ELSEVIER Journal of Geochemical Exploration 50 (1994) 1-11

JOURNAL O[ GEDCHUi L EXPLORATION

Indonesian mineral depos i t s - introductory comments, comparisons and speculations

Richard H. Sillitoe 27 West Hill Park, Highgate Village, London N6 6ND, UK

( Received 20 August 1993: accepted after revision 28 October 1993 )

Abstract

Indonesia possesses a spectrum of precious- and base-metal deposits typical of Cenozoic volcano- plutonic arcs. Porphyry Cu-Au, porphyry Mo, skarn Cu-Au, sediment-hosted Au, low-sulphidation epithermal Au, high-sulphidation epithermal Au and volcanogenic massive sulphide (VMS) Au are all represented, although the first, third and fifth of these are to date the most important economically. Members of the porphyry Cu-Au and skarn Cu-Au classes comprise world-class orebodies. The porphyry Mo and sediment-hosted Au deposits are among the few representatives of these deposit types known from island-arc settings. The VMS deposits are of high-sulphidation affiliation and constitute type examples for this recently recognized VMS variety.

1. Introduction

The Indonesian archipelago, 13,000 islands stretching for 5,200 km, contains an appre- ciable extent of Earth's Cenozoic volcano-plutonic arcs, and is the location of 15% of its historically active volcanoes. These arcs total approximately 9,000 km in length, with some 80% comprising segments containing known mineral deposits (Carlile and Mitchell, 1994). Halmahera and Irian Jaya may be considered as part of the circum-Pacific realm, whereas the remainder of the archipelago is related to complex convergence along the northeastern margin of the Indian-Australian plate (Hamilton, 1979).

In common with other subduction-related, I-type/magnetite-series volcano-plutonic arcs generated during the Cenozoic, Indonesian metallogeny is dominated by porphyry Cu and epithermal Au deposits. The metallogeny possesses close links with that of neighbouring mineral belts: Irian Jaya is the western extension of the major Au-Cu province of Papua New Guinea. North Sulawesi may be a southwestern continuation of the Philippines (eastern Mindanao) Au-Cu province (Carlile and Kirkegaard, 1985). Mineralization in West Kal- imantan extends to the Bau Au trend of Sarawak (East Malaysia). The Cenozoic arcs of

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Indonesia are constructed, in part, on cratonic crust which, in central Sumatra and its offshore islands, includes the southeastern part of the South East Asia Sn belt. Elsewhere, however, the arcs are more primitive and occur in oceanic settings (Carlile and Mitchell, 1994).

All major Au and Cu-Au deposits in Indonesia are Mio-Pliocene in age (Carlile and Mitchell, 1994), as they are in the island arcs of the western Pacific region (Sillitoe, 1989). High erosion rates consequent upon rapid uplift and high rainfall throughout much of the Indonesian-western Pacific region account readily for this restricted age distribution because of removal of most older deposits and exposure of the young ones.

This Volume provides a timely overview of Indonesian mineralization and metallogeny, with particular emphasis on Cu and/or Au deposits and prospects resulting from recent exploration efforts. In this introduction to the Volume, selected metallogenic features, inevitably reflecting personal interests, are highlighted briefly. The features are selected for mention because they either reinforce geological patterns and relationships in Cenozoic arcs elsewhere or have the potential to increase understanding of current metallogenic problems.

2. Au-rich porphyry Cu deposits

Three districts in Indonesia contain four Au-rich porphyry Cu deposits: Grasberg in the Ertsberg district of Irian Jaya (Van Nort et al., 1991; MacDonald and Arnold, 1994), Cabang Kiri and Sungai Mak in the Tombulilato district of North Sulawesi (Lowder and Dow, 1978; Carlile and Kirkegaard, 1985; Perello, 1994) and Batu Hijau in Sumbawa island ( Metdrum et al., 1994). Grasberg is the world's premier Au-rich porphyry Cu deposit and, with a geological resource of > 2500 tonnes of contained Au, qualifies as the world's largest hydrothermal Au deposit. The Ertsberg and Tombulilato districts are present in extensions of Au-Cu provinces in neighbouring countries, as noted above, whereas Sum- bawa, part of the Sunda-Banda arc, is a new location for major porphyry-type deposits. Indeed, some explorationists (including this one!) had wrongly assumed that subduction of the Indian-Australian plate may not have been conducive to porphyry Cu-Au generation.

The occurrence of porphyry Cu-Au deposits and prospects as clusters in the Ertsberg, Tombulilato and Batu Hijau districts is a common characteristic for this type of deposit, as exemplified also by those in the Philippines (Sillitoe and Gappe, 1984). The predominant intracontinental strike-slip fault - - the Sumatra fault - - in Indonesia does not appear to act as a control for major porphyry Cu deposits, in marked contrast to the analogous Philippine fault in the Philippines ( Sillitoe and Gappe, 1984) and also the West fault in northern Chile (Davidson and Mpodozis, 1991). Only the Au-poor Tangse porphyry Cu-Mo prospect (Van Leeuwen et al., 1987) is localized by the Sumatra fault.

The geological characteristics of the Au-rich porphyry copper deposits in Indonesia Carlile and Mitchell, 1994) are closely comparable to those which typify many such

deposits elsewhere (e.g., Sillitoe, 1990). Ore is confined to multiphase diorite to tonalite porphyry stocks and their immediately adjoining host rocks, Successively younger phases of porphyry possess progressively lower Cu and Au contents, and tend to be located centrally in the stocks to give rise to low-grade cores (e.g., Grasberg, Batu Hijau). Cu-Au mineral- ization accompanies K-silicate alteration unusually rich in early hydrothermal magnetite,

R.H. Sillitoe / Journal of Geochemical Exploration 50 (1994) 1-11 3

which is overprinted in the shallower parts of systems by intermediate argillic assemblages defined by sericite, chlorite and hematite after magnetite. The Au/Cu ratios in K-silicate alteration increase progressively downwards at Grasberg (Van Nort et al., 1991 ), Cabang Kiri (Carlile and Kirkegaard, 1985) and Batu Hijau (Meldrum et al., 1994). Shallowly eroded systems, commonly characterized by andesitic-dacitic volcanic rocks, display advanced argillic assemblages, especially quartz-alunite+ pyrophyllite, constituting the erosional remnants of lithocaps, as in the Tombulilato (Lowder and Dow, 1978) and Batu Hijau (Meldrum et al., 1994) districts.

Grasberg, in common with two (Ok Tedi, Bingham) of the other three giant Au-rich porphyry Cu deposits in the world, was generated in carbonate host rocks: a fact of obvious exploration utility in Indonesia and elsewhere, and unlikely to be fortuitous. Indeed, the high Cu and Au contents in the Grasberg stock may be ascribed to containment of metallif- erous fluids by the low-permeability carbonate sequence to achieve a sort of "pressure cooker effect".

Porphyry Cu deposits in the Cenozoic arcs of the western Pacific and Indonesian regions generally lack hydrothermal tourmaline (cf. Sillitoe and Gappe, 1984), in marked contrast to Cordilleran porphyry Cu deposits in the western Americas, some of which, for example in the central Andes, are extremely rich in tourmaline, especially associated with sericitic alteration and hydrothermai breccias. In view of the fact that B, essential for tourmaline generation, is supplied to arc volcanoes by the downgoing slab (e.g., Morris et al., 1990), it may be speculated that subduction beneath the western Americas was more conducive to B extraction, possibly because of greater involvement of pelagic sediments, than that beneath the western Pacific and Indonesian arcs. However, abundant dravitic tourmaline in the Cu- and Au-bearing, inter-mineral hydrothermal breccias in the Bulagidun porphyry system, North Sulawesi, provides a notable exception to this generalization (Lubis et al., 1994). Tourmaline is also recorded from the advanced argillic lithocap at Batu Hijau (Meldrnm et al., 1994). Could Bulagidun and Batu Hijau have been formed from magmas which incor- porated more subducted B than is usual in the western Pacific and Indonesian regions or was the necessary B extracted by magma or fluid interaction with compositionally distinctive island-arc crust?

Immature, incipiently developed zones of supergene copper enrichment underlie rela- tively thin leached cappings at Grasberg (Van Nort et al., 1991 ), Sungai Mak (C.J. Bryant and M.J. Wilson, unpublished report, 1984) and Batu Hijau (Meldrum et al., 1994). However, erosion rates are inferred to be too rapid for mature zones of cumulative enrich- ment to develop, although the Ok Tedi porphyry Cu-Au deposit in neighbouring Papua New Guinea contains a more substantial chalcocite blanket that influences profoundly the deposit's economics (Rush and Seegers, 1990).

3. Porphyry Mo deposi~

The porphyry Mo mineralization of F-deficient type at Malala in northwestern Sulawesi ( Van Leeuwen et al., 1994) joins Polillo in the northeastern Philippine archipelago ( Sillitoe and Gappe, 1984; Knit-tel and Burton; 1985) as a substantial Cu-poor porphyry-type Mo

4 R.H. Sillitoe/Journal of Geochemical Exploration 50 (1994) 1-11

concentration in an island-arc setting. Porphyry Mo deposits are more typical of Cordilleran arcs, especially in western North America.

The late-collision environment invoked during granitic intrusion and Mo introduction at Malala cannot be compared in detail with that at Polillo because of the lack of a proper tectonic understanding of the small island of Polillo. Nevertheless, the calc-alkaline intrusive suites at Malala and Polillo are both more fractionated than is usual for Cenozoic island- arc magmatic rocks, a fact which offers a ready explanation for the dominance of Mo over Cu. However, the Sra isotope ratios for the Polillo intrusive rocks (Knittel and Burton, 1985) are much lower than those for the Malala suite (Van Leeuwen et al., 1994), and preclude any involvement of sialic crust in magma genesis.

The widespread distribution of Mo mineralization at Malala as a sheet-like zone along the roof of the host intrusion is reminiscent of the geometry of the large F-deficient porphyry Mo-(Cu) deposit at Mount Tolman, Washington, northwestern U.S.A. Both deposits seem to be relatively low in grade (roughly 0 .1% Mo) because of the lack of focusing of the mineralizing fluids by a pipe-like intrusive geometry. Mount Tolman, related to a late granitic differentiate of a granodioritic intrusive suite, was, like Malala, generated during crustal thickening (Carlson and Moye, 1990).

4. Skarn Cu-Au deposits

The only large skarn Cu-Au deposits in Indonesia are those in the Ertsberg (Gunung Bijih) district of Irian Jaya, which are porphyry Cu skarns in the sense of Einaudi et al. ( 1981 ). The Ertsberg stock, only 1.5 km from that containing the Grasberg porphyry Cu deposit, hosts subeconomic porphyry Cu-type mineralization associated with K-silicate alteration.

The Ertsberg deposits are unusual among major Cu-Au skarns in being associated with magnesian rather than mainly calcic silicates. Forsterite and monticellite are abundant high- temperature skarn minerals, whereas talc, serpentine, tremolite and chlorite are common retrograde minerals (Mertig et al., 1994). The only other sizable porphyry Cu skarn con- taining large volumes of magnesian silicate alteration is Christmas, Arizona, U.S.A. ( Perry, 1969). The Ertsberg East (GBT) complex vies with Bingham, Utah, U.S.A. (Tooker, 1990) as the world's largest Cu-Au skarn deposit ( > 100 million tonnes), and attains its large size because of development by replacement of receptive lithologies over a vertical interval of at least 1300 m.

5. Sediment-hosted Au deposits

Disseminated Au deposits of replacement origin in carbonate rocks at Mesel and nearby prospects in North Sulawesi (Turner et al., 1994), at Wanagon in the Ertsberg district, Irian Jaya (MacDonald and Arnold, 1994) and in the Cikotok district, West Java (Carlile and Mitchell, 1994) possess many geological similarities with sediment-hosted, Carlin-type deposits in the western U.S.A. (cf. Berger and Bagby, 1991 ). Similarities at Mesel include:

R.14. Sillitoe / Journal of Geochemical Exploration 50 (1994) 1-11 5

lack of an unambiguous genetic relationship with intrusive rocks; control by high-angle faults; occurrence of Au in decalcified carbonate rocks as well as jasperoid; micron-sized Au; deficiency of base metals and Ag; Au in arsenian pyrite; and an Au-As-Sb-Hg-T1 geochemical signature. Mesel is the first large sediment-hosted Au accumulation ( > 60 tonnes Au) defined in an oceanic island-arc setting.

Mesel and nearby sediment-hosted mineralization may be associated genetically with low-sulphidation epithermal Au veins present elsewhere in the Ratatotok district (Carlile and Mitchell, 1994), but also with nearby low-grade porphyry Cu-Au mineralization. A distal position with respect to porphyry, skarn and carbonate-replacement types of Au and base-metal mineralization is documented for the sediment-hosted Au deposits of the Bau district, Sarawak (Sillitoe and Bonham, 1990), the Bingham district, Utah (Gunter et al., 1990) and other examples, and would also apply to the sediment-hosted Au at Wanagon in the Ertsberg district. Nevertheless, a transition between sediment-hosted and volcanic- hosted, albeit high-sulphidation, epithermal Au mineralization is also observed in a porphyry system at E1 Hueso, northern Chile (Sillitoe, 1991 ).

Some recent workers (Cox et al., 1991; Seedorff, 1991 ) distinguish two types of sedi- ment-hosted Au mineralization, one generated distally with respect to intrusion-centred districts (e.g. Sillitoe and Bonham, 1990; Berger and Bagby, 1991) and the other, exem- plified by major deposits in the Carlin trend and elsewhere in Nevada, U.S.A., tlae product of metamorphic dewatering of thick sedimentary sequences (e.g. Seedorff, 1991 ). A thick sedimentary sequence is absent from the island arc at Mesel ( Kavalieris et al., 1992), which implies that an intrusive affiliation may be assumed. However, the geological features capable of distinguishing between these two proposed categories of sediment-hosted Au deposits remain obscure. Indeed, the description of Mesel by Turner et al. (1994) empha- sizes its similarities with sediment-hosted Au deposits in Nevada, and strongly supports the notion that there is only a single, albeit broad genetic category of sediment-hosted Au deposits rather than two modes of origin leading to the same end product.

6. Low-sulphidation epithermal Au deposits

Low-sutphidation/adularia-sericite epithermal Au-(Ag) deposits are widespread in Indonesia, and those of vein type in Sumatra-West Java and Central and East Kalimantan predominate. Lebong Donok in Sumatra is a medium-sized (41.5 tonnes Au) bonanza Au deposit with textural and mineralogical similarities to the much larger Hishikari deposit in Kyushu, Japan (Van Leeuwen, 1994). The recently discovered Gunung Pongkor deposit, West Java, is substantially larger ( 102 tonnes Au), but also comprises classical sulphide- and base metal-poor, low-sulphidation veins ( Basuki et al., 1994). The low sulphide content ( < 1 vol.%) contrasts with those of the base metal-rich epithermal veins that are more common elsewhere in West Java (Marcoux and Milrsi, 1994) and in most other parts of Indonesia. Vein breccias characterize many of the low-sulphidation districts, most spectac- ularly at Lebong Tandai, Sumatra (Jobson et al., 1994) and Cirotan, West Java (Marcoux et al., 1993).

The only low-sulphidation epithermal deposit which approaches giant status (defined here as > 200 tonnes Au) is Kelian in East Kalimantan. However, this bulk-mined deposit is related to intrusive rocks, is rich in base metals and yields fluid inclusion temperatures

6 R.H. Sillitoe / Journal of Geochemical Exploration 50 (1994) 1-11

(up to 330°C) and salinities ( > 10 wt.% NaC1 equiv.) somewhat higher than typical for epithermal deposits ( Van Leeuwen et al., 1990). Consequently, a deeper level of formation, at least 900 m based on fluid inclusion geobarometry, was proposed by Van Leeuwen et al. (1990). Based on examination of recent mine exposures, this writer interprets the sedimen- tary rock-charged Muddy breccia at Kelian as a series of diatremes related to felsic plug- domes, and perceives similarities with the diatreme-hosted Au deposit at Montana Tunnels, Montana, U.S.A., also interpreted to have formed in the deep-epithermal environment (Sillitoe et al., 1985). Both Kelian and Montana Tunnels are rich in Zn, Pb and manganoan carbonates but lack appreciable quartz.

Most of the low-sulphidation epithermal deposits and prospects in Indonesia are associ- ated, at least spatially, with andesitic-dacitic volcanic rocks, which contrast with the felsic dome complex hosting the disseminated Au mineralization at Gunung Pani in western North Sulawesi (Kavalieris et al., 1990). The dome complex is part of a late Miocene-Pliocene felsic volcanic suite which is broadly coeval with the belt of intrusive rocks containing the Malala Mo deposit (see above; Kavalieris et al., 1992). The presence of both epithermal Au and porphyry Mo mineralization in association with the same magmatic suite may suggest that the dome-hosted Au was concentrated in the shallow parts of a concealed porphyry Mo system (T. van Leeuwen, written commun., 1992) if it is accepted that certain epithermal precious-metal deposits, some containing Mo, represent the tops of porphyry Mo systems (Sillitoe, 1992).

Furthermore, the enrichment of W ( as wolframite), Sn ( as cassiterite and Te-canfieldite ) and Ag ( including uytenbogaardite) in the low-sulphidation epithermal vein Au-Ag deposit at Cirotan, West Java (Marcoux et al., 1993), is attributed by Marcoux and Mil6si (1994) to its association with Pliocene dacitic magmatism shown, using Pb-isotopic data, to be of crustal origin. Similar Sn and Ag enrichment, as stannite, canfieldite and Ag sulphosalts, is also documented for the Mangani Au-Ag vein in Sumatra (Kieft and Oen, 1974). Such lithophile-element enrichment in precious-metal deposits related to magmatism of crustal parentage recalls the epithermal, Ag-rich (but Au-poor) tops to the Sn- and base metal- bearing vein and stockwork systems of the Bolivian Sn-Ag belt (Sillitoe, 1992).

7. High-sulphidation epithermal Au-(Cu) deposits

High-sulphidation/acid-sulphate epithermal prospects containing Au with enargite are present in various parts of Indonesia, but none is yet known to be large or high grade by world standards. Prospects include Motomboto in the Tombulilato district (Carlile et al., 1990; Perello, 1994), Miwah in Aceh province, northern Sumatra ( A. Williamson, personal commun., 1992) and possibly Binebase in Sangihe island (Swift and Alwan, 1990), part of the North Sulawesi-eastern Mindanao arc. To these undoubted high-sulphidation systems may be added several extensive zones comprising massive silicification of advanced argillic affiliation which, to date, have not been shown to contain any Cu or Au mineralization (e.g. Carlile and Mitchell, 1994).

Some of these high-sulphidation zones are observed to be the shallow parts of porphyry Cu systems, as at Motomboto (Perello, 1994), and others are suspected also to be parts of lithocaps above porphyry Cu mineralization by analogy with numerous similar examples

R.H. Sillitoe / Journal of Geochemical Exploration 50 (1994) I - I 1 7

in the circum-Pacific region (e.g. Sillitoe, 1989). However, the major zone of replacement and residual silica at Masupa Ria in Central Kalimantan (Thompson et al., 1994) is asso- ciated most obviously with low-sulphidation epithermal Au veins rather than with enargite- Au mineralization, as are much less extensive but apparently similar outliers of siliceous rock in the Mount Muro (Simmons and Browne, 1990) and Muyup (Wake, 1991) low- sulphidation districts in the same andesitic arc. This relationship leads Thompson et al. (1994) to propose superimposition of low- onto high-sulphidation mineralization during evolution of a single hydrothermal system at Masupa Ria. The same superimposition is also observed in parts of the Baguio Au district in the Philippines (Aoki et al., 1993). High- and low-sulphidation mineralization types, generally of unequal economic importance, are commonplace in the shallow parts of intrusion-centred systems (e.g. Sillitoe, 1989, 1992) although, as in the Tombulilato district (Perello, 1994), they are generally separated spa- tially: with low-sulphidation veins around and locally beneath a high-sulphidation centre. Similarly, at Batu Hijau, a swarm of low sulphidation veins is present distally with respect to the porphyry Cu-Au deposit and its partly preserved lithocap (Meldrum et al., 1994).

8. Volcanogenic massive sulphide Au deposits

The Lerokis and Kali Kuning Au-Ag-barite deposits in Wetar island, part of the Banda arc, are considered by Sewell and Wheatley (1994) to be generated at and immediately beneath the seafloor and therefore to be assignable to the Kuroko-type, volcanogenic massive sulphide (VMS) class. The stratabound bodies of Au- and Ag-bearing, ferruginous barite sand, claystone and siltstone at Lerokis and Kali Kuning are interpreted as exhalative, whereas the barite-rich veins and stockworks elsewhere in the eastern Banda arc and, possibly, also the stratabound replacement body at Binebase, Sangihe island (Swift and Alwan, 1990; Carlile and Mitchell, 1994) are epigenetic but thought to be generated in proximity to the seafloor. However, occurrence of both massive pyrite-marcasite containing zones enriched in enargite and tennantite beneath the barite-rich ore at Lerokis and Kali Kuning (Sewell and Wheatley, 1994) and alunite-bearing alteration assemblages suggest an affiliation with high-sulphidation epithermal deposits generated in a subaerial environ- ment (Carlile and Mitchell, 1994).

Enrichment of the Lerokis and Kali Kuning mineralization in the epithermal element suite, particularly Au, Ag, As, Sb and Hg (Sewell and Wheatley, 1994), is reminiscent of modern VMS deposits which accumulated ( and are accumulating) under relatively shallow- water conditions (say, < 1500 m) in western Pacific and other island arcs (Herzig and Hannington, 1992; Herzig et al., 1994) as well as of possible ancient analogues like the Eskay Creek precious- and base-metal deposit in British Columbia, Canada (Britton et al., 1990; Sillitoe, 1993). The most obvious modern analogue for these probable VMS deposits of high-sulphidation affiliation in Indonesia is the mineralization reported by Minniti and B onavia (1984) on Palinuro Seamount in the Tyrrhenian arc, Italy. The volcanic-exhalative mineralization is present beneath about 600 m of seawater and contains abundant barite, is enriched in Au (up to 7 ppm) and possesses Cu (up to 1%) as enargite, tennantite and chalcopyrite (Tufar, 1992).

8 R.H. Sillitoe/Journal of Geochemical Exploration 50 (1994) 1-11

The high-sulphidation characteristics of the Lerokis and Kali Kuning deposits and the possible modern analogue on Palinuro Seamount contrast with the low-sulphidation alter- ation and mineralization assemblages associated with most Phanerozoic VMS deposits and suggest that, as with epithermal deposits, two discrete categories of VMS deposits (high- sulphidation/acid-sulphate and low-sulphidation/adularia-sericite) may be distinguished.

It should be cautioned, however, that the presence of massive, pyritic sulphides displaying colloform texture and abundant barite are insufficient by themselves to denote a VMS environment because both features are commonplace in high-sulphidation deposits of unam- biguously subaerial origin. For example, the Tambo deposit, in the E1 Indio Au belt of northern Chile, includes barite-cemented hydrothermai breccias in which the barite and Au are associated intimately (Siddeley and Araneda, 1986).

9. Concluding remarks

Indonesia is developing into a major Cu-Au province comparable to those of the circum- Pacific and Alpine-Himalayan belts. Indonesian arcs contain about one-sixth of the world's known porphyry Cu-Au deposits, including the richest (Grasberg). Epithermal Au min- eralization is dominated by that of low-sulphidation type but also includes a number of high-sulphidation examples. The overall proportion of low- to high-sulphidation systems is similar to that in, for example, the western Pacific region (Sillitoe, 1989) or the western U.S.A. (Berger and Bonham, 1990). Skarn and sediment-hosted mineralization types are restricted by the limited distribution of appropriate host lithologies, but still include one of the world's principal Cu-Au skarn deposits (Ertsberg East) and the largest sediment-hosted Au deposit in an oceanic island-arc setting (Mesel).

Recent exploration successes in Indonesia have reaffirmed known metallogenic patterns (e.g. low-sulphidation epithermal Au at Gunung Pongkor) as well as extending knowledge of the region's metailogeny (e.g. porphyry Cu-Au at Batu Hijau; sediment-hosted Au at Mesel; high-sulphidation VMS Au at Lerokis and Kali Kuning). These extensions to understanding are taken to imply that further exploration surprises are in store and that rigid adherence to provinces and epochs defined using the currently incomplete metallogenic data base may be counterproductive. It is felt that any Mio-Pliocene arc terrane in Indonesia possesses exploration potential for the deposit types discussed above.

Acknowledgements

Opportunities to familiarize myself with Indonesian mineral deposits gained during asso- ciations with BHP Minerals Pacific, P.T. Aneka Tambang, P.T. Freeport Indonesia, P.T. Rio Tinto Indonesia and RGC (Indonesia) are acknowledged gratefully. The manuscript was improved by the comments of John Dow, Jeff Hedenquist, Larry James and Theo van Leeuwen who, as editors of this Volume, are thanked for the opportunity to contribute this introduction.

R.H. Sillitoe / Journal of Geochemical Exploration 50 (1994) 1-11 9

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