SPATIAL AND TEMPORAL EVOLUTION OF HYDROTHERMAL ALTERATION AT LAVRAS DO SUL, BRAZIL: EVIDENCE FROM DIOCTAHEDRAL CLAY MINERALS E VERTON MARQUES B ONGIOLO 1,3, *, P ATRICIA P ATRIER-MAS 2 ,A NDRE ´ S AMPAIO MEXIAS 3 , D ANIEL B EAUFORT 2 , AND M ILTON L UIZ L AQUINTINIE F ORMOSO 3 1 CPRM, Geological Survey of Brazil, Rua Banco da Provı ´ncia 105, 90840-030, Porto Alegre, RS, Brasil 2 Universite ´ de Poitiers, CNRS, HYDRASA, 40 Avenue du Recteur Pineau, 86022 Poitiers, France 3 Universidade Federal do Rio Grande do Sul, UFRGS, Instituto de Geocie ˆncias, Av. Bento Gonc ¸alves 9500, 91509-900, Porto Alegre, RS, Brazil Abstract—The Au-Cu (ÔPb, Zn, Ag) prospects of Lavras do Sul, southernmost Brazil, are hosted in Neoproterozoic granitic and volcanogenic rocks. Mineralization occurs in structurally controlled N40ºW to EÀW quartz veins; sericite (Ôchlorite) and sulfides are the main secondary minerals in associated wall rocks. In the present contribution we use petrography (optical microscopy and scanning electron microscopy (SEM)), mineralogy (X-ray diffraction (XRD) with polytypes, FWHM, decomposition of diffraction patterns), and crystal chemistry of samples from several prospects to document the spatial and temporal evolution of sericitic alteration of veins and wall rocks associated with gold. Hexagonal, coarse-grained 2M 1 phengite-rich alteration (Ô illite) is best developed with coarse-grained primary growth (comb) quartz + pyrite Ô Au veins and altered wall rock from the western portion of the granitic complex (phyllic alteration). Pure phengite was recognized by narrow XRD profiles (FWHM 40.2º2y CuKa) of the <5 mm particle-size fraction, non-expandable d 00l X-ray reflections and interlayer charge (IC) >0.9 per O 10 (OH) 2 . Towards the eastern zones of the granitic complex and in the volcanogenic rocks, wider XRD profiles (FWHM values 5 0.2º2y CuKa) were decomposed. They contain mixtures of coarse- to fine-grained, lath- like crystals of both 2M 1 and 1M illite (non-expandable d 00l X-ray reflections, IC between 0.85 and 0.89 per O 10 (OH) 2 ) with expandable d 00l reflections associated with lath-like, fine-grained crystals of ordered (R51) illite-rich I-S (80À90% of illite; IC of ~0.8 per O 10 (OH) 2 ), and minor amounts of regularly ordered (R = 1), illite-rich I-S mixed layers (75% of illite; IC of ~0.74 per O 10 (OH) 2 ). The dioctahedral clay association of illite + illite-rich I-S mixed layers (intermediate argillic alteration) is best developed in quartz + pyrite Ô Au veins, breccias, and wall-rock alteration from the eastern portion of the granitic complex and in the volcanic area. Quartz from veins and breccias has fine-grained primary growth, recrystallization, and replacement textures, similar to those in epithermal deposits. The overall distribution of the dioctahedral clays indicates that the study area represents a fracture- controlled, tilted, porphyry to epithermal deposit, with telescoping alteration features observed in the east of the mining district. Deeper levels of exposure of a large hydrothermal system are observed in the west of the mining district, as shown by higher-rank dioctahedral minerals (phengite) that crystallize at relatively high temperatures (T phe & 300ºC, phyllic alteration) associated with coarse-grained, primary-growth quartz veins, similar to those observed in porphyry deposits. On the other hand, shallower levels of exposure are observed in the east of the study area, associated with abundant, lower-rank dioctahedral clay minerals (illite + illite-rich I-S mixed layers, intermediate argillic alteration) that crystallize at relatively lower temperatures (T I-S & 120À200ºC). Available data show that gold is associated with phengite, but that lower-rank, overprinting alteration characterized by illite-I-S may have locally modified the original gold grades. Key Words—Brazil, Clay Minerals, Gold, Hydrothermal Alteration, Illite, Lavras do Sul, Mixed-layer Illite-smectite, Phengite, Polytypes, Prospects, X-ray Diffraction. INTRODUCTION Located in the Rio Grande do Sul state, southernmost Brazil (Figure 1a), the Lavras do Sul Au-Cu (ÔPb, Zn, Ag) mining district comprises a group of prospects hosted in Neoproterozoic granitic and volcanic rocks. Several investigations of gold prospects were conducted in this district by Brazilian public agencies in the 1940s and between 1974 and 1995. They estimated gold resources of ~3.5 t, 6 t, and 7.5 t for the Cerrito, Bloco do Butia ´, and Volta Grande prospects, respectively (Figure 1b, locations 8, 19, and 2, respectively). New exploration programs by mining companies are now underway to re-evaluate these reserves. Some work has been conducted on the alteration and mineralization features by Mexias et al. (1990, 2005), who suggested that the Lavras do Sul Intrusive Complex Clays and Clay Minerals, Vol. 56, No. 2, 222–243, 2008. Copyright # 2008, The Clay Minerals Society 222 * E-mail address of corresponding author: ebongiolo@pa.cprm.gov.br DOI: 10.1346/CCMN.2008.0560207