No description available.

No description available.

No description available.

The Abhainn Srathain copper mineralisation with at least 10 million tonnes of rock containing copper was worked during the Eighteenth Century and is situated 1-2km to the south of Meall Mór, South Knapdale. The mineralisation is hosted by epidiorites, quartzites and schists of the Upper Erins Quartzite Formation in which the levels of copper reach up to 2%, 1.3% and 0.8% respectively. The main suiphide phases, pyrite and chalcopyrite, occur in disseminations, in layers and as large crystals in quartz and/or calcite cross-cutting veins. The observed opaque mineral textures are due to recrystallisation, deformation and limited mobilisation indicating a premetamorphic origin for the mineralisatlon. Microscopic compositional variation of the minerals and isotopic geothermometry of analysed pyrite-chalcopyrite pairs suggest disequilibriun conditions during the regional metamorphism. Sulphides contain low minor element concentrations with a high Co:Ni ratio in pyrite (12.5:1). The mineralisation is associated with the local development of epidote, Mn-rich garnet, chlorite, muscovite and calcite and/or quartz cross-cutting veins which all resulted from premetamorphic alteration during ore formation. During this alteration CaO, Fe₂0₃, CO₂, MnO, Cu, S and some trace elements were added, Al₂0₃ was diluted and MgO, FeO, alkalis and some trace elements were removed. The isotopic composition of bacteriogenically reduced sulphur from sulphides throughout the Knapdale Pyrite Horizon ranges between o³⁴s=+4.5 and 12.8 per mil. The consistent isotopic values of the suiphides from the Abhainn Srathain copper mineralisation with an average of around +7 per mil regardless of location, depth, lithology and style of mineralisation suggest that the source of the hydrothermal sulphur is a mixture of inorganically reduced downward percolating Dalradlan seawater sulphate and sulphur leached from interbedded basic igneous rocks. Weak exhalative activity caused by the shallow intrusion of sill bodies into the wet unlithified sediments of the Lower and Upper Erins tiartzite accompanied the deposition of the Upper Erins Quartzite and is expressed by weak disseminated and stratiform pyrite with traces of chalcopyrite and sphalerite (Knapdale Pyrite Horizon). Increasing intensity of this exhalation was due to the creation of a geothermal system centred at the site of the present copper mineralisation. During this stage the hot ascending water reacted with the rocks causing local alteration and precipitation of pyrite and chalcopyrite as disseminations, layers and cross-cutting veins. At the same time cold water descended into the hot intrusives and altered the rocks by dissolving silica and precipitating calcite and oxides.

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O Sienito Pedra Branca, com cerca de 90 km2, é um plúton tardi-orogênico com idade de 610 Ma intrusivo na Nappe Socorro-Guaxupé, e é truncado a oeste por rochas alcalinas (fonolitos e nefelina sienitos) do Maciço de Poços de Caldas, de idade cretácea. O plúton é formado por quatro unidades sieníticas, que intrudiram em três pulsos principais. O primeiro pulso é representado pela unidade Sienito Laminado Saturado a Insaturado (SLSI) de tendência alcalina que aflora na parte N e S-SE do plúton. O segundo pulso consiste na intrusão do Sienito Laminado Supersaturado (SLS), que trunca o interior do SLSI. O último pulso trunca internamente SLS, e constitui o centro do plúton, denominado Sienito Supersaturado Interno (SSI). Externamente ao plúton, separado por um septo de granitos, está o Sienito Supersaturado Externo (SSE), que pode estar associado ao segundo ou terceiro pulso. O Sienito Pedra Branca é denominado por sienitos hipersolvus com índice de cor entre 20-25, nos quais as variações mineralógicas mostram diferentes condições de cristalização. O SLSI atingiu condições mais oxidantes que os sienitos supersaturados, como demonstrado por suas paragêneses máficas (egirina-augita + flogopita + hematita + magnetita versus diopsídio + flogopita ± hornblenda + ilmeno-hematita ± magnetita). Embora em todos os sienitos a cristalização tenha ocorrido sob condições oxidantes, o Mg# maior dos minerais máficos e as maiores proporções do componente hematita nos óxidos romboédricos indicam que a fugacidade de oxigênio foi mais elevada nos SLSI. Dois tipos principais de enclaves são encontrados. Enclaves máfico-ultramáficos são encontrados em todas as unidades, enquanto os microgranulares são exclusivos dos sienitos supersaturados, e têm composições vairáveis nos campos dos sienitos, monzonitos e dioritos. Os enclaves monzoníticos e dioríticos são interpretados com produtos de magmas básicos que interagiram com o sienito hospedeiro em câmaras magmáticas mais profundas, e foram trazidos durante a ascensão. Característica particular dos sienitos supersaturados é a presença de cristais de plagioclásio corroídos, englobados por feldspato alcalino, que possuem composições semelhantes à do plagioclásio presente nos enclaves dioríticos (\'An IND.25-35\'). Estimativas de temperatura liquidus utilizando a saturação em apatita mostraram altas temperaturas dos magmas sieníticos, entre 1040 e 1160º C. Já estimativas de pressão e fO2 foram frustradas devido à ausência de plagioclásio magmático nestas rochas (limita o uso do barômetro de Al na hornblenda) e à ausência de equilíbrio para os óxidos de Fe-Ti (pares ilmeno-hematita + magnetita). As unidades do Sienito Pedra Branca mostram uma pequena variação composicional e diferem principalmente conteúdo de \'Al\' IND.2\'\'O IND.3\', compartilhando assinaturas geoquímicas com elevados teores em LILE como Ba (4000-10000 ppm) e Sr (2000-4500 ppm), além de outros elementos como \'P IND.2\'\'O ind.5\' (1-2%) elementos terras raras leves (La= 100-380 ppm). As semelhanças geoquímicas entre as unidades indicam que, apesar de suas diferenças, elas têm origem comum. Os modelos de gênese de magmas potássicos da literatura sugerem como área fonte mais provável dessas rochas o manto enriquecido. As altas razões LILE/HFSE, demonstradas pelas pronunciadas anomalias negativas em Nb-Ta, Ti e Hf-Zr, indicam vinculo com áreas ativas onde a injeção de fluidos ricos em REE e LILE durante a subducção gera meassomatismo no manto superior. As assinaturas isotópicas com Nd(t) pouco negativo (-7.6 a -8.1) e razões \'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr IND.(t)\'+ 0.7077- 0.7078 associadas ao elevado conteúdo de Ba, Sr e ETH, reforçam que estes magmas estão associados à fusão parcial de horizontes enriquecidos do manto litosférico. A associção de sienitos insaturados e supersaturados em sílica, em paralelo com a presença de enclaves dioríticos exclusivamente nos sienitos supersaturados sugerem que a violação da barreira térmica ocorreu devido à interação de magmas saturados de tendência alcalina com magmas básicos. A falta de evidências de interação in situ (e.g. diques sin-plutônicos) e a ocorrência de rochas microgranulares mais máficas apenas como enclaves retrabalhados indicam que a interação entre os magmas e consequentes mudanças composicionais deve ter ocorrido em níveis mais profundos da crosta. / The Pedra Branca Syenite, with about 90 Km2 in area, is a 610 Ma late-orogenic pluton intrusive in the Socorro-Guaxupé Nappe and truncated to the west by alkaline rocks (phonolite and nepheline syenite) from the cretacic Poços de Caldas Massif. The pluton is made up of four syenite units that intruded as three main magmatic pulses. The first pulse is represented by the laminated silica-saturated to undersaturated syenite (SLSI) with alkaline tendency that occurs at N and S-SE of the plúton. The second pulse corresponds to the laminated oversaturated syenite (SLS), which truncates SLSI. The last pulse, named internal oversaturated syenite (SSI), cuts SLS internally and makes up the plutons core. Externally, separated from the main pluton by a granite septum, occurs the external oversaturated syenite (SSE) that may be associated to the second or third pulse. The Pedra Branca Syenite is dominated by hypersolvus syenites with color indices 20-25, whose mineralogical variations show contrasts in crystallization conditions. The SLSI reached more oxidizing conditions than the oversaturated syenites, as recorded by their mafic assemblages (egirine-augite + phlogopite + hematite + magnetite versus diopside + phlogopite ± hornblende + ilmeno-hematite ± magnetite). Although in all syenites the crystallization occurred under oxidizing conditions, the SLSI have higher Mg# in the mafic minerals and grater proportions of the hematite component in the rhombohedral oxides, indicating higher oxygen fugacity, close to the HM buffer. Two main types of enclaves are present in the Pedra Branca Syenite. The mafic-ultramafic enclaves occur in all units, while microgranular enclaves are exclusive of silica-oversaturated syenites, and have compositions varying in the compositional fields of syenites, monzonites and diorites. Monzonitic and dioritic enclaves are interpreted as products of basic magmas that interacted with the syenitic host in deeper magma chambers and were carried during ascension. Another particular feature of the silica-oversaturated syenites is the presence of corroded plagioclase included in alkali-feldspars, which have compositions similar to the plagioclase crystals from the diorite enclaves (\'An IND.25-35\'). Estimatives of liquidus temperatures using apatite saturation showed high temperatures for the syenitic liquids, between 1040 and 1160° C. Pressure and fO2 estimates were frustrated by the lack of magmatic plagioclase (restricting the use of the Al in hornblende barometers) and the lack of equilibrium between the Fe-Ti oxides (ilmeno-hematite + magnetite). The Pedra Branca Syenite units show small compositional variations, and differ mostly in \'Al IND.2\'\'O IND.3\' content (higher in silica-oversaturated syenites), sharing geochemical signatures with high contents of LILE such as Ba (4000-10000 ppm) and Sr (2000-4500 ppm), plus other elements as \'P IND.2\'\'O IND.5\' (1,0-2%) and light rare earth elements (La= 100-300 ppm). Geochemical similarities between the units indicate that despite their differences, they have common origin. Models of potassic magmas genesis in the literature suggest as most likely source areas the enriched mantle. The high LILE/HFSE ratios, highlighted by strong negative anomalies of Nb-Ta, Ti and Zr-Hf in spiderdiagrams are suggestive of links with active areas where rich-REE and LILE fluid injection during subduction caused metassomatism in upper mantle. Isotopic signatures with moderately negative Nd(t) (-7.6 to -8.1) and \'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr IND.(t)\'=0.70767 to 0.70779 associated with the high trace elements contents (Ba, Sr and REE), reinforces that these rocks are associated with partial melting of enriched horizons of the lithospheric mantle. The association of silica undersaturated and oversaturated syenites, and the presence of dioritic enclaves only in the oversaturated units suggests that the violation of the thermal barrier may be connected to the coexistence with basic magmas. The lack of evidence of in situ interaction (e.g., synplutonic dykes) and occurrence of more mafic microgranular rocks only as reworked enclaves indicates that the magma interaction and consequent compositional changes must have occurred at deeper crustal.