Intrusion-related gold systems: the present level of understanding

Abstract

This volume presents new data on a group of gold deposits that are hosted primarily within or in the immediate wall rocks to intrusions, and which have recently been suggested to comprise a distinct class of magmatic-hydrothermal system. These deposits have been called 'porphyry gold deposits' (Hollister 1992; Bakke 1995), 'intrusion-related stockwork-disseminated deposits' (Sillitoe 1991), 'plutonic-related gold deposits' (Newberry et al., 1988; McCoy et al., 1997) and 'intrusion-related gold deposits' (Thompson et al. 1999). Lang et al. (2000) preferred the term 'intrusion-related gold systems' because it reflects a tendency common to all magmatic-hydrothermal environments to form ores that manifest multiple styles, metal assemblages and spatial associations with their related intrusive centres. Although in its infancy, investigation and exploration of intrusion-related gold systems has accelerated markedly in the last five years, due in part to their global distribution and to the large number of deposits that contain a gold resource of >30 metric tons (Fig. 1). Major deposit examples include Fort Knox (~210 t Au), Donlin Creek (~315 t Au), Pogo (~160 t Au), and Dublin Gulch, True North and Brewery Creek (~40 t Au each) in Yukon and Alaska, as well as Mokrsko, Czech Republic (~120 t Au), Vasilkovskoe, Kazakstan (~300 t Au), Salave, Spain (~30 t Au), Korri Kollo, Bolivia (~160 t Au) and Kidston, Australia (~140 t Au). A paucity of detailed descriptions of individual intrusion-related gold systems, the plutonic provinces that host these systems, and the genetic processes critical to their formation currently limits our ability to either develop precise criteria for their definition or to formulate well-constrained geological and exploration models. The principal discussions (Sillitoe 1991; Hollister 1992; Newberry et al. 1988 and 1995; Lang et al. 1997; McCoy et al. 1997; Thompson et al. 1999; Lang et al. 2000; Goldfarb et al. 2000; Newberry 2000) suggest that there are several features common to most intrusion-related gold deposits and provinces, including: 1) metaluminous, subalkalic intrusions of intermediate to felsic composition that span the boundary between ilmenite- and magnetite-series, 2) carbonic hydrothermal fluids, 3) a metal assemblage which variably combines Au with elevated Bi, W, As, Mo, Te and/or Sb and low concentrations of base metals, 4) a low sulphide content (<5 volume %) with a reduced ore mineral assemblage that typically comprises arsenopyrite, pyrrhotite and pyrite, and which lacks magnetite or hematite, 5) areally restricted, commonly weak hydrothermal alteration, except in systems formed at the shallowest depths spanned by these deposits, 6) a tectonic setting of continental magmatism well-inboard of inferred or recognized convergent plate boundaries, and which commonly contains coeval intrusions of alkalic, metaluminous calc-alkalic and peraluminous compositions, and 7) a location in magmatic provinces best or formerly known for W and/or Sn deposits. Deposits that can be confidently included in the group formed during much of the Phanerozoic, but inclusion of some Proterozoic and even Archean deposits has also been proposed (e.g., Robert, this volume). The defining criteria as presently recognized show that intrusion-related gold systems and their associated plutonic provinces are globally widespread (Fig. 1), but adequate descriptions of the contained deposits are only now beginning to emerge. The Tintina Gold Belt of Alaska and the Yukon Territory in the northern part of the North American Cordillera (Fig. 1) is thus far the most extensively studied intrusion-related gold systems province. This belt is ~1000 km in length and contains gold deposits of Early Cretaceous to Eocene age (McCoy et al. 1997; Newberry et al. 1995; Lang et al. 2000; Goldfarb et al. 2000) that, as a group, span much of the globally recognized variation among these systems. As such, the Tintina Gold Belt is currently the primary standard against which deposits in other provinces can be compared, and is therefore emphasized in this introduction. This paper briefly considers the status of our knowledge of intrusion-related gold systems, and is intended only as an introduction to the major characteristics of these deposits rather than a comprehensive description. The discussion emphasizes the nature of the associated igneous rocks and their tectono-magmatic setting, the styles of deposits and their spatial distribution, the characteristics and evolution of hydrothermal fluids, and structural controls. It concurrently highlights some of the important gaps in our understanding of these systems that will be fruitful areas of investigation in ongoing and future research programs. The paper concludes with comments on deposit classification and the relationship of intrusion-related gold systems to other types of magmatic-hydrothermal systems.