José Carlos Frantz

The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil

The Cordilheira Intrusive Suite is made up of peraluminous two-mica leucogranites related to the Brasiliano Cycle. Located in the east region of the Sul-rio-grandense Shield, in southernmost Brazil, the granitic bodies were emplaced mainly inside mylonitic zones belonging to a transcurrent system with NE orientation. The structural relationship of this intrusion with shear zones indicates a syn- to late-kinematic emplacement. The Cordilheira Intrusive Suite rocks are mainly syenogranites and monzogranites, with inequigranular texture, sometimes modified by mylonitic deformational effects. Mineralogically they are composed of alkali feldspar, plagioclase, quartz, muscovite and biotite, with minor amounts of gamet, tourmaline, apatite, zircon, sphene and fluorite. The analyzed samples show a SiO2 range of 71.55 to 77.68%, and a reduction in the K2O/Na2O ratio with increasing differentiation. Trace-element contents, including REE, are similar to that of granites produced by partial melting of quartz-feldspathic rocks. Field relationships and geochemical data are suggestive of partial crustal melting along transcurrent shear zones developed during the late orogenic stages of the Brasiliano Cycle. The evidence considered in this paper shows the remarkable similarity of the Cordilheira Intrusive Suite to Hercynian granitoids of Portugal and France.

Geochemical modeling of gold precipitation conditions in the Bloco do Butiá Mine, Lavras do Sul/Brazil

A geochemical modeling of gold deposition was performed using the EQ3/EQ6 software package using conditions inferred from geological, petrographic, geochemical and fluid inclusion data from the Bloco do Butiá gold mine, Lavras do Sul, RS. Gold in the mine occurs only in the pyrite structure (invisible gold). The pyrite occurs associated with white mica (phengite) in the zone of phyllic alteration. The process of gold deposition showed to be related to temperature and pH decrease. The pH decrease was fundamental to gold deposition by destabilization of sulfur species [Au(HS)2- , HAu(HS)2(0) and Au(HS)0] dissolved in the aqueous solution, being Au(HS)0 the main gold transporting complex. The addition of KCl is hard to accept as cause of gold precipitation because no Cl- was detected in phengite. However, the geochemical mass balance calculation resulted in the gain of some potassium in the zone of phyllic alteration. The precipitation of pyrite (+/- auriferous) may have been strongly influenced by iron availability resulting from dissolution of ferrous chlorites by the fluids responsible for phengite deposition. The low salinity in quartz grain fluid inclusions from the propylitized wall rock also indicates the little importance of chlorine as gold transporting agent. Sulfur, and not chlorine, compounds must have dominated the gold transporting complexes in the Bloco do Butiá gold area.

Geocronologia U-Pb em zircão de rochas intrusivas e de embasamento na região do Vale do Jacurici, Cráton do São Francisco, Bahia

The Jacurici Complex, located in the NE of the Sao Francisco Craton, is constituted by several Cr-mineralized mafic-ultramafic N-S bodies, possible fragments of a single sill disrupted during deformation. Some works suggest it is intruded on the Serrinha Block while others consider it in the Salvador-Curaca Belt. The basement on this region is informally divided into paragneisses and orthogneisses; the latter is supposed to be younger considering it is less deformed. Petrography revealed that some of the paragneisses are alkali-feldspar granite strongly milonitized. The orthogneisses occur at the north and consist, at least in part, of monzogranites with heterogeneous deformation, locally of low temperature. U-Pb zircon dating were performed for five representative samples. Just three provided good concordia ages. A mafic rock produced a 2102 ± 5 Ma age and it is petrographically similar to the metanorites described in the Jacurici Complex, being interpreted as the record of the first pulses of the mafic magmatism. A monzogranite yielded a 2995 ± 15 Ma age, older than expected, related to the Serrinha Block. The alkalifeldspargranite yielded a 2081 ± 3 Ma age. The Itiuba Syenite and the pegmatites that crosscut the JacuriciComplex have similar ages. Considering the lack of information about the supracrustal sequence that hosts the intrusive alkaline and mafic-ultramafic rocks at the Ipueira and the Medrado areas, it is possible that part of the terrain belongs to the Salvador-Curaca Belt. We suggest that the Jacurici Complex could be intruded after the tectonic amalgamation between the Serrinha Block and the older part of the Salvador-Curaca Belt and, therefore, could be hosted by both terrains.

O corpo Várzea do Macaco e as mineralizações de cromo, níquel e cobre, Complexo Máfico-ultramáfico Jacurici, Cráton São Francisco, Bahia

The Jacurici Mafic-ultramafic Complex, located in the northeastern portion of the Sao Francisco craton, consists of several N-S oriented layered bodies that host the largest chromite deposit in Brazil. The Varzea do Macaco body is at the northern part and also host a Ni-Cu sulfide mineralization. This study describes this body and its mineralization and compare with the southern intrusions (Ipueira-Medrado) where a petrological evolution was previous established. The Varzea do Macaco is stratigraphically inverted and disrupted in five blocks, laterally dislocated by late faults. It is constituted by dunite, lherzolite, ol-webesterite, chromitite and gabbronorite with variable serpentinization. The sulfide ore (Po ± Pn ± Cpy) is concentrated close to the main thick chromitite layer and occurs as: primary magmatic with interstitial sulfides associated with olivine and pyroxene; and as a remobilized ore, with sulfides associated to metasomatic veinlets or lenses that crosscut the primary layering. The interval where magmatic ores occur is characterized by the presence of magmatic amphibole that possible favoured late metamorphism and metasomatism transformation, stronger in this interval. The remobilized sulfide is enriched in chalcopyrite showing an increase in the Cu/Ni ratio. Comparing to Ipueira-Medrado, the Varzea do Macaco is enriched in clinopyroxene, but it can be subdivided in the same Ultramafic and Mafic zones. Possibly, both bodies are part of a single intrusive system characterized by a primitive magma with high Mg and Ni contents. The chromite mineralization is considered to be triggered by crustal contamination. At Varzea do Macaco, sulfur saturation was reached.