Element mobility during regional hydrothermal sodium and magnesium alteration: implications for ore formation in the Bergslagen ore district, Sweden

Abstract

Ore-distal hydrothermal alteration zones are commonly suggested as a source of metals to ore-forming fluids. The Bergslagen ore district, Sweden exhibits extensive ore-proximal and ore-distal alterations and has been used as a typical locality for establishing the hydrothermal leaching model for volcanogenic massive sulphide (VMS) deposits. The ore-distal alteration in the region has been reported as depleted in ore-forming metals but robust mass change evaluations are lacking. Defining least-altered reference compositions is a major hurdle in Bergslagen due to compositional variation in the stratigraphy, extensive alteration, and high-grade metamorphic overprint. This study presents mass balance calculations for Na- and Mg-altered rocks in the Hällefors area using a set of systematically defined least-altered samples. Results show systematic mobility of light rare earth elements (LREE, here La-Eu; e.g., 80% of the Ce is mobilised during alteration which equates to 60 µg/g Ce), but no mobility of base metals. Precursor rock compositions have conspicuously low base metal concentrations (median: Zn 10 µg/g, Pb 2.5 µg/g; n = 13) compared to other volcanic centres in Bergslagen. Major base metal deposits occur in areas where least-altered volcanic rocks have higher base metal concentrations (e.g., Garpenberg; median: Zn 31.50 µg/g; Pb 11.75 µg/g; n = 10). The REE contents in least-altered rocks are relatively elevated in areas that host REE mineralisation such as the Riddarhyttan area. The results indicate that regional differences in metal fertility of the volcanic host succession may be a primary control on the metal enrichments, including REEs, occurring in the ore deposits throughout Bergslagen.