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Sari Gunay |
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Iran |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Sari Gunay epithermal gold deposit is located in central north-western Iran, 60 km north-west of the city of Hamedan in the province of Kordestan, Iran, approximately 300 km WSW of Tehran (#Location: 35° 12'N, 48° 05'E).
It is a sub-volcanic intrusion-related epithermal gold deposit hosted a by a 11.7 to 11.0 Ma Miocene, high K sub-alkaline complex, which belongs to the Takab belt of structurally controlled, middle Miocene, mildly alkaline volcano-plutonic complexes which are marginally younger than, and displaced to the south-west of, the extensive, but much less alkaline Eocene-Miocene Urumieh-Dokhtar calk-alkaline volcanic arc, which hosts large porphyry Cu deposits such as Sar Cheshmeh and Meiduk.
Both the Takab Belt and the Urumieh-Dokhtar volcanic arc lie along the Tethyan suture between Eurasia and Africa-Arabia and is the result of the closure of at least two Tethyan oceans, the Ordovician to Jurassic Palaeo-Tethys and the multiphase Neo-Tethys in the Cenozoic.
For detail of the regional setting see the Iranian Porphyry Copper and Epithermal Gold Province record.
The volcanic complex that hosts the Sari Gunay gold deposit forms a range of small hills that are ~10 km across, flanked to the east and south-east by shallowly sloping lahars, and block and ash-flow tuffs. The core of the system is locally eroded down to its hypabyssal intrusive roots, with vertically flow-banded necks forming prominent remnant spires. In addition, hydrothermal alteration, particularly silicification, has resulted in erosional resistance and accentuation of the Sari Gunay and adjacent Agh Dagh hills around 2 km to the south-east.
The mineralised volcanic complex unconformably overlies Jurassic schists and limestones on its eastern side and Eocene to Oligocene intermediate to felsic volcanic rocks and limestone to the south-east. The margins of the complex to the north, south, and west slope gently below Quaternary alluvial deposits. To the northwest, a small Quaternary volcanic complex projects through the alluvial cover and includes basanitic cinder cones and trachytic lavas.
The Miocene Sari Gunay volcanic complex and other Pliocene-Quaternary volcanic centres to the NNW are enclosed within a prominent major NNW-SSE trending structural corridor, the Takab structural and magmatic trend, which parallels other regional thrust and strike-slip faults that accommodate orogenic transpressional deformation in central Iran. In the Sari Gunay area, the corridor appears to have been sectioned by a series of late, small, NE-SW trending block faults, with small inferred vertical displacements. Two such faults have been locally mapped, one to the west of Sari Gunay and the other between Sari Gunay and Agh Dagh to the south-east, which is consistent with the apparent difference in erosional level in the two systems.
The Sari Gunay volcanic complex is a partially eroded composite volcano and its coeval volcaniclastic flank deposits. Intermediate lava flows slope steeply away from the prominent central peaks of Sari Gunay and Agh Dagh. These flows are locally intercalated with volcanic debris avalanche deposits containing juvenile volcanic fragments. Small peaks of erosionally resistant dacite to rhyolite domes or volcanic necks, with strong near-vertical flow banding, protrude through the more shallowly dipping lavas. These domes appear to represent pyroclastic flow-dome complexes. Volcanic rocks have porphyritic to vitrophyric textures with devitrification of glassy matrices in most instances and variable oxidation of mafic phenocryst phases.
Porphyritic trachyte lavas are the most abundant volcanic rock type in the complex. Hornblende is commonly the dominant ferromagnesian silicate mineral, although Biotite is also present. Plagioclase phenocrysts are abundant and commonly zoned and intergrown, producing a glomeroporphyritic texture. Clinopyroxene forms sparse clusters of small crystals. Rare rounded quartz and large sanidine phenocrysts occur in more felsic and potassic compositions, respectively. The matrix is very fine grained to glassy, with abundant feldspar crystals.
Rhyolite is less abundant, occurring as flow-banded domes or pyroclastic deposits that outcrop as prominent hills. Some of these rhyolites are significantly younger than the intermediate suite of volcanics. Quartz phenocrysts are abundant and are variably euhedral to rounded and embayed. Sanidine phenocrysts are also abundant, with minor plagioclase and biotite, while hornblende, where present, is oxidised. The matrix consists of variably devitrified glass, which commonly accentuates flow-banding textures.
Latite porphyry lavas contain abundant phenocrysts of zoned plagioclase, frequently in glomeroporphyritic clusters, and variably oxidised hornblende, with lesser, variably oxidised biotite, clinopyroxene and orthopyroxene. The matrix is fine grained or glassy, and contains abundant feldspar crystals and variably oxidised micro-phenocrysts of magnetite, with minor alteration carbonate.
The deposit occupies what is interpreted to be the eroded roots in the core of a large central composite volcano containing a variety of hypabyssal intrusive and fragmental rock types, that was breached by diatreme eruptions and subsequently by hydrothermal, quartz-tourmaline breccias. The majority of the intrusive rocks are biotite-plagioclase-phyric trachytes or dacites, with lesser trachytes containing large cm-sized sanidine phenocrysts, and minor plagioclase, biotite, hornblende and quartz.
These sub-volcanic rocks are crosscut by two steeply dipping, partial ring-shaped diatreme breccia and tuffisite bodies, one centred on Sari Gunay and the other on Agh Dagh. These fragmental intrusive bodies have been classified on the basis of clast composition and size. They range from igneous clast-dominated breccias known locally as dacite crystal tuff, to mixtures of igneous and basement clasts known locally as dacite lithic tuff, to basement clast-dominated breccias mapped as lithic tuff breccia. Peperites are also recorded. Clast sizes range from 1 to 20 cm across and are sub-angular to angular. Lithic clasts consist of any of the basement lithologic units, including schist, siltstone and earlier volcanic rocks. The matrix consists of disaggregated igneous crystals and may show strong flow-banding and bedding textures.
Quartz-tourmaline hydrothermal breccia bodies are spatially associated with, localised by and overprint the diatreme breccia. The largest of these hydrothermal breccias forms a north-northeast-trending zone of silicification extending for almost 2 km on the south-east flank of Sari Gunay and hosts the main high-grade ore zone and may have been controlled by the same planes of structural weakness that localised the diatreme breccias. The quartz-tourmaline breccias range from clast to matrix supported, while sedimented geopetal textures are locally observed, particularly toward the margins of the breccia systems. Clasts within the hydrothermal breccias are typically derived from the local host rock, suggesting limited vertical transport. The matrix is composed of a dark mixture of very fine grained blue-green tourmaline needles and quartz, with minor pyrite and rutile.
The earliest hydrothermal activity comprises weak potassic alteration and quartz-sulphide-magnetite veining with low grades of Cu and Au mineralisation and is only encountered at depths of >300 m below the surface. Paragenetically later quartz-tourmaline breccias and veins followed the SSW-NNE trend of the diatreme breccias and produced strong sericitic alteration and silicification in igneous clasts and wall rocks. Tourmalinisation and silicification, locally with fine-grained K feldspar, are characteristic of the inner zones of intensive alteration, veining and hydrothermal brecciation, with quartz and tourmaline representing the bulk mineralogy of the veins and breccia cements. A peripheral zone of propylitic chlorite alteration is extensively but weakly developed in the surrounding volcanic rocks and is manifested by modified ferromagnesian (hornblende and biotite) phenocrysts, with associated minor calcite-quartz-pyrite-galena ±sphalerite veining. The sericite alteration has been dated at between approximately 10.8 and 10.3 Ma and contains disseminated pyrite with small inclusions of chalcopyrite, galena and rare stibnite, and overgrowths of fine-grained arsenopyrite. Tourmaline occurs locally as fine green-brown-blue needles in the sericitic alteration.
Supergene oxidation extends to depths of 20 to 150 m, with partial oxidation locally as deep as 300 m, commonly channelled by veins, joints, and fractures. In the oxidised zone, sulphides are replaced by limonite, jarosite and rare alunite, whiled kaolinite locally replaces wall-rock feldspars or sericite. Supergene oxidation of the predominantly refractory primary gold enhances the metallurgical properties of the ore.
The Sari Gunay hydrothermal system contains a progression of veining, starting with high-temperature, porphyry-like veins, locally intersected at depth, followed by quartz-tourmaline brecciation and veining, and a final stage of epithermal-style quartz-pyrite-stibnite-realgar-orpiment veining, with which the bulk of the gold mineralisation is associated, as follows:
i). Porphyry-style quartz-sulphide-magnetite veins - occur as sheeted sets containing minor pyrite, chalcopyrite, tennantite and magnetite occurs at depth beneath the main zone of quartz-tourmaline brecciation and epithermal mineralisation. These veins are associated with sericitic and minor K feldspar wallrock alteration, and contain low-grade, ~0.25% Cu mineralisation and <0.5 g/t Au. With increasing intensity of veining, magnetite/hematite becomes dominant over sulphides and forms dark selvages to the veins.
ii). Quartz-tourmaline veins and breccias postdate the porphyry-style quartz-sulphide-magnetite veins and are associated with intense sericitic wall-rock alteration that forms a halo up to tens of metres wide around major vein and breccia sets. These veins followed the SSW-NNE trend of the diatreme breccias and contain only minor sulphides, mainly of pyrite, fine-grained marcasite, rutile and rare stibnite. These veins do not carry significant gold grades. Adularia which is present in small amounts in early quartz-tourmaline vein stages, becomes increasingly important in later stages, displacing tourmaline to form quartz-adularia intergrowths and vuggy fillings.
iii). Epithermal quartz-pyrite-stibnite-realgar-orpiment veins which contain the bulk of the gold mineralisation and largely followed the zone of structural permeability produced first by the diatreme breccias and then by the quartz-tourmaline breccias. The large zone of quartz-tourmaline brecciation on the south-eastern flank of Sari Gunay hosts and localised the emplacement the main body of gold mineralisation. The epithermal vein stage begins with the adularia-quartz veins characterised by the presence of "sooty pyrite" which gives the veins a dark grey to charcoal black appearance. The sooty pyrite is fine-grained (<10 µm) arsenical pyrite ± arsenopyrite formed at the vein margins and adjacent wall rocks, and commonly overgrows earlier pyritohedra. Gold is present in solid solution within the sooty pyrite, with high gold grades invariably associated with these veins. The epithermal veins are characterised by an increasing abundance of arsenic- and/or antimony-bearing minerals with time. Auriferous arsenical pyrite overgrows early coarse-grained pyrite, and is followed by arsenopyrite and minor stibnite, followed in turn by massive veins of stibnite and finally vuggy infillings of realgar, orpiment and locally cinnabar. Fine wire gold very rarely occurs in late vuggy cavities in stibnite or realgar veins.
iv). Base metal sulfide veins comprising Vuggy quartz-calcite-pyrite±galena±sphalerite veins with elevated silver concentrations are found mainly outside the main gold-mineralised zones in areas of propylitic alteration.
The dominant structural trend in the Sari Gunay hydrothermal system of both the quartz-tourmaline veins and breccias, and by the later epithermal quartz-stibnite veins is 20 to 30° (NNE-SSW), with dips of from 70°WNW to vertical.
The deposit contained a resource of 52 Mt @ 1.77 g/t Au at a 1.0 g/t Au cut-off within an area of 600 x 1200 m and to a depth of 350 m in 2004 (Richards, et al., 2006).
This description is summarised from Richards, et al., 2006
The most recent source geological information used to prepare this decription was dated: 2006.
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.
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Richards J P, Wilkinson D and Ullrich T, 2006 - Geology of the Sari Gunay epithermal gold deposit, northwest Iran: in Econ. Geol. v101 pp 1455-1496
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Richards, J.P. and Sholeh, A., 2016 - The Tethyan Tectonic History and Cu-Au Metallogeny of Iran: in Richards, J.P. (Ed.), 2016 Tectonics and Metallogeny of the Tethyan Orogenic Belt, SEG Special Publication 19, Ch. 7, pp. 193-212.
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