Placer gold grains in the Nilambur

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Ashtamudi estuary, situated on the southwest coast of India, is enormously affected by anthropogenic interventions. Physicochemical quality of water and sedimentological features of the estuary are evaluated during monsoon and nonmonsoon seasons to elucidate its quality variations and to link the same with existing environmental scenario. The whole data has been factorized using principal component analysis for extracting the total variability and linear relationships existing among a set of different physicochemical parameters of the backwater system. In PCA, high loadings were obtained for conductivity, salinity, fluoride, calcium, magnesium, sulfate, boron, and pH. The results were revealed that all the physicochemical processes depend upon seasonal fluctuation of freshwater input and seawater intrusion. Wide spatial concentration fluctuations of organic carbon and iron in bottom sediment have been noticed and both constituents reveal good correlation with sediment texture. The results showed high deterioration of the physicochemical quality of water during nonmonsoon season with respect to monsoon season.

Occurrence of dust, coarse grains and even large nuggets of gold in laterite weathering profiles has emerged as a recent challenge to metallogeny. Supergene gold concentrations, as in Nilam bur, South India, show morphological and textural characteristics indicative of chemical dissolu tion, migration and reprecipitation by low temperature solutions in weathering environments. The gold grains from the upper zones have near-spherical shapes with numerous etch pits, while those from the lower zones are of more irregular in shape and are less corroded. Textural features like filamental, dendritic or tuft-like patterns and the petaloids or coatings of "painted gold" over the primary grains indicate that they are the products of incipient growth of secondary gold. Based on their nature of occurrence and grain morphology, we develop a model involving the oxidation of pyrite to form an acidic profile which dissolved gold and transported it as chloride complexes from the primary veins. In the high Eh and neutral to alkaline environment of the oxidized zone, these complexes got dissociated and pure metal was precipitated. Chemical reprecipitation has often led to an increase in the "fineness" (purity) of gold formed in supergene environments.

No abstract