Santa Cruz, Argentina
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The Manantial-Espejo silver-gold deposit is located in Santa Cruz Province, southern Patagonia, Argentina, ~105 km NW of Puerto San Julián on the Atlantic coast and 340 km SSW of Comodoro Rivadavia (#Location: 48° 48' 12"S, 69° 30' 44"W).
Exploration at Manantial-Espejo commenced in 1989 when St Joe Minerals acquired title from local interests. It subsequently passed to Lac Minerals in 1991 and to Barrick Exploration in 1994. Triton Mining, through Minera Triton Argentina S.A., acquired 80% of the title in 1998. The remaining 20% is held by Compañía Minera Alto Valle S.A. The ownership has subsequently passed to Pan American Silver in 2006 and production commenced in 2008 with both underground and open pit mining.
Manantial-Espejo lies within the central southwest part of the Deseado Massif in the Chile-Argentina Patagonian Region. The geology of this region is dominated by the volcanic rocks of the Chon Aike large igneous province, represented by the Jurassic Bahia Laura volcanic complex. This complex comprises a lower package predominantly composed of andesitic volcanic rocks - mainly lavas - and an upper package of rhyolitic volcanic rocks - ignimbrites, epiclastics and lavas.
The regional setting and geology of the Deseado Massif is described in the Southern Andes and Patagonia and Cerro Vanguardia records.
Pre-Jurassic basement is revealed locally as a small outcrop of probable Palaeozoic granite and as lithic fragments of quartz-biotite schist and granite within Jurassic ignimbrites.
The oldest unit within the overlying Middle to Upper Jurassic sequence is composed of andesite and basaltic-andesite lava, with lesser andesitic agglomerate and clastic sedimentary interbeds. These are regarded as belonging to the Bajo Pobre Formation (e.g., Sharpe et al., 2002), or El Piche Formation (e.g., Guido and Jovic, 2014). They are followed by eight ignimbrite units of the Chon Aike and La Matilde formations, the first of which is of dacitic composition (dated at 156 Ma; U-Pb-SHRIMP; Moreira et al., 2009), whilst the rest are rhyolitic with interspersed tuff and tuffaceous sandstone. Thermogenic travertine deposits have been mapped within tuffs overlying the third ignimbrite unit, interpreted to be associated with a hot-spring environment (Echeveste, 2005) that was a precursor and surface expression of the main sub-surface silver and gold mineralising event. The Middle Jurassic volcano-sedimentary sequence to the top of the tuff overlying the third ignimbrite is cut by a suite of small rhyolitic domes and NNW striking cogenetic dykes with associated rhyolitic lavas and autoclastic facies. These ascending acid magmas interacted with groundwater and sedimentary interbeds to form hydroclastic breccia composed of lava fragments from the sequence traversed, and from surface deposits. Discontinuous outcrops of this breccia cover an area of ~18 km2. The overlying succession comprises at least five rhyolitic ignimbrite units, whose temporal relationships are not clear because of the lack of stratigraphic contacts. The seventh ignimbrite is overlain by further rhyolitic domes lavas and autoclastic facies, then an andesitic lava before the final rhyolitic ignimbrite that probably belongs to the La Matilde Formation (Echeveste et al., 2016).
Locally, the uppermost Upper Jurassic ignimbrite is capped by an unconformity, followed by Upper Oligocene to Upper Pliocene sedimentary rocks and basalts. Quaternary gravels and silts fill small depressions at the southern end of the property which is located in the plains.
Further afield, the Jurassic volcanism is succeeded by Upper Jurassic to Lower Cretaceous continental sedimentation, Upper Cretaceous basalts and Lower Tertiary sedimentary rocks and basalts.
The Manantial-Espejo mineralisation predominantly occurs as veins in faults with short strike-slip and larger down-dip displacements. Mineralisation styles include massive quartz veins, vein breccias, sheeted and stockwork veining, and minor dissemination. Quartz is the dominant infill, displaying distinctive textures indicating repeated overprinting hydrothermal events. These veins are located within fault zones that generally strike ~WNW, dipping near vertical. They have a cumulative strike length of 13.6 km and cover an area of ~50 km2.
The principal ore minerals within the quartz veins are argentite/acanthite, silver sulphosalts, electrum, native gold and silver, proustite, freibergite, uytenbogaardita, iodargirite, galena, sphalerite (mainly Fe-poor), pyrite, marcasite and chalcopyrite. The dominant gangue mineral is quartz, with lesser adularia, as well as bladed calcite and platy barite, both of which are largely replaced by quartz and adularia. The hypogene mineralisation is largely associated with crustiform grey and white banded quartz, late breccias, replacements, and open space infillings (particularly vugs), and occurs in six hydrothermal stages as described in the Maria Vein section below.
Supergene mineralisation and enrichment produced an assemblage of limonites (jarosite, goethite), cerussite, pyrolusite, psilomelane and hematitic oxides. The upper oxidation zone is variable, extending from the surface, locally to depths of as much as 200 m, controlled by fracturing.
Disseminated mineralisation includes microveinlets proximal to both main veins and within the stockwork veining, and in or proximal to breccia zones.
At least four vein zones are recognise within the Central Vein Zone, namely the Maria and "P" veins, Melissa, Concepción and Karina-Unión.
The Maria Vein continuously outcrops over an interval of >950 m, with a pinch and swell thickness that varies from <1 to ~20 m, and down dip continuity of >250 m. It trends at 120 to 130° and dips at 50 to 70°SW. It has good continuity, with little evidence for significant transverse fault offsets. Ore-grade mineralisation is not continuous throughout the vein. Open pit ore-grade zones range from tens to hundreds of metres along strike. Underground ore-grade zones are tens of metres in length, with >100 m of vertical extent in the Maria West area. Data indicates mineralisation continuity within the plane of the vein to average ~50 to 100 m for both gold and silver.
The major hosting lithologies are Bimodal Quartz Porphyry in the footwall and Lithic/Lapilli Tuff in the hanging wall. The vein has a number of textures and mineral parageneses interpreted from fluid inclusions to represent a depth of fluid entrapment for high-grade mineralisation of between 250 and 800 m. The main vein textures are white to grey crustiform quartz, massive white quartz with replaced bladed calcite, massive quartz, colloform banded quartz-adularia and diverse breccia types. The Maria vein shows the widest variety in textures and hydrothermal stages within the district, with cross-cutting relations in outcrops and drill core in two of the main ore bodies revealing a paragenetic sequence of six hydrothermal stages, as follows:
• Stage 1 which comprises crustiform banded fine-grained quartz carrying ore minerals, which mostly occurs in the uppermost (50 to 100 m depth) part of the Maria vein, but locally persists to a depth of ~200 m below surface.
• Stage 2 includes massive white quartz which has replaced bladed calcite, and massive to vuggy white quartz, with or without minor blades. This stage generally has a low Ag-Au grade, although later stage ore minerals partially fill vugs, accounting for some ore grade where this paragenetic stage is present.
• Stage 3 comprises colloform banded quartz and adularia - including generations of amethyst - replacing other minerals (e.g., calcite or barite) to form a needle-like and platy texture. Adularia becomes more abundant downwards, increasing to almost massive adularia below 150 m depth. Although this stage has low-grade Ag-Au, it serves as a favourable host for later hypogene stages and supergene mineralisation.
• Stage 4 is the main period of brecciation, representing various breccia types and events, although breccias were also formed to a lesser degree during all stages. It generally has low-grade associated Ag-Au mineralisation, although one event appears to carry high-grade Ag mineralisation, occurring as a dark pyrite- and Ag-mineral rich fine-grained material, occurring as breccia matrix, thin veins, open space infill and replacements. This mineralisation occurs at depths of >150 m below surface, and is responsible for the highest Ag grades in the Maria Vein (locally with >10 kg/t, containing native Ag).
• Stage 5 occurring as thin (<1cm) veins and open space fill base metal sulphides (mainly Fe-poor sphalerite and galena, and minor chalcopyrite). These veins cut stage 4 breccias.
• Stage 6 contributed post-mineralisation chalcedony and supergene alunite, mainly filling open spaces.
The "P" Maria Veins are less continuous than the main Maria Vein, both in overall extent and gold and silver mineralisation. Continuity of mineralisation is in the range of tens of metres. The P veins strike sub-parallel to the Maria Vein, occurring in the hanging wall of the main Maria structure. They have thicknesses that range from 0.6 to 18 m, averaging 4.2 m. With few exceptions, the "P" Veins are distinctly silver-poor compared to the Maria Vein.
The Concepción Vein System is composed of several sub-parallel veins with strikes of from 60 to 70° and a dip of 45°. The largest is ~600 m long and from 1 to 12 m thick, extending to depths of >200 m. Outcrop of these veins is not continuous. At surface, the main vein occurs at the contact of Bimodal Quartz Porphyry and Equigranular Quartz Porphyry lithofacies. The paragenetic sequence is similar to that described at the Maria vein, although some stages and texture are not represented. Vein filling is principally massive white to milky quartz with minor blades and adularia, probably corresponding to stages 2 and 3 of the Maria Vein. Ore minerals occur as spots and striae, representing partial crustiform banding, together with more transparent quartz cracking and healing brecciated white quartz. The main mineralising event may corresponds to the high-grade event in stage 4 of the Maria Vein. Like the Maria Vein, it shows good continuity, with little evidence of significant fault offsets. Open pit ore-grade zones have dimensions of tens to hundreds of metres along strike and tens of metres down-dip.
The Melissa Vein System strikes at between 90 and 115° with subvertical dips over a length of ~600 m and to a depth of ~100 m, and a thickness that varies between 0.5 and 3 m, averaging 1.5 m. At the surface, it is found along the faulted contact between Equigranular Quartz Porphyry and Lithic/Lapilli Tuff lithofacies. This vein system is characteristically gold bearing, with textures and paragenetic sequence similar to the Concepción System. It is dominantly brecciated, with strong associated oxidation. The blind North Vein is an extension of the Melissa Vein System. It contains moderate to high grade Ag-Au intersections continuously for 225 m of strike and 150 m down dip. A few small transverse fault offsets in the order of metres interrupt the structural continuity of the vein.
The Karina-Unión Zone is a complex of sheeted and stockwork veining, with dips from near vertical to 60°N. The Karina and Unión veins strike ENE and WNW respectively. They have a combined surface strike exposure of 850 m and persist to a depth of >150 m. The entire system extends over a width of >100 m;, with the thickest single vein being ~10 m. The textures and paragenetic sequence of these veins are similar to that of the Concepción Vein System. Silver and gold values vary from low to high grade. Due to the nature of the veining, there is not a consistent continuity of discrete structures. However, there is a continuity of aggregates of veins and veinlets with elevated gold and silver grades. These blocks have dimensions of ~25 to 90 m.
Other veins are also found within the Central Vein Zone.
Reserves and Resources
Mineral Resources and Ore Reserves (included in Mineral Resources) as published in December, 2005, prior to mining (Pan American Silver, 2006) were:
Measured + Indicated resources - 5.13 Mt @ 142.3 g/t Ag, 2.45 g/t Au;
Inferred resource - 0.73 Mt @ 123.5 g/t Ag, 1.44 g/t Au;
Proved + Probable open pit reserves - 1.757 Mt @ 118.1 g/t Ag, 3.07 g/t Au;
Proved + Probable underground reserves - 1.3316 Mt @ 202.5 g/t Ag, 3.15 g/t Au.
Concepción Vein System
Measured + Indicated resources - 0.87 Mt @ 194.5 g/t Ag, 2.03 g/t Au;
Inferred resource - 0.20 Mt @ 159 g/t Ag, 1.44 g/t Au;
Proved + Probable open pit reserves - 0.2282 Mt @ 237 g/t Ag, 2.76 g/t Au;
Proved + Probable underground reserves - 0.1480 Mt @ 359.8 g/t Ag, 3.03 g/t Au.
Melissa Vein System
Measured + Indicated resources - 0.25 Mt @ 466.9 g/t Ag, 6.58 g/t Au;
Inferred resource - 0.05 Mt @ 348.4 g/t Ag, 3.12 g/t Au;
Proved + Probable underground reserves - 0.2342 Mt @ 442 g/t Ag, 6.26 g/t Au;
Measured + Indicated resources - 2.48 Mt @ 184.5 g/t Ag, 1.84 g/t Au;
Inferred resource - 0.35 Mt @ 137.5 g/t Ag, 1.42 g/t Au;
Proved + Probable open pit reserves - 1.8917 Mt @ 208.4 g/t Ag, 1.96 g/t Au;
Measured + Indicated resources - 8.73 Mt @ 168.7 g/t Ag, 2.35 g/t Au - for 1473 t Ag and 20.5 t Au;
Inferred resource - 1.33 Mt @ 141.8 g/t Ag, 1.50 g/t Au - for 188 t Ag and 2 t Au;
Proved + Probable open pit reserves - 3.8768 Mt @ 169.1 g/t Ag, 2.51 g/t Au;
Proved + Probable underground reserves - 1.7138 Mt @ 248.8 g/t Ag, 3.56 g/t Au.
Remaining Mineral Resources and Ore Reserves (NOT included in Mineral Resources) as published in December, 2016 (Pan American Silver, 2007) were:
Measured resource - 0.1 Mt @ 125 g/t Ag, 1.65 g/t Au;
Indicated resource - 0.4 Mt @ 207 g/t Ag, 2.04 g/t Au;
Inferred resource - 0.5 Mt @ 211 g/t Ag, 2.60 g/t Au;
Proved reserve - 2.2 Mt @ 111 g/t Ag, 1.17 g/t Au;
Probable reserve - 0.50 Mt @ 244 g/t Ag, 3.32 g/t Au.
This record is largely drawn from: Steinmann, M., 2006 - Manantial-Espejo project, Santa Cruz Province, Argentina - an NI 43-101 Technical Report prepared by M3 Engineering & Technology Corporation for Pan American Silver Corp., 204p.
The most recent source geological information used to prepare this summary was dated: 2016.
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.
References to this deposit in the PGC Literature Collection:
Echeveste, H., Lopez, L. and Carlini, M., 2016 - Geology of the Manantial Espejo epithermal district, Deseado Massif, Patagonia Argentina: in Journal of Maps v.12, pp. 172-177.|
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