A Review of Conduit-Related Ni-Cu-(PGE) Sulfide Mineralization at the Voisey’s Bay Deposit, Labrador, and the Eagle Deposit, Northern Michigan
Edward M. Ripley, Chusi Li, 2011. "A Review of Conduit-Related Ni-Cu-(PGE) Sulfide Mineralization at the Voisey’s Bay Deposit, Labrador, and the Eagle Deposit, Northern Michigan", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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Mafic to ultramafic intrusive systems necessarily contain "conduits" through which magma has passed enroute to shallow levels of the crust. It is now commonly accepted that conduit systems are key sites for the accumulation of immiscible sulfide liquid and the generation of Ni-Cu-(PGE) deposits. Deposits such as those of Voiseys Bay in Labrador and Eagle in northern Michigan illustrate that sulfide deposits may form in both near vertical and horizontal portions of conduit systems. Although the Voisey Bay deposit is considerably larger than that associated with the Eagle intrusion, there are important similarities that emphasize the role of magma conduits in localizing sulfide mineralization. in addition to the similarities related to the physical collection of dense sulfide liquid, both deposits occur in provinces where anorthosites and early-stage picritc magmatism (Nain Plutonic Suite and the Midcontinent Rift system) are key elements of geochemical development. Geochemical evidence for the contamination of mafic magma by country rocks exists at both Voiseys Bay and Eagle. Conduit systems such as those of Voiseys Bay and Eagle provide an environment for enhanced magma-country rock interaction that may trigger the attainment of sulfide saturation of passing magma and initiate the ore-forming process.
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Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the