 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; June 2001; v. 29; no. 6; p. 539–542; 3 figures. 539 Rotund versus skinny orogens: Well-nourished or malnourished gold? R.J. Goldfarb U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA D.I. Groves S. Gardoll Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia, Crawley, WA 6009, Australia ABSTRACT Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800–2555 Ma and 2100–1800 Ma), a singular lack of deposits for 1200 m.y. (1800–600 Ma), and relatively continuous formation since then (after 600 Ma). The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma or earlier). This worldwide process allowed preservation of gold deposits in cratons, roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate tectonic regime led to the accretion of volcano-sedimentary complexes as progres- sively younger linear orogenic belts surrounding the margins of the more buoyant cratons. The susceptibility of these linear belts to uplift and erosion can explain the overall lack of orogenic gold deposits at 1800–600 Ma, their exposure in 600–50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma. Keywords: gold, geologic time, continental accretion, orogenesis, plate tectonics, ore deposits. INTRODUCTION Mineral deposits can be sensitive indicators of the predominant tectonic environment dur- ing their formation (Meyer, 1988). For ex- ample, specific mineral deposits (e.g., many Fe, Pb-Zn, or Cu deposits) can be related to the opening of internal oceans during the breakup of continents, whereas others (e.g., Au-Ag, Cu-Au) record the closure of external oceans (Barley et al., 1998). The temporal dis- tribution of different types of mineral deposits can therefore contribute substantially to our understanding of the supercontinent cycle, and the spatial distribution can potentially provide insights into evolutionary changes in the na- ture of the tectonic processes that contributed to the formation, and controlled the preser- vation, of the minerals that constitute the de- posit type. OROGENIC GOLD DEPOSITS IN SPACE AND TIME A problem with understanding the spatial- temporal distribution of many mineral-deposit types is that, although they are inherent to a particular plate tectonic setting, they form in environments in which they are highly sus- ceptible to erosion. For example, shallow- level porphyry Cu-Au to near-surface epith- ermal Au-Ag deposits are the products of rapidly uplifting magmatic arcs (Sillitoe, 1989), but their preservation in the geologic record before the Mesozoic is poor. Orogenic gold deposits are an important exception be- cause they form mainly at mid-crustal levels during the later stages of the evolution of mountain belts (Goldfarb et al., 1998; Groves et al., 1998) and, hence, have better preser- vation potential, as shown by their global abundance within deformational belts as old as Archean. These orogenic gold deposits formed from metamorphic (magmatic) fluids generated from deep sources in near-arc, volcano-sedi- mentary rock sequences; the deposits are as- sociated with plate subduction below, and/or terrane collision onto, older blocks of conti- nental crust (Goldfarb et al., 1988; Landefeld, 1988; Barley et al., 1989; Kerrich and Wy- man, 1990). The deposits represent focused fluid flow that is an inherent consequence of crustal heating during orogenesis (Thompson, 1997). Under typical mid-crustal temperature regimes, given sufficient availability of re- duced sulfur, gold will be mobilized in the up- ward-migrating fluid systems. The younger orogenic gold deposits, there- fore, coincide spatially with the margins of ex- ternal oceans where accretion of juvenile crust has taken place, as illustrated by the distri- bution of most major Mesozoic-Tertiary oro- genic gold provinces around the Pacific mar- gin (Fig. 1). They are commonly related to environments in which large thermal anoma- lies were caused by thrust-related thickening of relatively radiogenic crustal material in an orogen core (Jamieson et al., 1998) or by up- welling of asthenosphere due to processes that include subduction of oceanic ridges (Haeus- sler et al., 1995), subduction rollback (Lan- defeld, 1988), or lithospheric delamination (Qiu and Groves, 1999). Older orogenic gold deposits reflect similar regional-scale, crustal fluid-flux regimes developed in these tectonic environments. The formation and preservation of the gold deposits are thus sensitive indi- cators of continental growth back through geologic time. However, there are important orogenic belts that lack gold deposits, and this feature must also be addressed in evaluating the temporal distribution of gold. The abundance of recent geochronological data for orogenic gold deposits and the current availability of reliable resource data for areas in eastern and central Asia allow for a much improved estimate of the distribution of oro- genic gold through time (Fig. 2). There are