Geochemical effects of Alteration Associated with the Askot Sulfide Orebody
Hydrothermal alteration of the rocks in the area under investigation has caused considerable variation in the major and trace element distribution. A remarkably clear picture of cation migration emerges from the review of chemical changes through the successive zones of alteration (fig 1). The trends depicted reflect the fact that feldspars were readily hydrolysed by the initial hydrothermal fluids releasing potassium, sodium, calcium, aluminium, and silica, while the ferromagnesium minerals were broken down to relase magnesium and iron. These cations were redistributed in the zone causing chemical and mineralogical variations. Some elements were also added to the country rocks by the hydrothermal fluids.
The trends of silica show a depletion in the zones of sericitization and argillization. In the zone of silicification/feldspathization, however, the values are higher by about 7-8 percent as compared with the average silica values in fresh rock. This suggests that silica derived by the hydrolysis of the silicates did not crystallise at the site of alteration but migrated away from it.
The trends of alumina are nearly constant throughout the zone indicating that this cation was relatively immobile during the process of hydrothermal alteration.
Iron is generally depleted throughout the zone of alteration. In the inner zone of sericitic alteration, however, it registers a maximum increase of about 8.2 percent above the average for fresh rock. Iron derived by the breakdown of ferromagnesium minerals may have migrated towards the fissure and combined with hydrothermal sulfur to form pyrite which is very abundant in the inner zone of sericitic alteration. A possibility of minor additions of iron by the hydrothermal fluids cannot be ruled out.
Except in the zone of sericitic alteration where it registers an increase, the entire zone of alteration is depleted in magnesium. The Mg cation was released by the breakdown of ferromagnesian minerals and migrated away from the fissure, hence lowering the value of magnesium in the altered zone below average. The highest values of Mg in the inner zone are however, 27.5 percent higher than the average, and it appears that magnesium was added hydrothermally during the second phase of propylitic alteration, a fact supported by the presence of phlogopite associated with the propylitic assemblages.
The values of potassium are higher by about 24 percent in the zone of sericitic alteration, whereas in the zones of argillization and silicification/feldspathization, its values are lower by as much as 74 percent as compared with the average for fresh rock. Potassium was released by the breakdown of K-feldspars and micas and migrated towards the fissure to form sericite in the zone of sericitization. Small additions of potassium by hydrothermal solutions in this zone can be considered a distinct possibility.
Calcium and sodium were released by the breakdown of plagioclases in the zones of sericitization and argillization. These cations migrated towards the fresh rocks forming plagioclases ranging from calcic oligoclase to labradorite in the outer fringe of the zone of hydrothermal alteration. The average values of calcium and sodium in the fresh rock are 0.05 and 2.03 percent respectively. Both these elements are strongly enriched in the zones of argillization and silicification/feldspathization.
Behaviour of trace elements
Cu, Pb and Zn show high concentrations near the orebody . The concentrations of copper and lead drop sharply towards the fresh rock, and a few meters away from the sulfide zone the values become comparable to the average values of these elements in the fresh rock. High values of zinc, however, persist for considerable distances through the zone of alteration, and unlike the trends of copper and lead, the concentration of zinc registers a gradual decrease towards the fresh rock.
The average values of Cr, Be, Ni, and Co in the altered and fresh rock are identical (Table 1). These elements show no anomalous values in the alteration zone and their values remain nearly constant throughout.
Table 1. Trace element concentration in the fresh rock and altered zone.
Vanadium shows no noteworthy change throughout the zone of alteration. However, it appears considerably depleted in the altered zone since the average value of vanadium here is 65 ppm which is considerably lower than that of the fresh rock which contains an average of 106 ppm vanadium.
Lithium has a concentration of 42 ppm in the inner zone of alteration. This value drops gradually to 14 ppm near the outer fringe of the alteration zone. The average value of lithium in the fresh rock is 18 ppm.
The concentration of strontium in the fresh rock is 22 ppm. It is strongly enriched in the altered zone where its average content is 139 ppm. The trend of strontium in the altered zone is identical with that of calcium. The identical distribution of Sr and Ca has also been reported by Ambler (1979) in a mineralized dacite dike near Yeoval in New South Wales, Australia. The average values of Cu, Pb, Zn, Cr, Be, Ni, Co, V, Li, and Sr for fresh rock and altered zone are given in Table 1.
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