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A lithogeochemical study of hydrothermal alteration associated with Mafic Hosted and Besshi-type Massive Sulphide deposits Stoynov, Hristo Simeonov


Pearce element ratio (PER) analysis was proposed in 1968 as a graphical method to be applied in petrology. Using major element chemical compositions of rocks, it allows for the determination of primary mineral parageneses. The application of the technique was later expanded to include studies of hydro thermal alteration associated with mineral deposits. This study tests the applicability of PER analysis and the related generalized element ratio (GER) analysis methods to the study of alteration associated with two classes of Volcanic-Hosted Massive Sulphide deposits. Presented are case studies of the Chu Chua and the Konuto Lake Mafic-Hosted Massive Sulphide deposits and the Goldstream Besshi-type deposit. Molar element ratio techniques (PER and GER) are used to identify altered samples and to differentiate between individual chemical alteration types. It is demonstrated that the technique is capable of identifying the exact alteration reaction for an individual sample or for a coherently altered group of samples. The format in which the chemical analytical data are presented allows for a convenient interpretation of the mineralogical effects of alteration. Thus, the validity of the conclusions based on major element lithogeochemistry can be independently verified by petrographic methods. The most typical chemical alteration types found in the three studied deposits are silica mobility (loss and addition), loss of Ca and Na and addition of Fe and Mg. Mineralogically, the alteration process involves the destruction of plagioclase and pyroxene, local deposition of quartz and deposition of iron sulphides. At Chu Chua, extreme Fe-Mg addition caused the characteristic talc-magnetite assemblage whereas at Goldstream Fe and Mg-rich clays were deposited in a seafloor hydrothermal vent setting. Mobilized Ca, Na and excess Si02 were exhaled on the seafloor, giving rise to exhalative chert deposits, carbonate and clay minerals. The tested technique allows the degree of alteration to be quantified in each individual sample. The geographic coordinates of altered samples are then plotted to reveal the spatial patterns of alteration. The outlined alteration anomalies coincide spatially with the known ore bodies and are significantly larger in overall dimensions. Thus, in an exploration context, they would represent intermediate targets. The study proposes exploration parameters, which can be used in similar geologic settings.

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