Discovery Gold in Gosowong

.Journal of Geochemical Exploration 60 1998 207227

Discovery and exploration of the Gosowong epithermal golddeposit, Halmahera, Indonesia

J.C. Carlile a,), G.R. Davey, I. Kadir, R.P. Langmead, W.J. Rafferty ba PT Austindo Mining Corporation, 3rd floor Graha Irama, Jl. H.R. Rasuna Said Ka. 12, Jakarta 12950, Indonesia

b Newcrest Mining Limited, Leel 9, 600 St. Kilda Road, Melbourne, Victoria 3004, Australia

Received 3 June 1997; accepted 21 October 1997

Abstract

The Gosowong epithermal gold deposit, on the island of Halmahera in eastern Indonesia, is located in an area of primarytropical rain forest with no previous history of gold mining or record of gold mineralisation. The deposit occurs in a newlyrecognised mineral district which contains a number of epithermal vein systems and at least two centres of low-gradeporphyry style CuAu mineralisation. Several zones of argillic and advanced argillic alteration have been noted which maybe related to additional centres of mineralisation. Gosowong is classified as a low-sulphidation epithermal quartz vein.Bonanza-grade goldsilver mineralisation is developed in shoots over a 400-m strike section of the vein system. Three typesof veining are recognized; quartzadularia veins and breccias; quartzchloriteillite veins and breccias; and crystalline orchalcedonic quartz vein stockworks. The area was targetted using a simple geological concept and the deposit wasdiscovered and tested using basic exploration techniques commonly applied in the rugged tropical terrains of Indonesia.Sequential exploration methods comprised reconnaissance drainage sampling of stream sediment, BLEG and float media,ridge and spur soil sampling, prospect scale grid soil sampling, hand trenching and diamond drilling. Lapse time fromidentification of the initial reconnaissance anomaly to an inferred resource estimate of almost 1 million ounces of gold wasless than 3.5 years. This case history illustrates that very detailed exploration is necessary to locate high-grade vein-type golddeposits in a tropical environment, but demonstrates that such resources still remain to be discovered in the relativelyunder-explored Neogene magmatic arcs of Indonesia. q 1998 Elsevier Science B.V.

Keywords: epithermal; gold; discovery; vein; quartzadularia; Gosowong

1. Introduction

Gosowong is a grass roots discovery in an areaof moderately incised primary tropical rain forestwith no previous history of gold mining or record ofgold mineralisation.

) Corresponding author.

The deposit is located on the north arm of Halma-hera island, in the province of Maluku in easternIndonesia approximately 15 km inland from the

X X .coastal town of Malifut 1809.05 N, 127841.65 E .Gosowong is one of four major high-grade veindeposits in Indonesia. Other examples includeLebong Donok and Lebong Tandai in Sumatra, bothdiscovered and mined by the Dutch between 1900and 1940, and the Gunung Pongkor deposit in West

0375-6742r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .PII S0375-6742 97 00048-4

( )J.C. Carlile et al.rJournal of Geochemical Exploration 60 1998 207227208

Java which was discovered by PT. Aneka Tambang .Persero in 1988 and commenced production in

.1994 Fig. 1 .Ternate, the district capital for the Gosowong

project, is serviced by daily flights from Jakarta viaManado. From Ternate, the site is reached by aone-hour ferry trip to the mainland, a two-hour driveon the sealed coastal road and then one hour on footalong logging tracks. In the eastern Indonesian con-text, access and exploration logistics are relativelystraightforward.

The deposit is contained within the sixth genera-tion Contract of Work of PT. Nusa Halmahera Min-erals which is a company jointly owned by NewcrestMining Limited of Australia and PT. Aneka Tam-

.bang Persero , the Indonesian state-owned miningenterprise.

This paper summarises the vein characteristicsand its discovery history and outlines explorationmethodology and geochemical signatures of the de-posit.

2. Regional setting

Gosowong occurs within the Halmahera Neogene .magmatic arc Sukamto et al., 1981 that extends for

approximately 350 km from Bacan island in thesouth through the north arm of Halmahera to Moro-

.tai in the north Fig. 1 . The northern portion of thearc that hosts Gosowong is thought to be constructedon crust of intermediate composition formed fromoverlapping Upper Cretaceous to Miocene arc se-

.quences Hall et al., 1988 . Rocks of the Neogenearc comprise andesitic intrusions and eruptive rocksand sediments. A chain of active volcanoes forms theislands west of Halmahera and the belt of activevolcanism cuts across the mainland a few kilometresto the north of the Gosowong district.

Rocks in the district comprise mainly andesitic todacitic submarine lavas and volcaniclastics with mi-nor interbedded tuffs of probable Pliocene age. Thevolcanic sequence is locally intruded by porphyriticandesite and quartz diorites, and is partly covered byRecent ash flow and pumice tuffs. Interpretation ofLandsat imagery suggests that the Gosowong veinsystem lies near the southern margin of a circularfeature 34 km in diameter where it is intersected by

NNE and NW structures that may have acted as .conduits for mineralisation Fig. 2 .

Two broad styles of mineralisation are recognisedin the Gosowong district. Low-grade porphyry stylecoppergold mineralisation centred on porphyriticquartz diorites has been identified at Tobobo andBora, 2.5 km WNW and 2.5 km WSW of Gosowong,respectively. Aside from Gosowong, quartz veiningwith localised bonanza gold grades has been noted atToguraci where it is partially emplaced within theBora porphyry. A zone of quartz veining and silicifi-cation occurs at Ruwait, 1.5 km northeast ofGosowong, and several areas of advanced argillicalteration occur at higher elevations in the district;these and a number of other targets in the districtremain to be tested.

3. The Gosowong vein

The Gosowong deposit is classified as a low-sulphidation epithermal quartz vein system as de-

.fined by Hedenquist 1987 .Host rocks comprise volcaniclastic sediments,

ranging from fine-grained siltstones to pebbly sand-stones and conglomerates, lava flows, breccias andlapilli tuffs. Outcrops of ignimbrite occur in thesouthern part of the mineralised zone and ignimbritehas also been recorded in drill holes. The northernportion of the vein system is covered by Recent tuffs .Fig. 3 .

Gosowong has a northsouth strike length inexcess of 1.5 km with dips varying between 708Eand 358E, becoming shallower with depth. Bonanza-grade gold and silver mineralisation occurs in themain Gosowong vein and in a smaller hanging-wall

.vein in a series of shoots Fig. 4 . These bonanza-grade shoots occupy a 400-m section in the middleportion of the system that sub-crops on the easternflank of a small ridge extending southwards from the

.Tobobo river Fig. 3 .The Gosowong vein is up to 30 m wide at the

surface and has been intersected in drill holes todepths of over 200 m below surface equivalent to300 m down-dip. The vein structure continues for atleast a further 600 m to the south of the bonanzazone but grades are much lower. To the north, gradesare also low within a 3040-m-wide zone of quartz

( )J.C. Carlile et al.rJournal of Geochemical Exploration 60 1998 207227 209

Fig. 1. Location map of Gosowong superimposed on a simplified geological map of Halmahera.


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stockworking that persists for 150 m to the Toboboriver and then continues beneath Recent cover to thenorth of the river for at least 350 m.

Three styles of veining are recognised within thedeposit; these include quartzadularia veins andbreccias, quartzchloriteillite veins and breccias

Fig. 3. Gosowong surface and subcrop geology showing distribution of vein types. Cross-section 10,150N and 10,275N are presented in Fig.4.


. .Fig. 4. Cross-sections of the Gosowong vein showing drill intersections: A section 10,150N; B section 10,275N. Drill results arepresented in metres at gram Au per ton, gram Ag per ton.


and low-grade crystalline and chalcedonic quartz .stockworks Figs. 3 and 5 .

Massive banded adularia-rich quartz veining andvein breccias form the upper parts of the southern

portion of the deposit. The adularia-rich zone isapproximately 120 m long, 75 m in down-dip extentand up to 30 m wide with individual veins up to 11m wide. Vein material is white and chalky due to the

Fig. 5. Longitudinal grade vs. metres of contour sections of the high-grade part of the Gosowong vein. Approximate distribution of veintypes is also shown.


high adularia content, and readily breaks down withsurface weathering. Gold grades are consistentlyhigh, averaging around 60 grt Au with a gold tosilver ratio of around 2 : 1.

Semi-massive as well as stockwork type greenishquartzchloriteillite veins and vein breccias occurbelow the quartzadularia zone in the south, andextend northward for the length of the deposit to-wards the Tobobo river and merge with lower-gradecrystalline quartz stockwork mineralisation. Goldgrades are highly erratic but average around 30 grtAu. Gold to silver ratios are generally lower than inthe adularia-rich zone and decrease to the northranging from -1 : 1 to 1 : 10.

Low-grade crystalline and chalcedonic quartz veinstockworks envelop and are interspersed with thehigher-grade vein types. Gold grades vary between 1and 10 grt and the gold to silver ratio is highlyvariable. This style of mineralisation is visually in-distinguishable from poorly mineralised quartzstockworking which contains -1 grt Au and makesup the rest of the deposit.

Base metals, arsenic and antimony are low .throughout the system Table 1 although copper,

lead and zinc show a slight increase with depth andto the north, coincident with the transition fromdominantly quartzadularia veining to quartzchlo-riteillite veining. Copper and lead values are slightlyelevated in the vein and zinc is slightly depletedrelative to wallrock.

Argillic alteration extends up to 30 m into thehanging wall but is more restricted in the footwall,giving way over a few metres to a propylitic assem-blage. In the northern part of the vein zone theargillic envelope is limited to within metres or cen-timetres of the vein.

Petrological studies indicate that gold in the adu-laria-rich veining is finely dispersed and intergrownwith adularia, or with quartz intergrown with adu-laria, as small grains, generally less than 20 mm.Visible gold is occasionally seen in drill cores ofquartzchloriteillite veining, where it occurs as finedisseminations in sulphide-rich bands. Pyrite contentis low, generally less than 2%, and minor argentiteand rare gold tellurides have also been noted inpolished sections.

Limited fluid inclusion studies indicate formationtemperatures of 2002408C in the adularia-rich zone

Table 1Vein and wallrock geochemistry of the Gosowong vein

. . . .Element No. of samples Mean ppm Min ppm Max ppm Std. dev. ppm

VeinAu 127 32.2 1.05 1420 133Ag 127 57 -1 500 101Cu 127 153 4 2240 265Pb 127 70 -4 517 77Zn 127 80 5 820 89As 127 11 -2 49 8Sb 127 3 -4 9 1.8

WallrockAu 1071 0.11 -0.005 0.998 0.18Ag 1071 1 -1 22 1.7Cu 1071 128 3 1790 121Pb 1071 20 -4 1050 53Zn 1071 99 5 1210 76As 1071 13 -2 297 14Sb 1071 3 -4 13 1.5

Data included are derived from the first 20 drill holes.


and the presence of common vapour-rich fluid inclu-sions suggests boiling as the dominant mechanism ofgold deposition.

4. Exploration history

4.1. Preious exploration

Previous exploration in Halmahera was conducted .by PT. Rio Tinto Bethlehem Indonesia RTZ in

1975, by the Geological Survey of IndonesiarFederalInstitute for Geosciences and Natural Resources of

.West Germany GSI in the late 1970s, and by PT. .Citra Maluku Mining CSR between 1985 and 1987.

Both RTZ and CSR carried out reconnaissancesampling in the general Gosowong area. The Toboboriver, which drains the Gosowong deposit, was sam-pled for stream sediment by RTZ, who were target-ting a major outcropping porphyry copper deposit.They did not assay for gold. CSR obtained BLEGanomalies of 34 ppb Au from a number of streamsin the area but did no follow-up on these results andceased exploration in the Halmahera area shortlythereafter. The GSI subsequently discovered Kaputu-san, a coppergold porphyry on Bacan, and outlineda resource of 77 Mt grading 0.25 grt Au and 0.33%Cu; they did not sample the Gosowong district.

4.2. Area selection

Newcrest Mining Ltd., following a review of thegeology and mineralisation of Indonesia in 1991,demonstrated that all the known gold and coppergold deposits of any significance in Indonesia occur

within Neogene magmatic arcs Carlile and Mitchell,.1994 . Six such arcs were identified, of which the

Halmahera arc was targetted as of high priority.Those of PlioPleistocene age were interpreted to

be most prospective for epithermal deposits, particu-larly where PlioPleistocene volcano-sedimentaryrocks occur immediately adjacent to Quaternary orRecent volcanic cover rocks. The presence of theyounger cover rocks was interpreted to indicate thatdeposits, which formed near the syn-mineral surfaceat destructive plate boundaries, where rapid upliftand erosion occur, would have better chance of beingpreserved.

The Halmahera arc fulfilled all of the abovecriteria and also provided a unique opportunity toacquire title to an entire and relatively under-ex-plored prospective arc. Newcrest began initial explo-ration work in 1992.

4.3. Target generation

The chronology of the discovery and explorationof Gosowong and the surrounding district togetherwith selected key results are summarised in Table 2.

First pass drainage reconnaissance commenced inAugust 1992 and within three months had identifiedthe Tobobo and Bora catchments as anomalous ingold. Follow-up in-fill drainage sampling and map-ping and subsequent ridge and spur soil samplinglocated outcropping coppergold porphyry-stylemineralisation in both drainages some four monthslater in March 1993.

The first outcrop of the Gosowong system wasdiscovered six months later in September 1993 aspart of the Tobobo programme. During sampling ofareas adjacent to the porphyry, a zone of gold-bearingquartz stockworking was located 1.8 km downstreamfrom the Tobobo porphyry. This site subsequentlybecame known as the discovery outcrop. The Kwarsariver anomaly reflecting the Gosowong vein, and theRuwait vein system were discovered shortly there-after, but exploration remained focused on the cop-pergold porphyries for the remainder of 1993.

4.4. Target delineation

Prospect work was started at Gosowong in early1994. Ridge and spur soil and float sampling, pro-gressed outward from the discovery outcrop andpicked up the margins of subcropping high-grademineralisation. Grid soil and float sampling followedimmediately and identified a gold in soil anomalywith a northsouth strike length of over 250 m thatincluded some spectacular values both in soils andfloat.

Bonanza grades were returned in a programme ofhand trenching after three months. Drilling startedten months later after the vein had been exposed intrenches for a strike length of 350 m. The first drillhole GSD-1 returned a spectacular intersection of 18m with 125.7 grt Au and 73 grt Ag.


Table 2Chronology of discovery and exploration of GosowongDate Event

Aug. 92 Start of drainage reconnaissance on the north arm of Halmahera.Nov. 92 Discovery of drainage anomaly in S. Tobobo and S. Bora.

. . .Tobobo: BLEG 1.0 ppb Au , stream sediment 0.56 ppm Au , float 0.73 grt Au . . . .Bora: BLEG 9.1 ppb Au , stream sediment 0.44 ppm Au , float 3.16 grt Au .

Dec. 92 Start of detailed drainage sampling and mapping with subsequent follow-up ridge and spur soil sampling.Mar. 93 Discovery of porphyry CuAu mineralization in outcrop at Tobobo and Bora.Jun. 93 Start of prospect scale work at Tobobo, Bora and adjacent areas.Sep. 93 Discovery of quartz vein zone in outcrop in S. Tobobo, The Discovery Outcrop.

selective chip sample: 20 m at 4.51 grt Au.Oct. 93 Re-sampling of Discovery Outcrop

channel sample: 20 m at 0.68 grt Au and 8.0 grt Ag.Nov. 93 Discovery of Kwarsa river anomaly.

stream sediment: 4.06 ppm Au, pan conc.: 81 colours with 9 of )0.5 mm. .Discovery of Ruwait vein system

Feb. 94 Start of ridge and spur and subsequent grid soil sampling at Gosowong.Mar. 94 Discovery of soil anomaly and high-grade float at Gosowong.

.soil anomaly: 250=60 m at 0.1 ppm Au max. 6.9 ppm Au . float evidence: 38.3 grt Au and 74.6 grt Au.

Apr. 94 Start of hand trenching at Gosowong.Jun. 94 Discovery of bonanza grades in trenches at Gosowong.

15 m at 40.8 grt Au and 10 grt Ag.28 m at 50.7 grt Au and 18 grt Ag.

. .Aug. 94 Discovery of Toguraci vein system .Apr. 95 Start of drilling at Gosowong.May. 95 Discovery of bonanza grades in drill hole GSD-1 at Gosowong.

18 m at 125.7 grt Au and 73 grt Ag.Mar. 96 Inferred resource.

1.0 mt at 29 grt Au and 45 grt Ag containing 960,000 oz Au.

Ten months later, Newcrest announced an in-ferred uncut resource of 1 million tonnes at 29 grtAu and 45 grt Ag containing just under 1 millionounces of gold in March 1996. This confirmedGosowong as a major new discovery.

5. Exploration methodology

The exploration techniques used in the discoveryof Gosowong are those commonly applied in Indone-sia in the search for gold and coppergold minerali-

sation in rugged tropical terrains Carlile et al., 1990;.van Leeuwen, 1994 .

5.1. Drainage sampling

First pass drainage sampling was conducted usingcoastal vessels, vehicles or on foot. Stream sediment,

.bulk leach extractable gold BLEG and float sam-ples were taken in major drainages near the coastabove tidal influence andror where streams crossedroads and tracks.

Second pass stream sediment samples followed onfirst pass anomalies and were collected every 1 kmalong first-order drainages and above the flood limitat the mouth of second-order streams. At each site 20l of y1r8 inch sediment were collected from theactive channel, wet-sieved through an 80 mesh nylonsieve and bagged in kraft sample bags for dispatch.

Table 3 lists the analytical suite for all samplemedia and the applicable detection limits. Gold was


Table 3Sample media, analytical elemental suites and detection limitsSample media Au Ag Cu Pb Zn As Sb

Pan conc. nra nra nra nra nra nra nra .BLEG ppb 0.01 nra nra nra nra nra nra

Stream sediment 0.005 nra 210000 44000 210000 210000 nraSoil 0.02 nra 210000 44000 210000 210000 nraFloat 0.005 1100 210000 44000 210000 210000 410000Trench 0.005 1100 210000 44000 210000 210000 410000Drill core 0.005 1100 210000 44000 210000 210000 410000

All detection limits are in ppm with the exception of BLEG which is in ppb.

analysed by fire assay using a 50 g charge with AASfinish. Samples were also routinely assayed by AASfollowing a perchloric acid digest and secondary

hydrochloric acid leach for copper, lead and zinc.Arsenic was assayed by XRF.

At each sample site a further 20 l of y1r8 inch

Table 4Surface sampling statistics for the Gosowong district

. . . .Element No. of samples Mean ppm Min ppm Max ppm Std. dev. ppm

BLEGAu 16 4.2 1 23 5.2Stream sedimentAu 123 0.12 -0.005 4.06 0.41Cu 121 72.8 11 248 40.4Pb 119 13.4 -4 37 6Zn 118 98.2 33 192 32.3As 116 9.1 -2 20 4.9SoilAu 266 2.59 -0.02 109 11Cu 266 94 -2 513 75Pb 266 22 -4 616 44Zn 266 55 5 191 30As 266 13 -2 146 13VeintrenchAu 250 28.6 1 678 65.1Ag 250 16.7 -1 410 46.4Cu 250 49 3 319 52Pb 250 67 -4 849 114Zn 250 27 -2 1190 108As 250 17 -2 317 46Sb 87 3 -4 8 2WallrocktrenchAu 377 0.29 -0.005 0.99 0.26Ag 377 1 -1 46 3.15Cu 377 94 4 1730 114Pb 377 41 6 450 59Zn 377 32 2 281 38As 377 22 -2 267 34Sb 81 2.5 -4 6 1


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sediment was panned using a wooden dulang a.traditional Indonesian gold pan , examined for heavy

minerals and gold, and grain counts were recorded.Samples were not assayed but were stored for futureexamination and analysis.

Rock float, if it appeared to be altered or miner-alised, was also carefully examined, sampled anddispatched for assay. Samples were split, and as-sayed using similar methods as for stream sediments,but with an additional silver and antimony analysesby AAS and XRF, respectively. Sample residues andhand specimens were stored for reference purposes.

BLEG samples were taken only along major riversat some of the stream sediment sites. At each site 3kg of y1r8 inch active sediment was collectedtaking care not to loose fines but flocculent was notused in the initial survey. Total samples were roll-leached for 24 h with DIBK extraction andAASrcarbon rod finish.

After the discovery of the Gosowong vein anorientation survey was undertaken to help guide theexploration elsewhere. Stream sediment and BLEGsamples were taken in the same way as describedabove at regular 200-m intervals for 2 km down-stream and 500 m upstream of the vein.

5.2. Soil sampling

All soil samples in the Gosowong area were takenfrom the lower B horizon which ordinarily is be-tween 30 and 40 cm below surface.

Single point samples were taken at 25-m intervalsalong ridge crests with cross lines every 100 m alongwhich samples were taken at 10-m intervals.

Grid soil sampling was conducted at 25-m gridintervals. Three samples were taken at each site, oneat the 25-m interval and one sample 6 m on eachside of the 25 m point along the line. Samples werecomposed and results plotted at the 25-m point withthe objective of maximising coverage, minimisingany nugget effect and reducing laboratory costs.

Gold analysis was conducted on 50-g sampleswith a hot aqua regia digest and AAS finish. Copper,lead, zinc and arsenic were routinely analysed usingthe same methodology as for stream sediments.

5.3. Outcrop and drill sampling

All channel samples in trenches were cut using aportable diamond saw with a sample interval of 1 m

or less within the vein and 2 m in wallrock. Channeldimensions approximated the NQ diameter core sothe results could reliably be incorporated in resourcecalculations.

The vein was initially drilled at 50 m centres andthen drilled out at 25 m centres with 12.5 m in-fill in

particularly high-grade zones. The drill core HQ and.NQ was halved on site using a diamond saw and

sampled at 1-m intervals in the vein, or less at thevein margins, and at 2-m intervals in wallrock. Allcore intervals were sampled in the first 20 drill holesbut for the remainder of the program only mineral-ized intervals and selected wallrock intervals weresampled. All core splits were stored for referencepurposes.

Very high-grade mineralisation caused obviousconcerns about the potential for over-estimation ofthe grade. Pulps of all vein samples for the firstseries of 50-m spaced drill holes were split anddispatched to two laboratories for duplicate analyses.The correlations of results between the laboratoriesand within each laboratory were greater than 90%.

6. Exploration signatures

Surface sampling statistics for the district aresummarised in Table 4. The reconnaissance samplesthat identified the Tobobo catchment that containsGosowong were taken early in the programme andthe vein had been located in the outcrop before thereconnaissance programme was complete. Statisticalanalysis was therefore not applied in picking theinitial anomaly.

6.1. Drainage results

The first stream site to return anomalous goldvalues was located in the Tobobo river, 6.5 kmdownstream of Gosowong. The results from thestream sediment samples showed 0.56 grt Au in they80q200 mesh fraction and 0.13 grt Au in the

.y200 mesh fraction Fig. 6 , but the BLEG sampletaken at the same site showed a result of only 1.0

.ppb Au Fig. 7 . Four out of 25 float samplesreturned )0.1 grt Au, with a maximum assay of0.73 grt Au from a sample of quartz vein breccia.No visible gold was recorded in the panned concen-trate.


A stream sediment sample taken in the smallsecond-order Kwarsa tributary that runs parallel andimmediately adjacent to the southern part of the veinand is only 1 m wide at its mouth, returned 4.06 grtAu. In the pan, 79 gold colours were counted, in-cluding 9 of )0.5 mm, and numerous float samplesof quartzadularia vein material were noted. Onesample of the vein float taken from the Kwarsa rivershowed a value of 74.6 grt Au.

Second order streams draining the northern part ofthe vein zone also returned encouraging values of0.9, 0.65 and 0.2 grt Au in stream sediment but onlyminor amounts of fine gold were visible in thepanned concentrates.

Stream sediment samples collected for orientationin the Tobobo river after the discovery of mineralisa-tion returned only one anomalous result of 0.2 grt

.Au Fig. 6 , 1 km downstream of the vein, a lowervalue than that obtained 6.5 km downstream at theoriginal reconnaissance site. Two anomalous BLEGorientation results of 5.1 ppb Au and 5.2 ppb Au andone highly anomalous result of 23 ppb Au werefound for samples taken between 1.2 and 1.7 km

.downstream of the vein Fig. 7 . Other BLEG resultsaveraged 2.3 ppb Au, representing a low-orderanomaly. No high-grade vein float samples werefound in the Tobobo river during the orientationsurvey.

Copper, lead, zinc and arsenic were all at back-ground levels in stream sediments at the originalreconnaissance site as well as in closely spacedfollow up sampling in both first- and second-order

.drainages adjacent to Gosowong Fig. 8 . Arsenicwas slightly elevated in stream sediments in the area

.to the north of the vein Fig. 8D . Elsewhere in thedistrict, copper, lead and to a lesser extent zinc

.values in stream sediments Fig. 8A, B are elevatedin the vicinity of the Bora porphyry. In the streamsediment orientation survey after the discovery, thezinc value was anomalous both upstream and down-stream of the Gosowong vein.

6.2. Soil results

Gold in the soils is highly anomalous and clearly .outlines the vein Fig. 9 . Initial ridge and spur soil

sampling returned a maximum gold assay of 6.88grt Au and located the margin of the subcroppingvein zone. Grid soil sampling outlined two highly

anomalous gold zones of )10 grt Au, which corre-spond with the south and central high grade shoots,subsequently delineated in trenches and drilling.These were contained within a dispersion halo ap-proximately 100 m wide of )0.1 grt Au. Themaximum grid soil gold assay was 109 grt Au.

Copper and zinc are depleted in soils over the .vein zone Fig. 10A, C and for nearly 300 m to the

south of the southern limit of the soil gold anomaly.Typical values are generally less than 100 ppm and50 ppm, respectively. Lead shows three spot highsalong the vein with the strongest bulls-eye anomalycoincident with the northern section of the quartzchloriteillite veining. Arsenic values show no varia-tion against background, and no trend related to

.veining Fig. 10D .A Recent tuff cover completely masks any geo-

chemical signature of the vein in soils as demon-strated by the lack of gold in soils just north of the

.Tobobo river Fig. 9 .

6.3. Outcrop and drill results

Selective chip sampling of the veinlets at thediscovery outcrop, a 20-m-wide zone of quartz vein-ing exposed in the Tobobo river, returned 4.51 grtAu. A continuous channel cut over the same zoneusing a diamond rock saw returned 20 m with 0.68

.grt Au Fig. 11 .Channel samples cut with a portable rock saw

perpendicularly to the vein in trenches confirmed thetwo bonanza zones in the southern and central min-eralised parts of the vein. The bonanza zones are 120m and 40 m in strike length at surface, separated byapproximately 100 m of lower-grade chalcedonicquartz veining and flanked to the north by a 120-m

.quartz vein stockwork Fig. 3 . The first bonanzagrades of 15 m with 40.8 grt Au and 10 grt Ag,and 28 m with 50.7 grt Au and 18 grt Ag werereturned from the southern high-grade zone of

.quartzadularia veining Fig. 11 .A comparison of vein and wallrock results from

.trench sampling Table 4 shows that copper is rela-tively depleted and lead is enriched at the surface inthe vein. Zinc, arsenic and antimony values show nosignificant difference between vein and wallrock.

The first drill hole, GSD-1, was drilled at a 458declination to the west to intersect the vein at theperpendicular on section 10,150N approximately 50

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. . . .Fig. 8. Stream sediment sampling results for A copper, B lead, C zinc, and D arsenic for the Gosowong district.


m below the first high-grade trench intersection.GSD-1 returned 18 m with 125.7 grt Au and 73 grtAg from the quartzadularia veins. GSD-2 wasdrilled to intersect the vein 50 m below GSD-1 and

confirmed the down-dip extent of the vein but re-turned disappointing results of 2 m with 3.1 grt Auand 8 grt Ag, and 5 m with 2.19 grt Au and 13 grtAg. GSD-3 intersected quartzchloriteillite veins

Fig. 9. Gold in B horizon soils over the Gosowong vein. Contours are in ppm.


. . . .Fig. 10. Copper A , lead B , zinc C and arsenic D in B horizon soils over the Gosowong vein. Contours are in ppm.


and breccias 50 m further down-dip of GSD-2 on thesame section and returned an encouraging result of 7m with 19.6 grt Au and 26 grt Ag confirming the

.discovery Fig. 4 . Subsequent drilling demonstrateda 400-m strike length for the high-grade zone and

.has also delineated a hanging-wall vein Fig. 4B .

Fig. 11. Trench sample results for gold and silver in the Gosowong vein. Results are presented in metres at gram Au per ton, gram Ag perton.


7. Conclusions

Gosowong was discovered using standard explo-ration methods in an area with no previously knowngold mineralisation. The district was targetted usinga simple geological concept. The Neogene volcano-sedimentary rocks were the right age to host exposedepithermal mineralisation. In Indonesia, near activedestructive plate margins, Neogene rocks, and partic-ularly those of PlioPleistocene age, represent aprospective geological window.

The discovery history, exploration methodologyand results highlight a number of aspects relevant toexploration for low-sulphidation epithermal veinmineralisation in a tropical environment.

Anomalous gold in stream sediment and floateffectively located the mineralised catchment area.The multiple mineralisation sources in the Tobobocatchment include the Tobobo porphyry and theRuwait vein system as well as the Gosowong vein, Itis therefore not possible to definitively attribute thereconnaissance anomaly, or the subsequent BLEGresults, to the Gosowong vein.

The Tobobo porphyry was found first andGosowong was first discovered in outcrop during themapping and sampling of areas adjacent to the por-phyry.

The vein is readily detectable in the smallsecond-order drainage of the Kwarsa river, in allsample media including stream sediments, pan con-centrates and float. The Kwarsa river is immediatelyadjacent to the subcropping vein.

Despite its high surface grades, Gosowong is notreadily detectable near its source in the first-orderTobobo drainage which cuts the vein. The Toboboriver in the vicinity of the vein is constricted and fastflowing. The lack of a clear gold anomaly in proxi-mal stream sediments is probably due to regular andvigorous flushing out of gold by floods, compoundedby dilution from sediment brought down from thelarge upstream catchment area.

The site of the original reconnaissance anomaly is6.5 km downstream of the vein and occurs at a slopebreak where the stream course widens and the flowrate slows down. The stream sediment anomaly atthis location may therefore be explained by goldsettling out due to the change of conditions.

Orientation BLEG results obtained subsequent tothe discovery detected the vein, but only within 2 km

of the source; the reconnaissance BLEG sample wasnot anomalous. In the case of Gosowong, the initialBLEG sampling was not effective as a low-densityreconnaissance method. It should however be notedthat flocculent was not used and fine gold may havebeen lost.

No high-grade quartzadularia float has beenrecorded in the Tobobo drainage. This is probablyexplained by the association of outcropping high-grade gold mineralisation with a high adularia con-tent. Adularia readily weathers in the surface envi-ronment and breaks down during transport. There-fore, the high-grade float does not persist beyond theproximal second-order tributary into the main river.A lower-grade crystalline quartz float that is moreresistant does persist to the first order drainage and,in the case of Gosowong, proved to be an effectivepathfinder for adjacent high-grade quartzadulariaveining.

At a prospect level, ridge and spur sampling andgrid sampling of B horizon soils for gold proved tobe a very effective method for defining a subcropand even for picking subcropping high-grade veinshoots.

Copper, lead, zinc and arsenic were of little prac-tical use as pathfinders either in drainage or soilsampling, which reflects their low content in thevein.

The time lag between the geological concept andthe reconnaissance anomaly was 14 months, the timebetween the reconnaissance anomaly and the discov-ery drill hole was a further 30 months, and betweenthe discovery drill hole and the finding of a justunder 1 million ounce resource was 10 months.

The Gosowong case history demonstrates thatsubcropping high-grade vein deposits still remain tobe discovered, but clearly illustrates that very de-tailed exploration is required to locate such depositsin the tropical environment of Indonesia.

The potential for further discoveries of this typein the relatively under-explored Neogene arcs ofIndonesia is regarded as high.

Acknowledgements

As with any discovery, Gosowong was the resultof a team effort. The authors were all closely in-volved with the discovery. J.C. Carlile targetted the


district, secured the tenements and directed the ex-ploration program, W.J. Rafferty led the reconnais-sance survey that defined the initial anomaly and I.Kadir first found and sampled the discovery outcrop.G.R. Davey led the project through the semi-detailedexploration phase and R.P. Langmead was responsi-ble as the site geologist for delineating and detailedtesting of the deposit. Early field work by VivianPreston and field assistants Mursani and Hermanto isalso acknowledged. Senior management of NewcrestMining Limited and PT. Aneka Tambang stronglysupported the exploration effort and their supportand encouragement together with permission to pub-lish this paper are gratefully acknowledged. The helpof David Brame and Irsam who produced the dia-grams, Roosyanti and Ivonne Tumbelaka who typedand organised the manuscript, and Mike Andrewsand Colin Davies who critically reviewed the variousdrafts and provided many helpful suggestions arealso gratefully acknowledged.

References

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Carlile, J.C., Digdowirogo, S., Darius, K., 1990. Geological set-ting, characteristics and regional exploration for gold in thevolcanic arcs of North Sulawesi, Indonesia. In: Hedenquist,

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Discovery Gold in Gosowong

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