Tasks and problems of microscopical investigations

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, gold tellurides, auriferous sulphides; acid generating and oxygen consuming minerals such as pyrrhotite, marcasite, and melnicovite pyrite; cyanide soluble copper minerals such as chalcocite, digenite, covellite or stibnite; clay minerals; carbonaceous matter).

Textures and particle sizes of the gold minerals (free or locked gold; native gold at grain sizes that permit gravity concentration or sizes that require a leaching process)

It is often useful to study the intermediate products and tailings of a gold recovery process in order to discover the reasons for potential gold losses. When refractory ores are roasted prior to cyanidation “flash roasting” may convert the enclosing sulphide into a non porous iron oxide that shields the gold inclusions from cyanide solutions.

Microscopy of selected ores and products

Callion/Australia ++ (2 sections): Auriferous limonite concentrate; gold is associated with limonite in which it occurs as rather small inclusions; isolated colloform textures of the gold indicate a simultaneous precipitation of gold and iron hydroxides rather than in situ relics of gold inclusions in former pyrite.

El Callao/Venezuela +++ (7 sections): Gold in associated with quartz (3 sections) and pyrite (2 sections). Auriferous pyrite concentrates (2 sections); gold occurs intergrown with pyrite, both in coarse lockings and as tiny inclusions.

Hillgrove/Australia +++ (2 sections): Auriferous sulphide concentrates; gold occurs intergrown with accompanying pyrite, arsenopyrite, and stibnite.

Kotchbulak/Uzbekistan +++ (4 sections): Native gold and gold tellurides.

Rheingold/Germany +++ (2 sections): Gold gravity concentrate; flakes and grains.

Salsigne/France + (3 sections):

Sol de Oro/Peru ++ (4 sections): Auriferous crude ore; gold occurs as inclusions in primary ore (pyrite) as well as in supergene ore (quartz and limonite).

+ traces of gold

++ accessory gold

+++ abundant gold