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Origin of kaolin deposits in the ‘Los Menucos’ area, Río Negro Province, Argentina

¹ Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000 Bahía Blanca, Argentina, ² Investigador Independiente Comisión de Investigaciones Científicas de la Provincia de Bs. As., Argentina, ³ Investigador Principal CONICET, ⁴ Departamento de Geología, Universidad Autónoma de Barcelona, 08193 Bellaterra, Spain

^* E-mail: smarfil{at}uns.edu.ar

(Received 17 November 2004; revised 26 January 2005)

Kaolinite deposits, hosted by rhyolitic tuffs, are found in the SE of the ‘Los Menucos’ area (Province of Río Negro, Argentina). The most representative examples are from the Blanquita and Equivocada mines. To discover the origin of these deposits, a mineralogical, geochemical (major and trace element) and stable isotope study (O and H) of kaolinite was carried out. Mineralogically, Blanquita mine is characterized by the presence of dickite, kaolinite, alunite and pyrophyllite, whereas in Equivocada mine, kaolinite is accompanied by dickite and traces of alunite (without pyrophyllite). The trace element contents and ratios of kaolin samples (P vs. S, Zr vs. Ti, (Cr + Nb) vs. (Ti + Fe), and (Ce + Y + La) vs. (Ba + Sr)) suggest that they formed from the hydrothermal alteration of the enclosing rhyolites although a possible contribution of kaolinite from residual processes cannot be discounted. ¹⁸O values of kaolinite range from +4.8 to +10.3, and D varies from –88 to –116. The presence of pyrophyllite in Blanquita mine suggests temperatures of formation >350°C. The calculated isotopic composition of the fluid in equilibrium with kaolinite at 350°C varies from +3.6 to +9.2 for O and from –74 to –103 for H. These values are compatible with fluids of magmatic origin or with fluids of surficial origin isotopically equilibrated with a rhyolitic magma at high temperatures. The isotope composition of kaolinite from the Blanquita and Equivocada mines is clearly different from kaolinite of residual origin from deposits in the Chubut and Santa Cruz provinces. The isotopic signature has proved to be a useful tool in discriminating between different genetic processes in the formation of kaolin deposits.

KEYWORDS: kaolin deposits, stable isotopes, trace elements, Argentina

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