Alain Cheilletz

40AR/39AR DATING OF SHEAR ZONES IN THE VARISCAN BASEMENT OF GREATER KABYLIA (ALGERIA). EVIDENCE OF AN EO-ALPINE EVENT AT 128 MA (HAUTERIVIAN-BARREMIAN BOUNDARY) : GEODYNAMIC CONSEQUENCES

A ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the Alpine Maghrebides in Algeria. Dating of shearing movement is carried out by 40Ar/39Ar laser-probe experiments performed on synchronous neoformed micas. The proposed age at 128 Ma is constrained by a two-component mixing model for the saddle-shaped age spectra on phengites and by a plateau age at 128.3±0.3 Ma on muscovite. This Hauterivian–Barremian boundary age records a strong reworking of the Alboran plate (input of detritals in the flysch basin) and surrounding areas. It can be correlated with the J magnetic anomaly in the Atlantic and with the distensive events in the Pyrenees and external Alps. Therefore, isotopic ages (80–130 Ma) obtained on the Alpine–Mediterranean basement could reflect interferences between this event at 128 Ma and late-Alpine stages.

Fluid Composition, δD of Channel H2O, and δ18O of Lattice Oxygen in Beryls: Genetic Implications for Brazilian, Colombian, and Afghanistani Emerald Deposits

The fluid composition, δD of channel H2O, and δ18O of lattice oxygen have been determined in beryl and emerald from a variety of geological environments and used to constrain the origin of the parental fluids from which beryl has grown. Step-heating analyses performed by quadrupolar mass spectrometry were used to quantify the composition of the fluid phases in beryl from granitic pegmatites and greisens and emerald from Brazil, Colombia, and Afghanistan. An important conclusion is that beryl and emerald have a similar fluid composition, with concentrations of H2O being greater than 90% of the total water in the mineral irrespective of the age of formation (2.0 Ga to 32 Ma) and tectonic settings. However, the Brazilian Santa Terezinha shear-zone emerald deposit contains abundant CO2, up to 13 wt% of the total fluid. A second conclusion is that the channel H2O content for some Brazilian emeralds is higher than the range defined for beryl in the literature, especially for those related to the shear-zone type...

Curvature Attribute from Surface-Restoration as Predictor Variable in Kupferschiefer Copper Potentials

This work explains a procedure to predict Cu potentials in the ore-Kupferschiefer using structural surface-restoration and logistic regression (LR) analysis. The predictor in the assessments are established from the restored horizon that contains the ore-series. Applying flexural-slip to unfold/unfault the 3D model of the Fore-Sudetic Monocline, we obtained curvature for each restored time. We found that curvature represents one of the main structural features related to the Cu mineralization. Maximum curvature corresponds to high internal deformation in the restored layers, evidencing faulting and damaged areas in the 3D model. Thus, curvature may highlight fault systems that drove fluid circulation from the basement and host the early mineralization stages. In the Cu potential modeling, curvature, distance to the Fore-Sudetic Block and depth of restored Zechstein at Cretaceous time are used as predictors and proven Cu-potential areas as targets. Then, we applied LR analysis establishing the separating function between mineralized and non-mineralized locations. The LR models show positive correspondence between predicted probabilities of Cu-potentials and curvature estimated on the surface depicting the mineralized layer. Nevertheless, predicted probabilities are particularly higher using curvatures obtained from Late Paleozoic and Late Triassic restorations.

Paleocene adakite Au-Fe bearing rocks, Mezcala, Mexico: evidence from geochemical characteristics

The Au–Fe mineralized granitoids at Mezcala district have a porphyry texture with a quartz+feldspar microcrystalline matrix and phenocrysts of plagioclase, quartz (with reaction rims), hornblende and biotite. The primary minerals are oligoclase–andesine, microcline and h-quartz. The accessory minerals are biotite, hornblende and, in minor amounts, apatite+zircon+sphene+titanomagnetite. Some intrusive rocks present abundant hornblende autoliths. Based on the petrography and bulk geochemistry of these granitoids, they are classified as monzonite, tonalite (the most abundant) and granodiorite with a strong calc-alkaline trend in potassium (K2O=3.8% average). The bulk and trace elements chemistry is SiO2=63.8%, Al2O3=15.83%, Fe2O3+MgO+MnO+TiO=6.52%; V=76.7 ppm, Cr=50.2 ppm, Ni=19.7 ppm, Sr=694 ppm. These granitoids show a strong depletion in heavy rare-earth elements (HREE), with average values of Yb=1 ppm and Y=13 ppm, this being the characteristic geochemical signature for adakite. The trace elements content suggests that the adakite granitoids from Mezcala were formed within a tectonic framework of volcanic arc related to the interaction between the Farallon and North America plates. This interaction occurred during the Paleocene after the Laramide Orogeny (post-collision zone) in a fast convergent thick continental crust (>50 km) subduction regime. The original magma is interpreted as being the product of partial melting of an amphibolite–eclogite transition zone source with little contribution of the mantle wedge. Along with the hydration processes, a metallic fertility also took place in the area. The geochemical signature of the adakites within the

Muscovite recrystallization and saddle-shaped 40Ar/39Ar age spectra: example from the Blond granite (Massif Central, France)

Muscovites from the Blond granite (West French Massif Central) were dated by the 40Ar/39Ar single-grain method. The 40Ar/39Ar ages obtained vary from 305.5 ± 0.3 Ma to 311.3 ± 0.5 Ma, and most of the age spectra are slightly saddle-shaped. The analyzed muscovites show phengitic recrystallization under optical microscope observations, SEM images, and electron microprobe chemical analyses. It is proposed that the saddle-shaped age spectra result from a partial recrystallization, which produced three different isotopic reservoirs in the analyzed white mica single grains: domains of early muscovite, domains of neocrystallized muscovite formed by phengitic and Al-Fe substitutions, and “low-activation energy sites.”

The genesis of Colombian emeralds: a restatement

A renewal of metallogenetical studies of Colombian emerald deposits produced new geological and geochemical data that favour a hydrothermal-sedimentary genetic model for these deposits. A comprehensive model is presented which integrates both chemical and structural aspects and invalidates some aspects of the model recently presented by Ottaway et al. The deposits result from a two-stage process in which shortening tectonics affect the two borders of the Eastern Cordillera of Colombia and provoke decollement planes, thrusting, and thrust-fault related folds in the Early Cretaceous black shale series. Three major descriptive and interpretative aspects of the deposits are presented: (1) time relationships between the two different sets of barren and mineralized extensional veins; (2) the association of graphite and emerald mineralization, and (3) the thermochemical sulfate reduction reaction acting at the site of the emerald deposits to reduce sulfate of evaporitic origin from the hydrothermal brines into hydrogen sulfide by interaction with organic-rich strata.

On the non-existence of a Cadomian basement in southern France (Pyrenees, Montagne Noire): implications for the significance of the pre-Variscan (pre-Upper Ordovician) series

The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca . 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca . 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al. , 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregura Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican ( ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic ( ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.

The timing of W-Sn-rare metals mineral deposit formation in the Western Variscan chain in their orogenic setting: the case of the Limousin area (Massif Central, France)

Abstract New 40Ar/39Ar dating performed on Rare Metal Granites and W ± Sn deposits in the northern Limousin has provided evidence of two metallogenic episodes. An Early Namurian episode (c. 325 Ma) was contemporaneous with the emplacement of the large peraluminous leucogranite bodies, which are associated with small W ± Sn deposits, but also with some larger deposits, at Puy-les-Vignes (323.4 ± 0.9 Ma) and Moulin-Barret (323.7 ± 0.8 Ma) formed at a shallower level above cryptic granite plutons. These new data indicate that the metallogenic potential of the Namurian leucogranites might have been underestimated. Most other W ± Sn deposits in the northern Limousin area are attributed to a Mid-Westphalian episode (c. 310 Ma), and are contemporaneous with the emplacement of all the Rare Metal Granites. Both episodes were related to leucogranite emplacement and associated fluid circulations, but in two different geodynamic contexts. The Early Namurian episode may be related to syncollisional extension of the Variscan belt, whereas the Mid-Westphalian one occurs during generalized extension and rapid exhumation of the belt associated with the granulite-facies metamorphism of the lower lithosphere probably related to the delamination of the lower lithosphere. Thus, W ± Sn and rare metals (Ta, Nb, Be, Li) deposits are clearly temporally and probably genetically related to leucogranitic magmatism.

Emeralds in the Eastern Cordillera of Colombia: Two tectonic settings for one mineralization

Colombian emeralds are formed through a hydrothermal-sedimentary process. On the western side of the Eastern Cordillera, the deposits are linked by tear faults and associated thrusts developed during a compressive tectonic phase that occurred at the time of the Eocene-Oligocene boundary, prior to the major uplift of the Cordillera during the Andean phase (middle Miocene). On the eastern side of the Cordillera, emerald mineralization occurred earlier, at the time of the Cretaceous-Tertiary boundary, during a thin-skinned extensional tectonic event linked to evaporite dissolution. This event predates the Andean phase, during which this part of the chain was folded and thrust over the Llanos foreland.

Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: II. Thermal modelling and implications for the mechanisms of uranium mineralization

A two-dimensional (2-D) thermal modelling of the Saint Sylvestre—western Marche leucogranite complex (northwestern Limousin, French Massif Central, FMC) was conducted to help constrain the cooling history and the uranium mineralization postdating the time of intrusion by 40–50 m.y. Numerical simulation of the post-emplacement cooling of the complex (<700°C) indicates that the pluton thermally equilibrated with its country rocks (at around 360–400°C and a mean depth of 10.5 km) by conduction in as little as 4 m.y. after its intrusion age (324 ± 4 Ma). Integration of the regional 40Ar/39Ar muscovite data in the 2-D model with an assumed universal Ar closure temperature of 325 ± 25°C reveals several sub-stages in the subsolidus cooling of the complex. The cooling pattern regionally defined by the muscovite data indicates that cooling was driven by the extensional exhumation and erosion of the thickened crust (+ intrusion) at a mean denudation rate of 0.3 mm y−1 Complete cooling below 325°C ended at 301 Ma via a transient faster denudation rate of 1.5 mm y−1 at the Westphalian-Stephanian boundary. The genetic relationships between the fluid circulations, the mineralization, and the cooling history of the pluton are discussed with particular emphasis on the tectonic process driving exhumation (extension). The initiation of the regional uplift at ∼320 Ma triggered at depth the circulation of in situ derived low-density aqueous fluids that reacted with the granite to form large vertical dissolution conduits (episyenites) characterized by the strong leaching of SiO2. The hydrothermal alteration was further enhanced during uplift by the structurally focused throughput of large volumes of aqueous fluids along brittle faults cutting across the laccolith. These conduits acted more than 20–30 m.y. after the trap formation as preferential channelways for the U-ore deposition at 270–280 Ma, due to sustained hydrothermal circulation adjacent to the high-heat producing terminal injections emplaced along the vertical fault zones and the metasomatic columns.