GMPV5.2

GMPV5
Mineral deposits: systems, settings and processes 

Mineral deposits represent principal sources of metallic and non-metallic raw materials for our society. The implementation of new climate policies and the rise of green energy production and use will trigger an unprecedented demand increase for such resources. Formation of economic commodities requires component sequestration from source region, transport and focusing to structural or chemical barriers. These enrichment processes typically involve magmatic, hydrothermal, weathering or metamorphic events, which operate in diverse geodynamic settings and over various time scales. The scope of this session is to collect insights from diverse areas of mineral exploration, field, analytical or experimental studies of mineral deposits as well as resource characterization and extraction. We invite contributions from fields of economic geology, mineralogy and geochemistry in order to advance our understanding of ore-forming systems.

Co-organized by ERE5
Convener: Cyril Chelle-Michou | Co-convener: David Dolejs
vPICO presentations
| Thu, 29 Apr, 15:30–17:00 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Cyril Chelle-Michou, David Dolejs
15:30–15:35
15:35–15:37
|
EGU21-7903
|
ECS
Ville Virtanen, Jussi Heinonen, Nicholas Barber, and Ferenc Molnár

The timing and degree of immiscible sulfide precipitation in a magma effectively controls the formation of magmatic sulfide deposits and the budget of degassing sulfur species in volcanic systems. Besides the absolute sulfur (S) content, sulfide precipitation is strongly affected by the sulfur content at sulfide saturation (SCSS) in the host silicate melt. Assimilation of S-rich wall-rocks, such as black shales, effectively increases the S content in the magma, while simultaneously lowering the SCSS. Accordingly, assimilation has been identified as the most important process in the formation of many economically significant magmatic base metal sulfide deposit, especially in continental tectonic settings. Detailed understanding of the relation between wall-rock assimilation and sulfide saturation requires accurate thermodynamic models for open magmatic systems experiencing assimilation-fractional crystallization (AFC).

The Magma Chamber Simulator (MCS) is currently the only geochemical modeling software that considers the thermodynamic phase equilibria in open magmatic systems involving magma and wall-rock (and recharge) subsystems. We utilized the MCS to explore how assimilation affects the SCSS and S content of the magma. With the current lack of thermodynamic data for sulfides, we tentatively modeled S as a trace element and varied its compatibility to wall-rock in the different models. For a case study, we chose the mafic layered intrusions of Duluth Complex, Minnesota, which host some of the largest Cu-Ni sulfide deposits in the world. Assimilation of the adjacent black shale has been established as the main source for S in the deposits.

Our MCS models show in detail how continuous assimilation of the black shale lowers the SCSS of the melt. Partial melt from the black shale enriches the magma in SiO2, Al2O3, K2O, and H2O, while depleting FeO, MgO, CaO, and Na2O, which causes a first order decrease in the SCSS. The compositional change also replaces troctolitic cumulates (plagioclase, olivine ± clinopyroxene) with norite (plagioclase and orthopyroxene), which leads to more pronounced FeO depletion in the melt, further lowering the SCSS. On the other hand, the assimilated partial melt also increases the melt mass in the magma subsystem, which counteracts the S enrichment. Accordingly, in the model where S is compatible to the wall-rock residual, the degree of sulfide saturation only slightly increases relative to the same magma experiencing FC without assimilation.

More than half of the wall-rock S must partition to the assimilated partial melt in order to meet the S isotopic criteria of the modeled Cu-Ni-deposits. The main stage of sulfide precipitation is associated with ~30 wt.% crystallization of the assimilating host magma. The proportion of sulfides relative to silicates in these models is smaller than observed in the Duluth Complex deposits, which underlines the role of dynamic processes in concentrating sulfides from the silicate magma.

How to cite: Virtanen, V., Heinonen, J., Barber, N., and Molnár, F.: Modeling the sulfide saturation in continuously assimilating magmatic systems with the Magma Chamber Simulator, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7903, https://doi.org/10.5194/egusphere-egu21-7903, 2021.

15:37–15:39
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EGU21-12593
Svetlana Drogobuzhskaya, Tamara Bayanova, and Andrey Novikov

The laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) is a unique method for local analysis that allows studying mineral grains in situ. The aims of these geochemical researches are to estimate concentrations and distributions of PGE and other siderophilic and chalcophilic elements in ore minerals from complex deposits in the Arctic region (Fennoscandian Shield), using the LA-ICP-MS local analysis of trace elements. Pyrite, pentlandite, pyrrhotite and other sulfides are important for determining platinum-group elements.

In situ analyses of sulfide crystals were carried out on polished thin sections by ICP-MS. The electron (LEO-1415) and optic (LEICA OM 2500 P, camera DFC 290) spectroscopy was applied to study the morphology of the samples. Analytical points on sulfide minerals were selected using microelectronic and optical images.

PGE and other elements (As, Bi, Cd, Cr, Co, CuFe, Ni, Se, S, Sn, Sb, Pb, Re, Te, Tl, Zn, Hf, Th, U, REE) were measured by ICP-MS, using an ELAN 9000 DRC-e (Perkin Elmer) quadrupole mass spectrometer equipped with a 266 nm UP-266 MAСRO laser (New Wave Research). NIST 610, NIST 612 and tandem graduation (using solutions), considering sensitivity coefficients of isotopes were used to check the accuracy of estimations. Fe, Ni and Cu were used as internal standards, being most evenly distributed elements in minerals, when concentrations of elements in sulfides were calculated. The estimates were carried out, using inter-laboratory standards of chalcopyrite, pentlandite and pyrrhotite, which had been preliminarily prepared and studied using microprobe analysis (Cameca MS-46).

Data on concentrations of PGE, Au and Ag in sulfides, including data on their distribution in minerals, are crucial in studying the origin of noble metals in sulfide ores and interpreting formation settings of complex deposits. Estimated concentrations of other trace elements provide an essential supplement to geochemical data. Received data are new data (LA-ICP-MS) of Pt-Pd and Cu-Ni reefs of the Monchegorsk ore areas (2.5 Ga) with prospected commercial deposits. Elaborated LA-ICP-MS techniques were applied to provide in situ measurements of noble metals (PGE, Au, Ag), as well as siderophilic and chalcophilic elements, in sulfide minerals in order to study their distributions in chalcopyrite, pentlandite and pyrite from the Pechenga and Allarechka Cu-Ni deposits (1.98 Ga), Fedorova Tundra and Severny Kamennik PGE deposits (2.5 Ga).

 

The scientific researches are supported by RFBR Grant No 18-05-70082, scientific themes 0226-2019-0032 and 0226-2019-0053.

How to cite: Drogobuzhskaya, S., Bayanova, T., and Novikov, A.: Geochemical study in situ (LA-ICP-MS) of ore minerals from Paleoproterozoic layered PGE intrusions in the north-eastern Fennoscandian Shield, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12593, https://doi.org/10.5194/egusphere-egu21-12593, 2021.

15:39–15:41
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EGU21-1987
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ECS
Patrick Krolop, Kari Niiranen, Sabine Gilbricht, Bernhard Schulz, Marcus Oelze, and Thomas Seifert

Iron oxide-apatite (IOA) deposits are an important source of iron ore based on the modal abundance of magnetite > 90 vol.%. Further interest is generated due to the high variability of apatite and hematite in some of these ores. The origin of the so-called Kiruna-type deposits has been subject to controversy for more than a century. Models range from a purely magmatic origin to ore-forming processes that involve variable stages of hydrothermal fluid involvement to a not widely accepted sedimentary-exhalative origin. In contribution of understanding ore-forming processes of this deposit type, we performed mineral chemistry and trace element analyses on samples from the Per Geijer deposits. They account for the lesser studied deposits in the Kiruna district of northern Sweden. A comprehensive mineral-chemical dataset of magnetite and hematite obtained by electron microprobe analysis (EPMA) and LA-ICP-MS from representative drill core samples is presented. Magnetite and four different types of hematite constitute the massive orebodies: Primary and pristine magnetite with moderate to high concentrations of Ti (∼61–2180 ppm), Ni (∼11–480 ppm), Co (∼5–300 ppm) and V (∼553–1831 ppm) indicate a magmatic origin for magnetite. Hematite type I appears as a replacement of magnetite with high Ti (∼15,700–42,300 ppm), relatively constant V (∼1460–2160 ppm) and moderate Sn (∼29–105 ppm) concentrations. Moderate and variable Ti (∼369–12,490 ppm) and low Sn (∼1.4–19 ppm) concentrations are representative for hematite type II. Hematite type III has lowest Ti (∼99–1250 ppm) concentrations. Significantly high Ti concentrations (∼12,100–78,700 ppm), low V (∼132–381 ppm) and high Sn (∼129–456 ppm) concentrations account for type IV. The presence of fluorapatite and disseminated pyrite with high Co:Ni ratios (> 1–10) in massive magnetite ores are consistent with a high temperature (∼ 800°C) genesis for the deposit. The different and abundant types of hematite state subsequent hydrothermal events.

How to cite: Krolop, P., Niiranen, K., Gilbricht, S., Schulz, B., Oelze, M., and Seifert, T.: Trace element geochemistry of iron oxides from the Per Geijer apatite iron ores in the Kiruna district, northern Sweden: Implications for ore genesis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1987, https://doi.org/10.5194/egusphere-egu21-1987, 2021.

15:41–15:43
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EGU21-1190
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ECS
Eleanor Heptinstall and John Parnell

Carbonatitic magmatism and metasomatism are rare in Great Britain & Ireland. However, REE-Nb minerals were identified in impure marbles and calc-silicates on the Aird of Shin and Arscaig near Lairg, Scotland. They belong to the Palaeoproterozoic Shin Group, a supercrustal succession of amphibolites, banded silicic gneisses, marbles and calc-silicates. The Shin Group is one of several Palaeoproterozoic to Neoproterozoic marble inliers in the North Highlands of Scotland that present evidence of Caledonian orogen carbonatitic metasomatism. The Proterozoic Bayan Obo ore complex in China was similarly deposited as a dolomite-limestone and later subject to alkali intrusions and carbonatitic metasomatism during the Caledonian orogeny. The Bayan Obo complex hosts REE-Nb-Fe carbonatitic fine-grained dolomites, REE-Nb deficient coarse dolomites, carbonatitic dykes, limestones and dolostones. The Aird of Shin marble mineralogy comprises niobium and tantalum Ca-bearing oxides, scheelite, strontian barite, ilmenite, REE-bearing monazite, REE-epidote and Fe-Mo sulphides in an impure Na-Fe-K calcite fabric. The Arscaig Qtz-Na-K calc-silicates are enriched in REE-bearing monazite, Mg-Fe chlorite and Fe-oxides, with minor REE-bearing xenotime, Mn-Fe garnet and strontian barite. The REE-Nb Aird of Shin marble and the REE-phosphate bearing Arscaig calc-silicates are comparable with carbonatitic mineral phases in the Bayan Obo complex. This study adds support to previous recognition of Caledonian carbonatitic magmatism and carbonatitic metasomatism of Proterozoic limestones and calc-silicates in Scotland. 

How to cite: Heptinstall, E. and Parnell, J.: Carbonatitic REE-Nb-Fe mineralisation in the Palaeoproterozoic Shin Group marble and calc-silicates, Loch Shin, Scotland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1190, https://doi.org/10.5194/egusphere-egu21-1190, 2021.

15:43–15:45
|
EGU21-851
|
ECS
|
Highlight
Chetan Nathwani, Adam Simmons, Simon Large, Jamie Wilkinson, Yannick Buret, and Christian Ihlenfeld

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene-Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~67 and ~59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu-Mo deposits that formed at 57-54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U-Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (>0.3) and lower Ti (<9 ppm) compositions after ~60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu-Mo deposits at Quellaveco, Toquepala and Cuajone from ~57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

How to cite: Nathwani, C., Simmons, A., Large, S., Wilkinson, J., Buret, Y., and Ihlenfeld, C.: Petrological and zircon chemical record of arc magma evolution from long-lived batholith construction to giant porphyry copper deposit formation , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-851, https://doi.org/10.5194/egusphere-egu21-851, 2021.

15:45–15:47
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EGU21-249
|
ECS
Majid Soleymani, Shojaeddin Niroomand, Abdorrahman Rajabi, Thomas Monecke, and Soroush Modabberi

The Late Cretaceous to Eocene Sistan suture zone in southeastern Iran separates the Lut continental block in the west from the Afghan continental block in the east. A major belt of Oligocene to Miocene igneous rocks occurs between the cities of Zahedan and Nehbandan, stretching for ~200 km from south to north parallel to the border with Pakistan and Afghanistan. Known porphyry Cu mineralization is associated with the intrusions and intrusive complexes at Kuh-e Janja (16.5+2.0 Ma), Kuh-e Seyasteragi (19.2+ 1.4 Ma), Kuh-e Assagie (27.5+2.0 Ma), and Kuh-e Lar (32.8+3.0 Ma).

Small intrusions and intrusive complexes in the Zahedan-Nehbandan magmatic belt are mostly intermediate to felsic in composition and have calc-alkaline or shoshonitic affinities. Associated volcanic and volcaniclastic rocks are common. The igneous rocks are hosted by deformed late Cretaceous to Eocene flysch sequences that formed in the Sefidabeh forearc basin developed during the subduction and closure of the Sistan ocean. The geochemical composition of the intrusive rocks and their ages suggest that igneous activity and related mineralization in the Zahedan-Nehbandan magmatic belt may have formed as a result of post-collisional processes. The locations of the intrusive centers in the Kuh-e Assagie and Kuh-e Lar may be controlled by strike-slip faults, which are major post-collisional structures.

The recent discovery of the Janja porphyry Cu-Au-Mo deposit below Quaternary alluvial terraces highlights the exploration potential of the Zahedan-Nehbandan magmatic belt. In addition to post-collisional porphyry deposits, other deposit types such as skarns, polymetallic veins, or epithermal deposits may be hidden below the regionally extensive Quaternary cover.

How to cite: Soleymani, M., Niroomand, S., Rajabi, A., Monecke, T., and Modabberi, S.: Metallogeny of the Zahedan-Nehbandan magmatic belt and implications to porphyry Cu exploration in southeastern Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-249, https://doi.org/10.5194/egusphere-egu21-249, 2021.

15:47–15:49
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EGU21-3318
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ECS
Maria A. Kokh, Clement Laskar, and Gleb S. Pokrovski

Knowledge of molybdenum (Mo) speciation under hydrothermal conditions is a key for understanding the formation of porphyry deposits which are the primary source of Mo. Existing experimental and theoretical studies have revealed a complex speciation, solubility and partitioning behavior of Mo in fluid-vapor-melt systems, depending on conditions, with the (hydrogen)molybdate (HMoO4-, MoO42-) ions and their ion pairs with alkalis in S and Cl-poor fluids [1-3], mixed oxy-chloride species in strongly acidic saline fluids [4, 5], and (hydrogen)sulfide complexes (especially, MoS42-) in reduced H2S-bearing fluids and vapors [6-8]. However, these available data yet remain discrepant and are unable to account for the observed massive transport of Mo in porphyry-related fluids revealed by fluid inclusion analyses demonstrating 100s ppm of Mo (e.g., [9]). A potential missing ligand for Mo may be the recently discovered trisulfur radical ion (S3•-), which is predicted to be abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids [10]. We performed exploratory experiments of MoS2 solubility in model sulfate-sulfide-S3•--bearing aqueous solutions at 300°C and 450 bar. We demonstrate that Mo can be efficiently transported by S3•--bearing fluids at concentrations ranging from several 10s ppm to 100s ppm, depending on the fluid pH and redox, whereas the available data on OH-Cl-S complexes cited above predict negligibly small (<100 ppb) Mo concentrations at our conditions. Work is in progress to extend the experiments to wider T-P-composition range of porphyry fluids and to quantitatively assess the role of S3•- in Mo transport by geological fluids.

  • 1. Kudrin A.V. (1989) Geochem. Int. 26, 87–99.
  • 2. Minubayeva Z. and Seward T.M. (2010) Geochim. Cosmochim. Acta 74, 4365–4374.
  • 3. Shang L.B. et al. (2020) Econ. Geol. 115, 661–669.
  • 4. Ulrich T. and Mavrogenes J. (2008) Geochim. Cosmochim. Acta 72, 2316-2330.
  • 5. Borg S. et al. (2012) Geochim. Cosmochim. Acta 92, 292–307.
  • 6. Zhang L. et al. (2012) Geochim. Cosmochim. Acta 77, 175–185.
  • 7. Kokh M.A. et al. (2016) Geochim. Cosmochim. Acta 187, 311–333.
  • 8. Liu W. et al. (2020) Geochim. Cosmochim. Acta 290, 162–179.
  • 9. Kouzmanov K. and Pokrovski G.S. (2012) Soc. Econ. Geol. Spec. Pub. 16, 573–618.
  • 10. Pokrovski G.S. and Dubessy J. (2015) Earth Planet. Sci. Lett. 411, 298–309.

How to cite: Kokh, M. A., Laskar, C., and Pokrovski, G. S.: The effect of the trisulfur radical ion on molybdenum transport by hydrothermal fluids, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3318, https://doi.org/10.5194/egusphere-egu21-3318, 2021.

15:49–15:51
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EGU21-4813
Elena-Luisa Iatan

Voia deposit belongs to the Săcărâmb-Cetraș-Cordurea Miocene volcano-tectonic alignment of the South Apuseni Mountains, Romania. This large volcanic complex represents a Sarmatian-Pannonian magmatic-hydrothemal mega-system of around 5 km2 with an estimated 3–4 Ma time-space evolution, consisting of seven andesitic volcanic structures grouped in a circle, three subvolcanic andesite-quartz porphyry microdiorite and associated porphyry Cu-Au(Mo), pyrite Ca-Mg skarns and epithermal Au-Ag-Pb-Zn-Cu mineralizations.

The mineral assemblages of alteration and mineralization processes belong to several mineralized zones on a vertical scale, according to sampling evidence and laboratory studies. HS products are found in the upper part of the structure (300-500 m), with dominant advanced and intermediate argillic alterations and sulfide-sulfate gold-poor veins (pyrite, marcasite, base metal sulfides, Fe-Ti oxides, vuggy quartz, alunite, gypsum, anhydrite). Within the 500-1200 m depth, the HS mineral assemblages gradually decrease in favor of IS and LS products. It is characterized by the coexistence of gold-rich LS assemblage (native gold, base metal sulfide, adularia, sericite-illite, chlorite, carbonates ± anhydrite veins), with the IS assemblage (iron oxides, chalcopyrite, pyrite, quartz, anhydrite). These assemblages overprint the HS mineral associations, resulting in a transition zone characterized by gold - pyrite - chalcopyrite - iron oxides - quartz - anhydrite mineral assemblage characteristic for HS and native gold - pyrite - base metal sulfides - carbonates - quartz mineral assemblage corresponding to IS+LS type.

Gold is present in all of the identified mineralization forms: porphyry-epithermal Cu-Au, epi-mesothermal carbonate veins with gold - base metal sulfides, quartz veins with pyrite - chalcopyrite - magnetite ± hematite ± anhydrite, anhydrite veins with base metal sulfides and sulfosalts, anhydrite veins with pyrite - anhydrite ± quartz, vuggy quartz (silica residue) with gold-poor pyrite veins and impregnations in porphyry systems.

Drilling core samples revealed that in Voia deposit, gold is concentrated in chalcopyrite (drills no. 7, 19, 37) along with pyrite - magnetite - hematite - quartz assemblage from the late potassic stage. The major amount of gold associated with chalcopyrite tends to be mainly submicroscopic. Pyrite from anhydrite veins of the early potassic stage ± phyllic alteration is relatively poor in gold (drills no. 1-6, 8-14). However, the highest gold contents are present in pentagonal dodecahedron pyrites (drills no. 33, 38, 39) of pyrite-chalcopyrite-magnetite ± hematite-quartz assemblage from late potassic stage ± phyllic alteration. Pyrite associated with magnetite from anhydrite veins tends to be poor in gold (drills no. 8, 11, 15, 28, 29). A carbonate vein containing gold-bearing base metal sulfides that was intercepted at 960,00-960,30m depth by drill no. 17 is one of the richest in gold.

Native gold occurs as fine inclusions in ore minerals (5-20 μm). Large irregular grains of native gold (>50 μm) appear at mineral boundaries and along the fissures. The gold color is bright yellow and has a measured Au:Ag ratio of 5:1, suggesting that native gold has been formed at a relatively high temperature.

Acknowledgments: This work was supported by two Romanian Ministry of Research and Innovation grants: PN-III-P4-ID-PCCF-2016-4-0014 and PN-III-P1-1.2-PCCDI-2017-0346/29.

How to cite: Iatan, E.-L.: The occurrence of gold in Voia deposit, South Apuseni Mountains, Romania, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4813, https://doi.org/10.5194/egusphere-egu21-4813, 2021.

15:51–15:53
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EGU21-11150
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ECS
Erwin Schettino, Claudio Marchesi, José María González-Jiménez, Edward Saunders, Károly Hidas, Fernando Gervilla, and Carlos Jesús Garrido

Magmatic-hydrothermal gold deposits form clusters in the Earth’s crust and are heterogeneously distributed within lithospheric blocks. A global assessment of whole-rock gold abundances in mantle lithologies worldwide indicates that Au concentrations increase with increasing fertility of mantle peridotites, with median Au contents ranging from 0.50 ppb in dunites, 1.00 ppb in harzburgites, and up to 1.26 ppb in lherzolites. Of particular interest are those volumes of fertile Subcontinental Lithospheric Mantle (SCLM) veined by pyroxenites and wehrlites, usually the Au-richest lithologies in the mantle as they have 2.05 ppb median Au concentrations. Partial melting of SCLM domains endowed in gold seems to play a key role in the genesis of gold-enriched magmas parental to magmatic-hydrothermal gold deposits in continental arc settings. The mineralogical expressions of gold inventory in such fertile mantle rocks are accessory Ni-Fe-Cu sulfides and discrete micron-to-nano-sized Au mineral particles that control the extraction and transport of gold in the mantle. Mantle xenoliths from the Neogene Volcanic Province (NVP) of southeast Spain represent an excellent example of SCLM refertilized by gold-sulfide-rich silicate melts underlying a gold metallogenic province. Here we present mineralogical and compositional data of sulfides in mantle xenoliths from this area (Tallante volcanic center), which are anomalously rich in gold (up to 46 ppm) compared to sulfides from SCLM not associated with Au-metallogenic provinces. We propose that these gold-rich, fertile mantle sources may have melted during the Cenozoic evolution of the westernmost Mediterranean subduction system and fed the ore-productive volcanic activity in southeast Spain.

How to cite: Schettino, E., Marchesi, C., González-Jiménez, J. M., Saunders, E., Hidas, K., Gervilla, F., and Garrido, C. J.: Gold fingerprint of the SCLM beneath a metallogenic province, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11150, https://doi.org/10.5194/egusphere-egu21-11150, 2021.

15:53–15:55
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EGU21-12132
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ECS
Debasis Pal, Sakthi Saravanan Chinnasamy, and Sukumari Rekha

Dharwar Craton in southern India hosts several gold bearing greenstone belts including the well-known Kolar and Hutti. Among them, the Gadag greenstone belt in the western part of Dharwar Craton contains many potential gold mines. It has three different lode systems named western, central and eastern lodes. These lodes are spatially distributed as linear groups along the shear zone with distinct lithological assemblages. Tourmaline is one of the most common hydrothermal minerals present in the alteration zones apart from chlorite, muscovite and sericite. These tourmalines show two textural association (i) occur as isolated, euhedral grains along the mylonitic foliation defined by chlorite, muscovite, sericite, quartz and carbonates (ii) occurrences of anhedral bizarre shaped tourmaline grains along with carbonate and quartz. Though texturally different, compositionally both the tourmalines are similar. They are dravite in nature with high Altot (6.02 to 6.56 apfu), low Na (0.42 to 0.88 apfu) and medium X-vacancies (0.08 to 0.57 apfu). The predominance of Fe2+ (high Fe2+/Fe3+) and low Na in the tourmaline crystal structure indicates low saline, reduced ore fluid of metamorphic origin that is responsible for gold mineralization in Gadag.

Microthermometric study of aqueous, carbonic and aqueous-carbonic inclusions from the auriferous lodes at Gadag reveal low to medium saline (0.04 to 9.59 NaCl equiv.)  H2O-NaCl-CO2±CH4 ore forming fluid. Presence of trace amount of methane content within the carbonic inclusion indicates mineralization occurred at reducing environment. Thus, fluid inclusion results consistent with the tourmaline chemistry and strongly supports the metamorphic origin of ore fluids that responsible for gold mineralization at Gadag.

How to cite: Pal, D., Chinnasamy, S. S., and Rekha, S.: Ore fluid characteristics at Gadag Gold Field, Dharwar Craton, southern India: Evidences from tourmaline chemistry and fluid inclusion study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12132, https://doi.org/10.5194/egusphere-egu21-12132, 2021.

15:55–15:57
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EGU21-14847
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ECS
Brian Nharara, Eugenia Airiyants, Ol'ga Kiseleva, Dmitriy Belyanin, Petr Roshchektaev, and Sergey Zhmodik

Vladimirskoe gold-rare metal deposits are located in the Urik-Kitoiskaya gold zone, at the edges of the Neoarchean age Gargan microcontinent, in the southeastern part of the Eastern Sayan. Gold mineralization is localized in sheared, beresitised mineralized zones among granite gneiss of the Gargan Group (NARg). The width of the zones is 3-10 m and the length is up to 800 m. The material composition of the ores is sulfide-carbonate-quartz. The main ore minerals are pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, as well as tellurides and sulfosalts of silver, lead, and bismuth. The main minerals at the deposit are gold, associated - silver, and bismuth, associated with sulfide mineralization. Average gold grades in ores are 7-12 g / t.

Mineralized ore zones are associated with faults, and are often localized at their intersection with dike complexes. Several fault systems are identified in the deposit. The first-order fault system is a right-lateral dip-slip with a submeridional strike. The second-order system has a northwest strike and represents zones of viscous faults, expressed by zones of cleavage, beresitisation, silicification, and sulfidization, which are associated with gold mineralization.

There are two types of dike complexes within the region. The first dike complex of the barun-holba subvolcanic complex (O-S) has basic composition. Dikes are widespread throughout the entire area and are characterized by diabase porphyrites, metabasalts, and more rarely, basaltic andesite porphyrites. The rocks have a porphyry structure with phenocrysts (1-3 cm) characterized by plagioclase, altered to form epidote and muscovite. Large porphyry segregations up to 10 cm, bearing traces of deformation processes are observed in some cases. The groundmass has a fine-grained, microlepidogranoblastic structure and is composed of a secondary epidote-chlorite-albite aggregate.

The second dike complex is less pronounced and is characterized by felsic dikes belonging to the Early Paleozoic Holba complex. It is located in the southeastern part of the region and is characterized by granite-porphyries, leucocratic pegmatoid granites, and dacites. Dikes of felsic composition have a felsic structure caused by microliths of albitised plagioclase, biotite, and secondary minerals (chlorite, epidote, amphibole, calcite).

Dikes and dike belts are the ore-controlling structures of gold mineralization. In intersecting zones of a northwestern strike, gold mineralization is concentrated near dikes and gradually fades away as we move from them. The greatest development of mineralized zones and the associated quartz-vein ore mineralization can be observed at the intersection of fault zones with dikes. In this case, ore columns with a thickness of 20-50 m formed, extending to a depth of 3 km. Vladimirskoe deposits belong to vein-dike ore-magmatic systems, their source of ore matter is of deep origin.

This work is supported by RFBR grants: No. 19-05-00764 and the Russian Ministry of Education and Science. 

References:

Gordienko I.V. et al., // Geology Ore Deposits. 2016. V. 58, № 5, P. 405-429.

Seminsky Zh. V. et al. // Proceedings of the Siberian Branch of the Section of Earth Sciences of the Russian Academy of Sciences. T. 45, N. 2. 2014. P. 19-34.

How to cite: Nharara, B., Airiyants, E., Kiseleva, O., Belyanin, D., Roshchektaev, P., and Zhmodik, S.: Ore-controlling dike complexes of the gold-rare metal deposit Vladimirskoe (Eastern Sayan), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14847, https://doi.org/10.5194/egusphere-egu21-14847, 2021.

15:57–15:59
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EGU21-2806
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ECS
Matteo Luca Deidda, Antonio Attardi, Fabrizio Cocco, Dario Fancello, Antonio Funedda, and Stefano Naitza

The Rosas Shear Zone (RSZ) is a 1 km thick brittle-ductile shear zone that outcrops in the Variscan fold and thrust belt foreland of SW Sardinia, where several important ore deposits were mined in the last century. The RSZ lies in the footwall and strikes parallel to the NE-dipping regional thrust that separates the Variscan foreland from the nappe zone. Two thrusts that developed along the limbs of two km-scale overturned antiforms, with NE-dipping axial plane, bound the RSZ. The folds show a SW-facing direction and a well-developed axial plane cleavage, and affect a lower Cambrian-upper Ordovician stratigraphic succession mainly made, from bottom to top, by a sequence about 200 m thick of dolostones and massive limestone followed by 50 m of marly limestones overlain by about 150 m of sandstones, pelites and siltstones, finally unconformable capped by conglomerates and siltstones, ranging in thickness from a few to 200 m. Differently, within the RSZ the bedding is completely transposed along the cleavage and its internal structure is characterized by anastomosing thrusts that affect the stratigraphic succession defining map-scale slices mainly consisting of dolostones and limestones embedded into the siliciclastic formations. It is noteworthy the occurrence of a NE-dipping, up to 100 m thick gabbro-dyke that postdates the deformation phases and that can be related to the exhumation of the chain during late Carboniferous-Permian times.

In the whole area, contact metamorphic and metasomatic processes selectively affected the Cambrian carbonate tectonic slices, originating several skarn-type orebodies. Mineralized rocks display the mineralogical assemblages and textures of Fe-Cu-Zn skarns, with relicts of anhydrous calcic phases related to the prograde metamorphic stage (garnet, clinopyroxene, wollastonite), frequently enclosed in a mass of hydrous silicates (actinolitic amphibole, epidote) and magnetite related to the retrograde metasomatic stage, in turn followed by chlorite, sulfides, quartz and calcite associated to the hydrothermal stage. Metasomatic reactions also involved mafic rocks, producing a mineral association marked by clinopyroxene, amphibole, epidote, prehnite and Ba-rich K-feldspar. Sulfide ores are made of prevailing sphalerite, chalcopyrite and galena, with abundant pyrite and pyrrhotite and minor tetrahedrite and Ag-sulfosalts. Garnets are andraditic/grossularitic, distinctly zoned and optically anisotropic. Field surveys pointed out the tight structural controls on skarn and ore formation. On a local scale, the gabbro emplacement along high- to low-angle NNW-SSE structures bordering the carbonate tectonic slices accentuate the effects of contact metamorphism, and metric to decametric mineralogical zonation (garnet→pyroxene→wollastonite) are recognized. On a larger scale, extensive hydrothermal fluid circulations involved the structures of the RSZ. Infilling of metasomatic fluids in carbonate tectonic slices is fault-controlled and aided by the increase in permeability due to the alteration of prograde silicates. The causative intrusion related to skarn ores belongs to the early Permian (289±1 Ma) ilmenite-series, ferroan granite suite which intrudes the RSZ about 3 km east from the studied area. The Fe-Cu-Zn skarn ores of Rosas are best interpreted as distal, structurally-controlled orebodies, connected to large-scale circulation of granite-related fluids in the km-sized plumbing system represented by the RSZ.

How to cite: Deidda, M. L., Attardi, A., Cocco, F., Fancello, D., Funedda, A., and Naitza, S.: Shear zone development and structurally-controlled skarn ore mineralization in the Rosas district, SW Sardinia., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2806, https://doi.org/10.5194/egusphere-egu21-2806, 2021.

15:59–16:01
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EGU21-1584
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ECS
Nils Reinhardt, Axel Gerdes, Aratz Beranoaguirre, Max Frenzel, Lawrence D. Meinert, Jens Gutzmer, and Mathias Burisch

Europe´s major Sn and W resources are hosted by magmatic-hydrothermal ore deposits of the Variscan Belt: e.g. in Cornwall, the Erzgebirge, the Iberian Massif, and the French Massif Central. In the Erzgebirge, several major skarn bodies are located in the Schwarzenberg district (12 x 15 km). Although recent geochronological data relates (skarn) ore-formation to late- and post-orogenic magmatic-hydrothermal activity, details on the nature and duration of mineralization events remain insufficiently understood.

In this study we present innovative in-situ LA-ICP-MS U-Pb geochronology of garnet from several skarn prospects in the Schwarzenberg district, which is complemented with available geochronological data on intrusions and mineralization in order to constrain the timing of skarn formation within the Variscan orogenic cycle.

Eighteen garnet dates range from 338.2 ± 2.5 to 294 ± 8.3 Ma. Associated errors are in the range of 2.5 to 8.4 Ma, but generally tend to be <7 Ma. The oldest ages (338-331 Ma, stage I) are related to metasomatic garnets of the Globenstein skarn (n=5) – a skarn that is exceptionally enriched in W compared to the other skarn prospects in the same district. Conversely, the other skarns (Antonsthal, Breitenbrunn, Hämmerlein) are younger and range from 327 to 313 Ma (stage II) and 304 to 294 Ma (stage III), respectively. Stage I and II garnets lie within the range of available zircon ages of major intrusive bodies in this area (Aue-Schwarzenberg granite suite: 334-322 Ma; Eibenstock granite: 326-311 Ma). The third stage, in contrast, does not overlap with the age of any known granite intrusions in the Schwarzenberg district. However, it coincides with widespread early Permian volcanic rocks, which presumably have intrusive roots that are not yet exposed in the Erzgebirge region.

The distribution of garnet ages implies that skarn formation occurred episodically during the ~45 Ma life-time of the Variscan orogen, with the onset of magmatic-hydrothermal activity occurring significantly earlier than previously assumed – at 338 Ma, immediately after the peak of regional metamorphism. Tin and W deposits (skarn, greisen and vein-type) seem to have formed episodically over the entire 45 Ma orogenic cycle of the Erzgebirge – this is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal parts of the European Variscan Belt.

How to cite: Reinhardt, N., Gerdes, A., Beranoaguirre, A., Frenzel, M., Meinert, L. D., Gutzmer, J., and Burisch, M.: Constraining the time-span of magmatic-hydrothermal activity in the Variscan Orogenic Belt – U-Pb geochronology of skarn-related garnet from the Schwarzenberg district, Erzgebirge, Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1584, https://doi.org/10.5194/egusphere-egu21-1584, 2021.

16:01–16:03
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EGU21-2137
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ECS
Laura Swinkels, Jan Schulz-Isenbeck, Max Frenzel, Jens Gutzmer, and Mathias Burisch

The Freiberg district, located in the eastern part of the Erzgebirge, Germany, hosts one of the largest series of epithermal polymetallic vein deposits in Europe. The present study aims to decipher mineralogical and geochemical zoning on the vein- and district-scale and to constrain the underlying ore-forming processes. Detailed petrographic investigations, geochemical analyses and fluid inclusion studies are carried out on several vertical vein profiles within the Freiberg district in order to develop a district-scale metallogenic model. Five different mineral associations related to Permian magmatic-hydrothermal activity have been recognized within the Freiberg epithermal vein system exhibiting a distinct district-scale and vein-scale zonation. The central part of the Freiberg district is dominated by sphalerite-pyrite-quartz and galena-quartz±carbonate associations with a mean silver grade of 769 g/t (n=65). Similar base metal-rich assemblages also predominate the deepest vein intersections (>300 m below ground level) in the peripheral sectors of the Freiberg District. Vein infill at intermediate depth and peripheral positions in the district is, in contrast, dominated by a sphalerite-Ag-sulfides-carbonate association. This association is marked by an abundance of carbonate gangue and significantly higher silver grades (mean = 4800 g/t; n=25). Veins in the shallowest and most peripheral parts (depth <150 m b.g.l.) of the Freiberg district are dominated by a Ag-sulfide-quartz association with a mean Ag concentration of 4900 g/t (n= 56). Silver is mainly hosted by sulfosalts and fahlore but significant concentrations may also be associated to Ag-sulfide inclusions in galena. Even shallower, the veins comprise a stibnite-quartz association with distinctly low Ag contents (410 g/t Ag, n=4). Fluid inclusions related to the various associations yield consistent salinities in the range of 0.1 to 6.0 % eq. w(NaCl). The homogenization temperature, however, progressively decreases from about 320°C for quartz associated with proximal sphalerite-pyrite-quartz mineralization, down to ~170°C for quartz related to distal Ag-sulfide-quartz association. The general formation of the Freiberg epithermal veins is related to the continuous evolution of a magmatic-hydrothermal system in time and space. Silver deposition is most likely triggered by boiling and associated cooling and volatile-loss, which results in a distinct carbonate horizon (typically at ~500 m depth b.g.l. for peripheral parts) with significantly elevated Ag grades (sphalerite-Ag-sulfides-carbonate association).

How to cite: Swinkels, L., Schulz-Isenbeck, J., Frenzel, M., Gutzmer, J., and Burisch, M.: Spatiotemporal zonation of the Freiberg Ag-Pb-Zn-(Cu) epithermal system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2137, https://doi.org/10.5194/egusphere-egu21-2137, 2021.

16:03–16:05
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EGU21-3156
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ECS
Marie Guilcher, Richard Albert, Axel Gerdes, Jens Gutzmer, and Mathias Burisch

Hydrothermal Ag-Bi-Co-Ni-As±U (five-element) veins are particularly prevalent across Central Europe, where this type of mineralization has been mined throughout the ages for its high-grade resources of Ag, Co, Ni, and U. The timing and the detailed geodynamic setting in which this style of mineralization formed remains, however, insufficiently understood due to the limited amount of geochronological data. In this contribution, we report the results of innovative LA-ICP-MS U-Pb geochronology on the carbonate gangue of Ag-Bi-Co-Ni-As±U mineralization from six districts in the Erzgebirge/Krušné Hory metallogenic province of Germany and the Czech Republic, with the goal to constrain the timing of ore formation in the context of Central Europe's geodynamic framework.

In-situ U-Pb ages of twelve samples, including dolomite-ankerite, calcite, and siderite cogenetic with Co-Ni-Fe-arsenides, range from 129.4 ± 8.2 to 85.93 ± 3.4 Ma. The ages of Ag-Bi-Co-Ni-As±U and fluorite-barite-Pb-Zn veins from the same occurrence (Annaberg-Buchholz district) overlap each other, suggesting that these two styles of mineralization are genetically related and may form coevally. The compilation of geochronological data from other Ag-Bi-Co-Ni-As±U occurrences in Europe suggests that the origin of this style of mineralization in Central Europe can be related to continental rifting associated with the Mesozoic opening of the Atlantic and/or the Alpine Tethys (200-100 Ma). This provides for the first time evidence for the formation of Ag-Bi-Co-Ni-As±U vein mineralization across Central Europe in response to continental rifting.

How to cite: Guilcher, M., Albert, R., Gerdes, A., Gutzmer, J., and Burisch, M.: LA-ICP-MS U-Pb geochronology of carbonates from Ag-Bi-Co-Ni-As±U veins in the Erzgebirge (Germany and Czech Republic): New insights into the timing of mineralization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3156, https://doi.org/10.5194/egusphere-egu21-3156, 2021.

16:05–16:07
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EGU21-15930
Juan Gómez-Barreiro, Santos Barrios-Sánchez, José Manuel Compaña Prieto, Juan Morales Sánchez-Migallón, Kelvin dos Santos Alves, Magdalena Tettamanti, and Inés Puente Orench

The origin of gold nuggets (Au‐Ag alloys) is not completely understood. They crop out in placer deposits, potentially derived from a primary source (hydrothermal/magmatic). Meteorization, erosion and transport of primary gold deposits result in the liberation of a variety of particle size. Recent investigations suggest that both primary and secondary microstructural features may be preserved and could be related to deformation during transport, recrystallization and primary formation. Besides, the contribution of biological mechanisms (biomineralization) may have played an important role during secondary growth in some nuggets. In many cases, there is no clear evidence to distinguish between supergenic and hypogenic gold, so texture information could be excellent information to constrain the origin. Besides, it has been demonstrated that crystallography controls the de‐alloying processes in gold nuggets. This mechanism, that transforms the primary Au–Ag alloys into pure gold by preferential dissolution of Ag along crystal boundaries, could be determined by variations on texture, a factor never explored before, which may explain the dispersion in de‐alloying values in the same deposit.

 

In this case we have explored a selection of gold nuggets collected in the W sector of the Iberian Massif (Spain), representing the principal morphological types. As a non-destructive technique neutron diffraction appears as the technique of choice in this case. Beside, neutrons absorption is very low so that large samples could be investigated. Samples were analyzed in transmission at ILL (Grenoble) for texture. Quantitative texture and gold crystallinity was calculated using Rietveld method as implemented in Maud software (EWIMV). Mono- and polycrystalline nuggets and alloy composition were clearly identified in each particle with this technique. Our results show a close correlation between the morphology (i.e. transport length) of the particle and the crystallographic results, particularly for fibrous and discoid shapes (i.e. Zingg, Corey shape factor), what could be used to develop better transport models (distance-to-bedrock sources) and understand multisource gold placer assemblages. 

How to cite: Gómez-Barreiro, J., Barrios-Sánchez, S., Compaña Prieto, J. M., Morales Sánchez-Migallón, J., dos Santos Alves, K., Tettamanti, M., and Puente Orench, I.: Quantitative texture analysis of alluvial gold: primary and secondary signatures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15930, https://doi.org/10.5194/egusphere-egu21-15930, 2021.

16:07–16:09
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EGU21-12077
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ECS
Kelvin Dos Santos Alves, Santos Barrios Sánchez, Juan Gómez Barreiro, Raul Merinero Palomares, Rafael Pablo Lozano Fernández, and José Manuel Compaña Prieto

In this contribution we have investigated the Fresnedoso Quaternary gold placer (Western Spain), analyzing the morphotextural and microchemical evolution of gold particles. The statistical analysis has revealed the presence of two populations of particles being consistent with primary sources situated at a distal [20 - 50 km] and a proximal [2.5 - 10 km] range. The gold morphology and chemistry point to a recycling (and potentially undiscovered) Tertiary paleoplacers. The discovery of primary laminar morphologies points to lode deposits in small-flat veins hosted in Precambrian metasediments (Schist Greywacke Complex). All these findings suggest that the Fresnedoso gold deposit is formed by mono and polycyclic particles. We have tested previous transport distance vs Flattening indexes (CFI, Shilo) models resulting in useful framework for exploration of undiscovered ores, even with a small sample dimension. Chemical analysis of the different gold morphologies depicted that the Fresnedoso gold is a AuAg bimetallic alloy. Three groups were identified based on the texture and composition of the gold particles: Type 1 (Au1= Au89-94Ag11-6), Type 2 (Au2= Au99 Ag1) and Type 3 (Au3~ Au >99). Particle's cores (gold Type 1) show a compositional range that could be interpreted as differences in primary sources, spatial dispersion of sources or the actuation of secondary processes, probably in an orogenic gold context. Microchemical heterogeneity in the particles is probably due to secondary processes. A conceptual model has been elaborated to explain particle's microchemical domains represented by gold Type 2 (rim) and Type 3 (micro-aggregates) as the result of two different de-alloying stages: A) initial Ag-leaching at the rim and/or through microcracks and grainboundaries (Type 2), B) Total reset of the primary chemical fingerprint, with porous microtexture and the precipitation of gold with iron oxyhydroxides and clays (Type 3). This model suggests a silver de-alloying mechanism favored in a chlorine-iron-rich environment as in the case of laterites. Deformation and eventually recrystallization mechanisms associated with the fluvial transport (mechanical cold-work), cooperated in the evolution of the particles (Dos Santos et al. 2020).

References 

Dos Santos Alves, K., Barrios Sánchez, S., Gómez Barreiro, J., Merinero Palomares,R. and Compaña Prieto, J.M. (2020). "Morphological and compositional analysis of alluvial gold: The Fresnedoso gold placer (Spain)." Ore Geology Reviews : 103489.

How to cite: Dos Santos Alves, K., Barrios Sánchez, S., Gómez Barreiro, J., Merinero Palomares, R., Pablo Lozano Fernández, R., and Manuel Compaña Prieto, J.: Primary and Secondary Gold Sources of Quaternary Placers of Western Spain: a Morphotextural and Compositional Analysis of the Fresnedoso Deposit., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12077, https://doi.org/10.5194/egusphere-egu21-12077, 2021.

16:09–16:11
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EGU21-13358
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Highlight
Santos Barrios, Juan Gómez Barreiro, Rafael Pablo Lozano Fernández, Raúl Merinero Palomares, Mercedes Suárez Barrios, Kelvin dos Santos Alves, Juan Morales Sánchez-Migallón, and Jeremi Malecki

Gold placers are abundant and intensively surveyed in western Iberia since antiquity. Three Cenozoic gold placers covering Neoproterozoic-Lower Paleozoic basement rocks have recently been revealed, which stand out for the number and size of the samples recovered: Salvatierra de Tormes (ST), Santibáñez el Alto (SA) and Casas de Don Pedro-Talarrubias (CSDP). To date, primary sources remain undiscovered. We have combined microchemical, inclusion analysis and morphology of gold nuggets to define the placer gold signature and its relationship with bedrock primary sources and infer mineralization styles. Coarse gold particles prevent secondary resetting of source signature and increase the chances to investigate mineral assemblages. Nuggets morphology analysis have reveled that ST and SA deposit are fluvial "trunk" placers, while CSDP represents an autochthon or colluvial placer type. Four different types of gold have been defined in nuggets: core gold (T1), rim gold (T2) fine grained gold in Fe-oxyhydroxides aggregates (T3) and "mustard" gold (Au+Sb-Pb-Fe-oxides) (T4).

Based on those categories we have explored primary and secondary signatures in the deposits. Lode signature is observed in the core of nuggets (T1) with a fineness between 800 and 1000. Alloy composition ranges from binary (Au:Ag) in SA to ternary (Au:Ag:Cu) in ST and CSDP. Sulphides and sulfarsenides dominates inclusions association in ST, while Sb- and Sb-Pb-Fe phases appear in ST and CSDP respectively. CSDP primary gold shows a distinct Hg content. The identification of mineral phases non-compatible with supergene conditions in gold and textural remnants of annealing microstructures, point to an hypogenic origin of T1 in all deposits and coul be compatible with a mesothermal system (<400ºC) in which, CSDP represents the higher T and SA the lower T end. A hybrid hydrothermal-magmatic system is proposed.

Secondary signature is complex and reveals several stages. Older evidence of in-situ modification of primary gold was found in CSDP gold-bearing quart fragments, with pervasive alteration under oxidizing and alkaline conditions. This process liberated gold from T1 and primary phases (e.g., aurostibite), leading to the formation of auroatimonades and "mustard" gold (T4), showing a complex textural pattern. Gold particles were subsequently modified during fluvial transport and deposition through the interaction with fluids, which activated Ag-leaching processes, resulting in the development of gold-rich rims (T2). Partial dissolution and re-precipitation of gold in reduction conditions formed very fineness gold particles embedded in Fe oxy-hydroxides (T3).

How to cite: Barrios, S., Gómez Barreiro, J., Lozano Fernández, R. P., Merinero Palomares, R., Suárez Barrios, M., dos Santos Alves, K., Morales Sánchez-Migallón, J., and Malecki, J.: Tracing gold nuggets back to the source: a microchemical analysis of tertiary gold placers in Central Spain., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13358, https://doi.org/10.5194/egusphere-egu21-13358, 2021.

16:11–16:13
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EGU21-905
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ECS
Dominik Teskera, Hana Fajković, Nikolina Ilijanić, Nenad Tomašić, Nikola Gizdavec, Čobić Andrea, and Miko Slobodan

High demand for specific chemical elements from the group of rare earth elements (REE) has led to a detailed prospection and geochemical analysis of a previously known but unexploited bauxite deposit. The Upper Eocene karst bauxite within exploitation field Mamutovac, located in the municipality of Promina in the Dalmatian Hinterland (Croatia), is such a deposit, with estimated reserves of 112,000 tone.

In order to determine REE distribution pattern in the Mamutovac Ia deposit, a 25meter core was obtained by exploration drilling, down to the deposit footwall that is composed of Upper Cretaceous rudist limestone. For this study 23 subsamples were singled out, on average, per each meter of a core. 

The degree of lateritization is determined by the Al2O3–SiO2–Fe2O3 composition diagram (after Schellman, 1986), and lateritization varies from moderate to strong, with a lower degree of lateritization in a lower part of the core, down from 15 m.  Two different genetic classification systems indicate the origin of the bauxite is mafic, basaltic igneous rocks.            

Main mineral phases in the bauxite core samples were determined using X-ray powder diffraction (XRPD) analysis. The mineral phases through the whole core are similar, with boehmite, gibbsite, hematite, and anatase as the main phases. Additional mineral phases determined in the core are kaolinite, goethite, and rutile. 

Results of geochemical analysis obtained by inductively coupled plasma emission/mass spectrometry (ICP-ES/MS) indicate an inhomogeneous distribution of REE through the core, with two main trends: from 0-15m and from 15-25m, with some elevation of REE abundances in the lower part of the core. In the upper part of the core,  total REE content (∑REE), including Y and Sc, ranges between 352 and 630 ppm (average 500 ppm) and light REE (La-Sm) to heavy REE (Eu-Lu) (∑LREE/∑HREE) ratios reach up to 10.2. For lower part ∑REE (including Y and Sc) ranges between 569 and 813 ppm (average 676 ppm) and light REE (La-Sm) to heavy REE (Eu-Lu) (∑LREE/∑HREE) ratios are up to 9.82. Singificant enrichment of LREE compared to HREE is present due to the fact that HREE are highly mobile in an alkalic karst environment and consequently removed through drainage channels. The most abundant REE is Ce. Within interval, 0-15m Ce ranges between 149.3-264.9 ppm (average 210.7 ppm), while within the interval 15-25m Ce ranges 152.7-301.7 ppm (average 219.56 ppm).

Correlation analysis shows no correlation between Sc and other REE and no significant correlation between Ce and other REE or potential bearing oxides. The correlation between Sc and Al2O3 or Fe2O3 suggests that Sc is likely bound to Al-oxyhydroxides and Fe-oxyhydroxides. Correlation between REE (Sc free) and P2O5 indicates REE (Sc free) are probably contained in REE-bearing phosphates. 

This activity has received funding from the European Institute of Innovation and Technology (EIT), a body of the European Union, under the Horizon 2020, the EU Framework Programme and Research and Innovation.

How to cite: Teskera, D., Fajković, H., Ilijanić, N., Tomašić, N., Gizdavec, N., Andrea, Č., and Slobodan, M.: Geochemical characteristics of Croatian prospective Bauxite deposit Mamutovac, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-905, https://doi.org/10.5194/egusphere-egu21-905, 2021.

16:13–16:15
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EGU21-11122
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ECS
Hatice Nur Bayram, Ali Erdem Bakkalbasi, Zeynep Doner, Ali Tugcan Unluer, Huseyın Kocaturk, Demet Yıldırım, and Murat Budakoğlu

Mediterranean type karstic bauxite deposits are considered as the primary source for aluminum (Al) production in Europe. During the Al production, Gallium (Ga) is extracted from the so called Bayer-liquor during the processing of bauxite to alumina. Ga, a rare metal, is widely used in modern chemistry and electronic industry. During the past decades, the worldwide demand for Ga has been continuously increasing. In Turkey, karstic bauxite deposits are generally found with shallow marine carbonate rocks which were deposited during Mesozoic period and located in Tauride Carbonate platform. Most of these karstic bauxite deposits can be hosted considerable Ga enrichments, with other immobile elements such as rare earth elements (REE), titanium (Ti), lithium (Li), and iron (Fe). This work focuses on the revealing of the potential Ga enrichments in bauxides from different deposits of Turkey (Mortaş-Doğankuzu, Konya; Küçükkoraş, Karaman; Acıelma-Yoğunoluk, Kahramanmaraş bauxite deposits). Geochemical data of major and trace elements of studied bauxite deposits show that these deposits have significant Ga enrichments (up to 72.6 ppm), as well as the REE (up to 580 ppm), Ti (up to 1.8%), and Li (up to 428 ppm) enrichments. In addition, the Ga enrichments show strong positive correlation with heavy rare earth elements (HREE) and moderate positive correlation with Al, Fe, Ti, Li and Sn elements. In this context, it can be concluded that the most probable source for Ga is rock forming aluminosilicates of the source rock due to the substitution with Al3+ and Fe3+. During weathering process Ga exhibiting immobile behavior much like Al and Fe. Gallium is than incorporated into Al-bearing phases and thus enriched in the bauxite. Presence of Li content can be also interpreted as a contribution from micaceous source such as meta-carbonate rocks of Tauride platform. Moreover, geochemical association between Ga, Ti, Li, tin (Sn) and HREE can be explained by the redox and pH conditions causing other ions seperated from shallow environments.

How to cite: Bayram, H. N., Bakkalbasi, A. E., Doner, Z., Unluer, A. T., Kocaturk, H., Yıldırım, D., and Budakoğlu, M.: Tauride Carbonate Platform Bauxite Deposits (Turkey) as an Alternative Source for Gallium, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11122, https://doi.org/10.5194/egusphere-egu21-11122, 2021.

16:15–17:00