Geoochemistry of hydrothermal alteration in volcanic rocks. Chapter 3

MARFIL S.; MAIZA PEDRO

Geochemistry - Earth's System Processes

InTech, Published

Año: 2012; p. 39 - 60

Hydrothermal alteration is a chemical replacement of the original minerals in a rock by new minerals where a hydrothermal fluid delivers the chemical reactants and removes the aqueous reaction products. An understanding of hydrothermal alteration is of value because it provides insights into the chemical attributes and origins of ore fluids and the physical conditions of ore formation (Reed M. 1997). Within a mineral deposit, the solution channelways are usually obvious because precipitated minerals and altered wallrocks remain as evidence. The direction in which the solutions flowed, especially in flat-lying deposits, is usually less obvious but in many cases can be enferred from  mineral zoning or similar evidence (Skinner B. 1997). The mobility of major, minor and rare-earth elements (REE) during alteration processes in different environments has been documented by numerous authors and has been used to discriminate the origin of kaolin deposits. (Sturchio et al., 1986; De Groot & Baker, 1992; Gouveia et al., 1993; van der Weijden & van der Weijden, 1995; Condie et al., 1995; Dill et al.,1997, 2000; Galán et al., 1998, 2007; Pandarinath et al., 2008, among others). Terakado & Fujitani (1998) studied the REE and other trace elements in silicastones, alunites and related rocks in order to examine the behaviour of trace elements in the acidic hydrothermal alteration of silicic volcanic rocks. They found that most of the elements such as Na, Fe, Ba and LREE were leached from the silicastones, while HREE, Th, Hf and Zr were retained in the rocks, even under strongly acidic hydrothermal processes. Alunite samples have LREEenriched and HREE-depleted features. According to Dill et al. (1997), the ratios TiO2 + Fe2O3 vs. Cr + Nb, Zr vs. TiO2 and Ba + Sr vs. Ce + Y + La in kaolinites allow discrimination between hypogene and supergene kaolinization processes. The APS-bearing argillaceous zones that formed during supergene processes are significantly enriched in REE relative to hypogene equivalents (Dill, 2000).2 + Fe2O3 vs. Cr + Nb, Zr vs. TiO2 and Ba + Sr vs. Ce + Y + La in kaolinites allow discrimination between hypogene and supergene kaolinization processes. The APS-bearing argillaceous zones that formed during supergene processes are significantly enriched in REE relative to hypogene equivalents (Dill, 2000).