Innovative evaluation of field and spatial remote sensing data for analysis of ore-mineralization in the Andean region of Patagonia (Argentina)

DEL VALLE HF; HALLER MJ

Puerto Iguazú

Simposio; IX Simposio Latinoamericano de Percepción Remota; 2000

SELPER

Numerous occurrences of base metals minerals are hosted in the Andean segment comprised between 43º and 44º south latitude. The mineral deposits from the area are linked to Cretaceous magmatic and metallogenic events and have been included in the Andean Metallogenic Province. The aim of this study was to line out the methodology used for the characterization and digital mapping of ore-mineralized areas, based on the geological reality of the environment (field data). A highly detailed study based on field surveys allowed to establish that the ore-mineralization in the area occurs as: i) disseminated, with pyrite and gold; ii) vein-forming, with a) chalcopyrite, pyrite, sulfosalts, and gold; and b) galena, tetrahedrite-tennantite, sphalerite and sulfosalts. The predominant ore-associated hydrothermal alterations are quartz-sericitic, argillic and porpylitic. A close relationship exists among the regional structure, rock alteration and ore-mineralization. The GPS coordinates data allowed the accurate identification of the study sites on the georeferenced image, and later an unsupervised classification has been carried out. The cartographic accuracy of the classes was considerably increased combining the stratification obtained by visual interpretation helped by computer of the country rock of a satellite image (Landsat TM), ancillary data (geologic map), with a supervised classification of each hydrothermal alteration occurrence based on maximum likelihood. The Landsat data channels were also rationed to enhance the rocks and soil-regolith response. Landsat TM data added two very important bands for detecting the presence of mineral assemblages, band 5 and band 7. The additional measurement in band 1, the blue band also gave better discrimination of iron oxide abundance when ratioed with band 3. In band 5 rocks hydrothermally altered to clays have very high reflectances, while in band 7 most of the important spectral absorption features associated with micas and clays occurred. Different algorithms are presented: mineral composite (5/7, 5/4, 3/1), extensive and intense advanced argillic alteration (5/7, 5/1, 5/4), and hydrothermal composite (5/7, 3/1, 4/3). Some of the most spectacular contrast between surface cover were achieved with the inverse of the Principal Component Analysis (PCA) on an image of 4 bands (1+2+3, 4, 5 and 7). The combination that provides superior images for lithologic and alteration discrimination was PCA 2, PCA 3 and PCA 4. This methodological approach was used to develop a simple and generally applicable model for the efficient correlation of field and remote sensing data.