A black shale protolith for gold-tellurium mineralisation in the Dalradian Supergroup (Neoproterozoic) of Britain and Ireland

J Parnell,M Perez,J Armstrong,L Bullock,J Feldmann,A J Boyce

doi:10.1080/03717453.2017.1404682

J Parnell, M Perez + Show 4 more

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https://doi.org/10.1080/03717453.2017.1404682

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Abstract

ABSTRACTThe Dalradian Supergroup of Britain and Ireland is mineralised by gold-tellurium vein deposits. The host succession includes carbonaceous, pyritic shales (pelites) which were a source of trace elements, including gold and tellurium. LA-ICP-MS mapping of pyrite crystals shows that late stages are enriched in gold, tellurium and lead, representing concentration of these elements during metamorphism and related hydrothermal activity. The sulphur isotope composition of the pyrite varies with stratigraphic position, reflecting an origin for the pyrite in the depositional environment through microbial sulphate reduction. Where pyrite was converted to pyrrhotite, trace element contents are much lower, indicating element liberation during metamorphism. These observations are consistent with a model of black shale protoliths for orogenic gold deposits.

Highlights

The Neoproterozoic Dalradian Supergroup in Scotland and Ireland (Figures 1 and 2) is mineralised by gold (Au) at numerous localities, some of which are being commercially exploited at the time of publication (Figure 2)
The geochemical data for the Dalradian carbonaceous shales show that they are a viable protolith for Dalradian-hosted gold-tellurium mineralisation
In particular: (i) Trace element concentrations, including Au and Te, in early-stage pyrite show that ore metal concentration began in the organic-rich sediments of the depositional environment

Summary

Introduction

The Neoproterozoic Dalradian Supergroup in Scotland and Ireland (Figures 1 and 2) is mineralised by gold (Au) at numerous localities, some of which are being commercially exploited at the time of publication (Figure 2). Previous models for the Au mineralisation suggest that it may be attributed at least in part to magmatic fluids (Curtis et al 1993; Parnell et al 2000; Tanner 2012; Hill et al 2014; Jenkin et al 2017), or, alternatively, to meteoric fluids (Craw and Chamberlain 1996) or metamorphism (Pitcairn et al 2015). The Easdale Subgroup includes pyritic black carbonaceous shales over an outcrop length of at least 450 km outcrop length in Ireland and the Scottish mainland This is significant, as one model for Au mineralisation in orogenic belts is based on the uptake of trace elements onto organic matter from seafloor exhalative fluids, followed by incorporation into early pyrite and concentration during metamorphism (Large et al 2009). The black shales deserve assessment as a source for Au mineralisation

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