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REDOX ALTERATION OF SERRA GERAL BASALT IN THE FORMATION OF AMETHYST AND AGATE GEODES (Au-Ag) IN SOUTHERNMOST BRAZIL AND NORTHERN URUGUAY

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Abstract: The early cretaceous metallogeny of silicon in Serra Geral Group basalts was controlled by redox reactions. Lower pH led to solubilization and reprecipitation of silicon in the sandstone and overlying basalt. Oxidation and reduction in the first lava flows mobilized silicon from the Botucatu erg sand to generate agate and giant amethyst geodes. Epithermal Au-Ag-Cu deposits may also occur. This is a unique feature among LIPs in the continents. We studied the geology of the border swath of Brazil and Uruguay to identify singular processes related to geode formation and to the Fronteira Oeste Rift, Cuesta de Haedo, lava flows of the Serra Geral Group, Guarani Paleoaquifer, Botucatu erg, rhombohedral structural province, paleohot spring field, injection and effusion of sand, hydrothermal explosions, sinter, silicification of paleoerg, circular and elliptical lava lower-crust remnants, volcanic-hosted seal aquifer, transcurrent faults, a widespread hydrothermal breccia stockwork, and duricrusts. Alkaline plugs (60 Ma) occur in the rift in Uruguay. The integrated study techniques included satellite image interpretation, geological field survey, petrography, scanning electron microscopy, and whole-rock geochemistry. We focus here on one main result - the oxidation and reduction of the lava flows on a regional scale forming the geodes. Every flow in the rift was altered and mineralized in sequence after cooling by the infiltration of hot (100-250 °C) water from the underlying Guarani Paleoquifer. This alteration affected the studied flows that covered large extents of the Botucatu Paleoerg – Catalán Flow, Cordillera Flow, Muralha Flow, and Coxilha Flow. The upper half (Tier 2) of the flow is red (oxidized, hematite), either thoroughly (no vertical cooling joints) or patchily along cooling and younger fractures. The lower half (Tier 1) is gray – either graphite-colored or white and contains minor pyrite, calcite and fluorite. Tier 1 may be thoroughly altered to white smectite (some kaolinite). Tier 2 is barren, but a significant observation is the presence of world-class agate and amethyst geodes in Tier 1. In Uruguay, geode deposits are exploited from Tier 1 of the Catalán Flow, and additionally from the overlying Cordillera Flow. On the Brazilian portion of the rift, agate geodes are extracted from Tier 1 near the surface in garimpos. The Paleoaquifer was initially oxidizing (as it is today – rainwater) but turned reducing and more acidic flow-after-flow. Reduction and acidification were caused by: (1) dissolution of myriad of calcite nanocrystals adhered to sand grains, (2) dissolution of H2 by percolating hot aquifer water over fresh basalt surfaces; (3) reaction of aquifer water with myriad of organic molecules in the erg. The description of Tier 1 and Tier 2 in the 200 x 350 km Cuesta de Haedo may foster significantly the capability of pinpointing the hidden occurrence of giant-amethyst-geode deposits and also agate-geode deposits. The epithermal province is prone to Au-Ag-Cu deposits yet to be searched for. The singularity of processes among LIPs worldwide is explained by (1) pahoehoe lava effusion covering erg areas larger than 100 km x 100 km, and (2) hydrothermal processes originating in the heating of a large, flatlying paleoaquifer during raised geothermal gradient from mantle melting and sill cooling. References: Hartmann et al. (2023) Catalán Flow, JSAES 133, 104734; Hartmann et al. (2024) Muralha Flow, Anais ABC 96(1).

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KEYWORDS: Serra Geral Group; Fronteira Oeste Rift; redox of lava flow; agate and giant amethyst geodes; two tiers.