The Aguablanca Cu-Ni(PGE) intraplutonic ore deposit (Extremadura, Spain). Isotope (Sr, Nd, S) consfraints on the source and evolution of magmas and sulfides

The Aguablanca Cu-Ni(PGE) ore is a case of an intraplutonic ore deposito It is hosted by mafic and ultramafic cumulates of the Aguablanca Stock, which is part of the larger calcalkaline Santa Olalla Plutonic Complex of 359±18Ma age (Rb-Sr whole rock determination). Primary mineralization consists of pyrrothite, pentlandite and chalcopyrite and resulted from the crystallization of an inmiscible sulfiderich liquid. Several stages of hydrothermal alteration are superimposed. Isotope work on the host igneous rocks (Sr, Nd) and the ore (S), suggests that contamination with a crustalsource took place at some depth before final emplacement of the plutons (ε Nd360= -5.8 to -7.2; Sr(360) = 0.7082 to 0.7103; (δ14S(sulfides) near + 7.4‰). Assimilation - fractional crystallization (AFC) processes are invoked to explain de chemical variability of magmas

300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System

The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenitehosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events n IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event 1 fluids are of mixed origin. They are metamorphic fluids (H20-COrCH4-NaCl; δ18SO = 4.7 to 9.3‰; δD ab.-34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H20-NaCl, 4.5 6.4 wt% NaCl eq.; δI8O = 7.3 7.8‰; δD = -77 to -74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H20-NaCl, < 8 wt% NaCl eq.; δ18O = -0.4 to 3.4‰; δD = -75 to -58‰). They were probably formed by local hydrothermal cells that were spatially and temporally related to the youngest. Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events n nI) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H20-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ18O (-4.6 to 6.3‰) and δD (-51 to -40‰) values are consistent with a meteoric origin, with a δ18O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals such as feldspars, actinolite and chlorite had their δ18O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ18O< ≈ -4‰; δD = -65 to -36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post -Variscan sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative δD values (≤-52‰) whereas those of Early Mesozoic age ranged between -65 and -36‰

Granitoides peraluminosos con andalucita y cordierita magmáticas en la sierra de Velasco: implicancias para el orógeno famatiniano.

La combinación de estudios petrológicos, geoquímicos y de química mineral en el sector noreste de la sierra de Velasco, a la latitud de la localidad de Santa Vera Cruz, revelan la presencia de una unidad ígnea peraluminosa con andalucita (Si = 3,85, Al = 8,14 y Fe3+ = 0,04) y cordierita (K+Na = 0,33 y XFe = 0,40) magmáticas, la cual es denominada unidad Santa Cruz. La aluminosisad del magma (ISA = 1,30 - 1,42), evidenciada por la presencia de minerales aluminosos como cordierita, andalucita y muscovita y la geoquímica de roca total indican que esta unidad fue derivada de fusión parcial de una secuencia metasedimentaria (esencialmente pelítica) en la corteza superior, que se emplazó en un rango de presión - temperatura de 2 a 2,4 kbar y 670 a 720 ºC. Notablemente, las características geoquímicas de la unidad Santa Cruz sugieren una roca fuente diferente a la establecida para otras unidades peraluminosas del orógeno famatiniano. [ABSTRACT] Peraluminous granitoids with magmatic andalusite and cordierite in the Sierra de Velasco: Implications to the Famatinian Orogen. The combination of petrological, geochemical and mineral/chemical data for the NE sector of the Sierra de Velasco, at the latitud of the Santa Vera Cruz town, reveal the presence of peraluminous igneous unit with magmatic andalusite and cordierite, which has been named Santa Cruz unit. The aluminous of magma (ASI = 1.30 - 1.42), evident by the presence of aluminous minerals such as cordierite, andalusite and muscovite and the whole-rock geochemical, principally indicate that this unit was derived of the partial melting of a metasedimentary secuency (mainly pelitic) in the upper crustal, which was emplaced a pression - temperature range of 2 to 2.4 kb and 670 to 720 ºC. Remarkably, the geochemical characteristics of Santa Cruz unit suggest a different source rock than other peraluminous units founded in the Famatinian Orogen

The A-Type event in the eastern Sierras Pampeanas, La Rioja and Catamarca Provinces (Argentina): geochronological and isotopic considerations

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEsubmitte

Edad y origen de la fluorita del yacimiento de la Nueva (Cabalango, Córdoba, Argentina) en base a geoquímica de isótopos radiogénicos (Nd y Sr)

The fluorite mineralization from the La Nueva Mine, has a Sm-Nd age of 130 ± 19 Ma, and thus must be related to the Lower Cretaceous late-Gondwanic extensional and magmatic event that affected the Sierras Pampeanas Basement of Argentina. Hydrothermal fluids involved in the formation of fluorite were probably derived by mixing of two fluids, an ascending high 87Sr/86Sr one, probably equilibrated with basement metamorphics, as suggested by the very negative ENd(130) values, and a second, "descending", with a low 87Sr/86Sr value

A-Type granitoids in the eastern Sierras Pampeanas (Argentina): evidence for early Carboniferous aborted rifting?

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEsubmitte

K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatinian Arc

New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NWArgentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dates of 469.5 3.2 Ma and 470.1 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP zircon date of 468.3 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the Kbentonite and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean

Magmatically folded and faulted schlieren zones formed by magma avalanching in the Sonora Pass Intrusive Suite, Sierra Nevada, California

The southwestern margin of the Late Cretaceous Sonora Pass Intrusive Suite, northern Sierra Nevada, California (USA), preserves a densely populated zone of magmatic structures that record dynamic magmatic layer formation and deformation (faulting and folding) within a solidifying upper-crustal magma mush. This zone consists largely of coupled melanocratic (or schlieren) and leucocratic bands hosted within the 95.6 ± 1.5 Ma Kinney Lakes granodiorite (Leopold, 2016), with orientations approximately parallel to the intrusive margin and with inward younging directions. Schlieren consist of a high modal abundance of medium-grained ferromagnesian minerals (hornblende + biotite), zircon, sphene, apatite, opaque minerals, and minor plagioclase and interstitial quartz. Leucocratic bands are dominated by coarse-grained feldspar + quartz with minor ferromagnesian and accessory minerals. Whole-rock geochemical and Sr and Nd isotopic data indicate that the schlieren are derived from the Kinney Lakes granodiorite by effective mechanical separation of mafic minerals and accessory phases. We interpret that the schlieren zone at the margin of the Kinney Lakes granodiorite formed by large-scale collapse of crystal mush by “magma avalanching,” facilitated by gravity, local convection, and possibly by host-rock stoping at the margin. This process eroded a significant portion of the solidifying margin of the chamber and resulted in the formation of magmatically deformed layered structures, which experienced further mingling, re-intrusion, magmatic erosion, and recycling processes. We envisage that magma avalanching of magma mushes in plutons can be achieved by any unstable process (e.g., tectonic, fluid-assisted, stoping, or gravity-driven) in large, long-lived magma-mush chambers

Palaeostress and geotectonic interpretation of the Alpine Cycle onset in the Sierra del Guadarrama (eastern Iberian Central System), based on evidence from episyenites

Several episodes of hydrothermal activity related to periods of fracturing and/or reactivation of previous structures took place from 300 to - at least - 100 Ma, in the Sierra del Guadarrama, which is part of the crystalline axis of the Iberian Hercynian Fold Belt (Central-Iberian Zone). One of these episodes led to the formation of episyenites, which are de-quartzified and alkalinized granites. Episyenite formation took place on a regional scale and in a short period (approx. at 277 Ma). The episyenites were formed by the action of fluids at temperatures between 350°C and 650°C, at depths of about 6.5 km, and in microfractured dilatancy zones developed under a regional extensional regime. These zones are crosscut by normal faults, developed during the progressive deformation process accompanying the formation of the episyenites. The calculated regional palaeostress tensor has ~r I close to vertical and σ3 between NI0-20E and an average value of the stress ratio (Ф) of 0.19 [Ф = (σ2 - σ3)/(σ1 - σ3)]. Because σ1 is close to vertical the stress tensor is compatible with an extensional deformation field. The analysis also shows that most of the faults that slip under this stress field have an average coefficient of friction of 0.8. This extensional regime was probably accompanied by a regional thermal anomaly, as suggested by the high temperature of the fluids involved, which are amagmatic. This thermo-tectonic episode is interpreted as representative of the generalized extensional regime corresponding to the onset of the Alpine Cycle. The episode was preceded by a wrench-faulting event, equivalent to the Late Variscan event of Arthaud and Matte (1977), for which an age of - at least - 300-290 Ma is indicated by recent radiometric data. In its turn, this event was preceded by the regional extensional gravitative collapse of the Hercynian orogen. A correlation between evidence from the cover (stratigraphy and volcanism) and evidence from the basement (hydrothermal alterations, dyke injection episodes and granitic magmatism) is attempted on the basis of new available radiometric data