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Journal of Iberian Geology

, Volume 45, Issue 3, pp 443–469 | Cite as

The granite-hosted Variscan gold deposit from Santo António mine in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fluid inclusions, sulfur and noble gases isotopes

  • Ana M. R. Neiva
  • António MouraEmail author
  • Carlos A. Leal Gomes
  • Manuel Francisco Pereira
  • Fernando Corfu
Research Paper
  • 48 Downloads

Abstract

The study area is located in the Central Iberian Zone, a major tectonic unit of the Iberian Massif (Variscan belt). In this region the basement is composed of Cambrian-Ordovician sedimentary and minor volcanic rocks that underwent deformation and metamorphism during the Carboniferous. These metamorphic rocks host ca. 331–308 Ma granitic plutons emplaced during the D2 extensional and D3–D4 contractional deformation phases. The gold-bearing quartz veins from the Santo António mine (Penedono region) occur in granite formed at 310.1 ± 1.1 Ma and post-dated the peak of metamorphism. Gold–silver alloy is included in quartz, but mainly occurs in spaces between grains or micro-fractures within arsenopyrite of all three generations and less in pyrite. Late sulphides and sulphosalts were deposited along fractures mainly in arsenopyrite, and locally surrounding the gold–silver alloy grains. Ferberite, scheelite and stolzite replace arsenopyrite. The abundant aqueous carbonic fluids and the occurrence of a low-salinity fluid and their minimum possible entrapment temperature of 360–380 °C suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean δ34S values of arsenopyrite and pyrite are − 4.7‰ and − 3.8‰, respectively. He–Ar–Ne isotopic data suggest a crustal origin. The ascent of the granite magma has provided the heat for remobilization of gold, other metals and metalloids from the metamorphic rocks. This gold-arsenopyrite deposit has thus similar characteristics as other selected gold-arsenopyrite deposits from the Iberian Massif, but it contains tungstates.

Keywords

Gold Mineralogy Geochemistry Fluid inclusions S, He, Ar, Ne isotopes Variscan orogeny 

Resumen

El área de estudio está ubicada en la Zona Centroibérica, una importante unidad tectónica del Macizo Ibérico (cinturón varisco). En esta región el basamento está compuesto por rocas sedimentarias y volcánicas del Cámbrico-Ordovícico tectonizadas y metamorfizadas durante el Carbonífero. Estas rocas metamórficas sirven como caja de los plutones graníticos datados en torno a 331–308 Ma y que fueron emplazados durante la fase de deformación extensional D2 y las fases de deformación contraccional D3 y D4. Las venas de cuarzo ricas en oro de la mina de Santo António (región de Penedono) que aparecen en un granito datado a los 310.1 ± 1.1 Ma son posteriores al pico metamórfico regional. La aleación de oro y plata se incluye en el cuarzo, pero se produce principalmente en los espacios entre granos o micro-fracturas dentro de arsenopirita de las tres generaciones y menos en pirita. Los sulfuros y sulfuros tardíos se depositaron a lo largo de las fracturas principalmente en arsenopirita, y alrededor de los granos de aleación de oro y plata. Ferberita, scheelita y la estolzita sustituyen a la arsenopirita. Los abundantes líquidos acuosos carbónicos y la presencia de un fluido de baja salinidad y su posible temperatura de atrapamiento mínima en torno de 360-380 ºC sugieren que este evento de formación de oro comenzó durante las etapas finales de la orogenia varisca. Los valores medios de S de arsenopirita y pirita son − 4.7 ‰ y − 3.8 ‰, respectivamente. Los datos isotópicos de He–Ar–Ne sugieren que en el origen de los fluidos mineralizados participa la corteza continental. El ascenso del magma granítico ha provisto el calor para la movilización del oro, otros metales y metaloides desde las rocas metamórficas. Este depósito de oroarsenopirita tiene así características similares a otros yaciamientos con arsenopirita y oro del Macizo Ibérico, pero sin embargo contienen tungstates.

Palabras clave

Oro Mineralogía Geoquímica Inclusiones fluidas Isotopos de S He Ar y Ne Orogenia Varisca 

Notes

Acknowledgements

This research was financially supported by Fundação para a Ciência e Tecnologia through the projects GOLD-Granites, Orogenesis, Long-term strain/stress and Deposition of ore metals—PTDC/GEO-GEO/2446/2012: COMPETE: FCOMP-01-0124-FEDER-029192 and UID/GEO/04035/2013. Thanks are due to Colt Resources for having allowed sampling in the Santo António gold mine and Dr. Pedro Keil for having helped in this field work, Profs. Martim Chichorro, José Brandão Silva and Rubén Díez-Fernández for constructive discussions in the field, Dr. J.M. Farinha Ramos for helpful information, Prof. R. Machado Leite for the use of electron microprobe at LNEG, Eng. Fernanda Guimarães for having helped to obtain analyses with this equipment, Dr. Manuel Moreira for the He–Ne–Ar isotopic data, Prof. R.A. Creaser for the Re content of pyrite and Dr. Armanda Dória for having helped with the Raman analysis. Thanks are also due to the editor and reviewers for their comments to help improve this manuscript.

Compliance with ethical standards

Conflict of interest

There is no conflict of interest.

Supplementary material

41513_2019_103_MOESM1_ESM.docx (22 kb)
Supplementary material 1 (DOCX 22 kb)
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Supplementary material 2 (DOCX 21 kb)

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© Universidad Complutense de Madrid 2019

Authors and Affiliations

  1. 1.Geobiotec, Departamento de GeociênciasUniversidade de AveiroAveiroPortugal
  2. 2.Departamento de Ciências da TerraUniversidade de CoimbraCoimbraPortugal
  3. 3.Instituto de Ciências da Terra (ICT), Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de CiênciasUniversidade do PortoPortoPortugal
  4. 4.Departamento de Ciências da TerraUniversidade do Minho, GualtarBragaPortugal
  5. 5.Instituto de Ciências da Terra (ICT), Departamento de Geociências, ECTUniversidade de ÉvoraÉvoraPortugal
  6. 6.Department of Geosciences and CEEDUniversity of OsloOsloNorway

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