Abstract Epithermal, Carlin, and orogenic Au deposits form in diverse geologic settings and over a wide range of depths, where Au precipitates from hydrothermal fluids in response to various physical and chemical processes. The compositions of Au-bearing sulfidic hydrothermal solutions across all three deposit types, however, are broadly similar. In most cases, they comprise low-salinity waters, which are reduced, have a near-neutral pH, and CO 2 concentrations that range from <4 to >10 wt %. Experimental studies show that the main factor controlling the concentration of Au in hydrothermal solutions is the concentration of reduced S, and in the absence of Fe-bearing minerals, Au solubility is insensitive to temperature. In a solution containing ~300 ppm H 2 S, the maximum concentration of Au is ~1 ppm, representing a reasonable upper limit for many ore-forming solutions. Where Fe-bearing minerals are being converted to pyrite, Au solubility decreases as temperature cools due to the decreasing concentration of reduced S. High Au concentrations (~500 ppb) can also be achieved in strongly oxidizing and strongly acidic chloride solutions, reflecting chemical conditions that only develop during intense hydrolytic leaching in magmatic-hydrothermal high-sulfidation epithermal environments. Gold is also soluble at low to moderate levels (10–100 ppb) over a relatively wide range of pH values and redox states. The chemical mechanisms which induce Au deposition are divided into two broad groups. One involves achieving states of Au supersaturation through perturbations in solution equilibria caused by physical and chemical processes, involving phase separation (boiling), fluid mixing, and pyrite deposition via sulfidation of Fe-bearing minerals. The second involves the sorption of ionic Au on to the surfaces of growing sulfide crystals, mainly arsenian pyrite. Both groups of mechanisms have capability to produce ore, with distinct mineralogical and geochemical characteristics. Gold transport and deposition processes in the Taupo Volcanic Zone, New Zealand, show how ore-grade concentrations of Au can accumulate by two different mechanisms of precipitation, phase separation and sorption, in three separate hydrothermal environments. Phase separation caused by flashing, induced by depressurization and associated with energetic fluid flow in geothermal wells, produces sulfide precipitates containing up to 6 wt.% Au from a hydrothermal solution containing a few ppb Au. Sorption on to As-Sb-S colloids produces precipitates containing tens to hundreds of ppm Au in the Champagne Pool hot spring. Sorption on to As-rich pyrite also leads to anomalous endowments of Au of up to 1 ppm in hydrothermally altered volcanic rocks occurring in the subsurface. In all of these environments, Au-undersaturated solutions produce anomalous concentrations of Au that match and surpass typical ore-grade concentrations, indicating that near-saturated concentrations of dissolved metal are not a prerequisite for generating economic deposits of Au. The causes of Au deposition in epithermal deposits are related to sharp temperature-pressure gradients that induce phase separation (boiling) and mixing. In Carlin deposits, Au deposition is controlled by surface chemistry and sorption processes on to rims of As-rich pyrite. In orogenic deposits, at least two Au-depositing mechanisms appear to produce ore; one involves phase separation and the other involves sulfidation reactions during water-rock interaction that produces pyrite; a third mechanism involving codeposition of Au-As in sulfides might also be important. Differences in the regimes of hydrothermal fluid flow combined with mechanisms of Au precipitation play an important role in shaping the dimensions and geometries of ore zones. There is also a strong link between Au-depositing mechanisms and metallurgical characteristics of ores.

Abstract Ancient and modern types of sedimentary placer deposits formed in both alluvial and coastal environments have been signficant sources of the rare earth elements (REEs). The REE-bearing minerals in placer-type deposits are primarily monazite [(Ce,La,Nd,Th)PO4] and sometimes xenotime (YPO4), which are high-density (heavy) minerals that accumulate with the suite of heavy minerals. Monazite has been extracted from many heavy mineral placers as a coproduct of the economic recovery of associated industrial minerals, such as titanium oxide minerals (ilmenite, rutile), zircon, sillimanite, garnet, staurolite, and others. Xenotime has been produced from some alluvial deposits as a coproduct of tin (cassiterite) placer mining. Placers are mineral deposits formed by the mechanical concentration of minerals from weathered debris. Placers can be classified as eluvial, alluvial, eolian, beach, and fossil (paleo) deposit types. Monazite-bearing placer-type deposits can occur in residual weathering zones, beaches, rivers and streams, dunes, and offshore areas. The detrital mixture of sand, silt, clays, and heavy (dense) minerals deposited in placers are derived primarily from the erosion of crystalline rocks, mainly igneous rocks and moderate- to high-grade metamorphic rocks (amphibolite facies and higher). In fluvial settings, slope is an important factor for the concentration of heavy minerals from detritus. In coastal settings, the actions of waves, currents, tides, and wind are forces that concentrate and sort mineral particles based on size and density. Placer deposits containing monazite are known on all continents. In the past, by-product monazite has been recovered from placers in Australia, Brazil, India, Malaysia, Thailand, China, New Zealand, Sri Lanka, Indonesia, Zaire, Korea, and the United States. More recently, monazite has been recovered from coastal and alluvial placers in India, Malaysia, Sri Lanka, Thailand, and Brazil. In particular, along the southwestern and southeastern coasts of India, beach deposits rich in heavy minerals have experienced renewed exploration and development, partly to recover monazite for its REEs as well as its Th, to be used as a nuclear fuel source. Exploration designed to locate heavy mineral placers in coastal environments should identify bedrock terranes containing abundant high-grade metamorphic rocks or igneous rocks and identify ancient or modern coastal plains sourced by streams and rivers that drain these terranes. Trace elements associated with heavy mineral placers, useful as pathfinder elements, primarily include Ti, Hf, the REEs, Th, and U. Radiometric methods of geophysical exploration are useful in discovering and delineating deposits of heavy mineral sands. Several minerals in these deposits can produce a radiometric anomaly, but especially monazite, due to its high thorium content. Some beach districts in India and Brazil have been demonstrated as areas of high background radiation with potential dose exposure to humans and others, primarily due to the Th and U in detrital grains of monazite and zircon. Monazite- or xenotime-bearing placers offer several advantages as sources of REEs. Ancient and modern deposits of heavy mineral sands that formed in coastal settings can be voluminous with individual deposits as much as about 1 km wide and more than 5 km long. Grains of monazite or xenotime in placer deposits are mingled with other heavy minerals of industrial value. Monazite and xenotime are durable and often the heaviest minerals within the sand-silt deposit, which makes them relatively easy to mechanically separate. Thus, the REE ore minerals, monazite or xenotime, can be recovered from heavy mineral placers as a low-cost coproduct along with the economic production of the associated industrial minerals.