Supergene features and evolution of gossans capping massive sulphide deposits in the Iberian Pyrite Belt

Velasco, Francisco; Herrero, J. M.; Suárez, Saioa; Yusta, Iñaki; Alvaro, A.; Tornos, Fernando

doi:10.1016/j.oregeorev.2013.01.008

Cited by 71 publications

(65 citation statements)

References 39 publications

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“…In addition, although little is known about the Au supergene mineralization in this location, the features observed in the supergene Au ore at Las Cruces are inconsistent with those previously reported for the gossans (Williams, 1950;Arribas, 1998;Capitán et al, 2003;Capitán, 2006;Velasco et al, 2013). The main differences lie in Au grade, Au distribution pattern through the supergene profile, Au-bearing lithologies, and size of Au particles and their fineness.…”

Section: Introductioncontrasting

confidence: 55%

“…The original resources of gossan at Cerro Colorado (Riotinto) exceeded 100 Mt @ 1 g/t Au and 56 g/t Ag, and the gossan at Filón-Sur (Tharsis) contained 15.5 Mt @ 1.7 g/t Au and 29 g/t Ag (Velasco et al, 2013). Previous research on the Iberian Pyrite Belt Au-rich gossans has mainly focused on metallurgy and exploitation of these ores (García Palomero et al, 1986;Viñals et al, 1995;Sánchez et al, 1996;Roca et al, 1999), whereas little attention has been paid to the geologic features and genetic models of this type of weathering-related Au deposit.…”

Section: Introductionmentioning

confidence: 99%

“…However, Williams (1950) and Arribas (1998) reported Au-rich ores in Riotinto that mainly occur near the boundary between unweathered massive sulfides and oxidized sulfides, whereas in gossan of Filón-Sur (Tharsis), the main secondary Au enrichment occurs at intermediate levels associated with goethite (Capitan et al, 2003(Capitan et al, , 2006. Recently, Velasco et al (2013) reported that in the gossan deposits of the Iberian Pyrite Belt, there is scarce native free Au, which is very fine grained and of low fineness.…”

Section: Introductionmentioning

confidence: 99%

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Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

Yesares¹,

Aiglsperger²,

Sáez³

et al. 2015

Economic Geology

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The Las Cruces deposit is in the eastern end of the Iberian Pyrite Belt (SW Spain). It is currently being mined by Cobre Las Cruces S.A. The main operation is focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). An Au-Ag-Pb-rich gossan resource (3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag) occurs in the upper part of the deposit. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au mineralization in the upper part of the gossan linked to Fe-oxides lithofacies, (2) Au concentration in the lower part of the gossan associated with leached black shales, and (3) Au ore in the cementation zone related to subvertical fractures.A hydroseparation device has been used to obtain heavy mineral concentrates from selected samples of different ore types. Reflected-light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA) were used to study the separated Au particles. Significant differences between the defined ore types include the Au-bearing lithologies, mineral associations, textural features, particle sizes, morphologies, and fineness. Au-rich minerals include native Au, Au-Ag electrum, and Au-Ag-Hg amalgams. Gold-bearing mineral associations include Pb-oxihalides, Fe-oxides, galena, pyrite, cinnabar, and Ag-sulfosalts.The Au enrichment mechanism in the supergene profile involves (1) dissolution of Au from the primary sulfides as chloride-rich ionic complexes during the weathering of the deposit under subaerial exposure; dissolved Au is transported downward through the supergene profile under acidic and oxidized conditions; (2) destabilization of the Au complexes by Fe-controlled redox reactions; as a consequence, coarse-grained, high-fineness Au particles precipitated in association with Fe-oxyhydroxides. This resulted in secondary concentration in the upper gossan; and (3) after deposition of cover sediments took place a progressive change in the system conditions resulting in a later Au remobilization as hydroxidehalide, hydroxide, thiosulfate, and bisulfide complexes in the lowermost gossan and cementation zone. The main pathways for migration of enriched fluids to the cementation zone are secondary permeability zones linked to Alpine reactivated faults. Deposition of Au seems to be related to fluid interaction with reductant lithologies, including black shales and the primary sulfides.

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Section: Introductioncontrasting

confidence: 55%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

Yesares¹,

Aiglsperger²,

Sáez³

et al. 2015

Economic Geology

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“…These caps can range from meters to tens of meters in thickness and are known to contain elevated concentrations of gold and silver that were exploited during Roman times. More recently in mining history, gossans were used as indicators for targeting the primary ore bodies below the surface [37][38][39][40][41]. The San Telmo, San Miguel and Tharsis mines (near Huelva, Spain) once held important economic metals reserves including Ag and Au ( Figure 1A) [42].…”

Section: Study Sites and Sample Acquisitionmentioning

confidence: 99%

Biogeochemical Cycling of Silver in Acidic, Weathering Environments

et al. 2017

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Under acidic, weathering conditions, silver (Ag) is considered to be highly mobile and can be dispersed within near-surface environments. In this study, a range of regolith materials were sampled from three abandoned open pit mines located in the Iberian Pyrite Belt, Spain. Samples were analyzed for Ag mineralogy, content, and distribution using micro-analytical techniques and high-resolution electron microscopy. While Ag concentrations were variable within these materials, elevated Ag concentrations occurred in gossans. The detection of Ag within younger regolith materials, i.e., terrace iron formations and mine soils, indicated that Ag cycling was a continuous process. Microbial microfossils were observed within crevices of gossan and their presence highlights the preservation of mineralized cells and the potential for biogeochemical processes contributing to metal mobility in the rock record. An acidophilic, iron-oxidizing microbial consortium was enriched from terrace iron formations. When the microbial consortium was exposed to dissolved Ag, more than 90% of Ag precipitated out of solution as argentojarosite. In terms of biogeochemical Ag cycling, this demonstrates that Ag re-precipitation processes may occur rapidly in comparison to Ag dissolution processes. The kinetics of Ag mobility was estimated for each type of regolith material. Gossans represented 0.6-146.7 years of biogeochemical Ag cycling while terrace iron formation and mine soils represented 1.9-42.7 years and 0.7-1.6 years of Ag biogeochemical cycling, respectively. Biogeochemical processes were interpreted from the chemical and structural characterization of regolith material and demonstrated that Ag can be highly dispersed throughout an acidic, weathering environment.

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“…At slightly less acidic pH vales, the precipitation of Fe(III) minerals can drive forward the production of more acidity (Baker and Banfield, 2003;Nordstrom, 2003) and precipitate a variety of Fe(III) minerals. Poorly crystalline iron-oxide phases precipitate first, such as hydrous ferric oxides (HFOs), ferrihydrite ((Fe 3 + ) 2 O 3 $0.5H 2 O), schwertmannite (Fe 8 O 8 (OH) 6 SO 4 ), and jarosite group minerals (MFe 3 (OH) 6 (SO 4 ) 2 ), where M is a cation, most commonly K + but also possibly Na + or H 3 O + (Velasco et al, 2013). Several of these will transform to more ordered crystalline phases, such as ferric oxide (hematite, Fe 2 O 3 ), goethite (aFeO(OH)), and lepidocrocite (c-FeO(OH)).…”

Section: Introductionmentioning

confidence: 99%

Preserved Filamentous Microbial Biosignatures in the Brick Flat Gossan, Iron Mountain, California

et al. 2015

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A variety of actively precipitating mineral environments preserve morphological evidence of microbial biosignatures. One such environment with preserved microbial biosignatures is the oxidized portion of a massive sulfide deposit, or gossan, such as that at Iron Mountain, California. This gossan may serve as a mineralogical analogue to some ancient martian environments due to the presence of oxidized iron and sulfate species, and minerals that only form in acidic aqueous conditions, in both environments. Evaluating the potential biogenicity of cryptic textures in such martian gossans requires an understanding of how microbial textures form biosignatures on Earth. The iron-oxide-dominated composition and morphology of terrestrial, nonbranching filamentous microbial biosignatures may be distinctive of the underlying formation and preservation processes.The Iron Mountain gossan consists primarily of ferric oxide (hematite), hydrous ferric oxide (HFO, predominantly goethite), and jarosite group minerals, categorized into in situ gossan, and remobilized iron deposits. We interpret HFO filaments, found in both gossan types, as HFO-mineralized microbial filaments based in part on (1) the presence of preserved central filament lumina in smooth HFO mineral filaments that are likely molds of microbial filaments, (2) mineral filament formation in actively precipitating iron-oxide environments, (3) high degrees of mineral filament bending consistent with a flexible microbial filament template, and (4) the presence of bare microbial filaments on gossan rocks. Individual HFO filaments are below the resolution of the Mars Curiosity and Mars 2020 rover cameras, but sinuous filaments forming macroscopic matlike textures are resolvable. If present on Mars, available cameras may resolve these features identified as similar to terrestrial HFO filaments and allow subsequent evaluation for their biogenicity by synthesizing geochemical, mineralogical, and morphological analyses. Sinuous biogenic filaments could be preserved on Mars in an iron-rich environment analogous to Iron Mountain, with the Pahrump Hills region and Hematite Ridge in Gale Crater as tentative possibilities.

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Supergene features and evolution of gossans capping massive sulphide deposits in the Iberian Pyrite Belt

Cited by 71 publications

References 39 publications

Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

Biogeochemical Cycling of Silver in Acidic, Weathering Environments

Preserved Filamentous Microbial Biosignatures in the Brick Flat Gossan, Iron Mountain, California

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