Sundaland diamonds.pdf

PROCEEDINGS OF SUNDALAND RESOURCES 2014 MGEI ANNUAL CONVENTION 17-18 November 2014, Palembang, South Sumatra, Indonesia

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The Enigmatic Sundaland Diamonds – a Review

Theo van Leeuwen Jalan Haji Naim IIIB No 8, Jakarta, Indonesia

ABSTRACT

Alluvial diamond occurrences with no obvious primary sources are present in several scattered locations in Myanmar, Thailand and Sumatra (“Sibumasu diamonds”) and in four districts in Kalimantan (“Kalimantan diamonds”). Together they are referred herein as Sundaland diamonds. Their history of exploration and mining, geological setting, characteristics, and possible origins are discussed. This review is based on a literature search of both published papers and unpublished company reports, and information provided by geologists who have been involved in their exploration and research.

INTRODUCTION

Alluvial diamonds with no obvious primary sources (“headless placers”) are found in several scattered locations in Sundaland (Fig. 1). Some occurrences are associated with the Sibumasu terrane, and referred to in this paper as Sibumasu diamonds. More abundant and of greater economic significance are diamonds found in Kalimantan (Kalimantan diamonds). Together they are named herein Sundaland diamond.s Over the years many attempts have been made to locate their primary sources, all of which have met with failure. In this paper I review the mining and exploration history of the Sundaland diamonds, their geological setting and characteristics, and the various theories that have been advanced regarding their origin in terms of both formation and geographic location. For this I have used both published literature and unpublished company reports, information obtained from the internet, and many communications I have had with research and exploration geologists who have first-hand knowledge of the Sundaland diamonds. My own interest in the subject stems mainly from my involvement in Rio Tinto’s diamond exploration work in Kalimantan undertaken in the 1990s. I have included a fair bit of anecdotal information, as this may be of possible use in future exploration.

SIBUMASU DIAMONDS

Alluvial diamonds have been found in central Sumatra, southern Thailand and Myanmar (Fig. 1), mostly as the result of mining and exploration of alluvial tin, gold or gem stone deposits. Significant quantities have been recovered from Momeik and Theindaw in Myanmar, and the Phuket-Phangnga-Takuapa area of southwestern Thailand. History of Exploration and Mining

The occurrence of diamonds in southern Thailand has been known among local tin miners for over 130 years. They have been recovered in some concentrates and their presence reportedly often coincides with a change in colour of the concentrate and the presence of a shiny mineral (mica?) (C. Watson, written comm., 2014). Tin mining/exploration was also responsible for the discovery of diamonds in the Bangkinang district in central Sumatra during the early part of last century, when 150 small diamonds were collected from cassiterite-rich pockets at the base of a gravel bed (‘t Hoen, 1931). In Myanmar, discovery of alluvial diamonds took place much later. Following reports of alluvial diamond finds at Momeik in 1971, close to the famous ruby mines, De Beers carried out a brief inspection the next year (D. Lennie, written comm., 2014). After that the Geological Survey iniated an integrated program of geological, geochemical


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Figure 1. Map of Sundaland region showing the distribution of major alluvial diamond deposits/districts, and plate tectonic features linked to the origin of the Sundaland diamonds.

and geophysical studies, which was terminated in 1989, as it had failed to locate the primary source of the diamonds (Win et al., 2001). Local gold panners found five small stones 10 km SE of Taungo in 1980/81. This discovery was followed up by the Geological Survey in 1987/88 involving bulk sampling from 30 pits, but only a few stones were recovered (Win et al., 2001). At Theindaw, diamonds have been a by-product of alluvial tin mining, starting in 1985, with more than 3000 stones having been produced in the ensuing ten years, weighting in total >2000 carats (Win et al., 2001). Geological Setting The Sibumasu diamond deposits occur in a geological setting that is characterized by Phanerozoic sedimentation, magmatism and tectonic activity, in contrast to most other diamond provinces in the world, which consist of much older terranes. They are spatially associated with a belt of Carbo-Permian “pebbly mudstone”, which stretches from southern Tibet to Sumatra (Fig. 1). This belt has been

used to identify the Sibumasu, (Sino/Siam, Burma, Malaya, Sumatra) Terrane (Metcalfe, 1984). The pebbly mudstones have been interpreted as normal continental margin sediments deposited on the southern margin of Paleoeurasia (e.g. Garson et al., 1975; Alterman, 1986), or as diamictites formed in a glacio-marine environment on the northern margin of Gondwana that during Carboniferous to early Permian times was covered by continental glaciers and ice sheets (e.g. Stauffer, 1983; Staufer and Lee, 1984; Cameron et al., 1980; Barber et al., 2005). The presence of diamictites together with distinctive Gondwana land faunas with NW Australian affinities found on Sibumasu and Paleozoic palaeomagmatic data strongly suggests a NW Australian origin for the Sibumasu terrane ( e.g. Metcalfe, 1996). Separation of Sibumasu from Gondwanaland occurred in the late Early Permian as a result of the opening of the Meso-Tethys. The block moved rapidly northward and collided with Cathaysia towards the end of the Permian or at


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Figure 2. Diamonds from the Theindaw deposit in Myanmar showing highly resorbed rounded forms. Field of view is 1.3 cm. Photograph provided by W.L. Griffin. the beginning of the Triassic (Barber et al., 2005), or somewhat later (Metcalfe, 2013). The pebbly mudstones and associated sedimentary rocks are in many places intruded by Mesozoic tin-bearing granitoids (e.g Cobbing et al., 1986), which explains the common co-occurrence of diamonds and tin in the alluvial deposits. Small bodies of unspecified ultrabasic rocks are found in the Momeik and Taungo areas, in the former area together with dunite, pyroxenite and serpentinite, while dikes of minette and basaltic rock are present in the Theindaw and Phuket areas (Win et al., 2001; Griffin et al., 2001). Characteristics Detailed examinations have been carried out of 26 stones from Momeik, 111 stones from Theindaw, and 125 stones from the Phuket-Phangnga district in order to characterize their morphology, crystal forms, colour, degree of resorption, surface features, mineral inclusions, nitrogen content, internal structures, and isotope features (Win et al., 2001; Wathanakul et al., 2001; Griffin et al., 2001). An earlier study by Sunagawa et al. (1983) involved detailed surface microtopographic observations of 15 stones collected from a tin deposit on a small island off Phangga-Phuket.

The stones from the three locations show the following main features; 1) shades of brown and yellow are the most common colors; 2) most stones grew originally as octahedra, but now show very high degrees of resorption (Fig. 2); 3) etch features are abundant, and breakage and abrasion are common, due to alluvial transport; 4) brown radiation spots are also common; 5) both Type I and Type II diamonds (measurable and very low N contents, respectively) are presen;: and 6) syngenetic mineral inclusions are mainly of the peridotitic paragenesis, with a smaller number showing eclogitic characteristics; the proportion of diamonds of the eclogitic paragenesis appears to be higher in the Myanmar diamond population than in the Thailand population. In detail, the Momeik, Theindaw and south Thailand diamonds show some differences with regards to the ratio of yellow and brown stones, degree of resorption, percentage of stones with lamination lines and with radiation spots, and cathodoluminincscence (CL) features (Griffin et al., 2001).. Of particular interest are the results of a study carried out by Win et al. (2006) who investigated the trace element composition of 40 diamonds from these three areas using LA-ICP MS. The pattern of ‘”clear” diamonds show broad similarities, but also some distinct difference, while fibrous or cloudy diamonds are different from area to area. The authors conclude that “The trace element composition and plots, especially REE of diamond, provide not only information on the conditions and environment of diamond crystallization, but also the potential to identify diamonds from specific sources”. Sunagawa et al. (1983) drew attention to the fact that trigon morphologies on the (111) faces of one of the diamonds examined by them include negative trigon (NT) type and positive trigon (PT) type. The authors noted that such co-existence or the occurrence of PT type on its own had not been reported previously, suggesting unusual conditions under which diamonds from Thailand have experienced dissolution. Khokhryakov and Pal’yanov (2010) subsequently showed experimentally that


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Figure 3. Conceptual model for the origin of the Sibumasu diamonds. positive trigons form on diamond interacting with carbonate-rich melt. Such melts are now thought to transform into kimberlite by reaction with mantle peridotite (C. Smith, written comm., 2014). Diamond Indicator Minerals and Igneous Host Rocks. Despite intensive searches undertaken in the diamond-bearing areas by government agencies, several exploration companies and researchers virtually no “classic” indicator minerals (pyrope garnet, chrome diopside, chromite, magnesian ilmenite) have been recovered from any of the alluvial deposits, nor have any igneous rocks been identified that might be suitable hosts, like kimberlite or lamproite. Many large samples taken from the Momeik area produced only one magnesian ilmenite together with a number of non-kimberlitic chrome diopsides, which were probably derived from nearby calc-silicate rocks (D. Lennie, written comm., 2009). Origin Griffin et al. (2001) and Win et al. (2001) conclude that the Sibumasu diamonds are typically mantle derived rather than having a more unusual origin (e.g. subduction/

exhumation). This interpretation is based on the nature of their inclusions (largely of the peridotitic paragenesis), features that reflect resorption in a corrosive magma (rounded and polished surfaces), carbon and nitrogen isotope compositions typical of kimberlite and lamproite, and nitrogen aggregation states indicating long residence and/or deformation at mantle temperatures. Griffin et al. (2001) note that certain features (high incidence of plastic deformation, yellow-blue oscillatory CL, reversed UV zonation) are more typical of diamonds from lamproitic sources, rather than kimberlitic ones, whereas Sunagawa et al. (1983) suggest that “diamonds in Thailand came from kimberlite magma, whose chemistry was slightly different from that of kimberlites in other localities” (no explanation is given in the English abstract; the rest of the paper is in Japanese). Griffin et al. (2001) also note that the Sibumasu diamonds are isotopically distinct from the largely eclogitic Argyle lamproite diamonds, and from morphologically similar diamonds from eastern Australia. Extensive abrasion and abundant brown radiation spots observed in the diamonds, and the lack of associatedl mantle derived minerals in the alluvial deposits suggest that the stones have spent considerable time in high-energy surface environment and may have travelled far


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from their primary sources (Griffin et al., 2001). Proportional differences shown by populations from the main diamond-bearing areas in Myanmar and Thailand (see above) do not necessarily imply more than one primary source, as these differences may not be representative of the whole diamond populations from each area and/or may be the result of surface processes. The close spatial relationship between the Sibamasu diamond occurrences and pebbly mudstones (especially Bangkinang, southern Thailand and Theindaw) suggests that the stones were derived from this unit (e.g. Carson et al., Cameron et al., 1980), although to date no diamonds appear to have been recovered from it. According to Griffin et al. (2001) this association combined with the diamonds’ distribution within the Sibumasu terrane suggests that the stones were derived from primary sources in northwestern Australia or within the terrane itself, prior to its separation from the Gondwana margin. The concept of the formation of diamondiferous pebbly mudstones is illustrated in Figure 3. M.Crow (written comm., 2014) notes that the Sumatra occurrence is very close to the Median Sumatra Tectonic Zone, interpreted to be a faulted suture, and suggests that the diamonds may have been derived from ultrabasic rocks associated with this structure. An analogue could be the Yarlungzangbo and Bangong-Nujing sutures in Tibet, which mark collision zones between India and Eurasia. Over 100 diamonds have been recovered from these areas (Bai et al., 1993), although it is not entirely certain whether these are from peridotitic bedrock or from alluvial concentrations nearby (Gurney et al., 2005). The Phuket occurrences are also close to a significant structure, the Khlong Marui fault zone, but no ultrabasics have been found in the area (M.Crow, written comm., 2014).

KALIMANTAN DIAMONDS In Kalimantan , commonly thought to mean “River of (gold and) diamond”, alluvial diamond

occurrences are clustered in four districts, in this paper referred to as Landak, Puruk Cahu, Martapura and Kelian, which are located in West, Central, South and East Kalimantan respectively (Fig. 1). In the Puruk Cahu and Martapura districts diamonds are found in many localities, whereas their occurrence is more restricted in the other two districts. History of Exploration and Mining Kalimantan is the second oldest diamond producer in the world after India, where alluvial diamonds were discovered at least 3000 years ago (Hershey, 1940). Webster (1983) believes that Hindus may have been the first to find diamonds on the island and started their exploitation around 600 A.D. According to Schnubel, 1980 there is strong archeological evidence that diamond deposits in the Landak district were worked by Malays and Chinese during the Sung Period (960-1279 A.D.). In the 16th century, the Portuguese reached Borneo and provided the first written references of diamond workings in the Landak district (Spencer et al., 1988). By the 17th century, when the Dutch had colonized large parts of the Indonesian archipelago, the diamond industry was well established, with the Netherlands East Indies Company (the world’s largest trading house at the time) obtaining a monopoly on the diamond trade in Kalimantan in the mid-1600s This led to the eastablishment of a diamond-cutting industry in Amsterdam. Diamonds were subsequenly also discovered in the Martapura district towards the end of the 17th century, whereas in the other two districts this happened much later. The first report on the occurrence of diamonds in the Puruk Cahu district was published at the beginning of the 20th century (Hirshi,1908), and in the Kelian district diamonds have reportedly been known only since the early 1960s (M. Hartley, written comm., 2009). The only attempt made by the Dutch at mining diamonds in Kalimantan took place in the Martapura district intermittently between 1922 and 1935, during which time only about 4000 carats were recovered (van Bemmelen, 1949).


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Reliable data on local mining activity and production during the Dutch colonial era are scarce. According to Ubaghs (1941) up to 57 Chinese workings were active in the Landak district in the 19th century. Recorded annual diamond production from this area between 1876 and 1879 ranged between 4062 and 6673 carats, after which it started to decline steadily to 600 carats in 1907 due to a drop in prices following the discovery of diamonds in South Africa (van Bemmelen, 1949). From that year to 1940 less than 1200 carats were produced in the Landak district, and in the Martapura district production fluctuated between 250 and 3,292 carats during this time (van Bemmelen, 1949). Production (unrecorded) continued after the Japanese invasion (Spencer et al., 1988). The Dutch made many attempts to locate the primary source of the Kalimantan diamonds. In 1930, a small ultramafic breccia was found at Pamali in the Martapura district, from which 13 small stones were recovered (Koolhoven, 1935). This occurrence was subsequently often cited as an example of a diamondiferous intrusive peridotite formed in a mobile setting (see Bergman et al., 1987), but more recently it has been shown to be a cemented scree deposit (Bergman et al., 1987; Burgarth and Mohr, 1991). Renewed efforts to locate the primary source(s) of the Kalimantan diamonds took place between 1972 and 2000. Exploration programs included those by Allstate Resources (1972, Puruk Cahu), Anaconda-Aneka Tambang JV (1983-4) in two large blocks centered on the Landak and Puruk Cahu districts, covering in total 80,000 km², BP Minerals (1986-88, Puruk Cahu), Pelsart (1987-89, Puruk Cahu and Martapura), Ashton Mining (1990-1, Puruk Cahu), South Pacific Resources (1996-7, Puruk Cahu), and Rio Tinto (1998-2000, all four districts) . Like their Dutch predecessors the modern explorationists failed to locate a single primary source. It should be noted, though, that the exploration programs generally lasted less than two years. Lack of money or encouraging indications were the main reason for their short duration.

Alluvial diamonds were also a target. In the 1970s Aneka Tambang conducted extensive exploration on fanconglomerates along the flanks of the Meratus Mountains, with generally negative results (Spencer et al., 1998). Acorn Securities Ltd formed a JV with this company in 1985, targeting buried paleochannels in a swampy area (located downstream of, and surrounded by, old workings, beyond the reach of local miners), where reworking could be expected to have resulted in higher in-situ grades (Spencer et al., 1988). It is interesting to note that a similar concept had been developed 65 years earlier by Dutch geologists (Krol,1922). Trial mining started in 2002 and full-scale operations in 2004. The project was suspended in early 2009 because of low diamond prices. By that time ownership had changed hands twice ( BDI Mining and Gem Diamond). Total production from 2004 to 2008 amounted to 122,206 ct with an average recovered grade of 0.11 ct/bcm (L. Spencer, written comm., 2014). Possible offshore extensions of the paleogravels were tested by a consortium led by Ocean Resources in 1996/7 by using a bucket-ladder dredge to collect bulk samples. The area had been geophysically surveyed and Bangka-drilled 2 years earlier by another condortium member, PT Indo Mineratama, based on the concept that before the rise in sea levels since the last ice age large parts of the Sunda Shelf were land areas. The bulk sampling was discontinued due to negative results (Cronan, 1999). Meanwhile, investigations were carried out on an alluvial deposit in the Puru Cahu district, where in 1986 a German engineer acting for a domestic company estimated a resource of 600,000 m³ of gravels at 0.68 to 0.8 cts/m³ based on limited bulk sampling. A subsequent study by Ashton in 1990 estimated this resource to be 385,000 m³ at 1 ct/m³ (Lennie, 1997). During the post-war time artisanal mining activities continued, albeit at a checquered pace, mainly in the Puruk Cahu and Martapura districts.


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Geological Setting Large parts of Kalimantan are covered by Cretaceous to Recent sedimentary and volcanic rocks with older rocks exposed in several areas. Here I will briefly discuss two Pre-Cenozoic tectonic features, SW Borneo Block and Meratus Suture (Fig. 1), as these have been linked to the origin of the Kalimantan diamonds. Dutch geologists believed the soutwestern part of the island to be underlain by Paleozoic or older rocks based on the presence of undated metamorphic rocks, known as Pinoh Metamorphics (van Bemmelen, 1949). Following the advent of plate tectonic theory this “old core” was interpreted to represent a crustal fragment (“SW Borneo Block”), that had its origin in Eurasia (e.g. Hutchison, 1989; Metcalfe, 1996), or NW Australia, from which it rifted in early Jurassic times, followed by collision with Sundaland in the Early Cretaceous (e.g. Hall, 2009; Hall et al., 2009; Metcalfe, 2013). The proponents of a NW Australian origin cite the occurrence of diamonds in Kalimantan as their main evidence. In order to find more direct proof for the presence of old crustal rocks in SW Kalimantan, Davies et al. (2014) recently carried out a study in the northern Schwaner Mountains, which involved U-Pb dating of numerous zircons collected from the Pinoh Metamorphics , granitoids that intrude the metamorphics and associated volcanics, and active river sediments. The results confirmed that the igneous rocks are of Jurassic and Cretaceous age as previously reported by Haile et al. (1977) and Williams et al. (1989) based on K-Ar dating, but otherwise they were unexpected as they showed the Pinoh Metamorphics to consist predominantly of Cretaceous meta-pelites. Furthermore, despite a large number of samples having been processed, only very few older (reworked) zircons were found, with the exception of a quartzite sample, which contained 16 Paleozoic and 15 Proterozoic grains. Six river sand samples yielded only five grains older than Cretaceous (oldest Silurian). And lastly, in the igneous rocks xenocrystic cores (often taken to

indicate recycling of old crust) were not observed in any zircon grains. In the Meratus mountains, located to the east of the (proposed) SW Borneo Block, a northerly trending belt consisting of high-pressure metamorphic rocks, ultramafic rocks and mélanges, which are unconformably overlain by Upper Cretaceous sedimentary-volcanic formations (e.g. Sikumbang, 1990; Wakita et al. 1998). This belt forms part of the Meratus Suture, which is generally believed to represent an Early Cretaceous subduction complex (e.g. Wakita et al., 1998, Parkinson et al., 1998). Turning now to the mode of occurrence of the Kalimantan diamonds, they have been reported from Quaternary–Recent river gravels and terraces, Upper Miocene-Pliocene paleochannels, and Eocene and Upper Cretaceous conglomerates (van Bemmelen, 1949; Spencer et al., 1988; Lennie, 1997). The majority of the diamondiferous deposits consist of varying amounts of moderately to well-rounded metamorphic and igneous rock fragments, and chert and quartz pebbles. Those with rounded quartz pebbles as the dominant clast component are in part of marine origin (D. Lennie, written comm., 2009). The best studied paleoalluvial deposits occur in the Cempaka area, where two old channels have been mined, viz. the Danau Seran and Cempaka channels. These consist of three main sediment facies: palludial (swamp), sheet wash, and alluvial (Spencer et al., 1988). The paludal sediments form the bulk of the overburden, varying in thickness from <2m to 10 m at Danau Seran. Sheet-wash sediments are mainly found along the margins of the swamps. They are invariably clay rich and may contain diamonds. Alluvial gravels carry most of the diamonds. They are thought to have been derived from fanconglomerates formed at the base of the Meratus Mountains, which is defined by a growth fault. The fanconglomerates contain a minor proportion of flattened clasts, which may be derived from Upper Cretaceous (diamond-bearing) conglomerates in the hinterland (L. Spencer, written comm., 2010). The Cempaka


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channel, which is the largest of the two, is 11km long and 1.6 km wide, containing a resource of >1.4 million carats from a 6 km length of the channel (Antam, Annual Report 2005). Accompanying Minerals, Diamond Indicators and Igneous Host Rocks. The Kalimantan diamonds are commonly found together with a mineral suite that includes corundum, diaspore, zircon, chromite/spinel, pleonaste, rutile, and rare tektite (Ubaghs, 1941); Spencer et al., 1988; Lennie, 1997). The presence of corundum in particular has been regarded as a favourable indication by local diamond diggers. Esenwijn (1932) demonstrated that the co-occurrence of diamonds and these “guide minerals” is soleley the result of similar specific gravity and hardness features and bears no genetic relationship; e.g. corundum-diaspore rock is commonly formed by contact metamorphism of bauxite. At Cempaka, agates are a rough guide to higher grade gravels, and high volumes of rutile in the concentrates are often accompanied by an increase in the diamond grade (C. Watson, written comm., 2014). “Classic” diamond indicator minerals appear to be rare. Several unpublished company reports describe “lamproitic chromite” ,represented by chromites containing 62-65% Cr203, from several locations in the Puruk Cahu and Martapura districts (Lennie, 1997). Their identification was based on physical characteristics or major element geochemistry (Lennie, written comm., 2009). Lee et al. (2003) note that chromite morphology provides a powerful tool for discriminating between common chromites and chromites associated with kimberlite/lamproite, but that major element geochemistry is insufficient due to compositional overlaps with chromite from peridotite or komatite. “Kimberlitic chromite” has also been reported from Central and SE Kalimantan (Lennie, 1997), but no further details are available. Chrome diopside has been reported from two locations in the Puruk Cahu district (Lennie, 1997; Simanjuntak and Simandjuntak, 2000). As this mineral weathers

easily in tropical conditions a proximal source is indicated, but without chemical analysis the nature of the source rock can not be determined with confidence. RioTinto collected pan concentrate samples from several streams in the Kelian district from which local panners had reportedly obtained diamonds and pyrope garnet. A number of chromite grains were recovered displaying the low Al, and high Cr and Mg chemistry typical of diamond inclusions (“DI chromites”). Similar chromites were also obtained from several diatreme breccia bodies exposed in the catchments of these rivers. One of the diatremes was bulk-sampled, but did not produce any diamonds (van Leeuwen, 2001). RioTinto geologists also collected several eclogitic and peridotitic garnets ( G3, G6 and G9 groups according to the classification of Dawson and Stephens, 1975) together with a few near-DI chromites from a miner’s pit in the Riam Kanan in the Martapura district (van Leeuwen, 2001). Graham et al. (2007) have suggested that the zircons from Cempaka are geochemically very similar to zircons from lamproite sources. However, this interpretation has been questioned by Smith et al. (2009), who point out that zircons are very rare in lamproites, and if present, are likely to be xenocrysts derived from country rock sources, e.g. granitoids. Interestingly, there is no mention of microdiamonds (frequently defined as being <1mm in longest dimension) in the published literature and company reports, with the exception of some microdiamonds reported by Ashton from the Puruk Cahu district (D. Lennie, written comm., 2009). The apparent lack of microdiamonds in exploration samples has been cited as evidence for a distal source of the Kalimantan diamonds. However, the fact that microdiamonds have rarely been observed during the regional exploration programs is not all that surprising when taking into account sampling practices used at the time, which involved panning of relatively small samples on site (with a good chance of loosing most very fine material), instead of sieving larger samples at a lab as is common practice in Australia


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(Lennie, 1997). That microdiamonds are more common than previously thought is demonstrated by the work carried out recently in the Martapuru district where microdiamonds were commonly observed in the Cempaka gravels and at several locations in the Meratus Mountains (L. Spencer, written comm., 2009). Plotting of diamond numbers against size showed the classic tail of finer diamonds associated with a log normal distribution as observed in other alluvial diamond deposits (C.Watson, written comm., 2014). Ballas and bort have also been observed in the Martapura district (Spencer et al., 1988; Barron et al., 2008a). To date no true kimberlitic or lamproite rocks have been found in Kalimantan, although rocks approaching lamproite in affinity have been reported from several locations, including minette in West and Central Kalimantan (Bergman et al., 1988, and references therein), and kajanite from East Kalimantan (Wagner, 1986). Pelsart geologists described an ultramafic diatreme with kimberlite-like whole rock geochemistry from the Martapura district (Lennie, 1997). Subsequent follow-up, however, suggested this exposure to be part of the Meratus ophiolite suite (L. Spencer, written comm., 2010). Characteristics Stones sold by local miners are mostly in the 0.1 to 2.0 carat range. Diamonds over 5 ct are rare, but significant stones have occassionally been discovered as evidenced by a large collection of >10 ct stones kept in the National Museum in Jakarta (Spencer et al., 1988). The largest stone ever found was a pink diamond weighting 166.75 carats. It was recovered from an alluvial pit at Cempaka in 1965 and surrendered to government officials in Jakarta on the day that a coup plot against the Soekarno government was discovered. The stone was named “Trisakti” and sent to Amsterdam for cutting. Its present whereabouts are a mystery. Several stones over 100 ct reportedly once belonged the ruler of Landak and were likely from that district (Spencer et al., 1988). Dutch reports mention

stones weighting up to 40 ct from the same area ( Rutten, 1927) and similar sized diamonds have been found in the Puruk Cahu district (Lennie, 1997). Several studies of physical and geochemical properties of the Kalimantan diamonds have been undertaken over the last 25 years or so. Jaques (1988) examined two stones each from the Landak and Kelian districts, as did Taylor et al. (1990), who compared them with diamonds from Argyle, Ellendale and Copeton in Australia.. Smith et al. (2009) examined a total of 872 diamonds from all four districts under the binocular microscope, recording morphology details and surface texture features, and selected representative samples for cathodoluminescence (CL) imaging (147 samples) and mineral inclusion studies (32). The stones, which ranged 1 to 4 mm in size, had been obtained directly from local miners at active working sites. However, there probably was some bias in the parcels bought as only cheap (<200,000 rupiah), and hence poor quality (i.e. a lot of inclusions) stones were purchased (D. Hamid, written comm., 2010). Other studies have focused on diamonds from the Martapura district. Spencer et al. (1998) described the results of quality analysis of 6,766 stones recovered from a bulk sample pit at Cempaka (mostly gem quality). Graham et al. (2006) selected a parcel (P1) of 58 diamonds obtained from the Cempaka mining operations for detailed microscopic examination as they showed significant features under the hand lens, and compared them briefly with a parcel (P2) of 115 diamonds from shallow workings. Barron et al. (2008a) used Raman spectroscopy to identify inclusions and determine their internal pressure values; many inclusions tested gave no Raman response and were investigated optically. The latest report is by Sun et al. (2001), who bought 11 stones at the diamond-trading centre in Martapura (which may possibly sell diamonds from other regions), and another three at the mine site. They carried out examination of physical properties, CL imaging, and optical and Raman spectroscopy.


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The visual characteristics of the Kalimantan diamonds have been described by Spencer et al. (1988), Graham et al. (2006) and Sun et al., (2001) for the Martapura district, and by Smith et al. (2009) for all four districts. The stones are predominantly colourless (white), pale brown or pale yellow, and have predominantly shiny surfaces. Other colours include pink, blue, canary yellow, and dark yellow. The colourless and coloured stones have low and high nitrogen contents respectively indicating two distinct mantle source materials (Graham et al., 2006). The Cempaka stones are unsual in that a

significant number of the P1 stones studied show a ‘crazing’ on the diamond surface, representing activation of the [111] cleavage at and near the diamond surface (Graham et al., 2006). Most diamonds are intensively resorbed, dodecahedroids, transitional forms, or more rarely cube combination forms. Some of the diamonds display an evolution of shape during growth followed by subsequent resorption, e.g. from octahedron to dodecahedron. The stones are commonly plastically deformed and resorbed, and show variable amounts of inclusions.

Figure 4. Kalimantan diamond types. a) Cube combination form with prominent brown radiation spots; b) ‘Diver’s Helmet’; c) Brown dodecahedra; d) Amber-coloured variety; e) Yellow dodecahedra. Photographs provided by C.B. Smith.


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Figure 5. 52.97 carat carbonado recovered from Cempaka Diamond Mine circa 2005; kept at the Australian Museum in Sydney. Photograph provided by L. Spencer. Smith et al. (2009) note that CL analysis indicated that 80% of the stones examined are homogenous or have simple octahedral zonations. By combining the distinctive morphologies of the stones with the CL characteristics, and nitrogen abundances and aggregation characteristics, they recognized five main diamond populations, which they named “Browns”, “Yellows”, “Diver’s Helmet”, “Cube Combination”, and “Amber” (Fig. 4). All of these five groups occur in every district in Kalimantan, with the exception of the “Cube Combination” form that was observed only among the Kelian and Martapura diamonds. The characteristics of the five populations are summarized by Smith et al. (2009) in Table 1 of their paper. The authors note that the “Diver’s Helmet” and “Amber” types are rarely recorded around the world and therefore may be useful markers to trace the primary source of the diamonds. It is worth pointing out that the diversity in characteristics shown by the Kalimantan diamonds does not necessarily mean that they are derived from different primary sources, as diamonds can have diverse origins even within the same deposit (Gurney et al., 2005). Surface radiation damage was observed in a significant portion of the stones examined,

dominated by brown spots, indicating temperatures of at least 550-600o C (Vance et al., 1973). This implies deep burial in a sedimentary package >20 Ma ago (Graham et al., 2006; Smith et al., 2009). There is a notable absence of primary broken crystals or cleaved forms which are typical indications of a proximal source. However, the stones show on the other hand relatively low amounts of percussion marks, abraded edges and rhombic cracks, features that one would normally expect to see in transported crystals. Smith et al. (2009) suggest that this is probably because their commonly rounded shape made them less easily abraded. Carbonado diamonds In addition to the diamonds discussed above, a highly unusual type occurs in Kalimantan, known as “carbonado” or “black diamond”. A carbonado of about 53 ct (Fig. 5), containing molybdenum inclusions, and several smaller ones were recovered during the Cempaka operations (L. Spencer, written, comm., 2009, 2014). Another large carbonado was shown to C. Watson (written comm., 2014) by local miners in the Martapura district. There is mention of black diamonds in old Dutch reports, and Sun et al. (2001) spotted a 25 ct black stone at the Martapura diamond trading center; these may include carbonado. Elsewhere in the world this type has been found mainly in Brazil, where Portuguese colonialists coined the term carbonado in 1841, and the Central African Republic. There are largely unsubstantiated reports of small occurrences in several other countries. In all instances the carbonados are restricted to placers. Carbonados are typically pea-sized or larger porous aggregates of tiny black crystals, and have yielded Archean formation ages (Heaney et al., 2005). This type of poly-crystalline diamond is of considerable commercial value because it is extremely hard and abrasion-resistant compared with a single crystal diamond.


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Figure 6. Map of Puruk Cahu district showing distribution of alluvial diamond occurrences and major lineaments. Modified after Lennie (1997).

Several exotic hypotheses have been invoked for their origin, including metamorphism of the earliest subducted lithosphere, radioactive transformation of mantle hydrocarbons, and meteorite impact on concentrated biomass (see Heany et al., 2005 and references therein), and formation in stellar supernovae explosions (Garai et al., 2006). McCall (2009), who recently carried out a review of publications dealing with this type of diamond, concluded that it is very difficult to envisage any source other than derivation ultimately from the deep Earth mantle. The latest review of proposed origins is by Haggerty (2014), which includes the results of his detailed examination of 800 carbonado samples. Contrary to McCall’s (2006) view, he favours an extra-terrestrial model that invokes the formation of carbonado from magmatic carbon, with crystallization of diamonds in white stars. In his scenario deposition of carbonado on Earth took place during the period of “Late Heavy Bombardment” (4.1-3.8 Ga) in the early solar system. Guerney et al.

(2005) note that if an extra-terrestrial origin is true, an explanation is required as to why carbonados are only found in the same deposits as diamonds with normal crystal morphologies. Proximal or Distal Primary Sources Smith at al. (2009) concluded from the presence of the five diamond populations in all four districts, the common surface radiation damage shown by the diamonds, and lack of primary broken crystals, that the Kalimantan stones have been transported far from their primary sources, mixed, buried and recycled into the modern alluvial. The high proportion of gem quality stones observed in Kalimantan is consistent with this interpretation, because of preferential disintegration of weaker imperfect and inequidimensiol stones by impact during transport (Gurney at al., 2005). It has been suggested by some exploration geologists that as the occurrence of alluvial diamonds is restricted to four separate areas, each area


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must contain one or more primary sources, although it is generally accepted that within a particular areas there has been significant surface transport. It could be argued, though, that alluvial diamonds are in fact more widespread but have not been found yet/reported by the local miners or are very low grade. However, there is some evidence that seems to support the presence of a proximal source. In the catchment area of S. Gula – S. Lahung in the Puruk Cahu district (Fig. 6), diamonds are mostly associated with marine conglomerates consisting predominantly of quartz pebbles, but in one locality they are found in gravel composed of almost 100% angular volcanic fragments and in adjacent red soil that contains only some silicic sinter fragments (D. Lennie, pers. comm., 2009). Alluvial diamonds collected by BP Minerals geologists from the same area are described as consisting of two distinct populations, one “exhibiting excellent preservation of finest growth laminations and crystal growth imperfections”, suggesting “minimal travel”, the other showing significant wear features (PT Masupa Amin Sakti, 1988). More recent work undertaken at this site failed to identify any kimberlitic or lamproitic chromite, and no micro diamonds were found (M. Hartley, written comm., 2009). Another interesting feature observed in the Puruk Cahu district is that alluvial diamond occurrences are commonly aligned along NE/SW trends, forming district lineaments on SLAR imagery (Fig. 6). This suggests the possible presence of small diamond-bearing bodies, like dykes and plugs, along these lineaments (Lennie, 1997). BP Minerals geologists noted a plug consisting of an unusually mafic volcanic rock, described as “limburgitic”. They commented that the presence of such rock suggests direct mantle derivation, which in turn implies the presence of deep crustal structures that penetrate into the mantle (PT Masupa Amin Sakti, 1988). Of particular interest is L. Spencer’s comment (written comm., 2010) that based on examination of about 80,000 carats of production at Cempaka there appeared to be

two distinct populations: 1) partly abraded diamonds with significant percussion marks and internal fractures: and 2) sharp, euhedral crystals with little evidence of abrasion or reworking. A similar feature was also observable in microdiamonds recovered (C. Watson, written comm., 2014). This suggests a mixing of distal and proximal sources. Sun et al. (2001) also hint at this possibility based on the much smaller number of stones examined by them. It is interesting to note that zircons from Cempaka show two distict populations too: 1) a +1mm suite (Early Cretaceous FT age) containing some moderately rounded grains with well-developed percussion, suggesting a distal source, and 2) a sub-mm suite (Late Cretaceous FT age) consisting of sharp, angular grains, that retain fine magmatic resorption features, suggesting derivation from a proximal source (Graham et al., 2007). An Australian Connection? Several workers have compared diamonds from Kalimantan with diamonds from Australia. Jaques (1988) notes that the (few) Kalimantan stones examined by him clearly differ from Argyle diamonds (East Kimberley, Western Australia) , but bear similarities to Ellendale lamproite diamonds (West Kimberley, Western Australia) in terms of colour, form, nitrogen characteristics, and etch features. Taylor et al. (1990) agree that the Kalimantan diamonds studied by them (again, only a few stones) differ from Argyle stones, and have a closer affinity to diamonds from Ellendale, and also to diamonds from Copeton in eastern Australia (see below). Smith et al. (2009), based on their detailed studies of Australian diamonds (e.g. Hall and Smith, 1985), come to a different conclusion by noting that “The morphological shapes and primary etch features of the Kalimantan diamonds are quite different from any lamproite, kimberlite or alluvial diamonds known from the Kimberley Region of Western Australia”. Metcalfe (2013, referring to Taylor et al. (1990) and Smith et al. (2009), argues that similarities


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between the Kalimantan and Western Australian diamonds supports the hypothesis that the SW Borneo Block has its origin in NW Australia. However, the Ellendale diamond pipes were emplaced in the Early Miocene (Jaques et al., 1984), ie much later than the proposed separation of the block from Gondwana, and ,as noted above, Smith et al. (2009) emphasize that in fact there are no obvious similarities. C.Smith (written comm., 2014) notes that this does not necessarily precludes the Kalimantan diamonds having come from a former (rifted) part of NW Australia hosting diamond deposits, as “every diamond field in the world has its own characteristics”. Origin In this section I discuss three different settings that have been proposed for the formation of the Kalimantan diamonds: A) subcontinental lithospheric mantle (SCLM), B) subduction zone, and C) obducted mantle peridotite, and for each setting possible geographic origins. The different scenarios are shown in Figure 7. (A) The diamonds were formed in the sub

continental lithospheric mantle and transported to surface by kimberlite or lamproite.

This scenario is favoured by Smith et al. (2009) and Taylor et al. (1990). It is based on the following evidence (mainly from Smith et al., 2009): 1. The external and internal morphology of the

Kalimantan stones is similar to that of diamonds transported to surface by kimberlite or lamproite from sources in the subcontinental lithosperic mantle.

2. N content and aggregations characteristics suggest a long term growth and/or high temperatures of formation for many of the diamonds examined.

3. Syngenetic inclusions analyzed were 70% peridotitic and 30% eclogitic.

4. The composition of inclusion chromites is typical of world-wide diamond inclusion chromites derived from cratonic lithosphere.

5. Micro-inclusions of graphite in the crystal centre could be interpreted as inclusion-seeds for diamond nucleation, characteristic of kimberlitic diamonds

6. A model age of 3.1± 0.2 Ga obtained from Re/Os dating of a high Ni-sulphide inclusion in a Martapura diamond and an unradiogenic Os isotope ratio of the sulphide indicate a likely Archean age for the lithospheric mantle hosting the diamond.

7. Estimated PT conditions under which this diamond was formed ( pressures of 4.2 – 6 Gpa and possible temperatures of 930-1250o C) are similar to those of diamonds from African and Yakutian cratonic mantle lithosphere.

As far as the geographic origin of the cratonic diamonds is concerned, four scenarios have been advanced, named here: A1 allochthonous, A2 semi-allochthonous, A3 semi- autochthonous, and A4 autochthonous A1 Allochthonous (Fig. 7a). It has been suggested that the diamonds originated outside of Kalimantan and may share a common source region with the Sibumasu diamonds (Turner et al., 1985), such as south China, or may have been derived from the Sibumasu terrane itself before SW Borneo rifted from Indochina (Griffin et al., 2001). The presence of diamonds in conglomerates of Late Cretaceous age indicates that (part of) the diamonds had arrived in Kalimantan by that time. The Mesozoic stratigraphic record for Sundaland is limited but suggests that much of the region was emergent (Hall, 2009). The diamonds could thus have been transported from Eurasia to Kalimantan by large river systems. A2 Semi-allochthonous (Fig. 7b). In this scenario the diamonds originated in some ancestral craton from which SW Borneo was rifted (Hutchison, 1996), where they formed alluvial deposits by weathering of primary source rocks, and were subsequently redeposited as placers along the craton margin, which was followed by transportation to their


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Figure 7. Cartoon illustrating six different scenerios for the origin of the Kalimantan diamonds. A1) Diamonds were transported from Eurasia to Kalimantan by rivers in Late Mesozoic when Sundaland was largely emergent. A2) Diamonds were eroded from diamond-bearing pipes in a craton (e.g. Australia) and transported to Kalimantan on a fragment that rifted from the craton; A3) Fragment itself contains diamond-bearing pipes. A4) Diamonds were formed and transported in sub-cratonic mantle decoupled from overlying crust. B) Diamonds were formed in subduction zone and brought to surface by a non-kimberlite/ lamproite carrier. C) Diamonds associated with obducted ophiolite.


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present position on the rifted fragment. According to Kadarusman (2010), Parkinson et al. (2000) (not in his reference list) presented mineral inclusion parageneses and nitrogen isotope data for diamonds from the Martapura district, and suggested that there were similarities with Argyle diamonds, on the basis of which they proposed an Australian origin for the Martapura diamonds and transport to their present location by means of a microcontinent. A3 Semi-auchtochtonous (Fig. 7c). This scenario envisages that a crustal slice hosting one or more diamond-bearing pipes or dykes broke off from a craton, and these were still largely in tact on arrival at Kalimantan. Diamonds were subsequently released from their primary source rocks by erosion and weathering processes, and dispersed accross the island (White et al., 2014).. L. Barron (written comm., 2008) has questioned the viability ot this model as diamond-bearing cratons are so stable that that they resist getting broken off, except by a localized mantle plume, which would by its nature zap the diamonds. A4 Autochthonous (Fig. 7d). In this model the fragment did not contain any pipes prior to rifting but was underlain by diamond-bearing mantle from which diamonds are sampled by kimberlitic or lamproitic magma after arrival of the fragment in Kalimantan (White et al., 2014). A variant of this model has been proposed by Taylor et al. (1990). Based on very similar time-temperature characteristics observed in some Ellendale, Copeton and Kalimantan diamonds the authors suggest that these diamonds may have a common origin in ancient Gondwana subcontinental lithosphere, which has been dispersed by more recent tectonic processes. As ancient crustal rocks are not known to be present in Kalimantan and the Copeton area, they suggest that the crust in these two regions may have been decoupled from the underlying lithospheric diamond source region. They further suggest that based on an estimated extraction mantle age of 150-5 Ma for the Kalimantan diamonds the stones were

collected by kimberlitic or lamproitic magma from the mantle and brought to surface some time during the mid-Mesozoic to Late Tertiary. Discussion of scenarios A The (apparent) absence of kimberlitic and lamproitic rocks in Kalimantan is consistent with scenarios A1 and A2, but does not necessarily disprove A3 and A4. According to Kirkley et al. (1991) a typical kimberlite pipe could be completely eroded away in about 70 My, that is in less time than the age of at least part of the Kalimantan diamonds. This estimate is for diamond-bearing cratonic regions like in Australia and South Africa, whereas Borneo’s tectonic history has been far less stable than that of these regions. Hall and Nichols (2002) have estimated that at least 6km of crust have been removed from large parts of Borneo during the Neogene as the result of high weathering and ersosion rates. Thus most, if not all, Cretaceous diamond-bearing bodies are likely to have been either eroded away or covered by Neogene sediments. The near-absence of indicator minerals can also be readily explained. Scenarios A1 and A2 envisage transport of diamonds and indicators over long distances. There are a number of alluvial deposits in the world, such as those in Namibia, that lie off craton or along craton margins but are inferred to have a primary cratonic source (as proposed in A2), in which the indicator mineral suite is often absent or rare (Griffin et al., 2000). An interesting feature of the Namibia deposits is that microdiamonds are absent in gravel beds with coarse diamonds, but occur in huge concentrations in mineral sands (C.Smith, written comm., 2009). In all four scenarios, the near-absence of indicator minerals would fit with destruction during extensive weathering/transportation/re-cycling experienced by (a large proportion of) the stones within Kalimantan itself. As parallel examples, there are no indicators associated with the diamondiferous Nullagine conglomerates in W. Australia or the Witswaterand conglomerates in S. Africa (C. Smith, written comm., 2009).


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It is also theoretically possible that (some) of the Kalimantan diamonds are hosted by unconvential igneous rocks, as has been observed in other parts of the world. Examples inlude calc-alkaline lamprophyres at Wawa, Canada (De Stefano et al., 2006), metamorphosed ultramafic rocks of likely lamprophyric origin at Dachine, French Guyana (Smith et al., 2012), and shoshonite–picrite at Karashoho, Uzbekistan (Golovko and Kaminsky, 2010). This idea has been suggested by Simanjuntak and Simanjuntak (2000), who interpret Neogene potassic and ultra-potassic volcanic rocks occurring in parts of Kalimantan to be related to “hot spots” in an extensional tectonic regime and propose these to be a potential primary source of the Kalimantan diamonds (but obviously no tof the diamonds found in pre-Neogene conglomerates). A possible weakness of scenarios A3 and A4 is that there is no strong evidence to support the presence of a continental fragment in SW Kalimantan. As mentioned earlier, the main evidence presented by proponents of the continental fragment hypothesis is the widespread occurrence of diamonds in Kalimantan, i.e it becomes a circular argument. It has been suggested that the rare occurrence of Proterozoic zircons in the Schwaner Mountains (3.2) and Paleogene sandstones in NW Borneo (van Hattum et al., 2006) constitutes another piece of evidence (Hall and Sevastjanova ,2012; Davies et al., 2014), but like the diamond evidence it is circumstantial at best, as the zircons are detrital and may have gone through several cycles of weathering, transportation and deposition. However, if we accept the model proposed by Taylor et al. (1990), which is also favored by Smith et al. (2009), i.e. SW Kalimantan is underlain by a remnant fragment of SCLM mantle without corresponding crust (from which it was decoupled), then presence of ancient crust is not required. B The diamonds formed in a descending slab

during failed subduction at relatively low temperatures and depth under ultrahigh

pressure (UHP)conditions and were sampled by non-kimberlitic magmas (Fig. 7e)

This hypothesis has been developed by Barron et al. (2008a). It. is based on a similar model proposed by Barron et al. (1994, 2008a & b) for alluvial diamond occurrences at Copeton and Bingara in New South Wales (Australia). These New South Wales diamonds have unique growth structures, unusual inclusions (including “grospyte”), estimated PT conditions of formation of 30-60 kbar and 790-840o C, and high remnant internal pressures (Pr) on inclusions, such as 7.5-35.6 kbar on pyroxene, garnet and coesite, and formation temperatures between 250 and 800o C (Barron et al., 2008b, and references therein, especially Meyer et al., 1997, and Davies et al., 2003). Some diamonds from the Cempaka deposit have several features in common with the Copeton – Bingara stones, including overlapping nitrogen characteristics, second order Raman spectroscopy peaks that are suppressed (in contrast to cratonic diamonds) and preliminary Pr estimates of 8-19 kb (Barron et al., 2008a; Barron, written comm., 2010). The Pr estimates were obtained from a number of olivine inclusions in five diamonds using strain birefringence around the inclusions (see Barron et al., 2008a), which were confirmed by a 2d Raman scan over two of the inclusions (12-15 kbar); they are 2x to 4× higher than those reported from cratonic diamond but well within the range reported for clinopyroxene inclusions in the Copetan-Bingara stones (Barron, written comm., 2010). Based on the pressure preservation index model developed by Barron (2003) the much higher internal pressures indicate a much lower temperature of formation than for cratonic diamond, suggesting that at least some of the Cempaka diamonds are probably of UHP origin rather than cratonic (Barron, written comm., 2010). Both the Copeton-Bingara and Cempaka diamond deposits occur within Phanerozoic mobile belts. In SE Kalimantan, subduction took place during the Early Cretaceous (Wakita et al.,


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1998; Parkinson et al., 1998). The presence of diamonds in conglomerates of Late Cretaceous age would therefore suggest that delivery to the surface (by non-kimberlitic, as yet unidentified media) occurred soon afterwards. For the small fraction of diamonds appearing as ballas, this may have been by Alaskan-type alkali ultramafic arc magma as suggested by the composition of Cr-magnetite in ballas from Cempaka (Barron et al., 2008a; Barron, written comm., 2009). The presence of an Alaska-type ultramafic complex in SE Kalimantan has been postulated by Zientek et al. (1992). C The diamonds in S. Kalimantan are derived

from obducted ophiolite (Fig. 7f) Nixon and Bergman (1987) suggest that the primary source of the Kalimantan diamonds are ophiolites, which represent mantle material that has been thrust up unto the eart’s surface as the results of collisional tectonic processes. Ophiolites are found in the Meratus Mountains and several other locations in Borneo. A genetic link between the Kalimantan diamonds and peridotites was first proposed by Wing Easton (1933), an idea that was controversial at the time, but became more widely accepted following the discovery of the Pamali Breccia (see above). Bergman et al. (1987), referring to Strache (1976) and Koolhoven (1935), noted that in SE Kalimantan streams generally become diamond-bearing after crossing an Upper Cretaceous sedimentary-volcanics unit, the basal part of which is diamondiferous where it overlies ophiolite rocks, but barren where other rock types form the basement. Burgarth and Simandjuntak (1983), on the other hand, concluded from the near-absence of ultrabasic pebbles in three widely spaced alluvial diamond deposits in SE Kalimantan, that the ophiolites could not be the source of the diamonds. A similar debate took place many years ago among Dutch geologists, with some proposing a granite or metamorphic vein origin (abundant granite and schist fragments, few ultrabasic clasts), while the proponents of a peridotite origin pointed out that this rock type is prone to

weathering in the tropical environment (Ubaghs, 1941). Diamonds associated with alpine-type peridotites occurring in fold belts peripheral to cratons have been reported from a number of other regions in the world, but in most cases are either very sparse or graphitized as a result of the slow rate of obduction (e.g. Nixon, 1995; El Atrassi et al., 2011).

CONCLUDING REMARKS

The manner of origin of the Sundaland alluvial diamonds and the location of their primary source(s) still remain largely an unresolved mystery and the subject of much scientific speculation. There appears to be a general consensus among research geologists that the stones have predominantly a “classic” SCLM origin, but there is some evidence that suggests that some may have formed under relatively low temperature and ultra-high pressure conditions in a subduction environment. There is much less agreement on where they came from in a geographic sense and how they got there. One of the problems is that, with some possible exceptions, the Sundaland stones appear to have been subjected to two or more cycles of weathering, transportation and deposition. This makes tracing them back to their primary sources a challenging task, especially as these sources may have been largely eroded away or covered by younger volcanic-sedimentary formation. Furthermore, certain exploration methods that are commonly used elsewhere are less effective in this part of the world, like indicator minerals techniques (low preservation of most indicators in the tropical ;secondary environment; common presence of non-kimberlitic chrome diopside and chromite from sundry ultramafics), airborne EM (thick weathering profiles), and airborne magnetics (widespread occurrence of magnetic rocks). Our limited understanding of the nature of the basement and pre-Cenozoic evolution of the diamond-bearing regions is another handicap in developing effective ecploration strategies, particularly in the case of Kalimantan.


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At the present state of our knowledge it would seem possible that the Sundaland diamonds have more than one origin (genetic, temporal and/or geographic) and that more than one type is present in the same location, like in the Wellington alluvial deposits in New South Wales where cratonic and UHP diamonds occur together, with both groups showing surface features consistent with magmatic transport to the Earth’s surface (Davies et al., 1999; 2002). Diamond prices have been rising recently and De Beers predicts that there will be a supply shortage by 2020. This begs the question as to whether the Sundaland diamonds deserve another look. With this in mind I would like to end this review with making a few suggestions for future research, which may add a piece or two to the highly complex and fascinating puzzle presented by these stones. 1) Revisit the area in the Puruk Cahu district

from where two distinct diamond populations have been reported (“proximal” and “distal”). If the existence of such populations can be verified, subject them to a series of analyses to check whether they can also be distinguished on the basis of internal features, like inclusions, and nitogen and isotope characteristics. Ditto for the two populations (in terms of surface damage) reported from Cempaka .

2) Collect and date a statisically meaningful number of zircon grains from alluvial diamond occurrences in different districts, making sure that they come from the same bed as the diamonds (note: this was recently done for the Martapura district by White et al., 2014). Patterns displayed by pre-Cretaceous ages may give a clue as to whether more than one distinct source region is involved, and where these may be. In addition determine trace element compositions of the zircons, in particular pre-Cenozoic ones, in order to identify their likely original source rocks (cf. Belousova et al., 2002).

3) Carry out more detailed analysis on selected Martapura diamonds, including age dating, Pr measurements, presence versus near-

absence of a broad 2nd order Raman peak in UV-transparent diamonds, and determination of chromite composition in inclusions with the aim to test the hypothesis that both cratonic and UHP diamonds may be present.

4) Analyze the five diamond types from Kalimantan for trace elements using LA-ICP MS. The initial purpose would be to see whether they can be fingerprinted by this method and to compare the trace element pattern with those obtained from the Sibumasu diamonds.

Acknowledgements I thank Larry Barron, Bill Griffin, David Lennie, Lee Spencer, and Charles Watson for helpful comments on earlier versions of the manuscript and many stimulating discussions. I am especially grateful to Chris Smith for invaluable assistance given throughout the long process of writing the paper, and for his detailed and constructive review of a recent draft. I also thank Supriyadi who helped with the preparation of the figures, and Mark Hartley for improving my understanding of diamond exploration and the science behind it during field trips and stimulating debates.

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