Super Porphyry Cu and Au|
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The Trapiche porphyry copper-molybdenum deposit is located on the SW margin of the Andahuaylas-Yauri belt, on the eastern margin of the Western Cordillera, at an altitude of 3900 to 4700 masl in southern Perú, 515 km SE of Lima, 140 km SW of Cusco, 85 km south of Abancay.
(#Location: 14° 29' 54"S, 72° 52' 32"W).
The Middle Eocene to Early Oligocene Andahuaylas-Yauri belt porphyry copper province is situated in southeastern Peru, extending for ~300 km between Andahuaylas in the NW and Yauri to the SE. The belt includes both porphyry related iron-copper skarn mineralisation, such as Tintaya, Atalaya and Las Bambas, and typical porphyry style alteration and Cu±Mo±Au mineralisation, such as Antapaccay, Los Chancas, Trapiche and Cotabambas. Other mineralisation includes replacement, and sediment-hosted oxide zinc deposits, and minor epithermal vein-style ores.
For more detail of the geology of the Andahuaylas-Yauri belt see the separate Las Bambas and Cotabambas records.
The stratigraphy in the Trapiche district includes, from the oldest exposed,
Late Jurassic to Mid Cretaceous
Piste Formation - a lower ~200 m thick unit composed of carbonaceous black shales (>80%) intercalated with sandstone and black limestone, overlain by ~100 m of calcareous siltstones alternating with sandstones and siltstones;
Chuquibambilla Formation - ~600 m of predominantly (80%) thinly bedded sandstones, with calcareous siltstones to limestones and brownish-shales;
Soraya (or Hualhuani) Formation - sandstones to quartzites;
Mara (or Murco) Formation - sandstones and red shales;
Ferrobamba (or Arcurquina) Formation - limestone and marl;
Precursor 29.17 Ma Oligocene (U-Pb; Colombo 2012) calc-alkaline granodiorite, which evolved into a 28.9 Ma quartz monzonite porphyry associated with for the alteration and mineralisation, followed by late porphyritic dykes.
Miocene to Pleistocene
Tacaza Group - 50 to 80 m of andesitic to dacitic volcaniclastic rocks and the overlying dacitic tuffs of the Alpabamba Volcanics;
Barroso Group - basaltic andesitic lavas.
Cobertura Formation - 5 to 20 m of gravels, alluvium and moraines.
In the immediate Trapiche deposit area, only the Piste and Chuquibambilla Formation, Oligocene intrusions and unconformably overlying Tacaza Group are represented.
The Trapiche deposit is related to the emplacement of a quartz porphyritic quartz monzonite intrusion and breccia pipe. The deposit has a NNW-SSE elongated ovoid shape in plan, with dimensions of ~2.1 x 1.0 km, and has been traced to a depth of 500 m. It is composed of three contiguous mineralised zones, from SE to NW of the i). Trapiche porphyry, ii). breccia Pipe and iii). Cu oxide zone.
The country rocks bordering the west, north and east sides of the three zones consist of sandstones, siltstones, calcareous siltstones and chlorite-epidote skarn of the Piste and Chuquibambilla formations. The sequence dips gently to the north, with the finer and more calcareous sedimentary rocks of the Piste Formation on the SW margin of the deposit, and the arenaceous Chuquibambilla formation to the north and west.
The oldest intrusive is a 29.17±0.67 Ma porphyritic granodiorite with phenocrysts of quartz, orthoclase and biotite, a little disseminated pyrite, traces of chalcopyrite and molybdenite, and a few megacrystals of orthoclase, which is related to the formation of the prograde skarn alteration phase.
The subsequent Trapiche quartz monzonite porphyry, which has been dated to 28.95±0.50 Ma, is primarily responsible for the main mineralisation and alteration, and has a porphyritic texture with phenocrysts of plagioclase, orthoclase and biotite in a quartzofeldspathic matrix. The porphyry style mineralisation occurs as disseminations and in 'A' and 'B' type quartz veinlets containing chalcopyrite, pyrite, bornite and molybdenite. Its emplacement appears to be either passive, or it is a transitional phase of the earlier granodiorite.
Two late quartz dacite porphyry phases, which are weakly altered and poorly mineralised subvolcanic rocks, have been dated to 29.88±0.39 Ma 28.53±0.27 Ma. The older of these is found to the north of the main deposit, as a 500 m diameter exposure in the vicinity of the Millucucho skarn, whilst the younger occurs as a larger ~250 x 800 m, north-south oriented mass, intruding the southern section of the deposit, partly extending along the contact between the breccia pipe and the main mineralised Trapiche porphyry.
The emplacement of the breccia pipe then took place in three stages: i). magnetite-quartz breccia, mainly found lower in the breccia pipe; ii). quartz-chalcopyrite-molybdenite breccia, which is the most extensive and iii). tourmaline quartz breccia, found closer to the surface.
The porphyry and breccia host the bulk of mineralisation. The mineralised porphyry weakly alters the contact with sedimentary wall rocks, which remain barren.
All of the rocks above are intruded by late quartz-monzonitic porphyry dykes and minor dacite and andesite.
Finally, the deposit has been deeply eroded and undergone intense supergene processes, resulting in the breccia and porphyry sections undergoing: i). leaching, ii). supergene enrichment, with a transition to the hypogene mineralisation, and iii). a zone of exotic copper oxides which occurs downslope, to the west and northwest of the breccia zone, within sedimentary wall rocks, containing mixed copper oxides and lesser supergene sulphides.
The structural framework of the deposit is characterised by 4 sets of fractures and faults:
i). the main sinsitral NW-SE Andean structural trend, considered the most important, represented by the Cerro Colorado and Harp Orco sinistral faults which straddle the deposit to the NE and SW respectively;
ii). a NE set represented by the Abra and Central faults that follow the porphyry-breccia contact and cut the breccia zone respectively;
iii). an east-west sinistral set represented by the Trapiche and Camp faults that connect the NW-SE set and occur near the northern ans southern limits of the deposit; and
iv). a north-south set that is well developed in the southern part the breccia and the separate Millucucho area skarn 500 m to the north of the main deposit.
Hydrothermal alteration is distributed over a 2.5 x 2 km area and is related to the porphyry and breccia pipe. In the latter, alteration is vertically zoned, with a deep calcic-sodic magnetite-chlorite-quartz-actinolite assemblage, considered to be a precursor phase, evolving upwards into a potassic zone, characterised by quartz, K feldspar, anhydrite and secondary biotite, which is associated with a large part of the economic mineralisation, and an outer chloritic margin containing an assemblage of chlorite and pyrite.
This zoning has an upper, superimposed phyllic assemblage of quartz-sericite-illite that also contains significant economic mineralisation. The uppermost part of the breccia pipe has undergone strong supergene leaching, is virtually barren and is characterised by clay alteration.
The Trapiche quartz monzonite porphyry was initially subjected to potassic alteration, with a superimposed fracture controlled phyllic phase that decreases in intensity with depth. The phyllic phase is linked to an upper intermediate argillic alteration characterised by an assemblage of quartz, chlorite and illite. Both the potassic and phyllic/intermediate argillic zones contain significant economic mineralisation.
A peripheral halo of calc-silicate hornfels altered sedimentary rocks occur to the west, NW and north of both the porphyry and breccia pipe, whilst to the south, weak propylitic alteration has been imposed on the granodiorite intrusive to the SE.
Both to the west and north, copper oxides have been developed by complex supergene processes, usually developed over areas of skarn and argillised sedimentary protoliths.
Cu and Mo mineralisation occurs as hypogene and supergene copper sulphides, molybdenite and lesser copper oxides, but dominantly a sulphides within the Breccia Pipe and quartz monzonite porphyry followed the the oxide zone and sedimentary country rocks.
Within the breccia pipe, the mineralisation mainly occurs disseminated and as patches in the matrix, and as disseminations and veinlets in the fragments. In the porphyry it is found as disseminations and moderate to strong sheeted veins and lesser stockwork, whilst the Cu oxides occur as impregnations and in fractures.
Leaching and supergene enrichment formed sub-horizontal zones in the breccia and porphyry, from top to bottom of enriched to transitional and primary. The main oxides occur mostly west and north of the breccia as a result of leaching and transported downhill of the breccia sulphides to form a shallow exotic layer. The characteristics of the or zones is as follows:
• Leached zone - the uppermost, where mineralisation has been leached and deposited at deeper levels. This zone is developed as a layer sub-parallel to the surface, mainly characterized by goethite, hematite, jarosite (2 to 10 vol.%) and remaining oxides and enriched copper sulphides. In the Breccia Pipe, permeability enhances the leaching, which persists over thicknesses of 20 to 120 m. In the porphyry, leaching is <5 to 30 m, and in the exotic oxide zone leaching zones is variable and reaches between 10 and 40 m.
• Supergene enrichment zone - is best developed in the breccia pipe and to a lesser degree in Trapiche porphyry. In the breccia pipe, the sulphides are chalcocite, covellite and digenite (0.5 to 2 vol.%), partially coating or completely replacing chalcopyrite and/or pyrite. The Cc-Cv:Cp ratio is 1:1.5, with the enrichment zone ranging from 50 to 200 m in thicknesses. Sulphides in porphyry are the same, but also include bornite. The Cc-Cv:Cp ratio is 1:20, with the enrichment zone ranging from 20 to 125 m, being thickest in adjacent arenaceous country rocks.
• Transitional zone - which mostly occurs in the breccia between the primary and enriched zones, as thin layers and small zones. It is characterised by a mixture of chalcopyrite and supergene copper sulphides. The Cc-Cv:Cp ratio is 1: 3. In the Trapiche porphyry, it is area is more restricted.
• Hypogene zone - which occupies the deepest part of the breccia pipe, but is closer to the surface in the Trapiche porphyry. It contains 0.5 to 3.5% pyrite, 0.3 to 3% chalcopyrite, and low bornite and molybdenite levels. Accessory minerals are 1 to 3% magnetite, with pyrrhotite, quartz, calcite and anhydrite. The Py:Cp ratio in the breccia is 2.5:1, where it has been traced over thicknesses of 100 to 300 m. In the Trapiche porphyry, the Py:Cp ratio is 1.5:1, and mineralised thicknesses reach 300 m, with a characteristic mineralogy of chalcopyrite-quartz-pyrite as 0.3 to 1.0 cm thick sheeted veins, molybdenite veinlets and 'B' type quartz veinlets.
• Copper oxide zone - which occurs as an exotic transported layer that occurs for 200 to 600 m down the steep (30 to 45°) northern and western slope from the outer margin of the breccia pipe, mainly within more arenaceous units of the sedimentary country rock sequence. Mineralisation consists of tenorite, cuprite, Cu-pitch, malachite, chrysocolla and azurite and neotocite, with traces of native copper and remnant chalcocite, covellite and bornite. The thickness varies from 20 to 100 m parallel to the surface.
• Mixed zone - which occurs within the oxide layer, as slope parallel, but restricted sub-layers between layers of oxides over thicknesses of 50 to 75 m, characterised by a mixture of oxides and supergene copper sulphides.
At a 0.10% Cu cut-off, the entire orebody has plan dimensions of 1.10 x 0.80 km, enclosing a NE-SW elongated zone at the same cut-off of 900 m x 400 to 700 m in the breccia pipe, and 300 x 150 m in the Trapiche East porphyry. At a 0.5% Cu cut-off, the last two zones are 600 x 300 m and 170 m x 150 m respectively.
As of December 2012, the deposit had been evaluated by 57 405 m of diamond drilling in 188 holes, distributed over an area of 1.5 km2.
Based on this data, the following resource (Tejada et al., 2013) has been estimated:
Total resource - 581 Mt @ 0.47% Cu, 130 ppm Mo; including a
Leachable resource - 258.10 Mt @ 0.54% Cu.
This summary is translated and paraphrased from "Tejada, F.L., Mamani, J.V. and Espinoza, B.I., 2013 - Descubrimiento y Geología del Pórfido Cu - Mo Trapiche, Apurímac - Perú; delivered to the Perumin 31 Convention, Arequipa, 16 to 20 September, 2013. 13p."
The most recent source geological information used to prepare this summary was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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