Lateral metal zonation and stringer zone development, reflecting fissure-controlled exhalations at the Stekenjokk-Levi strata-bound sulfide deposit, central Scandinavian Caledonides

Abstract

The volcanogenic strata-bound sulfide deposit at Stekenjokk occurs in a felsic-dominated, rifted-arc sequence. An already spilitized volcanic pile was altered during mineralization, involving loss of sodium and a marked enrichment in magnesium and potassium. The massive strata-bound ore is capped by black phyllite with gabbro intrusions. A thinner postore sequence of felsic and mafic volcanics lacks the mineralization alteration described above and is of the normal spilitic type, similar to the preore volcanics away from the mineralization. Subsequent deformation led to regional stratigraphic inversion and rocks attained greenschist-grade regional metamorphism.Unfolding of the fold stack reveals lateral dimensions for the Stekenjokk-Levi ore layer on the order of 0.4 X 10 km. In most places, a massive (FeS 2 + ZnS + CuFeS 2 ) ore component is present, up to ca. 8 m thick. It is generally underlain (stratigraphically) by an irregularly disseminated type of ore (FeS + CuFeS 2 ) of more variable thickness (0-25 m). Locally, the latter extends beyond the massive ore layer; in places and especially at the southern end of the orebody, it may be absent. Metal zonation occurs in both vertical and lateral directions; higher Pb and lower Cu proportions as well as higher Ag values occur stratigraphically upward and southward along the elongated ore layer.It is inferred that the disseminated ore is indicative of a stringer zone whereas the massive ore is exhalative, both ore types being produced by hydrothermal solutions. These were controlled by a fissure system along which exhalations were expelled at several locations (feeder chimneys). The fissure system also controlled the bottom topography, creating local troughs in which the brines deposited their iron and base metal sulfides. The period of sulfide deposition occurred at a late stage of the arc development, probably during an extensional regime related to the incipient intra-arc rifting. The lateral metal zonation indicates a general southward migration of the expelled brines. Even if most of the mineralization indicates proximal conditions, the southernmost, Pb- and Ag-rich parts probably represent more distal deposition.