METAHALLOYSITE CLAYS OF POÇOS DE CALDAS ALKALINE COMPLEX, MG-BRAZIL
HANS D. SCHORSCHER AND RENATA A.R.N. DE-OLIVEIRA
Instituto de Geociências, USP, São Paulo, SP.
Presented by ANTONIO C. ROCHA-CAMPOS
Based on field work and mineralogical-petrographical optical, XRD and SEM methods, we compared a clay deposit (Mineração Varginha) located in the western part of the domain of 'potassic rock' hydrothermal alteration hosting the Osamu Utsumi uranium mine, SE-Poços de Caldas Complex (PAC), to the unweathered hydrothermalized protoliths. The deposit occurs in the swampy upper hillside of an S-to-N-draining open valley within an area of moderately hilly morphology with gentle slopes, open valleys and altitudes of 1280-1380m. Here, different generations of mutually intrusive leuco-to-hololeucocratic nepheline syenites and phonolites were transformed by potassic-pyritic alteration into hydrothermalites referred to as 'potassic rocks' (with K2O of 12.5-13.8wt.-%), having low-grade U-Th-REE-Zr-F-Mo mineralization. Deep chemical weathering originated the clay deposit and exhumed this part of PAC to a subvolcanic level.
Studied deposit hosts two types of clays: one is grey with millimetric to centimetric white argillized pseudoleucite phenocrysts (ACP) representing a weakly porphyritic pseudoleucite phonolite of fine matrix weathered in situ to clay, with preserved magmatic structure; the other is a homogeneous white clay (ABM) formed from a fine hololeucocratic aphyric phonolite. Combined optical, XRD and SEM studies of ACP and ABM clays and of unweathered 'potassic rocks' (hydrothermalites of hololeucocratic aphyric and pseudoleucite phonolites), considered as analogous to the clay protoliths, always showed highly crystalline hydrothermal kaolinite with the same habit of fine idiomorphic plates and booklets (Ø £ 5µm); equivalent idiomorphic hydrothermal illite is also omnipresent yet much coarser (Ø ³ 15-50µm), practically representing fine sericite/muscovite. Feldspar occurs only in the hydrothermalites, being a low-temperature pure K-feldspar of maximum triclinicity formed by hydrothermal alkali-exchange reactions (K for Na) and structural readjustments from magmatic sanidine.
Kaolinite and illite of low crystallinity occur only in the clay samples; the former predominates by far over the kaolinite of high crystallinity and proved to be in fact tubular metahalloysite (SEM). XRD studies with heating techniques at 60oC showed the illite of low crystallinity to be in fact (hydrated) halloysite partially preserved from dehydration (to metahalloysite) due to water-saturation in the clay deposit. (December 14, 2001) .
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
