A comparative study of APS minerals of the Pacific Rim fold belts with special reference to south American argillaceous deposits
Introduction
Kaolin deposits are widely acknowledged as of hypogene or supergene origin. Argillaceous rocks are extensively mined in many parts of the world and often represent the only source for ceramic goods, fillers and filter material. Moreover, kaolinitic rocks form part of alteration zones that surround metal deposits, together with other phyllosilicates such as smectite, pyrophyllite, and various types of mica. Aluminum-phosphate-sulfate (APS) minerals in many argillaceous rocks provide excellent insight into the nature of these argillites, but they are rarely studied in detail. The Pacific Rim fold belts host many kaolin deposits of hypogene and supergene origin that are located close together, especially in the South American Andean fold belt. Although the argillites in the immediate surroundings of some of the large base and precious metal deposits have been investigated, their APS minerals have not arrested much attention (Sillitoe and McKee, 1996, Richards et al., 1999). In this research, several kaolin deposits of different types, irrespective of their economic importance, were screened for APS minerals associated with phyllosilicates, particularly kaolinite-group minerals.
The APS minerals belong to an isostructural alunite–jarosite solid solution with the general formula AB3(XO4)2(OH)6, with A=Na, K, Ca, Pb, Ba, Sr, and REE; B=Al, Fe, Cu, Zn; XO4=PO4, SO4, and AsO4 (Strunz and Tennyson, 1982, Scott, 1987, Stoffregen and Alpers, 1987). Various elements may be accommodated into the crystal lattice to create extensive solid solution series minerals that are very attractive to economic geologists who use minerals of the alunite–jarosite family as a potential guide to precious metal mineralization, tools helpful in the evaluation of nonmetallic deposits, and commodities in themselves when alunite or REE-bearing APS minerals reach economic grade (Bove et al., 1990). The most significant species in the broad spectrum of APS minerals found in nature are listed with respect to their major components in Table 1 (Strunz and Tennyson, 1982, Scott, 1987). According to Strunz and Tennyson (1982) these minerals may be subdivided into the alunite, woodhouseite, and crandallite solid solution series (Table 1).
Much emphasis was placed on APS minerals during investigations into the origin of argillaceous deposits (Störr et al., 1991, Dill et al., 1995a, Dill et al., 1995b, Dill et al., 1997a, Schwab et al., 1996, Rojkovič et al., 1999). Because APS minerals are often more sensitive to changes in the physicochemical conditions than are the phyllosilicates hosting them, they offer a promising tool to determine the development of APS-bearing argillaceous mineralization. In places, the quality of the clay may be improved by APS minerals such as florencite which contains REE in considerable amounts (Maksimovic and Panto, 1995, Walther et al., 1995). However, the quality may be deteriorated and the use of clay restricted by elevated contents of heavy metals such as As and Pb. Both elements may be accommodated in the lattice of APS minerals and are widely known to be very harmful to living beings (Störr et al., 1991).
Argillaceous rocks are very widespread in alteration zones of igneous rocks, especially volcanic, subvolcanic, and pyroclastic rocks (Moretti and Pieruccini, 1968, Bristow, 1977). These altered igneous rocks are extensively mined, mainly for phyllosilicates such as smectite and kaolinite. In many parts of the world, such as the Pacific Rim fold belts, these volcanic-related argillites form the only source of ceramic raw materials, fillers and filter material to supply the domestic market (Naranjo et al., 1994, Martino, 1995). Moreover, argillization is well developed in some world-class precious and base-metal deposits around the Pacific Ocean (Sawkins, 1984, Mitchell, 1992, Kurosawa et al., 1994). This type of hydrothermal alteration enriched in APS minerals (mainly alunite) is associated with a group of epithermal deposits variously termed ‘acid-sulfate’ (Heald et al., 1987), ‘high-sulfidation’ (Hedenquist, 1987) and ‘kaolinite–alunite’ (Berger and Hemley, 1989). The presence of APS minerals in these ore deposits is related in time and space to gold and silver concentrations and therefore be used as an ore guide in the field of mineral exploration.
The geological, chemical and mineralogical data gathered throughout this investigation into APS mineral-bearing argillaceous deposits constitute the basis to constrain the physicochemical conditions under which these argillites were emplaced. Furthermore, these data are used in classifying these argillaceous deposits according to current classification schemes, determining the facies patterns of the APS-bearing argillaceous deposits, and establishing a paragenetic sequence of the APS mineralization that is applicable not only to the deposits under study, but also to volcanic-hosted ore deposits in fold belts elsewhere.
Section snippets
General pathway of mineralogical analysis
Samples were taken for mineralogical and chemical analyses from open pit and underground mines that are mainly operated for kaolin and smectite (Fig. 1). The samples, totaling as much as 10 kg in weight, were split and separated into various grain-size fractions. The particle size analyses and the separation of grain-size fractions appropriate for the identification and quantification of sheet silicates and APS minerals were performed by wet sieving and Atterberg settling methods with 0.01N NH4
Geology
Volcanic and volcaniclastic rocks of late Tertiary–Quaternary age cover almost 90% of El Salvador (Table 2). The igneous rocks rest upon Jurassic–Cretaceous series made up of clastic and calcareous sedimentary rocks interbedded with volcanic rocks (Weber et al., 1974). The Caribbean plate was pushed eastward, accompanied by intensive intra-arc rifting and arc volcanism (Pindell, 1995). The Cenozoic volcanic–volcaniclastic rock sequences in the Ahuachapan geothermal field that host the kaolin
The genetic environment of the APS-bearing argillaceous deposits
The argillaceous deposits discussed in this study closely resemble one another with respect to the genetic environment. Kaolin is the major commodity of the APS-bearing argillaceous mineralizations that were formed during the Cenozoic in modern fold belts and are hosted by acidic to intermediate volcanic and volcaniclastic rocks. Only in Chile coarser-grained subvolcanic rocks form part of a host rock lithology that is older than the other country rocks. These discrepancies, however, have no
Acknowledgements
I am grateful for the support of K.-H. Henning, J. Kassbohm, F. Nehring, T. Puff, M. Schäfer, M. Siebrand, and M. Zander (Greifswald University). Discussion with H.-R. Bosse (Federal Institute for Geosciences and Natural Resources) was very fruitful for this comparative study. Comments by H.H. Murray, U.E.D. Kelm, and another anonymous reviewer for the Journal of South American Earth Sciences are kindly acknowledged, as is the editorial handling by J.N. Kellogg.
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