C. R. De Souza Filho

Targeting key alteration minerals in epithermal deposits in Patagonia, Argentina, using ASTER imagery and principal component analysis

Principal component analysis (PCA) is an image processing technique that has been commonly applied to Landsat Thematic Mapper (TM) data to locate hydrothermal alteration zones related to metallic deposits. With the advent of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a 14-band multispectral sensor operating onboard the Earth Observation System (EOS)-Terra satellite, the availability of spectral information in the shortwave infrared (SWIR) portion of the electromagnetic spectrum has been greatly increased. This allows detailed spectral characterization of surface targets, particularly of those belonging to the groups of minerals with diagnostic spectral features in this wavelength range, including phyllosilicates (‘clay’ minerals), sulphates and carbonates, among others. In this study, PCA was applied to ASTER bands covering the SWIR with the objective of mapping the occurrence of mineral endmembers related to an epithermal gold prospect in Patagonia, Argentina. The results illustrate ASTERs ability to provide information on alteration minerals which are valuable for mineral exploration activities and support the role of PCA as a very effective and robust image processing technique for that purpose.

ASTER and landsat ETM + images applied to sugarcane yield forecast

This paper proposes a method to support sugarcane yield forecast using vegetation spectral indices, principal component analysis and historic yield data. The study area is located in the State of São Paulo, Brazil, and is divided into 11 production plots (108.75 ha), where sugarcane of the RB85 5536 variety is cultivated on red latossol (oxissol‐type) soil and flat topography. The data employed in the study include radiometrically and geometrically corrected enhanced thermatic mapper Plus (ETM+)/Landsat‐7 and ASTER/Terra images, acquired in June and April 2001, respectively, and historic harvest data measured in 2000 and 2001. The method comprises several steps: (a) enhancement of specific spectral responses of vegetation constituents; (b) reduction of spectral dimensions with prioritization of information and weighing of parameters related to foliar area; the data processed through these steps are reduced to a single image (the synthesis image), from which the mean DN (digital number) per cultivated area is calculated; (c) the image DNs are subsequently transformed into ton of stalk per hectare (t ha−1) through normalization, which requires knowledge of the previous years yield for the cultivated production plots under analysis. Yield estimates using the method showed greater precision in comparison to the ubiquitous visual methods employed by the sugarcane agro‐industry in Brazil. Using factual productivity data of the year 2000 harvest only, the method achieved estimate errors varying between 2.57% and 5.65%, compared with 9.06% expected by the sugar factory; whereas using data from the year 2001 harvest, error margins were remarkably lower, around 1%.

Neoproterozoic terrane assemblages in Eritrea: review and prospects

Abstract Four, perhaps five large terranes that are structurally and lithologically distinct constitute the Neoproterozoic of Eritrea. Two are composites of structurally bounded smaller blocks, many of which also show considerable lithological differences. Accretion along regional shear zones was dominated by sinistral strike-slip. The westernmost Barka Terrane is an upper amphibolite to granulite grade complex, characterised by polyphase ductile strains. It contains metasediments and undeformed felsic dykes that have no counterparts in the other terranes. The Hagar Terrane is dominated by immature supra-subduction zone volcanic rocks, but its western parts contain oceanic basalts and pelagic sediments, deformed basic-ultrabasic rocks and a major olistostrome. Parts of this tectonised ophiolite retain high P , low T assemblages, suggesting that the Barka Shear Zone (its western boundary) was initiated as a subduction zone. Other subduction zone metamorphic assemblages in the internal Hagar Terrane may represent tectonic underplating of an evolving volcanic arc. The Adobha Abiy Terrane is a highly deformed post-accretionary sedimentary basin, formed after the close of volcanism. The Nakfa Terrane is a collage of many lithologically different volcano-sedimentary blocks bounded by shear zones, in which metamorphism generally reached greenschist and lower grades. It also includes several mafic-ultramafic belts. Those blocks for which geochemical data are available suggest magmatism in supra-subduction zone environments. In its axial part, the Nakfa Terrane reveals a regional steep belt dominated by high-grade migmatites and gneisses that formed from volcanic rocks subjected to lower crustal conditions. Late syn-kinematic granitoid plutons are regionally concentrated along this steep belt. However, these high-grade rocks show a number of differences from isolated occurrences of gneisses in the Red Sea lowlands that might constitute a fifth entity, the Arag Terrane. Three major episodes of deformation probably correspond to protracted terrane accretion, shortening and thickening of new crustal materials, and crustal reworking dominated by sinistral transpression. The last affects post-accretion sedimentary sequences in the Adobha Abiy Terrane and similar strata in the Nakfa Terrane that show lower metamorphic grade than the earlier volcano-sedimentary associations. This sequence spans a ∼300 Ma period from about 850 to 550 Ma.

A Neoproterozoic supra-subduction terrane in northern Eritrea, NE Africa

The western part of a tectonic block in the complex Pan-African architecture of northern Eritrea contains partly re-equilibrated low-T, high-P assemblages (550°C, 14.5 kbar), basalts with oceanic geochemistry and a major olistostrome containing lenses of serpentinite. These features demonstrate subduction zone products. East of this accretionary wedge the block is dominated by island-arc volcanic and volcaniclastic rocks, and various calc-alkaline plutons. A bimodal volcanic suite of primitive andesitic basalts and evolved dacites and ignimbrites represents early submarine arc development and emergent volcanism, respectively. Intense, polyphase deformation culminated in sinistral transpression with crustal-scale imbrication between several east-verging thrusts. A tectonically bounded unit in the internal part of this arc association also contains relict high-P assemblages (650°C, 10–13 kbar) in oceanic basalts, which are possibly products of tectonic underplating as the arc developed. Oceanwards collapse of the tectonically inflating arc may explain formation of evolved, subaerial volcanic rocks within small extensional basins. Down-to-west extensional structures may have been inverted during terrane accretion to act as east-verging shear zones during late thickening. The area remains to be correlated with those areas in Arabia that were juxtaposed with it before Red Sea opening.

Brazil's 2001 energy crisis monitored from space

Data sensed by the US Air Force Defence Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) during the years 2000 and 2001 in Brazil were tested as a tool to monitor reduction of nocturnal lighting. This particular timing was examined as the Brazilian population and industry were forced to reduce electric power consumption by 20% during 2001, in relation to 2000, for a period of several months, starting officially on 1 June 2001. Large urban agglomerates were compelled to switch off city lights by at least the same amount. The Distrito Federal (DF), including the Brazilian capital, Brasilia, was one of the primary areas where the government actively sought electric power consumption reductions. Using the DF as a study case, we demonstrate that the mean grey levels derived from averaging DMSP-OLS data acquired over urban centres appear to be a useful index to monitor relative oscillations in energy consumption.

Geostatistical Simulation of Structurally-Controlled Low Grade-High Tonnage Gold Ores: A Strategy for Targeting Genuine Enriched Zones

This paper presents a geostatistical study of a classic, structurally-controlled, carbonaceous phyllite-hosted gold deposit with low grade/high tonnage characteristics. Gold in this mineralisation is hosted in boudinaged quartz veins that vary in size from 2 cm up to 60 cm. Larger quartz boudins are reported but only exceptionally. The boudins show maximum elongation along the regional mineral stretching lineation (N220) and an intermediate elongation perpendicular to the latter. Gold grades within zones rich in quartz boudins are known to be high, but are diluted as the phyllites comprising these mineralised veins contain little if any gold. Therefore, ore grades, though low, can vary considerably throughout the deposit.