San Jose - Huevos Verdes, Frea, Kospi, Odin, Ayelen
Santa Cruz, Argentina
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The San José low sulphidation epithermal silver-gold deposits are located in Santa Cruz Province, southern Patagonia, Argentina, ~20 km NNW of the Cerro Negro Au-Ag deposits, ~45 km ESE of Perito Moreno and ~225 km WSW of Comodoro Rivadavia (#Location: Vein Zone - 46° 39' 39"S, 70° 17' 24"W).
Resources within the San José Operation of Minera Santa Cruz include nine, NW-SE trending parallel to sub-parallel mineralised veins, the Huevos Verdes North, Huevos Verdes South, Huevos Verdes Central, Huevos Verdes Ramal, Frea, Kospi, Odín and Ayelén, Luli and Susana and Micaela distributed over an area of 2.5 x 2 km. One of the most important of these is the Huevos Verdes vein system which includes the first three veins listed above, representing separate developments within the same, 2 km long structural trend.
The first recorded exploration at San José was undertaken by Minera Andes Inc. in the late 1990s. Following a regional structural study and field mapping, a geochemical and geophysical program uncovered areas of Landsat colour anomalies, and coincident anomalous gold and silver values. By 2001, a number of prospects had been outlined, including the Huevos Verdes Zone. In 2001, a joint venture company Minera Santa Cruz, S.A. was formed between Minera Andes (49%) and Hochschild Mining PLC (51%). Subsequent exploration included definition of the Huevos Verdes Zone and underground development. A feasibility study was completed in 2005 and a decision to mine at Huevos Verdes, Kospi and Frea in 2006. Commercial production commenced on January 1, 2008. In January 2012, McEwen Mining Inc. acquired Minera Andes Inc. and its 49% interest in Minera Santa Cruz, S.A.
The San José district is located in the northwestern corner of the Deseado Massif of Patagonia. This massif comprises a Palaeozoic metamorphic basement unconformably overlain by Permian to Triassic continental sedimentary sequences and by Middle to Upper Jurassic bimodal basaltic-andesite and rhyolitic volcanic and volcaniclastic rocks of the Chon Aike Large Igneous Province. The latter was deposited in a cratonic extensional environment that presaged and accompanied the opening of the South Atlantic Ocean. Extension is reflected by normal faults, horst and graben formation, and block tilting. Several small basins formed after the main volcanic episodes, accommodating deposition of Upper Jurassic to Lower Cretaceous continental sedimentary rocks, comprising tuffaceous sandstones, tuffites, limestones, conglomerates and shales. Basaltic plateau volcanism was dominant during the Tertiary, accompanied by minor marine ingressions resulting in the deposition of sandstones, shales and fossiliferous limestones. Intrusive rocks are scarce in the area, represented by irregular bodies of rhyolitic porphyries that intrude the main silicic volcanic units, and by basaltic plugs that pierce the entire sequence.
The voluminous mafic to silicic volcanic rocks of the Jurassic Chon Aike Large Igneous Province make up the Bahia Laura Volcanic Complex. The sequence is subdivided into the lower Bajo Pobre Formation, predominantly composed of amgydaloidal basalts to basaltic andesites, discordantly overlain by the Chon Aike Formation, dominantly rhyolitic ignimbrites, with intervals of andesite, and the partly interdigitating La Matilde Formation, which is largely made up of volcaniclastic rocks. The principal host rock for silver and gold mineralisation in the San José district is the Bajo Pobre Formation, where veins are typically developed in competent basaltic-andesite flows, and to a lesser extent, in volcaniclastic units.
The location and regional setting and geology of both the overall and north-western Deseado Massif are described in the Southern Andes and Patagonia and Cerro Negro records.
The geology within the San José district is as follows, from the base:
Middle to Upper Jurassic Bahia Laura Volcanic Complex, divided into,
Bajo Pobre Formation, which is the lowermost stratigraphic unit exposed in the immediate San José district, and is assumed to underlie the entire deposit area. Rocks attributed to the formation comprise a lower volcaniclastic unit and an overlying lava flow, both of which have an andesitic composition, and a maximum combined thickness of 120 m. A restricted dacitic, hornblende-megacrystic lava flow or stock, the stratigraphic position of which is unclear, has also been included in the formation. The lower andesitic volcaniclastic unit is inferred to have been deposited in an epiclastic environment on the basis of its chaotic facies changes. It occurs as a grey to greenish volcaniclastic agglomerates with large andesitic clasts up to 90 cm in diameter. This unit is generally pervasively altered, commonly to at least a propylitic assemblage. Age dating indicates a hiatus of around 5 m.y. between the andesitic volcaniclastics and overlying lava flows. This hiatus is represented by the discordance between the two lithofacies.
The andesitic lava flow has a thickness of up to 50 m, and is massive, with columnar jointing and auto-breccia textures where weathering and hydrothermal alteration are concentrated. It is a black to greenish andesite with either porphyritic or aphanitic textures. The lower part of this andesitic lava flow unit is finer grained and more basaltic in composition than the upper portion. Rocks attributed to the Bajo Pobre Formation have been dated at between 151.3±0.7 and 144.7±0.1 Ma (40Ar/39Ar).
The Bajo Pobre Formation hosts the Au and Ag mineralisation at the Huevos Verdes, Frea, Kospi and all other vein deposits at San José, as well as regional prospects as far as 8 km radially outwards from the main vein cluster.
Bahia Laura Group, subdivided into,
• Chon Aike and La Matilde formations which discordantly overlie andesitic volcanic rocks of the Bajo Pobre Formation. Where locally exposed, the Chon Aike Formation comprises rhyolitic to rhyodacitic ignimbrites and subordinated agglomerates, volcanic breccias, tuffs and scarce lava flows. The ignimbrites have a porphyritic texture with a fine reddish tuffaceous matrix and phenocrysts ranging from 0.5 to 1.8 mm, principally of subhedral K feldspar, plagioclase, anhedral quartz grains, biotite and minor lithic fragments. In the Huevos Verdes area, the La Matilde Formation is poorly represented. Tuffaceous rocks attributed to the this formation range from quartz-rich crystal tuffs to fine-grained crystal poor tuffs. Crystal tuff contains up to 50% phenocrysts, mostly quartz and biotite while feldspars are normally altered or weathered to argillic aggregates. Crystal poor tuffs contain some phenocrysts, mostly biotite, but also glass shards. The thickness of the La Matilde Formation is inferred to be <100 m, and possibly <20 m, in the deposit area.
Differentiation of the Chon Aike and La Matilde formations is difficult at San José, with most authors grouping all of this section of the Bahia Laura Group into the Chon Aike Formation (e.g., Colquhoun et al., 2007). However, mapping by Dietrich et al. (2004) suggest these rocks represent a widespread tuffaceous unit overlying the Bajo Pobre Formation, defining the Saavedra West basin, interpreted to be a syn-volcanic graben, possibly a caldera, developed on the upper surface of the Bajo Pobre Formation and infilled by pyroclastics correlated with the La Matilde Formation (Puritch et al., 2014). Pebble dykes are abundant within the graben, and ignimbrites that may be correlated with the Chon Aike Formation occur along one edge. In the Saavedra Oeste area, 6 km to the south of Huevos Verdes, the thickness of the formation is around 80 to 100 m. However, at Huevos Verdes, and at La Sorpresa, 5 km further to the north, the thickness is only 15 to 20 m. In contrast, the Chon Aike Formation may be up to 1200 m thick elsewhere in the Deseado Massif. Pyroclastic rocks of the Chon Aike Formation are laterally extensive, with ignimbrites having been dated at 151 to 147 Ma (40Ar/39Ar) in the San José area, younger than the age of the volcaniclastic sequence of the Bajo Pobre Formation but older than the andesitic flows of the same unit. Where the Chon Aike/La Matilde formations are overlain by sedimentary rocks of the Cretaceous Castillo Formation, the contact is concordant.
The relationships described above would be consistent with the sequence interpreted at the Cerro Negro district (see the graphic stratigraphic column in that record), 20 km to the SSE. At the latter, a lower unit of basaltic-andesite lava flows and lahar agglomerates (the Bajo Pobre Formation) are overlain by a sequence of dacitic ignimbrites with rhyolitic lavas and domes at the base, then by a second unit of andesitic domes, lavas and pyroclastics (variously regarded as part of the Bajo Pobre or Chon Aike formations). These are followed by dacitic lavas and domes, rhyolitic ignimbrites, tuffaceous sediments and epiclastics (Chon Aike Formation), before the overlying tuffs and tuffaceous sediments of the La Matilde Formation.
• Castillo Formation, a 5 to 80 m thick, southerly thinning sequence of sedimentary rocks interpreted to have been deposited in small, normal block faulted depressions. It is divided into three members, with the lowermost redefined as a tuffaceous deposit belonging to the Chon Aike or La Matilde Formation.
• Tertiary-aged basalts are extensively developed within the north-western part of the Deseado Massif, with at least two basaltic episodes recognised in the San José region. The Upper Oligocene Alma Gaucha Formation occurs as uniform flat-lying flood basalts up to 30 m thick that cover a significant part of the district. Recent basaltic flows from the Cerro Portuguese volcanic centre form lava flow channels that overlie the earlier flood basalts.
Glacially-derived, unconsolidated till deposits up to 50 m thick occur predominantly in the Rio Pinturas valley a few kilometres to the west.
The location of mineralisation at San José is strongly influenced by structure. Mineralised veins appear to be restricted to a corridor formed by the sub-parallel, NNE striking, sinistral Rio Pinturas and San José strike-slip fault zones/lineaments, which are to the west and east respectively. These structures are ~10 km apart and can be traced for ~100 km, but are truncated ~10 km to the north of San José by the regional WNW-ESE trending Rio Deseado Lineament. The main structural trend of faulting and mineralised vein systems within this corridor at San José is WNW to NNW, with less prominent east-west and north to NE striking faults and veins.
The Huevos Verdes veins, which have the best developed and highest grade and width ore shoots, strike at 305 to 320°, dipping at ~65°NE and reflect sinistral displacement. This sense of movement is supported by the observation that vein segments with strikes of >325° and <310° usually lack significant mineralisation and are characterised by brecciation and fault gouge. In contrast, better grades and thicknesses are in segments that strike at <325° to >310°.
The regional scale structural history of the massif commenced with early, pre-mineralisation, NNW striking normal faults formed in response to Permian-Triassic rifting (Dietrich et al., 2005). This direction parallels the gross elongation of the exposed sections of the massif.
Subsequently, a new regime was established, comprising i). a dextral WNW (280°) trending dominant fault-set seen throughout the Deseado Massif, and ii). a sinistral NNW (350°) striking set of faults, the second most frequent structural trend of the massif, representing reactivation of pre-existing NNW-striking Triassic extensional structures. These two structural trends acted as a conjugate shear pair with an interplane angle of ~70°. The orientation of σ1, modelled as the acute bisector of the conjugate pair, is 315°. The majority of veins are oriented roughly parallel to σ1, occurring as sinistral extension fissures between the two conjugate fault sets. These observations are consistent with a Riedel shear model in which the sinistral NNE-striking Rio Pinturas and the San José strike-slip fault zones/lineaments represent the main shear plane, and the NNW and WNW structures represent R and R', respectively. All of the four structural trends (WNW, NW, NNW and NNE) are at least partially mineralised, indicating their existence at the time of mineralisation (Dietrich et al., 2008). However, as these structures developed during a period of extension, the principal driver of shearing and fracturing would be extension parallel to σ3 at ~45°.
Post-mineral faults crosscut the veins, with or without a mesoscopic displacement, and are frequent in the Huevos Verdes Vein System, although they apparently do not greatly affect the geometry of the vein. The orientation of these faults varies from 290 to 60°, with dips to the SW and NE, and have widths that vary from 2 to 100 cm.
Litho-stratigraphy also appears to play an important role in governing mineralisation where certain lithostratigraphic horizons favour the opening of fractures. Mapping shows the fracturing of rocks is far more intense in andesitic lava flows than in underlying volcaniclastic rocks. Fracture-controlled wall rock alteration and mineralisation is alsomore pronounced in the lava flows (Dietrich et al., 2008).
Vein Occurrence and Distribution
Numerous veins have been identified by exploration, but to 2015, only 5 have been extensively explored by surface diamond drilling and subsequently developed by underground mining, namley Huevos Verdes, Frea, Kospi, Ayelén and Odin. These may be summarised as follows:
Huevos Verdes Vein System - the most important system outlined to the end of 2014. It comprises of an array of sub-parallel
veins striking at ~325° with dips ranging between 45 and 75°NE that can be traced over an interval of almost 2 km along strike, divided into 4 mineralised intervals, as detailed below. Mineralisation is hosted by the Jurassic Bajo Pobre Formation, close to the contact between andesitic lava flows and the underlying volcaniclastics. The veins pinch and swell with numerous bends and jogs. Several sub-parallel veins and splays from the main vein have been outlined. The vein width is variable, ranging from <1 to ~15 m. With the exception of limited outcrops of the southern section of the structure, the remainder of the vein system is blind below up to 50 m of Tertiary basalt cover. Within the Huevos Verdes Norte and Huevos Verdes Sud zones, the strongest mineralisation is restricted to sub-vertical 50 to 80 m long ore shoots which can extend 50 to 200 m vertically, corresponding to structural bends and jogs. High-grade sections of the veins consist of banded to mottled quartz with irregular sulphide bands containing fine-grained argentite and pyrite. Ruby silver and native silver are locally observed. The base metal content (Zn-Pb-Cu) of the veins tends to increase with depth.
The four subdivisions of the Huevos Verdes vein system are:
• Huevos Verdes Norte - which has an irregular shape, pinching and swelling along its 400 m strike length. It varies from 0.5 to 4 m in thickness and dips between 65 to 70°NNE. The vein and surrounding host rocks have been subjected to strong illitic and argillic alteration with minor propylitic and
K feldspar remnants. This vein segment is the most weakly mineralised of the Huevos Verdes Vein System. The highest grades of gold and silver are
restricted to two principal sub-vertical shoots, which are each ~50 to 80 m long and can be traced for ~150 to 200 m vertically.
• Huevos Verdes Central - which has been traced for ~400 m along strike and ranges from 0.5 to 5.0 m in thickness. It dips at from 70 to 75°NNE. The strongest mineralisation is restricted to a gently plunging 40 to 70 m long ore shoot that has been traced for >300 m vertically.
• Huevos Verdes Sud - that persists over a strike length of ~520 m, with a thickness of from 0.5 to 3 m and dip of 42 to 75°NNE. Strike varies considerably, from ~280° in a splay on its northern margin, locally to 10°, but is overall ~345°. The variability in strike and hence occurrence of dilation zones, possibly explains the development of higher Au-Ag grade shoots. Four main sub-vertical shoots, each up to 80 m long and as much as 200 m vertically appear to carry the bulk of the mineralisation.
• Huevos Verdes Ramal - that occurs over an ~200 m interval along a WNW-ESE strike, sub-parallel to and ~100 NE of Huevos Verdes Norte. It ranges from 1 to 3 m in thickness and persists over a vertically extent of 250 m.
Kospi Vein - persists over an ~1300 m length along its NW strike of 308°, and has been traced to a vertical depth of ~230 m. It dips at ~70°SW and is 0.25 to 9.5 m thick. Its strike is sub-parallel to Huevos Verdes Norte and is ~500 m to the NE of the latter. It is completely masked beneath Cretaceous and Tertiary cover and was discovered in 2005 as a result of drilling an IP/resistivity target.
Micaela Vein - has been traced for ~1 km along an east-west strike and ranges in width from 1 to 4 m. The vein has been traced vertically for approximately 200 m. It extends from just east of the northern end of the Kospi Vein, to near the southern end of Frea.
Frea Vein - like all of the others listed here, is hosted in Jurassic volcanic rocks and is masked by Cretaceous sedimentary rocks and Tertiary basalts. It is controlled by northwest trending faults. The vein was discovered in 2003 by drilling an IP/resistivity target. It has been traced for ~1.2 km along strike and vertically to 200 m depth, and varies from 0.5 to 7 m thick, with a dip of ~52°NE.
Odín and Ayelén Veins - These two parallel veins are the two most north-easterly tested to 2016. They are ~400 and 500 m respectively to the NE of, and sub-parallel to, the Frea Vein. They have been traced over strike lengths of ~1.9 and 1.6 km respectively, with both dipping to the SW. The Odín vein has been tested to 200 m depth.
Mineralisation and Alteration
Mineralisation within the main San José deposit area is restricted to small erosional windows within the overlying Cenozoic basalt cover, with even the most economically important structure, Huevos Verdes, occurring over a length of <50 m of continuously exposed vein in the southern sector, and one small outcrop in the northern sector. The associated alteration halo is similarly masked by Tertiary basalts and soil.
Alteration principally comprises silicification which accompanies all of the veins and fractures, occurring as a narrow halo, generally surrounded by an extensive zone of intermediate argillic mixed with phyllic alteration. Strong argillic alteration is interpreted to be a supergene overprint of the sections of the propylitic halo containing disseminated pyrite.
Detailed underground mapping of the Huevos Verdes Vein System revealed nine different stages/styles of mineralisation and Ag-Au deposition which can be distinguished by textural features and composition of gangue and sulphides. Each shows different characteristics in mineralisation style and alteration. The only common characteristic is the main NW to NNW structure. The crosscutting relationships in the vein allowed a sequence to be established, commencing at the contact with wall rock and ending in the centre of the vein. Some zones can be divided into several minor bands. Most of the nine stages observed in the Huevos Verdes Central Vein also occur in both Huevos Verdes Norte and Huevos Verdes Sur, although three are restricted to either the south or north sector of the Huevos Verdes Vein System. Chalcedonic quartz and quartz matrix breccias are restricted to the north sector, whereas calcite veinlets are only observed in Huevos Verdes Sur.
The styles of mineralisation recognised in the Huevos Verdes Vein System define the following paragenetic sequence, from oldest to youngest (after Gutierrez, 2006):
• Chalcedonic quartz and early quartz - occurring in veins of massive, brown quartz and greenish chalcedonic quartz that average 2 m in thickness and are barren to only poorly mineralised. The later chalcedony injects and follows early quartz that is locally granular to lattice-textured and occasional microcrystalline with rare clay, limonite and pyrite. Early quartz is milky white to grey-brown, massive to granular, fine to very fine grained, and occasionally medium grained, and mostly occurs as compact anhedral grains. Rare small <5 mm fragments of banded quartz are surrounded by microcrystalline quartz. Chalcedonic quartz is mostly brown, and locally brown-red and brown-green, and is finelly crystalline to colloform crypto-crystalline, locally forming large masses (up to 3.5 m width) which are characterised by conchoidal fractures and locally has colloform banding. Contacts between quartz and chalcedonic silica are occasionally diffuse. Limonite occurs as patches in micro fractures and scattered spots after oxidised pyrite. Pyrite appears as disseminate fine grains, mostly as very fine pseudocubic grains scattered principally in grey quartz and less common as irregular patches in cavities.
This mineralisation style is interpreted to have been emplaced in two separate pulses, the first occurring preferentially as coarse-grained and granular quartz with massive to lattice texture; the second as a chalcedonic quartz matrix surounding host rock fragments forming a breccia by a late infill of remaining open space. This style of mineralisation has only been observed at Huevos Verdes Norte. Local gradational contacts of chalcedonic cross-cutting mottled quartz suggest a second pulse of chalcedony. The chalcedonic quartz does not contain ore, except where it is in gradational contact with mottled quartz, and where it is crosscut by sulphide veinlets.
• Grey massive quartz - comprising massive fine-grained grey quartz containing disseminated sulphides, and enclosing fragments of pervasively silicified fine-grained andesite and clear quartz. Rare medium-coarse grained quartz crystals are evident. There is a gradational contact between the grey quartz and pervasively silicified andesite fragments, with sporadic breccia texture with limonite and clay in micro veinlets. Andesite clasts contain argillised fine phenocrysts. Amorphous grains of clear quartz occur in andesite and silica forms comb druses. White clay is principally found as compact masses infilling cavities between druses. Pyrite crystals and grains are disseminated in grey silica and andesite. This mineralisation style is sporadically developed in Huevos Verdes Sur, and rarely in Huevos Verdes Norte, where it is associated with the mottled quartz unit. Veins are no more than 130 cm thick, averaging ~60 cm and are barren to very poorly mineralised.
• Mottled quartz with sulphides - comprising coarse granular, clear, white or grey quartz, with irregular bands and streaks of grey sulphides, commonly enclosing chloritised andesite fragments which contain disseminated pyrite and local clay and limonite. The characteristic mottled, patchy character is dependent upon the predominance of sulphides, quartz or andesite fragments. Cubic and irregular pyrite occurs in quartz as well as in andesite fragments. The fragments of chloritised andesite have diffuse contacts with enclosing quartz and sulphides. Sulphides vary from <5 to 80% in some bands, and include pyrite, galena, sphalerite, chalcopyrite and acanthite (Ag2S). The darkness of individual bands reflects the grain size rather than the quantity of sulphides. Bands with <15% 0.03 mm finely dispersed sulphides has a higher apparent sulphide content than bands with the same content of coarser sulphides. This mineralisation style is commonly in Huevos Verdes Sur and Huevos Verdes Norte. It occurs over widths that vary from 0.10 to 2.5 cm, and it is occasionally in transitional contact with the quartz-chalcedonic unit. The brecciated nature of clasts of banded to mottled quartz and greenish andesite set in a mottled quartz-sulphide matrix implies successive pulses of mineralisation and reworking. Textures indicates cyclic tectonic and fluid flow events, which incorporate previously formed mineralisation. Each tectonic episode resulted in opening and renewed fluid flow to produce the characteristic banding. This mineralisation style is a moderate to major contributor of grade.
• Colloform to massive quartz - which is predominantly white, except where stained by limonite, and only contains traces of sulphides that are disseminated as very fine grains and small patches that have gradational contacts with the quartz. It occurs as either a compact mass of anhedral, medium sized grains of massive milky quartz, or with millimetre-scale colloform banding. Lesser inclusions include limonite, pervasively altered aphanitic andesite fragments, rare pyrite and grey sulphides, and clay. The massive milky quartz locally has cavities filled by irregular colloform quartz, white clay and fine granular quartz fragments. It occurs at Huevos Verdes Norte, where it locally has a very notable lattice-type bladed texture and parallel-type bladed quartz. It is also found in the central and northern sections of Huevos Verdes Sur, and sporadically in Huevos Verdes Norte. It does not show a preferential association with other mineralisation styles, suggesting that it formed as an individual pulse. Only minor Au-Ag mineralisation accompanies this stage.
• Banded quartz with sulphide and chlorite/smectite - composed of alternating bands of white, clear to grey quartz, sulphides and chlorite-smectite. The quartz bands are mostly white and medium- to very fine-grained anhedral micro-sacharoidal aggregates containing very fine- to fine-grained disseminated sulphides. Grey quartz tends to be massive and has a gradational contact with sulphide rich bands. Less frequently, quartz occurs as comb, druse, colloform and crustiform textures. Sulphides include galena, sphalerite and chalcopyrite, but are mostly acanthite, and vary from grey to light grey in colour. Pyrite accompanies bands of these sulphides, but also occurs as very fine-grained individual bands, patches and disseminations of cubic and pseudocubic crystals. Clay is principally white, although it is often greenish due to contained smectite. It has been observed infilling cavities in quartz where granular or cavernous texture is more common. Banding is produced by alternating grey sulphides and clear to grey quartz layers, which can be in linear or undulose, and visible continuously over substantial intervals along the vein. Individual bands are from a millimetre to three centimetres thick, with quartz bands being the widest and are classified as 'ginguro bands' (after Izawa et al., 1990). This mineralisation style mostly occurs in the central core of Huevos Verdes Sur, where it is almost continuously formed along the mineralised structure. It is not common, but occurs sporadically in Huevos Verdes Norte, and is closely associated with sulphide veinlets in both of these veins. Locally it also has a gradational contact with the mottled quartz and sulphides style. The Au-Ag grade is high due to the sulphide content.
• Sulphides veinlets - which occur as centimetre-scale veinlets and as open space filling in any available cavity (e.g., druse cavities). Veining is predominantly composed of fine-grained sulphides with minor quartz. Sulphides include dusty acanthite, galena, chalcopyrite, sphalerite and pyrite. Sulphide minerals are mostly anhedral, with grain sizes usually of <1 mm. Macroscopic crystals of galena or sphalerite, native silver or crystallised acanthite are rare. Sulphide veinlets are dark grey to grey, depending on the quartz content, with yellowish patches or bands of pyrite. Sulphide veins and veinlets are principally associated with mottled quartz and banded quartz styles with 'ginguro' textures. Individual sulphide veinlets are <20 cm wide and occur in a discontinuous and irregular fashion. Mineralisation occurred in multiple pulses as indicated by fine banding with variable sulphide content. Quartz was precipitated first in each pulse, followed by pyrite, galena, chalcopyrite, acanthite and sphalerite with 'chalcopyrite disease'. Both early pulses of sulphide deposition tend to have low to no acanthite content, whereas the intermediate pulses contain most of the contained silver mineralisation. Pyrite is ubiquitous and the most common sulphide in each pulse. It is found between quartz crystals, and rarely occurs as euhedral crystals. Galena, which is principally found in triangular pits, infills open spaces and only rarely occurs as euhedral cubic crystals. It is also locally isolated with no other associated sulphide minerals. There is no evidence of silver impurities in galena, although Ag and Pb contents show a high correlation coefficient. Sphalerite is frequently intergrown with exsolved blebs of chalcopyrite as 'chalcopyrite disease'. Chalcopyrite also occurs as an anhedral mass, infilling open space between quartz grains. Chalcocite is distinguished by its bluish tints and red interference colours. Acanthite is commonly associated with chalcocite, but also occurs as anhedral aggregates, being recognised by its colour and irregular surfaces. This style of mineralisation is irregularly distributed in the Huevos Verdes Sur and Huevos Verdes Norte veins, being more frequent in the central-south sector of Huevos Verdes Sur associated with mottled and banded quartz. Sulphide veinlets also occur in the central sector of Huevos Verdes Norte where they are more restricted, enclosed within quartz chalcedonic host rock. This is the major ore stage.
• Clay with disseminated quartz and pyrite - comprising dusty textured white clay enclosing small amounts of anhedral to subhedral quartz crystals and minor disseminated and irregular patches of grey sulphides, pseudo-cubic and cubic medium size pyrite crystals and occasional iron-oxide. Vugs in neighbouring quartz are filled with clay and iron oxide. Iron oxide also occurs in fractures, microfractures and infills cavities between quartz grains. The most important and notable texture shown by this mineralisation style is its brecciated nature with irregular fragments of very altered volcanic rock. This mineralisation style principally occurs in the southern part of the Huevos Verdes Sur vein, and sporadically along Huevos Verdes Norte. Crosscutting relationships indicate this to be a post ore-stage.
• Quartz-matrix breccia - occurring as a clear, fine grained quartz matrix enclosing angular to sub-angular, 0.5 to 15 cm fragments of green-greyish (chloritic) aphanitic andesite which contain disseminated pyrite and hematitic borders, and occasional clay cavity infilling. Quartz also commonly has comb and drusy textures, and tends to have a grey colouration at the contact with andesitic fragments. The overall colour of this stage is a light grey-greenish. This mineralisation style is basically restricted to Huevos Verdes Norte, where it has been mapped over a 30 m interval along the contact between the quartz vein and host rock, and has been interpreted as a post-mineralisation event since it is crosscutting chalcedonic quartz and the fragments are unaltered.
• Calcite veins and veinlets - composed of euhedral to subhedral, <1 to >10 mm, white to tan, mostly rhombic, calcite crystals. Veins vary from 0.1 to 0.6 m in thickness and have a similar strike and dip to the main vein, commonly forming a breccia enclosing clasts of host rock. Calcite also occurs in <2 mm, chaotically aligned, stockwork-like, microveinlets filling fractures within the host rock, and representing one of the latest stages of hydrothermal alteration and mineralisation. These veins and veinlets only occur in the southern sector of Huevos Verdes Sur, which is characterised by the presence of rhodonite with associated silver sulphides, and andesitic host rock with calcite veinlets.
Ore Reserves, Mineral Resources and Production
Ore Reserves and Mineral Resources at San José at 31 December, 2016 (Based on Hochschild Mining 2017), were:
Measured resource - 1.64770 Mt @ 564 g/t Ag, 8.2 g/t Au;
Indicated resource - 1.89145 Mt @ 404 g/t Ag, 6.26 g/t Au;
Measured + Indicated resource - 3.53195 Mt @ 479 g/t Ag, 7.16 g/t Au - for 1695 t Ag, 25.35 t Au;
Inferred resource - 1.03836 @ 404 g/t Ag, 6.40 g/t Au - for 421 t Ag, 6.6 t Au;
Ore Reserves (included in Mineral Resources)
Proved reserve - 1.3 Mt @ 502 g/t Ag, 7.34 g/t Au;
Probable reserve - 0.6 Mt @ 401 g/t Ag, 6.57 g/t Au;
Total reserves - 1.8 Mt @ 465 g/t Ag, 7.06 g/t Au - for 840 t Ag, 12.9 t Au.
Cumulative production since commencement of operations in 2007 to 31 December, 2016;
4.46 Mt @ 434 g/t Ag, 6.16 g/t Au - for 1935 t Ag, 27.48 t Au;
TOTAL endowment - production + resources = 4051 t Ag, 59.43 t Au.
Metallurgical recoveries from 2010 to 2013 were: ~85 to 87% for Ag and ~86 to 89% for Au.
This record is largely drawn from:
• Puritch, E., Burga, D., Hayden, A., Pearson, J.L., Brown, A.H. and Duff, J.K., 2014 - Technical report on the San Jose silver-gold mine, Santa Cruz, Argentina - an NI 43-101 Technical Report prepared by P&E Mining Consultants Inc., for McEwen Mining Inc., 126p.;
• Puritch, E., Malloch, K., Armstrong, T., Hayden, A., Pearson, J.L., Brown, A.H. and Burga, D., 2010 - Cerro Negro Operations, Santa Cruz Province, Argentina - an NI 43-101 Technical Report prepared by P&E Mining Consultants Inc., for Minera Andes Inc, 186p.;
• Gutierrez, R., 2006 - Geology of the Huevos Verdes silver-gold vein system, San Jose District, Deseado Massif, Patagonia, Argentina; A thesis submitted to the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology), 189p.; and
• Dietrich et al. (2008) as cited below.
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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San Jose - Huevos Verdes
Dietrich, A., Gutierrez, R., Nelson, E.P. and Layer, P.W., 2008 - Geology of the Epithermal Huevos Verdes Vein System and San Jose District, Deseado Massif, Patagonia, Argentina: in PACRIM Congress 2008, San Jose, California, USA., March 28-30, 2008, Proceedings, pp. 109-113.|
Dietrich, A., Gutierrez, R., Nelson, E.P. and Layer, P.W., 2012 - Geology of the epithermal Ag-Au Huevos Verdes vein system and San Jose district, Deseado massif, Patagonia, Argentina: in Mineralium Deposita v.47, pp. 233-249.|
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