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Epithermal Gold 2012
Epithermal Gold Deposits of the Western Pacific
12 to 31 May, 2012
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CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Porter GeoConsultancy, continued its International Study Tour series of professional development courses during May 2012 by visiting a representative selection of the different styles, and the most important examples, of epithermal gold deposits on the western margin of the Pacific. The program included major low-, intermediate- and high-sulpidation deposits. A number of the high-sulphidation examples are closely associated with major porphyry Cu-Au mineralisation, although some are also accompanied by intermediate- to low-sulphidation vein systems.Japan - with Prof. Sachihiro Taguchi.
The mine visits were complemented by field & classroom workshops presented by experts from academia and industry.
The full tour commenced in Fukuoka, Kyushu, Japan on the evening of Friday 11 May, 2012 and ended in Port Moresby, PNG on the evening of Thursday 31 May, 2012.
Participants were able to take any 3 or more days, up to the full tour, as suited their interests or availability, with participants joining and leaving the tour at appropiate locations along the route.
The main components of the itinerary were:
Kasuga, Nansatsu district, Kyushu, Japan - high-sulphidation silica-gold. Hishikari, Kyushu, Japan - adularia-sericite low-sulphidation. Lepanto-Far South-East, Luzon, Philippines - high- & intermediate-sulphidation, and porphyry. Chatree, Thailand - low- to intermediate-sulphidation. Martabe, Sumatra, Indonesia - high-sulphidation. Tujuh Bukit, East Java, Indonesia - high-sulphidation and porphyry. Lihir, PNG - quartz-sulphide Au (-Cu) low-sulphidation. Hidden Valley, PNG - carbonate-base metal-Au low-sulphidation (to porphyry?). Porgera, PNG - high-sulphidation & porphyry Au.
Geological summaries of the deposits on the itinerary are as follows:
Kyushu Field Workshop .......... Saturday 12 to Tuesday 15 May, 2012.
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Kyushu, the southern most of the main islands of Japan, is the principal gold producing area in the country with several important deposits - the most outstanding of which is Hishikari - related to waning or extinct Quaternary geothermal systems. Cenozoic volcanism within Kyushu is related to the north-westward dipping subduction of the Philippine Sea Plate beneath the Eurasian Plate. Late Cenozoic volcanic activity is associated with large tectono-volcanic graben like depressions, representing multiple calderas, which migrated east to south-eastward with time, as did the associated geothermal activity and related gold mineralisation.
The epithermal gold mineralisation is typically, but not exclusively, quartz-vein type with associated adularia and minor calcite cutting both basement and the overlying Quaternary andesitic and lesser rhyolitic volcanics and lacustrine sediments. High temperature geothermal activity is currently observable close to active volcanoes on the island, mostly water dominated, controlled by fractures, such as those that are presently forming epithermal veins.
It is possible to observe currently active epithermal systems precipitating both high and low sulphidation epithermal gold mineralisation at different locations and to study the conditions and processes that are responsible for their formation in drill holes testing for geothermal power generation stations. Further to the west, older, now largely extinct systems will be visited which host significant gold deposits, such a the Kasuga high sulphidation and Hishikari low sulphidation gold deposits.
The Kyushu Field Workshop, which will be led by renowned international expert Professor Sachihiro Taguchi over a four day period, is designed to visit i). the Kuju Iwo-yama high temperature fumarolic area which is an active high sulphidation field, ii). the Hatchobaru geothermal power plant where gold mineralisation is currently being deposited, and magmatic hydrothermal activity (high sulphidation ) occurs in a low sulphidation hydrothermal field, iii). the Komatsu Jigoku low sulphidation steaming ground iv). the Aso volcano and museum/volcanic observatory, v). the Hishikari low sulphidation bonanza gold mine, v). the Kasuga high sulphidation mineralisation and vi). the active Sakurajima Volcano.
This workshop will provide a rare opportunity to see at first hand the processes involved in the formation of epithermal gold deposits and the results of these same processes as expressed by recent large deposits in the same district.
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Kasuga - Nansatsu district, Kyushu, Japan ...................... Monday 14 May, 2012.
Kasuga is one of a group of three main deposits (the other two being, Iwato and Akeshi), which are found in the Nansatsu district of southern Kyushu, Japan and have together produced more than 27 tonnes of gold. All three have similar geologic characteristics, known as the "Nansatsu-style", more commonly referred to as high-sulphidation epithermal Au mineralisation.
The basement rocks in the Nansatsu district are dominated by Cretaceous sandstone rich trench turbidites with minor submarine basalt and chert, overlain by Tertiary-Quaternary calc-alkalic volcanic rocks which are associated with caldera development and the outflow of 5.9 to 6.4 Ma pyroclastics emplaced shortly before the mineralising event. Across the district, there was an east-ward shift with time in the locus of calc-alkaline volcanism, which includes pyroxene-hornblende, hornblende- and pyroxene-andesite flows and their pyroclastic equivalents, and tuffaceous siltstones. Hydrothermal activity displays a similar eastward younging pattern to the volcanism.
The gold mineralisation is entirely restricted to the intensely silicified rocks which form the core of these deposits. The silica core of each system is enveloped by argillic alteration that is zoned from an inner alunite-kaolinite, through to an outer smectite and/or interstratified illite/smectite interval. This argillic zone is usually less than 1 to 10 m thick and grades outward into propylitic alteration and finally into fresh country rock. The silica zone is underlain by deep alteration characterised by abundant pyrite and typically Na rich alunite, while dickite dominates over kaolinite. At depth the alunite dickite grades into a sericite-chlorite assemblage.
The principal ore minerals are enargite and electrum, with rare luzonite in the upper sections and ubiquitous and dominant pyrite throughout. Gold grades, which are intimately associated with goethite as native gold in the upper parts of the orebody, are roughly concentric, decreasing outwards. The main high grade zone contains 5 g/t Au in the western part of the pit where the fracture density is highest.
The mine produces around 10 000 tonnes per month of silica flux which contains approximately 4 g/t Au retrieved during the copper smelting process for which the flux is utilised. The reserves at the end of WW2 were approximately 1.6 Mt of ore, with around 0.8 Mt now remaining. Larger tonnages of lower grade (around 1 g/t Au) is known nearby. Historic production from Kasuga between 1929 and 1994 amounteds to some 8.8 tonnes of Au and 5.0 tonnes of Ag. While small, the deposit is a good example to illustrate many of the characteristics of high sulphidation Au-Ag mineralisation in an intensively studied district.
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Hishikari - Kyushu, Japan ...................... Tuesday 15 May, 2012
Hishikari is an adularia-sericite low sulphidation bonanza vein epithermal gold orebody, with one of the highest average grades of gold recorded for a large deposits. It is located approximately 30 km north of the city of Kagoshima on the island of Kyushu in southern Japan (#Location: 32° 00', 25"N, 130° 41', 20"E).
In 2004 the total contained gold, both mined and in reserve, totalled 264 tonnes (8.5 Moz) comprising 3.5 Mt @ 60 to 70 g/t Au and 2 Mt @ 20 to 25 g/t Au. Annual production in 2011 was 0.205 Mt of ore averaging 37 g/t Au. Production to the end of 2010 had been ~193 t (6.2 Moz) of gold from 3.9 Mt of ore with an average grade of 49 g/t Au, with a remaining reserve of 150 t (4.8 Moz) of gold at similar grades (~3.5 Mt).
The geology of the deposit comprises a basement sequence of Cretaceous sediments and unconformably overlying Quaternary volcanics. The concealed vein system of the ore deposit is localised by this unconformity. The Cretaceous basement is made up of shale and sandstone, with minor tuffaceous shale and chert of the Shimanto Super Group. These sediments are heavily hematite stained for up to 20 m below the unconformity surface with the overlying volcanics, suggesting a palaeo-weathering surface, and form a restricted basement high in the vicinity of the orebody. The Quaternary volcanics, comprise sub-aerial deposited alternating andesites and dacites, both lavas and pyroclastics of the Hishikari Group, which are calc-alkaline in composition and have magnetic susceptibilities typical of the magnetite series. They are mostly Pleistocene in age (1.6 to 0.55 Ma) and comprise 5 units, with a basal andesite and an upper-most andesite, separated by two dacite and one andesite unit. They are in turn overlain by younger pyroclastics and alluvial accumulations.
The ore veins have been dated at between 1.04 and 0.78 Ma, in the Pleistocene, emplaced soon after the deposition of the Lower Hishikari Andesite, closely correlated with the overlying dacite unit.
The Hishikari deposit is composed of more than 125 veins (generally 1 to 3 m thick, to a maximum of 8 m), which strike at between 30 and 70°, distributed within an altered and mineralised corridor with a width of 500 to 1000 m and strike interval of ~3 km, and dip at from 70°NW to 70°SE. The individual veins are 300 to 400 m long, composed of crustiform banded quartz-adularia with subordinate smectite. The top of the vein system is generally >100 m below the surface, straddling the unconformity between the Shimanto Super Group and the mainly Pleistocene Hishikari Group. Some 60% of the ore is in the Cretaceous basement, the remainder being in the andesitic lowermost unit of the Hishikari Group.
The vein system forms two groups, the Sanjin-Honko section to the northeast, separated by a gap of ~300 m from the southwestern Yamada section. The Sanjin veins are a smaller set 100 to 200 m to the southeast of the main Honko vein swarm. The Honko-Sanjin deposit is predominantly within Cretaceous host rocks, where these form a basement rise, while a larger proportion of the Yamada section is in the Lower Andesite where this unit is thicker on the margin of the rise.
Hydrothermal alteration within the volcanic hosts can be grouped into four zones, namely a: i) cristobalite-smectite zone (cristobalite and/tridymite with smectite), ii) quartz-smectite zone (quartz with smectite and/or kaolin minerals, with pyrite), iii) a mixed-layer clay zone (chlorite/smectite and/or illite/smectite together with quartz, adularia, calcite, and laumontite) and iv) chlorite-illite (quartz-chlorite-adularia-calcite). Chlorite and illite/sericite are the principal alteration minerals surrounding the high grade Honko-Sanjin veins, while the lower grade Yamada vein extends into a smectite-mixed layer clay alteration phase. Mixed layer clays (zone iii) and quartz-smectite (zone ii) form a near horizontal layer of intense argillisation 50 to 100 m above the main mineralisation centre, with an argillic zone at the top consisting of quartz, kaolinite, and pyrite. These are followed by the shallowest and outermost alteration characterised by cristobalite-smectite (zone i), commonly with hydrothermal K feldspar, which in turn grades into unaltered andesite or dacite. The veins are mainly gold-silver bearing quartz-adularia containing electrum, naumannite and other silver sulphides, precipitated at temperatures of 215 to 175°C, and only contain a few percent sulphides, including prominent chalcopyrite, sulphosalts and selenides.
The original discovery is credited to the Metal Mining Agency of Japan in 1981, with the intersection of 15 cm @ 290 g/t Au, 200 m below surface. Subsequently the Sumitomo Metal Mining Co Ltd enlarged, proved and developed the deposit.
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Travelling from Japan to the Philippines ...................... Wednesday 16 May, 2012.
Lepanto-Far South-East - Luzon, Philippines ...................... Thursday 17 to Saturday 19 May, 2012 (includes Manila to Mankayan and return travel).
The Mankayan Mineral District is located within a well-defined, 150 km long belt of porphyry copper deposits in the Central Cordillera of northern Luzon, Philippines, approximately 250 km north of Manila, and embraces a mineralised complex composed of four related significant gold bearing deposits. These are the Lepanto high sulphidation epithermal enargite-luzonite-gold Au-Cu orebody; the Victoria and Teresa intermediate sulphidation Au-Ag veins; and the concealed Far South-east porphyry Au-Cu deposit (#Location: 16° 51' 30"N, 120° 47' 0"E).
Four main units have been recognised in the Mankayan district, namely:
(i) Late Cretaceous to middle Miocene basement consisting of the Lepanto metavolcanic, Apaoan volcaniclastic and Balili volcaniclastic rocks. The Lepanto metavolcanic rocks are the lowermost stratigraphic unit found within the Mankayan district, and are only exposed in the west. They are inferred to be of Cretaceous- Paleogene age, comprise tightly packed andesitic to basaltic lavas with minor turbiditic sedimentary rocks, and are cut by mafic dykes which have subsequently undergone greenschist facies metamorphism. The overlying Apaoan volcaniclastic rocks are present in the northeast and consist of green and red thin-bedded siltstone-sandstone. The Balili group, which contains fossils indicative of a late Oligocene to middle Miocene age, unconformably overlies both the Lepanto metavolcanic and Apaoan volcaniclastic rocks, and consists of mostly matrix- supported polymictic volcanic conglomerates;
(ii) the Miocene tonalitic Bagon intrusive complex, which has been dated at 12 to 13 Ma and intrudes the Lepanto metavolcanic rocks;
(iii) the Pliocene Imbanguila dacitic to andesitic porphyry and pyroclastic rocks, which predate the Far Southeast porphyry Cu-Au mineralisation, and host much of the Lepanto enargite Au deposit and the Victoria veins.
(iv) postmineralisation cover rocks, including the Pleistocene Bato dacitic to andesitic porphyritic lava and pyroclastic flow units, and the Recent Lapangan tuff.
Both the Imbanguila and Bato dacitic units, which are difficult to distinguish, are characterised by complex sequences of volcanic breccias, pyroclastic horizons, and massive porphyritic rocks, as well as dyke emplacement during deposit formation. The Imbanguila host rock returned ages of 2.19±0.62 to 1.82±0.36 Ma, while the Bato rocks are 1.18±0.08 to 0.96±0.29 Ma. The Imbanguila unit overlies basement rocks, and was apparently issued from two identified vents that are located above the subsequent Far Southeast porphyry alteration and mineralisation, which are centred on quartz diorite porphyry dykes that intruded to about 300 to 400 m below these vents. The distribution of the Imbanguila unit is spatially related to, but offset by the Lepanto fault, a splay of the major north-south Philippine fault system that extends across the length of Luzon. The Lapangan tuff forms a thin and discontinuous cover of poorly consolidated dacitic air-fall tuff, which is mainly present in the centre of the Mankayan district.
The Lepanto enargite-luzonite-gold orebody produced Cu and Au as early as the 14th century, with organised mining from 1865 and by Japanese companies during WWII. Total production from 1948 to its closure in 1996 was 36.3 Mt @ 3.4 g/t Au, 2.9% Cu, 14 g/t Ag, when the remaining resource was 4.4 Mt @ 2.4 g/t Au,1.76% Cu.
The distribution of the Lepanto epithermal ore is controlled by the NNW-SSE trending Lepanto fault and its intersection with the generally flat-lying unconformity at the base of the Imbanguila dacite. Mineralisation extends along the NW-SE trending Lepanto Fault over a strike length of >3 km to the northwest of the Far South-east porphyry orebody, and outwards along the unconformity over widths of almost 1 km. This mineralisation persists over a thickness of up to 100 m and >200 m in the core of the deposit. The high sulphidation orebodies at Lepanto are hosted within a composite advanced argillic alteration zone composed of hypogene alunite, dickite, kaolinite and pyrite, locally including diaspore and pyrophyllite at depth. Alteration generally comprises an upper layer of dickite-kaolinite, underlain by a core of silica±energite, underlain in turn by quartz-alunite, grading downwards to further dickite-kaolinite on the periperies and pyrophyllite-diaspore-kandite at depth within the Lepanto Fault zone. This argillic alteration zone has a strike length of >7 km and width that varies from ~0.5 to >2 km, persiting over vertical thicknesses of generally ~100 m, but following the Lepanto Fault to >400 m below the unconformity. Within the argillic zone, the ores are closely associated with the silicic alteration, comprising vuggy residual to massive residual quartz. The Lepanto fault hosts ~70% of the ore, with strong brecciation of the silicic ore and offsets of alteration zones possibly reflecting syn-mineral movement. The balance of ore is contained in the subhorizontal blanket of the lithocap that follows the unconformity, hosted by the lower Imbanguila dacite, Balili volcaniclastic rocks and upper Lepanto metavolcanic rocks.
Paragenetic studies of Lepanto mineralisation indicate early coarse pyrite generated during the largely Cu- and Au-barren leaching event, which produced a core of residual vuggy quartz surounded by the halo of advanced argillic alteration. This was followed by the high sulphidation state minerals enargite and luzonite with fine pyrite, largely hosted by the silicic core, and finally by tennantite, chalcopyrite, sphalerite, galena and tellurides plus selenides. The post-enargite sulphides are associated with the introduction of gold, accompanied by anhydrite plus barite gangue minerals. The tennantite of the gold event may be of intermediate- or high sulphidation state, although the presence of chalcopyrite indicates the former.
Far South-east (FSE) is a concealed, deep seated, bell-shaped porphyry Au-Cu deposit hosted by volcaniclastic rocks, centred on a late Miocene quartz-diorite intrusive complex. The geological resource at FSE (calculated in 1995; Lepanto Mining website, 2011) at a cut-off grade of 0.7% Cu equiv., is estimated at 657 Mt @ 0.94 g/t Au, 0.65% Cu (and at a 1.5% Cu equiv. cutoff is 180 Mt @ 1.70 g/t Au, 0.80% Cu). The top of the porphyry-type mineralisation is >650 m below the surface, and the deposit is generally elongated parallel to the regional NW-SE structural trend. It has approximate dimensions of >1000 m east-west, 800 m north-south and persists over a vertical interval of >900 m. Cu and Au mineralisation grade shells are concentric around the dykes and irregular intrusive bodies that comprise the melanocratic quartz-diorite porphyry complex, which was emplaced into the basement Lepanto metavolcanic rocks. The porphyry system has a core of potassic alteration, which consists of a biotite-magnetite±K feldspar assemblage associated with veins of vitreous, anhedral quartz, and accompanied by bornite-chalcopyrite-magnetite mineralisation. This alteration is partially to pervasively overprinted and fringed by alteration assemblages of chlorite+hematite and/or white mica sericite-clay-chlorite accompanied by chalcopyrite-pyrite. No definitive paragenetic evidence links Cu sulphide minerals to the early veins of vitreous, anhedral quartz veins, although petrographic evidence shows that Cu sulphides are mainly associated with a later event characterised by euhedral quartz crystals and anhydrite, and the early anhedral quartz has been shown to be overgrown by euhedral quartz, the latter associated with sulphide deposition and mineral inclusions of illite. Bleached centimetre- to metre-wide halos of illite accompany euhedral quartz-anhydrite-white mica-hematite-pyrite-chalcopyrite-bornite veins, both of which cut sericite-clay-chlorite alteration. Gold occurs as free grains of electrum associated with chalcopyrite and bornite, locally accompanied by Bi-Te-bearing tennantite. Upward and outward from the core of the mineralised porphyry system, the pervasive sericite-clay-chlorite assemblage grades from white mica-dominated, with minor local pyrophyllite, to an assemblage containing abundant pyrophyllite, variably accompanied by quartz, anhydrite and kandite minerals (dickite, nacrite, and kaolinite). This pervasively altered rock is overlain and locally cut by a silicic zone with local alunite that hosts the southeastern extent of the Lepanto ore deposit. The alunite halo includes a variable assemblage of anhydrite, diaspore, dickite, and/or pyrophyllite.
A late intra-mineral hydrothermal breccia, with altered and mineralised porphyry fragments cemented by sulphide minerals, outcrops directly above the northeastern portion of the FSE deposit. This breccia changes from fragment to matrix-supported with depth and is steeply dipping with an anastamosing form. It has a north-south elongated long axis of up to 400 m, and cuts Imbanguila dacites, Balil volcaniclastics and quartz-diorite dykes, passing through the centre of the porphyry deposit.
A well-defined diatreme breccia outcrops above the Lepanto orebody, ~1 km northwest of the surface projection of the FSE deposit where it cuts Imbanguila dacite. The breccia contains lithic fragments with biotite and magnetite alteration, and bornite-chalcopyrite mineralisation, suggesting derivation from an underlying porphyry deposit.
The Victoria intermediate sulphidation quartz-gold-base metal tensional vein system is located to the southwest of, and passes within a few hundred metres of the FSE porphyry. Veins generally trend NNE with a dip to the southeast, although some have arcuate strikes, curving from northeast to southeast. They are predominantly hosted by Imbanguila dacite porphyry and pyroclastic rocks, although some extend downward into the Balili volcaniclastic and Lepanto metavolcanic units. The Victoria veins are not exposed and pinch out upwards at ~200 to 250 m below the surface, with more than 8 mineralised zones having been identified. Grades of 3 to 9 g/t Au are continuous over a 400 m vertical interval, although high-grade >30 g/t Au ore is more restricted within up to 250 m vertical intervals. The main veins are being mined over a 300 m vertical interval, with widths of 2 up to 8 m, and strike extents of up to 600 m. The paragenesis of gangue and ore minerals in the Victoria veins comprises: (i) early quartz veins, associated with an intermediate sulphidation-state assemblage including chalcopyrite, tetrahedrite and low Fe sphalerite, as well as pyrite and galena; (ii) a carbonate stage of rhodochrosite, with similar sulphides; and (iii) a late, sulphide-poor, sulphate stage of anhydrite. Bornite and hematite are restricted to the early quartz veining, while gold was introduced during the later part of the quartz vein development and through the carbonate stage, ending during the sulphate phase. The northwesternmost of the Victoria veins (anhydrite+quartz+pyrite±illite) cut and overprint advanced argillic alteration and enargite (quartz±alunite±pyrophyllite±diaspore±dickite assemblages) related to the Lepanto deposit. Similar epithermal quartz veins have also been recognised cutting enargite mineralisation in the main Lepanto deposit, near its base.
At Teresa, 2 km south of the FSE porphyry deposit, and just to the southwest of the southern margin of the Victoria vein cluster, veins trend north-south and mineralisation tends to occur over wider intervals, at lower grades, in breccia zones. These veins are mined over an ~250 m vertical interval (although narrow widths persist at greater depths), and over similar strike extents to those at Victoria.
Both the Teresa and Victoria veins are reported to be enveloped by halos of illite, or locally chlorite, and are rarely in direct contact with propylitic-altered wall rock.
The initial resource estimate (1997) at Victoria was 11 Mt @ 7.3 g/t Au representing ~80 t of contained Au. Mining commenced in 1997. As of January 2011, the remaining proved + probable reserve at Victoria was 2.87 Mt @ 4.64 g/t Au; and at Teresa was 0.13 Mt @ 4.30 g/t Au for a total of 14 t of contained Au in the two vein systems (Lepanto Consolidated Mining, 2011).
Biotite from the FSE porphyry-related potassic alteration and alunite in the halo to the silicic host of the Lepanto high sulphidation orebody returned ages of 1.41±0.05 (n = 6) and 1.42±0.08 Ma (n = 5), respectively (Arribas et al., 1995), demonstrating the coeval age of potassic alteration in the porphyry deposit and its overlying advanced argillic alteration lithocap. Phyllic alteration overprint of the porphyry ore body followed, with illite ages of 1.37 to 1.22±0.04 to 0.10 Ma (n = 10; Arribas et al., 1995). Illite samples from the Victoria veins were dated at 1.31±0.02 Ma (Ar-Ar; Sakakibara et al., 2001) and 1.14±0.02 and 1.16±0.02 Ma (Hedenquist et al., 2001). Although alteration around the Victoria and Teresa veins appears similar, a sample of illite from the 900 m level of the Teresa vein was dated at 2.22±0.05 Ma (Ar-Ar; Chang et al., 2011) indicating that this portion of the vein system is older.
Shinohara and Hedenquist (1997) and Hedenquist et al. (1998) argued that the synchronous alteration events of the main porphyry (FSE) and lithocap (Lepanto) were related to a coupled hypersaline liquid and a low-salinity vapour, which formed by separation as the solvus was intersected by critical fluid at depth. The hypersaline liquid remained at depth and caused the potassic alteration, as evidenced by fluid inclusions, while the buoyant vapour ascended to shallower depths to form an acidic condensate that leached the rock and created the residual quartz (silicic) and quartz-alunite alteration. The phyllic alteration overprint of the porphyry ore body followed and has been demonstrated to also be of magmatic in origin (Chang et al., 2011). Stable isotopic studies of the biotite, alunite and illite indicated that all were formed from aqueous fluids with a dominantly magmatic origin, although the alunite-stable fluid was an acidic condensate of magmatic vapour with a variable meteoric water component, the latter progressively and regularly increasing with distance from the porphyry, to around 4 km to the northwest (Hedenquist et al., 1998).
The Guinaoang porphyry Cu-Au deposit (500 Mt @ 0.4% Cu, 0.4 g/t Au) is located 3 km southeast of FSE along an extrapolation of the Lepanto Fault. Mineralisation is largely hosted by an altered quartz-diorite intrusion 200 to 1000 m below surface, concealed by post-mineralisation rocks and a shallow-level advanced argillic alteration (quartz-alunite) lithocap which overprints phyllic alteration and copper sulphides of the upper sections of the porphyry system.
This summary is largely drawn from, and in places paraphrases, Hedenquist et al., 1998 and Chang et al., 2011.
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Travelling from Philippines to central Thailand, including half rest day in Bangkok ...................... evening Saturday 19 & Sunday 20 May. 2012.
Chatree - Thailand ............. Monday 21 & Tuesday 22 May, 2012.
The Chatree low sulphidation epithermal gold deposit is located in northern Thailand, 280 km north of Bangkok and 45 km southeast of the provincial capital of Pichit.
The mineralisation is located within a zone composed of parallel, north-south trending Upper Permian and Triassic volcanic arc sequences of island arc acid to intermediate volcanic and marine sedimentary rocks formed above a subduction zone prior to and during the collision of the Southeast Asian and Shan-Thai crustal blocks. It falls within the Loei-Pechabun Belt that extends from northern Laos to central Thailand which also contains low sulphidation epithermal gold, copper-gold skarn and porphyry copper occurrences. Multiple plutons and small batholiths of Late Carboniferous to Triassic granites occur within the arc.
In the Chatree district, the host sequence is composed of Middle Permian to Upper Triassic volcanic suites of rhyolite, dacite, andesite and basaltic andesite flows and their pyroclastic equivalents. Middle Triassic tuffaceous sediments, limestone and chert occur throughout the district. Bedding dips are of the order of 10 to 30°, while the dominant fault trends are north-south, NE and NW.
Two circular features, each with a diameter of 20 to 30 km have been detected in the Chatree district and are interpreted as possible calderas. Both are cut by NW and NE trending faults. Granodiorite and diorite porphyry stocks up to 4 km in diameter are mapped and interpreted from airborne magnetic data, while doleritic and dioritic dykes are common.
The Chatree gold deposit is hosted by Upper Permian to Upper Triassic volcanic arc porphyritic latite-trachyte flows, their pyroclastic equivalents and volcaniclastic rocks which have been and subordinate fine to medium grained volcaniclastics, all of which have been silicified over an area of 7.5 x 2.5 km.
The sequence is as follows, from the base:
- potash feldspar microporphyritic to porphyritic latite trachyte flows - undetermined thickness;
- interlayered and interfingered latite-trachyte flows and crystal/lithic/vitric tuffs and fine lapilli tuffs of porphyritic latite trachyte composition, with volcaniclastics, containing sandstones, siltstones, cherts and volcanic clasts - 200 m thick;
- lapilli tuff and clast supported agglomerate with subordinate crystal/lithic/vitric tuffs, volcanclastics and rare flows - up to 120 m thick.
Dykes of varying composition from andesite to basalt, as well as quartz monzonite, monzodiorite, latite and trachy-andesite, cut the sequence.
Overall, gold occurs in an adularia-sericite epithermal system within quartz-calcite-adularia veins, breccias and stockworks deposited in dilational zones during repeated reactivation of pre-existing structures.
Multiple phases of veining and alteration are recognised, generally being more intense in the vicinity of veins and breccias. There are at least 5 stages of veining in the lower levels of the deposit and three in the upper. Early vein assemblages are dominated by quartz, calcite, adularia and pyrite, followed by quartz, calcite, chlorite, ferroan dolomite, illite-smectite, pyrite and then quartz, calcite, pyrite. Post dyke alteration was less developed and mainly calcite with local epidote and adularia.
Each of the mineralised zones is associated with a major structure, trending at between 40 and 350° and dips that vary from vertical in some zones to as little as 20° in others. In each zone there has been repeated dilation to produce crustiform banding in the veins. Most veins have breccias developed on their margins, with a wide variety of angular to subrounded clast types. Stockwork and sheeted veins are common in the wall rocks. Individual veins may be a few mm to tens of metres in thickness and extend down dip for up to 400 m (in shallow dipping veins) to depths of as much as 170 m. The maximum depth of mineralisation is 280 m from the highest known point at 210 m ASL. The best grades are between 50 and 110 m ASL. Ore grades are found in quartz, carbonate veins, stockworks and breccias. There is around 2.5% sulphides which is almost exclusively pyrite with minor sphalerite, galena and chalcopyrite. The lateritised oxide zone averages 10 m in thickness, underlain by saprolite and a 30 m thick transition zone. In the primary zone the Ag:Au ratio averages 2.5:1 and 5:1 in the enriched oxides.
The most likely age of mineralisation has been taken to be Triassic.
Mining operations commenced in November, 2001 and from 2003 to 2011, 2.4 Mt of ore were treated per annum. The ore has a high metallurgical recovery (89 to 91%), with gold and silver extraction by standard Carbon in Leach (CIL) processing.
The total mineral resource, together with the previously mined ore, totals almost 150 t (5 Moz) of gold, at average grades of 1.2 g/t Au and 12 g/t Ag (Kingsgate Consolidate website, 2010).
In 2004 the project comprised five delineated prospects within an area of 7.5 x 2.5 km. Resources at that time totalled 31 tonnes of gold and 161 tonnes of silver. Grades varied over a wide range, but averaged 2.5 g/t Au. The Ag:Au ratio is 2.5:1 in the primary ore and 5:1 in the enriched zone. Gold mineralisation extends over a vertical interval of at least 280 m. Gold is visible and is accompanied by an average 2.5% sulphides, mainly pyrite, with associated galena and sphalerite.
Reserves and resources in September 2005 (Kingsgate Consolidated website, 2007) were:
Proven + probable reserve - 29.48 Mt @ 1.9 g/t Au, 18 g/t Ag = 54 t Au, 530 t Ag;
Measured + indicated + inferred resource - 52.05 Mt @ 1.9 g/t Au, 16 g/t Ag = 99 t Au, 833 t Ag.
Remaining reserves and resources at 30 June 2009 (0.5 g/t Au cut-off; Kingsgate Consolidated website, 2010) were:
Proven + probable reserve - 37.1 Mt @ 1.2 g/t Au, 12 g/t Ag = 45 t Au, 440 t Ag;
Measured + indicated + inferred resource - 81.7 Mt @ 1.18 g/t Au, 9 g/t Ag = 96.4 t Au, 750 t Ag.
Remaining reserves and resources at 30 June 2012 (0.3 g/t Au cut-off; Kingsgate Consolidated website, 2012) were:
Measured + indicated + inferred resource - 168.8 Mt @ 0.7 g/t Au, 5.8 g/t Ag = 118.5 t Au, 986 t Ag; including
Proven + probable reserve - 71.3 Mt @ 0.75 g/t Au, 7.8 g/t Ag = 53.8 t Au, 554 t Ag.
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Travelling from central Thailand to western Sumatra, Indonesia ...................... pm Tuesday 22 May & am Wednesday 23 May, 2012.
Martabe - Sumatra, Indonesia ....... Wednesday 23 and Thursday 24 May, 2012.
The Martabe group of gold deposits, including Purnama Pit 1 (Purnama Dalam Zone), Purnama Timur, Barani (Pelangi), Horas, Horas Barat, Ramba Joring (Baskara), Tor Uluala (Kejora), Tor Uluala Barat, Uluala Hulu Extension and Uluala Hulu, define a more than 7 km long, by ~1km wide mineralised corridor near the town of Sibolga in the North Sumatra Province of Indonesia, on the west coast of Sumatra Island, some 200 km south of Medan (#Location: 1° 31' 21"S, 99° 4' 3" E).
The main Pit 1 exploits the Purnama Dalam Zone and adjoining Purnama Timur deposit over a 1 km NNW-SSE trend. The Barani Pit is just over 1 km to the SE, while the Horas and Horas Barat deposits are just under 2 km SSE of the Barat Pit. The Ramba Joring Pit is just over 1 km NNE of the Main Pit 1. The Tor Uluala, Tor Uluala Barat, Uluala Extension and Uluala Hulu deposits are ~2 km north, ~2.2 km NNW, 2.5 km NNE and 3.5 km NNE of the main Pit 1.
These deposits comprise high sulphidation epithermal mineralisation within Tertiary volcanics and sediments proximal to a splay of the regional scale, NNW- to NW-trending, Great Sumatra Fault Complex. Wrench tectonics imposed by the oblique subduction of the Indo-Australian plate to the SW below the Eurasian plate of which Sumatra is part, resulted in episodic fault activity, pulses of high level magmatism, multistage phreatomagmatic breccias, flow dome complexes, hydrothermal alteration and gold mineralisation. Local structure reflects the regional dextral strike slip tectonics as NW to NNW fault sets, and related NE trending en echelon conjugate extensional faults to the east which together have promoted and localised fluid flow to produce zones of intense silicification and veining. A third, east-west set of contractional faults with a reverse component is also evident.
The geology of the Martabe district is divided into two domains by the NW to NNW trending Purnama Fault. To the SW are little disturbed and weakly altered mudstones, siltstones, sandstones and basaltic to andesitic lavas. In contrast, to the NE a comparable sequence of lithologies, with the addition of volcanic breccias, have been subjected to a complex series of multiphase magmatic, phreatomagmatic and hydrothermal influences.
The oldest rocks in the Martabe area are Palaeozoic meta-sedimentary rocks of the Tapanuli Group. The eastern part of the area is dominated by Triassic granites (correlated with the Sibolga granite batholith to the northwest), which have both intrusive and fault contacts with the older rocks.
The oldest of the overlying Tertiary units is the Barus Formation, which mostly comprises sediments, mainly conglomerates and sandstones, with minor siltstones and shales, that underlie much of the Martabe area. The Barus Formation sediments are overlain by the Miocene age Angkola Volcanics, a series of basaltic and andesitic lavas and breccias. The porphyritic andesite and volcanic breccia are significant host rocks to mineralisation.
A Late Tertiary dacitic dome complex, was emplaced along the faulted western margin of the granites. This complex comprises an early series of fault-controlled phreatomagmatic intrusive breccias followed by an intrusive dacitic flow dome complex that includes a dacitic porphyry, which is also a significant host rock, and a hornblende andesite, which is a late-stage intrusive and is generally unmineralised (except at Ramba Joring). Multiple diatreme facies overprinting each other are regarded as being coeval with the early dacitic intrusives in the north, while a second phase of multistage phreatomagmatic breccias have partially destroyed sedimentary sequences and define a concentric pattern around the late porphyritic hornblende andesite intrusive body.
Overlying all of these rocks is the Quaternary Toba Tuff, which forms a prominent plateau to the north of Martabe but is restricted to small erosional remnants in the Martabe area.
Mineralisation is dominantly hosted in steeply west dipping to subvertical breccia units and structural zones within the andesite host sequence that are presumed to be conduits for the hydrothermal fluids. The high sulphidation epithermal mineralisation followed extreme acid sulphate leaching of the wall rocks. This early stage leaching, which was focussed on the major structures, produced an advanced argillic assemblage with vuggy to massive silica, surrounded by a silica/dickite/alunite advanced argillic alteration envelope, grading outwards into silica-illite and peripheral kaolinite-illite argillic zones. The silicification (or removal of other minerals to leave silica) produced a vuggy, porous, brittle rock subject to fracturing which subsequently channelled mineralising fluids such that gold grades are directly proportional to the percentage of silica.
Individual deposits are composed of multiple mineralised zones, e.g., at Tor Uluala, mineralised zones are distributed over a strike length of over 1.2 kilometres, many of which outcrop at surface. This mineralisation is open at depth and is dominantly hosted in steeply west dipping to subvertical breccia units within an andesite/dacite host sequence.
Mineralisation within the corridor comprises:
i) an early weak (0.1 to 0.5 g/t Au) silica-pyrite phase immediately following the acid sulphate leaching;
ii) a low sulphidation phase of colloform banded chalcedonic silica veins with quartz and bladed carbonate 'boiling' textures with low (0.1 to 1 g/t Au);
iii) the main high sulphidation stage enargite/luzonite assemblage with associated covellite, native sulphur, pyrite, bismuthinite barite and marcasite in fractures and vugs that are mostly oxidised. In this latter stage, where un-oxidised, gold occurs as free grains and in enargite, while silver is in enargite/luzonite and as proustite/pyrargyrite inclusions in bismuthinite. The best gold is late in the hydrothermal event in late stage fracturing and crackle type brecciation of the silicified rocks marginal to the late clay-altered diatreme breccia;
iv) oxidation which is highly irregular and controlled by fracturing, has affected most of the fracture controlled mineralisation but does not seem to have resulted in any supergene upgrading. Minor to strong oxidation extending to depths greater than 100 m in places at Tor Uluala, although fresh sulphides can also be observed close to surface. While oxidation has not resulted in supergene upgrading, it has improved the metallurgical recovery of the refractory primary sulphide mineralisation.
At the end of 2003 the quoted resource at the Purnama deposit alone, amounted to 51.9 Mt @ 1.9 g/t Au for 98 tonnes (3.15 Moz) of contained gold (Newmont, 2004).
Resources (predominantly oxides) and reserves (all oxides), in Sept 2011 at a 0.5 g/t Au cut-off (G-Resources, 2011) were:
Proved + probable reserve at Pit 1, Barani and Ramba Joring - 45.5 Mt @ 2.1 g/t Au, 23.0 g/t Ag
Measured + indicated + inferred resources at Purnama, Purnama Timur, Barani, Ramba Joring, Tor Uluala and Uluala Hulu -
165.2 Mt @ 1.4 g/t Au, 13.6 g/t Ag (for 232 t, or 7.46 Moz of contained Au).
Measured+indicated+inferred resources (dominantly oxides) by deposit, in August 2012, at a 0.5 g/t Au cut-off (G-Resources, 2012) were:
Purnama - 82.04 Mt @ 1.7 g/t Au, 22 g/t Ag;
Purnama Timur - 4.97 Mt @ 1.5 g/t Au, 13.2 g/t Ag;
Ramba Joring - 38.35 Mt @ 1.0 g/t Au, 4.1 g/t Ag;
Barani - 16.9 Mt @ 1.2 g/t Au, 3.2 g/t Ag;
Tor Uluala - 31.5 Mt @ 0.9 g/t Au, 7.7 g/t Ag (inferred resources only);
Horas - 15.7 Mt @ 0.8 g/t Au, 1.7 g/t Ag (inferred resources only);
Uluala Hulu - 1.26 Mt @ 2.0 g/t Au, 24.0 g/t Ag (inferred resources only);
TOTAL - 190.72 Mt @ 1.31 g/t Au, 12.5 g/t Ag, for 250 t (8.05 Moz) of contained gold;
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Travelling from Sumatra to Bali within Indonesia ...................... Friday 25 May, 2012.
Tujah Bukit - East Java, Indonesia ....... Saturday 26 May, 2012.
The Tujuh Bukit group of telescoped epithermal - porphyry copper-gold-silver deposits are located in the province of East Java in Indonesia, ~205 km southeast of Surabaya, and 120 km due west of Denpasar in Bali (#Location: 8° 35' 21", 114° 01' 08" E).
The Tujuh Bukit deposits lie within the east-west trending Sunda-Banda magmatic arc, above the north dipping subduction zone at the convergent intersection of the Australian-Indian and the Eurasian plates. The main cluster of deposits and prospects at Tujuh Bukit includes Tumpangpitu, Candrian, Katak and Gunung Manis distributed over an area of ~5 km in diameter.
Tumpangpitu - comprises high sulphidation Cu-Au-Ag epithermal mineralisation that is telescoped onto a large underlying Au-rich porphyry Cu-Au-Mo system. Broadly, the overall mineralised system covers a northwest-southeast elongated area of ~2.4 x 1.4 km, and comprises a deep, magnetic, tonalitic intrusive complex that has been emplaced within an older and more extensive feldspar-hornblende diorite stock. The latter has in turn been intruded into a cover sequence of lithic and crystal-lithic volcanic breccias that are found at shallow levels of the deposit and conformably overlie a succession of sedimentary rocks that partially dip inwards towards the tonalitic intrusive centre. The contact between the tonalitic complex, believed to be the source of porphyry mineralisation, and the older intrusive, volcanic and sedimentary rocks, is characterised by the presence of one or more extensive diatreme breccia bodies and numerous smaller breccia masses. In particular, the upper margin of the tonalitic complex is intensely altered and fluid metasomatised, and is transitional with the intrusive breccia (which has upward entrained interstitial melt). This breccia is in turn transitional at shallower depths with hydrothermal breccias as fluids are interpreted to have progressively exsolved from the decompressed melt.
The shallow epithermal levels of the system are dominated by intense silica-clay-alunite-pyrite hydrothermal alteration of the andesitic lithic volcanic breccia, diatreme breccia, hydrothermal breccias and diorite over an area of ~4 x 2.5 km. This envelope is cross-cut by several northwest-trending (possibly structurally controlled) zones of hydrothermal breccias that have undergone advanced argillic alteration and form ~2500 x 300 m ridges that parallel the regional structure and the aeromagnetic grain. This resistant, more siliceous, advanced argillic alteration comprises vuggy silica, silica-alunite, silica-alunite-clay, silica-clay-alunite and silica-clay.
The deeper parts of the deposit are characterised by alteration and veins characteristic of porphyry systems, centred on the apex of a large, deep tonalitic intrusive complex. Porphyry-style Cu-Au-Mo occurs within a shell of magnetite, quartz-magnetite and quartz vein stockwork within and around the periphery of the tonalitic complex, overprinting the outer margins of the tonalite and adjacent intruded wall rocks. The mineralisation is dominantly within areas of phyllic alteration overprinting potassic assemblages, and to a lesser degree within zones of potassic alteration within the tonalite.
An ovoid, upward flaring diatreme breccia body, with a diameter of ~500 m, occurs within the central-western part of the deposit, and contain clasts of porphyry style mineralisation. It is composed of milled polymict breccia in its upper sections, with roots that penetrate down into the tonalite, where it has characteristics of an intrusion breccia. The diatreme breccia body is overprinted by steeply oriented structural feeders to high sulphidation mineralisation.
There are five zones of known near surface Au-Ag oxide mineralisation and deeper sulphide copper-gold mineralisation within Tumpangpitu, denoted A to E. In October 2012, a JORC compliant inferred resource for the porphyry sulphide mineralisation was released (Intrepid Mines) at a 0.2% Cu cutoff of: 1.9 Gt @ 0.45 g/t Au 0.45% Cu, 90 ppm Mo. This resource is within an area of 3.4 x 2.8 km and vertical extent of ~1.1 km, and has a high grade core of 1 Gt @ 0.61 g/t Au, 0.61% Cu,
Not included within the porphyry resource, is the Tumpangpitu oxide Au-Ag zone with a measured + indicated resource of 70 Mt @ 0.71 g/t Au, 21 g/t Ag and an inferred resource of 19 Mt @ 0.75 g/t Au, 21 g/t Ag for 64 t of contained gold (Intrepid Mines, Sept. 2012).
Drilling several hundred metres to the east of, and angled away from, this resource, has intersected additional porphyry mineralisation (with no overprinting high sulphidation-style) at a higher RL, apparently associated with a separate intrusion and accompanying stockwork vein system. The best intersection has been 689 m @ 1.0% Cu, 0.85 g/t Au, including 244 m @ 1.42% Cu, 1.06 g/t Au and 466 m @ 1.19% Cu 1.05 g/t Au (Intrepid Mines release Feb. 2012).
Katak - is located ~2 km northeast of Tumpangpitu where an 800 x 300 m magnetic diorite intrusion with associated stockwork chalcopyrite-pyrite mineralisation is developed.
Candrian - is a northwest-southeast elongated prospect which covers an area of 2.5 x 1 km, ~2 km east of Tumpangpitu and underlies soils rich in pyrophyllite, dickite and alunite. Drilling has encountered porphyry style gold-copper mineralisation with intersections grading ~0.8 g/t Au, 0.2% Cu, with cores of >1.5 g/t Au and >0.35% Cu.
Gunung Manis - is a low sulphidation epithermal system that lies ~3 km east of Tumpangpitu, 1 km east of Candrian and 2 km southeast of Katak, and is the basis of artisanal mining of visible gold in drainage systems. Mineralisation is associated with an area of argillic alteration within a diorite body that coincide with a magnetic low, and comprises narrow (1 to 10 mm thick) open space sheeted veins and fractures. Chalcedony occurs in many veins, with abundant bladed calcite. Locally quartz-sericite-pyrite veins are observed. Sulphides include pyrite, chalcopyrite, galens and tetrahedrite-tennantite.
Further porphyry style mineralisation is indicated by mapping, sampling and historic gold and copper workings at Salakan, ~ 5 to 6 km to the northwest of Tumpangpitu.
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Travelling from Bali, Indonesia, to eastern Papua New Guinea, including half rest day in Bali ...................... Sunday 27 & Monday 28 May, 2012.
Lihir - Papua New Guinea ........... Tuesday 29 May, 2012.
The Ladolam gold mine lies within the Luise Caldera, located on the eastern side of the north-south elongated, 20 x 12 km, Lihir Island within the Tabar-Lihir-Feni-Tanga chain of islands northeast of New Ireland (Location: 152°38'E, 3°08'S.
Lihir is a major bulk gold deposit with mineralisation represented by an earlier un-economic porphyry stage, and a later, overprinting low sulphidation advanced argillic phase that accompanied the introduction of the bulk of the gold mineralisation. The three main orebodies, Minifie, Lienitz and Kapit (the latter two connected by the Link zone) are located within a generally north-south elongated area inside the breached 5.5 x 3.5 km Luise Caldera on the east coast of Lihir Island. All fall within a 1.5 to 2 km radius of the centre of the caldera. Minifie has plan dimensions of the order of 700 x 400 m with mineralisation extending from 50 m above to 150 m below sea level. Lienitz is 600 x 300 m in plan and is mineralised from 140 m above to 250 m below sea level, although the bulk of the ore is from sea level to 200 m below.
Lihir Island is one of a series of four volcanic island groups which rise from a submarine platform and form a chain roughly parallel to and 50 km to the north-east of New Ireland in Papua New Guinea. This chain is also parallel to and 100 km south-west of the Kilinailau Trench where the Pacific plate subducted below the Melanesian Arc which is peripheral to the Australian Plate until the Mid Miocene. The magmatic arc of the Tabar-Lihir-Feni-Tanga chain of islands however, is apparently related to subduction below the New Britain Trench to the south, exploiting a pre-existing crustal weakness from the earlier subduction, and/or a transform fault associated with extension on the Manus Spreading Centre.
The 3 to 1 Ma Luise Caldera occupies the youngest of several Miocene to Holocene alkaline volcanoes developed on the island. The oldest rocks on the island comprise 350 m of Pliocene to Miocene mafic lavas and volcaniclastics of the Londolovit Block on the northern tip, while a NE-SW elongated, unconformably overlying and partly fault bounded block of Pliocene to Pleistocene mafic lavas, agglomerates and lahars around 500 m thick, the Wurtol Wedge adjoins, the Londolovit Block and stretches across the island to the west coast. The Londolovit Block and Wurtol Wedge probably represent portions of a deeply eroded Late Miocene to Pliocene volcano. Sub-economic gold with associated potassic, phyllic and argillic alteration is hosted by altered intermediate volcanics and intrusives. The southern part of the island is occupied by the Pliocene to Pleistocene Kinami Volcano lavas, pyroclastics, breccias and derivative epiclastics. This volcano has associated phyllic and argillic alteration, which grades into potassic zones at depth, which in turn pass out into propylitic zones. Low grade gold accompanies the phyllic alteration. The Pleistocene Huniho Volcano, which has a number of satellite cones and craters makes up the north-western segment of the island with tephras and lahars overlying mafic lavas. The late 5.5 x 3.5 km Luise Caldera is the remnants of a volcanic edifice on the central east coast, north of the Kinamo volcano and east of the Wurtol Wedge. The associated volcanics include trachy-basaltic lavas, pyroclastics and breccias. All three of the remnant volcanic centres have undergone some form of seaward collapse, particularly the most recent, the Luise volcano. Raised coral reefs fringe most of the island, but are absent in Luise Harbour, suggesting they predate the caldera flooding event.
Volcanic rocks predominate in the upper parts of the ore zones of the Luise Caldera, and the margins of the system, and are underlain by intrusives. The host volcanic sequence occupies most of the floor of the Luise Caldera and comprises intermediate (latitic, andesitic and trachytic) lavas, tuffs and volcanic breccias. These extrusives are intruded by a series of fine to medium grained, quartz poor but silica saturated monzonitic to monzodioritic porphyries ranging from pyroxene microdiorite to biotite syenite and some andesite porphyry.
Intrusion-related potassic alteration occurred in the period 0.917 to 0.342 Ma, while the epithermal gold mineralisation is dated at 0.336 Ma, possibly continuing to 0.1 Ma, although the geothermal system is presently active. While the Luise Caldera trends elongate NNE, it is cut by north-south structures associated with the deep fractures that are interpreted to localise the magmatism of Lihir Island. NW-trending fractures are also evident and host the NE-dipping Minifie mineralisation. The Luise volcano is interpreted to have collapsed sideways at about 0.34 Ma. NE-dipping listric-style faults are interpreted to have developed within the remaining underlying part of the edifice. This failure removed about 1 km from above an active porphyry Cu-Au deposit. In doing so, it is believed to have removed the confining pressure and allowed epithermal fluids to escape up the listric faults produced during the collapse, and as such initiated development of the epithermal mineralisation.
Mineralisation is coincident with a NNE trending zone of hydrothermally altered volcanics and breccias intruded at depth by porphyries. Three stages of alteration have been identified,
i). an early porphyry style accompanied by low grade Cu-Au-Mo mineralisation at depth, with a potassic core (mostly phlogopitic biotite-anhydrite with lesser orthoclase, magnetite with pyrite, chalcopyrite and molybdenite) and a peripheral propylitic phase (chlorite ±amphibole ±albite ±epidote ±calcite ±magnetite);
ii). A transitional zone, which forms the bulk of the ore at Minifie, characterised by pervasive adularia ±illite and fine grained, refractory auriferous pyrite within extensive shallow hydrothermal breccias located above biotite altered porphyritic stocks, and grading abruptly downward into sub-economic anhydrite-K feldspar ±pyrite alteration of the porphyry system; and
iii). a younger, overprinting and shallower low sulphidation epithermal style as in the Lienetz area, comprising advanced argillic (alunite ±opaline silica ±kaolinite ±sulphur), argillic (kaolinite ±smectite ±illite) and phyllic (illite ±K-feldspar ±silica). Alteration generally forms a horizontal layering, with porphyry style potassic and propylitic assemblages at depth, grading up through phyllic to epithermal argillic and advanced argillic phases nearer the surface.
Alunite and opal fill veins and stockworks near the surface, grade to quartz and adularia at intermediate depths, underlain by anhydrite and carbonate. Sulphides and gold mineralisation generally mimic the horizontal alteration pattern, although some follows steep 'feeder' fractures. The horizontal zonation represents a surface oxidation regime, related to mixing with ground water, passing down progressively into argillaceous altered and silica clay, through to a boiling layer and then the anhydrite sealed zone. Grades are best from the silica-clay to the boiling zones. Gold is predominantly fine grained and contained within pyrite and marcasite. The overall sulphide or reactive sulphur content averages 6%. Geothermal activity is still taking place.
The total Identified Mineral Resource at January 2004 (based on a 1.5 g/t Au cut-off) was 442.5 Mt @ 3.14 g/t Au for 1390 tonnes (44.7 Moz) of contained gold. Proved + probable reserves totalled 163.5 Mt @ 3.88 g/t Au for 635 tonnes (20.7 Moz) of contained gold.
The total Measured + Indicated + Inferred Mineral Resource at August 2011 (Newcrest Mining website, 2012) was 830 Mt @ 2.1 g/t Au for 1745 tonnes (56 Moz) of contained gold. Proved + Probable Reserves (a subset of the resources) totalled 400 Mt @ 2.4 g/t Au for 960 tonnes (31 Moz) of contained gold. The Lihir operation is owned by Newcrest Mining Limited.
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Hidden Valley - Papua New Guinea ................. Wednesday 30 May, 2012.
The Hidden Valley low sulphidation carbonate-base metal-gold epithermal deposits are within the Morobe Goldfield, in the Morobe Province of Papua New Guinea, 210 km NNW of Port Moresby. The total production from the Goldfield to 1975, which also includes the historic Wau and Edie Creek epithermal deposits and the extensive Bulolo placer accumulations, amounted to 115 tonnes of Au, of which approximately 15% was from lodes, the remainder being from alluvial operations.
The Morobe Goldfield lies within the Highlands Fold Belt of the New Guinea Orogenic Province, situated between the overthrust Papuan Ultramafic Belt to the north-east and the Neogene sediment filled Aure Trough to the south-west. The basement in the district, the Cretaceous Owen Stanley Metamorphics (which include blue-grey graphitic slate, chloritic and sericitic schist and phyllites, with minor quartzite and marble), is intruded by the 14.3 Ma Miocene Morobe Granodiorite Batholith. These are overlain by a Pliocene sequence that includes, the Bulolo Ignimbrite which is several hundred metres thick and dips at 40 to 65°E, and the Otibanda Formation comprising 700 m of poorly sorted auriferous conglomerate and sandstone with minor reworked tuff and ignimbrite near the base, and the Pliocene (3.8 to 3.4 Ma) intrusive dacite to andesite porphyry of the Edie Porphyry which has strongly altered (silica-sericite-pyrite) the surrounding metamorphics. These are all cut by the Namie Breccia of the Wau Maar complex which is generally coarse and poorly sorted and is believed to be of subaerial phreatomagmatic origin. Some of the Edie dacite porphyries in the district are present as domes emplaced during the formation of the maar and generally found along the ring fault. All of these are overlain in part by Pleistocene fluvial and fluvioglacial sediments. A major NNW trending, 10 km long, east dipping fault zone, the Escarpment Fault cuts most rocks up to the beginning of Maar Formation. Exposures of the fault show fault gouge, silicification, pyritisation, as well as hosting intrusions of Edie Porphyry and hydrothermal breccias similar to the Namie Breccia.
All of these rocks lie within the down faulted, NNW-SSE trending Bulolo Graben/Wau Basin, between two blocks of Morobe Granite. This graben is extensional, with listric faults, and some structures that, at stages, were subjected to reverse movement. These graben faults have influenced the distribution of breccia and intrusives and are believed to be in part reactivated older structures. It would seem that the graben was active prior to, during and probably after the Pliocene magmatism and mineralisation. Primary gold occurs in a number of forms in the district. These include i). Lodes and Veins - stringers and veins of quartz with gold on the margin of the Morobe Granodiorite, mainly in the Kaindi metamorphics, ii). mesothermal gold associated with the Pliocene dacite-andesite porphyries and iii). the bulk of the gold which is epithermal and hosted by rocks related to the Wau Maar.
The extensive vertical relief in the Morobe district and the graben preserves and exposes varying styles of intrusion-related low sulphidation epithermal gold mineralisation. At the deepest levels (e.g., the Hamata deposit), massive quartz-sulphide-gold style mineralisation is associated with magnetite-K feldspar porphyry style alteration; at mid-levels (e.g., Wau and Hidden Valley) carbonate-base metal-gold mineralisation occurs, while bonanza gold-grade epithermal quartz-Au-Ag mineralisation (e.g., Edie Creek) crops out at the highest topographic level. Most deposits display overprinting relationships (e.g., Edie Creek), while the Kerimenge prospect hosts three low sulphidation mineralisation styles, variably telescoped over a 250 m vertical extent.
In the Hidden Valley project area the Morobe Granodiorite batholith (locally a coarse grained monzogranite) is flanked by fine metasediments of the Owen Stanley Metamorphics. Both are cut by narrow dykes and small stocks of Pliocene porphyry (correlated with the Edie Porphyry) ranging from hornblende-biotite to feldspar-quartz porphyries. A number of commonly argillic altered and gold anomalous breccias are known, including both hydrothermal and overprinting structural breccias.
The Hidden Valley deposit area is dominated by a series of post Miocene faults controlling the gold mineralisation, including an early north trending set and the main NW faulting. The NW trending, 30 to 35° NE dipping Hidden Valley Fault, with its associated breccia, is the most important structure in the deposit area, forming a planar structural base underlying the main mineralisation. The main Hidden Valley Kaveroi orebody is broadly tabular, up to 100 m thick, and is localised immediately above and parallel to the Hidden Valley Fault.
Alteration is variable, ranging from pervasive to intense, accompanying veins and breccias. All rocks exhibit propylitisation (chlorite-epidote-pyrite), from pervasive to discrete alteration associated with structures. Argillic alteration (kaolinite-carbonate and occasionally smectite) is associated with breccias and veins. Adularia is present in carbonate veins, while an envelope of phyllic alteration with an illite-leucoxene-quartz assemblage commonly surrounds these veins, joints and breccias.
Gold and silver mineralisation is found within veins that occur within structurally controlled stockworks within the granodiorite on the hangingwall of the Hidden Valley Fault in three vein-alteration suites, as follows: i). quartz veins with weak pyrite-chlorite-epidote propylitic alteration, which are generally barren, and are thicker and more prevalent in the metamorphics than in the granodiorite; ii). chlorite veins with hematite-quartz-carbonate-pyrite, which are mainly found as either larger composite veins or a microveins in stockworks in the granodiorite, above and below the Hidden Valley Fault, often associated with strong propylitic alteration and are only rarely gold mineralised, and iii). carbonate veins which contain the main gold ore, and were developed in three pulses, namely a). irregular carbonate-quartz-adularia veinlets which are not strongly mineralised; b). carbonate-adularia-mixed sulphides-gold as a second pulse, occurring as carbonate veining (often with collorm banding) enveloped by quartz-adularia-sulphide selvages, from which a 4.15 Ma date has been obtained from adularia, with more sulphides (pyrite, sphalerite, galena and chalcopyrite) and rare visible gold, and c). carbonate-kutnahorite-gold representing a third phase which has visible gold and tetrahedrite infilling corroded earlier sulphides. Gold correlates best with the carbonate content of these veins rather than with adularia or base metal sulphides.
The Hamata deposit is hosted within the Morobe granodiorite, which it is cut by two types of andesite porphyry dyke, characterised by feldspar and hornblende phenocrysts respectively, and by dacite porphyry (correlated with the Edie Porphyry). The Edie Porphyry occurs as a larger, unaltered body to the west, and a series of altered, smaller dykes within the ore zone. Primary gold mineralisation at Hamata is related, and proximal, to the altered Edie Porphyry intrusions. It occurs in at least three, NE-striking and 45 to 50°SE dipping, sub-parallel zones (Masi, Lower and Eastern) that are each up to 50 m thick. They are composed of low density and diffuse zones of quartz-pyrite veining within K feldspar-sericite altered granodiorite, although at the upper contacts of each zone, 3 to 4 m wide reefs of pyrite-hematite±magnetite-quartz veins are well developed, and contain the main gold ore zones. Each reef zone contains one or more of these ore zones, with strike lengths of up to 600 m, down-dip extents of 200 to 300 m and thicknesses of 1 to 10 m. Three stages of mineralisation are recognised: Stage 1 - semi-regional thin magnetite-hematite-pyrite veins with K feldspar-sericite selvages; Stage 2 - coarse pyrite-hematite-magnetite-quartz fracture fill with sericite alteration and the bulk of the gold mineralisation, occurring as non-refractory 20 to 30 µm blebs in pyrite; and Stage 3 - shear veins of quartz-pyrite-arsenopyrite-marcasite with sericite and clay alteration, and only low, refractory gold.
The Hidden Valley Mine consists of the Hidden Valley Kaveroi and Hamata open pits, which are ~6 km apart, and an ore processing facility, situated in steep, heavily forested, mountainous terrain.
At the end of 2003 the total mineral resource at Hidden Valley and the nearby Hamata and Kerimenge prospects was: 53.3 Mt @ 2.7 g/t Au, 47.1 g/t Ag for 144 tonnes (4.72 Moz) of gold, with 1730 tonnes (55.7 Moz) of silver.
Reserves and resource at Hidden Valley Kaveroi as of June 2010 (Morobe JV, Newcrest Mining website, 2012) were:
Total Measured + Indicated + Inferred Mineral Resource - 96 Mt @ 1.65 g/t Au, 33 g/t Ag for 158 tonnes (5.1 Moz) of contained gold;
Proved + Probable Reserves (a subset of the resources) - 55.7 Mt @ 1.82 g/t Au, 35.6 g/t Ag for 101 tonnes (3.26 Moz) of contained gold.
Reserves and resource at Hamata as of August 2011 (Morobe JV, Newcrest Mining website, 2012) were:
Total Measured + Indicated + Inferred Mineral Resource - 6.66 Mt @ 2.4 g/t Au, for 15.9 tonnes (0.5 Moz) of contained gold;
Proved + Probable Reserves (a subset of the resources) - 5.6 Mt @ 2.12 g/t Au for 11.9 tonnes (0.38 Moz) of contained gold.
The operation is controlled by the Morobe Mining Joint Venture (Harmony 50%, Newcrest 50%).
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Porgera - Papua New Guinea ................. Thursday 31 May, 2012.
The Porgera gold deposit is located at an altitude of 2500 m in the remote Enga Province of western Papua New Guinea, 130 km west of Mount Hagen and 600 km northwest of Port Moresby, (Location: 5° 28'S, 143° 05'E).
The operation, which was commissioned in 1990, was based on the lower grade, open pit, Waruwari bulk gold orebody (originally calculated at 54 Mt @ 4.3 g/t Au) and the high grade, underground, tabular Zone 7 deposit (5.9 Mt @ 27 g/t Au).
The orebody is associated with the Late Miocene (6.0±0.3Ma), silica-poor, K-rich, shoshonitic Porgera Intrusive Complex (PIC) of micro-gabbro to diorite and feldspar porphyry, which was intruded at an estimated palaeodepth of ~2.0 to 2.5 km, into previously thrusted late Jurassic to Cretaceous pelitic sediments deposited on the shelf of the Australasian plate margin. Intrusion of the PIC magmas occurred slightly prior to, or during, the uplift of the Papuan Fold and Thrust Belt that was formed as a result of the collision of the Indo-Australian plate with the Bismarck Sea and Caroline oceanic plates to the north and east. The mineralising event occurred immediately after the emplacement of the PIC and was locally accompanied by intense phyllic alteration that affected the shallow-level intrusions. A late, steeply dipping normal structure, the Roamane fault, crosscuts all intrusions and hosts the richest gold bearing veins and breccias.
The host shelf sequence includes the Jurassic Om Formation, a thinly bedded, dark grey carbonaceous siltstone with pyritic calcareous nodules, overlain by the Cretaceous Chim Formation which comprises bioturbated and laminated grey calcareous siltstone and mudstone with local sandy horizons. Eocene to Miocene limestone crops out further south and east of the mine.
Regionally, these sediments are broadly folded about east to ESE trending axes, but dip steeply around the intrusive contacts, adjacent to which they are altered. The earliest faults are flat dipping and may be related to early mineralisation. Steeper NE and E-W faults post date the intrusive complex and are also mineralised. The main gold ore is associated with the E-W trending, 60 to 75° dipping Romaine Fault and steeper associated footwall and hangingwall fractures.
The immediate host package comprises Cretaceous sediments including mudstones, shales, siltstones and sandstones, which are often calcareous. Syn-mineral thrusting has placed calcarenite in contact with black shale, the main host rock in the mine area, while post-mineral thrusting on the Boundary Fault has resulted in the emplacement of the calcarenite - black shale package over fissile brown shale.
The gold mineralisation at Porgera displays a staged geological development associated with evolution of the PIC as a differentiating alkaline intrusion, intruded as stocks (<500 m diameter) and dykes. Aeromagnetic data indicate that the intrusions exposed at the mine occur as apophyses to a much larger (3.5 km diameter) buried magmatic source, which is inferred to have domed the host sedimentary rocks.
Two main mineralising events have been recognised, namely:
Stage 1 associated with the emplacement of generally porphyritic intrusions of, i). initial augite hornblende diorite, ranging from equigranular to porphyritic, with associated 'andesite' dykes and sills; ii). subsequent hornblende diorite with radiating hornblende rosettes in a fine-grained matrix.
Stage 2 mineralisation was associated with quartz-feldspar porphyry stocks and dykes emplaced as the most differentiated later stage of PIC magmatism and was initiated as localised crosscutting milled matrix fluidised breccia dykes, containing fine silica-pyrite, formed by explosive hydrothermal activity at an elevated crustal setting.
Three types of gold-bearing vein assemblages are recognised:
(i) Pre-Stage I magnetite-sulphide-carbonate veins with economically insignificant gold, localised in the central and deep parts of the mine, some 1000 m below the current workings. Magnetite is dominant, with associated secondary biotite and lenses of pyrite, pyrrhotite, chalcopyrite, and rare inclusions of gold in pyrite;
(ii) Stage I base metal-sulphide±gold-carbonate veins. Sulphide minerals are pyrite, sphalerite and galena, with rare chalcopyrite, arsenopyrite, marcasite, freibergite and proustite/pyrargyrite. The gangue consists of complex Mn-Ca-Mg carbonates and subordinate quartz. The veining carries refractory Au in arsenical pyrite, disseminated in zones of intense phyllic alteration, plus minor Au in the associated base metal sulphide veins localised by NE- to NNE-trending pre-mineral faults and the margins of intrusive bodies. Gold ocurs in pyrite, either in an invisible form, or as microscopic inclusions of native gold. This stage predominates in the Waruwari open pit area; and
(iii) Stage 2 quartz-roscoelite-pyrite-gold and breccia veins, which host the highest gold grades and are the economically most significant vein assemblage. Some pyrite is arsenian, with local arsenopyrite, although chalcopyrite is rare. The Ag-Au tellurides petzite, krennerite and calaverite, and the Ag telluride hessite are present, as well as coloradoite and altaite. Gold dominantly occurs in the native form and as electrum. Veins are strictly fault controlled, in particular within the late normal Romaine Fault.
Gold grades and grain size increase from Stages 1 to 2. In addition, Stage 2 mineralisation is characterised by high silica and low sulphur, and consequently by higher Au:S (10:1) ratios compared to Stage 1 (2:1) which has low silica and high sulphur.
The majority of gold is present as extremely fine grains contained within pyrite. The Romaine Fault is around 4 km long, with the main mineralised zone being over a strike length of 1 km, with a width of up to 20 m. The highest grade ore has a vertical extent of 240 m, and is found below a depth of 400 m from the surface. At depth the roscoelite mineralisation gives way to a gypsum-rich zone with less sulphide. Mineralisation is commonly associated with the altered contact zones of the intrusive complex.
At December 2007, reserves and resources were:
Proven + probable reserves of 97.96 Mt @ 3.98 g/t Au
Measured resources of 34.8 Mt @ 2.55 g/t Au
Indicated resource of 24.0 Mt @ 1.96 g/t Au
Inferred resource of 11.1 Mt @ 2.89 g/t Au
In 2010, proven + probabe reserves contained 230 t (7.4 Moz) of gold (Barrick website, 2012), presumably at similar grades to the 2007 reserve.
Barrick is the owner of a 95% joint venture interest and is the operator of the Porgera gold mine.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available from the Epithermal Gold 2012 Tour options page.
This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
Porter GeoConsultancy Pty Ltd|
6 Beatty Street
LINDEN PARK 5065
Telephone: +61 8 8379 7397
Facsimile: +61 8 8379 7397.