Black shales and mesozonal quartz vein‐hosted Au: The Truchas Syncline, Spain and the Harlech Dome, Wales, a comparative study

Abstract

A comparative study of quartz vein‐hosted gold occurrences associated with Palaeozoic metapelites in two areas of Wales and Spain combines new, previously unpublished and published data. Metamorphic grade is greenschist in both areas, but very low‐grade indicators in the host metapelites distance the environment from the greenschist/amphibolite transition zone required for some orogenic gold occurrences. Basin fertility for Au is indicated by the presence of auriferous pyrites in the protolith black shales in Spain. Only minor igneous activity has taken place in both study areas. Mineral parageneses are similar, with early sulphide phases characterized by As/Co and later auriferous phases by Cu/Pb/Zn sulphides. Mesozonal P–T conditions apply at deposition in both terranes. In Spain, mineralisation typically occurs in quartzites near to the metapelites, but not where the veins are in contact with them, and extensional faulting appears to be a stronger control over mineralisation than geochemical interaction with metapelite wall‐rocks. In Wales, both structural and geochemical factors (C content of the wall‐rocks and coupled oxidation of NH4 ions substituted in wall‐rock phyllosilicates to produce CH4 and N2) could have a role in Au deposition. In both areas, minor cross‐fault systems between larger faults are typical hosts of the mineralisation. Assignment is made to different subtypes of the orogenic gold model but these subtypes share the characteristic of a local source. This has implications for exploration methodology in epizonal/anchizonal metapelite‐dominated terranes, where indicators of basin fertility for Au within the protolith itself assume importance.

Gondwana platform paleoenvironment reconstruction: Insights from Ordovician black-grey shales in NW Iberia

“…However, other authors have proposed variable conditions with only temporally anoxic environments (Aramburu and García-Ramos, 1993). The scarcity/absence of organic matter observed in the SEM study, the low total organic carbon (TOC) measured in the Middle Ordovician shales (0.24%; Cunningham et al, 2023) and the Mo contents below detection levels (<1 ppm) (Table 2) indicate that oxygen conditions were not strictly anoxic. Therefore, the occurrence of framboidal pyrite in some of these sediments could be explained by its precipitation at reducing conditions during the diagenesis (Gómez-Fernández et al, 2021).…”

“…Temperatures and pressures estimated by Chl thermometry and pseudosections indicate metamorphic peak conditions of 1-2 kb and 385C Rodríguez Fernández et al, 2021). These pressure-temperature conditions are thought to be related to the gold mineralization present in this domain (Cunningham et al, 2023;González-Menéndez et al, 2021). Towards the south, away from the studied zone, and in the lower formation (Capas de los Montes), some shale layers with kyanite appear, indicating a higher-pressure metamorphic grade (Díez Montes, 2007 and references therein).…”