CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Image: A Golden Pagoda in the Forbidden City.
Porter GeoConsultancy continued its International Study Tour series of professional development courses by visiting a representative selection of the different styles, and the more significant gold deposits, in the major gold provinces from the north of China to neighbouring Laos and Thailand in the south.
in Beijing, China,
Jiaodong Goldfield on the Shandong Peninsular of eastern China,
Jinchangyu on the northern margin of the North China Craton, Hebei, China,
Qinling - southern margin of North China Craton, Henan, central China,
Zijinshan epithermal/porphyry deposit in Fujian, south-eastern China,
Chatree epithermal deposits, Thailand,
Sepon carbonate hosted and porphyry related deposits in Laos,
Phu Bia epithermal/porphyry gold-copper, Laos.
The tour commenced in Beijing, China on the evening of Saturday 6 October and ended in Bangkok, Thailand on the evening of Sunday 21 October, 2007. Participants were able to take any 5 or more days, up to the full tour, as suited their interests or availability.
The main components of the planned itinerary were:
Overview Workshop in Beijing ...................... Sunday 7 October, 2007.
A comprehensive workshop was run in Beijing, China at the beginning of the tour to provide a context to the tectonic, geological and metallogenic setting of the gold deposits to be visited in China and descriptions of and comparisons with other deposits in the region not on the itinerary.
The workshop was led by Dr Taihe Zhou of the Sino-QZ Group, an acknowledged international expert on the gold deposits of China, their setting, distribution, geology and mineralisation.
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Jiaodong Goldfield - Shandong Peninsular ...................... Monday 8 October, 2007.
The Cretaceous fault and shear zone controlled orogenic gold deposits of the 200 km long Jiaodong Gold Province are located on the Shandong (Jiaodong) Peninsular of eastern Shandong Province, northern China, approximately 450 km south-east of Beijing.
The surface expression, and an extensive suite of ore, mineralisation and alteration styles and host-rocks were studied from the significant mines operated by the Zhaojin Mining Industry Co. Ltd within the Jiaodong Goldfield. In addition the ore stockpiles and facilities at the company's Dayigezhuang mine were also visited.
The most important deposits in the goldfield include Sanshandao, Jiaojia, Xincheng, Linglong, Taishang, Dayigezhuang, Cangshang, Wangershan, Jiehe, Hexi, Fushang, Daingezhuang, Pengjiakuang, Denggezhuang and Jinqingding. They are some of the up to 20 significant and 300 smaller deposits and prospects that constitute the Jiaodong Gold Province.
Most of the deposits are clustered in an east-west trending belt distributed over a 75 km interval towards the western end of the gold province. Another significant cluster, the Rushan group, are some 100 km to the ESE.
The Shandong Peninsular is on the south-eastern portion of the North China craton, immediately north of the easternmost part of the ultra-high pressure belt that marks the Late Permian to Early Jurassic suture between with the Yangtze craton to the south. It is made up of a sequence that includes the late Archaean (2.94 to 2.67 Ga with 3.0 to 3.4 Ga components) tonalite-trondhjemite-granodiorite and mafic to felsic volcanic and sedimentary rocks all of which have been subjected to amphibolite to granulite facies metamorphism to form the granulite, gneiss, amphibolite and biotite-bearing schists of the Jiaodong Group. These Archaean rocks are unconformably overlain by a Proterozoic sequence comprising:
i). The Paleoproterozoic Jinshan Group, which is composed of a sequence of mainly silty clastics and calcareous-magnesian carbonates with intercalated mafic to ultramafic volcanics which have all been metamorphosed to a suite of mica schist, quartz-feldspar-biotite gneiss, marbles and graphitic rocks.
ii). The Paleoproterozoic Fenzishan Group, comprising fine-grained gneiss, mica-quartz schist, feldspathic quartzite, magnetite-bearing rocks, marble and graphitic rocks that are the result of upper greenschist to amphibolite facies metamorphism of silty clastic sediments, Mg-rich carbonates, and calcareous clastics and pelites.
These two groups were deposited between 2484 and 2381 Ma and metamorphosed during the Luliang Orogeny from 2224 to 1847 Ma, and are overlain by,
iii). The Neoproterozoic lower greenschist facies limestones, dolomite, slate and phyllite of the Panglai Group.
All of these rocks have been subjected to multiple stages of granulite- to greenschist-facies metamorphism over the timespan 2945 to 1674 Ma. They are overlain by Mesozoic shales, clastics and volcanics and are intruded by voluminous Mesozoic (Jurassic to Cretaceous) Yanshanian age granitoid intrusives, which occupy over 40% of the terrane, and were emplaced in two episodes, 164 to 155 and 130 to 126 Ma, the first of which is largely due to anatectic reactivation of the basement rocks. The mineralised Yanshanian granites are grouped into three types, i). the 160 to 150 Ma Linglong biotite granite with a gneissic structure; ii). the 130 to 126 Ma Guojialing biotite-hornblende granodiorite; and iii). the medium to coarse-grained massive Luanjiahe granite, all of which are multiphase batholiths containing numerous xenoliths of Jiaodong Group basement rocks. They are typically calc-alkaline in character and are interpreted to have formed from anatectic reactivation of basement volcanic and sedimentary rocks.
Two main stages of deformation have been defined in the Jiaodong gold province during the late Mesozoic, the first of which was dominated by NW-SE oblique compression, producing a series of NNEŠ to NE-trending brittle-ductile shear zones with sinistral oblique reverse movements, followed by a later reactivation, expressed by brittle structures and half-graben basins. These structures are believed to be subsidiary to the major Tan-Lu fault.
The gold deposits of the 200 km long Jiaodong Gold Province together contain more than 1600 t of gold and in 2000 produced 55 t of gold. Approximately 80% of the reserves are in the 3500 sq. km Zhaoyuan-Laizhou gold belt within this province. Around 85% of the gold resources are spatially associated with the 3000 sq. km, 165 to 125 Ma Linglong and Guojialing Granites. Only 5% are hosted by Precambrian metamorphic rocks. The emplacement of gold is largely constrained to the time interval between 130 and 120 Ma.
Two main types of gold deposit are recognised, namely: i). Linglong type large quartz vein systems, predominantly hosted within granites in second or third order brittle fractures, occurring as single or multiple, relatively continuous quartz veins; and ii). Jiaojia type defined by disseminated and small veinlets/stockworks along first order regional faults and shear zones, surrounded by broad alteration halos and often concentrated along granite-metamorphic rock contacts.
Both types of gold deposits are similar in their geology and geochemistry. Ore mineral assemblages are dominantly pyrite, with lesser galena, sphalerite, chalcopyrite, pyrrhotite and arsenopyrite. Four stages of mineralisation recognised include: i). pyrite-quartz; ii). gold-bearing pyrite-quartz; iii). gold-bearing multiple metallic sulphides and quartz; and iv). quartz-carbonate. Gold and silver are mainly present as native gold and electrum and are concentrated in stages ii and iii. Alteration minerals are sericite, muscovite, sulphides (mainly pyrite, pyrrhotite and arsenopyrite), carbonates, K-feldspar, chlorite, and kaolinite. The mineralisation has returned radiometric ages of 100±4 to 135±5 Ma for eight major gold deposits, 5 to 20 m.y. younger than those of corresponding granitic intrusions in the same belt.
Sanshandao, one of the largest of these deposits, is located around 30 km north of Laizhou City and is described herein as an example. The deposit is confined to the major NE trending Sanshandao fault which cuts the Sanshandao Granodiorite, a member of the Guojialing granodiorite suite. The Sanshandao Granodiorite is a small NE-SW trending stock emplaced within mafic to intermediate gneisses and migmatised amphibolites of the Archaean Jiaodong Group. The deposit comprises a high density concentration of high grade quartz-sulphide veinlets and stockworks. The bulk of the ore is disseminated within highly fractured and altered Mesozoic granodiorite. This alteration is characterised by strong silicification, sericitisation, sulphidation and K-feldspar alteration. Ore most commonly occurs as disseminated gold within sericite-, quartz- and pyrite-altered granodiorite along the Sanshandao Fault with lesser K-feldspar, carbonate and locally chlorite. Mineralisation is also found to a lesser degree as gold-quartz vein/veinlet stockworks, predominantly within the altered granodiorite and occasionally in the Archaean gneiss. There are four stages of vein development as follows: a). Quartz-K feldspar-sericite, b). Quartz-pyrite ±arsenopyrite, c). Quartz-base metal sulphide and d). Quartz-carbonate.
Structures associated with the fault zone are characterised by early shearing and late brittle deformation over a zone that is some 200 m wide and 5 km in length. Six orebodies have been delineated, concentrated within dilational intervals of the major fault zone. The largest of these orebodies is over 1000 m in length, averages 0.4 to 6.2 m in thickness and has been traced to a depth of around 900 m. The ore zones trend at 20 to 40° and dip at between 30 and 50° SE. Grades range from 3 to 10 g/t Au. The Sanshandao deposits are said to contain more than 60 t of gold and together with the Cangshang deposit 5 km to the south-west on the same structure have a resource of approximately 107 t Au at an average grade of 6.1 g.t Au (in 2001).
Tonnage & grade figures for the main deposits include:
Taishang - 108 t Au at an average grade of 4.9 g/t Au,
Sanshandao - 107 t Au at an average grade of 6.1 g/t Au (2001),
Xincheng - 85 t Au at an average grade of 7.2 g/t Au,
Jiaojia - 58 t Au at an average grade of 7.3 g/t Au,
Linglong - 58 t Au at an average grade of 7.3 g/t Au,
Dayigezhuang - 51 t Au at an average grade of 4.6 g/t Au,
Cangshang - 45 t Au at an average grade of 4.5 g/t Au.
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Travelling Shandong Peninsular to Jinchangyu ...................... am., Tuesday 9 October, 2007.
Jinchangyu - Northern Margin of the North China Craton ...................... pm., Tuesday 9 & Wednesday 10 October, 2007.
The Jinchangyu gold deposit is located in Hebei Province, about 180 km ENE of Beijing in northern China and has been mined for more than 1000 years.
It is one of a series of orogenic style lode gold deposits on the ~1500 km long northern margin of the North China Craton extending from the middle of Inner Mongolia, through northern Hebei and Liaoning, to Jilin Province. Together they account for more than 900 tonnes (30 Moz) of gold. The Yanliao Gold Province mainly within the Yanshan Mountains of Inner Mongolia and Shanxi Provinces, represents the central section of this larger gold belt and includes Jinchangyu, as well as the Dongping, Xiaoyinpan, Honghuagou, Jinchangouliang, Paishanlou, Yuerya, Niuxinshan, Baizhangzil, Wulong and Haigo deposits.
The bulk of the major orogenic gold deposits on the northern margin of the North China Craton are concentrated in the Yanshan Mountain area, where regional structures include both NNE striking faults and shear zones, and east-west trending folds and faults.
The northern margin of the North China Craton is characterised by east-west trending basement uplift blocks of metamorphosed Archaean and Paleoproterozoic gneiss, schist, granulite, amphibolite and banded iron formation that have been episodically uplifted during Variscan, Indosinian, and Yanshanian tectono-magmatic events. Slightly metamorphosed Mesoproterozoic to Neoproterozoic shallow marine quartzite, slate and limestone, and Paleozoic to Cretaceous shallow marine to continental sedimentary rocks, surround the uplifts. Most of the deposits are hosted by uplifted blocks of Precambrian metamorphic rocks, although Palaeozoic and Mesozoic felsic plutons are commonly found in close proximity and host around 30% of the mineralisation.
Gold deposits and granites are associated with both Variscan and Yanshanian tectonism, although broad scale regional deformation is mainly Variscan and is best characterised by east-west striking folds and fault zones formed during Permian early stages of ocean closure between the North China and Angara cratons. Locally, in the eastern part of the gold province, the Variscan structures are overprinted by Yanshanian NNE trending strike slip faults. The Late Jurassic to Early Cretaceous Yanshanian tectonism could have been caused either by the oblique subduction of the Izanagi oceanic plate underneath the North China craton and/or final closure of the Mongolia-Okhotsk ocean between the Angara and North China cratons.
Both Variscan gold deposits and granites are recognised in the western part of the Yanshan area, although only Yanshanian gold ores (e.g., Jinchangyu) are found to the east. The larger deposits are typically associated with the younger Yanshanian hydrothermaI systems.
Jinchangyu, and associated deposits, are localised within a 6 x 1 km, NNE-trending fault cutting Archaean metamorphic rocks. They comprise clusters of quartz veins and quartz albite veins over width of a few tens of metres, up to 300 m long, and continue to a depth of 550 m. The veins mainly contain pyrite, lesser gold, electrum, chalcopyrite, galena, pyrrhotite, magnetite, molybdenite and trace amounts of telluride minerals.
Many of the deposits show a spatial association with granites of both orogenies and some are all at least partly hosted by granites. This multi-episodic mineralisation correspond to episodic tectonic reactivation and associated magmatism along the northern margin of the North China craton.
As with the deposits hosted in metamorphic rocks (eg., Jinchangyu), the granite-hosted deposits are also structurally controlled, and commonly localised by brittle secondary structures related to a major regional fault or shear zone.
Jinchangyu is variously quoted as having contained >100 t of gold and having remaining resources of around 90 t Au with an average grade of 2 g/t Au, or 66 t Au with an average grade of 7 g/t Au.
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Qinling Mine, Xiaoqinling Goldfield - Southern Margin of the North China Craton ...................... Thursday 11 October, 2007.
The Qinling (Yangchaiyu-Sifangou-Jindongpen) and neighbouring Wenyu-Dongchuang gold mines are located approximately 150 km east of Xian in north-western Henan Province, central China.
These deposits lie within the Xiaoqinling Gold Province which extends from Tongguan in eastern Shaanxi Province to Lingbao in western Henan Province, and contains more than 100 orogenic gold deposits and >1200 auriferous quartz veins, with ores occurring in a number of distinct belts both in quartz veins and disseminated in altered metamorphic rocks.
The gold deposits generally lie about 30 to 50 km in board of the southern margin of the North China Craton, north of the suture with the Yangtze Craton to the south and are concentrated in three main goldfields, the Tongyu, Wenyu and Dahu goldfields.
The main hosts to mineralisation are late Archaean Taihua Group which comprises up to 4000 m of amphibolite, plagioclase gneiss and migmatite, with marble and quartzite, and the Mesoproterozoic metavolcanic rocks of the Xiong'er Group. Indosinian alkalic porphyries and dykes (213 to 202 Ma) and Yanshanian granites are widespread. The 130 to106 Ma Wenyu granite (just west of Tongguan) intruded the central part of the gold-rich area, and is exposed over an area of about 20 sq. km, although deposits are generally hosted in the Precambrian basement rocks hundreds of metres to as much as 10 km from the intrusions and their related hornfelsed aureoles. The dominant alteration assemblages include quartz, sericite, pyrite, and carbonate minerals.
Regional structures are dominated by the east-west trending, north dipping, >60 km long Maxundao deep fault zone (that extends from Tongguan to Lingbao), originally a compressional structure, which shows evidence for late extension. A series of large gold deposits, with total resources of 300 to 450 t of contained Au, occur at intersections of second order WNW to east-west striking faults with NE and NW striking faults to the north of the first-order Maxundao fault zone.
In the Wenyu-Qinling area, the Taihua Group is folded by the east-west trending Laoyacha anticline and adjacent Jinuobei syncline, bounded by the regional scale Taiyao Fault to the north and the Xiaohe Fault to the south. Three late Yanshanian (Cretaceous) age monzonitic bodies dated at between 130 and 110 Ma, the Niangniangshan, Wenyu and Huashan granites respectively, are distributed along the northern margin of the area.
Within the Wenyu-Qinling area, >100 gold bearing veins are concentrated in a number of gold deposits, the largest of which is the Wenyu-Dongchuang deposit with reserves of >150 t Au at grades of around 6 g/t Au. The Jingtongcha-Yangzhaiyu-Sifangou deposits are some 10 km to the ESE, while the Chucha-Luanshigou deposit is 15 km due east. All are gold-bearing quartz veins distributed around the southern margin and 2 to 4 km from the margin of the Wenyu Granite, hosted by amphibolite, plagioclase and migmatites of the Lijiayu Formation of the Taihua Group.
Veining is controlled by a series of east-west striking shear zones which typically extend over strike lengths of around 2 km, with the larger being up to 5 km long. These shear zones are characterised by mylonites, phyllonites and mylonitic schists. The gold veins and ore shoots are localised by the dip and strike direction of the main fault planes, with the richer and thicker shoots typically found where the shear zones dip at <45° and strikes are 140 to 160°. In contrast where the dips are >45° and the shear strikes at 80 to 100°, veins are generally absent and mylonites are found instead. The thicker ore shoots are mineralised over widths of 4 to 20 m.
The principal metallic minerals within the gold-quartz veins are native gold, electrum, pyrite, chalcopyrite, galena and sphalerite, with local magnetite, pyrrhotite, bornite and tetrahedrite. The gangue is dominated by quartz, siderite, ankerite, calcite and sericite, with minor chlorite, microcline, biotite and epidote. These minerals were introduced in four stages of veining as follows: i). Stage 1 - pyrite-quartz, characterise by white quartz with disseminated pyrite cubes and rare native gold in fractures within the quartz or in pyrite cubes; ii). Stage 2 - quartz-pyrite, occurring as abundant pyrite with minor grey quartz, with pyrite present as monominerallic bands paralleling the vein margins. High grade native gold and electrum occurs in healed fractures and as inclusions in pyrite crystals; iii). Stage 3 - carbonate-polymetallic sulphides, containing chalcopyrite, galena, sphalerite, pyrrhotite and fine pyrite with siderite, quartz and ankerite. This stage contains high grades, mainly as electrum and follows zones of thicker quartz veining and cut and brecciated zones of better developed to massive Stage 2 pyrite. iv). Stage 4 - quartz-calcite, occurring as fine grained crystals, and contains little gold.
The vein zones typically have altered selvages that are 2 to 10 m thick. In the outer parts of these selvages, hornblende is altered to epidote and chlorite and plagioclase is replaced by clinozoisite. Inwards from this outer zone, silicification increases, as does carbonate, pyrite and late hydrothermal quartz to obliterate the original rock texture, accompanied by an increase in Au, Ag and Pb. Further inwards, the alteration assemblage is mainly hydrothermal quartz, sericite and pyrite to form the mineralised veins, with a continuing increase in Au, Ag, Cu and Pb, and lesser Zn.
The Wenyu deposit contains approximately 50 t of Au at a grade of 6.5 g/t Au, while Dongchuang has 55 t of Au at a grade of 7 g/t Au. The Yangzhaiyu and Sifangou deposits of the Qinling Mine account for an additional 90 t of Au at a grade of 10 to 11 g/t Au.
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Travelling Xian, via Xiamen to Shanghang ...................... Friday 12 October, 2007.
Zijinshan - South-eastern China ...................... Saturday 13 October, 2007.
The Zijinshan mining district is located 17 km north of Shanghang and 190 km WNW of Xiamen in Fujian Province, south-eastern China (#Location: 25° 11' 12"N, 116° 24' 27"E).
The Zijinshan district contains five significant deposits that fall into different but related classes, as follows:
• Epithermal gold - Zijinshan and Bitian
• Porphyry copper - Zhongliao
• Transitional Porphyry-Epithermal - Wuzichilong and Longjiangting.
The oldest rocks in the district are Neoproterozoic phyllites and metsandstones, unconformably overlain by Devonian to Carboniferous shallow marine sediments with minor limestone containing Fe, Mn, Pb, Zn and Cu mineralisation on limestone sandstone contacts. Following Triassic tectonism these were intruded by Jurassic granites and composite stocks, and related volcanics. These rocks were in turn followed by an extensive Cretaceous volcanic suite controlled by a major NE trending deep fault zone and formed a 150 km long volcanic belt, including the Shimaoshan subaerial dacitic to rhyolitic volcanics which unconformably overlie the Jurassic granites. These were then overlain by Cretaceous red beds.
Copper-gold mineralisation in the Zijinshan district is developed within the 125 ±4 Ma Shimaoshan volcanics and related intrusives in a small fault bounded volcanic basin where the dacitic volcanic dome complex was formed. Dacitic porphyry occurs in a pipe like intrusion in the centre of the basin, surrounded by dacitic volcanic breccia, tuff and lava. A system of circular and radial faults and fissures developed around the dacite porphyry pipe are typically filled by hydrothermal breccias and veins.
High sulphidation, epithermal Au-Cu mineralisation at Zijinshan, the most important of the deposits, is found mainly within those hydrothermal breccias and veins which have filled a system of NW striking fissures, preferentially developed on the NW of the volcanic dome complex. The breccias contain wallrock and ore clasts which are generally <5 mm across, set in a very fine grained mineralised matrix composed mainly of quartz and alunite with minor sulphides (pyrite, covellite, enargite and digenite).
The hydrothermal breccias are mainly within the upper part of the Zijinshan deposit. At about 800 m below the surface drilling has encountered the 105 ±10 Ma Luoboling granodiorite porphyry which contains porphyry copper mineralisation beneath the dacite porphyry.
• Early, widespread silicification in the Jurassic granites only, with associated tin,
• Phyllic alteration within the Jurassic and Cretaceous granites, volcanics and dacite porphyry, grading into potassic alteration at depth in the porphyry copper zone.
• Dickite zone peripheral to the phyllic alteration, forming narrow (<200 m) elongated bodies above the main dickite zone at depth. Dickite overprints the phyllic minerals, but is in trun overprinted by the alunite phase.
• The alunite zone composed of 5 to 60% alunite, 35 to 90% quartz and <5% pyrite, is spatially associated with high sulphidation copper and characteristically occurs in the NW part of the Zijinshan deposit, and changes gradually to dickite at depth.
• Surficial late silicification in the central part of the Zijinshan deposit, which grades with depth into the funnel shaped alunite zone. This zone corresponds to the hydrothermal breccia development and the economic gold ores.
The high sulphidation Au-Cu mineralisation formed around the dacite porphyry pipe in the centre of the dacite volcanic dome complex, with ore in the NW striking hydrothermal breccias. The dacite porphyry grades down into a granodiorite porphyry which contains potassic alteration and porphyry Cu-Au style mineralisation.
The Zijinshan deposit comprises a separate upper gold, overlying and grading into the lower copper orebodies.
The copper orebodies total 1.45 Mt of contained Cu @ a grade of 0.63% Cu in 230 Mt of ore,
The gold orebodies contain up to 250 t of gold (depending on cut-off, Zijin Mining, 2004), as follows:
356 Mt @ 0.72 g/t Au for 254.5 t Au at 0.3 g/t Au cut-off
276 Mt @ 0.82 g/t Au for 227.1 t Au at 0.4 g/t Au cut-off
214 Mt @ 0.93 g/t Au for 199.5 t Au at 0.5 g/t Au cut-off
166 Mt @ 1.04 g/t Au for 174.0 t Au at 0.6 g/t Au cut-off
131 Mt @ 1.15 g/t Au for 150.7 t Au at 0.7 g/t Au cut-off
Production from Zijinshan in 2003 totalled 9.5 t Au from ore with a head grade of 0.82 g/t Au (Zijin Mining, 2004).
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Travelling Shanghang, via Xiamen and Hong Kong to Bangkok ...................... Sunday 14 October, 2007.
Chatree - Thailand .......... Monday 15 & Tuesday 16 October, 2007.
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 Bangkok to Sepon ...................... Wednesday 17 October, 2007.
Sepon - Laos .......... Thursday 18 October, 2007.
The intrusive-centred, sediment-hosted Au-Cu Sepon mineral field lies approximately 40 km north of the town of Sepon, in Savannakhet province of south-central Laos. The Sepon mineral field includes seven gold and one copper deposit (Khanong) which occur as separate adjacent deposits over a 5 km interval containing over 120 tonnes (4 Moz) of gold and 1.2 million tonnes of copper.
The 1.25 mtpa Sepon gold mine poured its first gold in December 2002.
At Sepon, gold occurs as fine disseminations hosted by strongly altered calcareous sedimentary rocks with similarities to the Carlin style gold deposits of Nevada in the USA.
The separate Khanong supergene-enriched, carbonate-replacement and sediment-hosted magmatic-associated Cu-Au resource is located immediately adjacent to and within the main 6x4 km cluster of gold deposits.
Sepon lies within the Truongson Fold Belt (or Annamite Cordillera) and the Kontum Massif. The Truongson Fold Belt is a NW elongated belt of Early to Mid Palaeozoic sediments and lesser volcanic rocks, interspersed with lesser tectonic slices of metamorphic terranes, bound to the north by the Song Ma-Song Da suture zone and to the south by the NW trending Truongson Fault. The Kontum Massif comprises an Upper Proterozoic basement of ortho- and para-gneiss, crystalline schist and migmatite which abuts the Truongson Fold Belt to the south.
In the Sepon district, the Truongson Fold Belt is represented by a series of E-W trending basins which are truncated by the NW-striking Truongson Fault. Granitic intrusions have been mapped in the district as well as rhyodacite and andesite dykes and stocks, interpreted to be part of the Late Palaeozoic Variscan Orogeny. At least three phases of around 300 Ma rhyodacite porphyry intrusion are recognised, including a late, quartz-stockwork veined phase, which is interpreted to be genetically associated with all known Cu and Au mineralisation in the field. Minor remnants of Jurassic to Cretaceous Khorat Basin continental sedimentary rocks unconformably overlie the Palaeozoic marine sedimentary sequences to the east and north, while west of the Kontum Massif and south of the Truongson Fault, Khorat sedimentary rocks are dominant. Cenozoic volcanic rocks unconformably overlie the Palaeozoic rocks to the north-east.
The stratigraphy within the Sepon district comprises Devonian to Carboniferous continental fluvial and shallow to deep marine sediments deposited in a half graben basin. Pre- to syn-rift, dominantly siliciclastic sediments, grade upwards to transgressive and regressive, carbonate-dominant sequences. This sedimentary sequence has been divided into nine lithology based formations. The lower and upper contacts of formation 6, a calcareous, carbonaceous mudstone, host most of the known gold mineralisation.
The sediments generally dip moderately north and NW, while the dominant fault directions are NW, parallel to the Truongson fault system, and east-west, parallel to the basin bounding faults. Intersections of east-west and NE trending faults are important in that they localise mineralisation and rhyodacite porphyry intrusive stocks and associated dyke and sill complexes of similar composition.
Four broad alteration/mineralisation styles are recognised namely: i). quartz stockwork porphyry Mo, ii). Cu-Au skarn, iii). Cu-Au carbonate replacement, and iv). sediment-hosted Au.
Individual deposits include the Padamand Thengkham porphyry Cu-Mo mineralisation within a rhyodacite porphyry intrusive plug, the adjacent Thengkham skarn Cu occurrences, the Khanong Cu carbonate replacement ore, the Vang Ngang silica-pyrite replacement Au deposit, and the Discovery and Nalou/Namkok jasperoid associated gold ores (adjacent to dykes and sills of rhyodacite porphyry), and finally Pb/Zn/Ag carbonate replacement mineralisation.
The gold mineralisation is localised by structural and stratigraphic fluid traps which are said to have close affinities with the sediment-hosted, 'invisible' gold deposits of the Carlin Trend, Nevada, USA. The mineralisation is finely disseminated and is closely related to decalcification and variable silica (jasperoid) replacement of carbonatic rocks following structures and lithological contacts. The geometry of the gold ore is controlled by a combination of anticlinal structures, shallow dipping stratigraphy, structure and porphyry sills.
The copper mineralisation includes both primary and secondary styles. Cu-Au skarns are developed along the contacts of the intrusive stocks. Spatially, the main hypogene mineralisation occurs as a lower temperature, silica-sulphide replacement style, semi massive sulphide body, hosted by shallow dipping, highly sheared carbonate rocks (calcareous cataclastitic, fault rocks, in the flat-lying, extensional, Khanong Fault), which occur immediately out-board of the skarnhornfels Cu-Bi front and in-board of sediment hosted Au-(As-Sb) mineralisation in a zoned intrusive-centred hydrothermal system. Weathering and in situ supergene enrichment of the hypogene sulphide protore produced a high-grade chalcocite-clay zone (approx. 10.3 Mt @ 4.6% Cu) overlain by leached limonite-clay, and underlain by a very high grade copper oxide zone, comprising malachite ±azurite ±cuprite ±minor native copper developed above an impermeable dolomite. Exotic Cu-Mn-wad mineralisation is developed for up to 1 km down the hydraulic gradient from the chalcocite-clay zone.
Reserves and resources in December 2005 (Oxiana Ltd, 2006) were:
6.30 Mt @ 1.99 g/t Au, 9.71 g/t Ag (Proved + probable reserves, Sepon Gold);
15.90 Mt @ 4.97% Cu (Proved + probable reserves, Sepon Copper);
38.1 Mt @ 1.27 g/t Au, 7.27 g/t Ag (Measured + indicated + inferred resources, Sepon Gold Oxide);
72.8 Mt @ 1.55 g/t Au, 7.82 g/t Ag (Measured + indicated + inferred resources, Sepon Gold Hypogene);
96.5 Mt @ 0.064 g/t Au, 5.52 g/t Ag, 2.03% Cu (Measured + indicated + inferred resources, Sepon Copper at Khanong, & Thengkham).
For detail consult the reference(s) listed below.
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Travelling Sepon to Bangkok ...................... Friday 19 October, 2007.
Phu Bia - Laos .......... Saturday 20 & Sunday 21 October, 2007.
The Phu Bia gold project comprises three gold deposits, the Phu Kham Gold Cap, Long Chieng Track and Ban Houayxai, and the larger Phu Kham Copper-Gold porphyry style deposit, all of which are located approximately 150 km north of Vientiane in the Xaisomboun Special Zone of Laos.
The geology in Laos is dominated by two volcano-sedimentary belts, the Louang Prabang and the Truongson Belts which converge in the Phu Bia district in north-central Laos. The two belts host all of the known gold and base metal deposits within Thailand and Laos. The Louang Prabang Belt contains the Chatree gold, Mae Sot zinc and Puthep copper deposits, while the Troungson Belt contains the Sepon gold and copper-gold deposits. All of these deposits are interpreted to be directly related to the emplacement of acid to intermediate intrusions into the Palaeozoic host rocks.
The Phu Bia gold project comprises three deposits, with oxide resources in all three and lesser primary ore in two. The known mineralisation is hosted by Palaeozoic sediments, tuffs and volcaniclastics, which are intruded by Permo-Triassic porphyry bodies believed to be the source of the gold mineralisation. The oxide zone of each of the deposits typically starts close to the surface and persists to depths of 20 to 60 m. The deposits are:
The Phu Kham Gold Cap deposit is the leached 'oxide cap' of the Phu Kham copper-gold deposit, in the outer zone of a porphyry-style hydrothermal copper-gold system, from which virtually all of the copper has been leached. The underlying hypogene copper-gold deposit comprises skarn and stockwork style mineralisation. The oxide gold cap predomiantly occurs as gossans formed after various types of skarn. Minor stockwork-disseminated style mineralisation occurs in the host rocks enclosing the gossans.
The Long Chieng Track stockwork vein style deposit appears to represent the high level expression of a conventional porphyry copper-gold system and includes both oxide and primary resources. The oxide zone ranges from 50 to 60 m in thickness, with the primary zone persisting to depth of at least 100 m, with a minimal supergene enrichment zone.
The Ban Houayxai deposit is a combination of stockwork vein and disseminated mineralisation interpreted to represent the distal expression of a porphyry copper-gold system. Gold predominantly occurs within the veins, although significant amounts are disseminated in the altered host rock. The ore zone comprises a 40 to 50 m thick oxide layer, overlying supergene enriched and primary mineralisation that persists to depths fo 50 to 100 m or more.
In addition, resources and reserves have also been outlined in the Phu Kham supergene enrichment and hypogene porphyry style mineralisation below the Phu Kham Gold Cap. The hyogene mineralisation comprises two styles, as follows:
i). Stock-work-disseminated sulphides - which comprises predominantly bornite and covellite with a relatively low associated content of chalcopyrite and averages around 0.5% Cu.
ii). Banded massive sulphides - which occurs as planar zones within the mineralised mass and is predominantly chalcopyrite rich, containing in excess of 1% Cu.
Published resource and reserve figures in 2005 at a 0.5 g/t Au cut-off include:
Measured + indicated + inferred oxide gold resources,
Phu Kham Gold Cap - 7.82 Mt @ 1.0 g/t Au
Long Chieng Track - 1.35 Mt @ 0.9 g/t Au
Ban Houayxai - 3.60 Mt @ 1.1 g/t Au
Total - 12.80 Mt @ 1.0 g/t Au
Measured + indicated + inferred supergene/hypogene sulphide copper-gold resources,
Phu Kham Copper-Gold - 277 Mt @ 0.51% Cu, 0.21 g/t Au (at a 0.2% Cu cut-off)
Phu Kham Copper-Gold - 131 Mt @ 0.75% Cu, 0.28 g/t Au (at a 0.4% Cu cut-off)
Measured + indicated + inferred hypogene gold resources,
Long Chieng Track - 2.31 Mt @ 0.9 g/t Au
Ban Houayxai - 2.34 Mt @ 1.2 g/t Au
Total resources - 4.65 Mt @ 1.1 g/t Au
The resources includes the following proved + probable reserves
Phu Kham porphyry Copper-Gold supergene/hypogene sulphide copper-gold reserves - 144 Mt @ 0.56% Cu, 0.25 g/t Au
Phu Kham Gold Cap oxide gold reserves - 6.60 Mt @ 1.0 g/t Au
Long Chieng Track oxide gold reserves - 0.62 Mt @ 1.0 g/t Au
Ban Houayxai oxide gold reserves - 1.33 Mt @ 1.3 g/t Au
Total oxide gold reserves - 8.55 Mt @ 1.1 g/t Au
Published mineral resource and ore reserve figures at 31 December, 2012 (PanAust Limited, 2013) were:
Ban Houayxai Gold Deposit - Measured + indicated + inferred resources for,
Oxide (0.25 g/t Au cut-off) - 12 Mt @ 0.66 g/t Au, 3.2 g/t Ag;
Transitional (0.35 g/t Au cut-off) - 16 Mt @ 0.93 g/t Au, 9.5 g/t Ag;
Primary (0.4 g/t Au cut-off) - 37 Mt @ 1.01 g/t Au, 7.4 g/t Ag;
TOTAL - 65 Mt @ 0.93 g/t Au, 7.2 g/t Ag;
Phu Kham Porphyry Deposit - Measured + indicated + inferred resources for,
TOTAL (0.25 g/t Au cut-off) - 227 Mt @ 0.48% Cu, 0.22 g/t Au, 2.0 g/t Ag.
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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 Orient Gold 2007 Tour options page.
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T M (Mike) Porter, of Porter GeoConsultancy
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