Types, genesis, and implications of phlogopite in the Jinchuan Cu-Ni (Pt) sulfide deposit, Gansu Province, China

doi:10.13745/j.esf.sf.2025.7.12

Abstract

Abstract:

The Jinchuan Cu-Ni-(PGE) sulfide deposit in China ranks as the world’s third largest and Asia’s largest deposit of its type, constituting a vital strategic resource reservoir. Significant debate persists regarding the genesis and metallogenic mechanisms of this deposit. Phlogopite is a common hydrous mineral within the rocks and ores of Jinchuan, and its relationships with rock-forming and metallic minerals can elucidate the interconnections between magmatism, mineralization, and fluid activity. Based on systematic microscopic observations of phlogopite coupled with elemental geochemical characteristics, this study identifies two distinct types within the Jinchuan deposit: Type A and Type B phlogopite. Type A phlogopite typically exhibits subhedral to euhedral habits with prominent deep yellowish-brown to colorless pleochroism. It predominantly occurs in barren to low-grade rocks, displaying symplectite/symplectic intergrowth with grains of olivine and pyroxene, indicating crystallization contemporaneously with these minerals from the mantle-derived ultramafic magma. Elemental analysis reveals its similarity to phlogopite found in kimberlites and lamprophyres from regions like the USA and South Africa. It was classified as ‘re-equilibrated’ phlogopite rather than primary magmatic phlogopite, signifying slight modification by mantle-derived melt-fluid flux. Type B phlogopite also commonly exhibits subhedral to euhedral habits but lacks pleochroism. It occurs in relatively higher-grade ores and more intensely altered rocks, developing between rock-forming minerals, sometimes in symplectite/symplectic intergrowth, and is often associated with metallic minerals or crosscut/replaced by them. It similarly displays symplectite/symplectic intergrowth with surrounding olivine and pyroxene grains, which are typically serpentinized or chloritized. Type B phlogopite was presumably also generated from magmatic crystallization. Elemental analysis indicates its resemblance to phlogopite in certain South African peridotites, classifying it as ‘neocrystallized’ phlogopite, which experienced stronger modification by mantle-derived melt-fluid flux and is associated with the main mineralization stage. Type A phlogopite formed under relatively high-temperature, high-Ti, and high-oxygen fugacity conditions characterized by low Si, Mg, and Na, but enrichment in Fe and Al, favoring the enrichment of metallic elements like Fe. Type B phlogopite formed through the modification of Type A phlogopite. Consistent with this, its elemental signature suggested that the modifying agent was a mantle-derived, Mg-, Si-, and Na-enriched, but Ti-, Fe-, and Al-depleted melt-fluid flux rich in volatile components such as Cl. During this process, rock-forming minerals absorbed Mg while releasing Fe, contributing to ore formation. The crystallization temperatures of phlogopite were calculated using Ti geothermometry. The continuous vertical variation in these crystallization temperatures likely represents prolonged, sustained melt-fluid flux, indicating that the Jinchuan ultramafic intrusion functioned primarily as a long-term conduit for melt-fluid flux. This persistent flux may be the key factor enabling the progressive enrichment of metallic elements and the formation of the deposit.

Key words: Jinchuan Cu-Ni(Pt) sulfide deposit, phlogopite, elemental geochemistry, melt-fluid flow

CLC Number:

YU Xianghui, LIU Cui, SU Shangguo, LIU Jixu, WANG Miao, GUO Xu, ZHOU Chenghao, GAO Yalin. Types, genesis, and implications of phlogopite in the Jinchuan Cu-Ni (Pt) sulfide deposit, Gansu Province, China[J]. Earth Science Frontiers, 2025, 32(6): 224-244.

Figures/Tables 16

Fig.1 Geological sketch map of the Longshoushan area. Modified after [19].

Fig.2 Geological sketch map with representative cross-section of the Jinchuan Cu-Ni deposit (a) and graphical capture of the 3D model of main orebodies (b). a modified after [19]; b modified after [28].

Fig.3 Field geological phenomena in the Jinchuan mining area

Fig.4 Ore types of the Jinchuan Cu-Ni deposit

Fig.5 Photomicrographs of rock and ore samples from the Jinchuan deposit

Fig.6 Photographs of phlogopite with distinct pleochroism in the Jinchuan deposit

Fig.7 Photographs of phlogopite lacking pleochroism in the Jinchuan deposit

Fig.8 Photographs of phlogopite exhibiting transitional pleochroism in the Jinchuan deposit

Fig.9 Classification diagram of phlogopite. Base map adapted from [30].

Fig.10 Ternary diagram of TiO2-FeO*-MgO for phlogopite. Base map adapted from [47].

Fig.11 Comparison diagram of elemental contents in contrasting phlogopite types from the Jinchuan Cu-Ni deposit

Fig.12 Discrimination diagram of (Fe2++Fe3+)-Mg for ferromagnesian micas. Base map adapted from [48].

Fig.13 Discrimination diagram of FeOT/(FeO+MgO)T-MgO for ferromagnesian micas. Base map adapted from [49].

Fig.14 In-sample comparison diagram of elemental contents in phlogopite from Specimen 18-0-2

Fig.15 Isopleth map of Ti-Mg/(Mg+Fe) for phlogopite thermometry. Base map adapted from [57].

Fig.16 Ternary diagram of Fe3+-Fe2+-Mg2+ for phlogopite in the Jinchuan deposit. Base map adapted from [58].

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