THE SOURCES OF METALS IN SULFIDE DEPOSITS IN THE HELGELAND NAPPE COMPLEX, NORTH-CENTRAL NORWAY - PB ISOTOPE EVIDENCE

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.