Geochemical anomaly separation by multifractal modelling

Geochemical anomaly separation using the concentration-area method as appli ed to two different geological settings in Portugal is investigated, Soil g eochemistry data sets were used in both settings. Geochemical surveys were conducted for the exploration of gold mineralisations in quartz veins assoc iated with major shear zones (Arouca, NW Portugal). Similar surveys were al so undertaken for V and Ti exploration related to the accumulation of oxide minerals in gabbroic rocks belonging to an exotic oceanic terrane (Mombeja ) and to a late calc-alkaline igneous complex (Odivelas), both in SE Portug al. Results have been interpreted using the extensive information available for each area, which includes detailed geological mapping, structural info rmation, whole-rock chemical analysis, and mineralogical data. Anomalous th resholds for each area were computed and compared mainly with whole-rock ch emical analyses and mineral chemistry data. In the Arouca area, anomalies a t the local scale were identified for Au (630 ppb), As (912 ppm), and Sb (6 30 ppm), and at a regional scale for As (96 ppm). In the Mombeja area, anom alies at a regional scale were identified for V (88 ppm), Cr (232 ppm), and Ni (76 ppm), and the available results for Odivelas area suggest the exist ence of local scale V anomalies, whose magnitude is generally above 400 ppm . In the Arouca area, the correlation between these threshold values and gr ades in rocks is clearly shown, especially by discriminating two different mineralising events. The second, and most important mineralising event is r esponsible for grades above 1 ppm for Au in the mineralised structures, con trasting with grades below 500 ppb from the first event. The anomalies iden tified in Mombeja reflect mainly a regional distribution of the outcropping rock types, mainly metagabbros and serpentinised peridotites, with special emphasis on the correlation between V and the gabbroic rocks. The threshol ds obtained for each element are always lower than their mean content in th e rocks. The Odivelas area shows threshold values for V clearly above the m ean rock content, being a consequence of the occurrence of anomalous accumu lations of oxide minerals within the gabbroic rocks. In order to overcome d ata scarcity, a scheme to super-sampling the population by using estimated values with ordinary kriging in the computation of the anomalous thresholds was essayed. These results, albeit needing further validation, gave some i ndicative references to the economic potential of this area for V mineralis ations. (C) 2001 Elsevier Science B.V. All rights reserved.