Marcus Vinicius Dorneles Remus

The link between hydrothermal epigenetic copper mineralization and the Caçapava Granite of the Brasiliano Cycle in southern Brazil

Abstract Base-metal deposits in the Cacapava do Sul Copper Province are hosted by both volcanosedimentary rocks of the Bom Jardim Group and by metamorphic rocks of the Passo Feio Formation, and show a spatial relationship to the Cacapava Granite. These associations have led to much controversy about the genesis of the base-metal deposits, which has been at least partly resolved by precise dating using SHRIMP (Sensitive High Resolution Ion Microprobe) U/Pb zircon studies combined with S, Pb, and Sr isotope trace studies. The Passo Feio Formation is Neoproterozoic in age and was derived from a complex continental source, as shown by the presence of xenocryst zircons of Archaean, Paleoproterozoic, and Neoproterozoic ages. It was metamorphosed at ca. 700 Ma. The syntectonic Cacapava Granite that intruded the supracrustal rocks of the Passo Feio Formation at 562 Ma was derived from an old sialic basement. Lead-isotope data are consistent with a 562 Ma age for the base-metal sulphide deposits sited in the Passo Feio Formation. The least-radiogenic compositions lie between the field of the isotopic compositions of the Cacapava Granite and rocks of the Passo Feio Formation, suggesting that Pb in the sulphide deposits may have been derived from both sources. The Pb, like that in the Cacapava Granite and Passo Feio Formation, was derived from a primitive crustal source. Sulphur isotope data from the base-metal sulphide deposits in the Passo Feio Formation are compatible with a mixed sedimentary and magmatic source. The most logical model for ore genesis, based on the isotopic data and spatial relationships, is that magmatic metal-bearing fluids from Cacapava Granite leached metals from the Passo Feio Formation and that the deposited sulphides therefore show mixed isotopic signatures. However, there is also some isotopic evidence from the Cacapava Granite itself that suggests assimilation of S-bearing rocks of the Passo Feio Formation during emplacement. Thus, isotopic signatures could have been inherited from assimilated metal sulphides at this stage, and deposition could have been entirely from Cacapava Granite-derived magmatic fluids. Importantly, the inferred 562±8 Ma age for the deposits in the Passo Feio Formation is younger than the well-constrained age of 594±5 Ma for the Camaqua/Santa Maria deposits. Thus, the epigenetic sulphides in the Passo Feio Formation cannot be the source of these deposits as previously suggested. Other isotopic data also argue against such a model.

Distal Magmatic-Hydrothermal Origin for the Camaquã Cu (Au-Ag) and Santa Maria Pb, Zn (Cu-Ag) Deposits, Southern Brazil

Abstract The Camaqua Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the largest base-metal deposits discovered so far in the sedimentary clastic sequences of the Neoproterozoic-age Camaqua Basin. The origin of the Camaqua-Santa Maria deposits has been the centre of dispute, with three alternative genetic hypotheses proposed: a syngenetic model, a diagenetic model, and a magmatic hydrothermal model. In detail, this mineralization has been suggested to be related to sedimentary clastic-diagenetic processes, volcanic-related processes, or deep granitic magmatism. Reevaluation of previous data and new studies in the area yield the following conclusions: (1) mineralization is fracture-controlled and magmatic-hydrothermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300°C; (3) the ∂ 34 S of sulphides of around 0% indicates an external magmatic-hydrothermal source of sulphur; (4) Pb isotope ratios of sulphides indicate that metals were derived at the end of the Brasiliano Cycle from a large crustal source with very primitive Pb and that (5) the age of mineralization is 594 Ma, as constrained by U/Pb SHRIMP (Sensitive High Resolution Ion Microprobe) determinations on zircons of the Lavras Granite. Thus, the Camaqua and Santa Maria deposits are interpreted to be of magmatic-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, late in the Brasiliano Cycle. The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits. However, the occurrence of mineralization along fractures within specific wall-rocks requires consideration of alternative exploration parameters. These include: (1) ancient EW- and NW-trending regional fractures and their intersections, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial porosity or secondary fracture-induced porosity or soluble rocks which are susceptible to replacement processes. Post-collisional plutonism of the Dom Feliciano Orogeny was the most likely heat source, and also the source of sulphur and base metals. Gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry-style Cu-Au deposits.

Structural evolution and tectonic setting of the Porongos belt, southern Brazil

The SW-NE-striking Porongos belt, located between juvenile Neoproterozoic rocks in the west and the Dom Feliciano belt, characterized by intense reworking of older crust, in the east, comprises a greenschist to amphibolite-facies metavolcano-metasedimentary succession (Porongos sequence) of unknown age with some exposures of Palaeoproterozoic gneisses (Encantadas gneisses). High-temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing (DT1-DT3) and can be attributed to the Palaeoproterozoic Trans-Amazonian orogeny. The deformation of the Porongos sequence occurred during the Neoproterozoic Brasiliano orogeny and comprises four ductile deformation phases (DB1-DB4), including two phases of isoclinal folding associated with shearing recorded in mylonitic layers, followed by closed NW-vergent folding and thrusting leading to formation of a thrust stack. Uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi-ductile to brittle block faulting in a sinistral strike-slip regime. The Porongos sequence can be subdivided into a southeastern and a northwestern part. Trace element analyses as well as Sm-Nd and Rb-Sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks. The metavolcanic rocks show eNd(t = 780 Ma) values of −20.64 and −21.72 (northwestern units) and −6.87 (southeastern sequence). The metasedimentary rocks were derived from late Palaeoproterozoic to Archaean sources, and the data indicate different sources for the northwestern and southeastern rock units of the Porongos sequence. eNd(t = 780 Ma) are −6.25 and −6.85 in the southeastern units, with TDM model ages between 1734 and 1954 Ma, and vary between −14.72 and −17.96 in the northwestern parts, which have TDM model ages between 2346 and 2710 Ma. High 87 Sr/ 86 Sr(t) values between 0.7064 and 0.7286 confirm reworking of older crust. Isotopic signatures of the Porongos sequence do not show indications for a significant contribution from a Neoproterozoic juvenile source. A passive margin or continental rift environment is suggested for the tectonic setting of the Porongos belt, which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust.

Geochemistry and Crustal Evolution of Volcano-sedimentary Successions and Orthogneisses in the São Gabriel Block, Southernmost Brazil — Relics of Neoproterozoic Magmatic Arcs

Abstract Precambrian metaplutonic rocks of the Sao Gabriel block in southernmost Brazil comprise juvenile Neoproterozoic calc-alkaline gneisses (Cambai Complex). The connection with associated (ultra-)mafic metavolcanic and metasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasiliano orogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambai Complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the Sao Gabriel block and its role during the assembly of West-Gondwana. The major element compositions of the igneous rocks (Palma Group and Cambai Complex) indicate a subalkaline character; most orthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-K tholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lower Palma Group and the Cambai Complex indicate origin at plate margins, i.e., in a subduction zone environment. This is corroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichment in LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continental arc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western Sao Gabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relics of oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemble the patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures of most igneous and metasedimentary samples and their low ( 87 Sr/ 86 Sr) t ratios suggest only minor contribution of old continental crust. A geotectonic model for the Sao Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneath the newly formed active continental margin between ca. 750 and 700 Ma. The Sao Gabriel block represents relics of an early Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de la Plata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in the western Brasilia belt (Goias magmatic arc).

REINTERPRETAÇÃO DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS, DE ACORDO COM OS SISTEMAS VULCANO-PLUTÔNICOS DE SUBSIDÊNCIA. PARTE 1: GEOLOGIA, GEOFÍSICA E GEOCRONOLOGIA (207PB/206PB/238U)

The Lavras do Sul intrusive complex (LSIC) and coeval volcanics, formed during the Late Neoproterozoic in the Sul-Riograndense Shield, are important because they contain historic Au-Cu occurrences. Field relationships, and gravity and magnetic data are consistent with the LSIC as nested intrusions of a volcano-plutonic subsidence system. These data also show the LSIC evolved from north to south, so we divided it into two sectors: the Tapera monzonite (TM) in the north, and the granitic body in the south. The Arroio do Jacques monzodiorite (AJM) occurs between the two sectors, surrounding the north-northeast border of the latter. TM is an intrusion emplaced during the formation and stabilization of an upper-crustal magma chamber linked to the coeval volcanics. Granites with distinct geochemical affinities forma concentric body that probably represents a resurgent pluton emplaced after some caldera collapse. Alkali-calcic granites are in the core above the main root zones, whereas alkaline granites occur as semicircular and peripheral bodies. 207 Pb/ 206 Pb and 206 Pb/ 238 U ages for several LSIC-units are comparable, and confirm that the intrusive complex is a multicyclic body, probably formed over a period of ca. 23 m.y. (606-583 Ma). Nonetheless, most of LSIC-units (TM, AJM and the core granites) have similar crystallization ages, of ca. 601 and 599 Ma, suggesting that this is the major period of igneous activity forming the volcano-plutonic system. Two distinct ages were obtained for alkaline granites, 207 Pb/ 206 Pb age of 598± 3 Ma (2σ) and 206 Pb/ 238 U age of 586.0 ± 2.8 Ma (2σ) respectively. Such difference in ages maybe resolved if the alkaline granites were emplaced in increments. The 207 Pb/ 206 Pb age of 567 ± 4 Ma (2σ) for the Jaguari Granite, situated to the west of LSIC, confirms it represents another younger alkaline event.

Controles mineralógicos e geoquímicos do níquel em rochas ultramáficas no escudo Sul-Riograndense

This work investigates the processes controlling the mobility and concentration of nickel in serpentinized ultramafic rocks of the Sul-Riograndense Shield (ESrg). Four ultramafic units were selected: Serrinha Complex, Cerro Mantiqueira Sequence, Pedras Pretas Massive and Cambaizinho Complex. Conventional techniques allow to chemically classify the rocks and to relate rock texture with the intensity and degree of the different events that contributed to changes in the mineralogy of the protholiths. Electron microprobe analysis of olivine and serpentine allowed determining the concentrations of nickel and its variations in all of the investigated units. The nickel-bearing minerals were identified, and established the relationship between geological events, element mobility and element concentration. The olivine of Pedras Pretas Massive contain lower levels of nickel with a range of 0.08 to 0.29%, and an average value of 0.17%, while olivines of Cerro Mantiqueiras Sequence contain higher NiO levels (maximum 0,44%), with an average value of 0.35%. The olivine composition of Pedras Pretas Massive and Cerro Mantiqueiras Sequence are homogeneous, while those of Cambaizinho Complex have a slightly higher range of variation with composition ranging between Fo92 and Fo84, but with NiO amounts between 0.17 e 0.30%. In Serrinha Complex, the olivine was not identified probably due to the intense serpentinization. Cambaizinho Complex and Cerro Mantiqueiras Sequence have serpentine minerals with a tendency to higher concentrations of nickel in relation to values obtained in the olivines, but some serpentines have lower values in the last sequence. By comparing the concentrations of nickel in each ultramafic unit, the olivines of Cerro Mantiqueiras Sequence have the highest NiO values, while Pedras Pretas Massive has the lowest values among the investigated ultramafic rocks. It is important to mention that the NiO concentration in the studied ultramafic units show equivalent values when compared to the other ultramafics around the world.