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The Aqishan-Yamansu belt in Eastern Tianshan (NW China) hosts several important Fe and Fe-Cu deposits, the origin of which is the subject of considerable debate. The coexistence of various types of ore-forming fluids makes it difficult to distinguish the genesis of the Fe-Cu deposits. We present detailed textural and compositional data on magnetite from the Paleozoic Shuanglong Fe-Cu deposit to constrain the formation of iron oxides and the evolution of the ore-forming fluids and thus define the genesis of the Fe-Cu ores. Based on the mineral assemblages and crosscutting relationships of veins, two mineralization stages were established, including the early Fe mineralization and late Cu mineralization stage. Three types of magnetite, i.e., platy (MA), massive (MB), and granular (MC) magnetite occur in the Fe mineralization. Backscattered electron (BSE) images identified display oscillatory zoning in an early hematite and transformational mushketovite phase (MA-I), characterized by abundant porosity and inclusions, as well as two later generations, including an early dark (MA-II, MB-I, and MC-I) and later light magnetite (MA-III, MB-II, and MC-II). The MA-I has extremely high W contents and mostly displays as micro-and invisible scheelite inclusions, which were probably caused by the W expulsion during mushketovitization. The texture and composition of magnetite suggest that the later light magnetite formed via dissolution and reprecipitation of the precursor dark magnetite, and the temperature and oxygen fugacity of fluids decreased over time. Our study also shows the MB-II magnetite and coexisting chlorite display synchronous oscillatory zoning, with the calculated temperature from 444 to 212 °C. Such variations could indicate the incursion of external low-temperature fluids with high salinity, which can dissolve the primary dark magnetite. This study provides a good example of using magnetite to trace the complex evolution and multiple sources of ore-forming fluids. [ABSTRACT FROM AUTHOR]

Abstract The Heijianshan Fe–Cu (− Au) deposit is located in the Aqishan–Yamansu belt in Eastern Tianshan, NW China. As a typical Fe–Cu deposit in the region, Heijianshan is hosted in the Upper Carboniferous Matoutan Formation submarine volcanic/volcaniclastic rocks. Alteration styles, mineral assemblages and vein crosscutting relationships divide the hydrothermal alteration and mineralization processes into seven stages, namely the chromite (Stage I), epidote (Stage II), magnetite (Stage III), pyrite (Stage IV), Cu (–Au) (Stage V), late veins (Stage VI) and supergene (Stage VII) alteration/mineralization stages. Magnetite mineralization comprises the hematite (Stage III–A) and main magnetite (Stage III–B) mineralization sub-stages. The Heijianshan magnetite ores consist of massive (with mushketovite (MOM) or sulfides (MOS)), disseminated (DO) and magnetite clasts (with chromite (MWC) or without chromite (MNC)) ores. Magnetite in massive- and disseminated ores is featured by (1) depletion in Zr, Nb and Ta; (2) low Ti (< 2 wt.%) and Al (< 1 wt.%); and (3) Ni/Cr ≥ 1, which all reflect a hydrothermal origin. Moreover, magnetite in massive- and disseminated ores has lower Cr (MOM: 0–13.2 ppm; MOS: 0–12.9 ppm; DO: 3.57–133 ppm) than magnetite clasts ores (MNC: 849–2544 ppm; MWC: 835–44,132 ppm). However, the high Cr in the magnetite clasts ores may have been inherited from the chromite they replaced. From the magnetite clasts to disseminated/massive ores, formation temperature decreased and f O 2 increased, which may represent the major controls on the formation of the various magnetite ore types. Compositions of the ore fluids and host rocks, formation of coexisting minerals and other physicochemical parameters (such as T and f O 2) may have variably influenced the magnetite geochemistry in the different Heijianshan ore types, with fluid compositions probably playing the most important role. Discrimination diagrams, for instance, Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr, may be useful for magnetite mineralization type differentiation in other submarine volcanic-hosted Fe/Fe–Cu deposits in the Aqishan–Yamansu belt. Geochemical discrimination diagrams, alteration and mineralization paragenesis indicate that the Heijianshan Fe–Cu (–Au) deposit is best classified as an IOCG-like deposit, which offers a new insight for classifying and characterizing ore genetic types for similar Fe and Fe–Cu deposits in Eastern Tianshan. Graphical abstract Image 1 Highlights • The Heijianshan magnetite from the massive-, disseminated- and most magnetite clasts ores are hydrothermal. • The Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr diagrams can discriminate the different magnetite ores. • Temperature, f O2 and other factors control the element distribution in magnetite. • The Heijianshan Fe–Cu (–Au) deposit can be classified as an IOCG-like deposit. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Magnetite is a common mineral in many ore deposits and their host rocks, and contains a wide range of trace elements (e.g., Ti, V, Mg, Cr, Mn, Ca, Al, Ni, Ga, Sn) that can be used for deposit type fingerprinting. In this study, we present new magnetite geochemical data for the Longqiao Fe deposit (Luzong ore district) and Tieshan Fe–(Cu) deposit (Edong ore district), which are important magmatic-hydrothermal deposits in eastern China. Textural features, mineral assemblages and paragenesis of the Longqiao and Tieshan ore samples have suggested the presence of two main mineralization periods (sedimentary and hydrothermal) at Longqiao, among which the hydrothermal period comprises four stages (skarn, magnetite, sulfide and carbonate); whilst the Tieshan Fe–(Cu) deposit comprises four mineralization stages (skarn, magnetite, quartz-sulfide and carbonate). Magnetite from the Longqiao and Tieshan deposits has different geochemistry, and can be clearly discriminated by the Sn vs. Ga, Ni vs. Cr, Ga vs. Al, Ni vs. Al, V vs. Ti, and Al vs. Mg diagrams. Such difference may be applied to distinguish other typical skarn (Tieshan) and multi-origin hydrothermal (Longqiao) deposits in the MLYRB. The fluid–rock interactions, influence of the co-crystallizing minerals and other physicochemical parameters, such as temperature and f O 2 , may have altogether controlled the magnetite trace element contents of both deposits. The Tieshan deposit may have had higher degree of f O 2 , but lower fluid–rock interactions and ore-forming temperature than the Longqiao deposit. The TiO 2 –Al 2 O 3 –(MgO + MnO) and (Ca + Al + Mn) vs. (Ti + V) magnetite discrimination diagrams show that the Longqiao Fe deposit has both sedimentary and hydrothermal features, whereas the Tieshan Fe–(Cu) deposit is skarn-type and was likely formed via hydrothermal metasomatism, consistent with the ore characteristics observed. [ABSTRACT FROM AUTHOR]