Gamma‑ray spectrometric evaluation in watersheds with gold anomalies in stream sediments, Passo Feio Complex, Caçapava do Sul region (Brazil)

Abstract

This study utilizes gamma-ray spectrometry to identify source areas of gold recognized in geochemical prospecting work in stream sediments. The local geological context is represented by the Passo Feio Metamorphic Complex, which contains gold deposits possibly hosted in quartz veins concordant with metamorphic foliation, composed of quartzites, schists, and amphibolites. The study followed the sequence: geological reconnaissance, drone survey, and gamma-ray spectrometric acquisition. Field reconnaissance indicated an alternation of rocks oriented in a WNW/ENE direction, with recognition of quartz veins embedded concordantly with the foliation of quartzites. The drone data enabled the generation of a Digital Elevation Model (DEM) and served as a basis for planning the gamma-ray spectrometry work. A total of 715 gamma-ray spectrometric readings were taken, with an average spacing of 40 m between points, followed by data processing and the generation of maps of K (0.5–2.5%) concentrations, eU (0.9–3.9 ppm), and eTh (2.8–24.4 ppm). The data obtained regarding the K (%) concentration values reveal a clear anomaly in the central region with WNW/ENE orientation. The concentration data obtained for U (ppm) and Th (ppm) show a similar pattern concerning the WNW/ENE anomaly in the center of the area. The overlap of positive anomalies served as an indication of gold source areas in two river valleys. The combination of detailed gamma-ray spectrometry and precision digital terrain modeling has enabled the delineation of primary targets potentially mineralized in gold, in a versatile and fast way, compared to traditional soil sampling techniques.