Accuracy of PIXE analyses using a funny filter

Light and heavy trace elements can be analysed simultaneously by particle-induced X-ray emission (PIXE) using an Al-funny filter (a filter with a hole drilled at its centre, placed in front of the detector window). The treatment of spectra, performed using the GUPIX code, requires the determination of an instrumental constant H. In theory and when using ordinary filters, H is a constant corresponding to the detection solid angle. In contrast, we have observed that H varies with X-ray energy using a funny filter. This is due to an inaccurate description of the geometry of the detection: detector-to-target distance, filter thicknesses and hole diameter of the funny filter. We develop a methodology to determine the effective values of the four geometrical parameters from a calibration of H using standards. In turn, H recalculated using these effective values is a constant equivalent to the effective detection solid angle

Quantitative analysis of fluid inclusions by particle-induced gamma-ray emission.

The particle-induced gamma-ray emission (PIGE) method, very sensitive for the light elements Li, Be, B, F, Na, is complementary to the PIXE method for the determination of trace elements in complex materials. The deep penetration (more than 50 mm) of 3 MeV protons in light matrices allows heterogenities situated under the irradiated surface of the target to be revealed. This facility is exploited in earth sciences mainly to characterize in situ the fluid inclusions placed on the ion beam path. The quantitative PIGE analysis of the fluid requires a calibration valid for any inclusion of any thickness and at any depth. In this paper, the concentration of a light element is calculated in an occluded fluid by comparison to that of a homogenous massive mineral taken as a standard. An application is given on the quantitative determination of F in fluid inclusions in corundum by comparison to a topaz gemstone standard. A discussion follows on the choice of the energies at which the stopping power of the traversed materials is taken for the calculation of the concentrations

Effet de la composition des micas dioctaédriques sur les distributions des éléments alcalins en traces

Experimental study of the distribution of trace amounts of Rb and Cs between some muscovites and hydrothermal solution. Octahedral aluminium of muscovites has been partly substituted by magnesium, and potassium partly by sodium. Rb and Cs distributions studied with these dioctahedral micas, have been compared to previous distributions study with «pure » muscovite KAl₃Si₃O₁₀ (OH)₂. Distributions have been expressed by the partition ratio : P[X] = (X/K)[aq] / (X/K)[m] (X for Rb or Cs). Octahedral substitutions have a greater effect upon P[Rb] and P[Cs] than Na-K substitutions. These results have been compared to previous results obtained with trioctahedral micas. It was observed that the increase of Mg content in mica promotes substitution of K by Rb or Cs. But natural muscovites have a fairly constant composition and then can be used, more safely than biotites, for geothermometric purposes.Étude expérimentale de la distribution de Rb et Cs en traces entre des muscovites et une solution hydrothermale. L'aluminium octaédrique des muscovites est partiellement substitué par du magnésium, de même le potassium est partiellement substitué par du sodium. On compare les distributions de Rb et Cs observées pour ces compositions de muscovites avec les distributions précédemment établies pour la muscovite type KAl₃Si₃O₁₀ (OH)₂. Les distributions sont caractérisées par les rapports de partage P[X] = (X/K)[aq] / (X/K)[m] (X pour Rb ou Cs). Les substitutions octaédriques ont plus d'influence sur P[Rb] et P[Cs] que la substitution de K par Na. On compare ces résultats à ceux précédemment établis avec des micas trioctaédriques. On note alors que Rb et Cs entrent mieux dans les micas qui contiennent du magnésium. Mais, comme les muscovites naturelles ont des compositions qui varient relativement peu, elles peuvent être utilisées plus sûrement que les biotites dans des applications géothermiques éventuelles.Volfinger Marcel. Effet de la composition des micas dioctaédriques sur les distributions des éléments alcalins en traces. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 98, 1, 1975. pp. 49-52

Contrôle structural du partage de Rb entre fluide et micas potassiques dans les pegmatites du Brésil.

L'étude géochimique des micas de pegmatites de l'état de Minas Gerais au Brésil a été effectuée à la microsonde électronique, afin de préciser les variations de teneurs de Rb, K, Al et F. Les résultats indiquent qu'il existe une relation linéaire décroissante entre les rapports [Rb/K] des micas et leurs teneurs en AlVI. L'interprétation est fondée sur l'hypothèse que le coefficient de partage CRb/Kmica/fl entre fluide et minéral ne varie pratiquement pas avec la température et la pression dans les conditions étroites de cristallisation des pegmatites. On montre que la relation obtenue, CRb/Kmica/fl=0,55×(5−[AlVI]), est d'ordre cristallochimique. Les micas à faibles teneurs en Al admettent de plus grandes teneurs en Rb, car les sites potassiques d'accueil de Rb sont plus grands. Cette relation fournit un autre moyen de calculer les valeurs [Rb/K] des fluides, connaissant Rb, K, Al et Si des micas. Cette cristallochimie permet aussi de prévoir une corrélation directe entre Rb et F dans les micas pegmatitiques

Le lithium dans une phlogopite de synthèse

We have studied the fixation of lithium in a phlogopite crystallized in the presence of a hydrothermal solution of LiCl. The chemical data and the strong decrease of the reticular distance d005 show that lithium enters the interlayer sheet, replacing potassium and not the octahedral sites as it does in lepidolites. Lithium cannot occupy the alcaline site in the interlayer sheet. We propose to localize Li+ in a cavity limited by the basal oxygens of two tetrahedra owing to two consecutive sheets. The favourable cavities are those limited by aluminous tetrahedra ; with this model, it is possible to predict the solubility limit of lithium, a function of AlIV content of the mica.On a étudié le mode de fixation du lithium dans une phlogopite cristallisée en présence d'une solution hydrothermale de LiCl. Le bilan chimique ainsi que la forte décroissance de la distance réticulaire d005 montrent que le lithium se fixe dans l'interfeuillet en remplacement du potassium et non pas dans les sites de la couche octaédrique comme c'est le cas dans les lépidolites. On montre que Li+ ne peut occuper entre les feuillets le site alcalin laissé vacant par le départ de K+. On propose la localisation de Li+ entre les oxygènes de base de deux tétraèdres appartenant à deux feuillets consécutifs. Les cavités de ce type (prismes droits à base triangulaire) les plus favorables sont celles que limitent deux tétraèdres alumineux. Ce modèle permet de calculer a priori la teneur limite en lithium, fonction de la teneur en AlIV du mica.Volfinger Marcel, Robert Jean-Louis. Le lithium dans une phlogopite de synthèse. In: Bulletin de Minéralogie, volume 102, 1, 1979. Les micas, chimie et cristallochimie

Étude expérimentale de lépidolites trioctaédriques hydroxylées

The extent of the solid solutions between phlogopite and each one of the three trioctahedral lepidolitic end-members — taeniolite, trilithionite, and polylithionite — has been determined experimentally as a function of temperature, between 400° C and 900° C, under a water pressure of 2 kbar. This study shows that none of the OH-lepidolitic end-members is stable under the chosen experimental conditions. The extent of the solid solutions studied decreases when temperature increases. The changes in the cell dimensions point out the octahedral position of lithium. The decrease of d005 and d060 with the lithium contents may be written as : d005 (Â) = 2.033 — 0.4135 • 10-6X d060 (A) = 1.535 — 1,235 • 10-6 X where X is the Li content (in p. p. m.). These two relations are valide for the three series of lepidolites. The stability at high temperature of a lithium rich mica, containing a large amount of vacant octahedral site is suspected, the composition of this micas beeing close to the join taeniolite-tetrasilicic magnesium mica.On a étudié l'étendue des solutions solides entre la phlogopite et trois pôles lépidolitiques trioctaédriques hydroxylés — la taeniolite, la trilithionite et la polylithionite — en fonction de la température, entre 400° C et 900° C, sous une pression d'eau de 2 kbar. On observe qu'aucun des trois pôles lépidolitiques n'est stable dans les conditions expérimentales adoptées. L'étendue des solutions solides étudiées croît lorsque diminue la température de synthèse. L'évolution des paramètres cristallographiques traduit nettement la fixation du lithium dans la couche octaédrique. Pour chacune des deux distances réticulaires mesurées, l'évolution en fonction de la teneur en lithium se fait avec la même pente dans les trois séries selon les réactions : d005 (À) = 2,033 — 0,4135 •10-6 X d060 (À) = 1,535 — 1.235 • 10-6 X où X est la teneur en lithium du mica exprimée en p. p. m. On soupçonne, enfin, l'existence à haute température d'un mica lithique lacunaire, proche du joint taeniolite -mica tétrasilicique magnésien.Robert Jean-Louis, Volfinger Marcel. Étude expérimentale de lépidolites trioctaédriques hydroxylées. In: Bulletin de Minéralogie, volume 102, 1, 1979. Les micas, chimie et cristallochimie

The occurrence of Gold-Rich Pyrite in the Itajubatiba Skarn deposit, Borborema province, Northeastern Brazil : a discovery by pixe analyses

The Itajubatiba deposit represents a gold skarn, in northeastern Brazil. Since its discovery in the 1940s, it was exploited during about 30 years and it has produced 5 tons of gold. Its average ore grade ranges from 0.5 to 2.0 ppm and has reached 6.3 ppm. Regional geologic units consist of a gneissic-migmatitic basement (Archean to Paleoproterozoic), a meta-sedimentary sequence (marble and schist) and Neoproterozoic intrusive igneous rocks. Later a metasomatism transformed marble, meta-tonalite (gneissic-migmatites), and meta-syenogranite into skarns. Gold mineralization is typically later, associated with sulfides, which appear either filling veins crosscutting the skarns or disseminated throughout the skarns. In order to verify this hypothesis regarding the formation of gold, a detailed search for the metal was carried out using several polished thin sections. Gold, however, was not observed. A subsequent detailed investigation of trace elements in sulfides of the Itajubatiba ore was conducted. Proton-Induced X-ray Emission (PIXE) analyses revealed that some elements occur in relatively high amounts in these sulfides, such as Mn (up to 0.9 wt.%), Au (up to 690 ppm), As (up to 1,360 ppm), Bi (0.5 wt.%), Pb (up to 1,870 ppm), Ni (up to 3,050 ppm), Co (0.6 wt.%), Se (up to 1,000 ppm), and W (up to 1,420 ppm). Pyrite shows the highest amounts of gold among the sulfides studied. Considering the quantity of gold detected in pyrite, the pyrite abundance in the study rocks, and the average densities of the main minerals occurring in skarns, their gold grade varies from 1.5 to 10.4 ppm (for gold hosted in pyrite only). The results reveal economically significant gold amounts linked to sulfides, and could provide a new perspective for the exploitation of the Itajubatiba deposit

W-Au skarns in the Neo-Proterozoic Serido Mobile Belt, Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit

The Serido Mobile Belt (SMB) is located in the Borborema Province in northeastern Brazil and consists of a gneiss basement (Archean to Paleo-Proterozoic), a metasedimentary sequence (marble, quartzites, and schists), and the Brasiliano igneous suite (both of Neo-Proterozoic age). In this region, skarns occur within marble and at the marble-schist contact in the metasedimentary sequence. Most of the skarn deposits have been discovered in the early 1940s, and since then, they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns, which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10- to 15-km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite-zoisite-sericite, calcite, and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble-schist contact. The main ore body is 5-120 cm wide and contains an average of 4.8-wt.% WO3, which occurs in the basal marble-schist contact. Fold hinges appear to control the location of high-grade scheelite. The late-stage gold mineralization contains bismite (Bi2O3), fluorine-bearing bismite, native bismuth, bismuthinite (Bi2S3), and joseite [Bi-4(Te,S)(3)], and also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold-bismuth-tellurium mineralization exhibits a typical late character and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that crosscut not only the banding in prograde skarn but also the retrograde alkali feldspar and clinozoisite-zoisite-sericite assemblage. The Au-Bi-Te-bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits