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Navidad |
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Chubut, Argentina |
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Ag Pb Cu
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Super Porphyry Cu and Au
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The Navidad low sulphidation epithermal silver-lead deposit is located in the central plateau of northern Chubut Province, ~360 km WNW of Puerto Madryn and 1580 km SW of Buenos Aires, Argentina (#Location: 42° 24' 55"S, 68° 49' 20"W).
The earliest recorded exploration in the Navidad area comprised an open-range regional geochemical reconnaissance program conducted by Normandy Argentina in mid-2000 to secure additional resources to complement their Calcatreu Au-Ag Prospect, ~80 km NW of Navidad. The program included 1200 bulk leach extractable gold (BLEG) stream sediment samples from watercourses draining Jurassic volcanic rocks in sections of Chubut Province. It produced anomalies that led to the discovery of the Navidad deposit (initially known as Sacanana). In early 2002, Newmont Mining acquired Normandy, and after a rationalisation of exploration projects decided to sell all of its Argentine interests. Following court proceedings related to disagreements between companies seeking to purchase Newmont's holdings, the Navidad Project passed to the 100% control of Aquiline Resources Inc. in 2008. Drilling by Aquiline from October 2006 resulted in a Mineral Resource estimate in 2009. In December 2009, Pan American Silver Corp. acquired Aquiline Resources and continued exploration and metallurgical testing. A law passed by the Province of Chubut in 2003 that prohibits open pit mining and the use of cyanide in mineral processing throughout the province, has effectively preventing the development of Navidad, and the project remains on care and maintenance.
Regional Setting
The Navidad deposit is located on the southwestern margin of the Somún Cura Massif in northern Patagonia, and lies within the major NW-SE trending transcontinental Gastre Fault Zone that defines the margin of the massif. This structure is believed to be the result of continental-scale NE to SW extension that produced a series of NW-SE trending half grabens and tectonic basins (von Gosen et. al., 2004). At Navidad these grabens/basins host the Jurassic Lonco Trapial Formation and overlying Cañadón Asfalto Formation, the latter of which hosts the Navidad mineralisation. Granitoid and metasedimentary rocks of the Palaeozoic basement in northern Chubut Province are locally exposed in windows through the overlying Mesozoic age volcanic and sedimentary rocks.
For more detail of the regional geological and metallogenic setting see the Southern Andes and Patagonia record.
Geology
The oldest rocks in the Navidad area are the Palaeozoic Mamil Choique Formation which outcrop in horsts along the southwestern side of the area, 6.5 km SW of the deposit. They comprise red and grey granitoids, cut by aplite dykes and quartz-rich pegmatites. These crystalline basement rocks are overlain by acid ignimbritic pyroclastics, volcanic agglomerates, and lavas of the Lonco Trapial Formation also to the SW of Navidad, succeeded, in turn, by sandstones, mudstones and limestones of the Cañadón Asfalto Formation which are the host for silver mineralisation at Navidad. Lonco Trapial ignimbrites outcrop again to the NE of the deposit. The Jurassic sequences are unconformably overlain by the Cerro Barcino Formation of the Cretaceous Chubut Group, comprising continental sandstones, conglomerates and tuffs. The youngest rocks are plateau basalts of the Miocene Pire Mahuida Volcanic Complex.
The Mesozoic sequence in the Navidad area was deposited within a graben structure cut by three NW-SE striking major fault zones, locally referred to as 'trends', from SW to NE the Argenta, Esperanza and Navidad trends. The Navidad Trend, which contains the bulk of the silver mineralisation, occurs in the immediate hanging wall of the major NE striking Sauzal Fault. Most of the economic mineralisation is hosted by the upper of two trachytic andesite lava flows which overlie an extensive andesite flow.
Mineralisation
The Navidad deposit occurs as eight individual mineralised zones in the three separate trends.
The Navidad Trend has six zones that occur over a strike length of ~5.8 km, and are essentially continuous. They comprise, from NW to SE:
• Calcite NW, which occurs as three long, tabular to slightly synformal bodies. The main body has a strike length of 1825 and extends from the surface to a depth of 130 m below, with an average overburden thickness of ~60 m. It has a down dip extent of 350 to 500 m, and a thickness of between 10 and 80 m, with a dip of from 1 to 5°, plunging gently to the NE. Towards the SE end of the zone, a smaller lens lies close to the surface parallel to the main body and about 80 m above it. It is ~275 m long, up to 250 m down dip and between 20 and 40 m thick. The third body is from 15 to 50 m below and parallel to the northern end of the main body, and is 1000 m long, 200 to 350 m down dip, and 10 to 30 m thick.
• Calcite Hill, is an irregular body with a narrow upper section that outcrops towards the western end, and merges with a larger mineralised lens down dip. It extends to a depth of ~250 m and forms a relatively flat structure that is 600 m long, with a down dip extent varying from 270 to 600 m. The lower part of this body has an irregular shape where two almost separate lenses merge with a variable thickness of between 150 and 20 m. Overall, the body plunges to the SW with a dip of 5°.
• Navidad Hill, where mineralisation trends for 520 m towards the NW to form an irregular globular shape ranging from 270 to 470 m down dip and 10 to 175 m in thickness. It dips shallowly to the SW and lies in the subsurface along the ridge crest, to ~50 m depth along the southern flank.
• Connector, which forms two intersecting, but distinct bodies that have a combined strike length of 670 m, and are between 240 and 590 m wide down dip. Mineralisation extends from the surface to a depth of 330 m.
• Galena Hill, with a strike length of ~900 and down dip extent of between 250 and 700 m. This zone, as defined by the 50 g/t Ag equiv. contour, forms a roughly stratabound body with a slight dip to the SW, and resembles an inverted shield with a flat top and a thicker (up to and >200 m) central portion that thins to a few metres on the margins. On almost every cross section, the mineralisation is affected by post-mineral displacement on NW-SE trending block faults with displacements of ~10 to 50 m. Those portions of the mineralisation located above horsts created by these faults are partly eroded whilst those portions to either side are fully preserved.
• Barite Hill, where mineralisation occurs as three lenses. The northern of these has a strike length of ~230 m, persists down dip for between 170 and 430 m, and is between 5 and 30 m thick. It dips to the SW, varying from ~3° where it outcrops in the north, to 25° in the SW
where it is ~120 m below surface. The second lens occurs towards the southern end of Barite Hill, and has dimensions of ~300 m along strike by 350 m down dip and thicknesses ranging from 4 to 32 m. It occurs at the subsurface on the crest of the ridge and plunges to the SW. The third mineralised body is characterised by high Ag values, and forms an irregularly shaped mass around 350 m long, between 100 and 400 m down dip, and 7 to 100 m thick, lying between 50 and 200 m below the second lens in southern Barite Hill with a dip of 30°WSW.
The Esperanza Trend hosts the
Valle Esperanza zone which occurs on the east flank of the trend, ~400 m SSW of Galena Hill. Two mineralised zones extend from surface to ~400 m below surface. The upper body is ~1100 m long and between 130 and 700 m down dip. The lower body, which lies ~50 m below the upper, is 800 m long and 140 to 500 m wide down dip. Both are between 5 and 30 m thick. The mineralised horizon strikes at ~290° with a variable dip between 70 and 10°NE, flattening towards the NE.
The Argenta Trend hosts the
Loma de La Plata zone, which is in the northern part of the Argenta Trend, and ~2.2 km SW from Calcite Hill. Two distinct mineralised bodies are recognised. The main zone is 850 m long with a north-south strike, persisting between 600 and 1200 m down dip and is 40 to 50 m thick, covering a surface area of 74 ha. The second body is considerably lower in grade and occurs ~60 m beneath the main mineralised zone. It has approximately the same surface area but an average thickness of only 5 m. The highest grade mineralisation occurs in the central and western side of the upper Loma de La Plata zone. The thickness of overburden varies from 0 to 50 m, with the dimensions of the high grade zone being 500 m north-south by 170 m east-west.
All of the Navidad mineralised zones have a gangue assemblage that is principally composed of calcite and minor barite, with silica being less important, occurring mostly as chalcedony and late amethyst. Visible ore minerals include native silver, clots of black sulphide comprising argentite/acanthite, discrete grains of sphalerite, galena, chalcopyrite, cuprite, bornite, native copper and copper carbonates (malachite, azurite). Most of the mineralised zones have a similar styles of mineralisation and paragenesis, although the proportion of sulphides varies considerably. Loma de La Plata is silver-rich, but is sulphide-poor and contains only very low levels of lead, zinc and copper. Multiple pulses of mineralisation are observed, principally at Galena Hill.
The dominant metal association is Ag-Pb, with other associations including Ag-Pb-Cu and Cu-Ag and, more rarely, Ag-Zn. Occasionally Ag occurs alone, or with Cu-Pb-Zn or simply isolated occurrences of individual base metals. This is taken to suggests deposition occurred through successive pulses of mineralisation. Gold appears to be totally absent from the system.
Mineralisation is mostly hosted in the upper trachytic andesite lava flows where cut by structures, although important mineralisation also occurs in the lower trachytic andesite lava flows at Galena Hill. In a few locations the underlying andesite also hosts high grade mineralisation. Zones with mostly trachytic andesite-hosted mineralisation include: Loma de La Plata, Valle Esperanza, Calcite Hill and Galena Hill. Sedimentary rocks and volcaniclastics that overlie or are laterally equivalent to the upper trachytic andesite also host significant mineralisation. Deposits where the mineralisation is dominantly hosted by these rock types include Calcite NW, Navidad Hill, Barite Hill, and Connector Zone.
High grade mineralisation is mostly found in permeable host rocks, where primary porosity is the result coarse volcaniclastic rocks and autobrecciated lava flows. Secondary porosity includes crackle brecciation of the brittle lava flows, hydrothermal eruption breccias, and tectonic brecciation. At both Valle Esperanza and Loma de La Plata, the autobrecciated upper trachytic andesite acted as an aquifer, sealed by overlying organic-rich sedimentary rocks (mudstones and limestones). The sedimentary rocks were unconsolidated and are commonly slumped. Mixing of reduced water, derived from the organic-rich sediments, and rising metal-laden hydrothermal fluid are interpreted to have triggered sulphide precipitation.
Mineral Resources
Mineral Resources at 31 December, 2016 (Pan American Silver Corp., Reserve and Resource Report, 2017) at 50 g/t Ag equiv. cut-off were:
Measured Resource - 15.4 Mt @ 137 g/t Ag, 1.44% Pb, 0.10% Cu (177 g/t Ag equiv.) for 2084 t Ag;
Indicated Resource - 139.8 Mt @ 126 g/t Ag, 0.79% Pb, 0.04% Cu (147 g/t Ag equiv.) for 17 573 t Ag;
Measured + Indicated Resources - 155.2 Mt @ 126 g/t Ag, 0.85% Pb, 0.05% Cu (150 g/t Ag equiv.) for 19 657 t Ag;
Inferred Resource - 45.9 Mt @ 81 g/t Ag, 0.57% Pb, 0.02% Cu (97 g/t Ag equiv.) for 2084 t Ag.
No reserves have been calculated.
This record is largely drawn from: Austin D.C.J., Wafforn, M., Welhener, H., Steinmann, M., Drielick, T.L. and De Mark, P., 2010 - Pan American Silver Corp., Navidad Project, Chubut Province, Argentina - an NI 43-101 Technical Report prepared for Pan American Silver Corp., by M3 Engineering & Technology Corporation, 200p.
The most recent source geological information used to prepare this decription was dated: 2010.
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.
Navidad
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Selected References:
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Bouhier, V., Franchini, M., Tornos, F., Rainoldi, A.L., Patrier, P., Beaufort, D., Boyce, A.J., Pratt, W. and Impiccini, A., 2023 - Genesis of the Loma Galena Pb-Ag Deposit, Navidad District, Patagonia, Argentina: A Unique Epithermal System Capped by an Anoxic Lake: in Econ. Geol. v.118, pp. 433-457. doi: 10.5382/econgeo.4968
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Dejonghe, L., Darras, B., Hughes, G., Muchez, P., Scoates, J.S. and Weis, D., 2002 - Isotopic and fluid-inclusion constraints on the formation of polymetallic vein deposits in the central Argentinian Patagonia: in Mineralium Deposita v.37, pp. 158-172.
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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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