Economic gold deposits may be considered to fall into four classes on the basis of the concentration and precipitation mechanism: (a) primary one-stage hydrothermal extraction. with precipitation in vein conduits or on the sea-floor, (b) hydrothermal leaching of materials which have undergone a prior Au enrichment (multistage). (c) alluvial concentrations via detrital transport ± groundwater Au contributions in aquifers. and (d) organic-dominated concentrations such as in coals.All gold enrichment involves the same chemical processes, solution-transport-deposition, but the ultra•low abundance of Au and its lode-association elements, including As, Sb, Wand B relative to transition and base metals. requires efficient leaching and/ or element selectivity. For instance, Archaean lode gold deposits of both vein and sedimentary types typically have enrichments of Au, As, Sb, B, W. Se. Te and Bi in the 10' to 10• range. coupled with low enrichments of the abundant base metals. Cu. Pb and Zn ( 101 to 10-•1. giving a dramatic separation of the rare 1•. abundant elements relative to background levels.
For significant concentrations of elements with ppb background abundances, there must be pervasive access of lluids to source rocks. coupled with effective leaching for efficient extraction, such that source volumes in the 100-1 000 km' range are required. From these considerations. metamorphic degassing at
400-6()()'-C is considered to be the single most important process in the leaching and transport of Au.
During dehydration reactions, fluids are present at all grain boundaries (given Pnuid;;,, P1oadl and most minerals arc subject to reactions such that both trace and abunda1'1.J;nctals arc accessible to solution. By contrast. in thermally-driven sea-water convection systems involved in base metal deposits. cell volumes are in the 100 to I 000 km1 range, such that the leaching of Cu and Zn. present at 105 times the Au abundance. must be inefficient - representing 0.04 to I %extraction. Fracture-controlled permeability in hydrostatic convective regimes constrains the volume of rock accessible to leaching.
Analytical and field evidence support metamorphic fluids as the transporting fluid reservoir for
Archaean lode gold deposits, 1•. marine water in base metal deposits. Fluids from which gold precipitated in veins were at 320-480° C. at lithostatic pressure. with 01xo=8-IO%.,, possessed significant CO,, salinity

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