The Purnama Gold Deposit in the Martabe District

of North Sumatra, Indonesia

Brian Levet

Manager, Exploration Business Development

Newmont Australia Ltd

10 Richardson Street, West Perth, WA

Martin Jones

Site Manager, P.T.Horas Nauli

Aek Pining, Batangtoru

Sumatra Utara, Indonesia

Bronto Sutopo

Project Geologist, P.T.Horas Nauli

Aek Pining, Batangtoru

Sumatra Utara, Indonesia

Key Words: epithermal; high sulphidation; acid sulphate alteration, phreatomagmatic breccias, felsic dome complex.

Abstract

The Martabe high sulphidation gold deposits are hosted in a sequence of Tertiary volcanic and sedimentary rocks proximal to a fault splay that forms part of the Great Sumatran Fault complex. Episodic fault activity, related to wrench tectonics associated with the oblique subduction of the Indian Australian plate below the Eurasian plate, has been responsible for pulses of high level magmatism and development of multi-stage phreatomagmatic breccias, flow dome complexes, hydrothermal alteration and gold mineralisation observed in the district.

Local structural architecture is consistent with the dextral strike slip tectonics observed on a regional scale with a major northwest to north-northwest fault set forming a prominent scarp that bounds the west side of the Purnama deposit. A well developed conjugate set of northeast extensional faults bisect the stratigraphy immediately to the east. These faults provide fluid channel ways that have localised and superimposed multiple alteration and mineralisation events on the rock mass.

As indicated, the principal host rocks are series of phreatomagmatic diatreme breccias and dacite flow domes. These have been intruded into and through a sequence of gently dipping mudstone, siltstone, sandstone and andesitic lava.

Multi-stage acid-leaching hydrothermal alteration events have produced widespread texturally destructive silicification. This has resulted in large volumes of vuggy silica with a tabular geometry cutting various lithologies and enhancing the permeability of these host rocks for later higher grade gold bearing fluids.

Gold mineralisation occurs in a number of deposits over a strike length of seven kilometres. The most significant and best defined of these is the Purnama deposit, where a resource of 66.7 million tonnes containing 1.74 g/t Au and 21.5 g/t Ag for a total of 3.7 million ounces of gold and 46 million ounces of silver has been defined by diamond drilling.

Introduction

Despite intense copper gold exploration efforts of the 1980s and 1990s in Indonesia, it is remarkable that the Purnama deposit was only discovered late in this period, two kilometres from the Trans Sumatra Highway near the town of Batangtoru with an population of 12,000 people.

In October 1993

Normandy Anglo Asian Indonesia made an application for a Contract of Work (COW) in the Sibolga District of North Sumatra. The COW, covering an area of 659,600 ha, was eventually granted in April 1997. Regional exploration and obligatory land relinquishment has since reduced land tenure to the present holding of 256,300 ha shown on Figure 1.

Figure 1: Location of the Martabe Gold Deposits, Sumatra, Indonesia

The Martabe gold system was discovered in late 1997 using regional BLEG (Bulk Leach Extractable Gold) stream sediment sampling techniques and the anomalies were defined using soil sampling on a 100 by 50 metre grid.

The best gold soil geochemistry was identified in talus below the Purnama hill on the western margins of the Purnama fault, which led to the discovery of the Purnama deposit. Exploration has also identified mineralisation within the Martabe District at Pelangi, Baskara, Kejora and Gerhana, as shown on Figure 2. These high sulphidation epithermal gold deposits are hosted in variable lithologies with strong structural controls.

Since 1998, exploration has focused on the Purnama deposit, which has been defined by 25,686 metres of drilling in 67 diamond holes on a 50 by 50 metre grid. The project is currently in Pre-Feasibility with a reported resource of 66.7 million tonnes at a grade of 1.74 g/t Au and 21.5 g/t Ag for a total of 3.7 million ounces of gold and 46 million ounces of silver.

Figure 2: Regional Geological Interpretation of the Martabe District

Geological Setting

The northwest trending fault to the west of the Purnama deposit is interpreted as being a subordinate strand of the Great Sumatran Fault and would be expected to have a history that included a significant component of lateral movement. Structural preparation introduced zones of high permeability, which allowed magma to be emplaced at high crustal levels. Structural field data supports this interpretation and does suggest that dextral displacement occurred along the fault.

The complex stress and resultant strain environment means that a vertical movement was likely and the juxtaposition of different rock types by northwest faults is evident. Although post-mineral faulting is also recognised, the amount of offset has yet to be quantified.

A major feature of this structural environment is the development of the conjugate sets of extensional en echelon faults that have been used as channel ways for hydrothermal fluids, now recognizable as zones of more intense silica alteration and veining. The steeply dipping fault orientations from north-northwest to northeast are very obvious and are clearly the major control on silicification and mineralisation. A third fault set oriented east-west is also observed, these are contractional with a component of reverse movement.

The northwest trending Purnama fault divides the geology of the Martabe district into two distinct geological domains. To the west, where outcrop and geological knowledge is limited, lithologies are relatively undisturbed, weakly altered and include a sequence of mudstone, siltstone, sandstone and basaltic to andesitic lava. To east of the Purnama fault where the geology is considerably more complex, similar lithologies have been subjected to intense multi-phase magmatic, phreatomagmatic and hydothermal events. The resulting stratigraphy is markedly different from that preserved to the west.

Figure 3 shows a simplified interpreted geology of the Purnama Deposit. The northwest trending fault set clearly juxtaposes unaltered basalt andesite to the west against highly altered stratigraphy to the east. Late movement on this fault is highly probable but has not been quantified.

The volcano-sedimentary sequence has been intruded by multi-phase phreatomagmatic breccias. The thicker breccia units show distinct facies variation. Lithic-rich facies at the base pass upwards through a finer grained, matrix supported, facies containing accretionary balls up to facies at the top of the sequence containing abundant wispy juvenile clasts of felsic volcanics.

The emplacement of these early phreatomagmatic diatreme breccias was followed by the intrusion of a dacitic flow dome complex. Multiple diatreme facies superimposed each other at Purnama are thought to be coetaneous with early dacitic intrusive activity to the north. Phreatomagmatic breccias of various ages that have partially destroyed the sedimentary sequences, are exposed in a concentric geometry around the late hornblende andesite intrusive.

To demonstrate the interpreted geology of the Purnama deposit, a series of cross-sections spaced 200 meters apart are shown in Figure 4. Each of the lithological units is briefly described below.

The basaltic andesite is green with an aphanitic to finely porphyritic texture.  Phenocrysts consist of fine to medium grained feldspars and this unit is unaltered to weakly chloritic.

The volcanic breccia consists of a sequence of fragmental units with intercalated, carbonaceous mudstone, siltstone and fine-grained sandstone. The principal rock type is a coarse heterolithic, matrix supported fragmental rock, with low matrix/fragment ratio. Clasts are predominantly of quartz-sandstone, siltstone and volcanic. The matrix varies from fine grained mud through to coarse sand. These fragmental rocks were originally thought to represent a volcaniclastic density flow unit deposited off an eroding arc. As the geology of Purnama becomes better understood the fragmental units are recognised as intrusive diatreme that has been injected along bedding planes within the sandstone, siltstone sequence and not volcaniclastic density flows as originally thought. The high proportion of sedimentary clasts, the muddy to sandy matrix and the concentric morphology of the units around the center of the diatreme complex would support this.

The porphyritic andesite is an easily recognisable unit, irrespective of the degree of alteration and is a useful marker unit within a stratigraphy made up of predominantly diatreme breccia facies. The rock has a porphyritic texture made up of approximately 25% phenocrysts, many of them plagioclase crystals. It is interpreted to be a lava flow, probably of submarine origin and is an important host rock for mineralisation because of its proximity to the mineralising fault system. The top and the base both show crackle brecciation, which is indurated by fluidised phreatomagmatic and hydrothermal breccias.

The phreatomagmatic diatreme breccia units show temporal and spatial variability in texture and composition. These multi-phase breccias, which vary in width and form, have been intruded into the sequence of siltstone and mudstone, both above and below the porphyritic andesite. Evidence for this is the high sedimentary composition of the breccia both as matrix and as clasts and the preservation of the mudstone and siltstone units both in and between phreatomagmatic units. Within the thicker breccia sequences, there are recognisable facies. The earliest phase of brecciation is expressed as a muddy matrix breccia in steep contact with pre-existing wall rocks and with numerous irregular fluidised breccia dykes that invaded the wall rocks along fractures and stratigraphic contacts. The main breccia body varies from clast-support to matrix-rich, all with a dark gray fluidised matrix, commonly with accretionary balls. The dark gray matrix is attributed to the abundance of comminuted carbonaceous mudstone wall rock that was incorporated in the breccia. A juvenile-rich facies, which occurs at the top of the sequence, typically contain lithic fragments with plastic deformation. Size of juvenile clasts varies from four centimetres to few millimetres. The juvenile component, which defines the direct magmatic input, is wispy-textured dacitic clasts. The textures in this breccia are very similar to those described from the diatreme breccia at Kelian (Davies et al, 1999), even though the style of mineralisation is different.

At least two ages of phreatomagmatic breccias are recognised and are distinguished by alteration type or by the composition of their clasts. Phreatomagmatic facies host high-grade mineralisation where they are in contact with mineralising fluids in zones of structural complexity.

Most lithologies at Purnama are cut by a late stage phreatomagmatic breccia, which contains clasts of both the earlier silicified phreatomagmatic breccias and dacitic lithologies. It is generally a coarse heterolithic fragmental breccia, typically containing andesite, siltstone, fine sandstone, silica and quartz vein clasts. This unit is often faulted, slickensides and gouge material are common.

The dacite porphyry outcrops to the east and north of Purnama deposit and is interpreted to be part of an extensive flow dome complex. The rock is predominantly porphyritic with 5-7% quartz eyes, 20% plagioclase crystals, with some biotite and hornblende. This unit includes autobreccias, flow banded and base surge deposits, as well as fine pyroclastic rocks with the same composition. The pyroclastic rocks are characterised by the presence of angular quartz crystal fragments. Volcanic dacite is locally altered to vuggy silica with advanced argillic and argillic alteration assemblages. It is interpreted that as groundwater was depleted during the cycle of brecciation, dacitic magma was able to rise to the surface as a flow dome which partly infilled the diatreme breccia column. Flow banding is observed on the margins of this unit.

The hornblende andesite is also a porphyritic rock and contains 10% hornblende crystals, 15% plagioclase crystals and less than 2% quartz phenocrysts. Fine-grained xenoliths of same composition are common and alignment in the crystals (flow foliation) can be identified in some intervals. This rock is crosscut by calcite and zeolite veinlets. Mapping has shown that the quartz-hornblende andesite occurs in the central part of Purnama area and may extend to the northeast. It has a circular geometry (about 500m diameter) that coincides with negative gold soil geochemistry. This unit, which can be interpreted as a dome, shows typical propylitic alteration, is unmineralised and considered to be late stage.

Alteration

Like many high sulfidation deposits, the epithermal mineralisation at Purnama follows extreme acid sulphate leaching of the wall rock. It is difficult to demonstrate clear alteration zonation at Purnama because multiple phreatomagmatic and alteration events have been superimposed on each other. The juxtaposition of phreatomagmatic rocks and timing of different alteration events means that the alteration contacts between breccias units are often sharp.

Hydrothermal alteration at Purnama is typical of many high sulfidation systems. The early stage acid-sulphate alteration event produce zoned advanced argillic alteration with vuggy to massive silica alteration enveloped by silica/dickite/alunite, grading out to silica – illite and peripheral argillic alteration zones as the initial acidic vapor phase was progressively neutralised by the wall rocks and groundwater. This style of alteration is focused around the major structures and the immediate wall rocks.

Figure 3: Geological Interpretation of the Purnama Deposit

Figure 4: Cross Sectional Geology of the Purnama Deposit

There is a very strong correlation between gold mineralisation and silicification as shown in Figure 5. The silicification has not only produced a vuggy permeable host, but also a host subject to brittle fracture created by subsequent tectonic events.

The term quartzification for this style of alteration is strictly more correct than silicification. Silicification implies the addition of silica rather than the depletion of all other elements leaving residual silica. Degrees of “silicification” can be attributed to both differences in permeability due to composition of the original lithologies and to the degree of acid leaching.

In some acid sulphate deposits, zones of barren granular silica have been recorded. This type of alteration is attributed to the alteration of the steam-heated zone above the paleo-watertable. At Purnama, there is some evidence of granular silica from outcrop, although more massive silica with very low gold occurs at the top of the deposit and may be attributed to a similar mechanism.

Figure 5

Section 167200N Showing the Relationship between Lithology, Structure, Alteration and Mineralization

Mineralisation

The silicification and the mineralisation are clearly controlled by both structure and permissive lithologies. The mineralised zone at Purnama extends about 1.2 km by 1 km in a zone between the southern margin of the breccia dacite dome complex and a north-northwest trending fault scarp. Several stages of mineralisation have been recognized:

Ø Early phase of silica - pyrite mineralisation with low gold grades (0.1 – 0.5 g/t Au) is associated with or immediately after the main acid sulphate alteration event.

Ø Colloform banded chalcedonic silica veins with quartz / bladed carbonate ‘boiling’ textures contain low gold grades (0.1 – 1 g/t Au). These silicified zones are characterised by locally dense zones of chalcedonic silica veins with individual veins varying from < 1 centimetre to over 1 metre wide. At Purnama these veins trend from almost north-south to 020 - 030° and are steep dipping. Although the colloform and crustiform banding and presence of quartz ‘boiling’ textures indicate potentially mineralised veins, for the most part these are apparently low-grade to barren of gold, with some apparent exceptions perhaps due to overprinting mineralisation. This well-developed, low sulphidation vein phase, which overprints the acid-sulphate alteration zone, is unusual in a high sulphidation system. An evolution of high sulphidation mineralization to intermediate and low-sulphidation stages is recognized at El Indio in Chile, and also at the Lepanto district in the northern Philippines where the veins develop along strike from the high-sulphidation mineralisation. At Martabe the low sulphidation system is superimposed on the high sulphidation system. One similarity with both El Indio and Lepanto is that the low sulphidation vein mineralisation is related to major strike-slip faults.

Ø The main-stage enargite / luzonite mineral assemblage marks a return to high sulphidation mineralisation. This mineralisation has associated with covellite, native sulphur, pyrite, bismuthinite, barite, and marcasite occurring in fractures and vugs which are mostly oxidized. In the sulphide zone gold is present as free grains and within enargite. The silver is also present in the enargite / luzonite and as proustite / pyrargyrite inclusions in bismuthinite. Consistent with many other high sulphidation deposits, the main stage of gold mineralisation is late in the evolution of the hydrothermal system. At Martabe the higher gold grades are associated with late-stage fracturing, and crackle-type brecciation of the silicified rocks marginal to the late clay-altered diatreme breccia. This fracturing and brecciation also clearly cuts the chalcedonic silica veins. It is interpreted that the emplacement of the later diatreme breccia was responsible for this fracturing and brecciation in competent (silicified) rocks around the diatreme margins and the diatreme itself became an impermeable hangingwall barrier due to the pervasive clay alteration. Similar clay alteration of syn- to pre-mineral diatreme breccias is recognised at Yanacocha and is attributed to a large influx of groundwater and neutralisation of the magmatic volatiles immediately following the breccia emplacement (S.J.Turner 2002).

Oxidation

The oxidation profile is highly irregular, reflecting the distribution of structural breaks and host rocks with good secondary permeability. Although some remobilisation of gold can be expected in the oxidised zone the lack of coarse, free gold and the presence of high silver values, indicates that there is likely to be little supergene upgrade of the gold values. Arguably, the high-grade zone probably reflects primary depositional gold grades rather than a zone of secondary enrichment.

The late-stage gold mineralisation is temporally distinct from the earlier silica – pyrite alteration and as it is predominantly fracture and breccia controlled is more susceptible to oxidation than the earlier silica pyrite. This means that the fracture-controlled mineralization is predominantly oxidised. The percentage of late stage iron oxide in fractures and crackle breccias rather than the proportion of sulphide in the rock is likely to be the best indicator of the gold leachability of the rock. Barite commonly occurs in these late-stage iron oxide fractures. Scorodite, from the oxidation of arsenical sulfosalts, may also be present. The presence of luzonite / enargite – covellite ± pyrite / marcasite in fractures represents the unoxidised equivalent of the late-stage mineralisation, which is likely to be refractory. In the geological model, sulphur in sulphide assays were used to define oxide, transitional and suphide interfaces.

Geochronology

Geochronological dating of lithology and alteration types is in progress. The granite horst to the east of the deposit has returned a date of 209 Ma. The following chronology has been therefore constructed from empirical relationships of magmatic, phreatomagmatic, alteration and mineralisation events observed in the core:

Date	Event	Empirical Relationship
Youngest Older	Hornblende Andesite	Cuts all lithologies, unaltered and unmineralised. Negative soil geochemistry
	Late stage Phreatomagmatic Diatreme Breccia	Clay-altered. Cuts dacite porphyry, not hornblende andesite. Muddy matrix with clasts of all lithologies except hornblende andesite. Unmineralized
	Main-Stage High Sulphidation Enargite Luzonite mineralization	In fractures, crackle breccias– also in dacite.
	Dacite porhyry	Late-stage felsic flow dome intruded into diatreme breccia column. Flow dome complex developed to the north.
	Hydrothermal brecciation	Contains low sulphidation veins and acid sulphate altered clasts.
	Low sulphidation veining and chalcedonic silica	Cuts and indurates the pervasive acid sulphate silicification
	Main-stage acid-sulphate alteration event	Strong to extreme acid leaching of early phreatomagmatic breccias and andesite units.
	Early Stage Phreatomagmatic Diatreme Breccia complex	Muddy matrix, clasts of mudstone, siltstones, sandstone and andesite. Shows facies variation in thicker sequences.
	Upper Mudstone/Siltstone
	Porphyritic andesite unit	Clear marker unit in the sedimentary sequence.
	Lower Siltstone/Sandstone unit
18 – 20 MA (fossils)	Carbonaceous siltstone / mudstone
	Basaltic Andesite	Uncertain age relationship.
209 MA	Granite

Acknowledgments

The authors would like to thank the management of P.T. Horas Nauli and Newmont Mining Corporation for the permission to publish this paper, to Brett Davies and Steve Turner for their major contribution and to John Hammond for his detailed review of the manuscript.

References

Davies A.G.S, Cooke D.R. and Gemmell J.B. 1999. Characteristics, Timing and Formation of Diatreme Breccias at the Kelian Gold Deposit, East Kalimantan, Indonesia.

Davies, B.M, 2002. Report on the Structural Review of the Martabe Project.

Internal Unpublished Report

Turner, S.J, 2002. Internal Memorandum.

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