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The Serra Geral Group has high Cu concentrations in southern Brazil, including the Realeza (Paraná) region where Au and Ag also occur. The basalts (46.7-50.8 wt.% SiO2) have breccias at the top of 10 out of 31 studied flows in the Realeza section and at the top of 5 out of 7 analyzed flows in the Capanema section. Sand injectites occur at the top of basalts in 18 out of 23 flows from the Realeza section. The average content of Cu in three samples of mineralized breccias is 3788 ppm. One sample from a silicified sandstone sill intruded at the top of a basalt flow has up to 0.66 ppm Au. K2O was not leached during basalt alteration. The hydrothermal breccias, particularly when sand is present, show enrichment in U which leads to higher values of scintillometry (rate of emission of K, U and Th). In the basalts, breccias and sandstones, Pd (17.5 ± 8.4 ppb) is higher than Pt (12.5 ± 5 ppb), suggesting a hydrothermal remobilization of the metals. The Serra Geral Group is thus shown to have high potential for precious and base metal deposits. [ABSTRACT FROM AUTHOR]

Abstract: We report the discovery of a large number (probably thousands) of gossans in the intraplate Paraná volcanic province in South America, based on observations from satellite images and field work supplemented with rock geochemistry and geophysics. We also provide a straightforward prospecting guide for agate and amethyst deposits. The study area is located on the border between Brazil and Uruguay, encompassing the Quaraí mining district and the Los Catalanes gemological district. Anomalies in Google Earth satellite images were identified above six underground mines in the Los Catalanes gemological district, characterized in the pampas of the region as irregular structures of intense green color and sometimes with some brownish tints. These vegetation, gamma-spectrometric, and geochemical anomalies occur at several stratigraphic levels in the volcanics. Three gamma-spectrometric sections performed on the Maurício mine of the Los Catalanes gemological district indicate low emission rates near 55cps (sd=4.7) in the gossan compared with the regional average of Cordillera flow (63cps). LOI higher than 2% is considered a strong indicator of intense hydrothermal alteration in the gossan. In the Quaraí mining district, gossans were studied in five areas, two in the Catalán flow, two in the Muralha flow, and one in Cordillera flow. Negative gama-spectrometric anomalies (higher than one standard deviation) occur in these gossans. The detailed study of one gossan included a gamma-spectrometric grid spacing of 50×50m (K, U, Th and total emission rate) and whole-rock geochemical analyses (ACME, Canada). The whole-rock geochemical analyses (20 samples) collected inside and outside the gossan demonstrate that these rocks are basaltic andesites of the low‐Ti Gramado chemical type. The samples within the gossan have high values of loss on ignition (2.3, 2.8, 2.9, 2.8, 2.9, 2.4, 2.6, 2.6, 2.3 and 2.3wt.%), whereas outside the gossan the values are lower (0.8, 2.3, 0.5, 0.5, 1.6, 0.5, 0.6, 0.9, 1.3 and 1.9wt.%). SiO2, K2O and Rb show strong negative correlations with a loss on ignition, whereas MgO shows a slight enrichment. The low gamma-spectrometric signal in the gossan is defined primarily by the lower K2O of the altered rock. We thus report the discovery of gossans above amethyst geodes in the world-class deposits of the Paraná volcanic province and present a first description of the structure. A straightforward prospecting guide will facilitate future exploration for new deposits. [Copyright &y& Elsevier]