Origin and Evolution of Magmas in the Porphyry Au-mineralized Javorie Volcano (Central Slovakia): Evidence from Thermobarometry, Melt Inclusions and Sulfide Inclusions

Abstract

<jats:title>Abstract</jats:title> <jats:p>The effect of magmatic sulfide precipitation on the potential of magmatic systems to produce porphyry-type ore deposits is still a matter of debate. In particular, we need to know whether magmatic sulfide precipitation has an impact on the Cu and Au content of the exsolving magmatic volatile phases and, by this way, on the Cu/Au ratio of porphyry deposits. The Javorie volcano is a perfect place to explore these questions. First, it hosts several Au-only porphyry-type mineralized occurrences which have among the lowest Cu/Au ratios reported in the literature. Secondly, the geology of the Javorie volcano and the timing of porphyry Au mineralization are well established.</jats:p> <jats:p>The evolution of the Javorie magmatic system was reconstructed by detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry analysis of minerals, melt inclusions and sulfide inclusions. The Javorie volcano was formed during the post-subduction magmatic activity affecting the Western Carpathians. It is a typical stratovolcano, composed dominantly of basaltic andesites and andesites which were intruded by several small stocks of dacitic to dioritic composition. According to our thermobarometric data, the volcano was fed by a transcrustal magmatic system in which two levels of magma chambers could be identified. Part of the magma evolved in the lower crust as suggested by the occurrence of magmatic garnet antecrysts in some of the studied rocks. The occurrence of magmatic sulfide inclusions in garnet indicates that sulfide saturation was reached in this lower crustal magma chamber. Most of the rocks crystallized in an upper crustal magma chamber (∼2 ± 1 kbar) that was fed by a basaltic to basaltic andesite magmas. A large variation in temperatures, ranging between 820°C and 1025°C, recorded by the extrusive and intrusive rocks suggest either that the upper crustal magma chamber was thermally zoned, or that the temperature of the whole magma chamber varied dramatically during its lifetime. Magmatic sulfide inclusions are present in all minerals and rocks of the upper crustal magma chamber, independent of their timing relative to porphyry Au mineralization (pre-, syn-, post-ore). These observations suggest that the magmatic system was sulfide saturated during its entire evolution. With very few exceptions, the precipitating sulfides were composed of monosulfide solid solution containing 0·2–9·2 wt % Cu and 0·05–11 ppm Au. The presence of these magmatic sulfides, together with results of a numerical model, suggest that the primitive magma feeding the upper crustal magma chamber contained less than 2·75 wt % H2O and that only a minor part of the magmatic sulfides was fractionated out of the system. Finally, the Cu/Au ratios measured in the magmatic sulfide inclusions and the ones predicted for the exsolved aqueous fluids are 10 to 100 times higher than the Cu/Au ratios of the porphyry deposits. Therefore, the extremely low Cu/Au ratios of the porphyry deposits must have been acquired during the hydrothermal stage.</jats:p>

Insight into volcanic garnet: origin and significance of garnet as exemplified by a detailed petro-mineralogical study of the Breziny andesite (Central Slovakia Volcanic Field, Western Carpathians, Central Europe)

“…Early crystallisation of apatite at presumably high temperature, prior to garnet growth, puts narrow limits to melt composition. Concerning zircon, Rottier et al [2019] estimated its crystallisation temperature, based on the Ti-in content method of Ferry and Watson [2007], to be equal to 763˘19 ºC. This value is almost the same than the T Zr saturation temperature (about 750°C) estimated for the bulk andesite, about 100°C lower than the minimum temperature suggested by conventional thermometry for garnet crystallisation.…”

“…However, higher values cannot be excluded. On the other hand, low-T conditions such as those suggested by Rottier et al [2019], down to 750°C, seem unlikely. Pressure conditions are not better known, as two problems severely limit the use of the ternary assemblage Grt-Opx-Pl for barometric purpose: 1) equilibrium conditions are not fully guaranteed for the different types of associations investigated; 2) above all, calculations strongly depend on Al content in orthopyroxene, and variations inside the same grain (Figure 5B) lead to differences in pressure as high as 0.5 GPa.…”

“…2) presence of high-grade garnet-bearing metasedimentary enclaves that could potentially be related to the genesis of the volcanic garnet megacrysts; 3) the ambiguous status of orthopyroxene, that is exceedingly rare as a phenocryst but forms frequent inclusions in garnet. Chemical composition of the Breziny garnet has been reported many times in the literature [Brousse et al 1972;Fediukova 1975;Irving and Frey 1978;Harangi et al 2001;Bouloton and Paquette 2014;Rottier et al 2019]. However, the origin and significance of this phase are still debatable.…”

“…Ultimately, they control the assemblages, proportions, and compositions of solid phases, along with the melt composition. Bearing in mind these limitations, the following discussion will try to: 1) examine whether or not the primary Grt-Opx-Pl assemblage that characterises SK8 may actually crystallise from a mantle-derived melt, as supposed by Rottier et al [2019]; 2) evaluate the possible role of a crustal melting event; 3) evaluate which type of crustal protolith might have been involved.…”

“…Garnet would have crystallised at high pressures (7-12 kbar) and temperatures (800-940°C), probably near the mantle-crust boundary zone [Harangi et al 2001]. However, it has been proposed very recently [Rottier et al 2019] that magmatic garnet in fact crystallised at "low" temperatures (down to~750°C) from an evolved, residual melt derived from fractionation of a primitive magma of mantle origin. This assumption, Corresponding author: jacky.bouloton@free.fr based in particular on the example of the Breziny andesitic dome, is at odds with former interpretations.…”