Abstract

The Au-Cu (±Pb, Zn, Ag) prospects of Lavras do Sul, southernmost Brazil, are hosted in Neoproterozoic granitic and volcanogenic rocks. Mineralization occurs in structurally controlled N40°W to E–W quartz veins; sericite (±chlorite) 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 2M1 phengite-rich alteration (± illite) is best developed with coarse-grained primary growth (comb) quartz + pyrite ± 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 ≤0.2°2𝛉 CuKα) of the <5 μm particle-size fraction, non-expandable d00l X-ray reflections and interlayer charge (IC) >0.9 per O10(OH)2.

Towards the eastern zones of the granitic complex and in the volcanogenic rocks, wider XRD profiles (FWHM values ≥ 0.2°2𝛉 CuKα) were decomposed. They contain mixtures of coarse- to fine-grained, lath- like crystals of both 2M1 and 1M illite (non-expandable d00l X-ray reflections, IC between 0.85 and 0.89 per O10(OH)2) with expandable d00l reflections associated with lath-like, fine-grained crystals of ordered (R ≥1) illite-rich I-S (80–90% of illite; IC of ~0.8 per O10(OH)2), and minor amounts of regularly ordered (R = 1), illite-rich I-S mixed layers (75% of illite; IC of ~0.74 per O10(OH)2). The dioctahedral clay association of illite + illite-rich I-S mixed layers (intermediate argillic alteration) is best developed in quartz + pyrite ± 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 (Tphe ≈ 300°C, 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 (TI-S ≈ 120–200°C).

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.

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