The origin of fluids involved in the formation of gold-bearing skarns of the Andorra granite (Central Pyrenees, Spain): Sulphur isotope data

E. Cardellach, C. Ayora, A. Soler, J. Delgado

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Abstract

Arsenopyrite-gold mineralization in the lower Paleozoic series of the eastern Pyrenees is of two types: disseminated in Hercynian metasediments, and veins cross-cutting the main foliation. In the central Pyrenees, gold-bearing arsenopyrite mineralizations have recently been discovered in skarns along the SW contact of the Andorra granite. The ore minerals are similar to those of the veins, with gold also being associated with arsenopyrite, bismuth and sulphides. The late Hercynian Andorra granite intruded Paleozoic metasediments of pre-Caradocian to Carboniferous age. Six types of skarns have developed at or close to the contact between the granite and Devonian limestones. Only three of these, the hedenbergite, pyrrhotite and arsenopyrite skarns, are sulphide-bearing. Gold grades in these skarns range from less than 0.8 g/t in pyrrhotite skarns to 7 g/t in arsenopyrite skarns. The Δ34S of arsenopyrite from disseminated and vein mineralizations show values from 11 to 15‰, indicating a sulphur source of crustal origin. Vein sulphides are considered to have formed after a metamorphic remobilization of the sulphide rich disseminations in the Paleozoic series. Sulphur isotopic composition of skarn sulphides is variable: pyrrhotite from pyrrhotite skarns has a Δ34S ≈ +‰, suggesting a sulphur of magmatic origin. Their gold grades are very low (>0.8 g/t). The heavier values of arsenopyrite from arsenopyrite skarns (δ34S ≈ + 10‰) and of galena and sphalerite from hedenbergite skarns (Δ34S ≈ + 7‰) correlate with the higher gold grades (7 and 5 g/t, respectively). This is interpreted to reflect a contribution of wall-rock sulphur-rich fluids generated from the thermal metamorphism of arsenopyrite-bearing wall-rock metasediments (δ34S ≈ + 12%o) during skarn formation or by assimilation of country rock by the granitic intrusion during its emplacement. The economic importance of these skarns appears to be related to the amount of crustal fluid involved in skarn formation. © 1992 Springer-Verlag.
Original languageEnglish
Pages (from-to)181-193
JournalMineralogy and Petrology
Volume45
Issue number3-4
DOIs
Publication statusPublished - 1 Sep 1992

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