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Abstract

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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