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Zarshuran, Agdarreh |
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Iran |
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Au
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Super Porphyry Cu and Au
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The Zarshuran and Agdarreh gold resource are located in north-western Iran, near Takab and is hosted by Precambrian sediments.
The two deposits occupy a 20x20 km area of the NW-trending Late Cretaceous to Tertiary Zagros orogenic belt produced by collision of the Afro-Arabian and Iranian plates along a NE-dipping subduction zone. They occur in a Tertiary volcanic belt that contains Miocene to Recent ore deposits and a geothermal field with high Au.
The host sequence, from oldest to youngest, is composed of a basement schist, the Chaldagh unit (limestone) and the Zarshuran unit (carbonaceous black shale with silica and carbonate intercalations). Gold is in hydrothermal veins of massive quartz (jasperoid) and quartz veinlets formed by carbonate replacement along high angle east-west and NW-SE faults in the Chaldagh limestones. It also occurs as disseminations in the carbonaceous, siliceous and calcareous beds of the Zarshuran black shale. A highly altered Oligocene-Miocene granite, which is also weakly mineralised, is found in assocaition with the mineralised Precambrian rocks.
At Zarshuran, the hosts are primarily composed of quartz, calcite dolomite and clays, with decalcification, silicification and argillisation characterising the mineralised rocks. The ore is mainly of orpiment and pyrite with lesser sphalerite, galena, realgar and stibnite, and subordinate cinnabar, lorandite, christite, coloradoite, getchellite, aktashite, baumhuerite, boulangerite, geochronite, plagionite and twinnite. Sulphide oxidation is mainly confined to veins, veinlets and fractures. The gangue minerals are primarily quartz, calcite, fluorite, hematite and barite, with accessory apatite, rutile, zircon and xenotime.
Although micron sized particles of gold are observed at Zarshuran, the quantities are insufficient to account for the assay grade, which has been shown to be due to the presence of invisible gold, particularly in pyrite, arsenian pyrite and other sulphides.
The Agdarreh disseminated gold deposit of is hosted by hydrothermally leached Miocene reef limestone in the Takab geothermal field, and is part of the Cenozoic UrumiehÐDokhtar volcanic arc of NW Iran. Alteration and mineralisation are largely bedding controlled blanket-like and include: i). pre-ore decalcification; ii). first-stage silicification associated with pyrite (early pyrite with 3 to 4 wt% As, late pyrite with <1 to 3 wt% As) and sphalerite; iii). second-stage silicification with precipitation of galena, Pb-Sb-As sulphides, sulphosalts, tellurides and native bismuth; iv). late-stage cinnabar and barite in vugs; v). oxide ore stage and carbonate alteration (complex Mn-Fe rich oxyhydroxides, arsenates, sulphates and rutile in residual leached rock and infill of karstic cavities). Gold is very fine grained and lies within the jasperoids where it is enriched in the Mn-Fe oxyhydroxide surface cap of the jasperoids. The gold is associated with the hydrothermal suite of As, Sb, Hg, Te, Se, Tl, Ba, Zn, Ag, Cd, Bi and Pb, characterised by very low Cu contents. Arsenian pyrite most likely carries most of the primary gold. Native gold occurs in association with the late-stage cinnabar and the oxide ore.
The Agdarreh deposit has many similarities to Carlin-type ore, interpreted to have resulted from near-surface hydrothermal activity related to the Cenozoic arc volcanism that developed within the extensional Takab graben. The extensive oxidation at Agdarreh may be partly due to the waning stages of hydrothermal activity. Active H2S-bearing thermal springs are locally depositing extremely high contents of Au and Ag, and travertine is present over large areas, suggesting that ore-forming hydrothermal activity occurred periodically from the Miocene to Recent in the Takab geothermal field.
Exploration had established a resource of approximately 2.5 Mt @ 10 g/t Au in 1999.
In 2007, Agdarreh had a resource of 24.5 t of Au in ore with an average grade of 3.7 g/t Au (Daliran, 2008). Total resource figures quoted in 2002 (Daliran, 2002) included Agdarreh - 50 t Au at grades of 3 to 4 g/t Au; Zarshuran - 80 t Au at grades of 3 to 4 g/t Au.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2008.
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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Selected References:
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Asadi H H, Voncken J H L, Kuhnel R A and Hale M 2000 - Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold : in Mineralium Deposita v35 pp 656-671
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Asadi H H, Voncken J H L, Hale M 1999 - Invisible Gold at Zarshuran, Iran: in Econ. Geol. v94 pp 1367-1374
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Daliran F, 2008 - The carbonate rock-hosted epithermal gold deposit of Agdarreh, Takab geothermal field, NW Iran, hydrothermal alteration and mineralisation: in Mineralium Deposita v43 pp. 383-404
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Mehrabi B, Yardley B W D, Cann J R 1999 - Sediment-hosted disseminated gold mineralisation at Zarshuran, NW Iran: in Mineralium Deposita v34 pp 673-696
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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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