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

• Type: Article in International Scientific Journal
• Date: 2018

Authors

Cruz, C (Author) Other; Noronha, F (Author) Other; Santos, P (Author) Other; Mortensen, JK (Author) Other; Alexandre Lima (Author) FCUP

Journal

Title: Mineralogical Magazine

Vol. 82

Pages: S307-S320

ISSN: 0026-461X

Publisher: MINERALOGICAL SOCIETY

Indexing

ISI Web of Knowledge - 1 Citation

Scopus - 1 Citation

Other information

Authenticus ID: P-00P-27B

DOI: 10.1180/minmag.2017.081.063

Abstract (EN): Several 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 Durico-BeirA 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 (similar to 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.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 14