Franciscus Baars - Academia.edu (original) (raw)

Papers by Franciscus Baars

The Namaqua Province of southwestern Africa is comprised of a number of distinct tectonostratigra... more The Namaqua Province of southwestern Africa is comprised of a number of distinct tectonostratigraphic subprovinces and terranes, which have in common a 1100-1200Ma structural and metamorphic imprint. In the western Bushmanland Subprovince, E-Wtrending belts of supracrustal gneisses are intruded by and infolded with granitic gneisses of varying ages. A central zone LIST OF PLATES Plate 1. Discordant contact of hornblende-biotite augen gneiss against homogeneous K-feldspar-quartz gneiss. Note the presence of recrystallized augen wrapped by the strong ferromagnesian mineral foliation. Hammer shaft is 29 em. Plate 2. The typical green, 'streaky' character of hornblendebiotite augen gneiss with refolded internal structure. The contact with homogeneous K-feldspar-quartz gneiss is commonly concordant. Hammer shaft is 26 em. Plate 3. Well-foliated, biotite-rich biotite-orthopyroxene granite gneiss at a concordant contact against biotite leucoparagneiss. Hammer shaft (27 em) is perpendicular to s 2 mineral foliation (s-surface), and pencil is parallel to the trace of the Nc-surface. Note the presence of a sheared, recrystallized parallel to the s2 mineral foliation. leucosome Plate 4. A stromatic migmatite root zone to a lenticular body of quartzofeldspathic anatectic granite (ca. 3 X 0.5 km) in biotite-orthopyroxene leucoparagneiss. Many of the dark spots in the leucosome assemblages are orthopyroxene. Hammer shaft is 26 em. Plate 5. Leucosome assemblages, containing garnet-quartz patches surrounded by anhydrous, quartzofeldspathic haloes, in a dyke of anatectic biotite granite that is intrusive into garnet-biotite quartzofeldspathic gneiss. The leucosomes comprise 50% of the rock volume. Hammer shaft is 26 em. Plate "6. Discordant leucosome, containing orthopyroxene (light green-brown), clinopyroxene (dark green-black) and plagioclase (white to pale blue), in hornblendepyroxene granulite gneiss. Hammer shaft is 83 em. Plate 7. An ovoid leucosome patch, containing rhombododecahedra of garnet intergrown with quartz, and surrounded by an anhydrous, quartzofeldspathic halo, in garnet-biotite quartzofeldspathic gneiss. Compass pouch is 8 em wide. Plate 8. Garnetiferous and entirely quartzofeldspathic leucosomes occurring as crenulated compositional bands in garnetbiotite metapelitic gneiss. At left, a coarse-grained garnetiferous. leucosome transgresses compositional banding and regional foliation. A K-feldspar-rich, pegmatitic vein occurs at the base of the outcrop, containing rootless stringers•of matrix gneiss. Hammer shaft is 29 em. Plate 9. Unzoned leucosomes occurring parallel to the regional, gneissic banding and foliation in garnet-biotite metapelitic gneiss. Note that many garnet grains intersect matrix-leucosome boundaries, but are always surrounded by, at least a narrow anhydrous halo. Hammer shaft is 29 em. Plate 10. Cross-section of a anatectic granite sill root zone in garnet-biotite metapeltic gneiss. Note the nebulitic internal structure of disjointed matrix stringers. Hammer shaft is 29 em. Plate 11. Coarse-grained orthopyroxene-K-feldspar leucosome patches in orthopyroxene-cordierite-phlogopite magnesian gneiss. Hammer shaft is 29 em. Plate 12. Transgressive, orthopyroxene-bearing quartzofeldspathic leucosome in biotite-orthopyroxene leucoparagneiss. Hammer head is 18 em. Plate 13. Vein and patch leucosomes, subparallel to regional foliation in biotite-orthopyroxene leucoparagneiss. The dark patches in both matrix and leucosome assemblages are orthopyroxene-biotite intergrowths. Hammer shaft is 29 em. Plate 14. Boudin-like, quartzofeldspathic sill in banded quartzofeldspathic gneiss. It is not clear whether these structures are igneous (i.e. pinch-and-swell) or truely tectonic (i.e. stretching boudins) in origin. Hammer head is 18 em.

Geochimica Brasiliensis, 1996

Transactions, Dec 1, 2006

The Carajá s Province presents several volcano-sedimentary sequences that comprise the Itacaiú na... more The Carajá s Province presents several volcano-sedimentary sequences that comprise the Itacaiú nas 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 Grã o 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 Carajá s 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 Carajá s represents an s-shaped synform-antiform pair, herein named the Carajá s Fold. This regional structure is partially disrupted by the Carajá s 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 highgrade 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 Carajá s 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.

Revista Brasileira de Geociências, Sep 1, 2000

On the basis of major and trace element analysis, the metabasalts of the Nova Lima Group, Rio das... more On the basis of major and trace element analysis, the metabasalts of the Nova Lima Group, Rio das Velhas Greenstone Belt, are divided into tholeiites and magnesian tholeiites. Using rare-earth element patterns, the rocks are subdivided into five geochemical populations (I-V). The chemical variations demonstrated on binary variations diagrams, using major and trace elements, proves this division. Population I is comprised of magnesian basalts and represents the most primitive terms of the sample population, with high MgO, Ni and Cr. Populations II, III, IV and V are differentiated terms evolved from compositions like those of Population I, with low compatible-element contents. The geochemical behavior, represented principally by the trace elements, shows that Population I may have formed by the partial fusion of a mantleplume source (P-MORB). The other populations are differentiated by fractional crystallization from a melt of composition similar to that of Population I. Populations IV and V present geochemical patterns reflecting mixing processes that have been caused by fractional crystallization together with contamination. The Nova Lima Group metabasalts are derived from an ocean-floor environment, as evidenced by structure (pillow lavas) and lithological associations (banded iron formation and metachert). The geochemical characteristics indicate that at least some of the metabasalts were formed by a mantle plume. These characteristics permit the identification of a submarine plateau environment. That some mafic-rock populations show evidence for contamination and the occurrence of felsic volcanic rocks suggest the presence of an island-arc or back-arc type environment.

Journal of South American Earth Sciences, 1997

Absrracr-The Ouro Fino Gold Deposit at the Riacho dos Machados Mine is located in the Aracuai Fol... more Absrracr-The Ouro Fino Gold Deposit at the Riacho dos Machados Mine is located in the Aracuai Fold Belt at the eastern margin of the S5o 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 Pa&s. 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 greywdckes. The dacitic and mafic/ultramafic rocks are of talc-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 SBo Francisco Craton and the Guanambi-Correntina Block. 0 1998 Elsevier Science Ltd. All rights reserved Resume-0 deposit0 de Ouro Fino da Mina de Riacho dos Machados esta locdlizado em rochas da Faixd de Dobramentos Aracuai, borda leste do C&on S%o Francisco. A mineraliza#o aurifera ocorre associada principalmente a urn quartzomoscovita xisto corn sulfetos, originado a partir da alteracrio hidrotermal de xistos peliticos e xistos quartzofeldspaticos do Grupo Riacho dos Machados (GRM). Associacoes minerais tipicas da facies anfibolito srio progressivamente alteradas para associacoes compativeis corn a facies xisto Verde, ao longo de zonas de cisalhamento. Ficd evidenciado pelos estudos que o process0 de segregacgo e concentracgo de elementos quimicos e minerais e o proprio processo da mineralizaeo, corn concentra@o de Au.

Banded Iron Formation-Related High-Grade Iron Ore

ABSTRACT Recent advances in geometallurgy have made nickel laterites a premier target for mining ... more ABSTRACT Recent advances in geometallurgy have made nickel laterites a premier target for mining and exploration companies. Parallel to this, a series of advances in remote sensing have become available at very low cost. Two of these advances, namely better topographical data, through NASA’s Shuttle Radar Topographic Mission (SRTM), and multispectral imagery from the ASTER sensor, have been combined to aid nickel laterite exploration in central Brazil. The Conceição do Araguaia region, located in Pará and extending into Tocantins state, is part of the Neoproterozoic Araguaia fold belt. The target area covers the Quatipuru mafic-ultramafic association, which includes serpentinites (metaperidotites and metadunites), talc schists, tremolite-actinolite schists and small volumes of pillow basalts, phyllites, BIFs, gabbroic and jasperoid rocks. These are enclosed regionally by slate to phyllitic rocks. Several other occurrences of mafic-ultramafic rocks, along with Quatipuru mafic-ultramafic association are interpreted as part of an ophiolitic complex. Low-grade (greenschist) regional metamorphism is dominant. Laterization has been active since the early Tertiary, resulting in an extensive regolith cover over the older rock units. In the present work, the Quatipuru mafic-ultramafic association was examined for nickel laterite mineralization by data compilation and remotely sensed image processing and interpretation. ASTER’s multispectral visible-shortwave infrared (SWIR) remote sensing capabilities were used to map areas of prospective mineral alteration and key mineral groups. Using spectral libraries for selected nickel-bearing minerals as standards, SWIR and visible-NIR bands of georeferenced mosaiced ASTER scenes were processed by feature-oriented principal component analysis (PCA), and the results converted in mineral-abundance images, based on statistical classification and pseudocoloring. The mineral abundance maps highlight areas most likely to contain minerals of interest. Processing was performed for the whole region of interest, and for its part in the central scene alone, which covers about 85 percent of the concession areas. In both cases, statistics for PCA were conducted on a subset of the data, minimizing extraneous factors such as large drainages and urban sprawl, and then applied to the whole region. Mineral abundance maps for the area have been built into a geographic information system (GIS), along with other remotely sensed data, public-domain regional geophysics, geologic, and infrastructure data, scouted geochemistry samples, and a leveled and continuous SRTM digital elevation model. Mapping for the occurrence of mafic-ultramafic rocks was achieved by a combination of PCs 1, 4 and 2 of ASTER bands 2, 4, 5, and 8. Clusters of anomalous contents of selected minerals are draped over the digital elevation model and indicate that the northeast-dipping rock units are covered by a laterized sequence constituting the main exploration targets. This targeting exercise revealed a number of favorable sites that are currently under exploration by mining companies.

Revista Brasileira de Geociências, 2000

Applied Earth Science, 2006

A genetic model for the genesis of high-grade (.65 wt-%Fe) Proterozoic, banded iron formation (BI... more 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 Carajá s (Brazil), the Aguas Claras, Conceiç ã o 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 faultmagmatic (Carajá s) 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.