Abstract

The Rio Maria granite-greenstone terrain is characterized by extensive surficial cover and a lack of outcrop. Therefore, airborne geophysical measurements play a major role in mineral exploration in this region. A high-resolution airborne survey was used to build a prospectivity model for gold targeting employing a fuzzy logic technique. Within the Rio Maria granite-greenstone terrain, a total of 57 new potential orogenic gold targets were identified. The ability of this processing technique to identify favorable targets with potential for economic gold mineralization was verified by comparing the new predicted targets with known gold occurrences (e.g., Mamão Mine and Lagoa Seca deposits). Geographic Information System (GIS)-based automated processing methods employing fuzzy logic techniques were used to derive spatial models for generating orogenic gold exploration targets.

Two metallogenic approaches were used. The first approach considers orogenic gold deposits hosted at the contact between mafic and felsic rocks. The second approach considers shear zone-hosted veins associated with mafic rocks and iron formations in the greenstone terrain. Detailed models were constructed for different blocks of the study area. A subset of these targets, i.e., Marcinho, Resende, and Votuporanga, were subsequently assessed using field evaluations that consisted of geological mapping and geochemical sampling. A follow-up drilling program is currently in progress and will be used to assess the main target areas where grid soil and rock sampling have indicated anomalous trends. The remaining predicted targets warrant further investigation.

Data integration using GIS modeling and interpretation resulted in the following main conclusions with respect to the orogenic gold exploration potential of the Rio Maria Province: (1) as shown using GIS-based prospectivity analysis, there is considerable potential for orogenic gold deposits along the Andorinhas greenstone belt, and several prospective areas are associated with mafic and iron formation units; (2) several deposits within the Rio Maria and Identidade greenstone belt are associated with felsic units, as demonstrated by the fuzzy logic models; (3) many of the previously known targets that have been re-identified should be reevaluated to identify those the most promising targets for the discovery of a gold deposit; (4) the final prospectivity model shows that many of the most important gold deposits known are located within areas of high favorability, and several other new potential gold-bearing targets were selected; and (5) the proposed method identifies 57 geologically consistent targets and led to the discovery of the Marcinho deposit.

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