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Kharmagtai |
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Mongolia |
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Cu Ag Au
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Super Porphyry Cu and Au
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The Kharmagtai Au-Cu porphyry district is located in the southern Gobi desert of Mongolia, 200 km north of the Oyu Tolgoi porphyry Cu-Au deposit and 430 km south of the capital Ulaan Baatar (#Location: 44° 0'N, 106° 10'E).
Like Oyu Tolgoi and Tsagaan-Suvarga, Kharmagtai lies within the southern Mongolia section of the Transbaikal-Mongolian orogenic belt, section of the Mid-to Late-Palaeozoic Kazakh-Mongol magmatic arc. The deposit is located within the Gurvansayhan island arc terrane, characterised by calc-alkaline to potassic-calc-alkaline igneous complexes and volcanic and sedimentary rocks ranging from Ordovician to Carboniferous in age.
During the Ordovician to Silurian, the area was occupied by an oceanic setting receiving mature sedimentation from a continental source or the eroded roots of an arc to the north. From the Devonian to Carboniferous, the region was dominated by island arc volcanism, with the ocean closing with arc collision during the Carboniferous. All terranes were welded by late Carboniferous to Permian continental granitic plutons indicating that amalgamation took place not later than the Carboniferous. Porphyry style mineral deposits hosted within the Gurvansaikhan terrane formed in two distinct tectonic settings, corresponding to:
• Late Devonian deposits formed early in the history of the island arc terrane (e.g. Oyu Tolgoi, Tsagaan Suvarga), and
• Early Carboniferous deposits formed in a collision-subduction setting (e.g. Kharmagtai, Shuteen and Oyut Ulaan).
Outcrop throughout the Kharmagtai district is sparse, although available exposure indicates large areas are underlain by a Devonian silty clastic sedimentary package which includes arenaceous volcaniclastic rocks with andesitic lava fragments, and plagioclase and hornblende crystals, as well as lesser felsic ash crystal tuffs, reworked tuffaceous sediments and locally, limited exposures of basaltic to andesitic volcanics. All of these lithologies have been intruded by high-K calc-alkaline diorite, quartz diorite, monzodiorite, monzonite, diorite porphyry and minor granodiorite, and syenite stocks of the Kharmagatai Igneous Complex. The magmatic arc has been eroded and unroofed in the district, with little evidence of comagmatic volcanic equivalents of the intrusive complex, although abundant roof pendants of sedimentary rocks within the stocks imply they are exposed at their upper-most levels. Preliminary Re-Os dates of 330.2 ± 1.0 Ma in the Carboniferous have been determined from samples of these intrusives at Kharmagtai, although Vigar, 2014 suggests and age of ~297 Ma.
The Kharmagatai Igneous Complex (KIC) is sub-elliptical in plan view, 6 km long x 2 km wide, trending WNW-ESE, extending from Tsagaan Sudal in the west to Chun in the east. The KIC was emplaced into a local volcano-sedimentary succession of ash siltstone, volcaniclastic sandstone, chert, andesite lava and andesitic volcanic breccia of Devonian-Lower Carboniferous age.
Six potential porphyry clusters have been identified, containing 19 individual porphyry and epithermal targets. At least four significant centres of porphyry related gold-copper mineralisation and alteration (reflected by coincident zones of IP anomalism), occur within an area of around 7 x 3 km. These include the Altan Tolgoi, Tsagaan Sudal and Zesen Uul centres which are located within a 700m radius of each other, and may be connected at depth and therefore cogenetic. A majority of the strongly mineralised porphyry complex lies under unexplored shallow post-mineral sediments. Porphyry mineralisation at Kharmagati is associated with widespread magnetite alteration and all the main prospects are associated with broad magnetic highs. A regional magnetic survey outlines numerous distinct circular magnetic anomalies in the vicinity of the Altan Tolgoi, Tsagaan Sudal and Zesen Uul prospects (Xanadu Mines web site 2017). Some 10 km to the west, distal gold-base metal-bearing breccia pipes has been recognised and several kilometres to the southwest again, complex, structurally controlled, gold bearing silicified zones occur within the volcano-sedimentary pile.
In 2016-17, testing of the Altan Tolgoi mineralised centre below superficial cover revealed significant depth extensions, with intersections such as 646 m @ 0.51% Cu, 0.87g/t Au (1.06% CuEq) from 16 m, within high-grade tourmaline breccia mineralisation, including 64 m @ 1.06% Cu, 3.15g/t Au (3.08% CuEq) from 18m. This tourmaline breccia complex is zoned vertically, from barren or weakly mineralised tourmaline breccia near‐surface, to chalcopyrite and chalcopyrite‐gold cemented breccia at increasing depths (Xanadu Mines ASX release, March 2017).
Six intrusive phases have been recognised within the KIC in the deposit area, comprising in order of emplacement: diorite, diorite porphyry, monzodiorite, quartz-diorite, quartz-diorite dykes and andesite dykes. Intense hydrothermal alteration, quartz stockwork emplacement and porphyry copper-gold mineralisation is associated with the dykes of quartz-diorite. Mineralisation is composed of four hydrothermal stages: Stage 1 includes the albitisation of feldspar and formation of magnetite±epidote and magnetite veins within the quartz diorite dykes. Stage 2 is characterised by quartz-chalcopyrite-pyrite±gold stockwork emplacment. Stage 3 is represented by an assemblage of chlorite, epidote, chalcopyrite ± magnetite alteration and Stage 4 by sericite, chlorite, pyrite-carbonate alteration. Late-stage copper-mineralised tourmaline breccia pipes are common throughout the Kharmagtai area and commonly comprise albite-altered diorite porphyry and quartz-diorite clasts set in a tourmaline, quartz, chalcopyrite, pyrite, epidote and calcite matrix. The deposit is unusually gold rich, with two trends in Au:Cu ratios that suggesting two separate mineralization events (Shin et al., 2005).
A representative example of one of these centres of porphyry related gold-copper mineralisation and alteration is represented by a 200 m diameter sub-cropping zone that was oxidised to depths of approximately 60 metres below surface. The oxide zone contains fracture controlled goethite, malachite, copper-wad, minor chalcocite and occasionally native copper. Drilling of the hypogene zone encountered chalcopyrite and subordinate pyrite associated with up to 10% stockwork quartz veins, mostly hosted in microdiorite, with lesser late chalcopyrite-rich sulphide veins (<5% sulphides) containing minor bornite. Rare molybdenite occurs as fracture coating and within milky quartz veins in the stockwork zone. Native gold is present as small inclusions in chalcopyrite grains and to a lesser extent in pyrite and as minor Au tellurides. In high-grade zones, the Au g/t:Cu% ratios frequently exceeds 2:1. Narrow zones (normally less than 10 metres in width) of hydrothermal tourmaline breccias contain quartz-veined microdiorite clasts in a matrix of quartz, tourmaline, sericite, pyrite and minor chalcopyrite. Magnetite is a major accessory mineral in this and all of the Kharmagtai mineralised porphyries, occurring within pre-mineral quartz veins and in sulphide-rich veins. Planar 'centre-line' and 'railroad textured' quartz - sulphide ± magnetite veins are characteristic of the Kharmagtai porphyry mineralisation. Drill intersections across this zone are generally of 80 to 100 m @ 0.5 to 0.8% Cu, 0.7 to 1.25g/t Au and define a 60° south dipping stockwork zone that has been traced to a depth in excess of 250 m below surface. Initial potassic alteration affected both intrusive and sedimentary rocks, represented by an assemblage of variably developed albite + biotite + K-feldspar + minor magnetite + quartz + pyrite with veinlets of quartz, magnetite, chalcopyrite, pyrite, apatite, epidote and chlorite. Propylitic (chlorite ± calcite ± pyrite) and phyllic (sericite + pyrite + quartz ± chlorite ± tourmaline ± chalcopyrite ± gold) alteration commonly overprints the potassic assemblages.
Some 1.5 km to the north, within another of the mineralised stockwork zones, the early potassic phase is overprinted by quartz-carbonate-sulphide veins and breccias with quartz, calcite, tourmaline, chlorite, sericite, pyrite, chalcopyrite, sphalerite, galena and tennantite-tetrahedrite enclosed by selvages of high-intensity texture-destructive phyllic type alteration in the form of quartz + sericite + pyrite + Ti-phase (rutile, leucoxene and anatase) minerals. Younger tourmaline breccias contain mostly pyrite. Late calcite - zeolite fracture fillings are also recorded, with trace pyrite, hematite, quartz and chalcopyrite.
The individual centres of copper-gold bearing porphyry related quartz stockwork mineralisation identified in the Kharmagtai district are all quite similar. Some occurs as broad planar zones, which in some cases are focused on narrow quartz diorite dykes within the diorite to quartz-diorite hosts, while others are more diffuse and irregular. It is considered possible that the individual stockwork zones may amalgamate at depth to form a single large porphyry deposit.
Published combined resource figures for the Altan Tolgoi, Zesen Uul and Tsagaan Sudal prospects (Xanadu Mines, technical report, 2014), at a cut-off of 0.3% Cu eq. above 1000 m RL, and 0.6% Cu eq. below 1000 m RL, were:
Measured resource - 33.572 Mt @ 0.63% Cu, 0.48 g/t Au;
Indicated resource - 17.593 Mt @ 0.46% Cu, 0.48 g/t Au;
Inferred resource - 81.933 Mt @ 0.24% Cu, 0.29 g/t Au;
TOTAL resource - 133.098 Mt @ 0.37% Cu, 0.36 g/t Au;
including
Measured + indicated resource at Altan Tolgoi - 44.753 Mt @ 0.55% Cu, 0.46 g/t Au.
Updated JORC compliant mineral resources in the same three deposits in March 2015 (Xanadu Mines, 2015) were:
Open cut @ 0.3% Cu cutoff
Indicated resource - 23 Mt @ 0.41% Cu, 0.55 g/t Au;
Inferred resource - 107 Mt @ 0.27% Cu, 0.24 g/t Au;
TOTAL open cut resource - 129 Mt @ 0.30% Cu, 0.30 g/t Au = 0.48% Cu equiv.;
Underground @ 0.5% Cu cutoff
Indicated resource - 24 Mt @ 0.43% Cu, 0.47 g/t Au;
Inferred resource - 51 Mt @ 0.42% Cu, 0.36 g/t Au;
TOTAL underground resource - 74 Mt @ 0.42% Cu, 0.40 g/t Au = 0.67% Cu equiv.;
Combined
Indicated resource - 46 Mt @ 0.42% Cu, 0.51 g/t Au;
Inferred resource - 157 Mt @ 0.32% Cu, 0.28 g/t Au;
TOTAL combined resource - 203 Mt @ 0.34% Cu, 0.33 g/t Au = 0.55% Cu equiv.
The section above on Kharmagtai has largely been drawn from Kirwin et al., (2005a) and references cited therein.
The most recent source geological information used to prepare this decription was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Kharmagtai
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Selected References:
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Kirwin D J, Wilson C C, Turmagnai D and Wolfe R, 2005 - Exploration History, Geology and Mineralisation of the Kharmagtai Gold-Copper Porphyry District, South Gobi Region, Mongolia: in Seltmann R, Gerel O and Kirwin D J, (Eds.), 2005 Geodynamics and Metallogeny of Mongolia with a Special Emphasis on Copper and Gold Deposits: SEG-IAGOD Field Trip, 14-16 August 2005, 8th Biennial SGA Meeting; CERCAMS/NHM, London, IAGOD Guidebook Series 11, pp 175-191
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| References in PGC Publishing Books: | |
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Seltmann R and Porter T M, 2005 - The Porphyry Cu-Au/Mo Deposits of Central Eurasia: 1. Tectonic, Geologic & Metallogenic Setting and Significant Deposits, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v2 pp 467-512
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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