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 for 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 evolution of the Helgeland Nappe Complex. Prior to peak Caledonian deformation, Zn-Pb sulfides were deposited from ore-forming solutions that derived 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 deposits were formed (lead isotope ratios of the sulfides are indistinguishable from the initial lead isotope ratios of the Bindal batholith), and the metals and ore-forming fluids were probably derived from the granitoid magmas. During the final uplift of the Caledonian nappes after orogenic collapse, structurally controlled base and precious metal sulfide deposits and occurrences were formed. The radiogenic Pb-206/Pb-204 ratios and low Pb-208/Pb-204 ratios indicate an ore-forming process involving convection of meteoric water and leaching of metals from the rocks in the brittle zone. This lead isotope survey of the sulfide deposits in the Helgeland Nappe Complex has revealed repeated ore-forming activity at previously mineralized sites which involved introduction of new lead when new metal reservoirs became available as a result of changing tectonic settings.