T. Bjerkgard et A. Bjorlykke, SULFIDE DEPOSITS IN FOLLDAL, SOUTHERN TRONDHEIM REGION CALEDONIDES, NORWAY - SOURCE OF METALS AND WALL-ROCK ALTERATIONS RELATED TO HOST ROCKS, Economic geology and the bulletin of the Society of Economic Geologists, 91(4), 1996, pp. 676-696
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.