Microscopy of Gold

    Gold minerals and especially native gold can be identify very well by ore microscopy because their distinctive optical properties. However gold microscopy has their peculiar difficulties. Above all ranges the usually low gold content of the ores which makes it hard to find any gold in polished section. This can be often achieved only after time consuming studies of several sections. Another complicating factor consists in the frequently small sizes of gold particles, which requires the use of high magnifications. Gold tends to show an irregular distribution in the ore. This is the case on macroscopic and even more so on microscopic scale. The fact that gold can not be detected in the very first section of high grade ore must not lead to the conclusion that it occurs in a submicroscopic form, e.g. in sulphides. Once detected it is possible find that there are local concentrations or clusters, often in association with specific properties of the ore such as siliceous or ferruginous parts or in zones of microfacturing. These environments can be locked after in order to observe microscopic gold more easily. Nevertheless, if no gold minerals can be found after the study of a reasonable amount of sections, electronoptical and microanalytical investigations should be considered. By using these tools submicroscopical or chemically bonded gold can be detected.

    An important aspect in mineral processing is the type of minerals present in a deposit, thus, it is possible find two categories, ore minerals (potentially valuable) and gangue minerals (waste), although this simplistic division occasionally leads to ambiguity in the classification of a particular mineral. The numbers and proportions of minerals vary throughout a mineral deposit, in some cases more or less, in other cases seemingly erratically.

    In the same way, mineral textures can be very different from one part of a deposit to another. Variations in mineralogy, mineral abundances and textures can impact severely on the economic viability of a deposit, thus emphasizing the need for a thorough understanding of the mineralogic characteristics of an ore deposit at the earliest possible stage of evaluation. The mineralogic characteristics of a deposit are essential for improving mineral recovery.

    The main task of gold ore microscopy with regard to metallurgical treatment is the assessment of all relevant parameters that influence the recovery process. The mineralogy of the ore determines the recovery process. The decisive mineralogical factors are in particular: the mineral assemblage (native gold, auriferous sulphides; acid generating and oxygen consuming minerals such as pyrrhotite, marcasite, and pyrite; cyanide soluble copper minerals such as chalcocite, digenite, covellite; clay minerals; carbonaceous matter). Texture and particle size of the gold minerals (free or locked gold, native gold at grain sizes that permit gravity concentration or sizes that require a leaching process).

    The next pictures are going to show some important occurrences of gold. It is not complete, but is very useful in order to know what it is possible find in a gold ore

    Photomicrograph illustrating liberated gold (yellow) measuring up to 65-mesh, galena and pyrite (gray metallic particles). 40x

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    Polished section showing gold (almost white) intergrown with pyrite crystal (cream colored) and chalcopyrite (yellow). Electron microprobe analysis of the gold showed that it is electrum analyzing 74% Au and 26% Ag. 200x
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    Photomicrograph showing gold particles (yellow), some with goethite coatings (brown) and mostly hematite (dark gray and brown). Minor sulfides (white metallic and oxide copper minerals (green).40x
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    Polished section showing native gold in association with goethite. Width of photomicrograph corresponds to approx. 250 microns.
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