Monazite in polymetallic chlorite-(tourmaline)-quartz-(fluorite)-cassiterite-sulphide lodes and its potential for constraining the chronology of magmatic hydrothermal mineralisation in Cornwall.

Abstract

Monazite [(Ce, La, Th, Nd, Y) PO4] occurs far more widely within the magmatic hydrothermal lode systems of Cornwall than has been previously recognised. In situ and mine dump specimens analysed by binocular and transmitted light microscopy, SEM, XRD and whole rock XRF reveal a near ubiquitous association of hydrothermal monazite with polymetallic chlorite-(tourmaline)-quartz- (fluorite)-cassiterite-sulphide mineralisation. The assemblage is interpreted to represent an early lode paragenesis formed by mixing of magmatic hydrothermal fluids with meteoric and/or metamorphic fluids derived from the country rocks. Such a model may explain why these assemblages are not observed in mines that have worked at greater depths within the granite. Major advances in understanding the chronology of granite magmatism across SW England using U-Pb monazite and xenotime methods have not, to date, been matched by the same level of success with respect to the development of the productive major lode systems. This discovery of widespread hydrothermal monazite raises the potential for high precision U-Pb dating of such mineralisation across the Cornubian Orefield. Where paragenetic relationships can be established it may also be possible to construct a fine-scale chronology for individual lode systems