Los Bronces-Rio Blanco is one of three giant Miocene to Pliocene Cu-Mo breccia/porphyry copper deposits in central Chile. It is located some 45 km (70 km by road) northeast of Santiago and is known to contain more than 50 Mt of economically recoverable copper metal.
The deposit is exploited by both the Disputada Los Bronces mine (~0.225 Mt of Cu pa in 2011) and by the Codelco Andina Division Rio Blanco operation (~0.20 Mt Cu pa in 2011).
As at El Teniente, 100 km to the south, mineralisation is generally contained as stockworks in the matrix of mineralised breccia pipes or is disseminated in host rocks surrounding the breccias.
Basement, exposed to the east and west, is composed of Palaeozoic metamorphosed sediments, volcanics and batholiths, overlain by Jurassic to Cretaceous marine strata, discordantly followed by Paleocene to Oligocene continental volcanics and lesser continental sediments, which following deformation, uplift and erosion were overlain by Oligocene to Miocene andesite flows and pyroclastics, and interbedded continental sediments. These volcanics were accompanied by extensive intrusive activity producing a number of batholiths, one of which, the 200 sq. km San Francisco Batholith, in part hosts the Los Bronces-Rio Blanco ore deposit. The remainder of the deposit is within the surrounding Miocene volcanics of the 21 to 11 Ma Abanico and/or Farellones formations.
The Miocene volcanics comprise 700 to 1200 m of sub-horizontally dipping andesitic lavas, some of which occur as large stoped blocks within the batholith. The 20.1 to 7.4 Ma (?) Ma San Francisco Batholith is a multi-phase intrusive with compositions including quartz-diorite, granodiorite, quartz-monzonite and quartz-monzodiorite with lesser granite and syenite. The contact between the lavas and the batholith below is sub-horizontal.
The Los Bronces-Rio Blanco deposit lies on the western margin of the San Francisco Batholith.
The earliest phase of alteration surrounding the Los Bronces-Rio Blanco deposit comprises a broad iron rich but sulphide poor zone within the San Francisco Batholith, characterised by disseminated hydrothermal actinolite ± magnetite ± titanite ± plagioclase, narrow veins of actinolite ± magnetite with halos of the same assemblage as the disseminations, and by breccias with magnetite ± clinopyroxene ± actinolite ± plagioclase matrix.
Approximately 50% of the copper ore in the deposit is present as breccia matrix, stockwork and disseminated mineralisation in a zone of potassic alteration associated with the emplacement of biotite rich breccias of the Rio Blanco Breccia Complex and quartz monzonite porphyry intrusions. This zone of strong potassic alteration is surrounded by a concentric shell of chlorite ± epidote and/or patches of sericitisation, which with the potassic alteration post dates the actinolite-magnetite phase. The potassic alteration and breccias has been imposed on both the granodioritic pluton and overlying andesitic lavas and comprises replacement of the protolith by biotite ± magnetite ± sulphides and the addition of K feldspar accompanied by the formation of stockwork veins with biotite ± K feldspar ± quartz ± magnetite ± anhydrite ± sulphides. In the current Rio Blanco underground mine, within the granodiorite, chalcopyrite dominates over bornite, pyrite and molybdenite. A high proportion of the potassic zone is composed of multiple mineralised biotite ± anhydrite ± tourmaline ± specularite breccias cutting and containing clasts of both andesite and granodiorite. In the magmatic breccias, which are volumetrically the most significant of these breccias, the clasts are enveloped by a dark matrix of fine grained biotite with associated tourmaline ± quartz ± anhydrite ± feldspar ± magnetite ± sulphides (chalcopyrite, bornite, molybdenite and minor pyrite). The other breccias of this zone have matrices dominated by either tourmaline, anhydrite and/or specularite with sulphides. These pipes extend for at least 800 m below the granodiorite-andesite contact. The potassic alteration has been dated at 7.4 to 5.4 Ma. Weakly mineralised Pliocene porphyries were emplaced both within and peripherally to the biotite breccias and potassic alteration.
The old Andina block cave mine exploited mineralisation within the andesites, while the current underground Rio Blanco mine extracts ore from the potassic phase alteration/mineralisation in the granodiorite of the San Francisco Batholith.
A second phase of mineralisation followed a period of uplift and erosion and comprised younger, tourmaline rich breccia pipes which contained the remaining 50% of the copper in the deposit. The most notable of these are the Sur Sur and Donoso breccias which have a matrix dominated by tourmaline with chalcopyrite, pyrite, molybdenite and minor bornite and may contain up to 10% Cu both within the matrix and altered clasts. Other less intensely mineralised breccias contain variable amounts of tourmaline, quartz, chlorite, sericite, anhydrite, specularite, sulphides and rock flour in their matrix. At least seven different breccias (based on matrix assemblage, clast size and type) are found to the west of the potassic zone, centred on the youngest of these, the tourmaline-sulphide Donoso breccia in the Disputada Los Bronces pit. The 5.2 to 4.9 Ma Donoso breccia has plan dimensions of 500x700 m with a vetical extent of more than 300 m and yields copper grades of around 1.2% in current operations. Another four or more breccia pipes are found to the south-east in the Codelco-Andina Sur Sur pit centred on the Sur Sur tourmaline-sulphide-iron oxide breccia pipe. The tourmaline phase breccias, developed over a length of more than 5 km striking at 150 to 170 degrees, are known to depths of more than 1000 m below surface and have been dated at between 5.9 and 5.1 Ma. In general they have sharp margins with the surrounding hosts. Some of the clasts contain earlier potassic phase stockwork mineralisation. The tourmaline breccias tend to be mono lithologic and are dominated by plutonic host clasts, although some andesite fragments are found. The clasts comprise 70 to 90% of the breccia, are angular, with little evidence of transport or abrasion and are from a few centimetres to tens of metres across.
Both the potassic zone of stockwork mineralisation and some of the younger mineralised tourmaline breccia pipes are cut by late Miocene to early Pliocene porphyries which were in turn followed by late tourmaline breccias. These intrusives include 5.2 to 3.9 Ma quartz monzonite porphyry dykes (which increases in thickness downwards to a deeper quartz monzonite stock) and a 6 km long north-south belt of dacitic to rhyolitic stocks, dykes, sub-volcanic plugs, domes and diatremes, and associated extrusives, the 4.9 to 3.9 Ma Don Luis Porphyry and La Copa volcanic complex.
Supergene enrichment has upgraded copper within fractures up to 200 m below surface in the area of the Rio Blanco underground mine, while leaching and enrichment has also affected some of the highly permeable breccias to develop chalcocite and covellite films on hypogene sulphide grains to 500 m below the surface.
The combined Los Bronces-Rio Blanco deposit is estimated to contain more than 5 Gt @ 1% Cu, 0.02% Mo.
At the Disputada Los Bronces mine, the reserves and resources in 2001 include:
Ore reserve - 475 Mt @ 1.03% Cu + 741 Mt @ 0.47% Cu leach ore, plus
Mineral resource - 200 Mt @ 0.95% Cu.
JORC compliant ore reserves and mineral resources at 31 December 2011 (Anglo American plc, 2012) were:
Proved + probable reserves, flotation - 1.4984 Gt @ 0.62% Cu, 0.014% Mo, plus,
Proved + probable reservesc, dump leach - 0.6837 Gt @ 0.33% Cu, plus,
Measured + indicated resources, flotation - 1.1339 Gt @ 0.43% Cu, 0.008% Mo,
Inferred resources, flotation - 3.1993 Gt @ 0.39% Cu,
Measured + indicated resources, dump leach - Nil,
Inferred resources, dump leach - 0.1144 Gt @ 0.26% Cu.
At the Codelco Andina operation in 2011 were (Codelco presentation 2011):
Total minerals reserves (proved + probable) and "identified mineral resources" - 5.888 Gt @ 0.78% Cu.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)