Fluorine-rich biotites and alkali-metasomatism as guided to massive sulphide deposits: an example from the Bleikvassli Zn-Pb-Ah-(CU) deposit, Norway
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Frie emneord: Sulfidmalmforekomst Fluorrike biotitter The Bleikvassli massive sulphide deposit is closely associated with a microcline gneiss interpreted as the metamorphosed equivalent of an alkaline magmatic rock of syenitic composition which yield a concordant U\/Pb zircon age of 481 +- Ma. A 30 metres wide zone along the margin of the microcline gneiss suffered intensive alkali-depletion and addition of iron and magnesium, when it was metamorphosed together with the massive sulphides. Iron and magne- sium partitioned in to biotite and phengitic mica and biotite show a conspi- cuous enrichment of fluorine and depletion of iron compared to the unaltered interior of the microcline gneiss. Partitioning of fluorine in favour of biotite is an indirect effect of the proximity to the massive sulphides. This is because an environment with a high fs2 wil favour the crystallisation of phlogopitic biotites which will enhance the substitutional exchange of F for OH. Because of this effect, fluorine can be used as a guide revealing the existence of unexposed massive sulphide deposits.