystallized from evolved Fe-rich MSS to Cu-rich ISS (intermediate solid solution) during sulfide fractionation, with those Type II and III magnetite having much higher Cu contents compared with that of Type I magnetite.

Platinum Group Elements in Geosphere and Anthroposphere: Interplay among the Global Reserves, Urban Ores, Markets and Circular Economy

“…For example, numerous intrusions containing the deposits of PGEs together with Cu and Ni are located in Russia at the Norilsk ore region [23][24][25][26]. Implications of ore-forming processes as well as magmatic evolution aspects are given in the literature [27,28].…”

“…Implications of ore-forming processes as well as magmatic evolution aspects are given in the literature [27,28].…”

Geochemistry and magnetite mineral properties in a porphyry copper prospect in A‐type granitoids: A case study from the Karbi Hills of Northeast India

“…The concentration of each element is ascertained by competition from co‐crystallizing sulphides in the copper‐rich intermediate solid solution (Dare et al, 2012). The TiO 2 content in magnetite is presumed to be directly related to the temperature at which it develops (Dare, Barnes, et al, 2014; Jiao et al, 2019; Nadoll et al, 2012, 2014). Mag‐II has less Ti (<1 wt.%) but more Fe than Mag‐I, suggesting their formation in a low‐temperature hydrothermal environment.…”

Formation of polymetallic ores in the metasedimentary rocks of Rangpo area, Sikkim Lesser Himalaya, India: Mineralogical and geochemical attributes

“…Magnetite associated with sulphide ores forms at high temperature and thereby occurs in magmatic, pyrometasomatic, or high temperature hydrothermal deposits (Schwartz & Ronbeck, 1940). A number of studies have been carried out on the chemistry of magnetite occurring in ore assemblages (Dare et al, 2012; Hu et al, 2017; Jiao et al, 2019, etc. ), suggesting that it can be formed from Fe‐rich mono sulphide solution at high temperature to later evolved Cu rich intermediate sulphide solution at relatively lower temperature.…”