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Inmaculada, Pallancata, Selene |
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Peru |
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Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Inmaculada, Pallancata and Selene silver-gold deposits are located in the Southern Andes of Peru, in the department of Ayacucho. Inmaculada is 520 km SE of Lima, 240 km NW of Arequipa, 210 km SW of Cuzco and 140 km east of the Pacific Ocean. Pallancata is 25 km NNE of Inmaculada, whilst Selene is a further 10.5 km to the NNE (#Location: Inmaculada - 14° 57' 30"S, 73° 14' 32"W).
These three deposits lie within a belt of Miocene epithermal mineralisation that extends from southern Peru to northern Chile and western Bolivia within the Western Cordillera of the Andes. They coincide with, and are related to, a Miocene magmatic arc that overlies a basement of Mesozoic sedimentary sequences. Miocene silver-gold deposits in Southern Peru have been worked since colonial times, e.g., the historic Arcata, Caylloma and Sukuytambo mines (Purser and Purser 1971). Inmaculada is the southern of a NNE-SSW aligned trio of mineralised centres that includes the Selene and Pallancata deposits. Extensive arc related Neogene ignimbrites are developed within this belt, and in some areas are overlain by recently or currently active shield volcanoes (Worner, Mamani and Blum-Oeste, 2018). Mineralisation has been attributed to the ignimbrite magmatic event. For an overview of the geology and mineralisation of this part of the Andes see the Central Andes and Bolivian Orocline record which includes a geological location map.
The age of Ag-Au mineralisation in the Explorador vein at the Selene Mine has been dated at ~14.2 Ma (Ar/Ar of alunite). Adularia samples from the Pallancata Mine yielded ages of ~13.4 Ma and ~12.4 Ma at Inmaculada. Therefore, there was a north to south migration of hydrothermal, and presumably causative magmatic activity in the district, from Selene to Pallancata to the Inmaculada Mine over a period of about 2 Ma.
The three deposits and districts are described separately below.
Inmaculada
The volcanic succession in the Inmaculada district has been summarised as follows, from the base:
• Cretaceous Ferrobamba Formation - marine carbonate and clastic strata.
• Aniso Rhyolite Volcaniclastics Domes and Tuffs - 0 to 700 m of 23.7 to 22.6 Ma (Ar/Ar) volcaniclastics and tuffs intruded by rhyolite intrusions. At Inmaculada these comprise a greenish volcaniclastic, moderately sorted, coarse to medium grained sandstone interbedded with greenish ash flow tuffs up to 100 m thick and interbedded with numerous thin-bedded reddish pumice-rich rhyolitic tuffs up to 10 m thick. They are intruded by at domes representing at least three rhyolitic lithofacies, namely sanidine-plagioclase rhyolite; sanidine-rhyolite; and sanidine rhyolite with silica veins. The phenocrysts include plagioclase-sanidine-biotite, but lacks hornblende, while quartz is likely to be present in small amounts. The sanidine-plagioclase rhyolitic dome is 1 x 0.5 km long, and crops out NW of the Inmaculada Mine where it is hosted in the volcaniclastic sequence.
• Pacapausa Andesite Lavas - which overlies the Aniso tuffs with an angular unconformity. It has not been dated and is 0 to 400 m thick. It consists of a series of 5 to 20 m thick pyroxene andesite lava flows, that grade upward to volcaniclastic sediments. The andesite lavas are generally sandwiched between zones of autobrecciated lava sediments that consists of reddish siltstone and greenish sandstone interbedded with crystal tuffs.
• Huallhua Rhyolite Ignimbrite - dated at 13.2 Ma, and varying from 0 to 850 m in thickness. It consists of four unwelded ignimbrites separated by fine-grained tuffs. The ignimbrites are whitish and made up of 30 vol.% pumice lapilli, as large as 3 cm in diameter; and up to 6 vol.% hydrothermally altered fragments up to 4 cm in size. The tuffs are not welded, are normally graded, poorly sorted, massive and matrix supported. This ignimbrite is massiver and generally strikes at 275 to 340° andd dips at 10 to 25°NE.
• Inmaculada Volcanics - dated at 13.2 Ma from base to top, and varying from 0 to 800 m in thickness, subdivided into:
Southern Volcaniclastic - overlies the Huallhua rhyolite ignimbrite with angular unconformity and comprises ~80 m of well to moderated sorted,
coarse-grained sandstone interbedded with fine-grained sandstone and whitish tuff. It grades upward to poorly sorted, matrix-supported, volcaniclastic sandstone with andesitic fragments up to 30 cm, which is interbedded with light brown andesitic lava flow.
Andesite Lahars and Lavas - comprising an up to 450 m thickness of andesite lahars (~70 vol.%) and pyroxene andesite ±hornblende lava (~30 vol.%). The lahar is matrix-supported, poorly sorted and polymictic, with rounded to subrounded pyroxene andesite fragments up to 2 m in diameter. It is interbedded with 5 to 20 m thick porphyritic pyroxene andesite lava flows. The andesite lahar and lavas dip gently and radially outward from the Huarmapata Hill and is the host to epithermal veins at the Inmaculada Mine.
Pyroxene Lavas - which overlies the andesite lahar and lava and is exposed to the north NE of Huarmapata Hill, also dipping gently and radially outward from the hill. These lavas are not interbedded with lahars. In the north, it is 200 m thick and consists of several pyroxene andesite lavas enclosed at the top and bottom by zones of auto-brecciated lava.
Andesite Ignimbrite - which conformably overlies the pyroxene lava and is exposed to the NE of the Inmaculada deposit. It dips gently to the east and NE and is up to 10 m thick with glassy groundmass and is only weakly affected by hydrothermal alteration.
Coarse Plagioclase Lava - which is 0 to 60 m thick and locally overlies the andesite lahars and lavas, and are, in turn, overlain by the Huancarama Rhyolite Ignimbrite. It is recognised by abundant, up to 5 mm long plagioclase phenocrysts. This unit also hosts epithermal
veining.
Andesite Sub-volcanic Plug and Dykes - The plug crops out on the Huarmapata Hill and is associated with numerous dykes radially distributed outward from that hill. The plug intrudes the Andesite Lahar and Lavas. The greenish pyroxene andesitic plug hosts calcite-pyrite veins. A zone along its western side has been subjected to local pervasive quartz-alunite alteration.
Quartz-sanadine Rhyolite Intrusion - that are exposed to the east of the Inmaculada Mine where they intrude both the Andesite Lahar and Lavas and the andesite ignimbrite. Most intrusions are not exposed at surface, but have been intercepted in drilling where they have been subjected to strong hydrothermal alteration that has converted sanidine to calcite and mafic minerals to chlorite. Although undated, these intrusions cut the ~13.2 Ma Andesite Lahar and Lavas and host ~12.4 Ma epithermal veins.
• Hornblende Andesite - dated at 10.1 Ma, and exposed over an area of 2.5 x 1 km to the NE of the Inmaculada Mine. It is crystal poor, only containing 10 to 12 vol.%, but comprises up to 20 vol.% vesicles, and has flow-banding with multiple orientations. No temporal relationship has been observed with respect to either the Andesite Ignimbrite or the ~23.7 Sanidine Rhyolitic Iintrusions, although it becomes glassy near the contact with the latter. Two groundmass hornblende andesite samples were dated at 10.12 ±0.01 and 9.98 ±0.01 Ma (Ar/Ar; Cuellar, 2019).
• Huancarama Rhyolite Ignimbrite - which is up to 40 m thick and has been dated at 9.17 to 9.13 Ma. This units crops out to the east of the Immaculata Mine, but it can also be found as remnants to the north and NW. It overlies the Aniso rhyolite volcaniclastics domes and tuffs, Huallhua rhyolite ignimbrite, Inmaculada volcanics, and the hornblende andesite unit, and has a basal, 3 m thick, vitrophyre that grades upward to a massive brownish
rhyolitic ignimbrite that is strongly welded. This ignimbrite covers the area like a blanket on a flat paleosurface.
• Chibchi Rhyolite Tuff - which is up to 40 m thick, likely of air fall origin, and has been dated at 6.9 Ma. It is exposed NW of the Inmaculada Mine where it overlies the Huancarama Rhyolite Ignimbrite, and the Inmaculada Volcanics, and is, in turn, overlain by Coñaccahua Andesite Lavas. It has a whitish colour, and Is unwelded and poorly indurated, well- to moderately-sorted, pumice supported, pumice rich, and thin-bedded with beds ranging from laminar to cross-bedded.
• Coñaccahua Andesite Lavas - which is 150 to 450 m thick and has also been dated at 6.9 Ma. This unit is the youngest volcanic sequence in the deposit area and is exposed to the NW of the deposit. Individual lava flows are 1 to 6 m thick with basal, clast supported,
monolithic breccias which have angular and subangular fragments. This volcanic sequence overlies the Chibchi Rhyolite Tuff and, locally, the Aniso Rhyolite Volcaniclastics domes and tuffs.
Cuellar (2019) proposes that a caldera, the Alpabamba Caldera, was located to the SW of the Inmaculada Mine. The same author suggests the faulted contact (the the San Salvador fault) between the 24 to 23 Ma Aniso Rhyolite Volcaniclastics Domes and Tuffs and the ~13.2 Ma Huallhua Rhyolite Ignimbrite reflects the margin of the proposed caldera. The ~13.2 Ma Inmaculada Volcanics, which range in composition from andesite to dacite and probably to rhyolite, are envisaged as having been erupted immediately following the caldera collapse. Two pyroxene barometry determinations (Cuellar 2019) suggest that the depth of pyroxene crystallisation of the Inmaculada Volcanics ranges from 10 to 22 km. Therefore, it is proposed that after the rhyolite pyroclastic eruption and the caldera collapse, intermediate composition magma rose from a source at 10 to 22 km and erupted as the Inmaculada Volcanics. The Inmaculada Mine is hosted in veins cutting the Inmaculada Volcanics.
The epithermal mineralisation of the Inmaculada deposit is predominantly hosted in the 13.2 Ma Andesite Lahars and Lavas, 13.2 Ma Andesite Ignimbrite and in the Coarse Plagioclase Lava units, all of which are members of the Inmaculada Volcanics. The mineralised ore shoots within the vein systems plunge very gently from ~0 to 10°NE. The Andesite Lahars and Lavas unit that hosts the bulk of the economic mineralisation, is the most altered unit, likely due to its higher permeability, although all volcanic units within the Inmaculada Volcanics have been subjected to some degree of hydrothermal alteration.
Veining at Inmaculada fills fault controlled open space related to two major structural events, namely: i). before 13.2 Ma, which strike at ~125° and dip at 65°SW to 52°N. These include the major San Salvador Fault System which hosts epithermal mineralisation to the NW of Inmaculada, as described below; and ii). between 13.2 and 12.4 Ma, which strike at 40 to 80° and dip both to the NW and SE and record small dextral strike-slip and normal displacements (Nelson 2005), without any appreciable offset to the mapped volcanic stratigraphy. These faults host the silver-gold mineralisation at Inmaculada.
Over 2 km to the NW of the Inmaculada vein complex, in the Minascucho-San Salvador and Tararunqui areas, high sulphidation Ag-Au mineralisation is evident, hosted within the older fault generation. Within the Inmaculada deposit area, the younger fault set hosts epithermal mineralisation that evolved from an early low to intermediate sulphidation Zn-Pb+Ag event with associated quartz-alunite alteration, to an overprinting low sulphidation Au+Ag mineralised suite accompanied by quartz-adularia-illite alteration. Only the latter produces economic Ag-Au mineralisation (Glave, 2013; Cuellar 2019; miningdataonline.com viewed 2022).
The low to intermediate sulphidation Zn-Pb+Ag veining is characterised by whitish quartz and chalcedony, with brecciation and banded, colloform, crustiform, granular and massive textures, and associated pyrite and base metals sulphides, including sphalerite, galena, chalcopyrite, minor argentite and scattered acanthite. These sulphides are variably oxidised to goethite, hematite and malachite with patinas of Fe and Mn oxides. Veining and alteration of this style forms a broad low grade envelope containing 0.2 to 1.0% Pb+Zn (Glave, 2013; Cuellar 2019; miningdataonline.com viewed 2022).
The low sulphidation Au+Ag veining comprises white chalcedonic quartz with carbonate replacement and banded colloform texture, and druses in cavities. These veins contain small amounts of disseminated pyrite, electrum, argentite, pyrargyrite and chalcopyrite, with rhodochrosite, adularia, and patinas of iron oxides, and locally marcasite in the upper levels (Glave, 2013; Cuellar 2019; miningdataonline.com viewed 2022). Two Ar-Ar ages of adularia from this second event at Inmaculada yielded 12.42 ±0.03 and 12.42 ±0.05 Ma (Cuellar 2019). This suggests the quartz-adularia veins formed ~0.8 Ma after the eruption of the dated Inmaculada Volcanic host rocks, but much earlier than the next magmatic event represented by the ~10 Ma Hornblende Andesite (Cuellar 2019).
Known mineralisation at Inmaculada is distributed over a NE-SW elongated area of ~6 x 1 to 2 km. At least 21 quartz vein systems had been delineated to 2013, mostly dipping to the SE. Of these, 7, namley Teresa, Lourdes, Juliana, Rebeca, Verónica, Marina and Organa are predominantly characterised by low to intermediate sulphidation assemblages with Zn-Pb+Ag mineralisation. The other 13 veins: Martha, Ángela, Roxana, Shakira, Lucy, Sara, Karina, Pirita, Jimena, Melissa, Kattia, Rosario and Claudia also include low sulphidation type with Au+Ag mineralisation (Glave, 2013).
Glave (2013) suggest that the Inmaculada veining occurred in an extensional setting, filling openings in the normal NE-SW faults that were the result of subsidence of blocks towards the SE. Initially these spaces were filled by base metal mineralisation (Zn, Pb, Ag). Continued activity on these vein-faults evolved to horizontal dextral displacement, coincident with emplacement of the second quartz-filled bonanza-type Au-Ag mineralising event.
Quartz-alunite alteration related to the Inmaculada veins complex is found to the SW of Cerro Huarmapata, the intrusive and structural centre of the immediate Inmaculada deposit area, as indicated in the geological descriptions above. This alteration occurs as an elongated 1 x 0.5 km zone with an area of ~ 0.34 km2. The associated epithermal veins have narrow alteration haloes of generally <2 m. Proximal to the veins, the host rock is strongly altered to quartz-illite-pyrite in which the groundmass/matrix is converted into equigranular quartz grains and feldspars are altered to illite. Narrow 1 to 2 mm thick, quartz-adularia veins were emplaced in the host rock near the earlier mineralised epithermal veins. More than 2 m distal from the latter veins, plagioclase is altered to calcite, and primary igneous mafic minerals to chlorite. The calcite-chlorite assemblage is taken to suggest a low water/rock ratio and near-neutral pH. Pyrite is the dominant sulphide in the altered wall rock and occurs as euhedral to subhedral grains disseminated throughout the host rock and alteration minerals (Cuellar 2019).
Cuellar (2019) describes the mineralisation within vein systems that have been been subjected to both the mineralising events, where silver-gold bearing veins are associated with low grade base metal mineralisation characterised by assemblages of sphalerite-galena-chalcopyrite-pyrite. These sulphides are found filling vugs in a crustiform quartz textured veins. Sphalerite is the dominant mineral phase and contains chalcopyrite disease blebs. The highest silver grades are associated with chalcopyrite-acanthite-calcite, which fills voids in crustiform quartz and appears to post-date the Zn-Pb base metal mineralisation. Chalcopyrite (confirmed by SEM) has a pinkish tarnish that resembles bornite, whilst sphalerite in contact with acanthite has a greenish halo.
Glave (2013) describe a vertical zonation within the Angela vein, one of the principal veins at Inmaculada. At high level elevations of 4650 to 4600 masl, grades are <1 g/t Au and <30 g/t Ag, with anomalous Hg, Te, Pb and Mn of >0.1 %. At intermediate and deep level elevations of between 4500 and 4200 masl, ore grade mineralisation is basically Au+Ag, with Au/Ag ratios of from 1/10 to 1/40, sometimes reaching 1/160 towards the NE. Gold is present as visible electrum. Silver occurs as pyrargyrite in blebs and colloform bands, accompanied by cubic pyrite. In hydrothermal breccias and/or stockwork veinlets, sphalerite occurs both as the iron-rich marmatite and yellowish-green iron-poor varieties. Less frequently galena and chalcopyrite is present, accompanied by disseminated cubic pyrite and weak pyrargyrite. In a longitudinal section of the vein, the vertical extent of mineralisation varies from between elevations of 4700 and 4300 masl in the SW, gently plunging to upper and lower limits of between 4400 and 4000 masl ~2 km to the NE.
Following the mineralising ~12.42 Ma event, the ~10 Ma Hornblende Andesite was emplaced, as exposed to the NE of the Inmaculada vein complex, likely as a subvolcanic intrusion. Subsequently, the ~9.2 Ma Huancarama Rhyolite Ignimbrite blanketed the deposit area, interpreted, on the basis of its plagioclase-quartz-sanidine-biotite-hornblende mineral assemblage, to have been erupted from an upper crust magma chamber. The igneous activity was terminated at ~6.9 Ma when both the Chibchi Rhyolite Tuff and the Coñaccahua Andesite Lavas erupted.
Ore Reserves and Mineral Resources as at 31 December 2020 (Hochschild Mining webpage viewed April 2022) were:
Proved + Probable Ore Reserve - 7.7584 Mt @ 116 g/t Ag, 2.8 g/t Au containing 900 t of silver and 21.7 t of gold;
Measured + Indicated Mineral Resource - 7.659 Mt @ 148 g/t Ag, 3.67 g/t Au containing 1130 t of silver and 28.1 t of gold;
Inferred Mineral Resource - 9.921 Mt @ 104 g/t Ag, 2.66 g/t Au containing 1032 t of silver and 26.4 t of gold.
NOTE: Resources are inclusive of reserves.
To date the Ángela vein has been the principal source of production which has totalled between 0.95 to 1.33 Mt of ore per annum from an underground, narrow vein operation at grades of 115 to 163 g/t Ag and 4.3 to 4.7 g/t Au (Hochschild Mining webpage viewed April 2022). Production commenced at Inmaculada in June 2015.
This summary is drawn from information contained within:
Cuellar Quispe, J.C., 2019 - The Geology, Geochronology, and Geochemistry of the Miocene Volcanic Rocks at the Inmaculada Ag-Au Mine, Southern Peru; a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, College of Earth, Ocean and Atmospheric Sciences, Oregon State University, USA. 149p.
Glave, J.V., 2013 - Geología del Yacimiento Inmaculada, Veta Ángela; Presentation to the ProExplo 2013 Conference, Lima, Peru; 23p.
Pallancata
Similar to the Inmaculada deposit, the Pallancata Vein mineralisation is hosted by a sequence of Miocene andesitic lava flows, rhyolitic tuffs and related volcaniclastic rocks. Volcaniclastic rocks include lapilli and crystal tuffs, volcanic breccias and conglomerates, and volcanic sediments intercalated with andesitic lava flows. A massive white rhyolitic tuff unconformably overlies the volcaniclastic rocks, forming much of the higher ground in the area. Rhyolite stocks have intruded the volcanic sequence to the north and east of the deposit, whilst a large porphyritic andesite laccolith has been encountered at depth. The volcanic rocks in the Pallancata area were deposited in a very active tectonic setting, with numerous small- and large-scale examples of marked depositional variations, largely across active faults. Some of these faults have a similar NW-SE orientation to the Pallancata Vein although many others strike near north-south. Widespread instability is indicated, particularly in the vicinity of the main Pallancata Vein structure, which is interpreted to most likely represent a major zone of collapse, either part of a caldera margin or a linear graben. Fault planes exhibit slickencrysts that invariably indicate sinistral displacement. A series of faults and veins in the Pallancata area form a continuous group of sub-vertical structures with varying degrees of sinistral strike-slip movement. Alteration and mineralisation appear to be controlled by these structures, with high grade zones likely associated with dilatational portions related to jogs and bends in these structures.
Six separate zones of veining have been defined within an area of 3 x 2 km in the Pallancata area. These comprise the ENE-WSW trending Virgen del Carmen/San Javier, Mariana, Mercedes and San Cayetano vein sets and the semicontinuous Pallancata and Huararani vein systems.
Within this series of mineralised zones, low-sulphidation epithermal silver-gold mineralisation occurs in a complex array of veins, breccias (both hydrothermal and tectonic), stockwork and silicified zones with multiple phases/pulses evident. Veins include vein breccia with in situ clasts of bladed carbonate replacement and partial leaching, and exhibit common crustiform and colloform banding. The vein matrix is generally composed of milky-white to coarse-grained semi-translucent drusy quartz. Breccias range from simple tectonic, to complex multiple-event hydrothermal types. These include black silica breccia that are rich in sulphides, to massive white quartz breccias.
The main Pallancata Vein is exposed over a strike length of ~1.5 km, with an ~110° trend and a subvertical dip, with a down-throw to the south. Vein thicknesses are structurally controlled, both horizontally and vertically, being developed over widths varying from 1 to 35 m. Individual veins or splays within the vein zone are more typically 0.5 to 3 m thick. It is the principal source of ore at Pallancata and represents a zone of complex, multi-phase veining and faulting within a generally well-defined structure that underwent major dilation. It pinches, splits and has a sinuous nature with an en echelon pattern, both horizontally and vertically. The vein has a distinct silicified halo which is more prominent on its northern side, while clay alteration grades outward from illite to smectite. The clay mineralogy is often hard to distinguish due to the high pumice content of host lapili tuffs. The structure hosting the Pallancata Vein is locally dominated by in situ quartz altered bladed carbonate. Elsewhere it includes jigsaw breccia, commonly with black
chalcedonic silica and sulphide fill, whilst other parts are true hydrothermal breccias with complex textures and multiple phases of banded chalcedonic silica and euhedral drusy quartz. Locally, the structure is filled by banded sulphide-rich chalcedony and broken crystals that were flushed into open spaces. Much of the high-grade mineralisation is hosted within the West Breccia Zone, found within the western section of the main Pallancata structure. It comprises an
intensely silicified body with a strong stockwork of quartz veins that typically strike NE-SW to north-south. High-grade mineralisation occurs where these stockwork veins intersect the NW-SE striking main Pallancata Vein structure.
Mineralisation is generally sulphide-poor and includes argentite, ruby silver (i.e., pyrargyrite/proustite), pyrite, marcasite, galena, electrum and rarely, native gold. The bulk of the visible silver mineralisation in Pallancata Vein occurs within the massive silica, including silica with carbonate replacement textures. In this mineralisation, silver occurs as argentite and pyrargyrite. Some argentite is accompanied by adularia after an early silicification and brecciation and an intermediate banded chalcedony phase. This is consistent with multiple episodes of silver deposition in the Pallancata Vein. Carbonate-replacement textures, the result of the silicification of laminar calcite, in combination with the presence of adularia, reflect boiling conditions during vein-breccia formation.
The Huararani stockwork outcrops immediately to the north of the western extremity of the West Breccia section of the Pallancata Vein, and is composed of much the same type of quartz veins and alteration as the Pallancata Vein, albeit at a higher structural level. Whilst these stockworks are moderately anomalous in precious metals at surface, drilling confirmed grades increase with depth where intervals of up to 2.2 m @ 1269 g/t silver and 4.7 g/t gold were intersected. The San Javier/Virgen del Carmen, Mariana, Mercedes and San Cayetano zones have similar styles of quartz veining to the Pallancata Vein.
Ore Reserves and Mineral Resources as at 31 December 2020 (Hochschild Mining webpage viewed April 2022) were:
Proved + Probable Ore Reserve - 0.6344 Mt @ 270 g/t Ag, 1.1 g/t Au containing 171 t of silver and 0.69 t of gold;
Measured + Indicated Mineral Resource - 2.145 Mt @ 300 g/t Ag, 1.31 g/t Au containing 644 t of silver and 2.8 t of gold;
Inferred Mineral Resource - 1.947 Mt @ 248 g/t Ag, 1.13 g/t Au containing 482 t of silver and 2.2 t of gold.
NOTE: Resources are inclusive of reserves.
Pallancata commenced production in 2007 at a rate of 0.25 to 1.1 Mt per annum from an underground operation with grades of 240 to 440 g/t Ag and 0.9 to 1.86 g/t Au. Ore from Pallancata is transported 22 km by road to the Selene plant for processing.
This summary is largely drawn from Henley, S., Knight, J. and Holloway, N., 2007 - Technical Report, Pallancata Project, Peru; A technical report prepared for International Minerals Corporation, by Mining Associates Pty Ltd; 84p.
Selene
The Selene mining district also lies within the belt of Neogene ignimbrites developed within the Western Cordillera of Southern Peru, and is the northern of the NNE-SSW aligned trio of mineralised centres that includes the Inmaculada and Pallancata mines. The three operations have been controlled by Hochschild Mining plc during most of their mine lives.
The ~50 km2 Selene mining district is closely associated with a Miocene volcanic centre and inferred caldera complex. The latter is reflected by two concentric elliptical ring structures developed in pyroclastic rocks that are ~5 to 7 km and 9 to 13 km in diameter respectively. These encircle both a central stratovolcano and most of the mineralised systems of the district. The most significant of these mineralised systems is the Explorador Ag-Au quartz-adularia vein system that includes the Selene Vein. The circular structures are developed in a sequence of rhyodacitic ash flow tuffs and pyroclastic rocks that underlie the entire district and beyond. The annulus between the two ring structures is also occupied by a number of dacitic to rhyolitic flow-dome complexes that host most veins and other mineralised structures within the district and are preferentially intruded along the ring structures, but also into the stratovolcanic edifice. All volcanic units of this basal suite within this part of the district, which total ~400 m in thickness, belong to the Sillapaca Group and comprise the Lower Miocene Alpabamba Formation, and the Middle to Upper Miocene Aniso Formation that define a concordant sequence of monotonous dacitic to rhyolitic breccia tuffs and pyroclastic rocks with intercalated lapilli to block tuffs (Guevara and Davila, 1983; Davila, 1991; Davila and Carrera, 1999) emplaced at 16.2 ±0.2 Ma (Dietrich, et al., 2005). This pyroclastic sequence rests unconformably on Mesozoic basement sedimentary rocks of the Ferrobamba, Mara, and Soraya formations (Davila and Carrera, 1999).
The stratovolcanic edifice unconformably overlies the pyroclastic sequence and is encompassed by the inner ring structure. It comprises a conical pile of andesitic epiclastic volcanic rocks, tuffs, and capping lava flows. The higher stratigraphic levels of the sequence has been preserved due to local subsidence of ~100 m within the inner ring structure. The eroded volcanic edifice has been estimated to have reached a height of up to 700 m. Capping andesitic lava flows have been dated at middle Miocene 15.2 ±0.1 and 15.1 ±0.1 Ma (Ar/Ar; Dietrich et al., 2005).
The shallow Explorador flow-dome complex intruded the eastern section of the stratovolcano and is partially exposed parallel to the inner ring structure. It is host to the Explorador, Tumiri, and Aycha vein systems. The complex represents a shallow intrusion with some possible extrusive portions and has a mushroom geometry in cross section, with stems that are 40 to 70 m thick. Fine and very planar flow banding can be observed from millimetre to metre scales and displays large scale flow folding. This is one of several rhyodacitic to rhyolitic flow dome complexes that intruded the stratovolcano and ring structures, and host clusters of low- to intermediate-sulphidation veins and a central high-sulphidation epithermal system with pervasive advanced argillic alteration. The others include the Parcachata flow-dome complex and the Pucanta, Huararani and Aycha domes. The peripheral Explorador and central Parcachata
flow-dome complexes are dated at 14.6 ±0.1 and 14.5 ±0.1 Ma respectively. The Aycha and Huararani domes developed as the outermost of these flow-dome complexes are more rhyolitic in composition, whilst thei cross-cutting relationship with the Explorador complex indicates they are a younger age (Dietrich et al., 2005).
The Selene district encompasses a wide spectrum of mineralisation and alteration styles ranging from high- to low-sulphidation, and from advanced argillic (alunite-dickite-kaolinite) through sericitic and argillic (illite-smectite) to propylitic. In the centre of the district, the Parcachata high-sulphidation system is characterised by deeply-rooted pervasive advanced argillic alteration (alunite-dickite-kaolinite) with cross-cutting alunite veinlets that passes radially into pervasive argillic kaolinite alteration. This system is weakly anomalous in Au, but is essentially barren. Sericitic alteration is best developed in epiclastic and tuffaceous rocks peripheral to the Parcachata flow-dome complex, grading out into the stratovolcano edifice but also occurs around the Aycha flow dome. The sericitic alteration appears to be truncated by illite-smectite alteration halos around the Explorador and Tumiri veins. The Pucanta sector to the SW of the Parcachata system comprises a cluster of high sulphidation replacement veins with structurally-controlled advanced argillic alunite-dickite-kaolinite-barite alteration along fractures with dissemination of sulphides and breccia-cementation. Gold and silver grades have a scattered distribution, with Au-dominated mineralisation at surface and adjacent to the Parcachata system, passing along strike and to depth into more Ag-dominated mineralization. Copper is anomalous, typically of the order of 50 to 200 ppm. The on-set of hydrothermal activity responsible for this high sulphidation mineralisation closely followed emplacement of the rhyodacitic flow-dome complexes and domes. Alunite veinlets of the central high-sulphidation
system gave a weighted mean Ar/Ar plateau age of 14.62 ±0.05 Ma that overlaps within the margin error the biotite ages of the Explorador and Parcachata flow-dome complexes (Dietrich et al., 2005).
Intermediate to low sulphidation style veins associated illite-smectite alteration characterise the Explorador flow-dome complex, the Tumiri vein, and several veins of the Aycha sector. The prominent Tumiri vein is up to 6 m wide and composed of several bands of massive quartz and quartz breccia whereas the other veins are characterized by banded quartz with adularia and Ag-Au bearing sulphide bands. Ag/Au ratios of intermediate to low sulphidation veins typically range from 40 to 400. These vein systems are distal to the central high sulphidation mineralisation and were developed towards the outer margin of the inner ring structure, interpreted to have occurred at around 14.2 Ma (Dietrich et al., 2005). Vein mineralisation around the Huararani (external to the inner ring structure) and Aycha domes is characterised by the most Au-pronounced metal content within the district (Ag/Au range: 5 to 30), and comprise banded veins of milky, massive to colloform chalcedony and chalcedonic bands with disseminated fine-grained pyrite. Carbonate replacement textures, silica-cemented vein breccias, and structurally controlled replacement mineralisation are locally present. Pervasive argillic kaolinite-group alteration occurs as a 0.2 to 1 km wide alteration halo along the contacts between the Huararani and Aycha domes across the inner ring structure.
Dietrich et al. (2005) concluded that the spectrum of mineralisation and alteration styles in the Selene district represents a zoned continuum of a single hydrothermal system with lateral and vertical fluid evolution away from a common source.
The Explorador Ag-Au quartz-adularia-sulphide vein system has a strike length of ~2400 m trend at 45 to 55°, and consists of the main vein plus an array of parallel quartz veins, structures and splays within a 200 m wide corridor that is interpreted to have been formed in a dextral strike-slip fault zone. Mineralisation has been delineated over a vertical interval of up to 350 m. The vein system is divided into a number of segments with varying continuity, number of strands, etc., and dips at from 75 to 80°NW. Ore shoots are largely sub-vertical to the north, but is generally subhorizontal to SW plunging in the central to SW portions of the vein system. The pattern of Ag/Au ratios is independent of grade distribution and varies from <50 near surface, whereas at depths of >300 m, it is >100, with a few fingers of lower values.
The paragenetic sequence within the vein system is characterised by early, banded quartz-sulphide with adularia and illite bands. Subsequent stages consist of lattice-textured quartz after calcite and massive milky quartz. The ore mineralogy, which is primarily associated with the early, banded quartz-sulphide stage, is characterised by ruby silver (proustite-pyrargyrite), argentite and minor tetrahedrite group sulphosalts (tennantite-tetrahedrite-freibergite) together with base metal sulphides.
Mining at Selene was commenced on a small scale in 1995, which was expanded in 2003. Production has since ceased, although the processing plant still (in 2022) treats ore from trucked 22 km from Pallancata. Resources, production and grades have not been encountered in sources sighted to date.
This summary is predominantly drawn from Dietrich, Andreas, Nelson, Eric P., Palacios, Celso, and Layer, Paul W., 2005 - Geology of the Explorador Ag-Au vein system and Selene mining district, Apurimac, Perú, in Rhoden, H.N., Steininger, R.C., and Vikre, P.G., (eds.), Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, p. 741–756.
The most recent source geological information used to prepare this decription was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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Inmaculada
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Dietrich, A., Nelson, E.P., Palacios, C. and Layer, P.W., 2005 - Geology of the Explorador Ag-Au vein system and Selene mining district, Apurimac, Peru: in Rhoden, H.N., Steininger, R.C. and Vikre, P.G., (eds.), 2005 Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, Geological Society of Nevada, Proceedings, pp 741-756.
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Gamarra-Urrunaga, J.E., Cadtroviejo, R. and Bernhardt, H.-J., 2013 - Preliminary Mineralogy and ore petrology of the intermediate-sulfidation Pallancata deposit, Ayacucho, Peru: in The Canadian Mineralogist v.51, pp. 67-91.
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