Abstract
Arsenic-bearing pyrite, known as arsenian pyrite, is the most important and in many cases the only Au-bearing mineral in Carlin-type and some epithermal deposits. Because it incorporates Au from solutions that are undersaturated with respect to native Au, precipitation of arsenian pyrite is the key to removal of Au from solution and formation of an economic deposit. Recent micro-scale studies of arsenian pyrite from a wide range of deposits have provided new information on its role as a host for Au and on the nature and significance of its growth zoning. Two types of arsenian pyrite have been recognized so far: 1) As (super 1-) -pyrite, in which As (super 1-) substitutes for S; it is found in Carlin-type and low-sulfidation epithermal deposits that formed in reduced hydrothermal systems. 2) As (super 3+) -pyrite, in which As (super 3+) and possibly other forms of oxidized As substitute for Fe; it is found in high-sulfidation epithermal deposits that formed in oxidized hydrothermal systems. Some arsenian pyrite also contains As (super 0) in nanoscale inclusions of amorphous (originally liquid) As-Fe-S. Although this As (super 0) is not part of the pyrite structure, it does contribute to the total As content of the pyrite. Most Au in arsenian pyrite is invisible, even in high-resolution TEM observations, and it is thought to be present in the crystal lattice of the pyrite. Entry of Au into the pyrite lattice is facilitated by As. Analytical compilations for Carlin-type deposits show that As and Au concentrations in As (super 1-) -pyrite plot in a wedge-shaped zone in Au-As space with an upper concentration (C) of Au defined by: C (sub Au) =0.02.C (sub As) +4X10 (super -5) , indicating a maximum Au/As molar ratio of approximately 0.02. Pyrite with As:Au ratios above 0.02 contains discrete grains of Au, usually in nanoscale particles. At least some As (super 3+) -pyrite appears to contain Au in solid solution above this solubility limit. Other elements that are found in arsenian pyrite in relatively high (ppm to low percent) concentrations include Ag,Bi, Te, Sb, Hg and Pb; of these, at least Sb appears to show solubility relations similar to those of Au. Trace elements are not homogeneously distributed in pyrite and, instead, form both sectoral and growth zones that show a wide range of concentrations. The growth zones are concentric and occupy all faces of a crystal; they can be correlated to provide an indication of hydrothermal flow patterns and connectivity, as well as the number and composition of fluid pulses that fed the hydrothermal systems. Arsenian pyrite varies in grain size from nanoscale to large crystals; the combined presence of As and smaller grain sizes cause the pyrite to decompose more rapidly, a factor that should aid in metallurgical processing. The ability of arsenian pyrite to scavenge Au from undersaturated solutions makes it the key factor in formation of many Au deposits. In exploration for Au deposits, attention should be given to factors that might cause deposition of arsenian pyrite.