Fernando Barra

Giant Kiruna-type deposits form by efficient flotation of magmatic magnetite suspensions

Kiruna-type iron oxide-apatite (IOA) deposits are an important source of Fe ore, and two radically different processes are being actively investigated for their origin. One hypothesis invokes direct crystallization of immiscible Fe-rich melt that separated from a parent silicate magma, while the other hypothesis invokes deposition of Fe-oxides from hydrothermal fluids of either magmatic or crustal origin. Here, we present a new model based on Fe and O stable isotopes and trace and major element geochemistry data of magnetite from the ~350 Mt Fe Los Colorados IOA deposit in the Chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits. We propose that concentration of magnetite takes place by the preferred wetting of magnetite, followed by buoyant segregation of these earlyformed magmatic magnetite-bubble pairs, which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults. Our data demonstrate an unambiguous magmatic origin, consistent with the namesake IOA analogue in the Kiruna district, Sweden. Further, our model explains the observed coexisting purely magmatic and hydrothermal-magmatic features and allows a genetic connection between Kiruna-type IOA and iron oxide-copper-gold deposits, contributing to a global understanding valuable to exploration efforts.

Timing and formation of porphyry Cu-Mo mineralization in the Chuquicamata district, northern Chile: new constraints from the Toki cluster

The recently discovered Toki cluster, which includes the Toki, Quetena, Genoveva, Miranda, and Opache porphyry Cu–Mo prospects, is located 15xa0km south–southwest of the Chuquicamata–Radomiro Tomic mines in northern Chile. These prospects occur in an area of 5u2009×u20096xa0km and are completely covered with Neogene alluvial deposits. Inferred resources for the cluster are estimated at about 20xa0Mt of fine copper, with Toki and Quetena contributing ∼88xa0% of these resources. Mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the Collahuasi Group and intrusions of the Fortuna–Los Picos Granodioritic Complex. Hypogene mineralization in the Toki cluster consists mainly of chalcopyrite–bornite with minor molybdenite with mineralization grading outward to a chalcopyrite–pyrite zone and ultimately to a pyrite halo. Alteration is dominantly of the potassic type with K-feldspar and hydrothermal biotite. Sericitic alteration is relatively restricted to late quartz–pyrite veins (D-type veins). Previous K–Ar geochronology for the cluster yielded ages within a range of 34 to 40xa0Ma. Four new Re–Os ages for Toki indicate that molybdenite mineralization occurred in a single pulse at ∼38xa0Ma. Re–Os ages for three different molybdenite samples from Quetena are within error of the Toki mineralization ages. These ages are concordant with a new zircon U–Pb age of 38.6u2009±u20090.7xa0Ma from the tonalite porphyry in Quetena. Two Re–Os ages for Genoveva (38.1u2009±u20090.2 and 38.0u2009±u20090.2xa0Ma) are also within error of the Toki and Quetena molybdenite ages. Four Re–Os molybdenite ages for Opache range between 36.4 and 37.6xa0Ma. The Miranda prospect is the youngest with an age of ∼36xa0Ma. Four new Re–Os ages for the Chuquicamata deposit range between 33 and 32xa0Ma, whereas nine new 40Ar/39Ar ages of biotite, muscovite, and K-feldspar range between 32 and 31xa0Ma. Analyzed molybdenites have Re and Os concentrations that vary between 21–3,099xa0ppm and 8–1,231xa0ppb, respectively. The highest Re and Os concentrations are found in the Toki prospect. Three new 40Ar/39Ar ages for the Toki cluster are younger than the Re–Os mineralization ages. The age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35xa0Ma that probably reflect a late hydrothermal phyllic event. The new geochronological data presented here for the Toki cluster indicate that molybdenite mineralization occurred within a very short period, probably within 2xa0Ma, and synchronously (at ∼38xa0Ma) in three mineralization centers (Toki, Quetena, and Genoveva). Furthermore, mineralization at the Toki cluster preceded the emplacement of the Chuquicamata deposit (35–31xa0Ma) and indicates that porphyry Cu–Mo mineralization occurred episodically over a period of several million years in the Chuquicamata district.

LA-ICP-MS zircon U-Pb geochronology to constrain the age of post-Neocomian continental deposits of the Cerrillos Formation, Atacama Region , northern Chile: tectonic and metallogenic implications

New U-Pb zircon dating of volcanic intercalations in the lower conglomeratic part of the Cerrillos Formation shows that its deposition extended in time at least from 110.7±1.7 to 99.7±1.6 Ma. The significantly younger U-Pb zircon age of 69.5±1.0 Ma obtained for the upper volcanic part of the Cerrillos Formation suggests recurrence of volcanism in the Late Cretaceous instead of continual volcanic activity. A minimum late Maastrichtian age for the Cerrillos Formation and its initial deformation was determined by the U-Pb zircon age range from 66.9±1.0 to 65.2±1.0 Ma for the lower part of the unconformably overlying Hornitos Formation. The new U-Pb data for the Cerrillos and Hornitos formations poses questions about the Campanian-Maastrichtian age range currently ascribed to the latter. The lower part of the Cerrillos Formation represents a major change in the sedimentary regime from previous marine carbonate sedimentation in a back- arc basin until the late Aptian to subsequent coarse alluvial sedimentation and volcanism since the early Albian. The lower part of the Cerrillos Formation is interpreted as the development of coalescent alluvial fans thinning inland, accompanied by volcanism. These developed as the result of transpressive deformation and uplift of the area of the current Coastal Cor- dillera by late Aptian, leading to subsequent active erosion and sedimentation inland, along with the eastward shift of the magmatic foci in the region. A mineralizing period of Cu-Au porphyries overlaps in time with the deposition of the Cerrillos Formation in northern Chile; marking also a significant change in the metallogeny of the Andes of northern Chile.

Alteration patterns of chromian spinels from La Cabaña peridotite, south-central Chile

La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3xa0kbar and 380°–420xa0°C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4xa0wt.%, with the exception of MnO (<0.9xa0wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cru2009+u2009Al) versus Mg# (Mg/Mgu2009+u2009Fe2+) and Fe3+/Fe3++Fe2+ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mgu2009+u2009Fe2+) below 0.5, and (Fe3+/Fe3++Fe2+) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe3+/Fe3++Fe2+)u2009<u20090.4. Accessory Cr-spinels from Centinela Bajo have Cr# and Mg# values that range from 0.55 to 1.0, and 0.6-u2009<u20090.1, respectively. The (Fe3+/Fe3++Fe2+) ratio is less than 0.4 in chromite cores and Fe2+-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.

“Invisible” silver in chalcopyrite and bornite from the Mantos Blancos Cu deposit, northern Chile

Relatively little is known about the mineralogical occurrence and geochemical controls on the incorporation of “invisible” (refractory) silver and gold in hydrothermal sulfide minerals. Secondary ion mass spectrometry (SIMS) analysis reveals that bornite (81–649 ppm Ag) and chalcopyrite (0.61–2211 ppm Ag) are major hosts for silver in the Mantos Blancos deposit (500 Mt, @1 wt% Cu), the largest Jurassic stratabound Cu-(±Ag) deposit in the Costal Range of northern Chile. Gold concentrations are generally two orders of magnitude lower, ranging from 0.05 to 1.66 ppm Au in chalcopyrite, and 0.08 to 2.38 ppm Au in bornite. In addition to precious metals, SIMS analysis shows significant concentrations of As (~100 ppm in chalcopyrite, <10 ppm in bornite), whereas other metalloids and chalcogens, such as Sb, Se, and Te, have highly variable concentrations ranging from tens of ppb to ppm levels. These microanalytical results are consistent with a two-stage hydrothermal evolution model, as recently proposed for the Mantos Blancos deposit. Within this context, Ag, Au, As, and base metals were most likely sourced from a Late Jurassic (~155 Ma) rhyolitic dome, and partitioned into bornite and chalcopyrite in quartz-sericite veins after cooling below ~430°C. This first hypogene Cu-Ag ± Au event was followed by a second, higher-temperature alteration phase (400–600 °C) related to the emplacement of diorite and granodiorite stocks (~141–152 Ma), in which Ag and Au were partitioned into fine-grained, porous chalcopyrite in potassic alteration vein assemblages. When coupled with recent studies in the area, results presented here confirm that the high Ag endowment of Mantos Blancos is the result of multiple pulses of hypogene mineralization followed by supergene enrichment of metals.

Late Cretaceous porphyry copper mineralization in Sonora, Mexico: Implications for the evolution of the Southwest North America porphyry copper province

Two porphyry Cu-Mo prospects in northern Sonora, Mexico (Fortuna del Cobre and Los Humos) located within the southwestern North American porphyry province have been dated in order to constrain the timing of crystallization and mineralization of these ore deposits. In Fortuna del Cobre, the pre-mineralization granodiorite porphyry yielded an U-Pb zircon age of 76.5u2009±u20092.3xa0Ma, whereas two samples from the ore-bearing quartz feldespathic porphyry were dated at 74.6u2009±u20091.3 and 75.0u2009±u20091.4xa0Ma. Four molybdenite samples from Los Humos porphyry Cu prospect yielded a weighted average Re-Os age of 73.5u2009±u20090.2xa0Ma, whereas two samples from the ore-bearing quartz monzonite porphyry gave U-Pb zircon ages of 74.4u2009±u20091.1 and 74.5u2009±u20091.3xa0Ma, showing a Late Cretaceous age for the emplacement of this ore deposit. The results indicate that Laramide porphyry Cu mineralization of Late Cretaceous age is not restricted to northern Arizona as previously thought and provide evidence for the definition of NS trending metallogenic belts that are parallel to the paleo-trench. Porphyry copper mineralization follows the inland migration trend of the magmatic arc as a result of the Farallon slab flattening during the Laramide orogeny.

Geochronological and thermochronological constraints on porphyry copper mineralization in the Domeyko alteration zone, northern Chile

2 ABsTrACT. At Domeyko, 40 km south of Vallenar in northern Chile (28°57S-70°53W), the Dos Amigos and Tricolor porphyry copper centers are located within a north-south-elongated hydrothermal alteration zone 6x1.5 km of surface dimensions. The centers are related to tonalite to granodiorite porphyry stocks displaying potassic alteration, which are surrounded by Lower Cretaceous andesitic volcanic rocks with sericitic, kaolinite-illite and propylitic alteration zones. The western boundary of the alteration zone is marked by the post-mineralization Cachiyuyo Batholith of granodioritic to dioritic composition. U-Pb zircon ages for the Dos Amigos porphyry are of 106.1±3.5 and 104.0±3.5 Ma; and 108.5±3.4 for the nearby Tricolor porphyry. The Cachiyuyo Batholith yielded U-Pb zircon ages of 99.6±1.8 and 99.1±1.9 Ma; and 40 Ar/ 39 Ar ages for biotite of 96.9±3.9 and 94.8±0.9 Ma. These dates indicate that batholith emplacement postdated the Dos Amigos and Tricolor porphyries, in agreement with geological relationships. Although copper mineralization is spatially and genetically related to the Lower Cretaceous (Albian) porphyry stocks, most of the dated hydrothermal micas from the Dos Amigos and Tricolor porphyries yielded 40 Ar/ 39 Ar ages between 97.1±2.5 and 96.0±1.4 Ma, which overlap within error with the cooling ages obtained for the neighboring batholith. 40 Ar/ 39 Ar dating of micas revealed significant disturbance of their K-Ar isotopic systematics that complicates accurate determination of the timing of hydrothermal activity at Domeyko. Nevertheless, the 40Ar/39Ar data establish a minimum Late Cretaceous age for this activity. A fission track age of 59.8±9.8 Ma of apatite from the Dos Amigos porphyry indicates cooling through the temperature range of the apatite partial annealing zone (~125-60°C) during the Paleocene; and an (U-Th)/He age of 44.7±3.7 Ma of apatite from the same porphyry sample shows cooling through the temperature range of the apatite He partial retention zone (~85-40°C) during the Eocene. These ages correspond to the exhumation of the porphyry, and the latter provides a maximum age for the supergene enrichment processes that formed the chalcocite blanket currently mined at Dos Amigos.

Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies, Central Chile

Chromitites (>80% volume chromite) hosted in two ultramafic bodies (Lavanderos and Centinela Bajo) from the Palaeozoic metamorphic basement of the Chilean Coastal Cordillera were studied in terms of their chromite composition, platinum-group element (PGE) abundances, and Re-Os isotopic systematics. Primary chromite (Cr# = 0.64–0.66; Mg# = 48.71–51.81) is only preserved in some massive chromitites from the Centinela Bajo ultramafic body. This chemical fingerprint is similar to other high-Cr chromitites from ophiolite complexes, suggesting that they crystallized from arc-type melt similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction mantle. The chromitites display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 180 and 347 ppb, as is typical of chromitites hosted in the mantle of supra-subduction zone (SSZ) ophiolites. Laurite (RuS2)-erlichmanite (OsS2) phases are the most abundant inclusions of platinum-group minerals (PGM) in chromite, indicating crystallization from S-undersaturated melts in the sub-arc mantle. The metamorphism associated with the emplacement of the ultramafic bodies in the La Cabaña has been determined to be ca. 300 Ma, based on K-Ar dating of fuchsite. Initial 187Os/188Os ratios for four chromitite samples, calculated for this age, range from 0.1248 to 0.1271. These isotopic compositions are well within the range of chromitites hosted in the mantle section of other Phanaerozoic ophiolites. Collectively, these mineralogical and geochemical features are interpreted in terms of chromite crystallization in dunite channels beneath a spreading centre that opened a marginal basin above a supra-subduction zone. This implies that chromitite-bearing serpentinites in the metamorphic basement of the Coastal Cordillera are of oceanic-mantle origin and not oceanic crust as previously suggested. We suggest that old subcontinental mantle underlying the hypothetical Chilenia micro-continent was unroofed and later altered during the opening of the marginal basin. This defined the compositional and structural framework in which the protoliths of the meta-igneous and meta-sedimentary rocks of the Eastern and Western Series of the Chilean Coastal Cordillera basement were formed.

Plume-subduction interaction forms large auriferous provinces

Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth’s crust, thus providing an alternative view to current crust-related enrichment models.The lithospheric controls on giant gold deposits remain unclear. Here, the authors show evidence for native gold in the mantle from the Deseado Massif in Patagonia demonstrating that refertilisation of the lithospheric mantle is key in forming metallogenic provinces.

A genetic link between magnetite mineralization and diorite intrusion at the El Romeral iron oxide-apatite deposit, northern Chile

El Romeral is one of the largest iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile. The Cerro Principal magnetite ore body at El Romeral comprises massive magnetite intergrown with actinolite, with minor apatite, scapolite, and sulfides (pyritexa0±xa0chalcopyrite). Several generations of magnetite were identified by using a combination of optical and electron microscopy techniques. The main mineralization event is represented by zoned magnetite grains with inclusion-rich cores and inclusion-poor rims, which form the massive magnetite ore body. This main magnetite stage was followed by two late hydrothermal events that are represented by magnetite veinlets that crosscut the massive ore body and by disseminated magnetite in the andesite host rock and in the Romeral diorite. The sulfur stable isotope signature of the late hydrothermal sulfides indicates a magmatic origin for sulfur (δ34S between −u20090.8 and 2.9‰), in agreement with previous δ34S data reported for other Chilean IOA and iron oxide-copper-gold deposits. New 40Ar/39Ar dating of actinolite associated with the main magnetite ore stage yielded ages of ca. 128xa0Ma, concordant within error with a U-Pb zircon age for the Romeral diorite (129.0u2009±u20090.9xa0Ma; mean square weighted deviationu2009=u20091.9, nu2009=u200928). The late hydrothermal magnetite-biotite mineralization is constrained at ca. 118xa0Ma by 40Ar/39Ar dating of secondary biotite. This potassic alteration is about 10xa0Ma younger than the main mineralization episode, and it may be related to post-mineralization dikes that crosscut and remobilize Fe from the main magnetite ore body. These data reveal a clear genetic association between magnetite ore formation, sulfide mineralization, and the diorite intrusion at El Romeral (at ~u2009129xa0Ma), followed by a late and more restricted stage of hydrothermal alteration associated with the emplacement of post-ore dikes at ca. 118xa0Ma. Therefore, this new evidence supports a magmatic-hydrothermal model for the formation of IOA deposits in the Chilean Iron Belt, where the magnetite mineralization was sourced from intermediate magmas during the first Andean stage. In contrast, the beginning of the second Andean stage is characterized by shallow subduction and a compressive regime, which is represented in the district by the emplacement of the Punta de Piedra granite-granodiorite batholith (100xa0Ma) and marks the end of iron oxide-apatite deposit formation in the area.