Mémoire (M.Sc.T.)-- Université du Québec à Chicoutimi, 1993. / Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Sc.T.)--université du Québec à Chicoutimi, 2000. / Document électronique également accessible en format PDF. CaQCU

The Beattie syenite is composed of a series of lenticular bodies of syenitic rocks, situated immediately north of the Porcupine-Destor fault zone in the town of Duparquet, approximately 32 km north of Rouyn-Noranda in the Abitibi Subprovince. The principal body is 3.3 km long and 470 m in width and is flanked by a series of smaller lenses to the south and northeast. The intrusion has been zircon dated at 2682±1Ma and 2682.9±1.1 Ma and hosted the major part of the Au-mineralization of the now defunct Beattie mine, which was an important producer of gold in the area from 1933 to 1956 (9.66 Mt at 4.88 g/t Au). A total of 5 petrographic units are defined here, on the basis of field relationships, macroscopic textures, petrology and mineralogy: 1- The porphyritic Beattie syenite unit is composed of 2% to 10% of tabular sub- to anhedral feldspar phenocrysts about 2 mm to 10 mm in size, set in a red feldspathic and aphanitic matrix. This unit can be strongly cataclastic with abundant hydrothermal minerals. 2- The equigranular magnetite-bearing syenite unit includes few feldspar phenocrysts, about 2 mm to 10 mm in a fine-grained matrix. It is characterized by unaltered titanite, epidote, hornblende porphyroblasts and is the only unit with actinolite replacing clinopyroxene. 3- The porphyritic Central Duparquet syenite unit contains between 2% - 25% of coarse equant euhedral feldspar phenocrysts, about 5 mm to 16 mm in size, in a red or sometimes grey aphanitic matrix. 4- The megaporphyritic syenite unit is composed of very coarse alkali feldspar phenocrysts, typically 1 cm to 6 cm across, in a red aphanitic matrix. The phenocrysts often form a glomeroporphyritic texture. 5- The lath syenite unit occurs as numerous ~m-width thin dykes, which cross-cut all other petrographic units. These dykes display a characteristic trachytic texture, as defined by the preferential alignment of alkali feldspar laths, which are typically 1 cm to 3 cm in a grey or red aphanitic matrix. Petrology and geochemistry investigations revealed that the syenite intruded into the older 2720-2718 Ma volcanic Deguisier Formation and the <2687±2 Ma sediments of the Porcupine Group. Detailed geochemical analysis revealed that the Deguisier Formation is composed of three units, forming a complete tholeiitic suite. The Deguisier Formation is proposed here to have been produced in a volcanic arc setting. Detailed petrological, mineralogical and geochemical studies indicate that, the syenite is part of the alkaline series and is metaluminous to peraluminous. The syenite intruded at shallow depths through a sequence of injections, yielding the five comagmatic units. The syenite and country rocks were then subjected to a series of hydrothermal alterations and brittle deformation events, combined with metamorphism to the greenschist facies. The Au-mineralization in this region is attributed to the hydrothermal alteration with brittle deformation.

M.Sc. (Geology) / Please refer to full text to view abstract

Submitted by Edisangela Bastos (edisangela@ufpa.br) on 2017-02-14T12:12:11Z No. of bitstreams: 3 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_GeologiaPetrogeneseGreenstone_V1.pdf: 71676863 bytes, checksum: 533a6c11ec5056e6d7d5e3c25ec0a9fc (MD5) Tese_GeologiaPetrogeneseGreenstone_V2.pdf: 25031034 bytes, checksum: a1f5f43677654e5f560b4c78f08e5e4f (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2017-02-14T12:28:05Z (GMT) No. of bitstreams: 3 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_GeologiaPetrogeneseGreenstone_V1.pdf: 71676863 bytes, checksum: 533a6c11ec5056e6d7d5e3c25ec0a9fc (MD5) Tese_GeologiaPetrogeneseGreenstone_V2.pdf: 25031034 bytes, checksum: a1f5f43677654e5f560b4c78f08e5e4f (MD5) / Made available in DSpace on 2017-02-14T12:28:05Z (GMT). No. of bitstreams: 3 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_GeologiaPetrogeneseGreenstone_V1.pdf: 71676863 bytes, checksum: 533a6c11ec5056e6d7d5e3c25ec0a9fc (MD5) Tese_GeologiaPetrogeneseGreenstone_V2.pdf: 25031034 bytes, checksum: a1f5f43677654e5f560b4c78f08e5e4f (MD5) Previous issue date: 1994-10-07 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / FINEP - Financiadora de Estudos e Projetos / Este trabalho trata da geologia e petrogênese do "greenstone belt" Identidade, situado entre as cidades de Xinguara e Rio Maria, SE do Estado do Pará. Os dados obtidos permitiram discutir a evolução geodinâmica do terreno granito - "greenstone" da região de Rio Maria, inserindo-a no contexto da Província Mineral de Carajás (PMC), SE do cráton Amazônico. O "greenstone" em lide compõe um cinturão "sinformal" direcionado WNW-ESE, correspondendo a um pacote metavulcãnico, com xistos ultramáficos (UM), basaltos (BAS) e gabros (GB) na base, e, no topo, rochas hipabissais dacíticas (DAC - ca. 2,94 Ga, Pb/Pb). O conjunto foi intrudido por metaplutônicas Mesoarqueanas, os tipos mais precoces sendo quartzo dioríticos, seguidos sucessivamente por granodioritos (com enclaves máficos), trondhjemitos / tonalitos e leucogranitos. O embasamento gnáissico (GN - aflorante a norte e reconhecido por conter uma fábrica mais antiga Sn-1/D1), o "greenstone" e os metagranitóides foram intrudidos no final do Paleoproterozôico por enxames de diques riolíticos (ca. 1,60 Ga, Rb/Sr) e diabásicos. O "greenstone" apresenta estruturas e texturas ígneas reconhecíveis, porém obliteradas em regiões de contato com metagranitóides e em zonas de cisalhamento. As ultramáficas ocorrem como tremolititos, tremolita - talco xistos e talco xistos; o anfibólio é bastante alongado e fino, comumente em arranjos paralelos, interpretados como fantasmas de texturas "spinifex". Os basaltos são maciços ou almofadados, freqüentemente variolíticos. Mostram diferentes graus de recristalização, sendo identificados restos de texturas hialofiticas, pilotaxíticas e traquitóides. Clinoanfibólio (hornblenda actinolítica), epídotos e plagioclásio (albita - andesina) são os minerais mais abundantes. Os gabros são maciços a porfiriticos, distinguindo-se relíquias de texturas subofiticas e granofiricas. Os dacitos são porfiríticos, com fenocristais de quartzo e plagioclásio (oligoclásio), além de hornblenda e nódulos máficos (biotita, clorita, opacos, epidotos, titanita, apatita) nas variedades menos evoluídas. Dentre os metagranitóides, os leucogranitos e trondhjemitos contêm biotita cloritizada, enquanto granodioritos e parte dos tonalitos portam biotita ou biotita + hornblenda (também em quartzo dioritos). O "greenstone" e os metagranitóides foram afetados por uma deformação dúctil, heterogênea, que evoluiu para zonas miloníticas. A estruturação da área é marcada por uma fábrica planar (Sn//Sm/D2) direcionada WNW-ESE a E-W, de mergulhos divergentes. Lineações de estiramento E-W, WNW-ESE ou NW-SE, meso e microestruturas assimétricas S-C, peixes de micas e de clinoanfibólios, e rotações de porfiroclastos a e 15 indicaram uma megaestrutura resultante de um binário com encurtamento NW-SE. A geometria atual do "greenstone" seria derivada de transpressão dextrógira, com o "greenstone" compondo uma estrutura em flor positiva. O regime transpressivo favoreceu a criação de regiões transtrativas, onde se alojaram plútons graníticos no NW, além de clivagens de crenulação extensional (Sn+i/D2) no SW. A quantificação da deformação revelou encurtamento da ordem de 60%, extensão subhorizontal, paralela ao "trend" do "greenstone", de 68 a 500%, e extensão vertical de 101 a 280%. O elipsóide de deformação variou de oblato a prolato, com mudanças de densidade e rotação do eixo de estiramento máximo (X) nas zonas miloníticas. A inversão da deformação permitiu reconstruir a forma original do "greenstone", que seria também alongada WNW-ESE, embora de excentricidade menor que a atual. Estes dados, juntamente com a petrofábrica do eixo c do quartzo, sugeriram que a deformação progressiva envolveu mecanismos de cisalhamento puro e simples, sendo o arcabouço final resultante deste último. Falhas e fraturas rúpteis diversas, afetando também diques riolíticos e diabásicos, marcaram o último evento (D3). As paragêneses minerais do metamorfismo principal (Mn/M2) originaram-se de recristalização estática, pré-tectônica, que modificou parte das texturas e quase totalmente a mineralogia das rochas do "greenstone". Formaram-se anfibólio verde azulado (hornblenda actinolítica), epídotos (pistacita predominante), titanita e quartzo em BAS e GB; tremolita, talco e clorita em UM. Saussuritização e sericitização de plagioclásio, biotitização de anfibólio, cloritização de biotita e transformação de hornblenda em titanita verificaram-se nos metagranitóides. A coexistência de hornblenda + plagioclásio (An> 17) e/ou hornblenda actinolítica + epidotos + clorita em rochas metabásicas mostrou que o evento supra foi de pressão baixa e temperaturas transicionais entre as fácies xisto verde e anfibolito. Este episódio essencialmente térmico refletiu o aquecimento crustal produzido pelo plutonismo do final do Mesoarqueano, tendo obliterado as associações prévias do metamorfismo de fundo oceânico. Ligeiramente concomitante a francamente subseqüente, houve um evento de recristalização dinâmica extensiva (Mm/M2) na fácies xisto verde, particularmente em zonas de cisalhamento e contatos litológicos. Em tais locais, existem evidências de aporte de fluidos (blastomilonitos xistosos e abundantes veios de quartzo) e remobilização da maioria dos elementos químicos (Al, Fe, Ca, K, Na, Rb, Sr, Zr). Em condições PT ainda menores, deu-se finalmente a ação de um evento discreto, relacionado com crenulações e formando clorita, epídotos e quartzo (Mn+1/M2). O evento M2, bem como aquele detectado somente em GN (M1 em fácies anfibolito), foram de natureza dúctil, o que os distinguiu nitidamente do último episódio (D3/M3). Este foi posicionado no final do Paleoproterozóico, tendo caráter hidrotermal e associado á feições rúpteis de alto nível crustal. A evolução progressiva do metamorfismo M2, com pico térmico precoce ao pico da deformação, sugeriu uma trajetória P-T-t anti-horária, correspondente á evolução metamórfica de bacias marginais fanerozóicas. Algumas análises químicas de rochas metavulcânicas permitiram a definição de séries magmáticas e discussão de modelos petrogenéticos. Reconheceram-se três séries geoquímicas, a saber, da mais antiga para a mais nova, komatiítica (UM), toleitica (BAS e GB) e cálcio-alcalina (DAC). A primeira corresponde a komatiitos peridotíticos, com MgO>18% em peso (base anidra), com um "trend" de enriquecimento em Al, tal como em Geluk e Munro, e menos cálcico do que Barberton. Os padrões de terras raras leves são irregulares, com razões (La/Sm)N entre 0,42 e 4,2 e anomalias negativas de Eu. Os terras raras pesadas pareceram menos afetados por processos pós-eruptivos, sendo planos ou ligeiramente fracionados (1,0<(Gd1Yb)N<2,3). Modelos quantitativos foram de dificil execução em virtude da remobilização de vários elementos, porém, em termos qualitativos, foi possível estimar cumulados ricos em olivina e ortopiroxênio. Dentre os toleítos, BAS e GB apresentaram padrões geoquímicos muito similares entre si. Ambos são toleítos de baixo potássio, comparáveis a toleítos arqueanos empobrecidos. Os elementos terras raras são quase planos, com valores 10X o condrito, e anomalias fracas ou inexistentes de Eu. Modelos preliminares sugeriram cumulados semelhantes para BAS e GB, compostos essencialmente de clinopiroxênio e plagioclásio. De acordo com alguns cálculos geoquímicos, a fonte dos magmas que originaram os komatiitos e toleítos seria o lherzolito a granada. Os DAC apresentaram características geoquímicas afins à metavulcânicas e metaplutônicas cálcio-alcalinas tanto modernas quanto arqueanas, seguindo o "trend" trondhjemítico. A diferenciação magmática teria decorrido por fracionamento de plagioclásio>quartzo>hornblenda>K-feldspato, com quantidades accessórias de biotita, magnetita, titanita, alanita e zircão. A fonte do magma dacítico seria crustal do tipo toleíto metamorfisado em fácies granada anfibolito e ligeiramente enriquecido em terras raras leves. No modelo geodinâmico proposto, já existia um embasamento gnáissico antes de 2,96 Ga. Entre 2,96 e 2,90 Ga, a conjugação de alto gradiente geotérmico com extensão litosférica provocou o rifteamento continental, formando bacias marginais, onde se daria a extrusão de komatiitos e toleítos. Em torno de 2,94(?)-2,90 Ga, geraram-se os DAC através de fusão de crosta oceânica em zonas de subducção, evoluindo por fracionamento a baixas pressões. Os mesmos mecanismos geradores dos DAC também seriam responsáveis pelo plutonismo cálcio-alcalino, culminando com a inversão estrutural do "greenstone", espessamento crustal e forma final do terreno granito - "greenstone" (transpressão dextrógira ca. 2,88-2,86 Ga). A região sofreu ainda um episódio de (rea)quecimento, detectado a nível de minerais, sem deformação e metamorfismo correlatos, ao final do Eoarqueano (2,69-2,50 Ga), e intrusão de enxames de diques riolíticos (1,60 Ga, Rb/Sr) e diabásicos ao final do Paleoproterozóico. A correlação com o conhecimento atual da PMC permitiu admitir que o terreno granito - "greenstone" de Rio Maria já estava configurado quando da implantação do Supergrupo Itacaiúnas (ca. 2,76 Ga) e da granitogênse alcalina na Serra dos Carajás. Assim, a transpressão sinistrógira que inverteu aquele supergrupo corresponderia a um evento posterior e bem distinto da transpressão dextrógira da região de Rio Maria. / This thesis deals to the geology and petrogenesis of the Identidade greenstone belt, located between Xinguara and Rio Maria towns, SE of Pará state. The data of this area permitted the discussion of the tectonic evolution of the gravite greenstone terrain of the Rio Maria region in the context of the Província Mineral de Carajás, SE of the Amazonian craton. The greenstone studied compose a synformal belt in the WNW-ESE direction, corresponding to one metavolcanic pile, formed predominantly by ultramafic schists (UM), basalts (BAS) and gabbros (GB) at the base, and hypabyssal dacitic rocks (DAC - ca. 2.94 Ga, Pb/Pb) at the top. The whole was intruded by metaplutonic rocks of Mesoarchean ages, the older one being quartz diorites, followed successively by granodiorites, trondhjemites / tonalites and leucogranites. The gneissic basement (GN - outcroping toward north and recognized for having an older fabric Sn-1/D1), the greenstone and the metagranitoids were intruded by hypabyssal rhyolitic (ca. 1.60 Ga, Rb/Sr) and basic dykes at the end of the Paleoproterozoic. The greenstone presents igneous structures and textures still recognized, although obliterated near the contacts with the metagranitoids and shear zones. The ultramafics occur as tremolitites, tremolite - talc schists and talc schists; the amphibole is very elongated and thin, commonly in parallel arrays, interpreted as ghosts of spinifex textures. The basalts are massive or pillowed and frequently variolitic. They show different degrees of recrystallization, with some relicts of hyalophitic, pilotaxitic and traquitoid textures. Clinoamphibole (actinolitic hornblende), epidotes and plagioclase (albite - andesine) are the most abundant minerais. The gabbros may be massives to porphyritics (plagioclase phenocrysts), still with some relicts of subophitic and granophyric textures. The dacites are porphyritic, with phenocrysts of quartz and plagioclase (oligoclase), besides hornblende and mafic clots (biotite, chlorite, opaque minerais, epidotes, sphene, apatite) in the less evolved samples. Concerning the metagranitoids, the leucogranites and trondhjemites have chloritized biotite, whereas the granodiorites and some tonalites comprise biotite or biotite + hornblende (also in quartz diorites). The greenstone and the metagranitoids were affected by one event of heterogeneous, ductile deformation, that evolved to mylonitic zones. The structural framework of the area is marked by a planar fabric (Sn//Sm/D2) in the WNW-ESE to E-W direction, with moderate to strong dips in a divergent fan. E-W, WNW-ESE or NW-SE stretching lineations, meso and asymmetric S-C microstructures, mica and clinoamphibole fishes, and rotation of o and i porphyroclasts indicated one megastructure resulting from a binary system with NW-SE shortening direction. The actual geometry of the greenstone would be derived from a dextral transpression, with the greenstone forming a positive flower structure. The transpressional regime favored the grow of transtensional cites and subsequent emplacement of granitic plutons on the NW contact, and extensional crenulation cleavage (Sn+1/D2) on the SW of the greenstone. Strain measurements displayed a ca. 60% shortening, subhorizontal extension of ca. 60 to 500% parallel to the greenstone trend, and vertical extension of ca. 101 to 280%. The strain ellipsoid may be oblate to prolate, with changes in density and rotation of the axis of maximum stretching (X) toward the mylonitic zones. The inversion of the deformation permitted the reconstruction of the original shape of the greenstone, that would be also elongated WNW-ESE, but with lesser eccentricity than today. These data, together with the quartz petrofabric, suggested that the deformation has been accommodated by pure and simple shear mechanisms, the final framework resulting essentially from the later. The last event (D3) are represented by faults and fractures which also affected the felsic and basic dykes. The paragenesis of the main metamorphic event (Mn/M2) is represented by static recrystallization, which modified some textures and almost ali minerais within the greenstone. The minerais formed phases were bluish green amphibole (actinolitic hornblende), epidotes, sphene and quartz in BAS and GB; tremolite, talc and chlorite in UM. The metagranitoids show transformations of plagioclase (saussurite, fine white mica), amphibole (to biotite and/or sphene) and biotite (to chlorite). The coexistence of hornblende + plagioclase (An>17) and/or actinolitic hornblende + chlorite in metabasic rocks shows that this event was of low pressures and temperatures in the transitional field of the greenschist and amphibolite facies. This episode should reflect a regional crustal heating produced by the plutonism at the end of the Mesoarchean, that obliterated the previous associations of ocean floor metamorphism. Slightly coeval to subsequently, it occurred one event of extensive dynamic recrystallization (Mm/M2) in the greenschist facies, specially within shear zones and lithological contacts. In these places, there are evidences of fluid incoming (schistose blastomylonites and abundant quartz veins) and remobilization of chemical elements (Al, Fe, Ca, K, Na, Rb, Sr, Zr). Finally, under lower PT conditions, it occurred a less expressive event related to crenulation cleavages and forming chlorite, epidotes and quartz (Mn+1/M2). The M2 event, as well as the one detected only in GN (M1 under amphibolite facies), was of ductile nature and cleary distinguished from the last one (D3/M3). The later was placed at the end of the Paleoproterozoic, being of hydrothermal character and associated to high crustal structures. The progressive evolution of the M2 metamorphism with its thermal peak predating the deformation suggested a counterclockwise P-T-t path, corresponding to the metamorphic evolution of Phanerozoic marginal basins. Some chemical analysis of the metavolcanic rocks permitted the definition of magmatic series and a discussion of petrogenetical modeling. It was possible to recognize three geochemical series, that is, from the older to the younger, komatiitic (UM), tholeiitic (BAS and GB) and calc-alkaline (DAC). The first one corresponds to peridotitic komatiites with MgO>18 weight % (volatile-free basis), with an enrichment trend in Al, such as in Geluk and Munro, and less calcic than the Barberton one. The light rare earth element patterns are irregular with (La/Sm)N ratios between 0.42 and 4.2 and negative Eu anomalies. The heavy rare earth elements seem less affected by post-eruptive processes, being plate or slightly fractionated (1.0<(Gd/Yb)N<2.3). The quantitative models were of hard execution due to the remobilization of several elements. It was possible estimate cumulates rich in olivine and orthopyroxene. With regarding to tholeiites, the BAS and GB showed very similar geochemical signatures, both being low potassium tholeiites comparable to depleted Archean tholeiites. The rare earth elements are almost plate, with values 10X the chondrite, and slight or no Eu anomaly. Preliminary modeling suggested similar cumulates for BAS and GB, composed essentially by clinopyroxene and plagioclase. The magma sources that originated the komatiites and tholeiites would be a garnet lherzolite. The DAC presented geochemical characteristics of modern and Archean metavolcanics and metaplutonics of trondhjemitic nature. The magmatic differentiation would be achieved by fractionation of plagioclase>quartz>hornblende>K-feldspar, with subordinated amount of biotite, magnetite, sphene, allanite and zircon. The source of the dacitic magma would be a tholeiite metamorphosed to the garnet amphibolite facies and somewhat enriched in light rare earth elements. The geodynamical model proposed admit the existence of a gneissic basement prior to 2.96 Ga. Between 2.96 and 2.90 Ga, the interplay of high geothermal gradients and lithospheric extension was responsible for extensive rifting, forming marginal basin systems, where extruded the komatiitic and tholeiitic rocks. At 2.94(?)-2.90 Ga, the DAC were generated from partia' melting of oceanic crust in subduction zone settings, and evolved by low pressure fractional crystallization. The same mechanisms that generated the DAC are extended also to the calc-alkaline plutonism, this one being responsible for the structural inversion of the greenstone, crustal thickening and final shape of the granite - greenstone terrain (dextral transpression ca. 2.88-2.86 Ga). The region still suffered a late episode (end of Eoarchean, 2.69-2.50 Ga) of (re)heating, registered only in sorne mineral, without any evidente of deformation and/or metamorphism. Finally, it occurred the intrusion of felsic (1.60 Ga, Rb/r) and basic dykes at the end of the Paleoproterozoic. The correlation with the actual understanding of the Província Mineral de Carajás permitted envisage that the Rio Maria granite - greenstone terrain was then configured at the moment of implantation of the Itacaiúnas Supergroup (ca. 2.76 Ga) and alkaline granitic plutonism at the Serra dos Carajás. So the sinistrai transpression that inverted that supergroup would correspond to a newer event, very distinct as regards as the dextral transpression of the Rio Maria region.

Geologically-constrained inversion of geophysical data is a powerful method for predicting geology beneath cover. The process seeks 3D physical property models that are consistent with the geology and explain measured geophysical responses. The recovered models can guide mineral explorers to prospective host rocks, structures, alteration and mineralisation. This thesis provides a comprehensive analysis of how the University of British Columbia Geophysical Inversion Facility (UBC–GIF) gravity and magnetic inversions can be applied to subsurface mapping and exploration by demonstrating the necessary approach, data types, and typical results. The non-uniqueness of inversion demands that geological information be included. Commonly available geological data, including structural and physical property measurements, mapping, drilling, and 3D interpretations, can be translated into appropriate inversion constraints using tools developed herein. Surface information provides the greatest improvement in the reliability of recovered models; drilling information enhances resolution at depth. The process used to prepare inversions is as important as the geological constraints themselves. Use of a systematic workflow, as developed in this study, minimises any introduced ambiguity. Key steps include defining the problem, preparing the data, setting inversion parameters and developing geological constraints. Once reliable physical property models are recovered they must be interpreted in a geological context. Where alteration and mineralisation occupy significant volumes, the mineralogy associated with the physical properties can be identified; otherwise a lithological classification of the properties can be applied. This approach is used to develop predictive 3D lithological maps from geologically-constrained gravity and magnetic inversions at several scales in the Agnew-Wiluna greenstone belt in Australia’s Yilgarn Craton. These maps indicate a spatial correlation between thick mafic-ultramafic rock packages and gold deposit locations, suggesting a shared structural control. The maps also identify structural geometries and relationships consistent with the published regional tectonic framework. Geophysical inversion provides a framework into which geological and geophysical data sets can be integrated to produce a holistic prediction of the subsurface. The best possible result is one that cannot be dismissed as inconsistent with some piece of geological knowledge. Such a model can only be recovered by including all available geological knowledge using a consistent workflow process.

Geologically-constrained inversion of geophysical data is a powerful method for predicting geology beneath cover. The process seeks 3D physical property models that are consistent with the geology and explain measured geophysical responses. The recovered models can guide mineral explorers to prospective host rocks, structures, alteration and mineralisation. This thesis provides a comprehensive analysis of how the University of British Columbia Geophysical Inversion Facility (UBC–GIF) gravity and magnetic inversions can be applied to subsurface mapping and exploration by demonstrating the necessary approach, data types, and typical results. The non-uniqueness of inversion demands that geological information be included. Commonly available geological data, including structural and physical property measurements, mapping, drilling, and 3D interpretations, can be translated into appropriate inversion constraints using tools developed herein. Surface information provides the greatest improvement in the reliability of recovered models; drilling information enhances resolution at depth. The process used to prepare inversions is as important as the geological constraints themselves. Use of a systematic workflow, as developed in this study, minimises any introduced ambiguity. Key steps include defining the problem, preparing the data, setting inversion parameters and developing geological constraints. Once reliable physical property models are recovered they must be interpreted in a geological context. Where alteration and mineralisation occupy significant volumes, the mineralogy associated with the physical properties can be identified; otherwise a lithological classification of the properties can be applied. This approach is used to develop predictive 3D lithological maps from geologically-constrained gravity and magnetic inversions at several scales in the Agnew-Wiluna greenstone belt in Australia’s Yilgarn Craton. These maps indicate a spatial correlation between thick mafic-ultramafic rock packages and gold deposit locations, suggesting a shared structural control. The maps also identify structural geometries and relationships consistent with the published regional tectonic framework. Geophysical inversion provides a framework into which geological and geophysical data sets can be integrated to produce a holistic prediction of the subsurface. The best possible result is one that cannot be dismissed as inconsistent with some piece of geological knowledge. Such a model can only be recovered by including all available geological knowledge using a consistent workflow process.

[Truncated abstract] The Laverton Greenstone Belt (LGB), located in the northeastern part of the Eastern Goldfields Province (EGP) of the Yilgarn Craton, Western Australia, has a total contained gold endowment of over 690t. An important feature of the gold deposits in the LGB is their close spatial association with granitoids, with many gold deposits located adjacent to, or hosted by, granitoids. Recently-proposed genetic models for Archaean orogenic gold deposits have emphasised the role of granitoids in the formation of ore-deposits, but differ significantly in the nature of that role. Some models suggest that the granitoids are a source of ore-fluids and solutes, whereas others suggest that granitoids exert an important structural control on gold mineralisation. Such competing genetic models for gold mineralisation variably propose either a proximal-magmatic or distal-metamorphic, or less commonly distal-magmatic, source for goldbearing fluids, or mixing of fluids from multiple sources. Isotope geochemistry and geochronological studies are used to constrain the source and timing of auriferous fluids at nine gold deposits in the LGB in an attempt to differentiate between conflicting genetic models. To overcome the lack of detailed deposit-scale geological constraints inherent to any regional study, hypotheses generated from regional datasets are tested in a detailed case-study of the Wallaby gold deposit. The Pb-isotope compositions of ore-related sulphides from deposits in the LGB plot along the line representing crustal-Pb in the Norseman-Wiluna Belt of the EGP, with individual deposits clustering with other nearby deposits based on their geographic location. This trend is similar to that recorded in the Kalgoorlie-Norseman region in the southern EGP, and is consistent with a basement Pb reservoir for gold-bearing fluids. As such, data are consistent with a similar fluid source for all gold deposits. The Nd and Sr isotopic composition of goldrelated scheelite in the LGB clusters very tightly. The inferred ore-fluid composition has a slightly positive εNd, similar to ore fluids at other gold deposits in the EGP for which a proximal magmatic source is highly improbable. As such, Sr and Nd data are consistent with a similar fluid source for the gold deposits analysed in the LGB, but cannot unequivocally define that source. The median S, C and O isotopic compositions of ore minerals from all nine different gold deposits studied in the LGB fall in a very narrow range

Thèse (D.R.Min.) -- Université du Québec à Chicoutimi, 2005. / Bibliogr.: f. 368-390. Document électronique également accessible en format PDF. CaQCU

The Kalahari Goldridge mine is located within the Archaean Kraaipan Greenstone Belt about 60 km SW of Mafikeng in the Northwestern Province, South Africa. Several gold deposits are located within approximately north - south-striking banded iron-formation (BIF). Current opencast mining operations are focused on the largest of these (D Zone). The orebody is stratabound and hosted primarily in the BIF, which consists of alternating chert and magnetite-chloritestilpnomelane-sulphide-carbonate bands ranging from mm to cm scale. The ore body varies in thickness from 15 to 45 m along a strike length of about 1.5 km. The BlF is sandwiched between a sericite-carbonate-chlorite schist at the immediate footwall and carbonaceous meta-pelites in the hanging-wall. Further west in the footwall, the schists are underlain by mafic meta-volcanic amphibolite. Overlying the hanging-wall carbonaceous metapeiites are schist units and meta-greywackes that become increasingly conglomeratic up the stratigraphy. Stilpnomelane-, chlorite- and minnesotaite-bearing assemblages in the BlFs indicate metamorphic temperatures of 300 - 450°C and pressures of less than 5 kbars. The BIF generally strikes approximately 3400 and dips from 60 to 75°E. Brittle-ductile deformation is evidenced by small-scale isoclinal folds, brecciation, extension fractures and boudinaging of cherty BIF units. Fold axial planes are sub-parallel to the foliation orientation with sub-vertical plunges parallel to prominent rodding and mineral lineation in the footwall. Gold mineralization at the Kalahari Goldridge deposit is associated with two generations of subhorizontal quartz-carbonate veins dips approximately 20 to 40°W. The first generation consists of ladder vein sets (Group lIA) preferentially developed in Fe-rich meso bands, whilst the second generation consists of large quartz-carbonate veins (Group lIB), which crosscut the entire ore body extending into the footwall and hanging-wall in places. Major structures that control the ore body are related to meso-scale isoclinal folds with fold axes subparallel to mineral elongation lineations, which plunge approximately 067°E. These linear structures form orthogonal orientation with the plane of the mineralized shallowdipping veins indicating stretching and development of fluid - focusing conduits. A second-order controlling feature corresponds to the intersection of the mineralized veins and foliation planes of host rock, plunging approximately 008°N and trending 341°. G0ld is closely associated with sulphides, mainly pyrite and pyrrhotite and to a lesser extent with bismuth tellurides, and carbonate gangue. The ore fluid responsible for the gold deposition is in the C-O-H system with increased CH₄ contents attributed to localized hydrolysis reaction between interbedded carbonaceous sediment and ore fluid. The fluid is characterized by significant C0₂ contents and low salinities below 7.0 wt % NaCl equivalent (averages of 3.5 and 3.0 wt % NaCl equivalent for the first and second episodes of the mineralization respectively) . Calculated values of f0₂. ranging from 10⁻²⁹·⁹⁸ to 10⁻³²·⁹⁶ bars, bracket the C0₂-CH₄ and pyrite-pyrrhotite-magnetite buffer boundaries and reveal the reducing nature of the ore fluid at deposition. Calculated total sulphur content in the ore fluid (mΣs), ranges from 0.011 to 0.018M and is consistent with the range (10⁻³·⁵ to 10⁻¹M) reported for subamphibolite facies ore fluids. The close association of sulphides with the Au and nature of the fluid also give credence that the Au was carried in solution by the Au(HS)₂ - complex. Extensive epigenetic replacement of magnetite and chlorite in BIF and other meta-pelitic sediments in the deposit by sulphides and carbonates, both on meso scopic and microscopic scales gives evidence of an interaction by a CO₂- and H₂S-bearing fluid with the Fe-rich host rocks in the deposit. This facilitated Au precipitation due to changes in the physico-chemical conditions of the ore fluid such as a decrease in the mΣs and pH leading to the destabilization of the reduced sulphur complexes. Local gradients in f0₂ may account for gold precipitation in places within carbonaceous sediments. The fineness of the gold grams (1000*Au/(Au + Ag) ranges from 823 to 921. This compares favourably with the fineness reported for some Archaean BIFhosced deposits (851 - 970). Mass balance transfer calculations indicate that major chemical changes associated with the hydrothermal alteration of BIF include enrichment of Au, Ag, Bi, Te, volatiles (S and CO₂), MgO, Ba, K and Rb but significant depletion of SiO₂ and minor losses of Fe₂O₃. In addition, anomalous enrichment of Sc (average, 1247%) suggests its possible use as an exploration tool in the ferruginous sediments in the Kraaipan greenstone terrane. Evidence from light stable isotopes and fluid inclusions suggests that the mineralized veins crystallized from a single homogeneous fluid source during the two episodes of mineralization under the similar physicochemical conditions. Deposition occurred at temperatures rangmg from 350 to 400°C and fluid pressures ranging from 0.7 to 2.0kbars. Stable isotope constraints indicate the following range for the hydrothermal fluid; θ¹⁸H₂O = 6.65 to 10.48%0, 8¹³CΣc = -6.0 to -8.0 %0 and 8³⁴SΣs = + 1.69 to + 4.0%0 . These data do not offer conclusive evidence for the source of fluid associated with the mineralization at the Kalahari Goldridge deposit as they overlap the range prescribed for fluid derived from devolatization of deep-seated volcano-sedimentary piles near the brittle-ductile transition in greenstone belts during prograde metamorphism, and magmatic hydrothermal fluids. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in