Sulfide deposits in Folldal, southern Trondheim region Caledonides, Norway; source of metals and wall-rock alterations related to host rocks

University of Oslo, Department of Geology, Oslo, Norway
Norwegian Geological Survey, Norway

Five strata-bound massive sulfide deposits of the Cu-Zn type are located at three different stratigraphic levels in the volcanogenic Fundsjo Group. The contents of and ratios between both the base metals Cu, Zn, and Pb and trace metals like Sb, As, Ag, Au, and Co vary according to the contents in the strata underlying each of the deposits. Alteration of the wall rocks associated with the deposits varies both in intensity and composition and is dependent both on host-rock composition and proximity to volcanic rocks. Correlation matrix calculation son all the metals analyzed show that the same physico-chemical conditions were responsible for metal deposition, both in the disseminated and massive parts of the deposits. Au and Ag were deposited together with Cu, whereas Zn and Pb were deposited together and separate from Cu. Geochemical analysis and Sr-Nd isotope data show that the zoned alteration (upward from Fe chlorite+quartz to quartz+sericite and to Mg chlorite+talc+carbonate+quartz) present beneath the Sondre Geitryggen deposit hosted in felsic volcanic rocks was the result of increased mixing of the metalliferous solutions with seawater. Cooling of the solutions led to a zonation in sulfides outward from the venting site, from chalcopyrite-pyrrhotite to sphalerite-pyrite-galena. Geochemical data indicate that the Fe-Mn chert (coticules) located above the Sondre Geitryggen deposit is the result of hydrothermal activity. Lead isotope data show that the source of lead was the underlying strata. The lead isotope data also show that two lead reservoirs were present during ore formation in the Folldal area, namely the mantle, represented by basalts, and the upper crust, represented by terrigeneous sediments intercalated in volcaniclastics. Sr and Nd isotope data from the extensively altered volcanic rocks indicate that seawater was the dominant source of the fluids. The differences in composition and extent of wall-rock alteration, metal zonations, shape, and size of the Folldal deposits indicate different modes of deposition of sulfides, i.e., distal vs. proximal deposition relative to the venting sites. Undersaturation in the hydrothermal solutions may explain the close relationship between the deposits and underlying strata reagarding metal content. Cu may have been leached mainly from the basalts whereas Pb and Zn may have partly come from higher strata. The deposits were formed in an island-arc setting close to a continent as revealed by stratigraphy and lead isotopes.

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