Unconformity-Related Hydrothermal Alteration Assemblages Including Clay Minerals at Tandilia , Argentina.

JUAN-C. MARTÍNEZ; JORGE A. DRISTAS; HANS-J. MASSONNE; THOMAS THEYE

Heidelberg

Congreso; Latin American Colloquium; 2011

Universidad de Heidelberg

An alteration zone, up to 30 m thick and rich in clay minerals, occurs at the unconformity between the Paleoproterozoic igneous-metamorphic basement and the Late Precambrian overlying sedimentary succession in the San Manuel hills (~37¡ã 47¡¯ S; ~58¡ã 50¡¯ W). Around 40 km to the NW, in the Barker area, two transitional alteration patterns rich in secondary clay minerals at such an unconformity have been established depending on pervasiveness and type of alteration (e.g. Mart¨ªnez et al., 2010). There, the most pervasive alteration is characterized by the common appearance of Na-rich dioctahedral white micas, Na and K-bearing pyrophyllite, and florencite-type aluminum phosphate sulfate (APS) minerals. A comparison between the Barker and San Manuel areas should reveal if there was a regionally widespread fluid flow alteration event where the most pervasively altered basement can be recognized at San Manuel. The basement protolith from San Manuel was a banded migmatite (Dristas and Frisicale, 1984) possibly similar to that outcropping at Barker. Bulk-rock analyses of the altered basement at San Manuel show an enrichment of LREE, a ¡ÆREE of up to ~1800 ppm and either positive (Ce/Ce* ~2.5) or negative (Ce/Ce* ~0.5) cerium anomalies. Light REE hosted in secondary APS minerals derived from monazite present in unaltered basement according to studies of the most altered basement at Barker (e.g. Mart¨ªnez et al., 2010). Pyrophyllite in the altered basement at San Manuel shows 0.02-0.18 K pfu and 0.07-0.12 Na pfu. Such values are notably lower than the Na (up to 1.0 pfu) and K (up to 0.6 pfu) contents detected in pyrophyllite from Barker (Mart¨ªnez et al., 2010). Secondary dioctahedral white mica of the altered basement at San Manuel has a paragonite content of up to 58 mol% remarkably higher than the content (45 mol%) determined at Barker (Mart¨ªnez et al., 2010). The Na contents in dioctahedral white micas and Na-K contents in pyrophyllite could be linked either to solid solution or to separate phyllosilicate phases so finely intergrown that this could not be resolved with the electron microprobe (Shau et al, 1991). According to common radiometric ages, the fluid flow causing the studied alteration acted during the Brasiliano orogeny (~ 600 M.a.). The corresponding fluids were either generated by basement uplift or hidden Neoproterozoic igneous bodies in the R¨ªo de la Plata Craton.