THE PACHUCA GROUP: A PETROGRAPHIC AND GEOCHEMICAL STUDY OF HYDROTHERMAL ALTERATION IN AN ACTIVE KAOLINITE MINE, VERACRUZ, MEXICO

Kaolinite is an important industrial mineral used in ceramics, glass, paper, rubber vulcanization, and as a filler in plastics. In order to be economically viable, a kaolinite deposit is easily beneficiated (separated from contaminants and size controlled) to a suitable purity and size grade, and geographically close to production plants to reduce transportation costs. Kaolin and other alumina-bearing minerals like pyrophyllite are a key ingredient in the manufacture of fiberglass, providing alumina and silica into the batch chemistry. Well-known deposits suitable for glass manufacturing are located in Georgia, China, and Mexico. One such deposit is a local alteration product of the Pachuca Group, a widely distributed body of extrusive andesitic and rhyolitic volcanic rocks.

An area of the mine in question was investigated for future production using drill core sampling and subsequent petrological and geochemical techniques. This section of the mine was disqualified based on high iron content but provided a suitable candidate for analysis of the hydrothermal alteration mode seen in the rest of the mine. The alteration mode was identified as an incomplete low temperature argillic hydrothermal alteration facies as evidenced by the presence of kaolinite, smectite, and opal with relic feldspar and volcanic glass. The variability of mineralogy with depth (a kaolinite rich upper weathering zone and kaolinite/smectite interstratifications in the lower weathering zone) suggests the hydrothermal water had a primary meteoric component as suggested by previous work. Whole rock stable isotope values were varied enough to make it difficult to constrain the origin of the water of formation. However, a select subset of results do suggest formation from meteoric water.

The use of this specific area of the mine as fiberglass raw material is not recommended, but other areas of the same mine may be more highly altered and are worth investigation.