Cerro Moro - Escondida, Gabriela, Esperanza-Nini, Carla, Deborah, Zoe, Martina
Santa Cruz, Argentina
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The Cerro Moro low sulphidation epithermal silver-gold deposits are located in Santa Cruz Province, southern Patagonia, Argentina, ~70 km SW of Puerto Deseado, 160 km NNE of Puerto San Julián and 250 km SSE of Comodoro Rivadavia (#Location: 48° 6' 30"S, 66° 39' 37"W).
Cerro Moro was discovered in 1993 by Mincorp Explorations S.A., during follow-up of regional Landsat Thematic Mapper satellite imagery colour alteration anomalies. The ensuing exploration program of geological mapping, rock chip geochemistry and drilling resulted in the discovery of >30 widespread quartz vein bearing structures that were variably mineralised over an area of >100 km2. The project was subsequently transferred to Cerro Vanguardia S.A., and was later optioned to eventually become 100% owned by Extorre Gold Mines Limited by 2007. A pre-economic assessment study was completed in October 2010 and an environmental impact assessment was approved in May 2011. Extorre was acquired by Yamana Gold in August 2012. Construction commenced in 2015. Production from the mine is expected to begin in 2018.
The Cerro Moro Ag-Au veins are situated towards the eastern margin of the extensive, ~60 000 km2 Deseado Massif that stretches across southern Argentina from the southern Andes in Chile to the Atlantic Ocean. This massif is largely occupied by rocks of the Jurassic Chon Aike Large Igneous Province, related to a late Triassic to late Cretaceous (230-65 Ma) extensional phase that culminated in the opening of the South Atlantic Ocean. The generally WNW-ESE aligned massif is flanked by the post-Jurassic subsiding Golfo de San Jorge and Austral sedimentary basins to the north and south, respectively. The massif is underlain by the Patagonian Terrane, an early Palaeozoic to Permian allochthonous terrane that collided with Gondwana in the early Permian. The Patagonian Terrane is represented by metamorphic rocks of the La Modesta Formation (also known as the Rio Deseado Complex), comprising phyllites, quartzites, schists, amphibolites, gneisses and migmatites. This formation is intruded by granites of Lower (Ordovician) to Middle (Late Devonian to Permian) Palaeozoic age. They are, in turn, unconformably overlain by continental sandstone of the Permian La Golondrina and Triassic El Tranquilo Formations, which were deposited in a series of graben and half-graben structures.
Extension during the Jurassic resulted in deposition of the ~990 m of epiclastic and pyroclastic volcanosedimentary sandstones and sandy tuffs of the Roca Blanca Formation. This succession is overlain by rocks of the Jurassic Chon Aike Large Igneous Province which dominates the geology of the Deseado Massif, and is represented by the Bahia Laura Group. The lowermost unit of this group is the widespread basaltic and basaltic andesite lavas and pyroclastic suite of the Mid-Jurassic Bajo Pobre Formation. These are overlain by the sequence of Mid- to Late-Jurassic rhyolitic to dacitic ignimbrites, tuffs and volcaniclastics of the Chon Aike Formation and volcaniclastics of the La Matilde Formation. The gold mineralisation at Cerro Moro is hosted by rocks interpreted to belong to the Chon Aike Formation.
During the Paleocene the region was draped with continental and marine sediments. More recently, during the Late Tertiary to Pleistocene, basaltic lava flows were extruded, although these are absent at Cerro Moro.
The regional setting and geology of the Deseado Massif is described in the Southern Andes and Patagonia record.
The main geological units in the Cerro Moro project area comprise:
Bahia Laura Volcanic Complex - of Middle to Upper Jurassic age, comprising from the base,
• Bajo Pobre Formation - tuffs, flows and sub-volcanic intrusives of andesitic to basaltic composition;
• Chon Aike Formation - the two principal components of which are a rhyolitic ignimbritic pyroclastic facies and a late intrusive/extrusive flow-dome facies,
• La Matilde Formation - characterised by more epiclastic and aerially deposited ash rich volcaniclastic deposits that are observed to overlie, or interdigitate with and form a lateral facies variation to the upper Chon Aike Formation sequence.
Cenozoic cover - around a third of the project area is covered by Tertiary marine sediments and Quaternary gravels.
Geological mapping of the project area (Callan, 2008) divided the sequence exposed into 14 stratified volcanic units that were grouped into five broad packages based on inferred ages, composition, lithological characteristics and spatial relationships. All belong to the Chon Aike Formation or laterally equivalent sections of the La Matilde Formation, as follows:
• The oldest of the packages is an extensive pile of coarse rhyolite clast-bearing ash-flow tuffs and remnant flow-domes, the upper part of which is punctuated by several closely associated welded ignimbritic horizons of rhyodacitic to locally more dacitic composition. These rocks form an extensive litho-structural domain covering a large part of the project area. Their southern boundary is defined by an arcuate, linked normal fault system formed by the interaction of the NW striking Escondida Fault to the SW and the NE striking Deborah South Fault to the SE. Subsequent rocks have shallow south to SE dips, and are generally confined to the corresponding down-thrown structural domain lying to the south of this linked normal fault system. Field evidence suggests that this linked fault system was active during deposition of the subsequent volcanic stratigraphy.
• The initial rhyolitic package described above is unconformably overlain by a compositionally distinct volcanic-sedimentary assemblage of andesitic to basaltic composition comprising bedded volcanic sediments, coarse volcanic breccias, flows and possibly concordant sub-volcanic intrusive units.
• These more intermediate rocks are, in turn, unconformably overlain by a bedded series of felsic air-fall tuffs and related epiclastic units, characterised by the presence of wood fragments, accretionary lapilli and local hot-spring 'sinter'.
• The bedded tuff and epiclastic unit passes gradationally upwards via a thin 'transitional' stratified sequence that is characterised by pumice-bearing vitric tuffs beds into the youngest mapped volcanic suite, a crudely stratified sequence of strongly welded ignimbritic tuffs of rhyolitic composition.
• A series of co-genetic, texturally diverse rhyolitic units are hosted within the stratified volcanic pile forming simple and composite intrusive flow-domes, as well as dykes on a variety of scales. Emplacement of these rhyolitic intrusives was strongly influenced by structure, and they are for the most part discordant with host volcanic rocks, although some locally sub-concordant relations were observed by Callan (2008). Cross-cutting relationships with the younger mapped stratigraphic units indicate much of the rhyolite intrusive activity occurred quite late in the geological evolution of the project area.
NOTE: This sequence is very similar to that described and illustrated by the stratigraphic column in the Cerro Negro record.
The Cerro Moro project area is characterised by a complicated structural architecture composed of a complex of steeply dipping to sub-vertical fault structures, with NW to WNW, WSW, NE and NE to NNE strikes. Several of the major mapped fault structures (e.g. the WNW to NW striking Escondida Fault, the NE striking Deborah South Fault, and the NW striking Deborah-Belen Fault) had a significant normal component of displacement over much of their kinematic history. This normal displacement is regarded to have exerted control on the primary distribution of volcanic units, Jurassic palaeo-topography, the emplacement and geometry of rhyolite flow-domes and related dykes, and more importantly provided sites for epithermal Ag-Au vein mineralisation. Post-mineral displacement also localised late rhyolitic intrusive activity, offsetting and brecciating vein mineralisation, and locally preserving high-level alteration types. This faulting is interpreted to represent a long-lived extensional regime that is manifested as widespread block-faulting with generally minor dip-slip or oblique slip on most faults, but with a locally more significant normal component of displacement across master faults (Callan, 2008).
Deposit Geology and Mineralisation
Polymetallic silver-gold mineralisation occurs as epithermal veins, with high-grades of gold and silver closely associated with sulphides such as: pyrite, sphalerite, galena, acanthite and chalcopyrite. Arsenic and mercury are at relatively low levels. The presence of yellow sphalerite and adularia are indicative of low temperature mineralisation. At least two mineralisation pulses have been observed. The first deposited clean white quartz veins with a low sulphide content and is generally poorly mineralised. It has been interpreted to be the product of shallow, circulating meteoric dominated water with scarce mixing of magmatic water. A second pulse, comprising black silica, is rich in sulphides and hosts high-grade mineralisation. Precious metal deposition is interpreted to be the product of mixing of magmatic dominated and meteoric waters. Boiling textures, vein breccias and repeated quartz overgrowths with dark 'ginguro' bands of fine-grained sulphides are also observed. Coarse acanthite and electrum have been observed in several mineralised shoots and are common in the high grade zones of the Escondida structure.
Some of the key vein systems are as follows:
• Escondida - the Escondida structure has been defined over a strike length of ~8 km on the SW margin of the project area and contains 6 main mineralised zones within the Escondida quartz vein system that occupies the fault zone and dips at 70 to 85°SW. The width of the mineralisation varies between 0.1 to 5 m on average, and continues to at least 250 m below the present surface. The structure has a general NW-SE trend, with high grade shoots being localised where the structure changes strike to east-west or WNW-ESE. The Escondida fault is interpreted to have pre-, syn- and post-mineralisation displacement. In the structural footwall to the north, the sequence consist of volcanics, whilst the hanging-wall rock to the south are sandstones. High-grade mineralisation is closely associated with a relatively late, black silica pulse of quartz which crosscuts and fills breccia voids within an earlier white quartz-adularia phase. The black silica is rich in coarse sulphides including galena, sphalerite, pyrite, chalcopyrite and acanthite.
The high grade Zoe shoot occurs along the southeastern end of the Escondida Vein System where the strike of the structure has rotated to be east-west. It is 3.4 and 2.4 km SE of the Escondida Far East and Martina shoots, respectively. Mineralisation has been traced for ~600 m along strike and to a depth of ~250 m, with thicknesses varying between 0.5 to 6 m. The main vein at Zoe marks the faulted contact between hangingwall andesitic sandstone and footwall rhyodacitic ignimbrites and intrusive felsic breccias. Mineralisation is associated with quartz-adularia and black silica material, rich in base metal sulphides. Coarse electrum and gold are observed in some of the high grade intersections.
• Gabriela - The Gabriela structure contains a NW oriented quartz vein dipping between 60 and 80°NE, 3 km NE of Escondida, located towards the centre of the project area and has shoots distributed over a strike length of at least 1.3 km. The mineralised zone varies from 10 to 20 m in width near surface in the central-southeastern part of the structure, and 3 to 5 m to the NW and further SE. Mineralisation has a substantially higher Ag:Au ratio, compared to other veins at Cerro Moro. The vein is mostly characterised by white quartz-adularia with disseminated fine pyrite (and possible acanthite). The vein has been tested to a maximum depth of 350 m from surface. Mineralisation cuts several volcanic units.
• Esperanza-Nini - Esperanza is a NW striking structure between Escondida and Gabriela, containing one or more quartz veins, which dip steeply to the NE. Veins at the northwestern end of the structure are enclosed within coherent andesite, whereas, to the SE, the main structure separates rhyodacite to the NE from rhyolite to the SW. Mineralisation in the latter area is characterised by a 15 m wide stock-work zone between several higher grade veins. The structure has a known strike length of ~2 km. Mineralisation persists to a vertical depth of at least ~120 m. The main veins are 1 to 3 m in width. Gold-silver mineralisation is associated with fine disseminated pyrite, copper, lead and zinc sulphides within quartz veins, with high grade zones accompanied by dark sulphidic 'ginguro' banding containing pyrite, chalcopyrite and sphalerite within quartz veining.
Nini is a major NW vein structure, known to be the NW continuation of Esperanza. It is partially covered by Tertiary sediments and has been traced over a strike length of ~1.2 km. In the NW the vein dips shallowly to the NE, with a hanging wall of dacitic ignimbrite and footwall of andesitic lavas. The central and south-eastern sectors have a dip change to 80°NE and the host rock is intermediate to rhyodacitic ignimbrite, in both the hanging wall and footwall. The thickness varies from 0.90 to 3.30 m. Mineralisation generally occurs within a single quartz vein, although vein breccias and stock-works with disseminated sulphides are locally evident.
• Loma Escondida - an east-west structure that dips steeply to the north and is hosted entirely within andesite. This vein is ~500 m north of the Escondida Vein System and has been interpreted as a secondary dilational structure formed by strike-slip movement of the major NW structures. The vein has been traced over a strike length of ~600 m. It is relatively narrow, ranging in thickness from 0.30 to 2 m. Pyrite, galena, sphalerite and acanthite are associated with high-grade shoots.
• Carla - is located ~2 km NE of the Martina shoot of the Escondida structure. It contains a quartz vein stockwork and breccia vein which is sub-parallel to a major NW fault that is interpreted as the same regional structure that controls the mineralisation at Esperanza-Nini. Mineralised clasts within a tectonic breccia are evidence that it has had been reactivated after the mineralisation. Patchy high grade mineralisation occurs over a length of 150 m and is truncated by post mineral faulting within 50 vertical metres of the surface. The thickness of the mineralisation varies from 1 to 10 m. High-grade mineralisation is associated with hydrothermal black silica breccias rich in sulphides, including pyrite, galena, sphalerite and acanthite. The mineralisation is emplaced close to a wide tectonic breccia zone that separates a felsic ignimbrite to the north and a coherent andesite unit to the south.
• Deborah - is located ~3 km NE of the Zoe Shoot which is on the eastern extension of the Escondida structure. The Deborah structure has a NE trend with a moderate NW dip. The structure separates felsic ignimbrite in the hanging wall from intermediate to rhyodacitic ignimbrite in the footwall. The vein has a total strike length of 950 m, with mineralisation being traced to a vertical depth of 80 to 100 m, over a thickness that varies from 0.50 to 5 m. To the NE the structure has been truncated by a NW trending fault with apparent dextral displacement. Better gold-silver grades are located at the north-eastern end and are associated with white quartz vein breccias with a black silica matrix and disseminated sulphides, mostly pyrite. The truncating NW trending structure (the Deborah Termination Structure) may represent a structure similar to that at Escondida, and has anomalous gold contents.
All of these examples are concentrated in the southwestern to southern section of the project area where veining is much denser. Others to the NW, north and SE appear to define the rim of an 11 km circular feature.
Mineral Resources and Ore Reserves
Published Mineral Resources for each of the main veins/shoots in 2012 (Guzmán et al., 2012) were:
Indicated resource - 0.62 Mt @ 18.8 g/t Au, 829.2 g/t Ag;
Inferred resource - 0.508 Mt @ 4.3 g/t Au, 164.8 g/t Ag;
Indicated resource - 0.044 Mt @ 18.4 g/t Au, 919.5 g/t Ag;
Inferred resource - 0.013 Mt @ 9.7 g/t Au, 595.4 g/t Ag;
Indicated resource - 1.642 Mt @ 1.5 g/t Au, 226.1 g/t Ag;
Inferred resource - 0.331 Mt @ 1.3 g/t Au, 219.7 g/t Ag;
Indicated resource - 0.105 Mt @ 27.2 g/t Au, 2614.5 g/t Ag;
Inferred resource - 1.248 Mt @ 4.1 g/t Au, 280.3 g/t Ag;
Indicated resource - 0.015 Mt @ 16.0 g/t Au, 701.2 g/t Ag;
Inferred resource - 0.002 Mt @ 9.5 g/t Au, 390.4 g/t Ag;
Inferred resource - 0.293 Mt @ 13.0 g/t Au, 60.3 g/t Ag;
Inferred resource - 1.773 Mt @ 1.8 g/t Au, 144.3 g/t Ag;
Inferred resource - 0.578 Mt @ 2.4 g/t Au, 48.1 g/t Ag;
TOTAL Indicated resource - 2.425 Mt @ 7.4 g/t Au, 498.8 g/t Ag;
TOTAL Inferred resource - 4.747 Mt @ 3.5 g/t Au, 172.0 g/t Ag;
TOTAL resource - 7.172 Mt @ 4.82 g/t Au, 282.5 g/t Ag - for 34.56 t Au, 2026 t Ag.
Published Mineral Resources and Ore Reserves at 31 December, 2016 (Yamana Reserve and Resources Report, 2017) were:
Indicated Mineral Resource - 3.321 Mt @ 2.23 g/t Au, 190.3 g/t Ag - for 7.40 t Au, 632 t Ag;
Inferred Mineral Resource - 4.427 Mt @ 1.96 g/t Au, 101.3 g/t Ag - for 8.68 t Au, 448 t Ag;
Probable Ore Reserve - 1.954 Mt @ 11.38 g/t Au, 648.3 g/t Ag - for 22 t Au, 1267 t Ag.
Note: Mineral Resources include Ore Reserves. The figures above have been double checked, due to the apparent inconsistency between resources and reserves.
This record is largely drawn from: Guzmán, C., Gosling, W., Coupland, D., Sanford, A., Sinha, K. and Gabora, M, 2012 - Preliminary economic assessment for the Cerro Moro Gold-Silver Project, Santa Cruz, Argentina - an NI 43-101 Technical Report prepared for Extorre Gold Mines Limites, 338p..
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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