Abstract Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: (1) crustal-scale fault-associated (CSF) subtype, (2) sediment-hosted orogenic gold (SHOG) subtype, (3) forearc (FA) subtype, and (4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age, districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

Abstract Phanerozoic lode gold deposits are invariably associated with convergent plate margins and occur within close proximity to major translithospheric structures or compressional to transpressional-transtensional shear zones. The deposits are almost entirely structurally controlled and the nature of the immediate host rock does not generally play an integral part in ore formation. Nonetheless and unlike the majority of their Archean and Proterozoic analogues, Phanerozoic lode gold deposits are primarily hosted in several kilometer-thick sequences of marine sedimentary rocks which accumulated on pre-collision continental margins and/or in prograding arc-trench complexes. The sedimentary successions are commonly under lain by, and interspersed with, bimodal volcanogenic rocks which formed as a result of magmatic processes related to spreading, arc formation, plate collision, and subduction. The largest Phanerozoic lode gold systems are found in sub- to medium-grade greenschist metamorphosed terranes which have been caught up in the accretion of one or more allochthonous microplates and associated oceanic crust to an active continental margin. Mineralization in these collisional settings closely follows peak meta-morphism of the immediate host rocks and is temporally associated with exhumation of the orogen and addition of heat into the thickened crust via lithospheric delamination processes. Generation of CO 2 -rich aqueous ore-forming fluids involves metamorphic devolatilization of subcreted hydrated crust and the devel op ment of laterally and vertically extensive hydrothermal plumbing systems. Rich Phanerozoic lode gold deposits display a very close spatial and temporal relationship with syn- to post-tectonic felsic intrusive rocks but generally predate the emplacement of the granitoids. The deposits typically consist of quartz gold lodes in fault and shear systems at or above the brittle-ductile transition and form at P-T conditions of 1 to 3kbars and 250° to 400°C: they are characterized by relatively straightforward parageneses and a lack of pronounced vertical mineral or ore zonation. Episodic brittle reactivation in response to short-lived tectonic pulses is common and can result in remobilization of pre-existing mineralization and the formation of secondary lode systems. Alteration halos around Phanerozoic lode gold systems vary from a few centimeters to several tens of meters and reflect variations in the host-rock lithology and reactivity, permeability and porosity, orientation of bedding in metasedimentary rocks relative to auriferous veins, and fluid composition. On the deposit scale, lithogeochemical information obtained from wall-rock alteration assemblages represents by far the most valuable exploration tool. Broad bleached zones characterized by carbonate, sulfide, and sericite altera tion surrounding mineralized zones provide an exploration target of increased magnitude. Geochemical traverses generally indicate depletion of Na 2 O and increased values of CO 2 , H 2 O, K 2 O, S, As, Au, and possibly Sb, within five to several tens of meters from the auriferous lodes.