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9. Mineral chemistry and thermobarometry of Jurassic arc granitoids: implications for petrotectonic and unroofing history of the southern Colombian Andes.

Author

Siachoque, Astrid, García-Chinchilla, Daniel Alejandro, Zapata, Sebastian, Cardona, Agustín, Vlach, Silvio Roberto Farias, Bustamante, Camilo, and Chavarría, Luisa Fernanda

Subjects
HORNBLENDE, MAGMAS, HYDROUS, ENSTATITE, PHLOGOPITE
Abstract

The Jurassic magmatic record in the southern Colombian (Northern Andes) includes numerous subduction-related I-type calc-alkaline granitoids with diverse structures and textures, formed in two main episodes at ∼195 to 165 Ma and ∼165 to 145 Ma. We provide new insights into the mineral chemistry, estimates of intensive parameters and petrogenetic processes of 12 plutonic occurrences in the region, grouped in 4 petrographic associations. Primary mineral assemblages include labradorite-to-oligoclase, alkali feldspars, ferroan enstatite, Mg-rich augite to ferroan-diopside, tschermakite to hastingsite and hornblende and Mg-rich annite; Fe-rich phlogopite and actinolite are post-magmatic phases. Amphibole chemistry indicates that the older (195–165 Ma) Jurassic bodies formed from relatively highly oxidized (f O2 values buffered at −0.1 ≤ NNO ≤ +1.4) hydrous (∼4 to 6 wt % H2O) magmas and their differentiation involves significant crustal assimilation and/or magma mixing, fractional crystallization and late-magmatic re-equilibration processes. In contrast, the younger (165–145 Ma) Jurassic intrusives, derived from subducted-modified mantle sources, record moderately lower oxidized hydrous conditions (f O2 values −0.7 to 0.8 ≤ NNO; ∼5 wt % H2O) with magma evolution mainly controlled by fractional crystallization and late-magmatic re-equilibration processes. Clinopyroxene-only, amphibole-only and amphibole-plagioclase thermobarometry estimations suggest that the Jurassic occurrences crystallized over variable temperature (647°C–1087°C) and pressure (0.7–6.3 kbar) conditions, corresponding to emplacement depths ranging from ∼15, ∼8 to 11, ∼5 to 7 and <4 km along the arc crustal column. The obtained data combined with time evolution allow the identification of exhumed and fragmented arc blocks in the Jurassic magmatic system and provide an essential link between the orogenic deformation event poorly constrained in the Northern Andes. [ABSTRACT FROM AUTHOR]

Published
2024
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20. Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications.

Author

Vlach, Silvio Roberto Farias, Jose Ulbrich, Horstpeter Herberto Gustavo, Ulbrich, Mabel Norma Costas, and Vasconcelos, Paulo Marcos

Subjects
*NEPHELINE syenite, *SANDSTONE, *BRECCIA
Abstract

The Poços de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium- to coarse grained and contain K-feldspar (Or95-83), secondary albite (Ab97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massif 's petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical 40Ar/39Ar plateau ages of 87.1(± 0.5) Ma and 86.6(± 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Poços de Caldas rocks are concentrated in the 75-86 Ma range interval. [ABSTRACT FROM AUTHOR]

Published
2018
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34. Mass balance, mineral chemistry and reflectance spectroscopy of the Pé Quente gold deposit, Alta Floresta Gold Province, MT

Author

Ana Patricia Caires Pereira, Xavier, Roberto Perez, 1958, Crósta, Alvaro Penteado, Vlach, Silvio Roberto Farias, Universidade Estadual de Campinas. Instituto de Geociências, Programa de Pós-Graduação em Geociências, and UNIVERSIDADE ESTADUAL DE CAMPINAS

Subjects
Espectroscopia de reflectância, Geotermometria, Geothermometry, Ouro - Minas e mineração - Alta Floresta (MT), Gold - Mines and mining - Alta Floresta (MT), Reflectance spectroscopy, Mineralogical chemistry, Química mineralógica, Depositos hidrotermais, Hydrothermal deposits
Abstract

Orientadores: Roberto Perez Xavier, Diego Fernando Ducart Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociências Resumo: O setor leste da Província Aurífera de Alta Floresta (PAAF), contexto meridional do Cráton Amazônico, centro-norte do Estado de Mato Grosso, hospeda mais de uma centena de depósitos e ocorrências auríferas que ocorrem na forma de veios, stockworks ou disseminados. Neste setor da província, o depósito Pé Quente representa um dos melhores exemplos de ouro disseminado associado à sulfeto. Descrições de testemunhos de sondagem e petrografia apontam biotita granodiorito como o litotipo hospedeiro da mineralização aurífera. A rocha hospedeira foi afetada por diferentes tipos de alterações hidrotermais espacial e temporalmente sobrepostas representadas por restrita alteração sódica com albita, ampla alteração potássica com feldspato alcalino, carbonatação, alteração fílica com sulfeto disseminado e venular, cloritização, silicificação e propilitização. As zonas mineralizadas, disseminada e venular, são compostas pela assembleia pirita, calcopirita, rutilo, galena e hematita, além de fases ricas em Ag, Te e Bi. Estas mineralizações são aparentemente sobrepostas por um veio de quartzo polimetálico (teor de Au > 30 ppm) com esfalerita, calcopirita, galena e pirita. Os dados obtidos no balanço de massa revelam que na alteração fílica, em comparação com a alteração potássica, houve acréscimo nos elementos móveis Ca, Fe, K, Mg, Na, Li, Ba, Rb e Sr e, imóveis Nb, P, Ta e Ti (exceto Hf, Th e Y). Análises de química mineral em conjunto com espectroscopia de reflectância em amostras mineralizadas provenientes da zona de alteração fílica identificaram as micas brancas fengita e muscovita indicando ampla ocorrência de reações do tipo Tschermak e comprimentos de onda da ligação Al-OH de 2211-2213 nm para os termos ricos em K+ e, entre 2214-2221 nm para os termos enriquecidos em Fe2+ e Mg2+. No grupo das cloritas foram identificadas as variedades: (1) ferro-magnesiana classificada como ripidolita/brunsvigita/diabanita e, clinocloro-Fe/ clinocloro Fe-Al com comprimentos de onda da ligação Mg-OH entre 2339-2349 nm e; (2) magnesiana classificada como brunsvigita/diabanita/penninita e, clinocloro-Fe/clinocloro com comprimentos de onda entre 2330-2338 nm. A geotermometria da clorita (1) Fe-Mg que se encontra em paragênese com o sulfeto na zona fílica forneceu a temperatura média de cristalização de 370-250 °C a qual pode ser estimada como sendo a temperatura de aprisionamento do fluido mineralizante. Os corpos de minério Barba, Goiano e Gabriel, do depósito de ouro Pé Quente, constituem um sistema magmático-hidrotermal único alojado no biotita granodiorito com mineralização aurífera disseminada e venular crono-correlata à zona de alteração fílica representada por muscovita-fengita, clorita Fe-Mg, quartzo e sulfeto (pirita) Abstract: The eastern sector of the Alta Floresta Gold Province (AFGP) located in the southern Amazon Craton, north-central portion of the state of Mato Grosso, hosts a significant number of disseminated and vein-type gold and gold-base metals occurrences. In this sector of the AFGP, the Pé Quente deposit is one of the best representative examples of disseminated gold-sulfide deposit group. Drill core logging and petrographic descriptions reveal that the deposit is essentially hosted by a biotite granodiorite. This host rock has been mainly affected by early and restricted sodic alteration with albite, followed by extensive potassium alteration with alkali-feldspar and carbonate alteration, which is then overprinted by phyllic alteration with white micas-chlorite-quartz. The latter hosts the disseminated gold-sulfide mineralization. Post-mineralization alteration includes chloritization, silicification and propilitization. Disseminated and veinlet ore zones are composed of pyrite, chalcopyrite, rutile, galena and hematite as well as Ag, Te and Bi rich-phases. These styles of mineralization are superimposed by a gold rich (up to 30 ppm) quartz polymetallic vein containing sphalerite, chalcopyrite, galena and pyrite. The mass balance data shows that the phyllic alteration when compared with the potassic alteration is marked by an increase in mobile elements as Ca, Fe, K, Mg, Na, Li, Ba, Rb and Sr, and increase in immobile elements like Nb, P, Ta and Ti (except Hf, Th and Y). Mineral chemistry combined with reflectance spectroscopy in mineralized samples from the phyllic alteration zone identified white micas as phengite and muscovite indicating wide occurrence of Tschermak¿s reactions. Al-OH absorption wavelengths are recognized between 2211-2213 nm to K+ rich terms and between 2214-2221 nm to Fe2+ and Mg2+ enriched terms. For the chlorite the following varieties have been identified: (1) iron-magnesian-rich classified as ripidolite/brunsvigite/diabanite and clinochlore Fe/clinochlore Fe-Al with Mg-OH absorption wavelengths between 2339-2349 nm and; (2) magnesian-rich classified as brunsvigite/diabanite/penninite and clinochlore Fe/clinochlore with Mg-OH absorption wavelengths between 2330-2338 nm. The chlorite geothermometry applied to the (1) Fe-Mg variety associated with the gold-sulfide mineralization in the phyllic zone yielded temperatures between 370-250 °C which can be linked accordingly to the precipitation of gold. The several ore bodies that comprise the Pé Quente deposit, such as Barba, Goiano and Gabriel, may have been the result of a single magmatic-hydrothermal mineral system in which the disseminated and vein-type gold mineralization is coeval to the phyllic alteration stage represented by muscovite-phengite, Fe-Mg chlorite, quartz and sulfide (pyrite) Mestrado Geologia e Recursos Naturais Mestra em Geociências FAPESP 2016/04370-5

Published
2017

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