Fluid evolution in gold‐bearing quartz veins associated with feldspar porphyry dikes at Vinliden in the Skellefte District, northern Sweden

Abstract

Abstract The Vinliden gold mineralization is located in the northwest‐em part of the Skellefte District in northern Sweden. The mineralization consists of a number of gold‐bearing quartz veinlets in a greywacke sequence. The quartz veins have been emplaced in small‐scale second order fault and shear zones within one of the major fault structures, which transects the folded supracrustal pile in the Skellefte District The Vinliden gold occurrence is spatially related to feldspar porphyry dikes which have intruded the greywackes. Alteration zones with albite or sericite surround the quartz veinlets. The mineralization also includes a sulfide assemblage dominated by arsenopyrite and pyrrhotite. Aqueous and carbonic (CO2‐CH4) fluid inclusions in the quartz veinlets indicate initial deposition of quartz and an early generation of arsenopyrite at a temperature exceeding 400 °C in association with a pressure drop from 5 to 1 kbar. Secondary fracturing opened passageways and allowed fluid pressure to adjust to hydrostatic conditions of c. 0.5 kbar, and subsequent sulfide and gold deposition occurred from a low salinity H2O‐NaCl‐CO2‐CH4 fluid at around 350–400 °C, accompanied by phase separation (boiling). In a post‐mineralizing stage, and after further fracturing of the quartz vein system, large volumes of a N2‐CH4 fluid (generated in the greywacke sediments) were introduced together with a brine with a complex CaCl2‐rich salt composition. Finally, a low to moderately saline aqueous solution filled larger cracks with calcite. The proposed fluid evolution X‐P‐T model at the Vinliden gold mineralization appears to be of regional significance and may be a distinct parameter for further comparisons in the Skellefte District as a whole. Broman, G, Bergström, U. & Lindblom, S., 1995: Fluid evolution in gold‐bearing quartz veins associated with feldspar porphyry dikes at Vinliden in the Skellefte District, northern Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 233–244. Stockholm. ISSN 1103–5897.