A. Birkeland et al., THE SOURCES OF METALS IN SULFIDE DEPOSITS IN THE HELGELAND NAPPE COMPLEX, NORTH-CENTRAL NORWAY - PB ISOTOPE EVIDENCE, Economic geology and the bulletin of the Society of Economic Geologists, 88(7), 1993, pp. 1810-1829
The Helgeland Nappe Complex of the uppermost Caledonian allochthons of
Norway is host to a number of different sulfide deposits. These occur
in epicontinental metasedimentary rocks close to, or at, the contact
with the granitoid Bindal batholith. Sixty-eight common Pb analyses fo
r six Zn-Pb sulfide deposits and one As-Au deposit make it possible to
recognize three distinct episodes of sulfide ore formation, which can
be related to successive stages of the tectonic and petrogenetic evol
ution of the Helgeland Nappe Complex. Prior to peak Caledonian deforma
tion, Zn-Pb sulfides were deposited from ore-forming solutions that de
rived their lead from a probable Late Proterozoic continental source,
as indicated by low Pb-206/Pb-204 and Pb-208/Pb-204 ratios and high Pb
-207/Pb-204 ratios. Shortly after peak metamorphism and emplacement of
the Bindal batholith, skarn related base metal and As-Au vein deposit
s were formed (lead isotope ratios of the sulfides are indistinguishab
le from the initial lead isotope ratios of the Bindal batholith), and
the metals and ore-forming fluids were probably derived from the grani
toid magmas. During the final uplift of the Caledonian nappes after or
ogenic collapse, structurally controlled base and precious metal sulfi
de deposits and occurrences were formed. The radiogenic Pb-206/Pb-204
ratios and low Pb-208/Pb-204 ratios indicate an ore-forming process in
volving convection of meteoric water and leaching of metals from the r
ocks in the brittle zone. This lead isotope survey of the sulfide depo
sits in the Helgeland Nappe Complex has revealed repeated ore-forming
activity at previously mineralized sites which involved introduction o
f new lead when new metal reservoirs became available as a result of c
hanging tectonic settings.