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187Re-187Os analysis of magnetite and other oxide minerals from various well dated geological settings (kimberlites, lamprophyres, Manicouagan impact melt rock, Great Bear magmatic zone iron ore deposits and Laramie Anorthosite complex) worldwide have been used to investigate the potential for precise geochronology. 187Re-187Os isotopes in groundmass oxide minerals from kimberlites and lamprophyres are not suitable for emplacement geochronology due to low Re contents and large quantities of un-radiogenic Os. TRD and TMA ages can be calculated however, and correlate well with previously published depletion ages and large magmatic events in the overlying crust, indicating the effectiveness of this technique. Geochronology using magnetite from the remaining settings is im-precise due to very low Re and Os contents but the ages produced are in the correct area. Young ages produced from the Laramie anorthosite complex indicate that magnetite may have a low closure temperature to Os diffusion (~200-300C).

In mineral deposits the application of the Re-Os system has evolved on two fronts; as a geochronometer in molybdenite, and as a tracer of the source of metals by direct determination of the source of Os contained in the ore minerals. Results obtained from a wide variety and types of mineral deposits indicate that ore minerals in most deposits contain a high initial osmium composition, compared to the mantle value at the time of ore formation. The Re-Os data presented here for the Platreef, South Africa, adds to the growing notion that the crust plays a fundamental role in the formation of mineral deposits and as a source of ore minerals. Additional data from the Zambian Copperbelt illustrate the utility of the Re-Os system as a geochronometer of sulfide mineralization. Two isochron ages of ca. 825 Ma and 575 Ma are consistent with a long-lived period of multistage mineralization linked to basin evolution and support a model where brines play a fundamental role in the formation of sediment-hosted stratiform deposits.Numerous new Re-Os molybdenite ages have recently been reported; however, the behavior of Re and Os in molybdenites is still poorly understood and controversy remains regarding the possible disturbance of the Re-Os isotopic system. Previous studies indicate that the Re-Os system in molybdenites, and in other sulfides, can experience disturbance by Re and Os loss or Re gain (both examples of open system behavior), and that the analysis of these altered samples yields equivocal ages. Through replicate analyses of samples and/or comparison with other robust dating techniques, such as the U-Pb geochronometer, it is possible to differentiate between Re-Os molybdenite ages reflecting a mineralization age or a post depositional event. Once the reliability of the Re-Os molybdenite analyses is proven, it is possible to constrain the timing of mineralization and the identification of multiple molybdenite mineralization events, information that is relevant in assessing the longevity of porphyry systems.The examples presented in this work support the use of the Re-Os isotopic system as an important geochemical tool in the understanding of mineral deposits.

Intraplate volcanism that has created the Hawaiian-Emperor seamount chain is generally thought to be formed by a deep-seated mantle plume. While the idea of a Hawaiian plume has not met with substantial opposition, whether or not the Hawaiian plume shows any geochemical signal of receiving materials from the Earth’s Outer Core and how the plume may or may not be reacting with the overriding lithosphere remain debatable issues. In an effort to understand how the Hawaiian plume works I report on the first in-situ sulfides and bulk rock Platinum Group Element (PGE) concentrations, together with Os isotope ratios on well-characterized garnet pyroxenite xenoliths from the island of Oahu in Hawaii. The sulfides are Fe-Ni Monosulfide Solid Solution and show fractionated PGE patterns. Based on the major elements, Platinum Group Elements and experimental data I interpret the Hawaiian sulfides as an immiscible melt that separated from a melt similar to the Honolulu Volcanics (HV) alkali lavas at a pressure-temperature condition of 1530 ± 100OC and 3.1±0.6 GPa., i.e. near the base or slightly below the Pacific lithosphere. The 187Os/188Os ratios of the bulk rock vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratio strongly correlates with major element, High Field Strength Element (HFSE), Rare Earth Element (REE) and PGE abundances. These correlations strongly suggest that PGE concentrations and Os isotope ratios reflect primary mantle processes. I interpret these correlations as the result of melt-mantle reaction at the base of the lithosphere: I suggest that the parental melt that crystallized the pyroxenites selectively picked up radiogenic Os from the grain boundary sulfides, while percolating through the Pacific lithosphere. Thus the sampled pyroxenites essentially represent crystallized melts from different stages of this melt-mantle reaction process at the base of the lithosphere. I further show that the relatively low Pt/Re ratios of the Hawaiian sulfides and the bulk rock pyroxenites suggest that, upon ageing, such pyroxenites plus their sulfides cannot generate the coupled 186Os-187Os isotope enrichments observed in Hawaiian lavas. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt-Re-Os isotope systematics of plume-derived lavas.

Mantle pyroxenites are important components of mantle rocks, because they provide important insights into bulk composition and heterogeneity of the Earth's upper mantle and therefore, direct evidence of mantle evolution throughout the Earth history. Studied pyroxenites from localities Bečváry (the Kutná Hora Complex), Níhov, Nové Dvory, Mohelno, Horní Kounice, Karlstteten and Meidling (the Gföhl Unit of the Moldanubian Zone) occur as dykes and/or layers within spinel and garnet peridotites from the Bohemian massif. Whole-rock concentrations of rare earth (REE) and other trace elements in studied pyroxenites yield extreme variability, which most likely reflect: 1) variable garnet/clinopyroxene ratios in bulk rocks, 2) different degree of fractionation of parental melts and 3) different concentrations trace elements in the source host material. Pyroxenites from locality Mohelno (LREE-depleted) may be derived from depleted or only slightly enriched suboceanic mantle. In contrast, pyroxenites from Karlstetten, Meidling, Horní Kounice and Nové Dvory (LREE-enriched) crystallized from the melts derived from enriched mantle source with possibly significant contribution of recycled crust. Studied pyroxenites are characterized by extremely variable 187 Os/188 Os ratios. While the pyroxenites from Mohelno and...

Submitted by Israel Vieira Neto (israel.vieiraneto@ufpe.br) on 2015-03-04T16:38:17Z No. of bitstreams: 2 DISSERTAÇÃO Emerson Marcello Ferreira Anastácio.pdf: 10089049 bytes, checksum: 0eb60791ae3e8e6cb281ce1df85c67e2 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) / Made available in DSpace on 2015-03-04T16:38:17Z (GMT). No. of bitstreams: 2 DISSERTAÇÃO Emerson Marcello Ferreira Anastácio.pdf: 10089049 bytes, checksum: 0eb60791ae3e8e6cb281ce1df85c67e2 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Previous issue date: 2013-07-18 / A área investigada situa-se entre Fagundes e Itatuba (PB), geologicamente inserida no Terreno Alto Moxotó (TAM), Subprovíncia Transversal (DZT), Província Borborema, Nordeste do Brasil. Localmente ocorre um conjunto de corpos tabulares, lenticulares e sub-elípticos, de uma suíte de rochas graníticas e metamáfico-ultramáficas com lentes de rochas metacarbonáticas associadas, além de skarns, encaixados em um embasamento gnáissico-migmatítico (Complexo Floresta). Lentes de rochas metamáficas-ultramáficas, pertencentes a uma série basalto-toleiítica com afinidades de MORB, hospedam importantes ocorrências de Fe-Ti. Os skarns abrigam uma associação de quartzo-epidoto-sulfetos (Fe-Cu), disposta em estruturas do tipo brechada e stockwork, formadas por hidrotermalismo tardio que pode ter gerado mineralizações com potencial interessante, (e.g. Fe, Cu, Au), como já constatado em outros skarns da Província Borborema. Visando elucidar de modo mais realístico o modelo genético da mineralização estudada, no contexto temporal do hidrotermalismo, foi realizado o estudo geocronológico da principal ocorrência de skarns mineralizados em Fe-Cu, utilizando-se o par isotópico Re-Os em calcopirita (sulfeto dominante na paragênese do minério estudado). Embora o erro nas análises das razões isotópicas entre Re e Os ter sido aceitável (< 1 %), as amostras de calcopirita analisadas registraram concentrações extremamente baixas de Re (7 a 20 ppb) e Os (6 a 98 ppb), que ocasionaram uma tendência não-linear no diagrama concórdia, não permitindo um cálculo preciso da idade. Assim, após várias tentativas de geração de idades interpretáveis diante do contexto geológico local, este cálculo foi feito individualmente para as quatro frações analisadas, sendo que as que forneceram dados plausíveis no contexto geológico da área retornaram idades entre 0,52 Ga (amostra YK-566-1) e 2,05 Ga (YK-566-4). Enquanto a idade obtida pela amostra YK-566-1 pode representar o momento da precipitação dos sulfetos na área, associado ao hidrotermalismo tardio, durante o Estágio 3 Cambriano (~520 Ma), a amostra YK-566-4 (2,05 Ga) provavelmente possui informações isotópicas herdadas do episódio juvenil mais antigo relatado até o momento (2308 ± 22,5 Ma) ou de seu metamorfismo posterior em 2,012 Ga. Embora os métodos utilizados não tenham permitido considerações e conclusões mais consistentes, face ao contexto geológico da área estudada como conhecido até o momento, os dados de Carmona (2006) combinados com os de Santos et al. (2013a), bem como os gerados neste trabalho, permitem, de fato, traçar uma evolução aceitável, onde a granitogênese neoproterozoica provavelmente seria a responsável pelo hidrotermalismo da área (~547 Ma), embora a mineralização primária de Fe-Cu provavelmente seja orosiniana (2,050-1,800 Ga).

Le géochronomètre Re-Os a fait ses preuves dans son application aux huiles et bitumes pétroliers. Cependant, l’application de ce système sur les fluides pétroliers souffre d’un manque de compréhension de la géochimie pétrolière de Re et Os, des mécanismes de remise à zéro du géochronomètre et par conséquent des évènements réellement datés. Plus précisément, l’utilisation du géochronomètre Re-Os nécessite de comprendre la manière dont le comportement du système Re-Os permet de remplir les conditions nécessaire au développement d’une isochrone. Ces conditions sont : 1) l’homogénéisation isotopique des huiles à l’échelle d’un champ pétrolier ; 2) le fractionnement de Re par rapport à Os pour obtenir des échantillons ayant différents rapport Re/Os ; 3) la fermeture du système jusqu’au moment de la datation. Des protocoles expérimentaux ont ainsi été mis au point afin de comprendre le comportement géochimique de Re et Os dans les huiles afin d’évaluer l’utilisation du système Re-Os en tant que géochronomètre dans le cadre d’une application directe sur les pétroles. Le fractionnement de Re et Os a été vérifié dans le cas de la perte d’asphaltènes au cours de l’évolution de l’huile par précipitation séquentielle en laboratoire. Des expériences de contact entre solutions aqueuses de Re et Os et huiles ont été conduites afin d’étudier la possibilité d’un transfert des métaux entre ces deux phases sur une large gamme de concentrations et de températures. Enfin, l’étude de spéciation de Re et Os dans les huiles artificiellement enrichies en Re et Os par spectroscopie d’absorption X (rayonnement synchrotron) a permis de proposer un mécanisme d’incorporation de Re et Os dans les huiles / The Re-Os radiogenic system is well adapted to the dating of oils and bitumen. However the meaning of the obtained age is ambiguous. This is mainly due to gaps in our knowledge of the geochemical behavior and the speciation of Re and Os in oils. Specifically, use of the Re-Os system requires an understanding of how Re-Os behavior can lead to the fulfillment of the conditions necessary for the development of an isochron. These conditions are: 1) the isotopic homogenization of oils at the scale of a petroleum field 2) the fractionation of Re from Os so as to obtain samples with various Re/Os ratios 3) the closure of the system during the period of radiogenic ingrowth of the daughter isotope, that is, from the time of the event of interest to the present day. Experimental investigation of the organic geochemical behavior of Re and Os in oils under various conditions, designed as analogs of the different stages of petroleum generation and evolution, were performed in order to evaluate the use of the Re-Os system as a geochronometer in the context of a direct use on petroleum. The possibility of Re-Os fractionation resulting from asphaltene loss during oil evolution was investigated by sequential asphaltene precipitation in the laboratory. The possibility of metal transfer from formation waters to petroleum was studied by performing contact experiments between oils and aqueous solutions of Re and Os of various concentrations over a wide range of temperatures and for varying periods of time. Finally, the study by X-ray absorption spectroscopy of Re and Os in oils artificially enriched in these elements contributed to the proposal of a mechanism for Re and Os incorporation at the oil-water interface

Le craton ouest-africain héberge de nombreux gisements et occurrences aurifères encaissés dans des terrains paléoprotérozoïques. Ces terrains birimiens se sont formés et accrétés lors du cycle orogénique éburnéen qui s’est déroulé entre 2,25 et 1,98 Ga. Six gisements d’or situés au Burkina Faso et au Ghana ont été étudiés dans le but d’établir une corrélation entre les différentes phases de l’orogenèse et le (ou les) événement(s) minéralisateur(s). L’étude détaillée de la minéralisation de ces six gisements a confirmé leur nature orogénique, mais a également révélé la présence d’un porphyre à Cu (±Au) dans le district minier de Gaoua, Burkina Faso. Par ailleurs, nos observations ont mis en évidence le caractère polyphasé de la minéralisation au sein d’un même gisement. Les datations Re-Os menées sur des sulfures directement liés à la minéralisation en or permettent de distinguer deux grandes périodes métallogéniques au sein de l’orogenèse éburnéenne. La première période se déroule lors de la phase d’accrétion magmatique D1 et de la phase de transition D2, soit entre 2200 et 2120 Ma. Cette période est caractérisée par une minéralisation à faibles teneurs disséminée dans les roches encaissantes. La deuxième période métallogénique prend place lors des stades cassants tardifs de l’orogenèse, aux alentours de 2050-2040 Ma. Elle est représentée par une minéralisation à or visible à plus fortes teneurs, concentrée dans des veines, des brèches et des zones de cisaillement. La mise en évidence de l’existence de minéralisations aurifères précoces pourrait avoir des conséquences sur la compréhension des gisements de type paléoplacer observés notamment au Ghana. Ces minéralisations primaires pourraient en effet constituer la source de l’or des paléoplacers tarkwaïens, mis en place à partir de 2130 Ma, qui demeure inconnue / The West African craton is a region enriched in gold deposits and occurrences which are hosted in Paleoproterozoic terrains. These Birimian terrains formed during the Eburnean orogeny which took place between 2.25 and 1.98 Ga. Six gold deposits situated in Burkina Faso and Ghana were studied in order to define a correlation between the different orogenic phases and the mineralizing event(s). The detailed study of the mineralization of the six deposits confirmed the importance of the orogenic gold deposits in the West African craton. Moreover, the existence of a Cu (±Au) porphyry deposit was revealed in the mining district of Gaoua, southwestern Burkina Faso. Our observations highlighted the polyphased character of gold mineralization within nearly all of the studied deposits. Re-Os dating performed on sulfides directly linked to the gold mineralization permitted two main metallogenic periods to be distinguished. The first period was coincident with the D1 and D2 orogenic phases, occurring between 2200 and 2120 Ma, and representing respectively magmatic accretion and transition towards a collisional regime. This period was characterized by low grade disseminated gold mineralization. The second metallogenic period took place during the later brittle deformational phases of the orogeny, ca. 2050-2040 Ma. This secondary mineralization contains visible gold concentrated in veins, breccias and shear zones, and displays higher gold grades. Geochronological evidence for the existence of early gold mineralization could have consequences for the understanding of paleoplacer-type deposits, observed primarily in Ghana. Indeed, this primary mineralization could represent a potential source for gold found in Tarkwaian paleoplacers, which formed after 2130 Ma, and for which the provenance of the gold remains unidentified

Subsurface data derived from ~388 ft of drill core from Martin County (TX) were used to understand the depositional setting of the Wolfcamp-D, a petroleum producing interval in the Midland Basin. Elemental geochemistry collected via x-ray fluorescence revealed a highly variable depositional history marked by the deposition of diverse siliciclastic and carbonate lithofacies. Integration of multiple datasets resulted in the interpretation of nine lithofacies, whose deposition appears cyclical. Correlations between molybdenum and total organic carbon indicate slow recharge of bottom waters and anoxic/euxinicconditions within the basin. The presence of phosphatic nodules coinciding with siliceous black mudrocks suggested high levels of primary productivity driven by upwelling. High-frequency sea level variability, driven by far-field glaciation and regional paleoclimate, were key controls on both the chemostratigraphy and lithofacies. Along-strike variability is seen throughout the basin due to paleobathymetry, proximity and connections to paleochannels, and localized structures. Rhenium-osmium (Re/Os) geochronology was conducted on siliceous mudrocks with high total organic carbon. A depositional age of 300 ± 18 Ma was obtained, partially confirming previous correlations to shelf biostratigraphic data. Scatter in the Re/Os data is likely due to mixing in the basin or non-hydrogenous Os incorporated into the analysis due to the method of preparation.

The Ertsberg-Grasberg Mining District of Papua, Indonesia (Western New Guinea) hosts the Ertsberg Cu-Au Skarn, the giant Grasberg Porphyry Cu-Au deposit, and several other orebodies. Two 1700-meter-long cores beneath the Kucing Liar ore skarn (KL98-10-22) and the Grasberg Igneous Complex (KL98-10-21) contain high concentrations of vein and disseminated molybdenite. KL98-10-22, the focus of this study, intersects two previously unencountered intrusions, the “Tertiary intrusion Kucing Liar” (Tikl) and “Tertiary Pliocene intrusion” (Tpi). An intense dilatational quartz vein stockwork cuts Tikl and Ekmai Sandstone (Kkes) units, predating Tpi intrusion. Prior to these ultradeep cores, which extend almost 3 km below pre-mining surface, molybdenite was rarely observed in the district. Geochemistry and isotopic data indicate that Tikl and Tpi intrusions originated from the same large magmatic system that emplaced other ore-forming Ertsberg-Grasberg district intrusions. Magma in a lower crustal chamber was recharged at least twice, according to Sr-Nd data. Laser-ablation inductively-coupled plasma mass spectrometry of magmatic zircons yields 238U-206Pb ages between 3.40 ± 0.12 Ma (Dalam Andesite) and 2.77 ± 0.15 Ma (Ertsberg intrusion), revealing a shorter period of igneous activity than previously measured by K-Ar and Ar-Ar dating. Analyses include composite ages of 3.28 ± 0.08 Ma for Tikl and 3.18 ± 0.11 Ma for Tpi. Inherited zircon cores indicate Precambrian (mostly Proterozoic) basement. Molybdenite veining beneath the Kucing Liar Skarn and Grasberg Igneous Complex postdates stockwork veining and occurred before the 2.99 ± 0.11 Ma Kali dikes. Only one molybdenite vein was observed cutting Tpi. Molybdenites yielded ~3 Ma Re-Os ages and anomalous >4 Ma and <0.5 Ma ages; anomalous ages were not reproducible in follow-up analyses (this study). Smearing deformation of molybdenite (through fault activity) causes crystal strain, likely leading to annealing recrystallization. Recrystallization possibly redistributes daughter-product Os, resulting in anomalous ages from annealed material. Fluids with high Mo/Cu ratios (which were likely supercritical) precipitated late-stage molybdenite deep in the system. These fluids developed through magma chamber crystallization, which concentrated molybdenum in the melt as an incompatible element, and stripping of Cu from the magma chamber during hydrothermal activity. / text