BEFORE THE POT: TRACKING THE POSSIBLE CLAY SOURCE OF

BERTOLINO, S.; ZIMMERMANN U.

Rio de Janeiro

Conferencia; XV International Clay Conference; 2013

AIPEA

In the search of the source materials of ceramics and paints used by the artisans of the Aguada Culture (300-1000 AC.), we studied different local clays at the Ambato Valley, Catamarca (Argentina) using mineralogy and trace element geochemistry. The Aguada Culture was the response to a socio-political and ideological change that occurred in the Ambato valley in the IV century AC; this chage promoted unprecedented institutionalization of social inequality, more specialized goods production and standardized work but a continuity of the earlier ceramic tradition has been suggested on previous studies. The valley limits with the Ambato Range to the west, the De las Higueras and Graciana (also known as Balcosna) Ranges to the east and the Aconquija Range to the north. Basement rocks, mainly banded gneises, migmatites, schists and phyllites outcrop at Ambato and Graciana, often intruded by pegmatites and tonalite-granodiorite bodies; quaternary sediments along with small relicts of tertiary sedimentary rocks fill the intermountain valley. Considering the regional area, several clay sources have been described and proposed as possible raw materials for both the ceramics and the pigments: Los Escobales (off-white and dark red) possibly Tertiary pyroclastic and sedimentary rocks, La Aguadita (reddish), El Rodeo (reddish brown) and Los Varela (reddish brown) Quaternary sediments, Humaya (red and ocher) possibly hydrothermal alteration of basement rocks and Andalgalá (greenish) phyllite. The mineral composition of clays sources is: 1) Los Escobales red: quartz, calcite, low albite, microcline, muscovite, I/S (35% I, R=0), illite, kaolinite and scarce biotite and hematite; offwhite: quartz, oligoclase-andesine, sanidine, smectite, biotite, amphiboles (hornblende), minor muscovite, chlorite, opal-C and clinoptilolite, and traces of pyrite, with almost pure smectite (with traces of I/S 15%I and feldspars) in the clay fraction. 2) La Aguadita: mainly micas, quartz, plagioclase, calcite and minor chlorite and hematite and I/S-I-Ch clay assamblage. 3) El Rodeo: micas, quartz, plagioclase with minor hornblende, clinoptilolite and chlorite and a clay suite of I/S and I. 4) Los Varela: micas, quartz and feldspars with dominant illite, scarce chlorite and I/S in the clay fraction. 5) Humaya red and yellowish have similar composition: quartz, calcite, micas (illite) and kaolinite, with minor hematite. 6) Andalgalá: quartz, muscovite, chlorite and minor plagioclase with a clay suite of well crystallized illite and chlorite. Clays show a large variation in silica even from the same exposures (Humaya red and yellowish) with differences of more than 10%. Al2O3 and Fe2O3 that are generally lower than in the ceramic while Ti2O is rather low. However, the clays vary from exposure to exposure significantly and cannot be evaluated as a group. The sample from Andalgala, in turn, is definitely silica-richer Trace element geochemistry shows that clays derived directly form the basement rocks are characterized by enormous high concentations in Cs. Remarkable are as well the high W, Bi, Sn and Sb in some samples, never observed in the higher metamorphic or low-grade metamorphic rocks. All other trace element abundances are typical for unrecycled UCC as it can be expected for continental clays. While the Cs anomaly is observed in all other country rocks of pre-Paleozoic age, the base metal anomalies need to be local. It seems that this signal is then transported into the ceramic.