Alexandre Raphael Cabral

Palladseite and its oxidation: evidence from Au-Pd vein-type mineralization (jacutinga), Cauê iron-ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract Palladseite (palladium selenide) and palladian gold occur as euhedral inclusions in specularite and as infill of microfractures and interstices in magnetite aggregates (host rock fragments) within speculariterich auriferous veins (jacutinga). Palladseite has minor amounts of Pt (0.6−3.5 wt.%), Cu (2.9−3.5 wt.%), Hg (0.9− 1.9 wt.%), and Ag (0.3− 0.5 wt.%). The palladian gold contains up to 6 wt.% Pd, and minor Cu and Ag. Isomertieite (Pd11Sb2As2) and sudovikovite (PtSe2) are also recorded. The veins cross-cut the main tectonic foliation of the wallrock (itabirite). The feature of fracture infill and the Se-Sb-As-Hg-Cu-Ag geochemical/mineralogical signature of the Au-Pd mineralization point to a post-ductile deformation and possibly low-temperature (epithermal) origin of the jacutinga mineralization style. Palladseite is replaced locally by a Pd-rich oxidation phase. Electron microprobe analysis of alteration halos around palladseite yields 76−80 wt.% Pd. Oxygen calculated by stoichiometry accords with a monoxide compound of the type PdO. However, the measured oxygen content is lower than expected for the PdO stoichiometry, suggesting loss of volatile species, and a more ‘hydrated’ compound, such as an oxyhydroxide.

Hydrothermal origin of soft hematite ore in the Quadrilátero Ferrífero of Minas Gerais, Brazil: petrographic evidence from the Gongo Soco iron ore deposit

Abstract The high-grade soft hematite ore from Gongo Soco, Minas Gerais, Brazil, consists of laminae of platy hematite that define a prominent foliation (S1), generally attributed to the Neoproterozoic Brasiliano Orogeny. The laminae alternate with bands (1-2 mm thick) of a porous goethitic matrix with S1-parallel crystals of platy hematite. Both the laminae and bands are overprinted by masses of fine-grained crystals (~2-20 μm in length) of platy hematite that pervade along the banding at low angle without obliterating the foliated aspect of the ore. The ore microfabrics are interpreted in terms of hydrothermal leaching of quartz and dolomite from precursor itabirite, with goethitic alteration and pervasive hematitisation. Pyrolusite and a Mn-Ba oxide, possibly romanèchite,occur as late open-space infilling.The ore has a constant Al2O3/TiO2 ratio; electron-microprobe analyses indicate that titanium is not associated with residual clay minerals, but substituting iron in platy hematite.

Palladian gold and palladium arsenide–antimonide minerals from Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract The present-day Gongo Soco iron ore deposit was one of the most famous gold mines in Brazil in the nineteenth century. The iron ore mining has exposed crosscutting, quartz–specularite auriferous ore shoots within Palaeoproterozoic banded iron formation. The coarse-grained gold particles are characteristically palladium-bearing and commonly have inclusions of palladium arsenide-antimonides, whose compositions are close to mertieite-II and isomertieite. Composite crystals with a mertieite-II core and isomertieite rim are interpreted as precipitates from a hydrothermal fluid of variable Sb : As ratio. The palladium content in gold appears to be correlated with the platinum content in the associated palladium arsenide-antimonide. Its platinum content, up to about 1 wt% Pt, seems to be related to the amount of arsenic. A myrmekitic intergrowth of an Fe-rich, goethite-like phase in a mertieite-II-bearing palladian gold nugget is recorded and is interpreted to be of low-temperature hydrothermal origin. A late-stage event, possibly of a low-temperature hydrothermal nature, deposited Pt–Pd alloy locally.

Talc mineralisation associated with soft hematite ore, Gongo Soco deposit, Minas Gerais, Brazil: petrography, mineral chemistry and boron-isotope composition of tourmaline

Talc mineralisation occurs as hematite–talc schist between soft hematite ore and dolomitic itabirite at Gongo Soco, Quadrilátero Ferrífero of Minas Gerais, Brazil. The hematite–talc schist and soft hematite have a prominent tectonic foliation of tabular hematite. Tabular hematite without preferential orientation is superimposed on the tectonic foliation. The talcose schist is enriched in F and has a constant Fe/S ratio. Electron-microprobe analyses indicate trace amounts of S in different generations of hematite. The whole-rock Fe/S ratio possibly represents sulfate S from hematite-hosted fluid inclusions. Fluid inclusions in foliation-overprinting hematite and chlorite geothermometry from talcose rocks suggest, respectively, temperatures from <200°C to ~300°C. Tourmaline, a rarely observed mineral in the hematite–talc schist, belongs to the alkali group and falls in the dravite compositional field. Boron-isotope determinations of tourmaline crystals, using secondary ion mass spectrometry, vary from −20‰ to −12‰ δ11B. This compositional isotopic range and the tourmaline chemical composition suggest a meta-evaporitic origin. A non-marine evaporitic setting is the most likely source of acidic, highly oxidising fluids, which resulted in the abundant F-bearing talc and the presence of otherwise immobile Ti in hematite. Oxidising brines were channelled along shear zones and converted dolomitic itabirite into the Gongo Soco soft hematite and the talc mineralisation. The latter is envisaged as the hydrothermal wall-rock alteration of dolomitic itabirite, which gave rise to the soft hematite ore.

Direct dating of gold by radiogenic helium: Testing the method on gold from Diamantina, Minas Gerais, Brazil

New analytical developments have made radiogenic helium ( 4 He) applicable to archeological gold artifacts for age determinations. Here we report the application of the U/Th– 4 He method to the direct dating of gold from the historically important gold deposit in Diamantina, Minas Gerais, Brazil. The U/Th– 4 He age of 515 ± 55 Ma for the Diamantina gold is corroborated by a new U/Pb age of 524 ± 16 Ma for rutile recovered from auriferous pockets. These ages tie the Diamantina gold mineralization to the Brasiliano orogenic event, in the context of the Gondwana amalgamation. Our results indicate that U/Th– 4 He dating of gold is possible, opening new perspectives for the dating of gold deposits without assuming contemporaneity between gold and datable hydrothermal minerals.

Poorly crystalline Pd–Hg–Au intermetallic compounds from Córrego Bom Sucesso, southern Serra do Espinhaço, Brazil

Potarite, ideally PdHg, is reported in the literature to have compositions varying from PdHg or Pd(Hg,Au) to Pd3Hg2. Such a Pd3Hg2 phase is unknown in the synthetic Pd–Hg binary system. For the first time, Pd–Hg grains recovered from the historical Bom Sucesso alluvium, regarded as the type locality of Pd, are shown to consist of arborescent and lamellar intergrowths of two intermetallic compounds, compositionally close to empirical Pd(Hg,Au), i.e. auriferous potarite, and (Pd,Au)3Hg2. The Pd–Hg–Au grains have a rim of palladiferous Pt. The otherwise sharp Pd–Hg–Au intergrowths become diffuse at the contact with the palladiferous Pt rim. Both the Pd–Hg–Au compounds and the palladiferous Pt rim did not diffract using the electron-backscattered diffraction (EBSD) and powder X-ray microdiffraction techniques, indicating that they are poorly crystalline. Their poor crystallinity and the diffuse zone between the Pd–Hg–Au core and the Pt-rich overgrowth are suggestive of electrochemical metal precipitation from dilute solutions within the alluvium.

Celebrating the Centenary of “The Geology of Central Minas Gerais, Brazil”: An Insight from the Sítio Largo Amphibolite

One century ago, in 1915, Harder and Chamberlin established the stratigraphy on which the geology of the Quadrilátero Ferrífero of Minas Gerais is based. One of their key observations was the occurrence of metavolcanic rocks in spatial association with the Itabira iron formation, an observation that has been ignored by most authors. Crucial for the understanding of the geology of the Quadrilátero Ferrífero, but equally ignored, is a 400-m-thick unit of amphibolite that rests immediately on the Itabira iron formation, also known as the Cauê Itabirite. Here, we revitalize the Sítio Largo amphibolite by indicating its basaltic affiliation. We present new results of U-Pb dating of zircon and rutile that show that the amphibolite protolith, which covered an erosional surface on the Cauê Itabirite, is ca. 2.18 Ga old and underwent metamorphic overprint at ca. 500 Ma.

Lead in diagenetic pyrite: evidence for Pb-tolerant bacteria in a red-bed Cu deposit, Quebec Appalachians, Canada

Abstract Diagenetic pyrite from the Silurian continental red bed-hosted Transfiguration cupriferous deposit in the Quebec Appalachians, Gaspé Belt, Canada, contains up to ~2% (m/m) Pb. This large Pb content in pyrite contrasts with experimental determinations that indicate solubility of <0.1% (m/m) PbS in pyrite at high temperature. The distribution of Pb in pyrite is heterogeneous, with plumbiferous domains occurringas patches and concentric growth layers alternating with Mn- and Mo-bearing zones. The plumbiferous pyrite is surrounded by As- and Cu-rich rims. This compositional heterogeneity, however, is elusive under normal backscattered-electron (BSE) imaging, but it can be recognized under high-gain BSE. Proton-induced X-ray emission (PIXE) confirms the presence of Pb. Plumbiferous pyrite with >0.1% (m/m) Pb has rarely been described; it is thus possible that plumbiferous pyrite may have been overlooked in metalliferous deposits worldwide. The plumbiferous pyrite from Transfiguration has a light S-isotope composition that is characteristic of bacterial sulphate reduction. We suggest that Pb in diagenetic pyrite indicates Pb-tolerant bacterial activity and, perhaps, constitutes a biosignature of bacterial tolerance to Pb in ancient sedimentary systems.

Evidence for metre-scale variations in hematite composition within the Palaeoproterozoic Itabira Iron Formation, Minas Gerais, Brazil

Abstract Hematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe2O3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Cauê (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as ‘jacutinga’), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to ~1.3 wt.% TiO2) is lepidoblastic and defines a pervasive tectonic foliation (S1). In contrast, Ti-free, chromiferous hematite (to ~6.4 wt.% Cr2O3) characteristically occurs as inclusions in palladiferous gold within S1-truncating ‘jacutinga’. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting ‘jacutinga’. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins (‘jacutinga’).

Empirical Pt 7Cu from an alluvial platinum concentrate and its significance for platiniferous quartz lodes in the lubero region, dr congo

An alluvial platinum grain from Lubero, DRC, is compositionally analogous to Pt 7Cu. This phase is so far unknown in nature, but is known from the binary platinum-copper system to have an ordered structure and formed below 500 °C, and as low as 100 °C. Such low temperatures rule out magmatic platinum mineralisation as source rock. The presence of previously reported inclusions of vein quartz and hematite in some alluvial grains of platinum, together with the relatively low formation temperatures of the Pt 7Cu compound, suggests that hydrothermal quartz lodes were the source of the platinum found in the Lubero alluvia.