Zhaochong Zhang

Lead (Pb) and zinc (Zn) are critical base metals whose sedimentary basin-hosted mineral systems form major exploration targets worldwide. Mississippi Valley-type (MVT) deposits represent one of the most important subtypes of sediment-hosted Zn-Pb mineralization and are concentrated within the Tethyan orogenic domain, where ore formation is commonly associated with fold–thrust belts and foreland basins developed along continental collision zones. The Southwestern Tianshan belongs to the southern Central Asian Orogenic Belt (Altaids). Although it is not part of the Tethyan domain, it experienced significant late-stage overprinting driven by far-field Tethyan tectonism that modulated regional ore formation. It hosts two types of sedimentary basins: Late Paleozoic remnant marine basins and Meso-Cenozoic foreland–intermontane basins. These contain Zn-Pb deposits hosted respectively in carbonate rocks (e.g., Hoxbulak) and clastic rocks (e.g., Uragen). This paper systematically reviews the geological and geochemical characteristics of these two deposit types, with emphasis on comparing their metal sources, ore-fluid properties, and mineralization ages. Carbonate-hosted Zn–Pb deposits include a Late Paleozoic sub-type (~258–260 Ma) and a Paleogene main stage (~55–45 Ma), whereas the clastic-hosted Uragen deposit formed mainly in the Paleogene (~55–45 Ma). The Paleogene event is broadly coeval with the India–Asia collision, while the Late Paleozoic sub-type may be related to the closure of the Paleo-Asian Ocean. Both are epigenetic MVT-type systems, but they represent distinct tectonic episodes. Mineralization represents a response to large-scale basinal fluid mobilization during continental collision. However, the two types differ systematically in metal sources: carbonate-hosted deposits derive Pb, Zn, and S mainly from the host carbonate strata themselves (stratabound), whereas the clastic-hosted deposit derives metals mainly from crustal materials supplied by orogenic denudation. Ore fluids in both types are dominated by basinal brines with participation of oil–gas fluids, but carbonate-hosted systems have moderate salinities (2.90–21.82 wt% NaCl eq.) with carbon from host carbonates, whereas clastic-hosted systems have higher salinities (7.22–20.29 wt% NaCl eq.) with carbon from organic matter. Carbonate-hosted deposits are primarily controlled by thrust fault systems and karst breccias, whereas the clastic-hosted deposit is controlled by synclinal structures and “source–reservoir–cap” assemblages. A four-element conceptual framework (tectonic activation, fault conduits, ore-hosting construction, and oil–gas reduction) is developed to organize the regional evidence and to generate testable exploration criteria. This framework is not presented as a validated predictive model, but as a working synthesis that links far-field collisional tectonics, basinal fluid flow, favorable host-rock traps, and hydrocarbon-driven sulfate reduction to Zn–Pb mineralization. Integrated exploration criteria are established, and favorable areas are delineated for future exploration in the Meso-Cenozoic foreland basins on the northwestern Tarim margin and the carbonate-dominated tracts in the South Tianshan margin and Kalpin uplift. The Southwestern Tianshan provides a key case for understanding the spatiotemporal coupling between continental collision and MVT mineralization in the Tethyan domain, with implications for exploration of similar deposits globally.

The burgeoning global demand for rare earth elements (REEs), driven by rapid technological advancements, has elevated their strategic significance. As a ubiquitous metallic mineral that crystallizes throughout the magmatic and hydrothermal stages, pyrite can potentially record the history of REE enrichment within magmatic–hydrothermal systems. The Bachu carbonatite-hosted REE deposit in Xinjiang represents a significant rare earth resource base in China. Elucidating its ore-forming mechanism is therefore crucial for understanding regional metallogenesis and guiding future exploration efforts. In this study, pyrite from this deposit is investigated in detail by employing an integrated analytical approach that includes scanning electron microscopy (SEM), electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace-element analysis, and in situ sulfur isotope analysis. Based on textural and petrogenetic relationships observed under microscopy, three distinct types of pyrite are identified: (1) pyrite coexisting with hydrothermal minerals such as barite and celestite; (2) pyrite showing evident replacement textures, despite the absence of direct intergrowth with sulfate minerals; and (3) pyrite displaying neither intergrowth nor replacement textures with sulfates. These textural distinctions, combined with variations in Co/Ni ratios, Co-Ni-As ternary plots, and Co/Sb vs. Se/As systematics, enable a clear discrimination between pyrite formed during the magmatic (Py1) and hydrothermal (Py2) stages. Pyrite from the magmatic stage (Py1) is characterized by positive δ³⁴S values ranging from 0.03‰ to 4.67‰, with a pronounced peak at 1.30‰, which is higher than the mantle δ³⁴S value (~0‰). This sulfur isotope signature suggests that crustal material was involved in the petrogenesis of the Bachu carbonatite. In contrast, pyrite from the hydrothermal stage (Py2) exhibits a bimodal distribution of δ³⁴S values. One group, peaking at approximately +0.93‰, indicates a magmatic sulfur source genetically related to Py1. The other group, however, displays distinctly negative values, with a peak around −4.93‰, which may be attributed to sulfur isotope fractionation processes during mineralization. Notably, barite from the hydrothermal stage exhibits consistently positive and elevated δ³⁴S values, ranging from 10.46‰ to 15.94‰ (average 12.82‰). The contrasting δ³⁴S values compared to the negative Py2 values manifest a clear “Tower Effect,” strongly suggesting that the extensive precipitation of barite was a primary driver of the negative sulfur isotope values observed in the composition of coexisting hydrothermal pyrite (Py2). The pyrite generations also establish a stage-specific link to REE mineralization: Py1 records the reduced magmatic interval of carbonatite differentiation and primary REE preconcentration, whereas Py2 is associated with the barite–celestine–fluorapatite–monazite assemblage of the REE-rich hydrothermal veins and records the later oxidized fluid overprint. By integrating detailed mineralogical and geochemical evidence, this study successfully distinguishes between magmatic and hydrothermal pyrite populations and provides critical constraints on the sources of ore-forming materials in the Bachu carbonatite-type REE deposit.

The origin of the compositional dichotomy between high‐Ti and low‐Ti basalts in continental flood basalt provinces remains debated. Most existing studies attribute this dichotomy directly to variations in primitive magma composition to circumvent the complexities of significant evolutionary processes, such as multiphase fractional crystallization. Here, we evaluate komatiite as a viable parental magma for the late Permian Emeishan Large Igneous Province to constrain its magmatic evolution. Olivine phenocrysts and spinel inclusions in the investigated picrites exhibit close affinities, consistent with derivation from a common komatiitic parental magma. Thermobarometric constraints define an olivine‐spinel‐clinopyroxene crystallization sequence and reveal two pressure regimes: 0–4.4 and 5.7–8.5 kbar. Thermodynamic modeling successfully reproduces the observed mineral assemblages and liquid lines of descent, accounting for two distinct adiabatic and multi‐stage evolutionary paths. We propose that extensive crustal differentiation along the liquid line of descent produced high‐Ti basalts, whereas limited evolution, melt‐cumulate re‐equilibrium, or incomplete crystal‐melt segregation generated low‐Ti basalts. These results demonstrate that contrasting crustal evolutionary pathways acting on a common komatiitic magma govern compositional diversities observed in global continental flood basalt provinces.

Nephelinites are a distinctive group of silica-undersaturated, strongly alkaline ultrabasic volcanic rocks that are occasionally associated with carbonatites. Their genesis has been attributed to the melting of a mantle source enriched by either recycled crustal materials or other metasomatic agents. This study investigates olivine nephelinites and constrains their mantle source from the West Qinling orogen, Central China—a locality noted for its unique occurrence of rocks previously classified as kamafugites or melilitites in China. Olivines in these rocks are categorized into Group I mantle xenocrysts (Fo_89–92) and Group II magmatic crystals (Fo_81–88). In situ δ¹⁸O values of the Group II magmatic olivines (5.04 ± 0.25‰; 1σ, n = 67) are slightly lower than those of Group I mantle xenocrysts (5.26 ± 0.20‰,1σ, n = 199). Measured ¹⁸⁷Os/¹⁸⁸Os ratios for these nephelinites are moderately to highly radiogenic (0.138–0.682), with age-corrected γOs values varying from +6.57 to +409. Magmatic olivines with elevated 100 × Mn/Fe (mean = 1.65), 100 × Ni/Mg (mean = 0.70), and high 100 × Ca/Fe (mean = 2.33) ratios fingerprint a metasomatized mantle peridotite source. Binary mixing indicates that the observed radiogenic Os isotopes can be explained by the incorporation of <30% melts/fluids derived from a subducted oceanic slab. Our findings suggest that mantle metasomatism, driven by recycled oceanic slabs, played a major role in alkaline enrichment observed in the West Qinling magmas, and may provide new insights into the crust–mantle interaction.

Water plays a critical role in the formation of large igneous provinces (LIPs) and deep-Earth material cycling. However, directly quantifying water content in ancient mantle sources remains a challenge. Here we developed a novel clinopyroxene-based hygrometer using a comprehensive dataset of 316 experimentally crystallized clinopyroxene samples with known equilibrium melt water contents. Based on this dataset, we trained and tested multiple machine learning models to predict water content from the major element composition of clinopyroxene. The S-MKL algorithm, a combination of a stacking-based representation learning framework (S-RL) and multiple kernel learning (MKL), achieved superior predictive accuracy and was selected as the optimal model. We applied this new hygrometer to clinopyroxene phenocrysts from the Permian Emeishan LIP in SW China. The results reveal a hydrous mantle source for the Emeishan LIP, with water contents comparable to those of backarc basin basalt. A pronounced decrease in the gradient of water content was identified from southwest to northeast, reflecting mantle source heterogeneity linked to Paleo-Tethyan oceanic slab subduction and stagnation in the mantle transition zone. Fractional crystallization modeling confirms that this gradient represents source-level variations rather than shallow processes. We conclude that fluids released from the stagnant slab metasomatized the mantle, enhancing its fertility and contributing to LIP generation. This study quantitatively establishes a link between slab-driven mantle hydration and LIP emplacement, highlighting the importance of plume-slab interactions and deep-Earth water recycling.

Magma mixing is widely recognized as a prevalent magmatic mechanism, yet its significance in the formation of coexisting nephelinite and melilitite has frequently been overlooked. This oversight stems from the conventional assumption that nephelinite forms either through fractional crystallization from melilititic melt or by direct derivation from the mantle with rapid crust transit. To bridge this knowledge gap, we provide a comprehensive petrological analysis of melilite-bearing nephelinite from Yongquan in Southeast China, which serves as a compositionally transitional facies between nephelinite and melilitite. In addition to the typical nephelinite assemblage of olivine, nepheline, clinopyroxene and titanomagnetite, this lava also contains melilite, monticellite, sodalite, cancrinite and perovskite. Such a mineral assemblage has rarely been documented in nephelinitic lavas. Crystals within this assemblage commonly display disequilibrium textures and intricate zoning patterns. Olivine microphenocrysts exhibit systematic textural and P compositional zonation. P-rich skeletal cores grade outward into P-rich polyhedral mantles and terminate in P-poor rims, while maintaining nearly uniform Fo (~77–81) values [Fo = 100 × molar Mg/(Mg + FeT)]. The mantle zones contain patchy domains of high-CaO olivine (Fo ~65–76), and the outermost rims are resorbed and commonly enveloped by monticellite. Clinopyroxene microlites preserve sector zoning, overprinted by pronounced oscillatory chemical bands (in Mg#, Cr, Ni and Mn) and resorption surfaces at core–mantle transitions, frequently accompanied by reverse zoning. The phosphorus zoning in olivine and sector zoning in clinopyroxene collectively suggest a progressive decrease in undercooling. Based on mineral textures and coexisting mineral phases, we infer that the unique mineral assemblage in the Yongquan melilite-bearing nephelinite resulted from mixing between a preexisting nephelinitic magma and a recharged melilititic magma. These observations demonstrate that magma mixing is the primary mechanism producing coexisting melilitite and nephelinite in Yongquan melilite-bearing nephelinite, rather than simple differentiation from a melilititic parent. The microphenocryst and groundmass microlites characteristics in these nephelinite magmas reveal short-timescale dynamics, highlighting the necessity of integrating textural data with bulk-rock compositions for precise interpretation of source signatures.

The petrogenetic link between carbonatites and associated silicate rocks remains a longstanding debate in igneous petrology. Some minerals, such as apatite and calcite, which crystallize across diverse lithologies during magmatic differentiation, can record geochemical changes in their crystallizing environments, thereby providing valuable insights into the genesis of these rock suites. The Wajilitage Carbonatite Complex (WCC) in northwest China, part of the Tarim Large Igneous Province, is a typical carbonatite–alkaline complex composed of calcite and dolomite carbonatites, aillikite, nephelinite, and nepheline syenite. In this study, in situ geochemical and C–O isotope analyses of apatite and calcite are utilized to investigate their genetic relationships. The Mg content of apatite (Mg_ap) serves as an effective recorder of the magmatic evolution of this carbonatite–alkaline complex. Apatite in the carbonatite has higher Mg contents than apatite in the nephelinite and nepheline syenite, precluding an origin via fractional crystallization or liquid immiscibility from these silicate melts. Although the Mg contents of apatite in the carbonatites overlap with those of apatite phenocrysts in aillikite, their distinct trace-element compositions (e.g. La, Sr, and Y) and δ¹⁸O_V-SMOW values rule out a direct genetic relationship between these rocks. We propose that the carbonatites formed independently by low-degree partial melting of a carbonated mantle source. Subsequent fractional crystallization of calcite and dolomite from this parental magma produced the calcite and dolomite carbonatites, respectively. Trace elements and δ¹⁸O_V-SMOW values of apatite suggest that nephelinite evolved to nepheline syenite, the latter of which assimilated the aillikite. Differentiation of the carbonatite magma generated distinct styles of rare earth element (REE) mineralization, with light REE (LREE) being enriched in dolomite carbonatite, and both LREE and heavy REE (HREE) being enriched in calcite carbonatite. This study presents an integrated petrogentic model for the WCC, highlighting the utility of apatite geochemistry in unraveling the complex magmatic evolution of carbonatite–alkaline complexes.

The role of fluids in the formation of large igneous provinces (LIPs) remains a topic of debate. The Emeishan Large Igneous Province (ELIP) in southwestern China, with a large volume of mafic intrusive and extrusive suites, provides an ideal window to explore this enigma. We predict the bulk rock characteristics of basalt and diabase in the ELIP using a model trained by machine learning methods (random forest [a collection of decision trees that work together to make predictions]) and using global data sets of basalts in diverse settings. The results indicate that the basalts and diabases in the ELIP generally exhibit some affinity to island arc basalt. Furthermore, there is a spatial decline in composition from west to east in the northern part of the ELIP and generally from south to northwest and northeast along the southern margin of the Yangtze craton in the southern part of the ELIP. This pattern may be linked to the subduction of oceanic lithosphere around the Yangtze craton during the Neoproterozoic. Our study reveals the interaction between the Neoproterozoic subducted slab and mantle plume and highlights the contribution of fluids from the slab in the formation of the ELIP.