Geochemistry of gold quartz vein walls from Jales (northern Portugal)

Abstract

The long hydrothermal quartz veins from Campo de Jales (northern Portugal) have been exploited for Au and form the main Portuguese Au deposit. Au and electrum occur included in sulfides, sulfosalts and quartz and in thin veinlets along fractures with sulfides, while Ag occurs in electrum, argentite, galena and most of the sulfosalts. The Campo gold quartz vein cuts mainly the Hercynian peraluminous medium- to coarse-grained porphyritic seriate two-mica granite and also the pre-Ordovician biotite-muscovite-andalusite schist with sillimanite. At 1–0.5 m from the vein the granite is altered into a porphyritic muscovite-biotite granite, while adjacent to the vein, up to 0.5 m, it is a muscovite granite. At 3 m from the vein the mica schist shows some metasomatic effects, but it is transformed into a biotite-muscovite-brunsvigite schist at 2 m from the vein and into a mica schist enriched in muscovite and sulfides adjacent to the vein in a zone of 0.1–0.5 m up to 1 m from the vein. There are progressive increases in SiO 2, Fe 2O 3, K 2O, H 2O +, S, As, Sn, Sb, Cu, Pb, Rb, Au, a negative Eu anomaly and progressive decreases in TiO 2, FeO, MgO, CaO, Cl, Li, Sr, Th and all REE with increasing degree of hydrothermal alteration of granite. K 2O, F, S, Rb and Cs progressively increase, while FeO, CaO, Zr and Sr progressively decrease with increasing degree of hydrothermal alteration of mica schist. At the direct contact with the vein the granite and mica schist show enrichment in K 2O, Fe 2O 3, H 2O +, Sn, Rb, S, As, Zn, Sb, Cu, Pb and Au, and impoverishment in TiO 2, MgO, FeO, CaO, Na 2O, Cl, Cr, V, Nb, Li, Ni and Zr, which gives information on the ore-forming fluids. The hydrothermal alteration of granite increases with depth, but seems to decrease with increasing distance from the granite-schist contact. The mineral data are used to show the evolution of hydrothermal alteration at the vein walls and to write the representative reactions of hydrothermal alteration. The unaltered granite was emplaced at ∼ 3 kbar and 690°C and f O 2 of 10 −18 bar, but was completely crystallized at 460–600°C. It was hydrothermally altered at 200–400°C and <1–2 kbar. log( f H 2 O f HF ) and log( f HCl f HF ) increase and log( f H 2 O f HCl ) decreases during the alteration. The mica schist was hydrothermally altered at ∼<200–480° C and ≤1 kbar with decrease in log( f H 2 O f HF ) and log( f HCl f HF ) .