Summary:
Deposit type
The Taca Taca deposit is a porphyry copper–gold–molybdenum system hosted principally within granitic plutonic rocks, with subordinate dacite, dolerite and rhyolite intrusions. Hydrothermal alteration forms kilometre-scale zones that grade outward from a central potassic core through phyllic and argillic assemblages. The propylitic zone is comparatively restricted for a deposit of this size. Phyllic alteration is the most pervasive and hosts most of the mineralisation. Late-stage argillic supergene processes have locally upgraded copper tenor.
Mineralisation comprises an upper leached horizon overlying a mixed supergene–hypogene zone. Copper occurs disseminated and within fractures, veinlets and quartz stockworks. Sulphide zonation consists of a chalcopyrite–bornite–molybdenite core yielding outward to a strong pyrite-rich halo. The overall sulphide assemblage is mixed and more variable than typically observed in porphyry systems.
Surface weathering and oxidation removed copper from oxide and hypogene copper minerals, producing a 150–300 m thick copper-depleted leached cap. The leached copper was remobilised and partially reprecipitated as secondary sulphides beneath the leached horizon, forming discontinuous supergene enriched zones dominated by fine-grained black chalcocite with lesser covellite. The boundary between hypogenedominant and supergene-dominant mineralisation is highly irregular and reflects alteration to depth along structures and within the host rocks. Copper grades within the supergene zones are typically higher than in the hypogene mineralisation.
Mineralisation
Most of the mineralisation is hosted by phyllic-altered Ordovician granite and associated aplite and minor dolerite dykes. Dolerite dykes tend to have relatively higher copper grades due to the abundance of ferrous iron in mafic minerals, which promotes copper precipitation from hydrothermal fluids. Mineralisation is subdivided into an upper leached zone and underlying mixed supergene and hypogene zones.
Leached horizon
A leached zone (also referred to as the “leach cap” or “leached cap”) ranging from 150 m to 300 m thick is almost completely depleted of copper mineralisation and is dominated by limonite assemblages consisting of hematite, jarosite, and goethite.
Refractory copper and remnant zones of copper oxide mineralisation are limited to sporadic metre-scale subhorizontal lenses.
Supergene gold mineralisation is also enriched near the surface above the thickest portions of the leached cap. It is not associated or encapsulated with chalcopyrite. Molybdenum enrichment is also evident. Since this zone will be mined during the initial mining of successive pit phases, opportunity exists to stockpile this gold-bearing material for later treatment.
Supergene-hypogene mineralisation
Hypogene copper mineralisation is dominated by chalcopyrite with lesser bornite, chalcocite, covellite and digenite. Copper sulphides occur predominantly as disseminations in sericite vein selvages, in microfractures and intergrown with quartz veins.
In the potassic zone, minor chalcopyrite and trace bornite are associated with secondary biotite. Most copper mineralisation is hosted in the phyllic zone and is related to the two phyllic alteration stages. Early greensericite alteration is associated with a chalcopyrite–bornite assemblage, higher copper grades and aboveaverage gold grades. Late white-sericite and quartz alteration is associated with pyrite–bornite and pyrite– chalcocite–covellite assemblages.
Molybdenite occurs as disseminations and in quartz vein stockworks, more commonly in K-feldspar altered granite and aplite dykes.
Fine-grained black chalcocite and lesser covellite are the principal secondary copper sulphides associated with supergene alteration. A discontinuous, sub-horizontal supergene enrichment zone occurs directly beneath the leached cap and has a higher copper tenor than the underlying hypogene mineralisation. This enrichment blanket is best developed in the northeastern part of the deposit. Supergene alteration associated with steeply dipping structures locally extends to depth, generating supergene pockets within hypogene zones. Copper mineralisation is observed as domains where either hypogene or supergene mineralisation is dominant, but overall, the deposit is characterised by mixed mineralogy with a variable chalcopyrite-to-chalcocite ratio.
The mineralisation has an arcuate geometry, reflecting the morphology of the igneous intrusion and the distribution of mineralising fluids.
Mineralisation and metallurgy
During early mining, feed to the plant from the upper part of the supergene zone will be mixed with surrounding leached material. In some areas, the upper 0-80 m of the supergene horizon contains discrete metre-scale leached lenses. This mixing is expected to introduce atypical levels of iron oxide minerals into early plant feed, which may require management during plant commissioning, although the volumes involved are small.
Early feed will also include variable amounts of soluble copper associated with localised oxidation in supergene-dominant zones. Depending on oxidation intensity and acid consumption by gangue minerals, this may require appropriate mine planning and processing strategies.
Supergene-enriched pockets within hypogene domains are common and observed at depth. In such areas, primary sulphide-dominant material contains subordinate but variable amounts of secondary sulphides (chalcocite).
Studies on coarse-crushed exposed material indicate no significant increase in soluble copper content over nine months under site conditions. However, drill core stored under cover at the Salta core shed shows frequent surficial oxidation with copper staining rimming chalcocite. This is attributed to hotter and more humid conditions at the core shed and extended exposure (>12 months). Long-term stockpiling or ponding of pit water over broken mineralised material should be avoided.
Trace nickel mineralisation has been reported in drill core. Nickel assay data indicate that grades will be insignificant at mining scale.