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Rio Blanco |
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Ecuador |
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Au Ag
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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 Rio Blanco low sulphidation gold-silver deposit is located in the Western Cordillera of southern Ecuador, ~300 km SSW of Quito, 130 km SE of Guayaquil and 18 km north of the Chaucha porphyry Cu-Au deposit at an elevation of 3400 to 4000 m asl (#Location: 2° 49' 51"S, 79° 22' 28"W).
Tectonic and Regional Setting
The bulk of Ecuador's porphyry Cu±Mo±Au±Ag and porphyry-related epithermal Au±Ag±Cu deposits are of Jurassic and Tertiary (mostly Miocene) age, that define two distinct and parallel, temporal metallogenic belts to the east and west respectively (PRODEMINCA 2000; Sillitoe and Perelló 2005; Chiaradia et al., 2009).
Rio Blanco is one of a number of Miocene porphyry and epithermal copper-gold deposits that occur in two main mineral districts, part of the broader Miocene metallogenic belt that follows the entire western Andean range or Cordillera Occidental. This belt is closely associated with arc magmatism resulting from the subduction of the Nazca plate below the South American margin (Sillitoe 1988; PRODEMINCA 2000; Sillitoe and Perelló 2005). Deposits include the Chaucha porphyry Cu-Mo, Gaby-Papa Grande porphyry Cu-Au, and the Quimsacocha high-sulphidation, Portovelo-Zaruma intermediate-sulphidation and Rio Blanco low-sulphidation epithermal Au±Ag±Cu deposits, all of which define the Azuay-El Oro District. The Junín porphyry Cu-Mo and Alpala porphyry Cu-Au deposits and other smaller occurrences make up the Imbaoeste District, ~440 km to the north. These two districts are located within 100 km of the Pacific coast, 200 km SSE of Guayaquil and 75 km north of Quito, in southwestern and northwestern Ecuador respectively.
For more detail of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador and the broader North Andes and Panama copper-gold province.
Geology
The Rio Blanco deposit is hosted by Oligocene rocks of the Rio Blanco Formation, a member of the Saraguro Group, and comprise volcaniclastic rocks, andesitic lavas, felsic ignimbrites and dacitic tuffs, surrounded and cut by by Tertiary diorite intrusions on three sides and by andesitic porphyry dykes (Appleyard, 2003). The Rio Blanco Formation is underlain to the west by Mesozoic rocks. Two intrusions cutting the Rio Blanco Formation have yielded U-Pb zircon ages of 35.7±0.2 Ma (Eocene) and 15.7±0.2 Ma (Miocene) respectively (Bineli-Betsi and Chiaradia, 2006). The Rio Blanco Formation andesitic to rhyodacite/dacitic volcanic rocks and the intrusions both have a calcalkaline affinity and fall in the geochemical field of volcanic arc rocks. Elevated 207Pb/204Pb values of 15.70 to 15.62 within the volcanic rocks of the Rio Blanco Formation and of the intrusions, suggest mixing between crustal and mantle sources, with a dominant crustal contribution (Bineli-Betsi and Chiaradia, 2006), most likely derived from black shales of the crystalline basement, which have suitable Pb isotopic compositions (Chiaradia et al., 2004). Lead isotope compositions of the Miocene intrusion are characterised by higher 206Pb/204Pb values than most of the volcanic rocks of the Rio Blanco Formation and the Eocene intrusion. However, volcanic rocks that are stratigraphically higher within the Rio Blanco Formation have similar isotopic signatures to the Miocene intrusions and may be coeval with the latter (Bineli-Betsi and Chiaradia, 2006).
Alteration and Mineralisation
All lithologies in the deposit area were subjected to an early stage pervasive biotite alteration, possibly related to the diorite intrusion, and are locally overprinted by a propylitic assemblage of chlorite, epidote, and actinolite. Numerous narrow, actinolite rich and weakly gold mineralised (200 to 500 ppb) veins, with quartz, chlorite, epidote, actinolite, magnetite, pyrite, pyrrhotite and minor gold, formed during this stage (Bineli-Betsi and Chiaradia, 2006; Sutcliffe, 2003).
This early gold phase was overprinted by a low-sulphidation main stage gold-silver event, when multiple episodes of chalcedony and cryptocrystalline quartz were introduced in sub-vertical, generally east-west striking, en echelon veins, hydrothermal breccias and silicified bodies. Individual veins are several hundred metres long, with a preferred ENE strike and dip of >45°SSE. Three main veins zones have been tested, the Alejandra North, Alejandra South and Dorada.
Structural analysis indicate these are extensional veins, developed in a dextral transtensional strike-slip regime. The resultant mineralised zone spans a vertical interval of >600 m, from the diorite roof zone at an elevation of 3400 m at Dorado, to within a few hundred metres to the palaeosurface, at 4000 m at San Luis. The main stage mineralisation is accompanied by illite to smectite clay alteration that has penetrated both the hanging wall and footwall of veins and vein breccias by up to 20 m, overprinting early-stage alteration minerals (Appleyard, 2003). At higher elevations, within permeable, weakly welded pumice tuff, the illite-smectite alteration envelope expands dramatically, with the altered tuffs containing strongly anomalous values of Au, As, Sb and Hg over several square kilometres (Bineli-Betsi and Chiaradia, 2006; Sutcliffe, 2003).
The main stage veins display a range of filling textures, including crustiform quartz, bladed calcite replaced by quartz and a variety of breccia types (monomictic, polymictic, matrix supported, clast supported and jig-saw breccias; Bineli-Betsi and Chiaradia, 2006).
Breccias are mostly composed of clasts of hydrothermal milky quartz, crustiform white and black quartz bands, quartz pseudomorphs after bladed calcite, adularia and lithic fragments, set in a matrix of fine-grained black quartz, sericite, carbonates and accessory chlorite and actinote-tremolite. Sulphide minerals are also contained within the breccia matrix, but total <2 %, occurring mainly as pyrite, pyrrhotite, pyrargyrite (58 to 65.3 wt.% Ag), tetrahedrite (22 to 33.6% Ag), As (up to 2 wt.%)-bearing tetrahedrite (5.7 to 20 wt.% Ag), arsenopyrite and accessory amounts of Fe-rich (10.0 to 10.5 wt.%) sphalerite, galena, chalcopyrite and electrum (40 to 56% Ag). Electrum is concentrated in volumes of few cubic centimetres within the breccia, mainly associated with pyrrhotite, but also with pyrite and pyrargirite, and occasionally with sphalerite, occurring in grains ranging from a few to several tens of µm. These grains are both interspersed within fine-grained quartz, together with the other sulphides, and in direct contact with or within pyrrhotite and/or pyrite. Gold-rich electrum occurs at deeper levels, whilst the highest Ag grades are found in shallower parts of the deposit (Bineli-Betsi and Chiaradia, 2006; Sutcliffe, 2003).
An example of one of the main stage veins, the Alejandra vein-breccia, which contains some of the most significant gold grades encountered, where pyrargyrite, gold and electrum occur as disseminations in hydrothermal breccias, and concentrated in millimetric bands that are present within crustiform and colloform quartz-adularia. The best intersections were from a true thickness of 27.2 m averaging 23.2 g/t Au and 250 g/t Ag, and 14.2 m averaging 18.6 g/t Au and 206 g/t Ag (Sutcliffe, 2003).
The age of mineralisation was not known in 2006. The diorite that intrudes the lower parts of the sequence, and which predates the early gold event, may be linked to the nearby Chaucha batholith that hosts the Chaucha porphyry Cu-Au deposit 18 km to the south, and has been dated by K-Ar at ~13 to 12 Ma.
Bineli-Betsi and Chiaradia (2006) record δ34S values for pyrite and galena from the veins ranging from -3 to +4‰, consistent with a magmatic source of sulphur. However, pyrrhotite from disseminated ore within an altered lava yielded a δ34S value of -23.2&permil indicating a contribution from sedimentary sulphur, probably related to black shales of the underlying basement, which are also found as clasts within the breccias. Pb isotope compositions of 6 vein sulphides and the lava-hosted pyrrhotite are identical and fall within the field of isotopic compositions of the Miocene intrusion.
Resources
According to Sutcliffe (2003), the three main zones, Alejandra North, Alejandra South and Dorada, contain 5 Mt @ 5.5 g/t Au and 42 g/t Ag, including the high grade Alejandravein-breccia at Alejandra North with an inferred resource of 1.5 Mt @ 11.2 g/t Auand 99 g/t Ag.
Bineli-Betsi and Chiaradia (2006) quote a reserve of 2 Mt @ 8.9 g/t Au and 68 g/t Ag, including bonanza shoots of up to 800 g/t Au (Appleyard, 2003).
Junefield Mineral Resources Holdings Limited quotes the following mineral resources (Website, 2016):
Measured + indicated resource - 2.15 Mt @ 9.5 g/t Au, 69 g/t Ag for 18.74 t Au and 135.65 t Ag;
Inferred resource - 3.62 Mt @ 3.0 g/t Au, 17 g/t Ag for 10.4 t Au and 56.02 Ag;
TOTAL contained metal - 29.14 t Au, 191.67 t Ag.
The most recent source geological information used to prepare this decription was dated: 2006.
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.
Rio Blanco
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Bineli-Betsi, T. and Chiaradia, M., 2006 - The low-sulphidation Au-Ag deposit of Rio Blanco (Ecuador): geology, mineralogy, geochronology and isotope (S, Pb) geochemistry: in 4th Swiss Geoscience Meeting, Bern 2006 Proceedings 2p.
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Sutcliffe, J., 2003 - The Rio Blanco low-sulphidation epithermal gold-silver deposit, southern Ecuador: in 10th Congreso Geologico Chileno, 2003 Universidad de Concepcion, Departamento de Ciencias de la Terra 1p.,
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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