corundum, geology.11, gems

Gemmological techniques are applied to the microscopic examination of heavy-mineral concentrates from stream sediments as a rapid means of assessing mineral potential and guiding exploration. Systematic sampling of stream beds and tributaries produces concentrates whose mineral assemblages reflect the rocks undergoing erosion within a watershed. Examination with a binocular microscope provides early information about possible mineralisation and checks the interpretation of subsequent geochemical analyses. Magnetic response and crystal habit help distinguish magnetite, ilmenite and chromite, while colour, transparency, cleavage, refractive index and other familiar gemmological properties assist identification of the accompanying grains. Accessory minerals can indicate source rocks, and assemblages containing pyrope garnet, chromium-bearing diopside and picroilmenite illustrate the use of indicator minerals in diamond exploration. Similar attention to associated minerals can help trace gold, cassiterite and scheelite to their sources. Interpretation must account for the contribution of superficial deposits, including glacial material, river gravels and volcanic ash, since stream concentrates may represent these rather than the underlying bedrock. The method is therefore most appropriate where active drainage exposes and erodes the materials of interest. Gold-particle morphology is discussed as evidence of transport and subsequent alteration: abrasion flattens and fragments grains, while dissolution and renewed growth modify their surfaces during residence in alluvial deposits. Attached gangue minerals may provide further clues to origin. Microscopic examination also reveals misleading contaminants, including mercury-coated gold resembling platinum, shotgun pellets capable of producing false lead anomalies, and iron welding droplets that might be mistaken for micrometeorites. Because visual identification alone may be insufficient for small ore-mineral grains, simple microchemical confirmation methods are described. Cassiterite is reduced to a metallic tin coating using zinc and hydrochloric acid, and treated grains can be located through their bleaching effect on starch–iodine paper. Scheelite may be detected by its bluish fluorescence under short-wave ultraviolet radiation or by an acid treatment producing a yellow coating that dissolves in ammonia solution. The article concludes that combining microscopy, mineral identification and selected chemical tests increases the information obtained from pan-concentrate surveys and offers gemmologists a practical contribution to mineral exploration.