and Mineralisation in the Busang Gold Prospect,
Terry M. Leach
Core samples, which were collected by
the author from the Busang prospect,
The early stages of hydrothermal activity were followed by an episode of quartz (± adularia) – carbonate – base metal sulphide veining, which was accompanied by argillic (illitic-kaolin clay) wallrock alteration. This event was sulphide-rich in the Central Zone and carbonate-dominated in the Southeast Zone. Gold mineralisation was observed in Central Zone drillcore 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. The latter two events are interpreted to be associated with the late-stage exsolution of metal-bearing brines from the felsic intrusion that formed the earlier porphyry quartz-molybdenite veins and widespread phyllic-propylitic alteration..
This paper is a summary of the results of
a petrological study that was carried out on a suite
of one hundred and twenty-nine core samples that were collected by the author
while on site at the Busang Prospect,
Samples were selected mainly from 10cm long skeleton core that was stored on site, although whole core samples were also collected from three holes that had not yet been sampled for assaying. A suite of forty-seven samples was collected from twenty-three holes that were drilled in the Central Zone, and fifty-three samples were selected from six holes in the Southeast Zone. The remaining twenty-nine samples were selected from Drillhole BDH 5, which was a 980m deep hole drilled under the Southeast Zone.
There has been a considerable amount of factually incorrect information made available over the last five years on the geological characteristics of the Busang prospect area. It is hoped that this paper will present a sound scientific approach that may help to balance out this lack of basic data.
prospect is located in
A 1km x 500m wide zone of alteration in the Central Zone (CZ) at Busang is thought to have been the focus of dilation of pre-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 to be related to movement on these transfer structures.
Alteration and mineralisation at Busang are hosted in a series of polyphasal porphyry intrusions that are dominantly dacite in composition in the Central Zone, and andesite in the Southeast Zone. These porphyry bodies have been emplaced into a sequence of intercalated carbonaceous sandstones and siltstones. Late stage flow banded feldspar porphyry dykes cut the dacite/andesite porphyry intrusions and result in visually sharp alteration contrasts where they cut previously altered hostrocks.
Diorite porphyry intrusions have been intersected at depth beneath the Southeast Zone and are inferred to be 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 not observed in the cores analysed during this study.
Intrusion, hydromagmatic, fluidised and vein/dilational breccias are common in the core collected for this study. Diatreme-like breccias were observed in outcrop during the site field work.
Very similar lithologies and breccias are host to mineralisation in the Kelian gold deposit (van Leeuwen et al., 1990).
Sequence of Hydrothermal Events
Three main stages of hydrothermal alteration (replacement and deposition) have been recognised at Busang (Figure 1). It postulated that these events are associated with the same overall hydrothermal system that evolved with time.
This stage of hydrothermal activity is characterized by an initial phase of porphyry-style quartz vein development, followed by an episode of phyllic and propylitic alteration and veining. These porphyry-related assemblages are most extensively developed in the drillcore from the Southeast Zone
In the deep drillhole BDH 5, centimetre-wide, sheeted to conjugate fracture sets host porphyry quartz ± anhydrite veins at depths of 500-700m beneath the central part of the Southeast Zone. These veins are characteristically grey due to the presence of abundant primary and secondary liquid- and vapour-rich inclusions.
Halite daughter crystals are present in some of the liquid-rich inclusions and these are indicative of periods when the fluids were hypersaline (>25 wt% equivalent NaCl). The deposition of the porphyry-related quartz veins was polyphasal, and locally extended to very shallow levels.
Anhydrite occurs as intergrowths with, and inclusions in some of the porphyry quartz veins, although in most cases anhydrite deposition post-dates the quartz.
Extensive zones of porphyry-related, propylitic and intense 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 are aligned along the NE-trending transfer structural zone.
The phyllic assemblage is dominated by coarse-grained 2M sericite (and locally muscovite) + quartz + pyrite. A purple anhydrite locally overgrows porphyry quartz in veins and is in turn overgrown by sericite. Dark blue-green tourmaline (schrol) is commonly associated with the quartz-sericite-pyrite wallrock alteration and overgrows sericite in veins. It is also associated with later dolomite-calcite deposition. Apatite occurs as minute grains with the phyllic alteration assemblage and commonly replaces wallrock mafic phenocrysts. .
The propylitic alteration / veining is characterized by the presence of epidote + quartz + chlorite + carbonate ± sericite and is peripheral to the phyllic alteration zones.
Fine grained milled matrix (fluidized) breccias locally cross-cut the porphyry-related quartz veins and are a pre-cursor to the later carbonate-base metal system. In places these breccias contain clasts of earlier quartz vein material. The clasts are sealed in a comminuted matrix that is altered to illitic clay and/or sericite ± quartz – carbonate – pyrite. It is speculated that these breccias may be related to the phreatomagmatic (diatreme) breccias observed at the surface. A compilation of field mapping and drill core logging is necessary in order to fully evaluate the presence of a diatreme-maar complex at Busang, 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. In outcrop, it was observed that the carbonate-base metal sulphide assemblages were also deposited along the fractured and brecciated contacts between the high level dacite intrusions and host sediments
Early quartz ± adularia lines the veins and is overgrown by pyrite, arsenopyrite, sphalerite, galena and rare tennantite. Carbonates are intergrown with, but mainly overgrow, the sulphide minerals and infill
the veins. This sequence of deposition is comparable to that described from many similar carbonate-base metal gold systems in the Southwest Pacific region (Corbett and Leach, 1998).
The carbonate base metal event extends into the Southeast Zone where it is present as base-metal-poor, carbonate-pyrite – marcasite stockwork veinlets and crackle breccia zones, and as rare discontinuous, sulphide-rich ‘psuedo’-veins. At depth in the SEZ, the carbonate-base metal veins cut both the quartz-molybdenite and massive pyrite veins. Quartz-sericite/illite alteration accompanies the sheeted carbonate base metal veins in the Central Zone; whereas widespread, lower temperature, intense argillic alteration (kaolin – illitic clays) is associated with the carbonate-rich veins in the Southeast Zone where it has overprinted the earlier porphyry-related phyllic assemblages.
As in other carbonate-base metal 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 with higher temperature sericite-quartz wallrock alteration and vein deposition; whereas later Fe-carbonates are associated with kaolinite and lower temperature illite assemblages.
The carbonate minerals are overall more Mn-rich in veins in the Southeast Zone, and more Mg-rich in veins in the Central zone. Abundant manganese oxide in outcrop in the Southeast zone attests to the abundance of the Mn-carbonate veins.
Trace barite locally fills open spaces in the carbonate-base metal veins.
This is the main gold mineralisation event at Busang.
Stage III : Epithermal Quartz Veins
Rare quartz-rich, locally crustiform banded veins occur in the Central Zone drillcore. These are accompanied by stibnite and realgar-orpiment mineralisation. These late stage epithermal quartz veins appear to post-date the carbonate-base metal event.
Stage IV : Weathering / Supergene
The samples collected for this study were selected in order to evaluate the primary or hypogene characteristics of the alteration and mineralisation 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 quartz form the porphyry veins indicative of two-phase (boiling) conditions during deposition. The liquid-rich inclusions homogenized at 260-446°C and freezing analyses indicated saline conditions (4-15 wt% equivalent NaCl). Some of the liquid-rich inclusions contain halite daughter crystals suggesting periodic hypersaline (>25 wt% equivalent NaCl) fluid conditions.
High temperature and salinity conditions during quartz deposition, the association with molybdenite mineralisation 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 these veins, homogenized over a comparable temperature range as the quartz, but under significantly more saline (3.1-5.9 weight percent equivalent NaCl) conditions. An increase in salinity of the ore fluid during carbonate – base metal vein deposition is characteristic of these styles of systems (Corbett and Leach, 1998), and indicates an influx of magmatic-derived, metal-bearing brines into a dilute circulating meteoric system. The mixing of these fluids has been interpreted (Corbett and Leach, 1998) to result in associated gold mineralisation.
Molybdenite is the only base metal sulphide mineral encountered in the porphyry-quartz veins, and occurs as fine-grained laths that are mutually intergrown with, and overgrowing, the quartz. Pyrite is virtually the only sulphide mineral associated with the phyllic and propylitic assemblages, however rare pyrrhotite, rutile and magnetite locally occur as inclusions in the pyrite.
Sulphides typically overgrow quartz in the carbonate-base metal sulphide veins and are intergrown with, and commonly overgrown by the carbonates. The sequence of sulphide deposition in these veins is from early to late as :
Pyrrhotite ± Magnetite
Sphalerite ± Arsenopyrite
Tennantite / tetrahedrite
Sphalerite typically overgrows the Fe-sulphides, and is generally iron-rich in the Central Zone samples and iron-poor in the Southeast Zone veins. The sphalerite in the Central Zone exhibits compositional zonations from dark red-brown, Fe-rich cores to yellow / colorless iron-depleted rims.
Marcasite, indicative of low temperature conditions, is common in SEZ core where it is generally intergrown with late stage Fe-carbonate minerals and/or kaolinite, and is typically hosted in thin discontinuous dendritic veinlets.
Mineragraphic analyses have shown that native gold/electrum mineralisation is associated with the carbonate-base metal veins and the late stage epithermal quartz veins in core from the CZ. Gold was however not observed in polished thin sections prepared from the Southeast Zone core samples.
In the carbonate-base metal veins, gold occurs either 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 / ovoid grains (200-250mm) that is mutually intergrown with late stage carbonate.
Over twenty-five gold grains were observed in six of the carbonate-base metal vein samples. The gold is not uniformly distributed along the veins, but typically occurs in discrete ‘clusters’ within one very small area. This ‘nugget’ effect is common in carbonate-base metal gold systems (Corbett and leach, 1998) and makes assay reproduction and resource calculations difficult.
Most of the gold grains observed in the core occur as minute (<20-40mm) inclusions in sulphides and therefore would be metallurgically refractory. However by volume / weight, the vast bulk of the gold (95% by volume) overgrows the sulphides and/or is intergrown with the carbonate minerals and therefore is expected to be relatively easily liberated during processing.
Based on data from electron micro-probe analyses, the gold exhibits a wide range in fineness (295-850), although the average fineness in each sample has a much narrower range of 562-774. The overall average of the fineness of the gold at Busang is 691 and this lies within the range of averages of other Southwest Pacific carbonate-base metal gold systems (Corbett and Leach, 1998).
A cluster of minute (6-80 mm) gold grains were also observed intergrown with quartz in a Stage III colloform banded quartz vein from one of the Central Zone core samples.
Multiple-element analyses was carried out by neutron activation and atomic activation on twenty-three vein core samples form the Central zone, and fifty-one vein and wallrock core samples from the four Southeast Zone drillholes.
The Central Zone vein samples graded up to 68.7 g/t Au (average 7.86 g/t Au), and 272 g/t Au, with a high Ag:Au ratio of generally > 10. There is an overall correlation between gold, and silver and base metal assays (Zn > Pb > Cu), although these are not directly proportional to gold grades. Elevated cadmium (up to 966 ppm), arsenic (up to 2.4%) and antimony (up to 380 ppm) are also associated with gold mineralisation.
Gold occurs in only trace amounts in the Southeast zone samples (average = 0.08 g/t Au), with grades up to 1.93 g/t Au associated with thin base metal carbonate veinlets. As in the Central Zone, there is a rough correlation of gold with base metal sulphides (Pb ³ Zn > Cu), arsenic (up to 0.6%) and antimony (up to 0.19%).
This study of the core samples from the Busang has recognized the progressive evolution of a large magmatic-related hydrothermal system (Figure 2) that is comparable to that encountered in many similar systems elsewhere in the Pacific region (Leach, 1999).
Early stage porphyry quartz–molybdenite veins formed under hot, saline conditions, and are probably related to the initial exsolution of fluids from a cooling magma. Felsic magmas are typically associated with porphyry-molybdenite systems (Carten et al., 1993). It is therefore postulated that the late stage rhyolite dikes, that have been recognized elsewhere at Busang, are apophyses of a felsic intrusion at depth under the Southeast Zone, and that this intrusion is the likely source of the hydrothermal systems at Busang. Similar late stage rhyolite intrusions are encountered at Kelian (van Leeuwen et al, 1990), and are genetically related to the gold mineralisation in that deposit.
The presence of tourmaline and apatite in
the extensive phyllic alteration assemblages in the Southeast Zone indicates that
volatile-rich magmatic fluids were channeled over a
large area. Similar large phyllic alteration halos have been recognized to be
commonly associated with porphyry systems around the
Quartz-adularia veins were deposited during the establishment of a circulating-meteoric-dominated hydrothermal system. Late stage exsolution of metal-bearing brines along the margins of this system, deposited carbonate-base metal veins as these fluids cooled at shallow levels in the Central Zone. There is evidence that there was an outflow of these fluids into the Southeast Zone. This outflow formed extensive late stage carbonate-pyrite-marcasite veins, local base metal-sulphide-rich carbonate veins, and widespread argillic alteration that overprinted onto the earlier porphyry-related assemblages.
As the hydrothermal system continued to wane, there was local deposition of epithermal quartz veins and associated Au-As-Sb mineralisation onto the earlier formed vein systems.
The author would like to acknowledge that the work was carried out, at least in part, at the request of, and the support of John Felderhof. Mr. Felderhof however, played no part in the preparation of this manuscript. The author would also like to thank Dr. Ray Merchant for reviewing the manuscript.
Carten, R.B., White, W.H., and Stein, H.J., 1993,
grade granite-related molybdenum systems :
Classification and origin. in Kirkman,
R.V., Sinclair, W.D.,
Corbett, G.J., and Leach, T.M., 1998, Southwest
Leach, T.M., 1999, Evolution of selected porphyry
copper-gold systems in the Southwest Pacific region
and its relevance to exploration : Pacrim ’99
Sillitoe, R.H., 2000, Role of gold-rich porphyry models in exploration, in S.G. Hagerman and P.H. Brown, eds., Gold in 2000, Reviews in Economic Geology, v. 13, p. 311-346.
Van Leeuwen, T.M., Leach, T.M., Hawke, M.M.
and Hawke, A.A, 1990, The Kelian disseminated gold deposit, East Kalimantan, Indonesia, in Hedenquist, J.W., White, N.C. and Siddeley, N., eds., Epithermal gold mineralisation of the Circum Pacific: Journal of Geochemical Exploration, v. 35, p. 1-61.