Fernando Tornos

Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile

We propose an integrated model that explains the magmatic and hydrothermal features of the unique El Laco iron deposit that is located in a Pliocene–Pleistocene volcano of the Chilean Andes. (Sub)volcanic crystallization of an iron-rich melt as massive magnetite promoted the exsolution of a small volume of a hydrosaline melt and of large amounts of vapor that led to the formation of an alkali-calcic hydrothermal assemblage replacing the host andesite; this assemblage is capped and overprinted by a large zone of acid-sulfate steam-heated alteration forming as a whole a protracted shallow-level magmatic-hydrothermal system. Oxygen isotopic data for the massive magnetite (δ18O: 4.3‰–5.0‰) and the alkali-calcic altered rock (diopside δ18O: 7.2‰–8.7‰; magnetite δ18O: 4.4‰–6.7‰) suggest that these rocks are genetically related to the host andesite (δ18O: 7.4‰–9.6‰). The estimated temperature of the mineral assemblage (>∼900 °C) may be the highest recorded in hydrothermal systems, is attributed to exsolution of fluids from the crystallizing iron-rich melts, and is considered unlikely to reflect the convection of surficial fluids.

Specific jarosite biomineralization by Purpureocillium lilacinum, an acidophilic fungi isolated from Río Tinto

Río Tinto (Huelva, southwestern Spain) is an extreme environment with a remarkably constant acidic pH and a high concentration of heavy metals, conditions generated by the metabolic activity of chemolithotrophic microorganisms thriving in the rich complex sulfides of the Iberian Pyrite Belt (IPB). Fungal strains isolated from the Tinto basin were characterized morphologically and phylogenetically. The strain identified as Purpureocillium lilacinum specifically induced the formation of a yellow-ocher precipitate, identified as hydronium-jarosite, an iron sulfate mineral which appears in abundance on the banks of Río Tinto. The biomineral was characterized by X-ray diffraction (XRD) and its formation was observed with high-resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled to energy-dispersive X-ray spectroscopy (EDX) microanalysis. Jarosite began to nucleate on the fungal cell wall, associated to the EPS, due to a local increase in the Fe(3+) /Fe(2+) ratio which generated supersaturation. Its formation has been also observed in non-viable cells, although with much less efficiency. The occurrence of P.u2009lilacinum in an ecosystem with high concentrations of ferric iron and sulfates such as Río Tinto suggests that it could participate in the process of jarosite precipitation, helping to shape and control the geochemical properties of this environment.

Zinc and sulfur isotope variation in sphalerite from carbonate-hosted zinc deposits, Cantabria, Spain

We studied zinc and sulfur isotopes and the chemical composition of sphalerite samples from Picos de Europa (Aliva mine) and sphalerite and hydrozincite samples from La Florida mine, two carbonate-hosted Mississippi Valley-type (MVT) deposits located in northern Spain; despite being close, they are hosted in carbonatic rocks of different ages, Lower Carboniferous and Lower Cretaceous, respectively. The two generations of sphalerite at Picos de Europa show different δ66Zn values (stage 1 sphalerite +0.24 per mil and stage 2 sphalerite from −0.75 to +0.08 per mil). Both generations also differ in the sulfur isotope composition (stage 1 has δ34Su2009=u2009+6.6 and stage 2 has δ34Su2009=u2009−0.9 to +2.9 per mil) and the chemical composition (stage 1 sphalerite, compared to stage 2 sphalerite, is significantly enriched in Pb, As, Mn, Sb, slightly enriched in Ag, Ni, and Cu and depleted in Co, Ga, Tl, Te, Ge, and Sn). We suggest that Zn isotope fractionation was controlled predominantly by pH and T changes. High Zn isotope values reflect rapid precipitation of sphalerite from higher-temperature acidic fluids that carried Zn mostly as chloride species after interaction with carbonate rocks while lower Zn isotope values most likely resulted from a longer precipitation process from fluid at higher pH and decreasing T that carried dominantly Zn sulfide species. At La Florida, sphalerite samples show light 66Zn-depleted signatures with δ66Zn values from −0.80 to −0.01 per mil (mostly between −0.80 and −0.24 per mil) and δ34S values from +10.7 to +15.7 per mil without any relationship between the δ66Zn and δ34S values. Here, the variation in Zn isotope values is interpreted as related to mixing of fluids from two reservoirs. The Zn was carried by a single deep-seated and higher T (~250–320xa0°C) fluid, and precipitation took place after mixing with a connate S-rich fluid in a system with mH2Su2009>u2009mZn2+ as a result of change in pH, T, and Zn predominant species. The light δ66Zn accompanied by heavy δ34S values resulted from fractionation of Zn aqueous sulfides at near-neutral pH and decreasing T. Hydrozincite samples show much heavier δ66Zn values (+0.21 to +0.33 per mil), consistent with fractionation during supergene processes.

Formation of recent Pb-Ag-Au mineralization by potential sub-surface microbial activity

Las Cruces is a base-metal deposit in the Iberian Pyrite Belt, one of the worlds best-known ore provinces. Here we report the occurrence of major Pb-Ag-Au mineralization resulting from recent sub-surface replacement of supergene oxyhydroxides by carbonate and sulphide minerals. This is probably the largest documented occurrence of recent microbial activity producing an ore assemblage previously unknown in supergene mineralizing environments. The presence of microbial features in the sulphides suggests that these may be the first-described natural bacteriomorphs of galena. The low δ(13)C values of the carbonate minerals indicate formation by deep anaerobic microbial processes. Sulphur isotope values of sulphides are interpreted here as reflecting microbial reduction in a system impoverished in sulphate. We suggest that biogenic activity has produced around 3.1 × 10(9)u2009moles of reduced sulphur and 10(10)u2009moles of CO2, promoting the formation of ca. 1.19u2009Mt of carbonates, 114,000u2009t of galena, 638u2009t of silver sulphides and 6.5u2009t of gold.

Metamorphic degassing of carbonates in the contact aureole of the Aguablanca Cu–Ni–PGE deposit, Spain

AbstractAnalysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic-metamorphic processes may be linked to global climate change and mass extinctions. Aguablanca, one of the largest Cu–Ni–PGE deposits in Europe, formed during the Variscan orogeny when a mafic magma intruded limestones and shales, creating a contact aureole composed of marble, skarn and hornfels. Our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit: The assimilation of terrigenous sediments supplied S to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma. The metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay. We modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature. The presence of clay, however, leads to the formation of calc-silicates and significantly enhances CO2 degassing. Our estimations suggest that degassing of the Aguablanca contact aureole released about 74.8xa0Mt of CO2, a relatively low volume that we attribute to the composition of the host rock, mainly a pure limestone. A far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of Panzhihua (part of Emeishan large igneous province, SW China). We propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces.n

Mineral chemistry of magnetite from magnetite-apatite mineralization and their host rocks: examples from Kiruna, Sweden, and El Laco, Chile

Interpretation of the mineralizing environment of magnetite-apatite deposits remains controversial with theories that include a hydrothermal or magmatic origin or a combination of those two processes. To address this controversy, we have analyzed the trace element content of magnetite from precisely known geographic locations and geologic environments from the Precambrian magnetite-apatite ore and host rocks in Kiruna, Sweden, and the Pliocene-Holocene El Laco volcano in the Atacama desert of Chile. Magnetite samples from Kiruna have low trace element concentrations with little chemical variation between the ore, host, and related intrusive rocks. Magnetite from andesite at El Laco, and dacite from the nearby Láscar volcano, has high trace element concentrations typical of magmatic magnetite. El Laco ore magnetite have low trace element concentrations and displays growth zoning in incompatible elements (Si, Ca, and Ce), compatible elements (Mg, Al, and Mn), large-ion lithophile element (Sr), and high field strength element (Y, Nb, and Th). The El Laco ore magnetite are similar in composition to magnetite that has been previously interpreted to have crystallized from hydrothermal fluids; however, there is a significant difference in the internal zoning patterns. At El Laco, each zoned element is either enriched or depleted in the same layers, suggesting the magnetite crystallized from a volatile-rich, iron-oxide melt. In general, the compositions of magnetite from these two deposits plot in very wide fields that are not restricted to the proposed fields in published discriminant diagrams. This suggests that the use of these diagrams and genetic models based on them should be used with caution.

Genetic constraints from paleomagnetic dating for the Aliva zinc-lead deposit, Picos de Europa Unit, northern Spain

Aliva has been the most productive Zn–Pb mine in the Picos de Europa Mississippi Valley-type (MVT) district of Spain’s Cantabrian Zone. The mineralisation is hosted in a Variscan thrust sheet by strata of the Late Carboniferous (~309u2009±u20093xa0Ma) Picos de Europa limestone–shale formation. Paleomagnetic and rock magnetic analyses of 194 specimens from 23 sites show that the sphalerite–galena–dolomite ore carries a stable characteristic remanent magnetisation (ChRM) in magnetite–titanomagnetite with minor pyrrhotite. Uncorrected for bedding tilt, the ChRM’s direction is declination (D)u2009=u2009347.2°, inclination (I)u2009=u200961.8° (Nu2009=u200920 sites, ku2009=u200975.4, α95u2009=u20094.3°). A negative paleomagnetic fold test shows that the MVT mineralisation is in one or more carbonate olistoliths of the Picos de Europa Formation in shales of the overlying Late Carboniferous Lebeña Formation and that the ChRM entirely postdates Variscan orogenic deformation. No plausible tilt correction of the ChRM’s paleopole supports a previously proposed late Variscan (Permian–Triassic) age for the genesis of the MVT mineralisation at Aliva. The paleopole does support an age of 112u2009±u20098xa0Ma for the hydrothermal dolomitisation and MVT mineralisation event with a subsequent increase in bedding tilt by ~10° NNE during the Oligocene–Miocene Pyrenean (Alpine) Orogeny. The 112u2009±u20098xa0Ma Aptian–Albian age ties ore genesis to the 35°u2009±u20092 counterclockwise rotation of the Iberian peninsula relative to stable Europe and strongly favours an origin in the rift’s flank associated with rift hydrothermal activity.

The high-grade Las Cruces copper deposit, Spain: a product of secondary enrichment in an evolving basin

The Las Cruces deposit (Iberian Pyrite Belt) includes a large, high-grade cementation zone capped by unusual rocks that contain carbonates, galena, iron sulphides, and quartz. Between the Late Cretaceous(?) and Tortonian, the volcanogenic massive sulphides were exhumed and affected by subaerial oxidation that formed paired cementation and gossan zones. Onset of Alpine extension produced accelerated growth of the cementation zone along extensional faults, leading to formation of the high-grade copper ore at ca. 11xa0Ma. Later, replacement of the overlying gossan by sulphide- and carbonate-rich rocks beneath sealing marl sediments is thought to have involved microbial processes, occurring between the Messinian (ca. 7.2xa0Ma) and today. Isotope data show that the cementation zone formed by the mixing of descending acidic waters derived from oxidation of the massive sulphides, with upwelling geothermal waters flowing at temperatures above 100xa0°C. The C, O, and Sr isotope values of the mineralization (87Sr/86Sr 0.7101–0.7104) and of the local groundwater (0.7102–0.7104) reflect equilibration with basement rocks, and indicate that influence on the ore-forming process by marl-equilibrated water (0.7091–0.7093) or Miocene seawater (0.7086–0.7092) was negligible. The high sulphur isotope values of the sulphides in the biogenic zone (most +19 to +24u2009‰) are well above those of the primary sulphides (δ34S ca. −6.8 to +10.3u2009‰) and likely reflect formation of the biogenic sulphides by reduction of aqueous sulphate in the groundwaters. Sulphur isotope values of the cementation zone (δ34S ca. −2.4 to +21.7u2009‰) are also consistent with some contribution of sulphur from the biogenic reduction of aqueous sulphate.

Geology of El Domo deposit in central Ecuador: a VMS formed on top of an accreted margin

El Domo is a Cu-Au-Zn-Ag volcanogenic massive sulfide deposit (VMS) within the Paleocene–Eocene submarine arc rocks of the Macuchi Formation in central Ecuador. The mineralization includes massive and semi-massive sulfides located at the interface between a submarine rhyodacite dome complex and overlying mafic volcanic and volcaniclastic rocks, including chaotic and unsorted debris flow deposits. Whole rock geochemical analyses show that the felsic rocks belong to a calc–alkaline sequence, whereas the mafic assemblages have a tholeiitic affinity, reflecting a magmatic association typically found in many VMS districts. This geochemical change temporally coincides with the formation of the massive sulfides. New zircon Hf isotopic data collected from a hanging wall rhyodacite suggest that the presence of an over-thickened crust of oceanic plateau affinity may have influenced the composition of the magmatic rocks. The timing of massive sulfide formation is constrained by an 40Ar/39Ar age of 41.49u2009±u20090.37xa0Ma (2σ) from the volcaniclastic sequence which hosts the massive sulfides and a U/Pb zircon age date of 42.13u2009±u20090.54xa0Ma (2σ) from the footwall rhyodacite; both ages are interpreted to provide minimum and maximum ages for mineralization. The massive sulfides are zoned and contain variable proportions of pyrite, sphalerite, and chalcopyrite. Minor accumulations of galena, bornite, tennantite, stromeyerite, and proustite are also present; the latter two minerals are the major carriers of silver whereas gold typically occurs as minute-free grains or in solid solution with silver. Ore minerals are intergrown with variable proportions of chlorite, barite, quartz, anhydrite, and smectite. The footwall rhyodacite hosts an irregular stringer zone which is rich in pyrite and gypsum and is characterized by intense quartz-sericite-pyrite alteration. The hanging wall volcaniclastic lithologies exhibit pervasive but vertically limited (8–10xa0m) silicification, chloritization, and argillization. Both felsic and mafic glass-rich rocks located near the contact between rhyodacites and mafic volcaniclastic rocks are replaced by massive to disseminated mineralization, likely controlled by their originally reactive and porous nature. Replacement appears to have taken place at very shallow depths, 10–20xa0m below the seafloor, as shown by: (1) the presence of massive sulfide clasts within the uppermost volcaniclastic horizons and (2) the absence of mineralization and alteration in the overlying debris flows. No evidence of exhalation on the seafloor has been found, and replacement below the sea floor was probably an efficient mechanism for preserving the VMS within such a highly dynamic volcanic system. Regionally, mineralization occurred within an extensional submarine arc setting whereby massive sulfides were preferentially controlled by the reactivation of NNE-SSW crustal-scale strike slip faults created during an earlier plate collision. Local stress regimes resulted in the formation of pull-apart basins within a regional transpressive setting and provided conduits for magmatic and hydrothermal discharge. The onset of volcanism and local extension coincided with an increase in the oblique convergence of the Farallon and the South American Plate.

Modelling of the Río Tinto Area

The Rio Tinto project area is located in the South Portuguese Zone, in the eastern part of the Iberian Pyrite Belt. The Iberian Pyrite Belt (IPB) is one of the world’s best-known ore provinces hosting volcanogenic massive sulphide deposit, formed in the latest Famennian (ca. 360 Ma) and subsequently folded and metamorphosed during the Variscan orogeny (330–300 Ma). The study area is located in the Rio Tinto syncline, with Carboniferous metasediments (Culm) in its core. The volcanic sedimentary complex (VSC) is overthrusted in the central part of the syncline forming the Rio Tinto anticline outcrop (an antiformal stack). The aim of this work was to construct a 3D geological model of the Rio Tinto mine area. To achieve this data compilation has been done including new geological mapping and structural interpretations, petrological and petrophysical sampling, drill hole logging, and geophysical data interpretation (gravimetric, magnetic and radiometric data). Complex surfaces were constructed using large data sets analysed by suitable geometrical techniques. The obtained 3D model shows the relationships between several lithologies, tectonic surfaces and mineralization zones, and is an example of reconstruction of complex geological units within the Iberian Pyrite Belt. In addition, in the Rio Tinto area it was possible to derive a predictive model defining four areas of high ore potential based on detailed geological field work, fracture analyses and geophysical studies related to the possible presence of massive sulphides and stockwork zones.