A comparative study of APS minerals of the Pacific Rim fold belts with special reference to south American argillaceous deposits

Journal of South American Earth Sciences 16 (2003) 301–320 www.elsevier.com/locate/jsames

A comparative study of APS minerals of the Pacific Rim fold belts with special reference to south American argillaceous deposits H.G. Dill* Federal Institute for Geosciences and Natural Resources, P.O. Box 510163, D-30631 Hannover, Germany Received 1 May 2001; accepted 1 April 2003

Abstract Aluminum-phosphate-sulfate (APS) minerals are of particular interest to exploration geologists in search of nonmetal deposits emplaced in volcanic or subvolcanic environments. These minerals are often more sensitive to changes in the physicochemical conditions during mineralization than phyllosilicates and thus can shed some light on the evolution of argillaceous deposits that may be of supergene or hypogene origin. The APS-bearing argillaceous deposits under study are located in El Salvador, Peru, Chile and Indonesia and occur in volcanic and pyroclastic rocks of the circum-Pacific Rim fold belts. Some sediment-hosted kaolin deposits from Chile where APS minerals were not found and volcanic-hosted Sb– Au deposits from Bolivia, which contain massive alunite as the only alteration mineral, were also discussed to connect this research to neighboring realms such as sedimentary host rocks and metallic deposits. The mineral assemblages in volcanic and pyroclastic rocks consist of alunite, woodhouseite, and crandallite solid solution series with Fe ˚ sheet silicates, siliceous compounds, zunyite and topaz. The APS-bearing argillaceous deposits disulfides, Fe oxide hydrates, 7, 10 and 14 A may be subdivided on the basis of their facies patterns (first-order-classification) and mineral zonation (second-order-classification). Facies patterns in the deposits vary with the distance of mineralization from the feeder channel system. A detailed study of these patterns provides an overview of temperature regime across the entire district of argillaceous mineralization. The APS-bearing argillaceous mineralization under consideration correspond to the following alteration patterns: advanced and intermediate argillic alteration, silicification, and sericitization, which is common to epithermal and porphyry-type ore deposits. The mineral zonation of APS mineral assemblages may be correlated with equivalent APS mineralization in ore deposits. The fourfold subdivision of APS mineralization reflects a decrease in the temperature of formation that controls the accommodation of bivalent (Sr2þ, Ba2þ) and trivalent (Ce3þ) cations in the lattice of APS minerals and is governed by the chemical composition of mineralizing fluids and preexisting element concentration in the volcanic host rocks. q 2003 Elsevier Ltd. All rights reserved. Keywords: APS minerals; South American Audes; Indonesia; Argillaceous deposits; Hypogeue; supergene

1. 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 * Tel.: þ49-511-643-2361; fax: þ49-511-643-2304. E-mail address: [email protected] (H.G. Dill). 0895-9811/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0895-9811(03)00099-3

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 ¼ Na, K, Ca, Pb, Ba, Sr, and REE; B ¼ Al, Fe, Cu, Zn; XO4 ¼ PO4, SO4, and AsO4 (Strunz and

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Table 1 The chemical composition of aluminum-phosphate-sulfate minerals. Crystallographic data and chemical composition according to Pabst (1947), Switzer (1949), Kato and Radoslovich (1968), Strunz (1974), Pouliot and Hofmann (1981), Scott (1987), and Michel and Van Everding (1987) Alunite–Woodhouseite group Alunite–Jarosite-solid solution series (ditrigonal–pyramidal/pseudocubic) AB3[(OH)6l(SO4)2] Alunite KAl3(SO4)2(OH)6 Na alunite NaAl3(SO4)2(OH)6 Jarosite KFe3(SO4)2(OH)6

Crandallite group Woodhouseite solid solution series (trigonal–rhombohedral/pseudocubic) AAl3[(OH)6l(SO4,PO4)2] Woodhouseite Ca Al3(PO4/SO4)(OH)6 Svanbergite Sr Al3(PO4/SO4)(OH)6 Hinsdalite PbAl3 (PO4/SO4)(OH)6

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 (Sto¨rr et al., 1991; Dill et al., 1995a,b, 1997a,b; Schwab et al., 1996; Rojkovic 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 (Sto¨rr 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,

Crandallite solid solution series (mostly ditrigonal-scalenohedral/pseudocubic) AB3H[(OH)6l(PO4)2] Crandallite Ca Al3H(PO4)2(OH)6 Goyazite Sr Al3H(PO4)2(OH)6 Gorceixite Ba Al3H(PO4)2(OH)6 Florencite REE Al3H(PO4)2(OH)6

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.

2. Methodology 2.1. 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 NH4OH solutions added to distilled water to prevent flocculation and disperse fine-grained aggregates. The powdered samples which constitute the size fractions , 2, 2– 6.3, 6.3– 20, 20– 63, and . 63 mm and whole-rock samples were prepared for X-ray diffraction (XRD) analyses with oriented slides and random samples. The slides were X-rayed in the air-dry state, after glycol treatment and heating the sample up to 550 8C for h, to identify and quantify

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Fig. 1. Sketch maps to show the position of mineral localities in El Salvador, Peru, Bolivia, Chile and Indonesia referred to herein.

the phyllosilicates (Cu Ka radiation, current: 30 mA, voltage: 40 kV). The XRD measurements were backed up by simultaneous differential thermoanalysis (DTA), including differential gravitational methods (TG). Further studies into the mineralogical composition of argillaceous samples involved a combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM – EDX). Diagnostic parameters for the identification of phyllosilicates that form the matrix of the APS minerals are described by Thorez (1976) and Weaver (1989). The study is aimed at precisely identifying the APS minerals present, mostly as minor constituents in the argillaceous rocks. Optical microscopy was applied to those hand specimens that, judging by their outward appearance, were enriched in APS minerals and most likely to contain APS minerals in a grain-size range amenable to optical methods (e.g. La Noemia, Socosmayo, Peru). In such cases, the petrographic microscope may decipher the complex texture of hypogene and supergene alteration. Moreover, it is a very useful optical tool to identify the relic minerals of the parent material of argillization (e.g. quartz, feldspar). 2.2. Detection and identification of APS minerals in argillaceous volcanic and volcaniclastic rocks Determination of APS minerals is not a routine mineralogical procedure because of their minute grain size and, in places, rare occurrence in the mineral assemblage. Some information on how to find and identify the APS

minerals is therefore provided in the following paragraphs. For further information on the technique, see Sto¨rr et al. (1991), who provide detail on APS mineral analysis involving the electron microprobe (EMP), SEM, and TEM. As far as the conversion of the analytical results into the mineral formulae of APS minerals is concerned, the proposals published by Kassbohm et al. (1998) were taken as guideline. Anomalously high contents of Sr, REE, Pb, S, and P obtained during XRD analyses of argillaceous material were used as major criteria to select samples for further studies by means of EMP, SEM, and TEM. All XRD runs were reviewed in the ranges 2.93 – 2.97, 1.89– 1.90, and 2.74– ˚ , which contain the reflections most diagnostic for the 2.75 A identification of APS minerals. The (001) spacing is the most diagnostic criteria to identify and quantify alunitegroup minerals in XRD runs. Reactions at temperatures from 540 to 570, 635 to 670, 750 to 770, and 850 to 890 8C, which were encountered throughout DTA/TG investigations, may be attributed to APS minerals. Reactions in the temperature range 130 – 150 and 200 –230 8C, however, may have been provoked by a reorientation in the crystal lattice of aluminum sulfates (AS), which are devoid of alkaline and earth alkaline elements (e.g. alunogen, meta-alunogen, aluminite). Thermal reactions of APS and AS minerals throughout DTA/TG investigations are caused by a significant weight loss. Metaalunogen was spotted in some samples but is not discussed further because it did not form during supergene or

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hypogene mineralization. It is considered to have resulted from dehydration during laboratory treatment or sample storage. Mineral chemical analysis of APS minerals was performed by means of a SEM. The probe current was usually 200 pA and the accelerating voltage 18 kV. Using a working distance of 39 mm, it was possible to distinguish among different types of grains by their morphology and acquire energy-dispersive X-ray spectra. Due to the small dimension of the phosphate grains, the intensity of the Xrays emitted is low. To increase the countable intensity, the detector window was placed as close as possible to the stage.

3. Results 3.1. Argillaceous deposits in the Ahuachapan geothermal field, western El Salvador 3.1.1. 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 deposits are related to these geodynamic processes. Two kaolin deposits, Agua Shuca and Cerro Blanco, are situated approximately 5 km SE of Ahuachapan (898500 W 148000 N) (Dill et al., 2000). At Agua Shuca small cavities and pockets measuring a few meters in depth are still being filled with kaolin. They are randomly scattered in an active geothermal field. In contrast, the fumaroles in the Cerro Blanco kaolin deposit are extinct. This kaolin deposit is situated on top of a mountain ridge and is also structurally controlled. Cerro Blanco is a shallow deposit that contains approximately 100,000 m3 of kaolin and kaolinitic tuffs. Both deposits are located in an area that is underlain by andesitic agglomerates and lavas as much as 900 m thick (Aunzo et al., 1989). Another kaolin mineralization has been discovered recently during metal exploration near San Isidro. The San Isidro kaolin deposit is bounded by faults cutting through felsic lavas and agglomerate tuffs in an area measuring approximately 400 m £ 100 m. The size and orientation of the zone of kaolinization are controlled by the strike and dip of underlying quartz veins, which strike NW –SE. The unaltered parent material has not been penetrated by exploration holes that go down to a depth of as much as 60 m.

3.1.2. Mineralogy Kaolinite prevails among the phyllosilicates identified in the mineral assemblages of the nonmetallic deposits in western El Salvador. In places, zones of strong kaolinization include blocks of andesitic parent rocks, which were subject to smectitization. In addition, cristobalite, poorly crystallized siliceous compounds, and goethite occur in various amounts in both kaolin deposits (Table 2). At San Isidro, kaolinite and quartz predominate, followed by lepidocrocite, pyrite, and relic feldspar. The APS minerals make up approximately 12% by volume of the Agua Shuca kaolin deposit and 7% of the Cerro Blanco deposit. No APS minerals were found at San Isidro. Despite the lack of APS minerals in the kaolin-quartz mineralization at San Isidro, this deposit is discussed for a full picture of the hydrothermal mineralization that caused the strong argillization in the study area (Fig. 2). The prevailing APS mineral series belongs to the alunite group (Table 3). In neither Agua Shuca nor Cerro Blanco has As substituted for P or S in the (XO4)—anion complex (Tables 3 and 4). Such substitutions have been experimentally investigated by Herold (1987) and in nature studied by Hak et al. (1969), who found the mineral kemmlitzite (Sr, Ce, La)[(OH)6(P,S)O4AsO4] in kaolin deposits. The APS mineral assemblage at Agua Shuca is more abundant in minerals of the alunite group than is the mineral assemblage of the adjacent Cerro Blanco deposit. Conversely, minerals of the crandallite group and AS, such as alunogen and aluminite, are more widespread in Cerro Blanco than in Agua Shuca (Table 4). The predominance of alunite over woodhouseite þ svanbergite indicates that the APS mineral assemblage of Agua Shuca is SO4 selective. The amount of jarosite in Agua Shuca is very low. Only in few samples were jarosite crystals were discovered at the edge of APS minerals. Alunite from the active fumaroles at Agua Shuca has higher K/Na ratios than that at Cerro Blanco. The intimate intergrowth between APS minerals and their zonation within single crystals can be made visible by means of chemical mapping using SEM – EDX. Ca and P are mostly found in the center of APS mineral aggregates. Alunite-group minerals and crandallite sensu stricto precipitated first in the center of APS minerals. Al and S are more or less homogeneously distributed across the mineral grain. In sites of the chemical map where concentrations of Al coincide with those of S and other elements do not show any significant enrichment, AS are suspected of prevailing in the mineral aggregate. Fe is homogeneously distributed across the mineral grain. Ba, Ce, Ca, and Sr, together with some P, are concentrated at the edge of APS mineral. The concentration of REE and earth alkaline elements with P led to the precipitation of crandallite solid solution series, which formed later in the succession. Locally, Fe is concentrated in rhomb-shaped crystals at the edge of the mineral aggregate. These sites,

Diorite, andesite

Andesites, trachyandesites (lavas and tuffs) Diorite, dacite

La Vanguardia, Chile, Dill et al. (1995b)

Desa Toraget, Indonesia, Dill et al. (1995c)

s.s.s., solid solution series.

Tertiary Volcanic and pyroclastic rocks, of rhyolitic to andesitic composition

Cosun˜o and Milluri, Bolivia, Mylius et al. (1994a,b) Dill et al. (1997b)

Tertiary

Rhyolite, andesite (ash flow tuffs)

Tertiary, (Pliocene) –Quaternary Tertiary

Rodalquilar, Spain, Arribas et al. (1995)

Baguio—Northern Luzon, Philippines Aoki et al. (1993)

Tertiary

Rhyolite, andesite, dacite, latite, trachite (lavas and tuffs)

La Noemia, Sangal, Socosmayo, El Sol 3, La Providencia, El Guitarrero, Peru, Dill et al. (1997a) Cretaceous– Paleocene

Tertiary (Pliocene) – Quaternary

Sericite, pyrophyllite, dickite, diaspore, tourmaline chlorite, quartz, adularia, calcite, rhodonite, rhodochrosite, Cu–Pb –Zn sulfides, tellurides, gold Complex Pb –Zn– – – (Cu–Ag–Au) and Au(Cu–Te–Sn) minerals, diaspore, zunyite, pyrophyllite, hematite, amorphous silica, quartz, kaolinite, dickite, illite, sericite, pyrite, iodide, tellurates Ag –Au–Zn–Bi–As minerals, stibnite, quartz, Fe-sulfides, dickite, kaolinite, nontronite

Aluminum sulfates, native sulfur, kaolinite

Quartz, opal CT, cristobalite, amorphous matter, alunogen, aluminite, kaolinite, goethite, lepidocrocite, smectite, feldspar, pyrite Quartz, tridymite, cristobalite, opal CT, muscovite, halloysite, metahalloysite, kaolinite, dickite, pyrophyllite, smectite, barite, smectite-illite m.-l., zunyite, topaz, anatase Quartz, muscovite, kaolinite

Associated non-APS minerals

Lithology

Age of formation

APS mineralization

Host rocks

Andesitic lavas Cerro Blanco and Agua Shuca, San Isidro, and tuffs El Salvador, Dill et al. (2000)

Locality, reference

Tertiary

Tertiary

K- and natroalunite, woodhouseitesvanbergite, crandallite, florencite, jarosite

Alunite

Tertiary

Tertiary– Quaternary (?)

Tertiary– Quaternary (?)

Tertiary (Miocene)

Tertiary— recent

Hypogene hydrothermal low-sulfidation Pb –Zn– (Cu –Ag–Au) and highsulfidation Au– (Cu–Te–Sn) superimposed by a supergene APS mineralization, 230 –330 8C Hypogene steam-heated, high sulfidation epithermal Sb-(Au) mineralization, ,200 8C

Hypogene hydrothermal high-sulfidation epithermal Au mineralization 200 –300 8C

Hypogene steam-heated kaolin deposit, ,60 8C

Hypogene hydrothermal and steam-heated kaolin and alunite deposit with a zone of supergene alteration superimposed on them, ,325 8C Hypogene hydrothermal kaolin deposit, ,200 8C (?)

Hypogene-hydrothermal and steam-heated kaolin deposit linked to a solfataric vent systems, 140– 250 8C

Age of formation Origin, temperature of formation

Svanbergite, hinsdalite, minamiite, woodhouseite, natroalunite, alunite

Ca–Pb-bearing alunite, woodhouseitehinsdalite, K-bearing svanbergite – woodhouseite, K-REE-bearing woodhouseite, Ca–Sr-bearing gorceixite, Ca-Sr-bearing florencite, Ca–Sr-bearing florencite Pb-bearing alunite, Pb-bearing, alunite –woodhouseite s.s.s

Alunite, svanbergite– woodhouseite s.s.s., P-bearing alunite, REE-bearing woodhouseite- svanbergite- s.s.s., florencite-bearing crandallite-goyazite s.s.s. florencite-goyazite

Alunite-crandallite s.s.s., alunite – woodhouseite s.s.s, gorceixite, florencite, jarosite, dussertite

Mineral assemblage

Table 2 Mineral assemblages of kaolin deposits in El Salvador, Peru, Chile, and Indonesia in comparison with high sulphidation-type ore deposits in Spain (Arribas et al., 1995), the Philippines (Aoki et al., 1993), and Bolivia (Dill et al., 1997b)

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Fig. 2. Diagrammatic section through the APS-bearing argillaceous deposits in El Salvador, Peru, Chile, and Indonesia. (The position of the individual deposits is not to scale; the vertical distance between the various stages of mineralization lies in the range of 100 m.) The wedge-shaped ‘APS-max’ shows which way the quantity of APS minerals increases. The various stages of the volcanic mineralization are named according to the most abundant minerals in the individual stages (e.g. kaolinite –cristobalite). On the right side, correlations with types of alteration in italics. The various deposits are shown according to their relative position within the hydrothermally altered volcanic edifice and to their mineralogical facies pattern (e.g. Cerro Blanco ¼ kaolinite–cristobalite).

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Table 3 Distribution of APS and AS minerals in nonmetallic deposits. The classification scheme matches the classification scheme portrayed in Table 1. The chemical composition of each mineral is abbreviated to the most diagnostic elements (e.g. KAl3 ¼ alunite) Deposit

Country

Alunite Jarosite serises

KAl3 El Giutarrero El Sol 3 Socosmayo La Providencia Sangal La Noemia La Vanguardia first stage La Vanguardia second stage Agua Shuca Cerro Blanco Desa Toraget

Peru Peru Peru Peru Peru Peru Chile Chile El Salvador El Salvador Indonesia

þþþ þþþ P q þþ þ þþþ þþþ P q þþ þþþ þþ þþþ P .

NaAl3

KFe3

Woodhouseite series

Crandallite series

CaAl3

Sr Al3

CaAl3H

þþ þ

þ

þþþ

þ

þ þ (þ ) þ þ

(þ )

SrAl3H

Aluminum sulfates

BaAl3H

CeAl3H þþ

þþ

(þ )

(þ )

(þ) þ

(þ )

þ (þ ) (þ )

þ (þ) (þ)

(þ ) þ þ

þþ þ , abundant; þþ , common; þ, rare; and (þ ), trace.

abundant in Fe, match elevated contents of O and S, whereas Al and K contents in the same place are conspicuously low. The contents of Ba, Ce and P are only moderately increased in this aggregate. Late-stage accumulation of jarosite, gorceixite, and florencite account for that element concentration at the margin of the mineral aggregates. The APS mineralization occurred later than the mineral assemblage of Fe-sulfides and kaolinite. The entire mineralization may be chronologically constrained to the period from the Pliocene through the recent based on the basis of geological data. 3.2. Argillaceous deposits in the Cajamarca Province, western Peru 3.2.1. Geology Several nonmetallic deposits mined mainly for kaolin and alunite are located in the Cajamarca Province, western Peru (Dill et al., 1997a) (Fig. 1). Part of the kaolin deposits is found in Lower Cretaceous sequences of alternating claystones, siltstones, and sandstones. The deposit near Cambulay (788000 0000 W 78470 1200 S) may be taken as reference for these type of kaolin deposits, which rarely contain APS minerals (Fig. 3). Locally dark carbonaceous

shales bearing seams of coalified matter are interbedded in the orthoquartzites. Most of the deposits are hosted by lavas and pyroclastic rocks (e.g. lapilli tuffs, ash tuffs, ash flows, ignimbrites) of rhyolitic, andesitic, and dacitic composition (Table 2). In one deposit, Sangal (788500 3600 W 78080 2400 S), latitic and trachitic lavas form the parent material for kaolinization. The argillaceous mineralizations in Peru are very different in their outward appearance. La Noemia deposit, which is situated on the Cerro Urusculle 4200 m above m.s.l. (788380 4200 W, 78510 3600 S), displays a circular shape in plan view (Fig. 4). Because it is not under operation during the period of investigation, its vertical extension cannot be determined with certainty. Studies at outcrop and under the microscope reveal welded textures in the pyroclastic host rocks, typical of ignimbrites. Boulders are scattered all over the Cerro Urusculle and conceal part of the outcrop of the kaolin deposit. The rocks display a strongly brecciated texture that attests to explosive events during the formation of these agglomerates. The Sangal deposit originated from an alteration of ash and lapilli tuffs, which display vitroclastic textures from unwelded to slightly welded. Pumice particles are flattened and oriented along the bedding planes of the primordial pyroclastic rock. The primary grain size, which is still

Table 4 Range of modal abundance and alkali chemical composition of APS minerals in the Agua Shuca and Cerro Blanco kaolin deposits, El Salvador. Based on mineral chemical data of eight samples Alunite group

APS-mineralstot

Agua Shuca (vol%) Cerro Blanco (vol%)

Cation variation in alunite

Alunogen þ aluminite

Alunite group

Crandallite Group

Woodhouseite þ Svanbergite

Alunite

Jarosite

Ba

K

Na

0–10 0–60

90– 100 20– 100

0–40 0–60

0–20 0–0

80–100 50–100

0–5 0–20

0–20 0–20

70–100 50–100

0– 25 0– 70

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Fig. 3. Sketch redrawn from photos to show stratiform kaolin deposits in volcanic rocks bound to an old peneplain (e.g. La Providencia, Peru) and interbedded with arenaceous and argillaceous clastic rocks (e.g. Cambulay, Peru).

discernible despite intensive kaolinization, and the pyroclastic textures point to an ash flow deposit. In the Socosmayo (788490 4800 W, 78050 4800 S) and El Sol 3 (788320 2400 W, 78010 0900 S) deposits, relic textures that escaped kaolinization indicate a sand-sized tuffaceous parent material of rhyolitic composition. The host rock is intensively brecciated and penetrated by a set of fissures. On an old peneplain china clay is currently worked in an open cast mine named La Providencia (788330 3300 W, 68590 5100 S) (Fig. 3). The surface of denudation has truncated volcaniclastic rocks, which are strongly brecciated. The parent rock of probably intermediate composition was a lithic unwelded crystal tuff-lapilli ash tuff. In vertical section through the deposit, conspicuous textural changes may be recognized in the loamy substrate. Rock sections rich in kaolinite are crudely bedded to nonbedded. Interlayers some decimeters thick with abundant smectite and siliceous matter are internally well stratified. The kaolin deposit near El Guitarrero (788300 3000 W 78110 4200 S) exhibits kaolinization that has affected the topmost parts of a coarse-grained, feldspar-rich crystal tuff (Fisher and Schmincke, 1984). The pyroclastic flow deposits filled a paleovalley incised into quartzites of Lower Cretaceous age. China clay accumulations measuring a few meters thick appear close to the present-day surface and therefore are worked by open cast mines or small galleries.

the various deposits is illustrated in Fig. 5. Various kanditegroup minerals (Weaver, 1989), nacrite excluded, have been identified in the Peruvian kaolin deposits under study, including dickite (La Noemia), well-ordered kaolinite (El Sol 3, Socosmayo, Sangal, La Noemia), moderately well to poorly ordered kaolinite (La Providenica, El Guitarrero), and halloysite to metahalloysite (El Sol 3, La Providenica). Halloysite-type phyllosilicates may easily be determined with TEM by their tube-like structure. Second in abundance among the non-APS minerals are siliceous compounds such as quartz (El Guitarrero), cristobalite, and opal CT

3.2.2. Mineralogy The minerals found in the Peruvian deposits are listed in Table 2; the variation of the mineral assemblage in

Fig. 4. Plan view of the alunite–kaolinite breccia pipe structure at La Noemia, Peru (contour elevation 10 m). Massive kaolinite – alunite mineralization and kaolinite –alunite breccia are exposed; kaolinite–alunite boulders are covered and mantled by boulder-sized debris.

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Fig. 5. Mineral assemblage and facies pattern of Peruvian kaolinite–alunite deposits. Gold is shown in brackets because, though elevated amounts could be determined, no discrete carrier minerals of gold were found. Na –K and P denote the major elements of alunite solid solution series, and Sr and REE denote the major elements of APS minerals. The temperature gives the maximum temperature of formation for the mineral assemblages in the various deposits shown below.

(El Sol 3, La Providencia). In El Sol 3, kaolinite-group minerals stands out from the other deposits for its elevated gold content of as much as 1.05 ppm Au. In the La Noemia (Fig. 6a), Sangal, Socosmayo (Fig. 6b) and El Sol 3 deposits, alunite is the sole representative of APS mineralization. In La Noemia, two different generations of Na- and K-alunite could be determined under the microscope. First-generation alunite is aligned parallel to the bedding planes of the parent rock (Fig. 6a) and is similar in its outward appearance to first-generation alunite from Sangal. Alunite exhibits a featherlike habit. In places, this sulfate pseudomorphs phenocrysts of rock-forming minerals that cannot be identified. Second-generation alunite forms spherical aggregates and clusters scattered among firstgeneration alunite. Kaolinite, which is interstitial to alunite, postdates sulfate precipitation. In Socosmayo, El Sol 3, and La Providencia, metahalloysite and halloysite, in addition to kaolinite, occur in the interstices. In La Providencia, alunite is associated locally with APS minerals that form part of the svanbergite – woodhouseite solid solution series. The APS mineral assemblage at El Guitarrero is depleted in sulfate. The primary APS mineralization contains minerals of the woodhouseite-svanbergite solid solution series, which gradually convert into florencite-type minerals at the edges of the mineral aggregates. Ongoing addition of REE to the APS mineralization and further depletion in sulfur gave rise to a florencite-bearing crandallite-goyazite solid solution series. Alunites from La Noemia kaolin-alunite deposit yield K/Ar ages of 11.5 ^ 0.7 and 13.3 ^ 0.4 Ma. The APS

mineralization in the volcanic-related argillaceous deposits in Peru is younger than the Middle Miocene. 3.3. Argillaceous deposits in central Chile 3.3.1. Geology An outline of the kaolin deposits in Chile was presented for the first time by Tabak (1968). Of these kaolin deposits, some in central Chile have been sampled for APS minerals. In the ball clay mining district of Pichilemu, in the Chinito (348250 0200 S 718580 5900 W) and San Francisco (348270 4200 S 718570 2900 W) silty to arenaceous layers are pervasively kaolinized. The Paleozoic series were described by Gonzales-Bonorino (1970). In the neighboring Rosario lo Solis region, the Blanquita (348050 0100 S 718310 1500 W) and San Ramon (348140 5100 S 718380 0300 W) mines were the target of this investigation. The uppermost parts of the Paleozoic Rosario granite were strongly altered, and the crystals of primary feldspar were almost totally converted into kaolinite-group minerals (Fig. 8). Near Combarbala´, a volcano-sedimentary sequence of Cretaceous-Paleocene age was subject to strong kaolinization (Rivano and Sepulveda, 1991) (Fig. 1, Table 2). The geological map by Rivano and Sepulveda (1991) shows several irregularly shaped patches of intensive hydrothermal alteration along a N – S-trending fault zones. An argillized fault zone as much as 20 m thick (Fig. 2) is exploited in the open pit at La Vanguardia (31830 2500 S 71870 1200 W). North of the La Vanguardia kaolin deposit,

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Fig. 6. Minerographs of thin sections displaying structures typical of alunite minerals from Peruvian kaolinite– alunite deposits. Scale bar 200 mm. Crossed polars. (a) High-sulphidation type alteration zone with two generations of P-free alunite. First-generation alunite is aligned parallelsubparallel to the former bedding of volcaniclastic parent rocks. Phenocrysts of second-generation alunite crystallized across the layering starting out from fissures. La Noemia, Peru. (b) Ash tuff of probably rhyolitic composition that underwent strong kaolinization and alunitisation. The irregularly textured matrix of kaolinite and alunite encloses two large quartzose vitroclasts (g). Socosmayo, Peru.

two of the located; the world-class Cecioni and

most important mineral belts in Chile are Maricunga and the El Indio belts both host Au – Ag deposits (Vila and Sillitoe, 1991; Dick, 1992).

3.3.2. Mineralogy In the Pichilemu and Rosario lo Solis regions, a broad spectrum of kandite-group minerals, such as kaolinite, halloysite, and metahalloysite, was identified in addition to montmorillonite, muscovite, and restite quartz and feldspar (Fig. 7). However, APS minerals were not detected. In the La Vanguardia deposit, two different stages of alteration are encountered (Dill et al., 1995b). Stage I shows enrichment of quartz (, 60%), as well as some muscovite (35%) and minor kaolinite (, 5%) (Fig. 8). Stage I alteration evolved proximal to the fault zone and is surrounded by the younger stage II alteration zone. In this distal alteration zone, relative to the siliceous alteration zone, quartz is less abundant (30%), and kaolinite is more

Fig. 7. Minerographs of argillaceous rocks from Chilean kaolin deposits. Scale bar 1 mm, TEM. (a) Tubular crystals of metahalloysite associated with platy xenomorphic crystals of muscovite and some goethite. Pichilemu. (b) Kaolinite, muscovite and smectite with some tubular crystals of halloysite. Chinito.

abundant. This argillaceous alteration zone is also host to muscovite (, 25%) (Table 2). In the silicification/sericitization zone, a complex solid solution series consisting of K-alunite, woodhouseite, and hinsdalite predominates among the APS minerals (Fig. 9). This complex is replaced by an APS mineral, identified as K – Sr-bearing woodhouseite. In the Stage II alteration zone, the quantity of APS minerals increases by three orders of magnitude relative to Stage I alteration. The APS mineralization begins with a woodhouseite – hinsdalite – alunite solid solution series. The APS mineral closely resembles that solid solution series found in the zone of silicification, yet the amount of alunite increases in the solid solution series. This alunite-enriched solid solution series is accompanied by various APS minerals that are strongly depleted in sulfate: K – REE-bearing woodhouseite, Ca – Sr-bearing gorceixite, Ca – Sr-bearing florencite, and crandallite– goyazite.

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3.4.2. Mineralogy The matrix of the APS mineral assemblage is very uniform. It consists of 90% kaolinite by volume. Quartz is second in abundance, making up 8% of the clay mud (Table 2). The APS mineralization is also rather uniform, dominated by alunite that bears some Pb and Ca. The APS minerals form pseudocubic crystals as they are commonly observed in end-member alunites. Increasing contents of Ca at the expense of K and of phosphate at the expense of sulfate led to the precipitation of woodhouseite –alunite solid solution series and native sulfur, which are present at the edge of the caldera.

4. Discussion 4.1. The genetic environment of the APS-bearing argillaceous deposits

Fig. 8. Minerographs of thin sections displaying the initial stages of alteration of parent rocks in Chilean kaolin deposits. Scale bar 0.5 cm. Thin section viewed under oblique light. (a) Incipient stages of kaolinisation of feldspar (white) of the Paleozoic Rosario granite. Dark spots are xenomorphic quartz crystals. Rosario lo Solis. (b) Silicification zone composed of fine-grained quartz intimately intergrown with muscovite in the matrix. Stockwork-like veinlets are filled with palisades and aggregates of granular quartz. La Vanguardia.

While the sequence of mineralization may be easily established, it is difficult to constrain the age of formation of this APS mineralization geochronologically (Tertiary – Quaternary?). Dating of K-bearing minerals in the alteration zone failed. 3.4. Argillaceous deposits in the north arm of Sulawesi, Indonesia 3.4.1. Geology The kaolin deposit under consideration is located in the Neogene north Sulawesi arc (1248500 0000 E, 1870 3000 N), which was brought about by the interaction of the SE Asian, Pacific, and Indian –Australian plates (Silver et al., 1983). The sampling site lies in the Tondano Caldera, which is rimmed by Quaternary stratovolcanoes, some of which are still active (Kavalieris et al., 1992) (Fig. 2), as are some of the fumaroles that produce the nonindurated clay mud in the Desa Toraget deposit.

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 impact on the mineralogical composition of the APS mineralization investigated in Chile. All argillaceous deposits are located in geothermal fields, some of which are still active, as are the metal deposits that were taken for reference (Table 2). The APS-bearing argillaceous mineralizations in Peru and El Salvador display a considerable depth zonation that is interpreted as a function of the proximity of the mineralization to the vent system. The volcanic-related hypogene APS mineralizations that were recorded from the various mining districts in Peru, El Salvador, Indonesia, Chile, and Bolivia are similar with respect to their structural features, porosity/permeability, and paleohydrology. The same sort of influence may be expected to have been exerted by these lithological and hydrological factors on the vertical and horizontal distribution of hypogene APS phases in the sites under study. Sandstone-hosted mineralizations recorded from Peru and Chile have been assessed but are excluded from the discussion for mineralogical and structural reasons. These stratabound argillaceous deposits differ greatly from the volcanic-related deposits in their porosity and paleohydrology. 4.2. Temperature of formation The answer to the question of the temperature of formation of the mineralizing fluids is fraught with difficulties because two different approaches must be taken to obtain reliable results. Direct measurements were

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Fig. 9. SEM micrograph and spectrum of alunite from La Vanguardia kaolin deposit.

carried out in still active fumaroles (e.g. Indonesia). In extinct vent systems, however, paragenetic mineral sequences indicate the physicochemical conditions in which these mineralizations were emplaced (e.g. Peru). From the brine pools at Agua Shuca which are being filled with clay mud abundant in APS and AS minerals, a temperature of 140 – 250 8C was reported by Ripperda et al. (1991). In the APS- and AS-bearing clay muds at Desa Toraget, a temperature of as much as 60 8C was measured at outcrop. The temperature ranges are representative of the temperatures of formation of the APS- and AS-bearing kaolin mineralizations in both sites under study. The precise range of the temperature of formation for the APS- and AS minerals in Desa Toraget and Agua Shuca, cannot be determined. The temperature conditions during the formation of the K – Na, alunite – kaolinite deposit at La Noemia were assessed by considering the mineral paragenesis of dickite and pyrophyllite. Pyrophyllite forms from kaolinite and quartz above 375 8C (Berman, 1988); above 400 8C at Pload . 1 kbar it is replaced by andalusite (Winkler, 1976; Shelley, 1993). The minimum temperature is approximately 300 8C. The presence of zunyite and topaz lend further support to the idea that the La Noemia K – Na, alunitekaolinite deposit was emplaced at fairly high temperatures (Fig. 5). Experimental studies of well-ordered kaolinite, dickite, and pyrophyllite corroborate this temperature determination. Alunite evidently formed below 325 8C (Dill et al., 1997a). According to the crystal habits, the argillaceous mineralization studied in the Peruvian deposits may have formed from fluids between 205 and 350 8C.

The Sangal alunite-kaolinite deposit, which lacks pyrophyllite and dickite, has formed at T values below 300 8C (Fig. 5). Socosmayo, El Sol 3, and part of the mineralization in La Providencia are characterized by the presence of halloysite instead of kaolinite and siliceous compounds in place of quartz. Their mineralization is likely to have formed below 200 8C. In the Illapel Province, Chile, no independent mineralogical data was obtained for the temperature of formation of the APS mineralization. Neither the phyllosilicates (kaolinite, muscovite) associated with the APS mineralization nor the microcrystalline quartz is suitable for temperature determinations. A rough estimation of the temperature regime under which the Chilean APS mineralization came into existence is only possible based on data from literature. Wohletz and Heiken (1992) have published a compilation of data about the alteration in geothermal fields. Following their descriptions, silicification zones texturally similar to those studied in Chile formed at temperatures below 200 8C. 4.3. APS-bearing argillaceous mineralization and facies patterns The La Noemia alunite –dickite – pyrophyllite mineralization is interpreted, on the based on its circular structure (Fig. 4), brecciation of pyroclastic rocks, and content of topaz and zunyite, as a central vent facies of a diatreme fill (Figs. 2 and 5). The Sangal mineralization, which is younger and devoid of high-temperature minerals, is representative of a more distal vent facies.

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The proposed depositional environment of El Sol 3, Socosmayo, and part of La Providencia is based on geological evidence interpreted in terms of an ephemeral maar lake or hot spring facies. The filling of the depression consists of well-bedded ash lapilli tuffs. The hillocks are made up of gently dipping layers of subaerial siliceous sinter. Socosmayo containing alunite and kaolinite is likely to have formed closest to the feeder channels, whereas the silica-halloysite mineralization of El Sol 3 and La Providencia formed at a more distal position relative to the feeder channels and closer to the margin of the shallow lake. Part of La Povidencia and the entire mineralization of the El Guitarrero deposit originated from chemical weathering that started from an ancient peneplain truncating the Tertiary volcanic series. The hypogene mineralization at La Noemia is representative of the deepest erosion level and may be assigned a mid-Miocene age of formation (Fig. 2). Highsulfidation epithermal mineralizations of Sangal, El Sol 3, and Socosmayo are younger and formed at a shallower level. Part of La Providencia and El Guitarrero came into existence close to the present surface and resulted from supergene alteration. In El Salvador, the quartz-kaolinite mineralization at San Isidro, which is representative of the deepest erosion level, gives way through a kaolinite – cristobalite mineralization poor in APS at Cerro Blanc to an APS-rich argillaceous mineralization at Agua Shuca. The latter is the present-day representative of the solfataric vent system that started off as early as the Late Tertiary (Table 2). In La Vanguardia, the situation is still much simpler. A quartz-mica mineralization low in APS minerals is followed outwards and upwards by a kaolinite –quartz mineralization abundant in APS minerals (Fig. 2). In Desa Toraget, the hydrothermal system is represented by a single uniform kaolinite mineralization (Fig. 2). 4.4. APS mineralization and mineral zonation To what extent the APS minerals contribute to this argillaceous mineralization and where the APS minerals accumulated within the facies patterns may be deduced from the horizontal bars and arrows in Figs. 2 and 5. In El Salvador, Chile, and Indonesia, APS minerals increase towards the top. In Peru, APS mineralization decreases towards the top and outwards from the feeder system. Other than in Peru, APS mineralization is a unidirectional process that becomes most prominent during the waning stages of high-sulfidation-type mineralization. This APS variation in time and space might lead to the conclusion that APS mineralization in El Salavador, Chile and Indonesia is of supergene origin, whereas in Peru, a supergene APS mineralization was superimposed on a hypogene APS mineralization. Such an interpretation would, however, ignore the mineralogical results of these studies and data

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derived from mineralogical investigations (e.g. Aoki et al., 1993). Thermodynamic studies were performed by Stoffregen and Alpers (1987) and Barth-Wisching et al. (1990). On the basis of activity diagrams showing the stability relations of the alunite-montorillonite-amorphous silica system, the applicability of temperature zonation and the APS zonation in the supergene and hypogene environments may be validated. To provide a more detailed insight into the mineralogical variation of the APS mineralization, a four-fold subdivision has been proposed (Table 5). The zonation of APS mineralization is a function of temperature, chemical composition of the mineralizing fluids, and the physicochemical conditions under which the APS minerals (and the associated host minerals) formed. This subdivision and correlations based on it may help bridge the gap between nonmetallic and metallic deposits. The APS mineral zonation may be used in the same way for mineral exploration as alteration patterns are used in searches of porphyry copper systems. 4.4.1. Stage I During Stage I, alunite and Ca-enriched APS minerals develop (Table 5). Both types of minerals have been encountered in samples from western El Salvador. APS minerals of Stage I are detected as relic phases in the center of APS mineral aggregates. Stage I mirrors a low-sulfate, Ca-enriched prestage prior to the main alunite mineralization (Stage II). Acidic hydrothermal solutions percolating through the volcanic host rocks decomposed the rockforming mineral apatite during prestage I and provided phosphate and some calcium (Reaction (1)) to form APS minerals. Destruction of alkali feldspars leds to the release of aluminum, alkaline earth, and alkaline elements necessary for the buildup of APS minerals (Reaction (2)) (Nriagu and Moore, 1984). apatite þ 3AlðOHÞ4 þ 9Hþ ) crandallite þ 4Ca2þ þ HPO2þ 4 þ 7H2 O 3AlOðOHÞ þ ðK; NaÞþ þ 3Hþ þ 2SO22 4 ) alunite

ð1Þ ð2Þ

4.4.2. Stage II Stage II alunite, which is the main sulfate-bearing mineral in the kaolin deposits under study, forms from kaolinite according to Reaction (3) 2kaolinite þ 2ðNa=Kþ Þ þ 6Hþ þ 4SO22 4 þ 9H2 O ) 2ðK; NaÞalunite þ 6H4 SiO4 ðaqÞ

ð3Þ

In some sites in El Salvador (e.g. Agua Shuca) extensive ˚ smectitization predates kaolinization. The preexisting 14 A ˚ sheet silicates when sheet silicates were converted into 7 A the pH value of mineralizing solutions lowered (Reaction ˚ sheet silicates altered to alunite (4)). Later on these 7 A

Hypogene sulfate

Hypogene– supergene transition

Supergene alteration

II

III

IV

alunite (K, Na) (SO4)

s.s.s., solid solution series.

Hypogene sulfate –phosphate

I

Jarosite (K, Na) Fe(SO4)

Alunite (K . Na)(SO4) svanbergite – woodhouseite s.s.s. (Sr, Ca) (SO4, PO4)

Woodhouseite – svanbergite, crandallite– florencite (Ca, Sr, Ce) (SO4, PO4) Na-bearing alunite (Na . K)(SO4)

Stages Interpretation Western Potosi Rodalquilar, Province, Bolivia Spain

Minamiitewoodhouseite s.s.s (Caz. @ K, Na) (SO4) –(Ca)(SO4, PO4) Na-bearing alunite (Na . K)(SO4)

Svanbergitehinsdalite (Sr, Ba, Pb) (SO4, PO4)

Baguio District, Philippines

jarosite þ dussertite (K, Na)Fe(SO4) BaFe(PO4)

(Aluminum sulfates þ native sulfur)

Pb-bearing alunite (Pb, K, Na)(SO4) Pb-bearing alunite – woodhouseite s.s.s. (Pb, K, Na)(SO4) – (Ca)(SO4, PO4)

Northern Sulawesi, Indonesia

Aluminite þ alunogen Al2SO4(OH)4· 7H2OAl2SO3 (OH)3·17H2O

Alunite – woodhouseite s.s.s. (K, Na) (SO4) –(Ca) (SO4,PO4)

Alunite –crandallite s.s.s. (Na q K) (SO4) –(Ca)(PO4)

Western El Salvador

Gorceixite þ florencite Ba (PO4) þ Ce(PO4)

Florencite–goyazite (Ce, Sr)(PO4)

REE-bearing woodhouseite– svanbergite- s.s.s. (Ce, Ca, Sr) (SO4, PO4); florencite-bearing crandallite– goyazite s.s.s. (Ce, Ca, Sr)(PO4 s SO4) Florencite–goyazite (Ce, Sr)(PO4 s SO4)

Alunite (Na q K)(SO4) A vanbergite– woodhouseite s.s.s. (Sr, Ca) (SO4, PO4) P-bearing alunite (K)(PO4 p SO4)

Cajamarca Province, Western Peru

Woodhouseite – hinsdalite –alunite (Ca, Pb, K)(SO4 q PO4) K-bearing svanbergite – woodhouseite s.s.s. (K, Sr, Ca) (SO4, PO4) K–REE-bearing woodhouseite-(K, Ce, Ca)(SO4,PO4) Ca–Sr-bearing gorceixite (Ca, Sr, Ba)(PO4)Ca– Sr-bearing florencite (Ca, Sr, Ce)(PO4 s SO4) crandallitegoyazite (Ca, Sr) (PO4)

Illapel Province, Central Chile

Table 5 Mineral zonation of some Pacific Rim-hosted APS mineralizations in nonmetallic argillaceous deposits of Tertiary–Quaternary age in comparison with high sulphidation-type ore deposits in Spain (Arribas et al., 1995), the Philippines (Aoki et al., 1993) and Bolivia (Dill et al., 1997b)

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corresponding to Reaction (3) 2K-smectite þ 2Hþ þ 23H2 O ) 7kaolinite þ 2Kþ þ 8H4 SiO4 ðaqÞ

ð4Þ

Smectite may immediately be transformed into APS minerals without an intervening stage of kaolinization when the activity of Kþ rises and the pH value of the solutions containing sufficient H2SO4 is substantially lowered, as in Reaction (5) 3K-smectite þ 4K þ þ 24Hþ þ 14SO22 4 þ 66H2 O ) 7K-alunite þ 33H4 SiO4 ðaqÞ

ð5Þ

Experimental studies on hydrothermal alteration of latitic volcanites by acidic solutions strongly enriched in SO22 4 ˚ sheet silicates lend support to this transformation of 14 A into APS minerals (Barth-Wirsching et al., 1990). Alunite, encountered in the mineral assemblages of Stage I and the initial phases of Stage II is abundant in Na. This is demonstrated by the findings in Cerro Blanco, El Salvador, as well as by the Na alunite from La Noemia, which contains alunite as the sole representative of an APS mineral. By the definition proposed herein, the lack of Caenriched APS minerals in La Noemia rules out any attribution of the above mineralization to the Prestage I mineral assemblage. Pb-bearing alunites and hinsdalite-woodhouseite-alunite solid solution series emerged early in Stage II. The initial charge is conserved when Pb2þ substitutes for 2Kþ in the Asite of the formula AB3(XO4)2(OH)6. Hinsdalite and plumbian alunite were described, among other APS minerals, in the gossan of Pb – Zn mineralization at Mount Isa, northwest Queensland (Scott, 1987). Their presence is compelling evidence that the volcanic rocks in Sulawesi and the Illapel Province, Chile, contain Pb or base metal mineralization that formed prior to APS mineralization and the elements that were scavenged by the aqueous fluids percolating through the host rocks in deeper strata. Its occurrence is not linked to pH or Eh changes in the mineralizing fluids. P-bearing alunite appears rather late in Stage II as a consequence of coupled substitution of (Ca, Sr)2þ þ (PO4)32 for Kþ þ SO2þ 4 . This substitution in the alunite lattice heralds the passage from alunite into an alunite – woodhouseite – svanbergite solid solution series, which is second in abundance only to alunite minerals in Stage II. It reflects a depletion of the aqueous solutions in sulfate. According to Stoffregen and Alpers (1987), only a few thermodynamical constraints can be placed upon the stability field of this type of APS mineral solid solution series. Woodhouseite-svanbergite solid solution series can coexist with alunite, kaolinite and muscovite. This APS mineral solid solution series becomes the most typical solid solution series among the APS minerals, as far as the advanced argillic alteration is concerned.

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4.4.3. Stage III During Stage III, the pathway of APS mineralization ramifies, as shown by the evolution in the kaolin deposits in western El Salvador. One branch of the mineralization becomes enriched in (PO4)32 and depleted in (SO4)22. The other branch is characterized by (SO4)2 as the only anion complex. As a consequence of rephosphatisation, crandallite-group minerals from in Stage III instead of woodhouseitesvanbergite solid solution series that dominate the end of Stage II. Ba and REE are the most typical cations that occupy the A-site of formula AB3(XO4)2(OH)6. Experimental studies by Schwab et al. (1993) suggest that the stability field of crandallite becomes enlarged with falling temperature and that the structure of crandallite solid solution series is stabilized by the incorporation of bivalent ions larger than Ca2þ, such as Ba2þ, Sr2þ, or trivalent cation such as Ce3þ. The occurrence of gorceixite, goyazite, and florencite in the crandallite solid solution series of Stage III are a direct response to falling temperatures during APS mineralization and the introduction of these elements. The reintroduction of (PO4)32 may be explained by admixing of phosphate derived from pedogenetic and weathering processes to hydrothermal solutions. The amount of (PO4)32 in the APS solid solution series increases during waning phases of Stage III and toward the present-day surface. At the edge of the Agua Shuca fumaroles, aluminum sulfates are being precipitated. A similar situation may be observed in northern Sulawesi, where aluminum sulfates and native sulfur may be encountered in outcrops adjacent to the Desa Toraget kaolin deposit (Table 5). Alunogen and aluminite mirror a peculiar pathway of mineralization when compared with other APS mineralizations under study. The reactive fluids have been drastically depleted in phosphate, alkaline, and alkaline earth elements in strongly oxidizing conditions. Aluminum sulfate minerals can only form when the activity of H2SO4 is persistently high, so that pedogenetic processes in any climate are impeded and no rephosphatization can take place. This is the case close to the vents of active or extinct fumaroles or where seasonal migration of phosphate is hampered by special climatic conditions, such as under permafrost conditions (Apollonov et al., 1994). In a tropical climate, the occurrence of aluminite and alunogen reflects a near-vent facies of the hypogene-supergene Stage III. Strongly oxidizing conditions, a constantly high level of SO22 4 maintained by the feeding fumaroles, and consequent retarded soil-forming processes are prerequisites for the formation of massive aluminum sulfate mineralizations. 4.4.4. Stage IV Stage IV has only been recognized in kaolin deposits from western El Salvador, where elevated amounts of Fe in the APS mineral assemblages point to late stage mineralization. Trivalent Fe accommodated in the lattice of the APS

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mineral may have been derived from decomposition of Fesulfides and FeOOH. An antithetic trend between trend occurs between sulfides and APS minerals in the Salvadorian kaolin deposits. San Isidro, which is barren as of APS minerals, contains pyrite instead. In addition to this decomposition of Fe disulfides, jarosite may have also resulted from goethite or lepidocrocite decomposition, both of which are present in the Cerro Blanco and Agua Shuca kaolin deposits in El Salvador, as shown in Reaction (6): 6goethite=lepidocrocite þ 2Kþ þ 2Hþ þ 4HSO2 4 ) 2jarosite

ð6Þ

Judging by the log SSO4 vs. pH diagram elaborated by Stoffregen (1987) for the superficial jarosite mineralization at Summitville, Colorado, jarosite-goethite mineral assemblages need a relatively high sulfur molality and a very low pH. Although often found in the same deposit, the stability relations of jarosite/natrojarosite and alunite eliminate the chance that they formed simultaneously and precipitated from the same solutions (Rye et al., 1993; Stoffregen, 1993). Stage IV mineralization, with jarosite prevailing over dussertite, is fostered by fumarolic processes in where aluminum sulfates dominate under the precursor minerals (Table 5) 4.5. Comparison of APS mineralization in nonmetallic and metallic deposits Kaolin deposits encountered in sedimentary sequences of alternating argillaceous and arenaceous beds and residual deposits, which developed on top of granitic batholits, are devoid of APS minerals. In Rosario lo Solis, strong weathering of the granitic parent rock brought about kandite-group minerals; in the Pichilemu district, supergene decomposition of Paleozoic slates caused deep saprolite to develop in the topmost part of the basement. In both deposits, the physicochemical conditions were favorable for the formation of kaolinite-group minerals. The pH values and the elements available were, however, unfavorable for the precipitation of APS minerals. The meteoric solutions in the two Chilean kaolin districts had only low contents of sulfur and phosphorus. Noble elements are unknown. This result may be applied to many of the supergene kaolin deposits studied in Peru in the course of this project. The relationship of APS mineral formation to precious and base metal mineralization in major metallic districts has not been investigated intensively in South America, though a few studies have dealt with this type of mineralization in Chile and Bolivia (Drake et al., 1997; Rice et al., 1998; Richards et al., 1999). Despite studies focused on the mineralogy of porphyrytype and epithermal deposits, a precise identification of the varied APS species and investigations of their variation in time and space are scarce. The Rodalquilar deposit,

enriched in precious and base metals (Arribas et al., 1995); the Baguio district, with its high sulfidation epithermal Au mineralization (Aoki et al., 1993); and the epithermal Sb – (Au) mineralizations at Cosuno and Milluri in Bolivia (Dill et al., 1997b) have been taken for comparison to bridge the gap between the nonmetallic deposits studied and various types of metal deposits. Stage I mineralization occurs in argillaceous deposits in which sulfate-bearing solutions had immediate access to the accessory rock-forming phosphates of the acidic and intermediate igneous rocks (Table 2). This is not the case in the deposits in Bolivia, were discrete bodies of massive alunite formed. Stage II forms the main stage and is characterized by high-temperature, Na-enriched alunite (Aoki et al., 1993). Experiments and published data on the chemistry of natural alunites suggest that precipitation of sodic alunite is favored at higher temperatures than Kenriched alunite (Stoffregen and Cygan, 1990). Stage III forms at the transition zone of hydrothermal alteration to more intensive chemical weathering, with P introduced by pedogenic processes (Ilchik, 1990). The La Providencia, Peru, opencast mine is situated on an old peneplain with a well developed kaolinitic saprolite truncating volcaniclastic rocks that host a small hypogene APS mineralization. This is also true for El Guitarrero, Peru. Both sites host a supergene APS mineralization that was superimposed on a hypogene mineralization. In samples from El Salvador and Chile, stage III APS minerals formed at the edge of stage II APS mineral aggregates. In Sulawesi, Indonesia; Agua Shuca, El Salvador; and Rodalquilar, Spain, sulfur-enriched mineralization evolved proximal to the vent system. The Fe-bearing APS minerals of stage IV, which formed at the periphery of APS aggregates with goethite and lepidocrocite, are produced by supergene alteration. 4.6. The classification and origin of APS-bearing argillaceous deposits Epithermal deposits, some of which also bear APS minerals, are variously termed ‘acid-sulfate’ (Heald et al., 1987), ‘high-sulfidation’ (Hedenquist, 1987), and ‘kaolinite – alunite’ (Berger and Hemley, 1989). They were intensively studied by Cunningham et al. (1984), Rytuba et al. (1990), Rye et al. (1992), and Rye and Stoffregen (1995). An idealised alteration patterns for high sulphidation deposits was published by White (1991) (Fig. 10). Rye et al. (1992) have made an attempt to classify and discuss the systematics of alunite mineralization. Magmatic hydrothermal alunite (APS minerals included) is related to intrusion-driven hydrothermal systems, which contain magmatic components in hydrothermal fluids. Steam-heated alunite directly precipitates from the vapor phase. This sort of alunite may be brought about by geothermal systems in which ascending H2S gas is oxidized to create H2SO4. Hayba et al. (1986) call this type of APS mineralization

H.G. Dill / Journal of South American Earth Sciences 16 (2003) 301–320

Fig. 10. Idealized alteration patterns for high sulphidation deposits (White, 1991).

‘primary supergene.’ ‘Secondary supergene’ APS mineralization, by definition, formed from surficial or atmospheric oxidation of primary sulfides in a weathering environment. To avoid any confusion, the following terminology is applied to the APS mineralizations under consideration: 1. hypogene-hydrothermal APS mineralization, 2. hypogene-steam-heated APS mineralization, and 3. supergene APS mineralization. The classification scheme is mainly based on proposals by Rye et al. (1992). The environments of formation of the various APS mineralizations are distinguished by their temperature of formation, textural and structural parameters visible in the field and in hand specimens, and mineralogical relations (Aoki et al., 1993). Aoki (1991) observes, in alunites from hydrothermal magmatic systems, a zonation closely resembling that determined in alunite from Cerro Blanco. The cores of these alunites are commonly enriched in PO4 and multivalent cations (e.g. Ca, Sr) and usually rimmed by minamiite (Ca-analogue of alunite) and Na alunite. In contrast, alunite, which precipitated in a steam-heated, acidsulfate environment overlying boiling, neutral pH, geothermal waters, lacks such a complex core zonation. It has a range of composition limited to the alunite-natroalunite solid solution series (Aoki et al., 1993). The La Noemia and Sangal, Peru (Fig. 2); Cerro Blanco, El Salvador (Fig. 2); and La Vanguardia, Chile (Fig. 2) nonmetallic deposits are classified as hypogene-hydrothermal APS mineralizations, which are metallic deposits (Aoki et al., 1993; Arribas et al., 1995; Table 2). These mineralizations display conspicuous zonation on various scales from the considerable fractionation of the fluids throughout mineralization. A complex core zone is rimmed by rhythmic bands of APS minerals of various chemical

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composition (Figs. 2 and 3). Their mineral assemblages developed at the highest temperature. This theory is corroborated by the presence of minerals such as zunyite, topaz, pyrophyllite, or dickite. This magmatic mineralization occurs at the deepest erosion level of the mineral province or the lowest mining level of the deposit. In the vicinity of the APS-bearing argillaceous mineralizations in Peru, El Salvador, and Chile, elevated concentrations of base and precious metals have been detected. Research into this kind of high-sulfidation-type mineralization has shown that they may overlie deeper porphyry Cu or porphyry Mo systems (Sillitoe and Bonham, 1984; Bove et al., 1990). The mineralizations at Socosmayo, El Sol 3, and part of La Provindencia in Peru are hypogene steam-heated APS mineralizations. The aqueous fluids that discharged in these depressions gave rise to subaerial siliceous sinter with alunite, cristobalite, opal CT, tridymite. As opposed to the hypogene-hydrothermal environment, in the steam-heated environments, sulfuric acid is produced by oxidation of H2S distilled off an underlying hydrothermal system above the water table. A similar situation occurs in the geothermal fields of western El Salvador, where steam-heated, APSbearing argillaceous mineralization is still going on in the active fumaroles and in Desa Toraget in Sulawesi. An extinct geothermal system, where acid-sulfate mineralization was produced by steam-heated waters, is indicated by the mineralogy and geological setting of the stibnite-bearing alunite deposits studied in Bolivia by Dill et al. (1997b); Table 2). The steam-heated APS occurrences are less diversified with regard to their mineral associations and mostly show a massive, mushroom-like or layered structure instead of the more vertical extension of the magmatic hydrothermal APS mineralizations. Kaolin deposits with few APS minerals, as at El Guitarrero, result from chemical weathering of pyroclastic rocks. Their textures have been interpreted in terms of the evolution of a paleosol or saprolite. Precipitation of REEenriched APS minerals unequivocally formed from meteoric waters. In La Providencia peneplanation, to which APS mineralization may be attributed, there is compelling evidence for supergene mineralization superimposed on a primary APS mineralization. The supergene APS mineralizations were not related in space or time with any precious or base metal mineralization in the area. These duricrusts came into being in a way similar to that of the well-known Lam Lam (Thies) deposit in Senegal, which overlying phosphorites (Flicoteaux, 1982; Nriagu and Moore, 1984) or uraniferous phoscretes on top of Pbearing graphite schists at Kurun, Sudan (Dill et al., 1991). They are a direct mirror image of the phosphatebearing parent material below, with no hydrothermal influx and exclusively controlled by the climatic conditions during formation. Descending fluids from a peneplain provoke this earthy and encrusted APS mineralization.

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To draw a broader picture of this mineralization and open the chance for exploration geologists to use APS minerals as an additional ore guide in the field of metal exploration, the APS mineralization studied herein has been correlated with the international adopted classification of hydrothermal wall rock alteration (Sawkins, 1984; Mitchell, 1992) (Fig. 2). The APS mineralizations studied correspond to that what might be called advanced argillization (e.g. La Noemia) and part of silicification-sericitization (e.g. la Vanguardia; Fig. 2). The intermediate argillic alteration is barren of APS minerals (e.g. San Isidro; Fig. 2). AsO4 may substitute for PO4 in the lattice of APS minerals, which would result in the formation of the complex APS mineral kemmlitzite {H(Sr, Ce, La, Nd) Al3[(OH)6 ({As, P, S, Si}O4)2]}, provided that arsenic is anomalously enriched in the parent material (Hak et al., 1969). Arsenic may attain contents of as much as 21.57 wt% As2O5 in this phosphate-sulfate-arsenate-silicate solid solution series. This is also valid for plumbojarosite (PbFe6[(OH)3(SO4)]4) and argentojarosite (AgFe3[(OH)3 (SO4)]), which may contain up to 19.84 wt% PbO and 18.00 wt% AgO, respectively, and precipitate on parent material abundant in Pb and Ag. Such elements, which are undesirable in kaolin used for nonmetallic purpose, introduce a new method for studies oriented toward metallic mineralization (Jandova et al., 1996; Sasaki et al., 1996; Hochella et al., 1999), especially in searches for Asenriched Cu-Au mineralization in Chile and Pb- and Agenriched mineralization in Bolivia. 4.7. Conclusion The tripartite classification scheme (hypogene-hydrothermal, hypogene-steam-heated, supergene mineralization) elaborated for epithermal metallic deposits may be applied to nonmetallic argillaceous deposits bearing APS minerals. Hypogene-hydrothermal APS-bearing argillaceous deposits in a mineral district formed at elevated temperatures as high as 325 8C are abundant in alunite with a high Na/K ratio and/or SO4-enriched APS species. Late-stage sulfatisation results in the formation of S-enriched APS minerals or secondary alunite. They are accompanied, in places, by minerals bearing Cl and F. Accumulations of precious and base metals may be encountered in the area of these nonmetallic deposits. Hypogene-steam-heated APS-bearing argillaceous deposits in a mineral district formed at lower temperatures (, 200 8C). They are abundant in APS species that, towards the waning stages of mineralization, become enriched in REE. They bear alunite with K prevailing over Na and some PO4 substituting for SO4. Late-stage sulfatisation results in the formation of aluminum sulfates. Supergene APS-bearing argillaceous mineralization without any hypogene precursor mineralization is subordinate in volcanic and volcaniclastic rocks.

The facies pattern of APS-bearing argillaceous deposits (first-order classification scheme) in the mineral district under study is controlled by the position relative to the feeder channel system and it provides an overview of the temperature regime across the entire district of mineralization. The APS-bearing argillaceous mineralization may be correlated with the common alteration patterns elsewhere in metal-mining districts (mainly advanced argillic alteration and, to a lesser extent, silicification and sericitization). The mineral zonation of APS minerals assemblages in APS-bearing argillaceous deposits (second-order classification scheme) provides a more detailed subdivision of the mineral assemblage and correlation with equivalent mineralization in metallic deposits. The fourfold mineral zonation is a function of the temperature of formation that controls the accommodation of bivalent (Sr2þ, Ba2þ) and trivalent (Ce3þ) cations in the lattice of APS minerals. It is also governed by the chemical composition of mineralizing fluids and preexisting element concentrations in the host rock, such as Pb and P, which may be proxies for elements such as K and S in the APS minerals. Facies pattern and mineral zonation, as introduced herein, may be used as possible guides in search of metal and nonmetal deposits in modern fold belts.

Acknowledgements I am grateful for the support of K.-H. Henning, J. Kassbohm, F. Nehring, T. Puff, M. Scha¨ fer, 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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