Wheeler, J.¹, Henley, R. W.², Gardner, J.¹, Mernagh, T.², Leys, C.³, Troitzsch, U.², Bevitt, J.⁴, Brink, F.², Knuefing, L.², Limaye, A.² Turner, M.² & Zhang, Y.²

¹ Department of Earth, Ocean, and Ecological Sciences, University of Liverpool, Liverpool, UK

 ² Australian National University, Canberra

³ P.T. Freeport Indonesia, Papua

⁴ Australian Nuclear Science and Technology Organisation, NSW 2234, Australia

Porphyry copper deposits are not formed just by crystallisation of ores from Cu-bearing hydrothermal fluids; metasomatism can be involved. We present a metamorphic point of view of the Grasberg porphyry deposit in Papua, which is hosted by intensely altered calc-alkaline plutonic rocks characterised by albite and anhydrite. We propose that plagioclase reacts with magmatic SO₂ to form anhydrite and albite, and this releases H₂S that plays a major role in Cu ore formation [1]. We split a complex set of reactions into conceptual “building blocks”: these did not happen in a particular order but help to explain our observations.

• SO₂ (in the volcanic gas) reacts with water to becomes H₂S and H₂SO₄.
• H₂SO₄ reacts with Ca from plagioclase to form anhydrite (in veins) c.f. [2].
• Albite is left over.
• H₂S reacts with Cu (in the volcanic gas) to form Cu minerals.
• Fe from biotite similarly reacts to form CuFe minerals.
• K-feldspar is left over.

Partly because of these reactions the potassic zone contain more K-feldspar than the protoliths. This is not due to the introduction of magma-derived K by metasomatism; a large chemical dataset shows that unaltered and altered rocks have similar major element bulk compositions. The “HSC Chemistry” package which includes thermodynamics of gases with varied chemistry has been used for preliminary models of reaction.

Electron Backscatter Diffraction work on the new albite shows it is replacing plagioclase inheriting the crystallographic orientation. This resembles microstructures in “coupled dissolution precipitation” reactions [3] though we are not implying the reaction mechanisms are necessarily the same. The thermodynamics and kinetics of the plagioclase breakdown will affect the overall amount of copper ore formed.

[1] Henley et al. JVGR (2022) 432: 107710.

[2] Henley et al. Nat Geosci (2015) 8: 210.

[3] Gardner et al. Lithos (2021) 396-397.

How to cite: Wheeler, J., Gardner, J., and Henley, R.: Genesis of porphyry copper deposits: key roles for plagioclase and anhydrite in metasomatism, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15902, https://doi.org/10.5194/egusphere-egu25-15902, 2025.

Earth’s enriched lithospheric mantle is postulated to be a natural repository of gold and rare earth element (REE) concentrations. We reviewed evidence for gold and REE enriched mantle from the Jiaobei and Luxi terranes in the North China Craton (NCC), which are the world’s third largest gold province and the China’s third largest REE deposit, respectively. In both terranes, extensive Archean tonalite–trondhjemite–granodiorite (TTG) suites are exposed, but whether their mantle source and partial melting pressure are different that caused diverse metallogeny remains ambiguous. Based on a comprehensive analysis of geochemical data, zircon U–Pb, and Hf isotopic compositions from the TTGs, we evaluate the petrogenesis, crustal–mantle evolution, and the role of source magma composition in the formation of crust as well as gold and REE mineralization. Zircon U–Pb–Hf isotope systematics reveal that magma emplacement occurred during three major pulses at ca. 2.9 Ga, 2.7 Ga, and 2.5 Ga in the Jiaobei Terrane, whereas magmatism in the Luxi Terrane was largely concentrated from ca. 2.7 to 2.5 Ga. Geochemical and isotopic data show that the ca. 2.9 Ga and ca. 2.7 Ga TTGs in the Jiaobei Terrane are inferred to have been generated by high- and low-pressure partial melting of an enriched mantle wedge and mafic crust of a thickened arc. The ca. 2.6 Ga and ca. 2.5 Ga TTGs in the Jiaobei Terrane were generated from low- to medium-pressure partial melting the crust of a continental arc. The mantle was gradually metasomatized by slab–derived fluids in the Jiaobei Terrane during ca. 2.7–2.5 Ga, and by additional melts from sedimentary protoliths in the Luxi Terrane during ca. 2.6–2.5 Ga. The spatial distribution of isotopic and geochemical patterns of TTGs reveals the presence of a heterogeneous enriched lithospheric mantle beneath the Jiaobei and Luxi terranes, formed by variable degrees of metasomatism and experienced variable degrees of partial melting. We propose that mantle metasomatism induced by melts derived from sedimentary precursors and low-pressure partial melting played an important role in the formation of the REE deposits and gold fertility within the SCLM.

How to cite: Li, S., Qiu, K.-F., Kusky, T., Wang, L., Yu, H.-C., Wu, M.-Q., Frimmel, H., Gao, Y.-X., Zhou, T., Yang, Z.-Y., Xi, Z.-C., and Deng*, J.: Mantle metasomatism facilitates the formation of continental crust and metal enrichment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8010, https://doi.org/10.5194/egusphere-egu25-8010, 2025.

New discoveries of ore deposits are essential to secure our future demand on raw materials. Exploration for major hidden ore deposits at depth requires novel exploration concepts based on mineral system analyses beyond the deposit scale. Such approaches seek to develop a fundamental understanding of the process chain of coupled physical and chemical interactions between magmas, fluids and rocks that lead to the formation of large ore deposits. Numerical models will play a key role in bridging spatial and temporal scales varying by orders of magnitude. In this contribution, we present new numerical constraints on the formation of porphyry Cu-Mo and epithermal Au-Ag deposits. The model can simultaneously resolve both magma (Navier‐Stokes) and hydrothermal (Darcy) flow. It further uses realistic non-linear properties of crystallizing magmas and saline fluids, dynamic permeability feedbacks including fault structures, and proxies for metal transport.  The simulations describe the interplay of episodic sill emplacements, magma convection, focused volatile degassing, hydraulic fracturing, fluid phase separation and mixing. The model further simulates the fate of chemical components like salts and metals, considering fluid-melt and vapor-brine partitioning, as well as precipitation and remobilization. The simulation results show that the coupled physicochemical interactions of all of these processes can self-organize into the accumulation of voluminous hydrous magma reservoirs, distinct stages of degassing and ore precipitation by interaction with groundwater convection in typical porphyry ore shells (e.g. Gruzdeva et al., 2024). The modelled temporal and spatial evolution of the magmatic-hydrothermal system successfully reproduces and explains many observations at porphyry and epithermal deposits worldwide. Combining these first-order constraints from simplified numerical models with geochemical and geophysical data provides a promising avenue for the development of multi-method approaches to develop robust exploration criteria for future discoveries of critical mineral deposits.

Gruzdeva, Y., Weis, P., Andersen, C. (2024): Journal of Geophysical Research: Solid Earth, 129, 7, e2023JB028433. https://doi.org/10.1029/2023JB028433

How to cite: Weis, P. and Gruzdeva, Y.: Numerical Modelling of Coupled Interactions of Magmas, Fluids and Rocks in the Formation of Porphyry Copper Deposits, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5671, https://doi.org/10.5194/egusphere-egu25-5671, 2025.

Reported Paleoproterozoic rare earth element (REE) deposits worldwide are quite limited. Here, we present the first detailed studies on a volcanic-sedimentary metamorphic type REE deposit, namely the Shengtieling REE deposit in NE China, to give a further case. The Shengtieling REE deposit (central Liaoning) occurs in the Lieryu Formation of the South Liaohe Group and is located in the Paleoproterozoic Jiao-Liao-Ji tectonic belt. The BSE images, combined with the monazite LA-ICP-MS in-situ trace elements, show that the main REE minerals are monazite, xenotime, and apatite from the magnetite leptynite. According to zircon and monazite U-Pb geochronological results, the maximum depositional age for magmatic clastic zircon is 1.95Ga, but both metamorphic zircons and monazites give similar ages of 1.90~1.87Ga. Whole-rock trace elements data suggest a continental island arc origin for the magnetite leptynite samples, thus indicating that the protolith may be arc-related sedimentary rocks. Notably, the 1.90~1.87Ga metamorphism is consistent with the timing of regional metamorphism, confirming the existence of arc-continent collision during the formation of the Jiao-Liao-Ji tectonic belt. Thus, based on the above new results, the Shengtieling REE deposit should be a typical Paleoproterozoic volcanic-sedimentary metamorphic type REE deposit. More attention should be paid to exploring similar REE deposit types in NE China and elsewhere.

How to cite: Liu, B., Ju, N., and Liu, X.: Zircon and monazite U-Pb geochronology and trace elements unravel a Paleoproterozoic volcanic-sedimentary metamorphic type rare earth element deposit in NE China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3683, https://doi.org/10.5194/egusphere-egu25-3683, 2025.

Despite their close temporal and spatial relationships, the effects of tectono-thermal events on ore formation remain obscure. To better understand this process, a comprehensive geochemical investigation was conducted on syn-tectonic pegmatite and quartz veins associated with the Devonian subduction and Permian collision of the Chinese Altai. We found that the Devonian fluids were organic alkanes-CO₂-S-Ca-Mg-rich saline fluids with variable CO₂/CH₄, lower F⁻/SO₄²⁻ and Al³⁺/Mg²⁺ ratios, whereas the Permian fluids are immiscible fluids including CO₂-C₄H₁₀-CO-rich oxidized gas bubbles and CH₄-C₃H₈-C₂H₆-Ca-Na-K-Al-S-Cl-F-rich reduced saline fluids with lower CO₂/CH₄ (mostly <1), higher F⁻/SO₄²⁻ and Al³⁺/Mg²⁺ ratios. The Devonian and Permian fluids also have similar δ¹³C-CO₂ values of −3.5~−23.8‰ and −3.7~−16.5‰, repressively. These data suggest that both fluids derived mainly from devolatilization and dehydration melting of metasediments but the Permian fluids likely involve more muscovite dehydration and biotite melting in the shallower and deeper crust, respectively. Besides, the Devonian fluids contain more meteoric components whereas the Permian fluids contain more mantle-derived components. Base metal-dominated Devonian mineralization occurred as the deep-sourced organic matter-S-rich fluids promote base metal migration whereas the relatively oxidized fluid conditions inhibited mineralization of many other metals. By contrast, the more reduced and F-rich Permian fluids with more mantle contributions facilitated the extraction of Au and uptakes of rare metals from reworked metasediments and promoted their mineralization. These findings provide more complete pictures of how tectono-thermal events fertilize the crusts and demonstrate that syn-tectonic fluids can serve as proxies for metallogenic processes during orogenic cycles in general.

How to cite: Xiao, M., Zhao, G., and Jiang, Y.: Syn-tectonic fluids decoding effects of tectono-metamorphic cycles on regional metallogenic evolution of the Chinese Altai, central Asia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15366, https://doi.org/10.5194/egusphere-egu25-15366, 2025.

Thermodynamic modelling of H₂O, HCl and NaCl ionization at magmatic-hydrothermal conditions provides essential basis for understanding speciation, acidity changes, metal complexation and ore-deposit formation. We critically evaluate modelling of H₂O, HCl and NaCl ionization using pre-existing approaches in literature, that include: (i) electrostatic models [1,2]; (ii) models based on semi-empirical logarithmic correlations with water density [3]; (iii) models based on virial expansion [4]; (iv) approaches based on stepwise hydration of solute [5], and (v) various statistical-mechanics-based theories [6]. Simulations of H₂O, HCl and NaCl ionization from individual models are consistent up to 400 °C and pressures above 3 kbar. However, significant discrepancies emerge with increasing temperatures and decreasing pressures. Electrostatic and virial models at low pressures (below 300 bar) and at high fluid density poorly perform partly because loose physical significance. Hydration models and approaches based on mean spherical approximation inaccurately describe pressure-temperature dependence. Density models emerge as the most accurate in ionization predictions and therefore, as promising approach for constructing new equations of state for ionic species. We develop a new low-parametric density model to depict the thermodynamic properties of aqueous species in low-density hydrothermal fluids with a more rigorous theoretical framework that incorporates intrinsic properties of aqueous solute (entropy, enthalpy, heat capacity and hardcore volume) and accounts for solute-solvent interactions (via volume compression). Our new density model offers improved accuracy and performance in the temperature-pressure space requiring fewer equation parameters in comparison to existing models in literature. The new density model is applied for the prediction of H₂O, HCl and NaCl speciation along four fluid-flow paths representing distinct crustal settings, specifically: transcrustal metamorphic devolatilization, intrusion-related lateral, vertical and adiabatic flow. Simulations reveal that metamorphic fluids have ionization capability by four orders of magnitude greater than the upper-crustal magmatic fluids. This demonstrates the superior effectiveness of high-pressure fluids in the transport of ionic species and acidity generation. Fluids exsolved from upper-crustal magmatic sources during lateral or vertical flow exhibit mutually comparable behavior upon cooling and progressively ionize species and produce significant acidity from 400 °C.  By contrast, speciation occurring during adiabatic flow is mainly controlled by decreasing fluid density, and as the fluid cools and expands solute species remain completely associated. Overall, the simulation of the four thermal gradients highlights the major impacts that pressure and fluid density have on H₂O, HCl and NaCl ionization and the various efficiency for acidic alteration and mineralization of hydrothermal fluids along their specific pathways and hydrodynamic conditions.

References:

[1] Tanger IV J C, Helgeson H C (1988) Am J Sci 288: 19-98

[2] Shock E L et al. (1992) J Chem Soc Faraday Trans 88: 803-826

[3] Marshall W L and Franck E U (1981) J Phys Chem Ref Data 10: 295-304

[4] Akinfiev N N and Diamond L W (2003) Geochim Cosmochim Acta 67: 613-627

[5] Djamali E and Cobble J W (2009) J Phys Chem 113: 2398-2403

[6] Lvov S N et al. (2018) J Molecul Liq 270: 62-73

How to cite: Salomone, F. and Dolejs, D.: Ionization of H2O, HCl, and NaCl in low-density crustal fluids: thermodynamic modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11313, https://doi.org/10.5194/egusphere-egu25-11313, 2025.

Vein-type gold orebodies in hydrothermal gold deposits normally develop vein-type orebodies normally composed of quartz and metal sulfides. Calcite, as one of the common minerals in vein-type gold deposits, commonly formed in the late mineralization stage. The Phapon gold deposit, located in northern Laos, is a unique deposit that is characterized by calcite as the main gangue mineral, free gold coexist with iron oxide, very low content of quartz and metal sulfides, and no spatial correlation with intrusive rock. The auriferous veins are hosted in lower Permian carbonate rocks and controlled by subparallel, NNW-trending brittle faults. The gold orebody is composed of a 0.3–2.0 m wide auriferous calcite vein that fills the fault zone, and the surrounding siderite and hematite alteration zones, with sparsely disseminated silification and sulfidation. The auriferous calcite vein consists of calcite (~90 vol%), subsequent siderite (~5 vol%) and hematite (~3 vol%), and a small amount of quartz, realgar, magnetite, orpiment, and traces of pyrite. In the siderite and hematite alteration zones, the hydrothermal mineral assemblage is similar to the veins, with less quartz, realgar, and orpiment and lacking pyrite.

Based on detailed field investigation, and microscopic and CL studies, three ore-forming stages were recognized as the pre-ore calcite(Cal-1)±quartz±pyrite veins, main-ore calcite(Cal-2)-siderite-hematite-realgar±orpiment-gold veins, and post-ore calcite(Cal-3) veins. The primary metal sulfides are mostly replaced by goethite during secondary oxidation. Gold normally formed as free gold that occurs in microcracks or along grain boundaries of Cal-2, or coexisted with goethite and fibrous hematite aggregates.Fluid inclusion petrography and microthermometry study suggested that the ore-forming fluids belong to a median-low temperature (180–240°C) and low salinity (3–10 wt% NaCl eq.) NaCl-H₂O-CO₂ system. Gold precipitation was mainly related to fluid immiscibility caused by pressure drop. Considering the coexistence of pyrite and iron-oxides in gold ores, gold deposition may be also related to changes of Eh and pH during the hydrothermal processes. Calcite LA-ICP-MS trace element analysis suggests inheritance between hydrothermal calcite and carbonate wall rock. Cal-2 shows higher REE, Mn, and Fe concentrations and the most obvious LREE-enrichment patterns compare to Cal-1 and Cal-3, indicating the ore-forming fluids in the main-stage are more acidic and have more intense fluid-rock interaction at the deposit trap. Vein calcite was dated by LA-SF-ICP-MS and obtained a lower intercept U-Pb age of 221.6 ± 7.6 Ma, which is interpreted as the Au mineralization age for the Phapon deposit. This age indicates that the epizonal orogenic gold mineralization event continued in the Late Triassic along the northwestern margin of the Indochina Block, postdating the late Permian–middle Triassic low-sulfidation epithermal and porphyry-skarn Au mineralization events and corresponds to the collision between the Sibumasu Terrane and the Indochina Block.

No similar deposit has been described until now, further study on the P-T-Eh-pH controlling factors during gold enrichment and precipitation process of Phapon will probably help to establish the metallogenic model of this kind of calcite-iron oxide vein-type gold deposits that spatially unrelated to intrusive rock mass, and further enriches the metallogenic theory of hydrothermal gold deposits.

How to cite: Guo, L.-N., Kolb, J., and Tang, Y.-W.: Gold enrichment and precipitation in the unique calcite-iron oxide vein-type Phapon gold deposit, Laos, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7713, https://doi.org/10.5194/egusphere-egu25-7713, 2025.