Gold in Arsenopyrites: Crystal Chemistry, Location and State, Physical and Chemical Conditions of Deposition
Michel Cathelineau, Marie-Christine Boiron, Philippe Holliger, Philippe Marion, Michel Denis, 1989. "Gold in Arsenopyrites: Crystal Chemistry, Location and State, Physical and Chemical Conditions of Deposition", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Arsenopyrite constitutes the main gold-bearing mineral in various hydrothermal deposits since its average gold content may reach 1,000 to 2,000 ppm. However, arsenopyrite crystals exhibit heterogeneous gold contents covering a wide range from a few ppm to more than 1 percent. Gold distribution within crystals is relatively poorly known since the in situ analysis and Au mapping by standard analytic techniques is difficult at low Au contents. Since the knowledge of the Au distribution within crystals is of critical importance for ore exploration and beneficiation, a multidisciplinary approach using combined electron microprobe analysis, secondary ion mass spectrometry, atomic absorption, and Mössbauer spectroscopy was carried out on arsenopyrite samples representative of various localities and of a large range of gold contents.
The Mössbauer data indicate that gold does not only occur as native gold but also very often and predominantly in a combined state, through a probable Au-As bond, within arsenopyrite crystals. QEM data and SEM images show clear patterns of mineral growth characterized by alternative enrichments in As or Sb-S. Ion images have confirmed the presence of gold in the As-rich zones of the crystals. Thus, gold is heterogeneously distributed and is specifically enriched in growth zones or overgrowths having a relatively homogeneous Au content, a high As content, and low Sb and S contents. However, gold may also occur as irregular patches and in enriched strips or microcracks filled by Au-rich arsenopyrites. Average gold content in arsenopyrite is thus highly variable in a deposit, as a function of the abundance of the Au-rich arsenopyrites and of the distribution of the enriched zones within crystals.
Au-rich arsenopyrites crystallize for the most part from aqueous solutions at low fo2 (around that fixed by the Ni-NiO oxygen buffer), low pH, and frequently at relatively low temperatures ranging from 170° to 250°C. The efficiency of the simultaneous coprecipitation of As and Au seems to be subject to variations of the local physico-chemical conditions controlling the sulfide deposition, especially the stability of the As-Au complexes in relation to the activity of Sb complexes and fs2.
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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in