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Jeronimo |
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Chile |
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Au
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Super Porphyry Cu and Au
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The Jeronimo carbonate repalcement gold deposit is within the Potrerillos Mining District, in the Atacama region of northern Chile, approximately 4 km ESE of the Potrerillos (Cobre) porphyr Cu-Mo-Au deposit - see also the separate Potrerillos Mining District record.
Pre-Jurassic basement units in the Potrerillos district include Carboniferous to Triassic granite, diorite, tonalite, and quartz syenite, metavolcanic rocks, and minor quartz-muscovite schist. These basement lithologies are overlain by the Jurassic, Tarapaca back-arc basin sequence of more than 400 m of calcareous mudstone and limestone of the Montandon Formation, grading upward into the 290 m of mixed carbonate and clastic sedimentary rocks with interbedded andesite and basaltic andesite flows of the Asientos Formation, which are in turn is conformably overlain by siltstone and limestone with inbterstratified basaltic andesite flows of the Late jurassic to Early Cretaceous Pedemales Formation. These sequences were overlain by Palaeocene bimodal andesitic and rhyolitic volcanics which are up to 350 m thick.
Eocene to Oligocene transpression, linked to deformation along the major regional, north-south trending Domeyko fault system, produced a belt of asymmetric folding, and thrust to reverse faulting in the Potrerillos district, including the Potrerillos Mine fault, an east-southeast-vergent thrust fault with approximately 1.5 km of displacement. The Potrerillos porphyry deposit and the El Hueso Au deposit are both located in the upper plate of the Potrerillos Mine fault. The Jeronimo deposit is in the footwall plate, 1.5 km east of El Hueso.
The mineralisation at Jeronimo occurs as irregular stratabound lenses within specific bioclastic limestone units of the Jurassic Asientos Formation. These manto-shaped zones of mineralisation are distributed over an area of approximately 2.0 x 1.3 km and average around 6 m in thickness.
Both the mineralization and alteration are focused on subvertical fractures and joints within the favourable bioclastic units. Alteration includes decarbonatization followed by the assemblages: i). intense, pervasive, replacement silicification; ii). carbonate, comprising Mn carbonate (rhodochrosite and kutnohorite) in the centre of the orebody and calcite-dolomite on the margins, largely restricted to vugs; and iii). argillisation, as widespread disseminations and veinlets of illite and vug fillings of kaolinite in the centre of the deposit. Other common alteration minerals include apatite, rutile, monazite, and barite.
The ore comprises a suite of pyrite, arsenopyrite, sphalerite, lead sulphosalts, orpiment and realgar, with minor coloradoite, altaite, cinnabar, and cassiterite. Gold is dominantly present as submicron-sized grains, ranging from 140 nm to 1.13 µm, encapsulated in pyrite, arsenopyrite, quartz and realgar and also occur within vugs in the silicified matrix.
Prior to development the resource at Jeronimo was: 16.5 Mt @t 6.0 g/t Au = 100 t Au.
The most recent source geological information used to prepare this decription was dated: 2003.
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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Thompson J F H, Gale V G, Tosdal R M and Wright W A, 2004 - Characterisitics and formation of the Jeronimo carbonate replacement gold deposit, Potrerillos District, Chile: in Sillitoe R H, Perello J and Vidal C E, 2004 Andean Metallogeny: New Discoveries, Concepts and Updates, Society of Economic Geologists, Denver, SEG Special Publication 11 pp 75-95
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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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