Supergene gold enrichment in the Castromil-Serra da Quinta gold deposit, NW Portugal

C Cruz,P Santos,J K Mortensen,F Noronha,A Lima

doi:10.1180/minmag.2017.081.063

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Open Access

https://doi.org/10.1180/minmag.2017.081.063

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Abstract

ABSTRACTSeveral gold deposits hosted mainly by Variscan granites and Precambrian to Palaeozoic metasediments occur in the northwestern part of Portugal. Most of these deposits were mined by the Romans (in the period I BC to II AD) as open pits and surface galleries. The Castromil-Serra da Quinta gold deposit is an important example of such a mined site; it occurs in the Dúrico-Beirã Au province located in the Central Iberian Zone (CIZ) in the western branch of European Variscan belt, mainly on the eastern flank of the Valongo anticline. Open pits and underground galleries at Castromil-Serra da Quinta exploited the gossan formed from the weathering of primary mineralization. The gossan is composed essentially of goethite, scorodite and clay minerals. A recent drilling campaign at Castromil-Serra da Quinta has provided samples of the primary mineralization below the oxidation level. Different modes of gold occurrence are defined based on metallographic studies of both the gossan and drill cores. Gold I occurs encapsulated in primary sulfide minerals, mainly arsenopyrite and pyrite; Gold II is also associated with the main primary sulfides, but occurs along grain boundaries and in microfractures of the sulfides or in associated quartz veins; and Gold III occurs as free gold particles in iron oxides within the gossan. In the gossan samples, it is difficult to distinguish whether the gold particles hosted in oxides correspond to Gold I, Gold II, or both, so these particles are described as Gold I–II and they are commonly surrounded by very much smaller particles of Gold III. Scanning electron microscope (SEM) and electron probe microanalysis (EPMA) data for the different gold particles reveal that Gold I is poorer in Ag (~15.5–39.76%) than Gold II (37.46–51.45%), whereas Gold III corresponds to native gold (<16.11% Ag). Gold III is thought to reflect gold enrichment in the upper level of the deposit, resulting from weathering processes that affected the primary Au (Bi) mineralization.

Highlights

IntroductionMany have been exploited since Roman times; they include different types of
The northwestern Iberian Peninsula contains numerous gold deposits
We have focused in particular on the nature and origin of gold hosted in Fe oxides, and whether they might be related to a supergene enrichment process

Summary

Introduction

Many have been exploited since Roman times; they include different types of. This paper is part of a special issue entitled ‘Critical-metal mineralogy and ore genesis’. The Applied Mineralogy Group of the Mineralogical Society and the IMA Commission on Ore Mineralogy have contributed to the costs of Open Access publication for this paper. Deposits, including alluvial gold as well as gold in gossan and in quartz veins (Fuertes-Fuente et al, 2016). A gossan is defined as iron-bearing weathered rocks usually overlying sulfide-bearing deposits, and predominantly consisting of iron oxides and hydroxides, exhibiting a massive or powdery appearance with about 30–50% porosity and around 45 vol.% Fe (Jensen and Bateman, 1979; Taylor et al, 1980; Taylor and Thornber, 1992)

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