Abstract
Au and sulfide mineralizations occur in the Sul-Rio-Grandense Shield, and important occurrences of these metals are reported and attributed to the Lavras do Sul Shoshonitic Association (LSSA). Several authors point to a significant participation of andesitic and lamprophyric rocks in the formation of these deposits. The Tupanci area has the presence of andesites and lamprophyres, stratigraphically correlated to LSSA volcanic rocks (HilĂĄrio Formation), representing the northernmost exposures of this magmatism. The LSSA is one of the most complete Neoproterozoic Shoshonitic associations (ca. 592-590 Ma) in southern Brazil, containing plutonic, volcanic and hypabyssal rocks, related to the post-collisional stage of the Brasiliano/Pan-African orogeny. The andesitic rocks of the Tupanci area occur in the form of the lava flows, overlayed by volcanoclastic deposits, product of the reworking of the volcanic unit. Lamprophyres occur in the form of dykes, with N-NW direction, which are intruded in rocks of the metamorphic basement and in andesitic lava flows, being texturally separated and classified as spessartite and lamprophyric lapilli-tuff. The singular occurrence of the lamprophyric lapilli-tuff stands out, is analogous to Kimberley-type pyroclastic kimberlitic intrusions, depicting explosive manifestations linked to LSSA lamprophyres. The andesites and lamprophyres that ocour in Tupanci area show characteristics of high-K calc-alkaline or shoshonitic rocks, evidenced by LILE and LREE enrichment, negative Ti and Nb anomalies, plus positive Pb anomalies and the absence of Eu anomalies. These attributes reinforce the relationship of andesitic and lamprophyric rocks with related LSSA rocks. The presence of Au and Pt grains in the matrix of lamprophyric lapilli-tuff, highlights the economic importance of these rocks for the mineral occurrences in the region, where future studies can better define the metallogenetic potential for the rocks of the Tupanci area.
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