Journal of Iberian Geology
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Journal of Iberian Geology The granite-hosted Variscan gold deposit from Santo António in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fluid inclusions, sulfur and noble gases isotopes --Manuscript Draft-- Manuscript Number: JIBG-D-18-00058R1 Full Title: The granite-hosted Variscan gold deposit from Santo António in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fluid inclusions, sulfur and noble gases isotopes Article Type: Research Paper Funding Information: Fundação para a Ciência e a Tecnologia Prof. Antonio Moura (UID/GEO/04035/2013) Fundação para a Ciência e a Tecnologia Prof. Ana Neiva (PTDC/GEO-GEO/2446/2012) Prof. Antonio Moura Prof. Manuel Francisco Pereira Abstract: A B S T R A C T 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 to 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 360-380ºC suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean 34S 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. Corresponding Author: Antonio Moura, PhD Universidade do Porto Faculdade de Ciencias Porto, PORTUGAL Corresponding Author Secondary Information: Corresponding Author's Institution: Universidade do Porto Faculdade de Ciencias Corresponding Author's Secondary Institution: First Author: Ana Neiva First Author Secondary Information: Order of Authors: Ana Neiva Antonio Moura Carlos Leal Gomes Manuel Francisco Pereira Fernando Corfu Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Order of Authors Secondary Information: Author Comments: Good evening, We sent the modified text plus figures and tables, according to the suggestions of the two reviewers. A. Moura Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Manuscript Click here to access/download;Manuscript;TEXT- PENEDONO.docx Click here to view linked References 1 A.M.R. Neiva *a, A. Moura b, C.A. Leal Gomes c, M.F. Pereira d, F. Corfu e 1 2 2 3 4 3 The granite-hosted Variscan gold deposit from Santo António in the Iberian Massif (Penedono, NW 5 6 4 Portugal): constraints from mineral chemistry, fluid inclusions, sulfur and noble gases isotopes 7 8 5 9 10 6 a Geobiotec, Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal; Departamento 11 12 7 de Ciências da Terra, Universidade de Coimbra, 3030-780 Coimbra, Portugal 13 14 8 b Instituto de Ciências da Terra (ICT), Departamento de Geociências, Ambiente e Ordenamento do Território, 15 16 9 Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4099-007 Porto, Portugal 17 18 10 c Departamento de Ciências da Terra, Universidade do Minho, Gualtar, 4710-057 Braga, Portugal 19 20 11 d Instituto de Ciências da Terra (ICT), Departamento de Geociências, ECT, Universidade de Évora, 7000-671 21 22 12 Évora, Portugal 23 24 13 e Department of Geosciences, University of Oslo, PB1047 Blindern, N-0316 Oslo, Norway 25 26 14 27 28 15 * Corresponding author: Antonio Moura E-mail address: [email protected] 29 30 16 31 32 17 ORCID numbers: Ana Neiva- 0000-0001-7808-9461; António Moura- 0000-0002-2906-3078; Carlos Leal 33 34 18 Gomes- 0000-0001-6854-5398; Manuel Francisco-0000-0001-9032-2318; Fernando Corfu-0000 0002 9370 35 36 19 4239 37 38 20 39 40 21 41 42 22 Acknowledgments 43 44 23 This research was financially supported by Fundação para a Ciência e Tecnologia through the projects 45 46 24 GOLD-Granites, Orogenesis, Long-term strain/stress and Deposition of ore metals – PTDC/GEO- 47 48 25 GEO/2446/2012: COMPETE: FCOMP-01-0124-FEDER-029192 and UID/GEO/04035/2013. Thanks are due to 49 50 26 Colt Resources for having allowed sampling in the Santo António gold mine and Dr. Pedro Keil for having 51 52 27 helped in this field work, Profs. Martim Chichorro, José Brandão Silva and Rubén Díez-Fernández for 53 54 28 constructive discussions in the field, Dr. J.M. Farinha Ramos for helpful information, Prof. R. Machado Leite for 55 56 29 the use of electron microprobe at LNEG, Eng. Fernanda Guimarães for having helped to obtain analyses with 57 58 30 this equipment, Dr. Manuel Moreira for the He-Ne-Ar isotopic data, Prof. R.A. Creaser for the Re content of 59 60 61 62 1 63 64 65 31 pyrite and Dr. Armanda Dória for having helped with the Raman analysis. Thanks are also due to the editor and 1 2 32 reviewers for their comments to help improve this manuscript. 3 4 33 5 6 34 A B S T R A C T 7 8 35 The study area is located in the Central Iberian Zone, a major tectonic unit of the Iberian Massif (Variscan belt). 9 10 36 In this region the basement is composed of Cambrian-Ordovician sedimentary and minor volcanic rocks that 11 12 37 underwent deformation and metamorphism during the Carboniferous. These metamorphic rocks host ca. 331 to 13 14 38 308 Ma granitic plutons emplaced during the D2 extensional and D3-D4 contractional deformation phases. The 15 16 39 gold-bearing quartz veins from the Santo António mine (Penedono region) occur in granite formed at 310.1 1.1 17 18 40 Ma and post-dated the peak of metamorphism. Gold-silver alloy is included in quartz, but mainly occurs in 19 20 41 spaces between grains or micro-fractures within arsenopyrite of all three generations and less in pyrite. Late 21 22 42 sulphides and sulphosalts were deposited along fractures mainly in arsenopyrite, and locally surrounding the 23 24 43 gold-silver alloy grains. Ferberite, scheelite and stolzite replace arsenopyrite. The abundant aqueous carbonic 25 26 44 fluids and the occurrence of a low-salinity fluid and their minimum possible entrapment temperature of 360- 27 28 45 380ºC suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean 29 30 46 34S values of arsenopyrite and pyrite are -4.7 ‰ and -3.8 ‰, respectively. He-Ar-Ne isotopic data suggest a 31 32 47 crustal origin. The ascent of the granite magma has provided the heat for remobilization of gold, other metals 33 34 48 and metalloids from the metamorphic rocks. This gold-arsenopyrite deposit has thus similar characteristics as 35 36 49 other selected gold-arsenopyrite deposits from the Iberian Massif, but it contains tungstates. 37 38 50 39 40 Keywords: Gold, Mineralogy, Geochemistry, Fluid inclusions, S, He, Ar, Ne isotopes, Variscan orogen 41 51 42 43 52 44 45 53 1. Introduction 46 47 54 Gold-only hydrothermal deposits include orogenic gold, intrusion-related gold systems (which may include 48 49 55 veins, stockworks, skarns, disseminations, epithermal and replacement types) and Carlin-type gold deposits. In 50 51 56 gold-only hydrothermal deposits, gold is very effectively fractionated from other metals (e.g. Garofalo and 52 53 57 Ridley 2014). Orogenic gold deposits are shear-hosted deposits developed along strike-slip fault systems linked 54 55 58 to late-stage orogenic events (e.g. Groves et al. 2000). Intrusion related gold deposits are developed in 56 57 59 magmatic-hydrothermal environments (e.g. Langard and Baker, 2001). Variscan gold deposits occur in western- 58 59 60 central Europe, e.g. in the Iberian Massif (e.g. Cotelo Neiva and Neiva 1990; Neiva 1994; Vallance et al. 2003; 60 61 62 2 63 64 65 61 Couto et al. 2007; Neiva et al. 2008; Gómez-Fernández et al. 2012; D’Angelico et al. 2016; Fuertes-Fuente et al. 1 2 62 2016), Massif Central and Bohemian Massif (Martínez Catalán et al., 1997; Goldfarb et al. 2001). The fluid 3 4 63 origin of gold deposits has been debated for a long time (e.g. Burrows and Spooner 1989; Lang and Baker 2001); 5 6 64 Pitcairn et al. 2006; Lawrence et al. 2013a, b; Rauchenstein-Martinak et al. 2014; Goldfarb and Groves 2015). 7 8 65 Some of the postulated mechanisms involve both magmatic and metamorphic, gold-bearing hydrothermal fluids, 9 10 66 but others are associated with regional metamorphic fluids, indicating that gold deposits can form from a variety 11 12 67 of fluid and metal sources (Lawrence et al. 2013b). Efforts have been directed at understanding the fluid origin 13 14 68 of gold deposits particularly from the standpoint of isotopic information including He-Ar isotopic data of fluid 15 16 69 inclusions in pyrite (e.g.