Lydia Maria Lobato

Structure and iron mineralisation of the Carajás Province

Abstract The Carajás Province presents several volcano-sedimentary sequences that comprise the Itacaiúnas Supergroup. The rocks represent bimodal volcanism, and clastic and chemical sedimentation in relatively unstable basins subject to recurrent structural events with subsidence and volcanism. The Grão Pará Group represents one of these sequences with mafic volcanic rocks that enclose discontinuous jaspilite lenses, with development of large high-grade orebodies (Fe>65%). This unit is named the Carajás Formation and presents peculiar characteristics compared with other iron districts in the world, thereby departing from the classic Lake Superior or Algoma iron formation types. Owing to the dense vegetation, lack of outcrops and harsh landscape, the structural analysis of the district can only be accomplished by combination of regional field work, detailed structural work on the open pits and remotely sensed image interpretation, in this case Landsat ETM7, JERS-1 images and the shuttle radar topography mission (SRTM) digital terrane model. The regional trend of the several sequences of the area is approximately N–S and the structure is dominated by a flattened flexural fold system with axes moderately dipping WNW, intersected by several strike-slip faults subparallel to their axial plane. The Serra dos Carajás represents an s-shaped synform-antiform pair, herein named the Carajás Fold. This regional structure is partially disrupted by the Carajás Shear Zone that divides it in the northern (Serra Norte) and the southern (Serra Sul) Ranges and also probably prepared the terrane with the development of pathways for mineralising hydrothermal fluids forming large high-grade massive iron bodies. Syntectonic granitic bodies played an important role in the structural evolution of the area as well. They caused localised ductile flattening deformation and thermal contact metamorphism in surrounding terranes. The Estrela pluton, for instance, was responsible for the discontinuity of the regional trend between the Serras do Rabo and Leste. The Carajás Province is regionally the upper crustal product of a very shallowly eroded dome-and-keel geometry of Neo-Archaean volcano- sedimentary units intruded by syntectonic calc-alkaline intrusives and overlying pre-existing infracrustal rocks. The lithostructural data suggest continental-margin back-arc basin development and closure in an oblique collision belt, with N–S shallow crustal shortening, parallel to the inferred pre-existing strike of the stratigraphy.

Controversy in genetic models for Proterozoic high-grade, banded iron formation (BIF)-related iron deposits - unifying or discrete model(s)?

Abstract A genetic model for the genesis of high-grade (>65 wt-%Fe) Proterozoic, banded iron formation (BIF)-related iron deposits such as the Tom Price, Mount Whaleback deposits in the Hamersley Province (Australia), the N4E, N4W and N5 deposits at Carajás (Brazil), the Aguas Claras, Conceição and Casa de Pedra deposits in the Iron Quadrangle (Brazil), the Donkerpoort West, Kwagas East deposit in the Thabazimbi iron ore district (South Africa) or the Saksagan ore field at Krivoy Rog (Ukraine), needs to take into account the: (a) timing of regional metamorphism with respect to the upgrade of BIF to high-grade iron ore; (b) significance and relationship of the protoore to the hydrothermal alteration minerals and zonation and the process of iron enrichment; (c) source and oxidation state of the hydrothermal fluids that caused the iron enrichment; (d) types of geochemical processes that caused the hydrothermal alteration and iron enrichment; and (e) significance of the widespread carbonatisation of the host rocks as a prerequisite for high-grade BIF-related iron mineralisation. We argue that a unifying model for high-grade BIF-related iron deposits cannot, presently, adequately explain the geological and geochemical characteristics observed in these deposits and therefore provide two end-member models that take into account the diversity of geological and geochemical ore deposit features. Our discrete model for the genesis of high-grade, BIF-related iron deposits is based on the interpreted tectonic setting, distinct hydrothermal fluid source of, and processes observed in, major BIF-related iron deposits. We divide them into two end-members: Proterozoic deep fault-magmatic (Carajás) type and Proterozoic rift-basin (Hamersley) type. Both end-members experienced varying degrees of deep weathering episodes that effected the hypogene enrichment and ultimately, yielded the high-grade iron ore mined in many places today.

In situ LA-ICPMS U-Pb dating of detrital zircons from the Cercadinho Formation, Minas Supergroup.

In this paper, new geochronological data obtained for the detrital rocks of the Cercadinho Formation, basal unit of the Piracicaba Group, Minas Supergroup, are presented. U-Pb Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) analysis in detrital zircon from quartzite samples provided a new maximum depositional age for the Cercadinho Formation. The youngest zircon population, among the dated samples, provided an average age of 2680 ± 24 Ma. These rocks have an expressive contribution of Meso- to Neoarchean zircons, with ages between 2812 ± 19 and 2909 ± 19 Ma, and older populations between 3212 ± 18 and 3272 ± 16 Ma, which occur mainly in the core of younger zircon grains. Comparing the U-Pb ages obtained in this work with previously published geochronological data for the basal units of the Minas Supergroup (Moeda Formation) an aging of the source for the detrital rocks of Cercadinho Formation can be observed, with a major contribution of zircons from TTG rocks crystallized between the Meso- and Neoarchean.

Iron isotope and REE+Y composition of the Cauê banded iron formation and related iron ores of the Quadrilátero Ferrífero, Brazil

The Minas Supergroup banded iron formations (BIFs) of the Brazilian Quadrilátero Ferrífero (QF) mineral province experienced multiple deformational events synchronous with hypogene mineralization, which resulted in the metamorphism of BIFs to itabirites and their upgrade to high-grade iron ore. Here, we present rare earth element and yttrium (REE+Y) compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during BIF deposition and the effects of hypogene iron enrichment. The REE+Y characteristics of itabirites (positive Eu anomaly and LREE depletion) indicate hydrothermal iron contribution to the Minas basin. Iron isotope data and Ce anomalies suggest BIFs were precipitated by a combination of anoxic biological-mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen (such as an oxygen oasis), perhaps related to increase in oxygen concentrations before the Great Oxidation Event (GOE). The similarity of the REE+Y composition of the itabirites from the different QF deformational domains, as well as to other Superior-type BIFs, indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original BIFs. However, iron isotope compositions of iron ore vary systematically between deformational domains of the QF, likely reflecting the specific mineralization features in each domain.

The petrochemistry of the auriferous, volcanosedimentary Riacho dos Machados Group, Central-eastern Brazil: geotectonic implications for shear-hosted gold mineralization

Abstract The Ouro Fino Gold Deposit at the Riacho dos Machados Mine is located in the Aracuai Fold Belt at the eastern margin of the Sao Francisco Craton. The gold is shear zone-hosted and associated principally with a sulphide-bearing, quartzmuscovite schist, derived from the hydrothermal alteration of pelitic and quartzofeldspathic schists of the Riacho dos Machados Group (RMG). Along the shear zones, mineral associations typical of the amphibolite facies are progressively altered to assemblages typical of the greenschist facies. It has become evident from the studies that the process of segregation and concentration of chemical elements and minerals is the very process of mineralization, involving the concentration of Au. The RMG comprises ultramafic, mafic and acid metavolcanic rocks intercalated in a sequence of metapelites. The metapelites have textures and geochemistry compatible with greywackes. The dacitic and mafic/ultramafic rocks are of calc-alkaline and tholeiitic subalkaline association, respectively. The RMG probably formed within a continental volcanic arc. The thrust faults that establish the regional architecture, and which host the Riacho dos Machados gold mineralization, are related to a continental collision of unknown age between the precursor to the Sao Francisco Craton and the Guanambi-Correntina Block.

Metaturbidite-hosted gold deposits, Córrego do Sítio lineament, Quadrilátero Ferrífero, Brazil

A study of geology and rocks paragenesis has been conducted at the Corrego do Sitio auriferous lineament, containing the Cachorro Bravo, Laranjeiras and Carvoaria metaturbidite-hosted lode-gold deposits located in the Quadrilatero Ferrifero Region, Minas Gerais, Brazil. These representative deposits are described to illustrate the essentially similar general character of all the deposits of lineament as well as the wide compositional and mineralogical differences in the ore of the different deposits, where, for the Cachorro Bravo deposit, a geological mapping included two underground mine galleries. The Corrego do Sitio unit is a metamorphosed turbidite in an alternating sequence of metagraywackes and phyllites, with parallel to discordant metamafic dikes and sills. The ore zone is predominantly hosted at the stratigraphic break between metasedimentary and metamafic rocks. Four deformation events affected the mine sequence. Mineralized veins and veinlets are considered to have formed within a brittle-ductile shear-zone environment and occurred in multiple episodes. Different vein types are recognized, but the most important volumetrically is a S1-concordant type characterized by smoky and milky quartz-carbonate-sulfide ± sulfosalts veins. Veins are dominated by quartz, but locally they are characterized by carbonate and a large variety of sulfide and sulfosalt minerals. Pyrite is the commonest associated sulfide mineral, followed by arsenopyrite and pyrrhotite. An extensive mineralogical study of polished sections has confirmed different generations of sulfide minerals. The sulfides and sulfosalt minerals are interrelated in the veins and disseminated on wall rocks. The data are consistent with a genetic models related to other Archean lode-gold deposits.