Departmentof Earth Sciences, University of Waikato, Hamilton, New Zealand
Core samples that were collected from the Busangprospect, East Kalimantan, reflect a progressive evolution of a largemagmatic-related hydrothermal system that is comparable to that encountered inmany similar systems elsewhere in the Pacific region. The initial, localizeddeposition of porphyry quartz-molybdenite veins took place at high temperaturesand salinity at deep levels. This was followed by an extensive phase of phyllic(sericite-quartz-pyrite ± tourmaline-apatite) wallrockreplacement and vein formation. Propylitic (epidote-chlorite-carbonate-quartz) alterationwas formed marginal to the phyllic assemblages. These porphyry-related eventswere centred in the Southeast Zone, and are postulated to be associated withthe emplacement of a felsic intrusion at depth.
The early stages of hydrothermal activity werefollowed by an episode of quartz (± adularia) carbonate basemetal sulphide veining, that was accompanied by argillic (illitic-kaolin clay)wallrock alteration. This event was sulphide-rich in the Central Zone andcarbonate-dominated in the Southeast Zone. Gold mineralisation was observed inCentral Zone core to be associated with both sulphide, as well as carbonate,deposition.
A final stage of epithermal-style quartz ±stibnite realgar veins are, in the Central Zone, locally gold-bearing. Thelatter two events are interpreted to be associated with the late-stageexsolution of metal-bearing brines from the felsic intrusion that formed theearlier porphyry quartz-molybdenite veins and widespread phyllic-propyliticalteration.
This paper is a summary of the results of apetrological study that was carried out on a suite of one hundred andtwenty-nine core samples that were collected by the author while on site at theBusang Prospect, East Kalimantan, during April, 1997.
Samples were selected mainly from 10cm long skeletoncore that was stored on site, although whole core samples were also collectedfrom three holes that had not yet been sampled for assaying. A suite offorty-seven samples were collected from twenty-three holes that were drilled inthe Central Zone, and fifty-three samples were selected from six holes in theSoutheast Zone. The remaining twenty-nine samples were selected from DrillholeBDH 5, which was a 980m deep hole drilled under the Southeast Zone.
There has been a considerable amount of factuallyincorrect information made available over the last four years on thecharacteristics of the Busang area. It is hoped that this paper will present asound scientific approach that may help to balance out this lack of basic data.
The Busang prospect is located in East Kalimantan, approximately 200km northof the coastal city of Samarinda, The hydrothermal system at Busang has beenlocalised at the intersection of the NE-trending Kalimantan suture and NW-strikingtransfer structures. A similar tectonic setting hosts gold mineralisation atthe Kelian gold mine approximately 150km to the southwest of Busang, at the Mt. Muro gold mine, as well as othermajor gold prospects in Kalimantan such as Muyup, Masupa Ria,Miwah and Gunung Mas (van Leeuwen et al, 1990).
A 1km x 500m wide zone of alteration in the CentralZone (CZ) at Busang is thought to have been the focus of dilation ofpre-existing EW fractures by dextral movement on the transfer structures. NW-trending alteration in the Southeast Zone(SEZ) extends over a strike length of more than 3km and is postulated also tobe related to movement on these transfer structures.
Alteration and mineralisationat Busang are hosted in a series of polyphasal porphyry intrusions that aredominantly dacite in composition in the Central Zone, and andesite in theSoutheast Zone. These porphyry bodies have been emplaced into a sequence ofintercalated carbonaceous sandstones and siltstones. Late stage flow bandedfeldspar porphyry dykes cut the dacite/andesite porphyry intrusions and resultin visually sharp alteration contrasts where they cut previously alteredhostrocks.
Diorite porphyry intrusionshave been intersected at depth beneath the Southeast Zone and are inferred tobe the deeper equivalents of the shallower level andesite porphyry bodies.
Basic sub-volcanic dykes crosscut the andesite /dacite intrusions and range from basaltic andesite to basalt in composition.Some of the basic dykes are pre-mineral, whereas others are post-mineral. Late,but pre-mineral, rhyolite dykes have been previously recorded, but were notobserved in the cores analysed during this study.
Intrusion, hydromagmatic, fluidised andvein/dilational breccias are common in the core collected for this study.
Very similar lithologies and breccias are host tomineralisation in the Kelian deposit (van Leeuwen et al., 1990).
Three main stages of hydrothermal alteration(replacement and deposition) have been recognised at Busang (Figure 1). Itpostulated that these events are associated with the same overall hydrothermalsystem that evolved with time.
This stage of hydrothermal activity is characterizedby an initial phase of porphyry-style quartz vein development, followed by anepisode of phyllic and propylitic alteration and veining. Theseporphyry-related assemblages are most extensively developed in the drillcorefrom the Southeast Zone
In the deep drillhole BDH 5, centimeter wide,sheeted to conjugate fracture sets host porphyry quartz ±anhydrite veins at depths of 500-700m beneath the central part of the SoutheastZone. These veins are characteristically grey due to the presence of abundant primary and secondaryliquid- and vapour-rich inclusions.
Halite daughter crystals are present in some of theliquid-rich inclusions and these are indicative of periods when the fluids werehypersaline (>25 wt% equivalent NaCl). The deposition of theporphyry-related quartz veins was polyphasal, and locally extended to veryshallow levels.
Anhydrite occurs as intergrowths with, andinclusions in some of the porphyry quartz veins, although in most casesanhydrite deposition post-dates the quartz.
Extensive zones of porphyry-related, propylitic andintense phyllic alteration occur over an area in excess of 700m x 3.5km and to depths of more than 400m in the Southeast Zone, and arealigned along the NE-trending transfer structural zone.
The phyllic assemblage is dominated bycoarse-grained 2M sericite (and locally muscovite) + quartz + pyrite. A purpleanhydrite locally overgrows porphyry quartz in veins and is in turn overgrownby sericite. Dark blue-green tourmaline (schorl) is commonly associated withthe quartz-sericite-pyrite wallrock alteration and overgrows sericite in veins.It is also associated with late dolomite-calcite deposition. Apatite occurs asminute grains with the phyllic alteration assemblage and commonly replaceswallrock mafic phenocrysts. .
The propylitic alteration / veining is characterizedby the presence of epidote + quartz + chlorite + carbonate ±sericite and is peripheral to the phyllic alteration zones.
Fine grained milled matrix (fluidized) brecciaslocally cross-cut the porphyry-related quartz veins and are a pre-cursor to thelater carbonate-base metal system. In places these breccias contain clasts ofearlier quartz vein material. The clasts are sealed in a comminuted matrix thatis altered to illitic clay and/or sericite ± quartz carbonate pyrite.It is speculated that these breccias may be related to phreatomagmatic(diatreme) events. A compilation of field mapping and drill core logging isnecessary in order to fully evaluate the presence of a diatreme-maar complex atBusang, and this lies outside the scope of this study.
Sheeted carbonate base metal veins occur in the Central Zone to the north of the porphyry system, and are inferred to be genetically related to the dextral rotation on the NE-trending accretionary structures. Inout crop, it was observed that the carbonate-base metal sulphide assemblages were also deposited alongthe fractured and brecciated contacts between the high level dacite intrusionsand host sediments
Early quartz ± adularia lines the veins andare overgrown by pyrite, arsenopyrite, sphalerite, galena and rare tennantite.Carbonates are intergrown with, but mainly overgrow, the sulphide minerals andinfill the veins. This sequence of deposition is comparable to that describedfrom many similar carbonate-base metal gold systems in the Southwest Pacificregion (Corbett and Leach, 1998).
The carbonate base metal event extends into theSoutheast Zone where it is present as base-metal-poor, carbonate-pyrite marcasite stockwork veinlets and crackle breccia zones, and as rarediscontinuous, sulphide-rich pseudo-veins. At depth in the SEZ, thecarbonate-base metal veins cut both the quartz-molybdenite and massive pyriteveins. Quartz-sericite/illite alteration accompanies the sheeted carbonate basemetal veins in the Central Zone; whereas widespread, lower temperature, intenseargillic alteration (kaolin illitic clays) is associated with thecarbonate-rich veins in the Southeast Zone where it has overprinted the earlierporphyry-related phyllic assemblages.
As in other carbonate-basemetal systems, a wide variety of carbonate species are present. These
range from early mixed Mn-Mg-Fe-Ca carbonates(kutnahorite, ankerite), followed by Fe-rich carbonates (siderite, Fe-dolomite)and late stage clear calcite. Mixed carbonates are commonly associated withhigher temperature sericite-quartz wallrock alteration and vein deposition;whereas later Fe-carbonates are associated with kaolinite and lower temperatureillite assemblages.
The carbonate minerals areoverall more Mn-rich in veins in the SoutheastZone, and more Mg-rich in veins in the Central zone. Abundant manganese oxide in outcrop in theSoutheast zone attests to the abundance of the Mn-carbonate veins.
Rare quartz-rich, locallybanded veins occur in the Central Zone drillcore. These are accompanied bystibnite and realgar-orpiment mineralisation and the late stage quartz veinsappear to post-date the carbonate-base metal event.
The samples collected for this study were selected in order to evaluate the primary or hypogene characteristics of the alteration and mineralization and therefore attempted to exclude the supergene effects of weathering. However it was noted that the oxidation,by groundwaters, of sulphide minerals in fractures and veins in places extended to depths of greater than 150m.
Fluid inclusion heating and freezing analyses were carried out on quartz from Stage I porphyry veins and on quartz and carbonate from Stage II veins in samples from the Central Zone.
Both liquid- and vapour-rich inclusions were observed in the quartzform the porphyry veins indicative of two-phase (boiling) conditions duringdeposition. The liquid-rich inclusions homogenized at 260-446°Cand freezing analyses indicated saline conditions (4-15 wt% equivalent NaCl).Some of the liquid-rich inclusions contain halite daughter crystals suggestingperiodic hypersaline (>25 wt% equivalent NaCl) fluid conditions.High temperature and salinity conditions during quartz deposition, the association with molybdenite mineralization and the sharp contacts of the sheeted veins are characteristic of B-type porphyry veins.
Quartz, deposited during the early stages of the Stage II carbonate-base metal veins, contains only liquid-rich fluid inclusions and was deposited over a temperature range of 230-311°C (average of sixty-five measurements = 262 °C ) from a relatively dilute(1.7-2.4 equivalent weight percent NaCl) meteoric fluid. Inclusions in carbonate minerals that overgrow the quartz in theseveins, homogenized over a comparable temperature range as the quartz, but undersignificantly more saline (3.1-5.9 weight percent equivalent NaCl) conditions.An increase in salinity of the ore fluid during carbonate base metal veindeposition is characteristic of these styles of systems (Corbett and Leach,1998), and indicates an influx of magmatic-derived, metal-bearing brines into adilute circulating meteoric system. The mixing of these fluids has beeninterpreted (Corbett and leach, 1998) to result in associated goldmineralisation.
Molybdenite is the only sulphide encountered in theporphyry-quartz veins, and occurs as fine-grained laths that are mutuallyintergrown with, and overgrowing, the quartz. Pyrite is virtually the onlysulphide mineral associated with the phyllic and propylitic assemblages,however rare pyrrhotite, rutile and magnetite locally occur as inclusions inthe pyrite.
Sulphides typically overgrow quartz in thecarbonate-base metal sulphide veins and are intergrown with, and commonlyovergrown by the carbonates. The sequence of sulphide deposition in these veinsis :
a)Pyrrhotite ± Magnetite
b) Pyrite
c)Sphalerite ± Arsenopyrite
d) Galena
e)Chalcopyrite
f)Tennantite / tetrahedrite
g)Marcasite
Sphalerite typically overgrows the Fe-sulphides, andis generally iron-rich in the Central Zone samples and iron-poor in theSoutheast Zone veins. The sphalerite in the Central Zone exhibits compositionalzonations from dark red-brown, Fe-rich cores to yellow / colorlessiron-depleted rims.
Galena occurs as rare inclusions inpyrite and sphalerite, but commonly overgrows these minerals and is locallyfound as intergrowths in carbonate. Chalcopyrite occurs as blebs and stringersin sphalerite, but more commonly overgrows other sulphide minerals and istypically intergrown with carbonate. Tennantite occurs in only trace amounts insome of the SEZ core where it overgrows chalcopyrite, and contains smallamounts (up to 1.2%) of silver.
Marcasite, indicative of low temperature conditions,is common in SEZ core where it is generally intergrown with late stageFe-carbonate minerals and/or kaolinite, and is typically hosted in thin discontinuousdendritic veinlets.
Mineragraphic analyses has shown that nativegold/electrum mineralization is associated with the carbonate-base metal veinsand the late stage epithermal quartz veins in core from the CZ. Gold washowever not observed in polished thin sections prepared from the Southeast Zonecore samples.
In the carbonate-base metal veins, gold occurseither as minute (4-40mm) inclusions in pyrite,sphalerite and galena; as larger grains (up to 100-200mm)that overgrow the sulphide minerals and extend into cavities and fractures,where it is intergrown with carbonate; and as a single large rounded / ovoidgrains (200-250mm) that is mutually intergrownwith late stage carbonate.
Over twenty-five gold grains were observed in six ofthe carbonate-base metal vein samples.The gold is not uniformly distributed along the veins, but typicallyoccurs in discrete clusters within one very small area. This nugget effectis common in carbonate-base metal gold systems (Corbett and leach, 1998) andmakes sampling and resource estimating difficult.
Most of the gold grains observed in the core occuras minute (<20-40mm) inclusions in sulphides andtherefore would be metallurgically refractory. However by volume / weight, thevast bulk of the gold (95% by volume) overgrows the sulphides and/or isintergrown with the carbonate minerals and therefore is expected to berelatively easily liberated during processing.
Base on data from electron micro-probe analyses, thegold exhibits a wide range in fineness (295-850), however the average finenessin each sample has a much narrower range of 562-774. The overall average of thefineness of the gold at Busang is 691 and this lies within the range ofaverages of other Southwest Pacific carbonate-base metal gold systems (Corbettand Leach, 1998).
A cluster of minute (6-80 mm)gold grains were also observed intergrown with quartz in a Stage III colloformbanded quartz vein from one of the Central Zone core samples.
This study of the core samples from the Busang hasrecognized the progressive evolution of a large magmatic-related hydrothermalsystem that is comparable to that encountered in many similar systems elsewherein the Pacific region (Leach, 1999).
Early stage porphyry quartzmolybdenite veins formedunder hot, saline conditions, and are probably related to the initialexsolution of fluids from a cooling magma. Felsic magmas are typicallyassociated with porphyry-molybdenite systems (Carten et al., 1993). It is thereforepostulated that the late stage rhyolite dikes, that have been recognizedelsewhere at Busang, are apophyses of a felsic intrusion at depth under theSoutheast Zone, and that this intrusion is the likely source of thehydrothermal systems at Busang. Similar late stage rhyolite dikes areencountered at Kelian (van Leeuwen et al, 1990), and are probably geneticallyrelated to the gold mineralization in that deposit.
The presence of tourmaline and apatite in theextensive phyllic alteration assemblages in the Southeast Zone indicates thatvolatile-rich magmatic fluids were channeled over a large area. Similar largephyllic alteration halos have been recognized to be commonly associated withporphyry systems around the Pacific rim (Sillitoe, 2000).
Quartz-adularia veins were deposited during theestablishment of a circulating-meteoric-dominated hydrothermal system. Latestage exsolution of metal-bearing brines along the margins of this system,deposited carbonate-base metal veins as these fluids cooled at shallow levelsin the Central Zone. There is evidence that there was an outflow of thesefluids into the Southeast Zone. This outflow formed extensive late stagecarbonate-pyrite-marcasite veins, local base metal-sulphide-rich carbonateveins, and widespread argillic alteration that overprinted onto the earlierporphyry-related assemblages.
As the hydrothermal system continued to wane, therewas local deposition of epithermal quartz veins and associated Au-As-Sbmineralization onto the earlier formed vein systems.
The author would like to acknowledge that the workwas carried out, at least in part, at the request of, and the support of JohnFelderhof. Mt. Felderhof however, played no part inthe preparation of this manuscript. The author would also like to thank Dr. RayMerchant for reviewing the manuscript and for helping to manage the workcarried out on the Busang core.
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