Tia Maria, La Tapada
Super Porphyry Cu and Au|
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The exposed Tia Maria and adjacent concealed La Tapada porphyry copper deposits are located in the province of Islay, Arequipa Region, southern Peru, within a range 10 km to the north of the town of Cocachacra, ~70 km SSW of Arequipa and 18 km from the Pacific Ocean coast (#Location: 17° 0' 48"S, 71° 46' 6"W).
The deposit lies in the centre of a more widespread, scattered cluster of gold bearing, quartz-pyrite-specularite-chalcopyrite veins, the abandoned Rosa Maria workings (Bellido and Guevara, 1961; Clark et aI., 1990). La Tapada is a concealed southern extension of Tia Maria, discovered by drilling under ~60 m of (semi)consolidated younger clastics.
The Tia Maria deposit lies within the southwestern part of the Bolivia Orocline, in the northern section of the Central Andes. For a regional perspective, see the separate Central Andes and Bolivian Orocline record.
Southwestern Peru is predominantly underlain by granulite to amphibolite grade gneisses of the Arequipa Massif, characterised by juvenile magmatism and metamorphism between 1.9 and 1.8 Ga, rejuvenated at ~1.0 Ga during a regional high-grade metamorphic event (Ramos, 2008). It is predominantly composed of biotite gneiss which grades to migmatite and granitic rocks, intersected by phyllonite and schistose gneiss. According to Ramos (2008;2010) the Arequipa terrane was accreted directy onto the Amazon craton. Docking is estimated to have occurred at ~1.05 Ga (Loewy et aI., 2004).
Extension during the early Mesozoic produced a fault controlled trough within the upper crust, the southern extension of the Huarmey-Cañete trough that follows the Pacific coast north to the Ecuador border. This linear basin appears to be floored by oceanic crust as far south as the NE-SW trending Abancay Deflection which marks the northern limit of the Arequipa Massif. South of this line, it is floored by attenuated crustal rocks of the massif. The trough was filled by volcanic and volcaniclastic sequences, and by sedimentary rocks deposited during repeated marine transgressions. These occur as patchy remnants of the Triassic-Jurassic Yamayo Group, predominantly composed of andesitic volcanic sand. A Cretaceous compressional event inverted the extensional basin at ~100 Ma, followed by repeated compressional and extensional episodes, linked to changes in the convergence rate between the continental and oceanic plates, throughout the Cretaceous and Paleogene (Pitcher, 1984; Jaillard and Soler, 1996).
The Arequipa Massif is intruded by the southern sections of the composite Peruvian Coastal Batholith that extends along the entire coastal belt from the Ecuadorian border in the north, to Toquepala, near the Chilean border, in the south. It has a length of almost 2000 km, but is seldom wider than 65 km. It has been divided into 5 sub-divisions, the last two of which, the Arequipa and Toquepala segments, intrude the Arequipa Massif and overlying rocks. The emplacement of the Coastal Batholith took place through much of the Mesozoic and Cenozoic. While the bulk of intrusion was between 100 and 60 Ma, the Punta Coles suite of the southern Toquepala segment is as old as 190 Ma. In the Tia Maria district the batholith is represented by ~140 Ma Cretaceous diorite, monzonite, granodiorite and granite. Proterozoic to Lower Palaeozoic shear zones cutting the Arequipa Massif rocks are intersected by numerous Cretaceous to Lower Tertiary brittle faults, breccias and crush zones. Tertiary and Quaternary continental sediments provide discontinuous cover.
The bulk of the mineralisation associated with the batholith is restricted to the southern Arequipa and the Toquepala segments where they intrude the Arequipa Massif. Intrusion-related mineralisation includes mid-Cretaceous, gabbro associated amphibole-magnetite-chalcopyrite veins and late Cretaceous granite associated quartz-specularite-chalcopyrite veins, chalcopyrite-molybdenite-scheelite skarns, quartz-calcite-auriferous pyrite veins and porphyry type chalcopyrite-molybdenite-pyrite disseminations and stockworks. The Arequipa segment contains a cluster of Mid- to Late-Cretaceous porphyry-type mineralization in the Rio Pisco area (Agar, 1981). The Toquepala segment hosts the Late Paleocene Cerro Verde, Toquepala, Cuajone and Quellaveco porphyry copper deposits (Vidal, 1985). Rosa Maria type quartz-specularite-chalcopyrite veins are found widely within the coastal section of the Toquepala segment of the batholith, but are only rarely economic (Chavez, 2000).
Mid- to Late-Cretaceous volcanic sequences host volcanogenic, layered barite-base metal sulphide and stratabound amphibole-magnetite-chalcopyrite deposits.
Geology, Alteration and Mineralisation
In outcrop and sub-outcrop, the Tia Maria deposit is hosted by granulite to amphibolite facies gneisses of the Proterozoic Complejo Basal de la Costa, part of the Arequipa Massif. These gneisses have a granitic to granodioritic composition. At depth mineralisation is also hosted by granitic rocks of the ~160 Ma (40Ar-39Ar; Clark et aI., 1990) Jurassic Fiscal Pluton that outcrops to the west of the exposure of the deposit. This pluton, and both Rosa Maria and Tia Maria veins are cut by ~145 Ma monzonite dykes (Clark et aI., 1990).
The main Cachuyo Zone mineralisation at Tia Maria occurs as quartz-chalcopyrite-pyrite stockwork veins that crosscut earlier biotite and
iron sulphide (now oxidised) veinlets. The quartz-sulphide stockwork was emplaced as a single event. Individual veins rarely exceed 4 cm in thickness and are developed within a relatively low density set of fractures. No sheeted dykes or mineralised breccias have been observed. In contrast, the later, crosscutting, gold bearing quartz-pyrite-specularite-chalcopyrite Rosa Maria veins are up to several metres thick and are much more widely distributed. These later veins are vuggy, have euhedral quartz crystals and multiple crosscutting generations (Clark et aI., 1990).
The Tia Maria veins are composed of >90% anhedral quartz crystals up to 0.5 mm in size. Rutile needles and wall rock inclusions are common within the veins. The quartz crystals all show evidence of fracturing and planes of secondary inclusions are very abundant. Patches or balls of very fine quartz with rounded edges and highly undulatory extinction are evident, apparently the result of shearing, possibly caused by the reactivation of the hosting fault/fractures. Secondary hematite, apparently replacing primary sulphides, occurs in varying amounts, from the occasional crystal to a solid band running through a vein, always occurring at a late paragenetic stage, localised in vein centres. Patches and veinlets of chlorite with no particular paragenetic relationship constitutes up to 10% of the veins and appear to be alteration related. No vugs or open spaces occur within the stockwork veins other then those due to the weathering out of sulphides.
The Rosa Maria veins are also dominantly quartz, characterised by euhedral, bladed crystals up to 2 mm long. Vugs and open spaces are abundant. The veins have abundant associated hematite and some chrysocolla, both of which are due to oxidation of primary sulphides. Fine disseminated gold occurs within the masses of dark red to black seconday hematite. Some of the hematite, occurring as specularite crystals up 1 cm across, appears to be primary.
The deposit has been oxidised to depths of ~200 m, and locally to 300 m. Alteration and mineralisation are generally subdued at the surface, although both are more evident in the subsurface. Supergene oxides occur throughout the oxidised zone in fractures and joints. While there are between two and four oxidation palaeohorizons, there is no hematite/goethite leached cap or enrichment blanket associated with the Tia Maria mineralisation. In contrast, there is abundant supergene hematite in the vicinity of outcropping Rosa Maria veins.
The Tia Maria copper oxide mineralisation is dominantly composed of chrysocolla and lesser malachite, concentrated in joints and fractures in the host rocks. This mineralogy is interpreted to be the result of a neutral pH, and in-situ oxidation of a chalcopyrite dominated (CuSx : FeSx > 5:1) protore. Chrysocolla mainly occurs as a fracture filling or a coating on other supergene fracture filling minerals. Malachite is found as fracture and box work filling, as botryoidal encrustation in outcrop and sprays of small (~1 mm) clystals growing on other frachure filling phases. Subsidiary copper bearing phases include pseudomalachite, occurring as sprays of green crystals in fractures, and neotocite that is mainly apparent as black spots on fracture coatings. Atacamite and brochantite and azurite are also present. The bulk of this oxide assemblage is pseudomorphic after the Cu sulphides, or only slightly displaced and infiltrated. The Supergene hematite within the deposit area can contain up to 5 wt.% Cu, but is visually indistinguishable from barren hematite. Other fracture filling minerals include berlinite, goethite, kaolinite, illite and chlorite.
Alteration throughout the deposit is characterised by early incipient patchy K-silicate stable alteration, occurring as shreddy hydrothermal biotite replacing hornblende and magmatic biotite, and minor mafic groundmass in gneiss and diorite. This is followed by fracture-controlled biotite and K feldspar. K feldspar and to a greater extent plagioclase, are replaced by variable amounts of fine white phyllosilicate, or sericite, occasionally accompanied by silicification. The mafic phases are variably replaced by sericite and patches of granular quartz, ranging from incipient, to complete replacement. The degree of phyllic alteration of the mafic phases broadly matches amount of feldspar present. These assemblages are overprinted by a more extensive propylitic alteration halo in the form of chlorite±rutile±epidote±magnetite replacing the bulk of the hornblende and magmatic biotite as well as some of the hydrothermal biotite. A faint argillic overprint, predominantly kaolinite, mainly occurs in the groundmass and along the edges and crystal boundaries of larger phenocrysts.
The deposit is regarded to represent a low-grade, low total sulphide and somewhat diffuse and erratic fracture-controlled hypogene porphyry Cu system, overprinted by widespread secondary oxidic Cu minerals developed in a desert environment by near neutral groundwater circulation.
Fluid inclusions from the Tia Maria veins show homogenisation temperatures of from 323 to 397°C and average 360°C and pressures of ~350 bar. Salinity ranged from 3.6 to 13.1, but average 7.7 wt.% NaCI equiv. (Mioduchowski, 2001).
Published ore reserves of leachable material at 31 December, 2015 were (Southern Copper Website, viewed November, 2017):
Proved reserves - 222.149 Mt @ 0.321% Cu - in the Tia Maria deposit,
Probable reserves - 542.863 Mt @ 0.357% Cu - in the La Tapada deposit,
TOTAL ore reserves - 765.013 Mt @ 0.347% Cu,
TOTAL waste - 694.017 Mt, for W:LO of 0.91.
INGEMMET Peru quotes a reserve of 640 Mt @ 0.39% Cu, 0.19g/t Au.
This description is largely drawn from information in: Mioduchowski, A.P. 2001 - Geochemistry and Petrology of tlte Tia Maria Prospect, Department of Arequipa, Peru; A thesis submitted in partial fulfillment of the requirements for a Degree of Master of Science in Geology, Department of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, USA., 123p. and from a description in Data Metallogenica following a visit by Laznika, 2012.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 7/11/2017
This description is a summary from published sources, the chief of which are listed below.
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