Federico Saponara, PhD

F. Saponara^{1, 2}, F. Lajoinie^1,2, M. Lanfranchini^{1, 3}, C. Spencer⁴, E. de la Cal⁵, M. Gonzalez^1,3, B. Marone^{1, 2
1}Instituto de Recursos Minerales (INREMI)(FCNyM-UNLP-CIC), La Plata, Buenos Aires, Argentina
²Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Ciudad Autónoma de Buenos Aires (CABA), Buenos Aires, Argentina
 ³Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA), La Plata, Buenos Aires, Argentina
⁴Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada
 ⁵ROCH S.A., Ciudad Autónoma de Buenos Aires (CABA), Buenos Aires, Argentina

Abyssal pegmatites represent the final crystallization stage of melts from the Paleoproterozoic basement of the Tandilia System. These bodies intruded in biotite-garnet gneisses exhibit vein morphology and appear as zoned or unzoned. This study aims to define the mineral composition, classification and genesis of the first zoned pegmatite described in this basement. Field relationships, petrographic analysis and advanced techniques, including XRD, SEM-EDS and δ¹⁸O isotopes, reveal that pegmatite can be divided into three zones: 1) Black feldspar core, 2) White feldspar-tourmaline, and 3) Biotite. Zone 1 is dominated by coarse black feldspar (10-20 cm in size), interstitial quartz and altered plagioclase. The feldspar is perthitic to microperthitic microcline with tartan twinning and sulphide inclusions of galena, pyrite and argentite. Its chemical composition (EDS determination) is 14-16% K₂O and 0.56-1.83% Na₂O with δ¹⁸O close to +5‰. Zone 2 contains white microcline (15-30 cm in size), biotite, muscovite, quartz, apatite, fluorite, fluorapatite, chalcopyrite, molybdenite, argentite, arsenopyrite and gold. Feldspar composition shows 13-14.2% K₂O, 1.95-2.20% Na₂O and an average δ¹⁸O around +7‰. This zone also includes tourmaline (chorlo variety) with grain-sizes of 8-10 cm and FeO_tot values of 15.40-19.09%. Zone 3 comprises crenulated biotite of ~5 cm, inequigranular texture and fine carbonate and muscovite veinlets containing sphalerite. The pegmatite exhibits distinct zonation, driven by the interaction between different processes. XRD analysis reveals that black feldspar in Zone 1 contains albitic components, while white feldspar in Zone 2 does not.  The increased K₂O content in the core, along with petrographic texture, suggests that black microcline formed from decomposition of plagioclase by hydrothermal fluids, which is consistent with lower δ¹⁸O values ​​in Zone 1, indicating extensive fluid interaction, and higher values ​​in Zone 2 reflecting minimal source rock interaction and assimilation. Tourmaline chemical analysis suggests a magmatic or partial melting origin, in concordance with previous studies of similar rocks in the region. Additionally, the presence of base and precious metals, along with fluorine, chlorine and carbonate/muscovite veinlets highlights the role of hydrothermal fluids complexes in metal redistribution. These results let us classify this pegmatite as a black and white microcline-quartz-tourmaline zoned vein-type pegmatoid using the CMS (Chemical composition - Mineral assemblage - Structural geology) classification. Detailed studies performed on the pegmatites in the Tandilia System basement, such as those presented here, will provide further insights into the composition, evolution and role of late Paleoproterozoic hydrothermal fluids in the region.