External Sulfur Addition in the Generation of Sulfide-rich Ni-Cu-PGE Deposits: The Importance of Focused Magma Flow

Abstract:

Sulfide-rich Ni-Cu-PGE orebodies hosted in mafic to ultramafic igneous rocks require focused magma flow and vigorous interaction with country rocks to liberate sulfide, as well as to produce traps for immiscible sulfide liquid. In the 1.1 Ga Midcontinent Rift System (MRS), Ni-rich sulfide deposits occur in conduit systems. Variations in S and Os isotope ratios indicate that magmas which followed different crustal pathways were focused into a central conduit that supplied overlying flows and sills. The 1.3 Ga Voisey’s Bay deposit in Labrador represents sulfide liquid collection in a conduit system which includes dike-like bodies and larger sub-horizontal chambers. Variable d³⁴S values again strongly suggest that focused magma flow and turbulence in the conduit resulted in the input of magmatic pulses that had undergone S isotopic homogenization even though pelitic country rocks are characterized by a range in S isotope values from -17 to +18 ‰. A very similar physical setting characterizes the sulfide-bearing Duke Island Complex, a Cretaceous – aged Ural-Alaskan intrusion in an arc setting. Magma pulses of variable sulfur isotopic compositions were focused into a central chamber where sulfide-bearing magma spread laterally. Trapped silicate liquid was efficiently expelled, leaving sulfide-bearing ultramafic cumulates. A less turbulent environment is indicated for sheet-like intrusions that carry disseminated sulfide mineralization in the Duluth Complex within the MRS. However, the potential ore sequences were built from multiple pulses of magma of distinct S isotope values that had interacted with sulfidic country rocks characterized by different S isotope compositions. Hence, the focusing of magmas from different pathways has been essential for the generation of potential sulfide-rich ore bodies in the Duluth Complex as well.