Mobility of gold during metamorphism of the Dalradian in Scotland

Abstract

Mobility of Au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic Au deposits. This study investigates the mobility of Au, As, and Sb during metamorphism of the Dalradian metasedimentary rocks of Scotland. The metamorphic processes in the Dalradian of Scotland are extremely well studied, and the terrane is an ideal area to investigate mobility of these metals. Our results show that of the 25 major and trace elements analysed, only Au, As, Sb, S and volatile contents as shown by loss on ignition (LOI) values show systematic variation with the metamorphic grade of the samples. Average Au concentrations decrease from 1.1±0.55ppb and 0.72±0.34ppb in chlorite and biotite zone rocks down to 0.4±0.22ppb and 0.34±0.13ppb in kyanite and sillimanite zone rocks. Average As concentrations decrease from 4.8ppm (range 0.5 to 17.8ppm) and 1.96±1.9ppm in chlorite and biotite zone rocks down to 0.24±0.15ppm and 0.2±0.12ppm in kyanite and sillimanite zone rocks. Average Sb concentrations decrease from 0.18±0.15ppm and 0.11±0.10ppm in chlorite and biotite zone rocks down to 0.04±0.02ppm in both kyanite and sillimanite zone rocks. Sulphur and LOI concentrations also show significant decreases. Mass balance calculations indicate that compared to chlorite and biotite zone samples, sillimanite zone samples have an average mass loss of 62±14%, 94±4% and 74±14% for Au, As, and Sb respectively. Every 1km3 of chlorite–biotite zone mixed psammitic–pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1.5t Au, 8613t As, 270t Sb, and 1.02 Mt S. The mobility of these elements is strongly controlled by the paragenesis of sulphide minerals. Pyrite, sphalerite, galena and cobaltite (as well as gersdorffite) decrease in abundance with increasing metamorphic grade in the Dalradian metasedimentary rocks. A critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite. This transition is complete by mid greenschist facies in the Loch Lomond samples but is more gradual at Glen Esk occurring between biotite and sillimanite zones. The Au, As, and Sb content of the sulphide assemblage also decreases with increasing metamorphic grade, and we suggest that this is a controlling factor on the mobility of these metals from the Dalradian metasedimentary rocks during metamorphism. Chlorite may be an important host mineral for As in the greenschist facies rocks. Breakdown of chlorite indirectly drives the mobility of Au, As, and Sb, as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite.We suggest that there is potential for significant undiscovered mineralisation in the Central and SW Highlands of Scotland. However, as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the Otago and Alpine Schist's, New Zealand, very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the Dalradian of Scotland.