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Título

PETROGRAPHIC CHARACTERIZATION OF MATARAZZO MARBLE SKARNS, DOM FELICIANO BELT: INSIGHTS INTO FORMATION MECHANISMS AND MINERALIZATION

Texto do resumo

From eastern Uruguay to southernmost Brazil, the Punta del Este Terrane (PET) records important essential features of the geotectonic evolution of the Dom Feliciano Belt. In its Brazilian extension, near the city of Arroio Grande, Rio Grande do Sul, ophiolitic and supracrustal rocks associated with the Arroio Grande Complex are exposed. The Matarazzo Marble, a high-grade metamorphic sequence of the Arroio Grande Complex, outcrops as two main bodies, each one around one km² in size, with slightly elongated to NE-trend, resembling a roof pendant structure. These bodies are surrounded by Arroio Pedrado orthogneiss, metasedimentary rocks from Telho Complex, and Cryogenian to Ediacaran granitoid rocks. In the two Matarazzo Marble quarries, subordinated metabasics (basalts and gabbro protolith, sometimes with melt reactions producing quartz syenites) and also metric diorite and tonalite intrusions. All these rock relationship contacts are fertile key areas for skarn reactions as a result of distinct rock interactions. The main reactions are generated by the interaction of marble (calcite) and amphibolite (Plg + Hbl + Bt), which can be favorable for developing different types of mineralization (e.g., Au, Cu, Fe, Mo, Sn, W, Zn). Therefore, this study uses optical microscopy and SEM-EDS analysis to characterize these occurrences through petrographic evidence from samples collected at Matarazzo quarries. Field relationships and structural data indicate that both lithologies responded differently during the peak of the orogeny and shear-zone influence, as evidenced by differential rheological behavior from ductile to ductile-ruptil. This main skarn reaction is barren and is often associated with boudins, folds, and the border of angular amphibolite fragments. This reaction generates a centimetric metasomatic zoning of green to reddish color column. The green zone is identified as endoskarn, with a paragenesis dominated by clinopyroxene (Di50Hd50) + epidote ± fluorapatite, while the reddish zone is called exoskarn, consisting of grossular + calcite + clinopyroxene ± titanite. The second and subordinate skarn reaction occurs by the interaction of marble and quartz-syenite crystallizing wollastonite and garnet, representing a prograde paragenesis. Garnet and diopside veins are frequently concordant with the marble foliation and in the near-angular amphibolite blocks. Portions of centimetric sulfide concentration are found at the skarn reaction among the marble with amphibolite, quartz syenite, and also with late diorite and tonalite intrusions. In this reaction, the predominant sulfides are pyrrhotite, pyrite, and chalcopyrite. This mineralization is associated with retrogressive overprint paragenesis dominated by clinozoisite, diopside serpentinization, and well-formed amphibole crystals. Furthermore, native gold is associated with a calcite grain and fine sphalerite, galena, and greenockite. The origin of these skarns is controversial due to the complex tectonic evolution of the area, the discontinuity of intrusions, and the heterogeneity of structure. Their origin may be induced by metamorphic fluids in a high-temperature environment, given their association with deformational processes associated with amalgamation events related to the closure of the South Adamastor Ocean.

Palavras Chave

Skarn reactions; Metasomatism; Punta del Este Terrane; Dom Feliciano Belt