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��Magma fertility for porphyry ore genesis

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(Ivan Espinel Pachon, Michael Schirra and Alexandra Tsay; PI: Zoltan Zajacz, current funding: ERC Consolidator Grant OXYGEN and SNSF MINT)

Magmas fertile for porphyry ore genesis are characterized by a particular set of geochemical characteristics that are a final consequence of complex processes occurring upon the generation of their parental melt in the Earth’s mantle and its subsequent evolution during ascent and storage in the Earth’s crust. We aim to decipher the key processes in this complex evolution and their respective effects on the key geochemical indicators (e.g., Sr/Y, La/Yb, Dy*/Dy, Zr, V/Sc) and variables that directly affect ore-fertility (fO2, H2O, S, Cl and ore metal abundances). A key tool in our arsenal is the use of silicate melt inclusions in minerals to reconstruct a record of the evolution of all above variables in time and space in ore forming or analogue magmatic systems. Current work is focused on the Taapaca and Parinacota volcanoes in the Central Volcanic Zone of the Andes, and a number of volcanoes at the northern termination of the Southern Volcanic Zone of the Andes.

��Architecture of magmatic-hydrothermal systems

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Rosia Poieni open pit, Apuesni Mountains, Romania, offers an excellent picture of the transition between porphyry Cu-Au mineralization and associated potassic and sericitic alteration and shallow high-sulfidation epithermal veining with advanced argillic alteration��

(PI: Kalin Kouzmanov; post-doc: Alexandre Cugerone, MSc students: Mariana Segovia More, Noah Martinet, Florian Sfalcin; current funding: CODELCO, Dundee Precious Metals; collaborations: ETH Zurich; AV����Ƶ of Lausanne; Istituto di Geoscienze e Georisorse-CNR, Pisa; Geological Institute-BAS, Sofia; Universidad Catolica del Norte, Antofagasta; Akita AV����Ƶ; Pontificia Universidad Catolica del Peru)

Architecture of magmatic-hydrothermal systems results from complex overprinting of discrete magmatic and hydrothermal events, mineralization and alteration styles. Reconstruction of their spatial relationship and relative timing is possible only after detailed mapping, combining various traditional with modern techniques on different scales - from district to ore body, down to a hand sample. Our field work is mainly focused on some magmatic-hydrothermal systems where a relatively “deep” porphyry environment (2-3 km) is overprinted by polymetallic and precious metal mineralization formed in near-surface epithermal environment (<1.5km). 2D and 3D mapping and modeling of geological features in magmatic-hydrothermal systems results from quantitative field work that has direct implications in exploration for mineral deposits. Combining quantitative field data with quantitative distribution of ore, gangue and alteration minerals, textural analysis, mineral geochemical signatures and bulk-rock composition allows developing novel concepts in target evaluation (including re-evaluation for strategic and critical metal distribution in ore bodies) and vectoring in mineral exploration.

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Magma degassing at depth: mobilization of ore metals

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(Ivano Gennaro and Alexandra Tsay; PI:�� Zoltan Zajacz, current funding: SNSF MINT)

Magmas contribute most of the metal budget of magmatic-hydrothermal ore deposits; therefore, the understanding of the timing and efficiency of metal extraction from the magma by exsolving fluids is essential for refining the genetic model of such deposits. Considering that the source magmas are thought to undergo a complex differentiation process through a vertically extensive section of the crust, fluid/melt partitioning models that work over a broad P-T-compositional range are required for understanding the extraction of the key ore metals (e.g. Cu, Au, Mo, Ag, Sn, W). We conduct high-pressure-temperature experiments in which we sample the fluids in the form of synthetic fluid inclusions in quartz or in the quenched silicate melt and subsequently analyze them using LA-ICP-MS. The data obtained is used to develop partitioning models applicable to natural magmatic-hydrothermal systems.

Chemical mass transfer by slab derived fluids

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(Zsofia Palos and Alexandra Tsay; PI:�� Zoltan Zajacz, current funding: ERC Consolidator Grant OXYGEN)

The oxidation state and the endowment of magmas in key ore-forming constituents (e.g. S, Cl, ore metals) are largely influenced by the input of subducted slab-derived fluids into their source region. We experimentally investigate the redox state and mobility of key heterovalent (S and Fe) and other major and trace elements in slab-derived fluids. To sample equilibrium fluids at these extreme conditions, we trap them as synthetic fluid inclusions in crystalline SiO₂ by forming fractures in situ during the experiment at the quartz-coesite phase transition. The concentrations of S, Fe and other elements in the synthetic fluid inclusions are subsequently determined by LA-ICP-MS.

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Micro-analytical method developments

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(Alexandra Tsay, Stefan Farsang, Zoltan Zajacz)

As our field-based research projects largely rely on the analysis of silicate melt inclusions in minerals and one of our key in-house technique is LA-ICP-MS, most analytical developments are aimed to increase the quality and information content of the LA-ICP-MS analysis of melt inclusions. We optimized our system to yield extremely low limits of detection for key ultra-trace ore metals (e.g. Au and Pt), we have established methodologies for the correction of polyatomic interferences on affected elements (e.g. Ag, Au, Pt), and we optimized our system to minimize gas blanks and memory effects affecting the quantitative analysis of S, Cl, Br ad I. We have also optimized analysis conditions to obtain the lowest matrix and plasma loading effects on the relative sensitivity factors for elements. Recent developments include improved S and Se analysis in MS/MS mode using reaction gases. We also developed in house methods for whole rock analysis by fusing rock powders without flux and by measuring pressed nano-powder pellets by using LA-ICP-MS.

In addition, we have also configured a Raman spectrometer that is capable to obtain spectra with high signal to noise ratio at magmatic temperatures from samples only a few micrometers in diameter (e.g. synthetic fluid inclusions in quartz).

Near-infrared microscopy and microthermometry – method development

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(Kalin Kouzmanov)

We have developed a novel analytical approach for microanalysis of “real ore-precipitating fluids” – well constrained primary fluid inclusion assemblages trapped in opaque ore minerals (Kouzmanov et al., 2010). The method consists of combining near-infrared (NIR) transmitted-light microscopy with laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS), allowing successful analysis of concentrations of major and trace elements in single primary fluid inclusions hosted in opaque ore minerals (e.g., pyrite, enargite, stibnite, wolframite; Casanova et al., 2018; Ortelli et al., 2018). The results obtained suggest some episodicity and potentially short duration of ore-forming pulses in hydrothermal systems and address some fundamental questions regarding metal transport and factors controlling ore precipitation mechanisms.