SULFIDE DEPOSITS IN FOLLDAL, SOUTHERN TRONDHEIM REGION CALEDONIDES, NORWAY - SOURCE OF METALS AND WALL-ROCK ALTERATIONS RELATED TO HOST ROCKS

Five strata-bound massive sulfide deposits of the Cu-Zn type are locat ed 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 t o the contents in the strata underlying each of the deposits. Alterati on of the wall rocks associated with the deposits varies both in inten sity and composition and is dependent both on host-rock composition an d proximity to volcanic rocks. Correlation matrix calculations on all the metals analyzed shaw that the same physico-chemical renditions wer e responsible for metal deposition, both in the disseminated and massi ve parts of the deposits. Au and hg were deposited together with Cu, w hereas Zn and Pb were deposited together and separate from Cu. Geochem ical analysis and Sr-Nd isotope data show that the zoned alteration (u pward from Fe chlorite + quartz to quartz + sericite and to Mg chlorit e + talc + carbonate + quartz) present beneath the Sondre Geitryggen d eposit hosted in felsic volcanic rocks was the result of increased mix ing of the metalliferous solutions with seawater. Cooling of the solut ions 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 Ge itryggen 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 or e formation in the Folldal area, namely the mantle, represented by bas alts, and the upper crust, represented by terrigeneous sediments inter calated in volcaniclastics. Sr and Nd isotope data from the extensivel y altered volcanic rocks indicate that seawater was the dominant sourc e of the fluids. The differences in composition and extent of wall-roc k alteration metal zonations, shape, and size of the Folldal deposits indicate different modes of deposition of sulfides, i.e., distal vs. p roximal deposition relative to the venting sites. Undersaturation in t he hydrothermal solutions may explain the close relationship between t he deposits and underlying strata regarding metal content. Cu may have been leached mainly from the basalts whereas Ph and Zn may have partl y come from higher strata The deposits were formed in an island-are se tting close to a continent as revealed by stratigraphy and lead isotop es.