Geochemical Signature and Magnetic Fabric of Capinha Massif (Fundão, Central Portugal): Genesis, Emplacement and Relation with W–Sn Mineralizations

The Fundão–Serra da Estrela–Capinha (FSEC) region is characterized by peraluminous to metaluminous Variscan granites intrusive in a complex and thick metasedimentary sequence. This work seeks to characterize the Capinha granite (CG), understand its spatial and genetic relationship with the host Peroviseu–Seia (PS), Belmonte–Covilhã (BC) and Fáguas granites, and evaluate its metallogenic potential. To achieve these goals, a multidisciplinary approach was undertaken, including field work and identification of the petrography and microstructures, whole rock geochemistry and anisotropy of magnetic susceptibility. Four distinct and independent differentiation trends were identified in the granites, namely, PS, BC, Fráguas and CG. The PS and BC played a role as host rocks for the W and Sn mineralizations. The Fráguas granite is anomalous in Sn and spatially related to the Sn–Li mineralizations, while the CG is anomalous in W and spatially related to W–Sn mineralizations. The post-tectonic CG is a peraluminous ilmenite-type whose ascent and emplacement were tectonically controlled. The Capinha magma used the intersection between the 25° N and 155° N strike–slip crustal scale faults for passive ascent and emplacement during the late-Variscan extensional phases. The magnetic fabric was drawn using an asymmetric tongue-shaped laccolith for CG. CG experienced two brittle deformation stages that marked the maximum compressive rotation from NE–SW to NNW–SSE.

Metamorfismo de alto gradiente P/T en la Sierra de Pie de Palo (Sierras Pampeanas, Argentina): modelado de equilibrio de fases minerales e implicancias geodinámicas en el antearco famatiniano

The Sierra de Pie de Palo (SPP, Western Sierras Pampeanas) shows evidence of two regional metamorphisms: one Mesoproterozoic attributed to the Grenvillian orogeny and other of Ordovician age related to the Famatinian orogeny. The Neoproterozoic-to-Cambrian sedimentary successions that cover the Grenvillian basement only record the Ordovician event. One staurolite-schist from the Ediacaran Difunta Correa Metasedimentary Sequence collected in the southeastern side of the SPP allows to constrain, by means of pseudosections, a prograde evolution from ca. 3 kbar and 515 ºC up to ca. 9 kbar and 640 ºC corresponding to a high P/T gradient. The SPP and the immediately east Loma de Las Chacras outcrop were part of the famatinian forearc which shows a progressive decrease of P (from ca. 13 kbar to 6 kbar), T (from ca. 900 ºC to 450 ºC), and P/T gradient (from ca. 85 ºC/kbar to 35 ºC/kbar) towards the active continental margin on the west. The Caucete Group, in the western side of the SPP, represents the westernmost part of the forearc, near to the active continental margin. Metamorphism was apparently coeval with the Famatinian magmatism and with ductile underthrusting at ca. 470-465 Ma, which led to burial of the forearc beneath the magmatic arc.

Geological Study of the Lesser Himalaya in the Kusma-Baglung Area, Western Nepal

Geological study was carried out along the Kaligandaki and Modi Khola valleys in west Nepal. The area comprises the rock of Lesser Himalayan metasedimentary sequence. Main rock types present in the area are quartzite, phyllite, metasandstone and psammatic phyllite. Schist and gneiss are present in some parts. Ulleri Gneiss is explored in the area. Kusma-Baglung area comprises the spectacular terrace deposits formed at the Quaternary time by the Kaligandaki River and the Modi Khola. The terrace extensively comprises clast of limestone, marble, gneiss, quartzite, schist, phyllite ranging in size from fine matrix to huge boulders derived from the Higher Himalaya. Bulletin of Department of Geology, vol. 20-21, 2018, pp: 29-36

Thematic interpretation of heliborne magnetic data for delineation of concealed regional structural fabric in the Narnaul-Palsana tract, North Delhi Fold Belt, India

The North Delhi Fold Belt (NDFB) exposure of the Delhi Supergroup of rocks is significant for its structurally controlled uranium mineralization. The Narnaul-Palsana tract within the Khetri subbasin of the NDFB comprises the arenaceous Alwar and argillaceous Ajabgarh Groups of the Delhi Supergroup. The metasedimentary sequence has been subjected to polyphase deformation and igneous intrusion. We used heliborne magnetic data to enhance our geologic understanding of the area. Total magnetic intensity data are gridded and enhanced to resolve the magnetic anomalies. The regional magnetic signature reveals a deep-seated fracture. Varying concentrations of magnetic minerals in different lithologies are reflected in the magnetic response and provide clues to the formational trends. Trend lines and breaks are extracted from the magnetic signature. Thematic analysis of trend lines reveals formational trends that indicate an antiformal and synformal fold pattern in different sectors of the study area. The spatial correlation of the fold patterns is used to decipher the tectonic sequence. Superimposition of antiformal folding over earlier antiform-synform structure and displacement due to later faulting is inferred. Magnetic data analysis is used as a tool to unravel the regional structural fabric of the area that is widely concealed below soil cover.

TERRENOS KHONDALÍTICOS: PRINCIPAIS DOMÍNIOS PARA MANGANÊS, GRAFITA, OURO E ZINCO NO ESTADO DO RIO DE JANEIRO E REGIÕES LIMÍTROFES

ResumoTerrenos khondalíticos representam um significativo componente de áreas de fácies granulito/UHT e os gnaisses que ocorrem associados a esses complexos são, principalmente, tipos aluminosos. Estes terrenos representam uma história de acresção crustal, ocorrendo nas zonas de junções/suturas das massas continentais como, por exemplo, nas que propiciaram a formação do supercontinente Gondwana. Nos Terrenos Khondalíticos presentes no Estado do Rio de Janeiro e em suas regiões limítrofes, registram-se diversas ocorrências de Mn/grafita, Au e Zn/Ag, cujas expressões podem ter caráter tanto regional quanto local. No geral, as mineralizações citadas estão situadas em regiões formadas por intercalações de gnaisses aluminosos, com ou sem sillimanita, e de rochas ortogranulíticas. Nesses terrenos, afloram camadas de gnaisses grafitosos (teor de C~2%). Essas camadas podem estar acompanhadas de corpos de gonditos, alterados para óxidos-hidróxidos de manganês, como nas faixas khondalíticas Marangatu e Palma-Guaçuí, ou somente mineralizados em grafita, como na Faixa Khondalítica São Fidélis. Na Faixa Khondalítica Palma-Guaçuí, particularmente no segmento entre Palma (MG) e Laje do Muriaé (RJ), ocorre um pacote de gnaisse grafitoso sulfetado, interpretado como um antigo folhelho negro, com cerca de 20 km de extensão, contendo mineralizações auríferas. O ouro ocorre na forma livre e como inclusões (ouro invisível) em piritas dos tipos framboidal (singenética), flor de girassol (diagenética) e maciça (metamórfica). A sequência metassedimentar à qual se relaciona a mineralização de zinco de Rio Claro (possivelmente uma extensão da Faixa Khondalítica Marangatu) é bastante semelhante àquelas encontradas em depósitos canadenses sedimentares-exalativos de Zn-Pb-Ag, do tipo Shuswap e Monash. As rochas sílico-carbonáticas da Unidade Lídice, que encerram as zonas sulfetadas, foram geradas em ambiente plataformal raso, possivelmente com um componente evaporítico.Palavras Chave: Terrenos khondalíticos, manganês, grafita, ouro, zinco.AbstractKHONDALITIC TERRAINS: MAIN DOMAINS FOR MANGANESE, GRAPHITE, GOLD AND ZINC IN THE STATE OF RIO DE JANEIRO AND SURROUNDING REGIONS. Khondalite terrains represent a significant element in areas with granulite/UHT facies and the gneiss rocks that occur associated to these complexes are, especially, aluminous type. These terrains represent a history of crustal accretion, occurring in suture zones of continental masses as, for example, in those which contributed to the formation of the supercontinent Gondwana. In khondalite terrains in the State of Rio de Janeiro and in its neighboring regions, there are several occurrences of Mn/graphite, Au and Zn/Ag, whose expressions may have both regional and local character. In general, those mineralizations are located in regions consisting of intercalation of aluminous gneiss, with or without sillimanite, and of ortogranulite rocks. In those terrains, layers of graphitic gneiss (C content ~ 2%) arise. Such layers may be accompanied by gondite bodies, changed to oxide-hidroxide of manganese, as in Marangatu and Palm-Guaçuí Khondalitic Belts, or only mineralized in graphite, as in São Fidelis Khondalitic Belt. In Palma-Guaçuí Khondalite Belt, particularly in the segment between Palma (MG) and Laje do Muriaé (RJ), there is a set of sulphide graphite gneiss, considered as an ancient black shale, about 20 km long, containing auriferous mineralizations. Gold occurs in free form and as inclusions (invisible gold) in pyrites of framboidal type (syngenetic), sunflower type (diagenetic) and massive type (metamorphic). The metasedimentary sequence to which Rio Claro zinc mineralization relates (possibly an extension of Marangatu Khondalitic Belt) is quite similar to those found in sedimentary-exhalative Canadian deposits of Zn-Pb-Ag, Shuswap and Monash types. Silic-carbonate rocks of Lidice Unit, which delimit the sulfide zones, were generated in shallow platformal environment, possibly with a evaporitic component.Keywords: Khondalitic terrains, manganese, graphite, gold, zinc.