A Special Issue on Archean Magmatism, Volcanism, and Ore Deposits: Part 1. Komatiite-Associated Ni-Cu-(PGE) Sulfide and Greenstone-Hosted Au Deposits Preface (1,2) PATRICK MERCIER-LANGEVIN, 1,† MICHEL G. HOULÉ, 1 BENOÎT DUBÉ, 1 THOMAS MONECKE, 2 MARK D. HANNINGTON, 3 HAROLD L. GIBSON, 4 AND JEAN GOUTIER 5 1 Geological Survey of Canada, 490 rue de la Couronne, Québec, Québec G1K 9A9, Canada 2 Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401 3 Department of Earth Sciences, University of Ottawa, 140 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada 4 Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada 5 Ministère des Ressources naturelles et de la Faune, 70 avenue Québec, Rouyn-Noranda, Québec J9X 6R1, Canada Introduction THE ARCHEAN, which represents more than a third of Earth’s history, contains some of the largest and richest ore deposits of the world. Numerous cratons of variable size and age have been mapped (Fig. 1) and others are still being discovered or delineated largely due to the widespread use of high-preci- sion U-Pb geochronology. One of the specific characteristics of Archean cratons is the coexistence and juxtaposition of dif- ferent and major styles of base and precious metals deposits formed at different times and crustal levels (Poulsen et al., 2000; Groves et al., 2005a; Robert et al., 2005). It has long been recognized that Archean cratons are extremely well endowed in metals (e.g., Meyer, 1981; de Wit and Thiart, 2005), especially in komatiite-associated Ni-Cu- (PGE), greenstone-hosted Au and volcanogenic massive sul- fide (VMS) deposits. Most of these deposits were formed in periods of supercontinent(s) or supercraton(s) formation (e.g., ~2800-2500 Ma Kenorland: Williams et al., 1991; Aspler and Chiarenzelli, 1998; ~2600-2200 Ma Sclavia: Bleeker, 2003). Although some of these deposit types (greenstone-hosted Au and VMS) were formed episodically at various epochs in Earth’s history (cf. Goldfarb et al., 2010), Archean examples appear to be more common, and in many cases larger and/or richer. Although common in the Archean, the deposits are not uniformly distributed among the cratons, highlighting the fact that the metal endowment of specific assemblages reflects both secular variations and craton- to district-scale controls. For example, the Yilgarn craton is very well endowed in komatiite-associated Ni deposits, in contrast to the Superior craton, which contains more significant VMS deposits. However, the study of Archean deposits and their environ- ment is challenged by superimposed tectonic and metamor- phic modifications. Thus, it is necessary to combine various techniques to understand the evolution and primary architec- ture of the host successions and to generate better exploration vectors in those challenging Archean environments. Major differences between Archean and younger metal- logenic processes or secular changes have been discussed in detail elsewhere (e.g., Kerrich et al., 2000, 2005; Groves et al., 2005a, b; Goldfarb et al., 2010; Huston et al., 2010; Naldrett, 2010). However, diverse styles of mineralization are being recognized among recently discovered deposits and pros- pects that are currently being explored in different parts of the world. Documenting the intrinsic characteristics of these deposits and better defining their genesis and evolution will help advance our understanding of metallogenic processes in the Archean. The objective of this part of the special issue on Archean magmatism, volcanism, and ore deposits is to high- light some of the metallogenic and associated processes that were active during the Archean and that have implications for genetic and exploration models, with a focus on komatiite- associated Ni-Cu-(PGE) sulfide and greenstone-hosted Au deposits. The second part of the special issue on Archean mag- matism, volcanism, and ore deposits will discuss specific pro- cesses related to base and precious metal-rich VMS deposits, their characteristics, host environments, and genetic models. Overview of the Special Issue Komatiite-associated Ni-Cu-(PGE) sulfide deposits Approximately 30% of the world’s Ni resources are hosted in sulfide deposits, whereas the remaining 70% are found 0361-0128/12/4032/745-9 745 † Corresponding author : e-mail, pmercier@nrcan.gc.ca (1) Geological Survey of Canada Contribution 20120033 (2) Ministère des Ressources naturelles et de la Faune Contribution 8439-2012-2013-02 BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS VOL. 107 August NO. 5 ©2012 by Economic Geology, Vol. 107, pp. 745–753