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 Taiwan and the Southern Alps of New Zealand are both young transpressive orogens characterized by rapid uplift and exhumation, high heat flow and vigorous surface processes. However, the distribution of heat flow, hot springs and veins in the two orogens is different. Taiwan has higher heat flow, distributed hot springs and localized veining. The Southern Alps has a narrow heat-flow anomaly, localized warm springs and widespread veining. Both orogens have two fluid-flow systems centred about the drainage divide. Shallow topographically driven meteoric water is restricted to the top 2–4 km. Deep flow is dominated by mineralizing rock-exchanged fluids. Extensional deformation occurs in the divide region of both orogens. At depth, vertical stretching produces subhorizontal veins. At shallower levels, stretching is horizontal and veins are steep. Veins in Taiwan are rare with zones of intense veining where flow has been localized into one site during exhumation from metamorphic to near-surface conditions. Fracturing and veining of the initially weak Slate Belt rocks causes a rheological change, increasing the tensile strength and making it more prone to fracturing, thus focusing fluid flow into the same locale. More uniform rheology in the Southern Alps leads to distributed veining.

Abstract The Southern Alps mountain range, New Zealand, is a collisional orogenic belt in which rapid uplift (near 10 mm/yr) has caused high near-surface geothermal gradients which commonly exceed 50°C/km. Thermal modeling suggests that conductive heat flow perturbations reach a steady state after about 2.5 Ma, with temperatures as high as 300°C within 5 km of the surface. The high heat flow results in enhanced fluid flow in the upper 6 km of the crust. Uplifting greenschist facies rocks release gold-bearing metamorphic water produced during metamor-phism by dehydration of chlorite. Gold precipitation occurs due to cooling and/or dilution where this>300°C fluid with salinities up to 4 equiv wt percent NaCl mixes with convecting lower salinity <250°C meteoric water. Uplifting amphibolite facies rocks release relatively small quantities of an aqueous fluid which is CO 2 -bearing but contains little or no gold. Influx of this fluid results in alteration of biotite to chlorite, causing progressive decrease in X H2 0 to values as low as 0.6. The resulting CO 2 -rich fluid mixes with meteoric water while migrating to the surface, where CO 2 -bearing hot springs emanate. Minor base metal mineralization occurs throughout the zone penetrated by meteoric water. The mineralization model can be extended to the Archean, when higher geothermal gradients prevailed. For an initial geothermal gradient of 40°C/km, uplift rates as low as 1 to 2 mm/yr extended over about 5 Ma could have produced near-surface thermal anomalies similar to those observed in the Southern Alps.