Cu Au Mo
Super Porphyry Cu and Au|
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The Taca Taca porphyry copper-gold-molybdenum deposit is located in northwestern Argentina, ~230 km WNW the city of Salta in Salta state, 165 and 300 km ESE of Escondida and Antofagasta respectively. The deposit is immediately west of, and partially covered by, the Salar de Arizaro, the largest salt lake in Argentina (#Location: 24° 42'S, 68° 0'W).
The deposit is hosted in the southern half of a 50 km long Ordovician batholith that forms the core of the Sierra de Taca Taca mountain range, and comprises coarse-grained granite, cut by several aplite dykes. The batholith is overlain by Late Permian sediments and volcaniclastic rocks, while narrow, north-south striking, steeply dipping, Permo-Triassic rhyolitic dykes outcrop throughout the region. The porphyry copper mineralisation and alteration at Taca Taca is directly related to Oligocene rhyodacitic intrusions of the Santa Inés Formation.
Late Tertiary red-bed sedimentary rocks, which are widely distributed in the region, but are most abundant to the east of Salar de Arizaro, are possibly the basal section of the sedimentary sequence that fills the salar basin. Pliocene to Pleistocene lavas from stratovolcanoes are exposed to the west and north of Taca Taca. Large evaporite deposits composed of alternating salts and sand occur in regional intermontane basins that constitute the present-day salars (Almandoz, 2008).
The Sierra de Taca Taca is interpreted to represent an uplifted block. Oligocene volcanics exposed to the west of the deposit dip to the west, suggesting that the block was uplifted with an eastern vergence along a major, high angle reverse fault located near the western border of the Salar de Arizaro. Regional evidence suggests uplift occurred during the Oligocene (Almandoz, 2008).
The host to the Taca Taca deposit is a pink, coarse grained, Ordovician age (441.5±3.4 Ma) porphyritic granite (Chavez, 2008) or granodiorite (Cornejo, 2008), which has an equigranular texture, and is composed of phenocrysts of plagioclase, quartz (2 to 4 mm 'eyes'), K feldspar and rare rutile after mafic minerals. This intrusion is cut by several aplitic and aplo-granitic dykes, coeval with the Ordovician granites. Minor foliated dolerite dykes are interpreted as the final stage of the batholith (Sillitoe, 2008). Narrow, north-south striking rhyolitic dykes in the eastern part of the deposit area appear to be related to the Permo-Triassic Choiyoi volcanic event.
The Permo-Triassic dykes are cut by a number of NE-SW striking, steeply dipping porphyritic rhyodacitic dykes which have an Oligocene age (29.30±0.57 Ma - U/Pb dates in zircons) that are contemporaneous with the porphyry copper mineralisation. These latter dykes are characterised by large plagioclase, K feldspar and quartz phenocrysts. Two different Oligocene intrusive events have been recognised:
i). Early-stage rhyodacite, characterised by crowded phenocrysts (up to 1 cm) of feldspar and quartz, hosted in a biotite-rich groundmass, cut by a strongly developed stockwork of early white to grey quartz veins;
ii). Late-stage intermineral rhyodacite that exhibits fewer phenocrysts of feldspar and quartz hosted in an aplitic groundmass, cut by only a weakly developed quartz stockwork.
On the eastern side of the altered area, the outcropping dykes appear to coalesce at shallow depth to form a wide zones of rhyodacite, underlying the core of the Taca Taca alteration system, and have been interpreted to be responsible for development of the porphyry deposit.
The structural fabric of the Ordovician granite is characterised by discrete but widespread NNE and NW trending, steeply dipping proto-mylonite to mylonite zones, which seem to have influenced the emplacement of the Oligocene rhyodacitic dykes, quartz veining related to the porphyry system, fractures and small scale faults. A NNW striking, vertical, normal fault, located in the western part of the deposit area (the West Fault), was probably active during Miocene times, and may have controlled development of the zone of supergene copper enrichment. The rocks to the west of the fault have a thinner leached cap and are uplifted relative to those to its east.
Hydrothermal alteration associated with the Taca Taca copper-gold-molybdenum porphyry is as follows, beginning with the earliest phase and progressing to assemblages that overlap or occur later in the development of the hydrothermal system (after Almandoz, 2008):
Potassic alteration, characterised by abundant, flaky secondary biotite replacement of mafic minerals and rare secondary K feldspar, occurring as selvages to early veins. Potassic alteration is fund as remnant rafts in the central part of the mineralised zone after a strong phyllic (sericite-quartz) alteration overprint;
Propylitic alteration, peripheral to the hydrothermal system, characterised by chlorite-illite alteration of feldspars and mafic minerals with minor epidote alteration of plagioclase. Pyrite is common and varies from 3 to 5%, but locally up to 10%;
Phyllic alteration, occurring as sericite-quartz alteration, which is the most widely distributed and pervasive alteration phase associated with the Taca Taca porphyry copper-gold-molybdenum mineralisation. It is exposed over an area measuring 3.5 x 2 km. Two stages of phyllic alteration are evident: i). an early phase is characterised by the presence of pale green sericite and quartz. The pale green sericite is related to an intermediate sulphidation mineral assemblage, which formed an assemblage of chalcopyrite, minor pink bornite and virtually no pyrite. The highest hypogene copper grades are directly associated with this alteration type; ii). a late phase of phyllic alteration overprinting potassic, propylitic and green sericite phyllic alteration stages, represented by coarse white sericite that completely replaces feldspar and mafic minerals, with common associated disseminations and veinlets of pyrite. The white sericite indicates a change in the sulphidation state of the mineralising fluid from intermediate to high sulphidation. This change may be explained by progressive cooling that produces more acidic hydrothermal fluids.
A well-developed, 150 to 300 m thick leached cap overlies the hypogene porphyry copper-gold-molybdenum mineralisation, characterised by abundant secondary kaolinite and hematite-jarosite fractures replacing pre-existing sulphide veins. Copper oxides are rare although brochantite is common at the base of the leached cap and within a restricted area about the summit of Cerro de Cobre. The base of the leached cap is sub-horizontal and well defined. Some sub-vertical structures accompanied by supergene alteration have been encountered at depths of as much as 800 m below the surface, accompanied by secondary kaolinite, silica (chalcedony), alunite and chalcocite.
Re-Os dating of the molybdenite has shown that the porphyry mineralisation is Oligocene in age. There are five main styles of mineralisation associated with the Taca Taca copper-gold-molybdenum porphyry:
Hypogene Porphyry Copper Mineralisation
Hypogene sulphide mineralisation comprises chalcopyrite and pyrite with lesser bornite, chalcocite, digenite and molybdenite as disseminations and in quartz vein stockworks. The bulk of the mineralisation is hosted by the Ordovician granite and associated aplogranite and aplite dykes, with molybdenite more common in the latter. The centre of the system, has a relatively low 3 to 5% total sulphide content, passing out into the main hypogene copper-molybdenum mineralisation, to a peripheral pyritic shell (Almandoz, 2008). The hypogene porphyry mineralisation appears to have a north-easterly trend with dimensions of 2.5 x 1.5 km, although there is some indications the hypogene mineralisation may have a annular geometry centred on the concealed core rhyodacitic intrusion. The hypogene mineralisation is open at depth and to the NW, north, NE and south, and possibly also to the SE in 2011.
During the potassic alteration phase, minor chalcopyrite with subordinate bornite is associated with secondary biotite, and the associated strong A-type quartz vein stockwork is essentially barren of sulphides. Milky quartz B-veins commonly contain molybdenite with subordinate chalcopyrite. The two phases of phyllic (quartz-sericite) alteration are accompanied by the highest hypogene copper and gold grades, with sulphides commonly disseminated in sericitic vein selvedges, microfractures and intergrown with the quartz veins. Chalcopyrite-bornite is associated with early green sericite and has the highest copper grades, as well as above-average gold grades. The late quartz and white sericite phase is associated with pyrite-bornite and pyrite-chalcocite-covellite sulphide assemblages. Copper grades decrease slightly and the gold grades are approximately half that in the early green phyllic alteration phase.
Supergene Enriched Mineralisation
In the central part of the porphyry system the preserved leached cap is strongly developed, although the supergene enrichment blanket is thin (<5 m) to virtually absent. Two thicker zones (>100 m) of supergene enrichment have been outlined. One of these is a narrow, elongate zone developed along the 'West fault' in the south and the second is located over a broader area in the north. The northern zone is up to 300 m thick with a radius of ~1 km, but remains open to the west, north and east. Similarly, the southern zone is open to the south and SW. Supergene copper mineralisation is dominantly fine-grained, black chalcocite with minor covellite. In addition to the supergene enriched sulphide blankets, several deep (>500 m), steeply dipping supergene enriched structures have been outlined. The >0.5% Cu supergene blanket zone covers an area of ~1500 x 500 m.
Remnant Oxide/Supergene Mineralisation within the Leached Cap
The 150 to 300 m thick leached cap to the porphyry deposit, was almost entirely depleted of copper mineralisation, and is dominated by limonite assemblages of hematite jarosite and goethite. Remnant chrysocolla, atacamite and brochantite copper oxide mineralisation occurs as limited, small sub-horizontal lenses up to several tens of metres across. Molybdenite and gold, which are relatively immobile in supergene weathering environments, have grades in the leached cap that are approximately the same as the hypogene values, with, in general, the highest gold concentrations corresponding to the thickest portions of the leached cap, above the best hypogene copper-molybdenum mineralisation. The >:0.2 g/t Au zone covers and area of ~1700 x 1000 m, within the >200 m thick leached cap, centred on the >300 m contour.
Hematite-Quartz Copper-Gold Veins
Numerous parallel, steeply dipping, north-striking quartz-pyrite veins that were oxidised to quartz-jarosite and quartz-hematite veins occur in a zone covering an area of ~2.75 x 2 km immediately to the NW of the main hypogene porphyry copper deposit. Veins are 0.5 to 2 m thick and consist of quartz with massive to semi-massive pyrite (commonly with chalcocite coatings) or hematite-jarosite with minor alunite. In outcrop, the chalcocite coatings have been oxidised to produce chrysocolla and brochantite. Argillic alteration envelopes composed of sericite and kaolinite commonly extend for several metres beyond the vein margins into the country rock. The copper found in this zone is secondary in nature, apparently having migrated northward from the porphyry and been re-precipitated on sulphide grains associated with the vein's alteration selvages as well as within distal pyrite-bearing alteration phases (largely propylitic).
Exotic Copper Deposits
An exotic copper accumulation has been detected over an area of ~3 x 1.25 km, extending to the SE from the eastern margin of the hypogene deposit. Mineralisation comprises chrysocolla, copper wad and native copper, with intersections such as 6 m @ 1.2% Cu within basal gravels beneath the salar.
Published NI 43-101 compliant mineral resource figures as at November 2012, include (after Sim, Davis and Larson, Lumina Copper Corp., Jan. 2013):
Sulphide copper resource
0.2% Cu equiv. cutoff
Indicated resource, hypogene + supergene ore - 2.817 Gt @ 0.38% Cu, 0.011% Mo, 0.07 g/t Au;
Inferred resource, hypogene + supergene ore - 1.396 Gt @ 0.31% Cu, 0.012% Mo, 0.05 g/t Au;
0.3% Cu equiv. cutoff
Indicated resource, hypogene + supergene ore - 2.165 Gt @ 0.44% Cu, 0.013% Mo, 0.08 g/t Au;
Inferred resource, hypogene + supergene ore - 0.921 Gt @ 0.37% Cu, 0.012% Mo, 0.05 g/t Au;
0.5% Cu equiv. cutoff
Indicated resource, hypogene + supergene ore - 997 Mt @ 0.61% Cu, 0.016% Mo, 0.11 g/t Au;
Inferred resource, hypogene + supergene ore - 302 Mt @ 0.52% Cu, 0.019% Mo, 0.07 g/t Au;
0.7% Cu equiv. cutoff
Indicated resource, hypogene + supergene ore - 454 Mt @ 0.80% Cu, 0.020% Mo, 0.14 g/t Au;
Inferred resource, hypogene + supergene ore - 76 Mt @ 0.68% Cu, 0.016% Mo, 0.07 g/t Au;
Oxide gold resource in the leached cap
0.1 g/t Au cutoff
Indicated resource - 799 Mt @ 0.18 g/t Au;
Inferred resource - 213 Mt @ 0.14 g/t Au;
0.2 g/t Au cutoff
Indicated resource - 243 Mt @ 0.27 g/t Au;
Inferred resource - 17 Mt @ 0.23 g/t Au.
This summary closely follows the principal source of information, "Sim, R. and Davis, B., 2012 - Taca Taca Property, Porphyry Copper-Gold-Molybdenum Project, Argentina; An NI 43-101 Technical Report prepared by Sim Geological Inc., for Lumina Copper Corp., 115p.".
The most recent source geological information used to prepare this summary was dated: 2012.
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.
Sillitoe, R.H., 1977 - Permo-Carboniferous upper Cretaceous and Miocene porphyry copper-type mineralization in the Argentine Andes: in Econ. Geol. v.72, pp. 99-109|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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