Using Mineral Chemistry to Characterize Ore-Forming Processes: An Introduction

This Special Issue brings together nine studies that employ mineral-based investigations to advance the understanding of ore-forming processes. The contributions demonstrate how integrated approaches combining microtextural analysis, in situ geochemistry, and isotopic systems can unravel complex mineralization processes across diverse geological settings. These studies successfully trace fluid evolution pathways, constrain metal sources, determine precise mineralization ages, and reveal petrogenetic processes controlling metal endowment. The findings demonstrate how mineral-scale signatures can reveal large-scale geological processes, from crust–mantle interactions in magmatic systems to fluid–rock reactions in hydrothermal environments. The methodological advances in micro-analytical techniques presented here enable the resolution of previously undetectable chemical signatures and temporal relationships in complex ore systems.

Li et al. (Contribution 1) conducted a comprehensive petrographic and geochemical study of magnetite from the Wuan orbicular diorite in Hebei Province, China. Their results indicate that the core magnetite crystallized from iron-rich melts formed through liquid immiscibility within the magma chamber. This immiscibility was triggered by a significant increase in oxygen fugacity, caused by CO₂ released during the reaction between ascending silicate magma and carbonate xenoliths. The study proposes a stepwise metallogenic model where Fe²⁺ in the original magma is oxidized under high oxygen fugacity, leading to the separation of an Fe-rich oxide melt and the early crystallization of iron oxides like magnetite, alongside iron-poor silicate minerals such as tremolite and actinolite. The compositional variations in magnetite suggest that the formation of Hanxing-type Fe deposits is closely linked to magmatic–carbonate rock interactions.

Filimon et al. (Contribution 2) employed a suite of microtextural and geochemical techniques—including SEM-EDS, EPMA, and LA-ICP-MS—to investigate sulfide minerals from the Matra As–Sb deposit in Alpine Corsica, France. Their results indicate that hydrothermal mineralization occurred within a system of reactivated faults. Sulfide precipitation is interpreted to have been primarily controlled by variations in sulfur fugacity (fS₂), driven by the influx of oxidizing meteoric fluids. This process led to a progressive evolution in arsenic speciation, from reduced forms (As¹⁻ in pyrite) to oxidized species (As²⁺ in realgar, As³⁺ in orpiment, and As⁵⁺ in hörnesite). The authors identified a diagnostic trace-element signature of As–Sb–Tl–Ni in pyrite and As–Fe–Tl in stibnite, and compared the mineralization with Tuscan Carlin-type deposits, highlighting shared tectonic settings.

Sun et al. (Contribution 3) carried out an integrated mineralogical study of pyrite and quartz from the Qiubudong Ag deposit in the central North China Craton. Their analytical approach included crystal morphology, trace-element geochemistry, thermoelectric and thermoluminescence properties, and fluid inclusion microthermometry. The study concludes that the deposit formed from medium–low temperature, F-rich, low-salinity hydrothermal fluids of volcanic–magmatic origin, with limited meteoric water input. Key mineralogical indicators for mineralization comprise the predominance of pentagonal dodecahedral pyrite, elevated Au content, low Co/Ni ratios, and complex thermoluminescence patterns in quartz.

Ouyang et al. (Contribution 4) performed zircon U–Pb geochronology and whole-rock geochemical analyses on a newly identified biotite granite porphyry from the Zhuxi W–Cu polymetallic deposit in Jiangxi Province, South China, and compared it with previously documented diorite porphyrite and biotite quartz monzonite porphyry in the region. They conclude that these three intrusive rocks were emplaced contemporaneously around 160 Ma but are not comagmatic. It is inferred that the biotite granite porphyry originates from partial melting of the thickened lower crust, the diorite porphyrite from melting of the delaminated lower crust, and the biotite quartz monzonite porphyry through mixing of mantle and crustal derived melts, all formed in a post-orogenic extensional tectonic setting.

Xiang et al. (Contribution 5) present new geochemical data from Early Cretaceous Kongco granitic porphyry dykes in the northern Central Lhasa Microblock, Tibet. These dykes are classified as A2-type granites, formed by partial melting of the lower crust induced by subduction-related slab break-off and asthenospheric upwelling during the collision between the Qiangtang and Lhasa terranes. The close spatial relationship between the dykes and copper mineralization suggests a genetic link between this magmatic event and regional metallogenesis.

Khedr et al. (Contribution 6) report mineralogical and geochemical analyses of titaniferous iron ores and their host gabbro, along with interstitial clinopyroxene, from the El-Baroud Layered Gabbros. The iron ores are mainly composed of titanomagnetite, ilmenite, and magnetite. Geochemical data suggest that the gabbro formed in an arc rifting environment, and crystallized under high oxygen fugacity conditions. The Fe-Ti oxide ores are interpreted to have originated from in situ crystallization and liquid immiscibility from ferropicritic parent melts. The distribution of the ores is inferred to be structurally controlled.

Li et al. (Contribution 7) presented in situ LA-ICP-MS U–Pb ages of carbonate minerals from the Lannigou Carlin-type Au deposit, Lanmuchang Hg-(Tl) deposit, and Sixiangchang Hg deposit in South China. The results constrain gold mineralization to ca. 137 ± 9 Ma, Hg-Tl mineralization to ~97 Ma, and Hg mineralization to 454 ± 21 Ma. Integrating these data with previous geochronology, the authors identify two main episodes of low-temperature Au–As–Sb–Hg–Tl mineralization in the Youjiang metallogenic province. The study confirms the utility of carbonate U–Pb dating to constrain the timing of low-temperature mineralization.

Fan et al. (Contribution 8) conducted a petrographic and mineral chemical study of amphiboles from the Dalaku’an mafic–ultramafic intrusion in the western East Kunlun orogen. The amphiboles are classified into three types: tschermakitic hornblende and magnesio-hornblende of mantle affinity, magnesio-hornblende with lower Al–Ti contents, and actinolitic hornblende indicative of crustal input. Thermobarometric estimates reveal declining temperatures and pressures, consistent with multi-stage magma chamber evolution at different depths. High oxygen fugacity and abundant water suggest fluid metasomatism related to subducted oceanic crust. These findings collectively indicate that the Dalaku’an intrusion formed in a post-collisional extensional setting with contributions from both mantle and crustal sources.

Zhang et al. (Contribution 9) present a comprehensive study incorporating geological mapping, geochronology, and isotope geochemistry of the Tietangdong breccia pipe in the Yixingzhai gold deposit, Central Taihangshan District (CTD) of the North China Craton. Three breccia facies are identified: massive skarn breccia, polymictic skarn matrix-supported breccia, and polymictic intrusive rock cement chaotic breccia. ⁴⁰Ar/³⁹Ar dating of adularia yields an age of 136 ± 1.5 Ma, synchronous with post-breccia felsite dike emplacement. Pyrite δ³⁴S values (~2.7‰) support a magmatic–hydrothermal origin. The results establish a genetic relationship between the breccia pipe and adjacent lode gold mineralization.

We hope this Special Issue serves as a valuable reference and inspiration for researchers in economic geology and related fields. Looking ahead, several exciting research directions stand out. First, continued improvements in micro-analytical tools—like high-resolution imaging and in situ chemical analysis—will allow us to study mineral formation in greater detail than ever before. Second, combining mineral chemistry data with advanced computational methods, such as thermodynamic modeling and machine learning, opens new ways to understand ore-forming processes and uncover hidden patterns in complex datasets. Finally, linking fine-scale mineral observations to large-scale tectonic and geodynamic settings will be key to building reliable models of how different types of ore deposits form. By bringing together these interdisciplinary approaches, mineral chemistry will continue to play a central role in unraveling the origins of ore deposits and guiding more effective mineral exploration in the future.