Paragenetic zoning and genesis of Cu-Zn-Fe-Pb-As sulfide skarn ores in a Proterozoic rift basin, Gruvaasen, western Bergslagen, Sweden

Abstract

The sulfide skarn ores of the Gruv•en mine area, western Bergslagen, Sweden, occur in a sequence of marbles and metatuffites intruded by basic sills and dikes. The mineralized area is exposed in a block-faulted and tilted structure within the Saxf• sedimentary rift basin. Sulfide mineralization occurs in zones within an area of intense potassic alteration. The central zone of the mineralized area is typified by Cu-W-Mo-bearing microcline skarns, Sn-bearing Cu-Zn-Fe sulfide skarns, a high Fe/Mg atomic ratio in the skarn-forming silicates (actinolite, ferrosalite), and a high Cd content in sphalerites. The peripheral zones are typified by ZnFe-As-Pb sulfide skarns, a low Fe/Mg atomic ratio in the skarn-forming silicates (tremolite, diopside, forsterite), and a low Cd content in sphalerites. The zoning crosscuts the lithologic boundaries, and transitions exist from massive sulfide ore bands through disseminated sulfide ore to fracture-filled skarn and barren host rock. Silicate, sulfide, and oxide assemblages indicate three stages of alteration and mineralization. During stage 1, potassic alteration and zonal precipitation of sulfides occurred. Basic sills and dikes close to the ores underwent strong potassic alteration and slight sulfide mineralization, and were altered to skarn; metatuffites underwent potassic alteration and sulfide mineralization, and were altered to skarn; and dolomitic limestones underwent dedolomitization and sulfide mineralization and were altered to skarn. During stage 2 the potassically altered tuffites were replaced by infiltrational-metasomatic amphibole skarns, which were subsequently replaced by pyroxene (-forsterite) skarns at temperatures of about 550øC. The late stage 2 pyroxene skarns are reaction skarns that are largely synkinematic with respect to boudinage of sulfide skarn layers and development of foliation in the marbles. Amphibole and pyroxene skarn relationships suggest a telescoping effect during the formation of the skarns. Silicate assemblages in metatuffites indicate metamorphic pressures of about 2.5 kb. During stage 3, retrogressive alterations, crystallization of pyrite, and remobilization of sulfides occurred largely syn-to postkinematically with respect to boudinage. Implications from this study suggest that the potassic alteration of country rocks, the increase in Fe/Mg atomic ratio in silicates, and possibly the increase in Cd content in sphalerites can be used to locate the Cu-W-Mo-bearing central zone of a central Swedish type of sulfide mineralization.

The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

“…Hydrothermal alterations are a hallmark of the Bergslagen Group. Generally, seawater and hence a (shallow) marine environment is assumed as the source of the fluids (e.g., Baker & De Groot, 1983;Hellingwerf, 1984;Hellingwerf & Oen, 1986) which were responsible for widespread but strongly variable Mg-metasomatism and K/Na exchange in the Bergslagen Group. Terrestrial crater lakes exhibit an enormous variation in chemistries, some being just as magnesian as seawater (c. 1280 ppm; Varenkamp, 2015).…”

“…Although our emphasis herein is on the Grythyttan Slate Belt, we concur with Sundius' (1923) interpretation that the Grythyttan and the Saxån Slate Belts originally formed a single lithostratigraphic unit. Hellingwerf (1984Hellingwerf ( , 1986Hellingwerf ( , 1987 and Hellingwerf & Oen (1986) have shown that the SSB was affected by a synsedimentary hydrothermal system, fed by seawater, by analogy with widespread Na/K exchange reactions and Mg-metasomatism superimposed onto many of the volcanic rocks in western Bergslagen. The obvious consequence that most or all of the Grythyttan Slate Formation was deposited in a marine environment will be discussed later on.…”

Conditions of formation of the iron-containing minerals, Hällefors silver mines, Bergslagen, Sweden

“…H 2 or CH 4 etc. ; fugacity coefficients of H20 gas (Hellingwerf, 1984;Kalogeropulos, 1983), but recent fluid inclusion studies reveal that for 473 K and the CO2 isochore 0.925, the pressure should be about 1.5 kbar. Assuming the same isochore for 573 K, the pressure should not exceed 2.25 kbar and for 443 K about 1.2 kbar.…”