Diablillos - Oculta
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The Diablillos high-sulphidation epithermal silver-gold deposit is located ~160 km SW of the city of Salta, along
the border between the Provinces of Salta and Catamarca, Argentina.
Modern exploration in the Diablillos area began in the 1960s with a regional reconnaissance for porphyry-style Cu/Mo mineralisation by Dirección General de Fabricaciones Militares, an agency of the Argentine military. Exploration at Diablillos commenced in ~1971, when the Secretaría de Minería de la Nación carried out geological and geochemical reconnaissance work in the area. On December 31, 1971, the prospect was included in a federal mineral reserve area for copper-molybdenum porphyry deposits. This reserve expired in 1984. The area was subsequently acquired by Abra de Mina, an Argentinean prospecting partnership. The title was subsequently explored by a Shell-Billiton joint venture from 1984 to 1987. Geochemical rock sampling and three shallow drill holes led to the recognition of a high sulphidation epithermal mineralisation and alteration system. The US owned Ophir Partnership Ltd optioned the property in early 1987 and drilled several holes that reportedly intersected anomalous to significant gold and silver values. Minera Utah International Ltd (BHP Ltd) began reconnaissance exploration in the area during the following year, and by late 1989 had concluded an agreement with Ophir and Abra de Mina to explore the Diablillos area. During a drilling campaign in December 1990, BHP drilled the discovery hole in the Oculto area, intersecting 105 m @ 2.91 g/t Au and 250 g/t Ag. Following this drilling, BHP estimated a total reserve of 4.1 Mt @ 2.4 g/t Au and 141 g/t Ag in a series of prospects, including the Oculta, Laderos, Vicuna and Corderos mineralised zones. By September 1991, the option agreement with Abra de Mina was terminated.
In 1992, Pacific Rim SA optioned the property from Abra de Mina, and completed the requirements to acquire 100% of the property on July 1, 1997. Pacific Rim continued exploration work until 1996, when acquired by Barrick Exploraciones Argentina S.A. Barrick continued delineation drilling and initiated preliminary environmental impact and metallurgical studies. Silver Standard Resources Inc. acquired all assets of Pacific Rim SA from Barrick in December 2001 and continued testing to outline a resource and conduct metallurgical and environmental testing. In 2009, Indicated Resources of 21.6 Mt @ 111.0 g/t Ag, 0.92 g/t Au and Inferred Resources of 7.2 Mt @ 27.0 g/t Ag, 0.81 g/t Au were estimated. Diablillos was subsequently purchased by Angel Bioventures Ltd in 2016 with Indicated Resources of 27.7 Mt @ 91.20 g/t Ag, 0.85 g/t Au and Inferred Resources of 1.09 Mt @ 43.9 g/t Ag, 0.87 g/t Au.
The Diablillos deposit is located in the Puna Region of northern Argentina, which constitutes the southern extension of the Altiplano of southern Peru, Bolivia and northern Chile, and is a high plateau separating the Cordillera Oriental (or Precordillera) to the east, from the Occidental (or Andean) Cordillera to the west. The Cordillera Occidental is largely occupied by a Cenozoic volcanic arc formed on the margin of the South America plate, above the subducting Nazca Plate. The Cordillera Oriental is predominantly composed of uplifted Proterozoic to Palaeozoic rocks, extending for ~1000 km from the Argentina-Bolivia border to Neuquén. Each of these domains are separated by north-south trending regional faults, the dominant structural features of the entire region. During the mid-Miocene Quechuan Orogeny, one of a series of pulses of Cenozoic contraction, the rate of convergence between South America and the Nazca Plate increased, and extensive late Miocene to Pliocene volcanic activity took place along the western margin of the Puna Plateau and along NW-SE conjugate structures. East-west to NW-SE compression led to reverse fault-bounded intra-arc basins, and uplift which began in the Early Miocene, but increased in intensity from the Middle Miocene. Subsequently the southern Puna has undergone uplift of ~2.5 km to an average elevation of ~4000 masl, with peaks of ~5000 masl.
Diablillos is located close to to the eastern margin of the Puna, near the intersection of the north-south Diablillos-Cerro Galán fault zone and the NW trending Cerro Ratones lineament. The former is one of the dominant north-south brittle to ductile shear zones in the Puna described above, each of which are hundreds of km in length and up to several km wide. These structures were formed during Neoproterozoic and lower Palaeozoic tectonism, and reactivated during the Mesozoic and Cenozoic. They are composed of multiple anastomosing shears, sometimes enclosing lenses of undeformed country rocks.
The geological succession in the Puna is composed of (after Ronning 1995 and Rennie, 2016):
Proterozoic to Lower Palaeozoic - representing the pre-Ordovician basement of the eastern Puna, known as the Pachamama Igneous-Metamorphic Complex. It comprises three subparallel north-south belts, each of which is ~200 km long. Diablillos is located towards the western margin of the eastern belt, which comprises pelitic, psammitic and granitic metamorphic rocks that have been intruded by younger granitoids of the Faja Eruptiva (described below).
• Metasedimentary rocks - a sequence of which in the vicinity of Diablillos comprise phyllites, metasiltstones and quartzites. Further north, apparently equivalent rocks contain late Ordovician fossils, which is inconsistent with the middle Ordovician radiometric ages for the Faja Eruptiva which intrudes them near Diablillos.
• Faja Eruptiva - magmatic rocks of Ordovician age that are widespread throughout northwestern Argentina. These include a belt known as the Faja Eruptiva de la Puna Oriental that extends from ~22°S in southernmost Bolivia to 27°S in Argentina. In the Diablillos area, the Faja Eruptiva is spatially coincident with the Diablillos-Cerro Galán fault zone and occurs as large, elongate bodies of porphyritic and equigranular, partly hypabyssal granitoids rich in sedimentary xenoliths. In the vicinity of Diablillos, the porphyritic intrusions contain feldspar phenocrysts up to 4 cm long. They follow a calc-alkaline differentiation trend and are peraluminous, and based on five U-Pb age determinations are of Middle Ordovician age.
• Cerro Ratones Volcanics - dated at 30±3 Ma Oligocene, although more recently a flank unit returned a late Miocene 7 Ma (40Ar39Ar biotite) age, suggesting an Oliocene to Late Miocene age range for the volcanic suite.
• Sub-volcanic Stocks and Extrusive Domes - generally Middle Miocene, 15 to 12 Ma, K2O rich dacitic rocks with biotite and occasional amphibole mafic phenocrysts, and accessory apatite, ilmenite, allanite and tourmaline, frequently associated with tephra deposits from low volume, plinian to phreatomagmatic eruptions.
Diablillos is one of a number of mesothermal and epithermal disseminated and vein occurrences in the northern and central Puna, characterised by base metal, gold, silver, tin and antimony mineralisation commonly associated with small, potassic-rich, Middle Miocene stocks and extrusive domes dated at 15±2 Ma (Sillitoe, 1977, in Coira et al., 1993, quoted in Ronning, 1997). These deposits occur along the trend of the Diablillos-Cerro Galán fault zone, and include the Diablillos, Incahuasi, Cóndor Yacu, Inca Viejo and Centenario.
At Diablillos, four main zones of high-sulphidation epithermal alteration and mineralisation, with strong supergene overprinting, have been delineated. The largest of these, which comprises the total resource at 2016, is the Oculto zone. It is hosted by a subaerial volcanic sequence, ranging in composition from pyroxene-hornblende to biotite-hornblende andesite, dated as Middle Miocene and assigned to the Tebequincho Formation (Stein, 2001). Basement rocks comprise Ordovician alkali-feldspar, porphyritic granite of the Complejo Eruptivo Oire, part of the Faja Eruptiva, and Neoproterozoic to Cambrian metasedimentary rocks of the Complejo Metamorfico Rio Blanco. Small, altered dacitic bodies have also intruded the basement and andesitic sequence (Stein, 2001).
The Cenozoic volcanic rocks are restricted to the east by the Pedernales fault, a structure associated with the broader Diablillos-Cerro Galán fault zone. These volcanics have been split into two groups by a NNE-SSW structure, the Jasperoide fault, 1 to 2 km to the west of the Pedernales fault. It seperates younger andesite flows and tuffs to the west from older pyroclastics and apron-bedded breccias to the east. The interval between the Pedernales and Jasperoid faults bracket a wedge-shaped graben that ranges from 2.7 km wide at Oculto to 800 m wide at Pedernales ~4.5 km to the north. It downthrows and preserves the bulk of the altered older volcanic rocks.
Hydrothermal breccias occur as pipes and dykes throughout the older volcanic suite from the Jasperoide Fault to another structure to its east, the Demonio fault.
The basement complex is exposed in most areas, except to the west of the Jasperoide Fault in the area of the younger andesitic flows and tuffs.
Phyllitic facies of the younger part of the basement complex are restricted to the northwestern section of the deposit area, and also to the east of the Demonio fault. These rocks contain ~2 vol.% boudinaged quartz containing molybdenum and iron oxide staining.
The intrusive basement granites of the Faja Eruptiva occupy a 1.5 km wide north-south strip in the centre of the deposit area, enveloping numerous xenoliths of quartz-mica schist similar to the intruded schist sequence of the basement sequence. These granitoids are principally found between the Pedernales fault to the west and the Demonio fault to the east. They are locally sheared to ultra-mylonites, which have been subsequently pervasively silicified and injected with sheeted quartz veins. The largest of these shear zones forms a prominent ridge known as Morro Eco near the Cerro Viejo prospect. The country-rock schist has been substantially deformed into tight small-scale folds, enhanced on exposed surfaces by differential weathering of the layers. Where altered, the schist are white, reflecting alteration of the dark micas into light-coloured clays or white micas. Where more intensely altered, the schist is completely silicified, producing a sugary quartzite appearance on broken surfaces, although the relic folded texture is maintained, particularly on weathered surfaces.
Cenozoic stocks and dykes intrude the basement complex, overlain by their extrusive equivalents. The oldest stratigraphic units encountered between the Jasperoide and Pedernales faults comprise fragmental andesitic tuffs that are generally strongly clay altered and not naturally exposed. These fragmentals are believed to be overlain by a lithic pyroclastic similar to one found on top of the Oculto mineralised zone, where it is observed resting on top of the andesite fragmental.
The overlying uppermost unit of the Cenozoic volcanic stratigraphic column to the west of the Jasperoide Fault, is composed of heterolithic, locally well-bedded apron breccias that form prominent exposures. The distribution and attitude of these breccias suggest a source to the east. They comprise a minimum of two distinct phreatic events, the first of which is dominated by andesitic clasts, followed by a more heterolithic clast event which includes blocks from the underlying older andesitic fragmental tuffs. Locally, the apron breccias exhibit evidence of sedimentary reworking with channels and cross bedding.
The hydrothermal breccia pipes and dykes that occur from the Jasperoide fault to the Demonio fault cross-cut all lithologies with the exception of the younger andesites west of the Jasperoide fault and the basement phyllites. The clasts in these hydrothermal breccias strongly reflect the host rock into which they were injected, although they almost always also include Faja Eruptiva porphyritic K feldspar granite. The hydrothermal breccias are differentiated from the apron breccia, which they can closely resemble, by their cross-cutting nature, occuring as isolated round to elongate pipes, grading into dyke-like structures. The largest of the exposed pipes is ~70 x 150 m, and is located at the northern end of Cerro del Medio, whilst the largest of the dyke-like hydrothermal breccias is discontinuously exposed over a strike length of 550 m. The dykes form three sub-populations based on their strike and alteration, namely: i). striking at 76° with strong silica-alunite alteration; ii). striking at 100° with strong silica alteration; and iii). striking at 167°, with mixed silica and silica-alunite alteration. The first two trends are concentrated in the central part of the group of deposits, whilst the third is restricted to the far eastern portion of the area.
The Cenozoic intrusives, which have a close spatial relationship to the hydrothermal breccias, are predominantly quartz-feldspar porphyries, occuring as small dykes and stocks. However, the breccias contain no clasts of the porphyry even where enveloped by it, suggesting they predate intrusion.
Mineralisation and Alteration
Seven known mineralised blocks within the four main mineralised and altered zones make up the Diablillos deposit cluster. Oculto, the most important and best explored, comprises the bulk of the delineated resource, and includes the Zorro and Cerro Bayo subzones; The other zones are Fantasma; Laderas; Pedernales including the Pedernales Sur and Pedernales Norte subzones; Cerro del Medio; Cerro Viejo and Cerro Viejo Este.
Oculto is a high-sulphidation epithermal silver-gold deposit related to remnant hot spring activity following Cenozoic magmatic and volcanic activity. It is expressed at surface by a broad zone of intense acid leaching located on the flank of Cerro Bayo, although no economic mineralisation is evident in outcrop. The deposit has undergone strong oxidation to depths of as much as 300 to 400 m below surface. In the resultant oxide zone, precious metal mineralisation occurs as native gold and chlorargyrite, with relatively less common iodargyrite, and locally common bismuthinite (Stein, 2001). These minerals occur as fine grained fracture-filling and vug lining associated with quartz, jarosite, plumbojarosite, hematite and goethite. Other accessory minerals include alunite, barite, native sulphur and bismoclite. Stein (2001) reported a high-grade zone of native gold, native silver and acanthite with accessory chlorargyrite, iodargyrite and jalpäite in the southwest extremity of the deposit. Gangue minerals include quartz, alunite, jarosite and iron oxides, accompanied by intergrowths of barite.
Hypogene mineralisation below the oxide zone occurs as vein- and breccia-hosted sulphides and sulphosalts. Primary sulphide and sulphosalt minerals include pyrite, galena, enargite, chalcopyrite, sphalerite, tennantite and matildite, with accessory barite and alunite. Stein (2001) observed incipient supergene enrichment where covellite partially replaces chalcopyrite and polybasite replaces tennantite.
The precious metal mineralisation throughout the hypogene deposit occurs as extremely fine grains along fractures and in breccias or coating on vug walls and weathered cavities. Mineral grains are not visibly identifiable to the naked eye, only by electron microscope or microprobe.
The dominant controls of alteration and mineralisation are both structural and lithological. Fluid flow has propagated along predominantly easterly and northeasterly trending steep fractures that host tabular silica veins and disseminations in bleached and altered wall rocks, as well as along more horizontal tabular bodies following the unconformable contact between basement granites and phyllites and the overlying Cenozoic andesitic pile. The latter contact comprises an erosional palaeosurface upon which pebble to cobble size granitic and metasedimentary detritus was deposited. This permeable conglomerate unit varies in thickness and distribution but forms an important host to mineralisation, hosting siliceous breccias.
The deposit has a broadly northeasterly trend and occurs within a vertical range of from 3965 to 4300 masl, predominantly between elevations of 4050 and 4250 masl.
The hypogene mineralised late Cenozoic volcanic and older intrusive rock have been subjected to alteration, which differs slightly, depending upon the lithology, as follows:
Volcanic rocks are altered to the following facies,
• Propylitic assemblage, characterised by chlorite, usually with significant accompanying clay minerals, observed in outcrop at the Pedernales Sur zone and in the subsurface at the Laderas and Oculto mineralised zones;
• Intermediate Argillic overprint, which is more abundant than the propylitic alteration, and has dominant clay minerals;
• Advanced Argillic that occurs in most mineralised zones, typically comprising clay minerals, but at the Oculto and Pedernales zones includes some alunite;
• Quartz-Alunite, where alunite is typically the dominant or sole alteration mineral, sometimes completely replacing the protolith. Associated minerals include dickite, pyrophyllite and diaspore;
• Vuggy Silica that is strongly developed in the central core of the Oculto deposit, probably temporally related to late-stage boiling
epithermal fluids and steam alteration. Vugs may be lined or partly filled by pyrophyllite, dickite and diaspore, or by alunite.
Intrusive Rocks are altered to the following facies,
• Silicification, which is most pronounced adjacent to the interpreted main hydrothermal fluid pathways. Resultant tabular bodies of silica have the appearance of quartz veins or veinlets, but are actually silicified granitoid rocks.
• Alunite, which occurs as fine-grained to microcrystalline masses replacing the feldspars and mafic minerals of the granitic rocks. It also occurs with quartz as veinlets, at times with jarosite.
• Argillic assemblage, which is found away from the loci of hydrothermal activity and occurs as clay alteration of feldspars and biotite alteration of mafic minerals.
The Upper Cenozoic host rocks in the central and eastern portions of the deposit area show evidence of a late, shallow, steam-heated alteration, overprinting the earlier hypogene alteration. These late stage altered rocks have a light grey colour and porous texture with abundant kaolinite and white, finely crystalline alunite, minor opal and occasional native sulphur.
At Oculto, this alteration pattern is represented by a series of roughly concentrically zoned assemblages, passing outwards from the deposit core, which is predominantly vuggy silica±alunite, into a surrounding zone of pervasive alunite and clay alteration, in turn grading outwards into kaolinite with illite, smectite and chlorite (Stein, 2001). Pervasive chlorite alteration underlies the mineralisation in the southwestern part of the deposit. A steam-heated zone of alunite-clay-opal is preserved above 4330 masl and occurs in outcrop in the central portion of the deposit.
The current Indicated Resource for the Oculto deposit, as estimated in November 2016 (Abraplata Resources Corp. website, viewed December, 2017), was:
27.7 Mt @ 91.2g/t Ag, 0.85g/t Au, containing 2530 t Ag and 23.5 t Au (ie, 59 t Au Equiv. or 4170 t Ag Equiv.).
The information in this summary is drawn from: Rennie, D.W., 2016 - The Diablillos Project, Salta Province,
Argentina; NI 43-101 Technical Report prepared for Angel Bioventures Inc by Roscoe Postle Associates Inc., 134p.
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.
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