The genesis of gold in Sb-As and Sb-Au ore veins, new constraints from the study of the La Bellière and La Lucette districts (Armorican Massif, France) (original) (raw)
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Geochemistry of Gold ores Mined During celtic times from the north- Western french Massif central
Scientific Reports, 2019
The Celtic culture of Western Europe left magnificent gold objects, such as jewellery and weapons from nobility graves and hoarded coins, as well as field evidence of pre-Roman gold mining and metallurgical workshops that attest to the mining of local ores. This is the case of Central France where many precious metallic ores have been mined throughout the ages from the Prehistoric times onwards. One of the lingering problems in assessing the provenance of gold artefacts and coins is the lack of relevant data on the isotope geochemistry and mineralogy of ore sources. Forty gold ores samples were collected and studied from Limousin (French Massif Central), a very significant gold mining district from the Celtic times. Their Pb isotope compositions clearly show a local dichotomy i.e. two distinct groups of ores, one of Late proterozoic to early paleozoic pb model age and another associated to Variscan ages and consistent with field relationships, mineralogy and elemental analyses. The use of Cu and Ag isotopes, and their coupling with Pb isotopes, will refine the tracing of future metal provenance studies, but also highlight some metallurgical practices like deliberate metal additions to gold artefact or debasement of gold coins. The newly acquired Pb, Ag, and Cu isotopic data on gold ores improves our understanding of ore deposits geology and provide clarifications on the provenance of Celtic gold from this area and its economic importance. Gold and silver coins and gold artefacts from early Bronze Age to Antiquity have been discovered in burial sites and hoards throughout Europe 1-3. Archaeometallurgists evaluate the role of these metals in human societies from two complementary perspectives: i) the chain of manufacturing technologies from mines to artefacts 4-6 , and ii) the provenance of ores which provided the metal used to manufacture a particular artefact 7-11. Manufacturing is key to the understanding of alloying techniques, monetary practice, in particular debasement 12,13 , while assessing the provenance of ores can reveal significant information on ancient sources of wealth and on routes along which metals were traded. Establishing the provenance of gold is a particular challenge in archaeometallurgy for several reasons. Historical texts, where they exist, provide only a fragmented view on ancient mining and metallurgy, and the locations of many ancient mining sites are not recorded in preserved texts or went unrecorded 14. Several gold ore deposit provinces are known in Europe, e.g., the Iberian belt, the Massif Central of France, the Carpathian belt with notably the Apuseni Mountains in Romania. Most existing provinces focus on ore mineralogy and genetic interpretations and very few of them explore the archaeological aspects. Thus, the community of researchers working on the provenance of gold use these available mining geological data because of the lack of mining and/ or metallurgical referentials previously studied by archaeology. Many historically important mining areas, such as the Iberian Pyrite Belt, have experienced multiple periods of activity extending even to the present day, which makes assessing archaeological contexts difficult 15. The original size of each mined ore deposit and its mining history is usually not known with precision 2. When evidence for multiple periods of mining activity in a single mining site, notably during Roman and Medieval times, is occasionally revealed by archaeological work, the record is
Tectonophysics, 2001
The relative chronology of¯uid migration, quartz and Au-deposition in a silici®ed fault from the main Au-district (Laurieras, St Yrieix district) from northern French Massif Central has been determined from microstructural,¯uid inclusion, isotopic and ore mineral evidences. Three main stages of¯uid circulation, microfracturing and quartz crystallization, and ore deposition were distinguished on the basis of textural relationships and the pressure, temperature and composition of the palaeo-¯uids: (1) a series of early¯uid events was responsible for the localized drainage of retrograde metamorphic¯uids along the main fault and the subsequent sealing by milky and microcrystalline quartz preceeded the main Au-ore stages. Early¯uids were aqueous± carbonic, trapped under lithostatic to sublithostatic pressures at temperatures in the range 350±5008C. Subsequently, several types of microstructures were developed in the early quartz matrix. (2) NS microfractures ®lled by clear quartz, arsenopyrite and boulangerite (I) contain signi®cant refractory gold concentrations. Clear quartz formed from aqueous±carbonic¯uids of lower densities than those of the earlier¯uids. Signi®cant pressure drops, down to pressures around 55 MPa were responsible for a local immiscibility of the aqueous±carbonic¯uids at temperatures of 340^208C. (3) The main ore stage is characterized by the formation of dense sets of sub-vertical (EW) microfractures, healed¯uid inclusion planes in quartz, and ®lled by ore minerals (native gold, galena and boulangerite II) when they crosscut earlier sul®des. The¯uids are aqueous with low and decreasing salinity, and probable trapping temperatures around 2308C. Isotopic data, obtained on micro®ssured quartz, indicate these dilute aqueous¯uids may be considered as meteoric waters that deeply in®ltrated the crust.
Editorial: Orogenic gold deposits
Frontiers in Earth Science, 2024
Editorial on the Research Topic Orogenic gold deposits This Research Topic comprises papers submitted on the geology, geochemistry, and mineralogy-petrography of orogenic gold deposits. The accepted papers include detailed mineralogical analysis and regional-scale contributions from different geological terranes, which aim to bridge the gap between laboratory studies and fieldwork of different scales (e.g., He; Zhen et al.; Li et al.; Yan et al.). Orogenic gold deposits are found in metamorphic terranes spanning from the Paleoarchean to the Tertiary. These deposits contribute approximately one-third of the world's total gold production (Goldfarb and Groves, 2015; Groves et al., 2020; Nassif et al., 2022). Orogenic gold deposits occur in different tectonic settings in subduction-related accretionary to collisional terranes. Worldwide exploration of gold deposits has focused on the orogenic belts of different ages, geological backgrounds, tectonic settings, and formation processes (e.g., Hronsky et al., 2012). Since the 1980s, studies from different parts of orogenic terranes focused mainly on the controls on ore formation, the source of metals, the ore-forming fluids, and the mineralizing processes. Several studies have presented evidence for and against certain models of orogenic gold ore genesis in recent years. This Research Topic focuses on the source of ore-bearing fluids and discusses the main controls and genesis of orogenic systems and associated ore deposits. The area of research covers a broad range of geographical locations. The goal of this Research Topic is to better understand the geological processes triggering the transport and deposition of metals (Au, Ag, Sb, As, Hg, etc.) over space and time, main controls on ore, and alteration of mineral assemblages. The topics related to SI are summarized below:
Mineralium Deposita, 2004
The Moulin de Che´ni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Maold peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early ''mesozonal'' stage of quartzsulfide (Fe-As-S) deposition, usually devoid of gold and a late ''epizonal'' stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400-450°C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100-50 MPa, sug-gesting rapid uplift of the basement up to 7.5 km depth;
Mineralium Deposita, 2004
The Moulin de Chéni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Ma-old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early “mesozonal” stage of quartz-sulfide (Fe-As-S) deposition, usually devoid of gold and a late “epizonal” stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400–450 °C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100–50 MPa, suggesting rapid uplift of the basement up to 7.5 km depth; (3) The resulting extensional tectonic leads to the deposition of gold, boulangerite, galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process. Temperature and salinity decrease from 280 to 140 °C and 8.1 wt% eq. NaCl to 1.6 wt% eq. NaCl, respectively; (4) Sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids (1.6 wt% eq. NaCl to 0.9 wt% eq. NaCl) under hydrostatic conditions at 200–150 °C.