Las Bambas - Ferrobamba, Chalcobamba, Sulfobamba
Cu Au Mo
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The Las Bambas group of porphyry/skarn copper-molybdenum-gold deposits, Ferrobamba, Chalcobamba and Sulfobamba, are located some 72 km SW of Cusco and around 530 km SE of Lima in southern Peru. It is approximately 90 km north-west of the Tintaya porphyry/skarn deposits (#Location: 14° 3' 36"S, 72° 20' 44"W).
The Andahuaylas-Yauri belt covers an area of ~25 000 km2 in southern Peru and extends for ~300 km between the localities of Andahuaylas in the NW and Yauri in the SE, ~250 to 300 km inland from the present-day Peru-Chile trench.
The region is underlain by 50 to 60 km thick continental crust (James, 1971), and straddles the transition zone between the southern, normal subduction regime of southern Peru and northern Chile, and the current northern flat subduction zone of central and northern Peru (Cahill and Isacks, 1992). Basement in the region comprises Precambrian to mid-Palaeozoic gneisses, ~130 km to the northwest of Cusco (Carlotto, 1998), probable extensions of the Marañón massif to the north, overlain by >10 000 m of Palaeozoic rocks, which include >10 000 m of volcanosedimentary, marine and continental rocks of Cambrian to Early Permian age (Marocco, 1978; Carlotto et al., 1996; Carlotto et al., 1997). The upper section of the pre-Andean basement is dominated by >1000 m of volcanic and clastic rocks of the Permian to Early Triassic Mitu Group in the Mitu extensional graben (Perelló et al., 2003).
The Mesozoic stratigraphy of this part of Perú is largely composed of Jurassic and Cretaceous sedimentary sequences deposited in a setting dominated by two main basins, the Arequipa basin and the Eastern Peruvian foreland basins, separated by the Cusco-Puno basement high (Carlotto et al., 1993; Jaillard and Soler, 1996). The Arequipa basin and Cusco-Puno high are respectively the southeastern extensions of the Pucará basin and Marañón Arch of the main Peruvian Andes to the NW of the Abancay Deflection. The Arequipa basin (to the west; Vicente et al., 1982), corresponds to the present-day Western Cordillera, and contains a Middle Jurassic to Late Cretaceous sedimentary pile that is >4500 m thick, with a lower turbidite dominated section, a middle quartz arenite, and an upper sedimentary sequence with abundant limestone (Vicente et al., 1982; Jaillard and Santander, 1992). The northeastern edge of the Arequipa basin, overlaps the Andahuaylas-Yauri region, and includes the Lagunillas and Yura groups (Marocco, 1978), made up of Early Jurassic limestone and Middle to Late Jurassic quartz arenite and shale, with a total thickness of approximately 800 m. The top of the sequence contains the massive micritic limestone, black shale, and nodular chert of the Ferrobamba Formation (Marocco, 1978; Pecho, 1981). The Cusco-Puno high is overlain by ~900 m of Late Jurassic to Paleocene terrigenous red beds interbedded with shale, limestone and gypsum (Carlotto et al., 1993; Jaillard et al., 1994).
The Mesozoic sequence is unconformably overlain by two main units, specifically the Eocene to early Oligocene San Jerónimo Group and the dominantly volcanic Anta Formation. San Jerónimo Group consists of two main formations (Kayra and Soncco), with a total thickness of ~4500 m, made up of red bed terrigenous (sandstone, shale, pelitic sandstone, and volcanic microconglomerate) strata, interbedded with tuffaceous horizons near the top. The Soncco Formation includes horizons of stratabound copper mineralisation, up to several metres thick, with hypogene chalcocite and bornite, and supergene copper oxides (Cárdenas et al., 1999). The Anta Formation is a >1000 m sequence with a lower member containing andesite lava flows and dacite pyroclastic flows locally interbedded with alluvial conglomerate, and an upper member of fluvial conglomerate with interbedded andesite and basaltic andesite flows (Perelló et al., 2003).
These rocks are succeeded by Oligocene to Miocene sedimentary rocks of the 1500 to 5000 m thick Punacancha and >1100 m thick Paruro formations, dominated by coarsening-upward red shale and sandstone, with gypsum and conglomerate being characteristic in the upper parts of the sequences. Oligocene and Miocene volcanic rocks in the region are largely the calc-alkaline sequences of the Western Cordillera and Altiplano, and include the OligoceneTacaza (trachyandesite, andesite and rhyolite tuff) and Miocene Sillapaca (mainly dacite flows with subordinate andesite) groups. In addition to these, a series of scattered, small shoshonitic volcanic centres of Pliocene to Quaternary age are mapped (Perelló et al., 2003).
The northeastern margin of the Western Cordillera in the region is underlain by large bodies of intrusive rocks collectively known as the Andahuaylas-Yauri batholith (Carlier et al., 1989; Bonhomme and Carlier, 1990), interpreted to be coeval with the middle Eocene to early Oligocene Anta Formation described above. It is composed of multiple intrusions outcropping discontinuously over an interval of >300 km between the towns of Andahuaylas in the northwest and Yauri in the southeast. Its width varies from ~25 km in the Tintaya area in the far SE, to ~130 km along the Chalhuanca-Abancay area to the NW. The batholith evolved through the following stages: i). early-stage, predominantly ~48 to 43 Ma, cumulates (gabbro, troctolite, olivine gabbro, gabbrodiorite, and diorite) followed by; ii). rocks of intermediate composition (monzodiorite, quartz diorite, quartz monzodiorite, and granodiorite; Carlier et al., 1989; Bonhomme and Carlier, 1990; Carlotto, 1998) emplaced mostly between ~40 and 32 Ma; and iii). subvolcanic rocks of dominantly granodioritic/dacitic composition, locally associated with porphyry-style mineralisation, which represent the most evolved and terminal stage of the batholith-wide fractionation trend. The bulk of the batholith is middle Eocene to early Oligocene age, between ~48 and ~32 Ma, although there is evidence of considerable temporal overlap between the more mafic and the more felsic intrusions of the younger group (Perelló et al., 2003). Post-batholith intrusive activity in the region includes a series of small ~28 Ma syenitic stocks (Carlotto, 1998), part of a larger alkalic magmatic province that also includes the basanites, phonotephrites, and trachytes of the Ayaviri region, with ages between 29 and 26 Ma (Carlier et al., 1996, 2000).
The region has been affected by several Late Cretaceous to Pliocene tectonic events (Marocco, 1975; Pecho, 1981; Cabrera et al., 1991; Carlotto et al., 1996) of which the Eocene to early Oligocene (Incaic) and Oligocene to Miocene (Quechua) pulses are the most important. A salient feature of the belt is the spatial distribution of porphyry copper stocks around the edges of the main intrusions that constitute the Andahuaylas-Yauri batholith, as exemplified by the Las Bambas cluster (Perelló et al., 2003).
According to Perello et al. (2003), the Andahuaylas-Yauri belt is defined by >30 centres that exhibit porphyry-style alteration and mineralisation, including 19 centres that are grouped in 5 main clusters, plus another 12 separate centres, with hundreds of occurrences of magnetite-rich, skarntype
Fe-Cu mineralisation. Porphyry copper stocks are dominated by calc-alkaline, biotite- and amphibole bearing intrusions of granodioritic composition, with local monzogranitic, monzonitic, quartz-monzonitic, and monzodioritic stocks.
The three Las Bambas project deposits, Ferrobamba, Chalcobamba and Sulfobamba are distributed over an approximate east-west interval of 15 km, with Chalcobamba being 8 km NW of Ferrobamba, and Sulfobamba, 5 km west of Chalcobamba. Mineralisation at all three deposits includes components of classic porphyry-style (stockwork veining, sheeted veins and disseminations) and skarn alteration hosted mineralisation. Skarn-hosted mineralisation occurs as disseminated to clotty copper sulphide minerals, with superimposed porphyry-related sheeted veins (Kelley et al., 2016). The fourth significant member of the cluster is Haquira, which is located ~7 km south of Chalcobamba.
All of the skarns are hosted by the Ferrobamba Limestone unit and are accompanied by porphyry copper style mineralisation associated with monzonite and quartz monzonite phases derived from stepped igneous differentiation between 42 and 40 Ma at Ferrobamba and 38 to 36 Ma at Chalcobamba. The lower Ferrobamba Formation horizons, near their contact with the Mara Formation, are the preferred host rocks. All three deposits lack associated late-stage hydrous epithermal events.
At Ferrobamba, the evolution of the deposit and intrusive complex commenced with emplacement of early precursor, non-mineralised 39 to 36 Ma Pionero and Taquiruta stocks. These were followed by the relatively hornblende-rich Ccomerccacca and Jahuapaylla stocks in the centre of the deposit, which are related to the main 34.56 to 34.29 Ma stage of Cu mineralisation within the skarn alteration (Kelley et al., 2016). Forrestal, (2005) shows skarn alteration occurring as a series of coalescing lenticular zones from a few tens to >100m thick at the contacts between both mineralised and unmineralised multi-dyke-like, multiphase quartz-monzonite to dacite intrusives with associated andesitic dykes, intruded into marbles and limestones of the lower Ferrobamba Limestone. According to Perelló et al. (2003), the mineralisation-related intrusions are dacitic in composition and intrude both unmineralised intrusions and skarn altered wall rocks of the lower Ferrobamba Formation with associated contact breccias. The porphyry style mineralisation is accompanied by early potassic alteration, with secondary biotite dated at 35.6±0.9 Ma (K-Ar; Perelló et al., 2003), accompanied by quartz veins and veinlets, overprinted by a sericite-clay-chlorite assemblage, with chalcopyrite and lesser bornite as the principal copper sulphides. Forrestal, (2005) shows Exo- and lesser endoskarn alteration with variable associated chalcopyrite >bornite mineralisation is sporadically exposed over a U-shaped, NE-SW elongated zone with a total strike length of around 3.5 km. Gold values are elevated to between 0.1 and 0.3 g/t. The USGS MR Database suggest this structure represents a NE plunging, tight anticline with a steep dipping axial plane, developed around what Forrestal, (2005) shows as a larger mass of un-mineralised intrusive, cored by the smaller mineralised intrusions.
According to Kelley et al. (2016), most of the disseminated to clotty mineralisation at Ferrobamba is associated with pockets that are interstitial to subhedral garnet crystals, developed through a variety of open-space and solid-state replacement processes within the skarn host. The main processes that form these pockets are those that produce variable reactivity and increased permeability of the rock. Pocket-filling assemblages comprise calc-silicate (garnet, clinopyroxene), hydrous calc-silicate (epidote, amphibole), hydrothermal (quartz, carbonate, biotite, K feldspar, specular hematite) and sulphides (bornite, chalcopyrite, molybdenite). Many pockets show evidence of successive replacement and most sulphide mineralisation is parageneticaly late. Much of the high-grade mineralization at Ferrobamba is hosted in garnet skarn that has been overprinted by clinopyroxene. Late-stage bornite replaces and upgrades earlier chalcopyrite mineralisation (Kelley et al., 2016). Kelley et al. (2016) suggest pocket-forming mechanisms may include: i). remnants of carbonate protolith or early clinopyroxene skarn; ii). later, interstitial, fine-grained prograde garnet skarn or clinopyroxene skarn superimposed on garnet skarn; iii). later retrograde (hydrous) skarn replacement; iv). filling or replacement by hydrothermal quartz and carbonate; and v). volume creation by brecciation and fracturing. Brecciation may be related to volume reduction during skarn mineral replacement, explosive hydrothermal brecciation, or local structural adjustments.
These observations would suggest largely barren, brittle, prograde skarn was formed by more than one phase of the intrusive complex, including the precursor, higher temperature mafic and intermediate diorite to granodiorite pulses. These skarn altered rocks then acted as a focus of mineralisation that accompanied late, evolved, and terminal stage porphyry intrusions, which during their emplacement, induced fracturing, brecciation and production of open space within these chemically receptive lithologies. The process of mineralisation then produced a mainly retrograde skarn assemblage, overprinting the more extensive earlier prograde skarn. This retrograde skarn assemblage accompanied disseminated to clotty ore and sheeted veins, surrounding the disseminated, veinlet and sheeted vein porphyry mineralisation within the late stage intrusions.
At Chalcobamba, Forrestal, (2005) shows the Cu-Mo stockwork as a late stage north-south elongated 700 x 200 to 400 m zone of mineralised multiphase porphyry (granodiorite stock cut by dacite and associated andesitic dykes) cutting an earlier barren porphyry and a more extensive, zone of earlier developed, variously mineralised skarn alteration. This stockwork zone is developed on the eastern limb of a north-plunging syncline defined by a U-shaped zone of skarn altered marbles and limestones of the lower Ferrobamba Limestone unit. The porphyries have undergone early potassic alteration, with secondary biotite dated at 35.6±0.9 Ma (K-Ar; Perelló et al., 2003) overprinted by sericite-clay-chlorite. As at Ferrobamba, gold values are elevated to between 0.1 and 0.3 g/t.
According to Kelley et al. (2016), unlike at Ferrobamba, Chalcobamba is dominated by chalcopyrite hosted in garnet and magnetite skarn. Skarn is characterised by early dark green, coarse-grained garnet that was brecciated and cemented by fine-grained pale reddish garnet, followed by magnetite replacing garnet. This retrograde skarn development appears to be genetically associated with the Vizcacha monzodiorite porphyry stock at the centre of the deposit. A major north-south elongated breccia pipe formed after skarn development to the NNE of this stock. This breccia mass comprises ~30% of the deposit, which reworked early skarn into the breccia before being cross cut by monzodiorite dykes of the Chuspiri swarm.
At Sulfobamba, a broad zone of NE-SW elongated 500 x 1300 m zone of Cu-Mo stockwork mineralisation is developed within a porphyry mass, with associated zones of skarn alteration (Forrestal, 2005).
According to Kelley et al. (2016), copper mineralisation has been dated at 34.09 to 34.58 Ma (Re-Os molybdenite) and occurs within an elongate garnet and magnetite skarn at the contact with the older ~36.11 Ma Chonta monzodiorite. The ratio of chalcopyrite to pyrite is nearly equal. Immediately to the north, 32.99 Ma (Re-Os) porphyry style mineralisation followed skarn formation. Several breccia bodies have been identified associated with the mineralisation, including syn-mineral hydrothermal breccias and a late intrusive breccia pipe. NE-striking post-mineral monzogranite dykes cross cut all of the units. Magmatic biotite associated with early mineralisation yielded an age of 35.2±0.9 Ma (K-Ar; Perelló et al., 2003).
The principal alteration type in all of the Las Bambas deposit cluster is potassic, directly associated with mineralisation, occurring early in the evolution of each porphyry system, and consists of quartz, biotite and K feldspar. Hydrothermal biotite replaces ferromagnesian components, typically magmatic hornblende and, less commonly, magmatic biotite. It also occurs in the groundmass of porphyry stocks and in veinlets, either alone or accompanied by other silicate phases. Major quantities of quartz were introduced as either uni- or multidirectional veinlets during potassic alteration, comprising A-type veinlets which carry significant mineralisation in the form of chalcopyrite and/or bornite at Ferrobamba and Chalcobamba in particuar, while Chalcobamba also has B-type veinlets, characterised by semicontinous centrelines filled by mm- to cm-sized grains of bornite and chalcopyrite (Perelló et al., 2003).
The Las Bambas deposits have also been subjected to significant sericite-clay-chlorite alteration. This assemblage imparts a pale-green overprint to potassic alteration and gives a soft aspect to the rock (cf. Sillitoe and Gappe, 1984). It generally modifies, but preserves some of the original original rock textures. It varies in both intensity and mineralogy, although assemblages always include one or more association of sericite (fine-grained muscovite), illite, smectite, chlorite, calcite, quartz and varied proportions of epidote, halloysite, and albite. Plagioclase (both phenocrysts and groundmass) is replaced by a pale-green, greasy sericite assemblage which also includes illite and, locally smectite. Amphibole and biotite, the latter of magmatic and/or hydrothermal origin, are characteristically replaced by chlorite. Calcite is common as a replacement of plagioclase, (Perelló et al., 2003).
Propylitic alteration (chlorite, epidote and calcite) is mainly found as part of the outer halo confined to noncarbonate wall rocks, and at the Las Bambas deposits occurs
within porphyry copper ore zones in late-mineral stocks and dykes. In both cases, ~1% disseminated and veinlet pyrite is common.
Calc-silicate alteration is an important associate of mineralization at the Las Bambas skarn-porphyry cluster. Garnet, diopside, epidote and actinolite are the characteristic alteration assemblages (Terrones, 1958; Santa Cruz et al., 1979). The bulk of the Cu(-Au, -Mo) mineralisation at Las Bambas was introduced during prograde events, typically as chalcopyrite and, less commonly, bornite. The distal skarn mineralisation is richer in Pb and Zn, and also contains structurally and lithologically controlled, yellow-brown jasperoid developed in limestone beyond the skarn front (Zweng et al., 1997).
The bulk of the Las Bambas cluster mineralisation formed during a brief temporal interval of ~0.5 m.y., in the latter part of the evolution of the Andahuaylas-Yauri batholith that had been intruded over a 10 m.y. period (Kelley et al., 2016).
The depth of partial to complete oxidation of sulphides is generally 30 to 50 m but, varies considerably due to the steep topography. Economically significant zones of supergene enrichment are absent, largely because of the high degree of neutralisation of both potassic alteration zones and particularly the carbonate country rocks. Consequently, most cappings are immature, and typically goethitic, with the development of malachite, chrysocolla, neotocite, pitch limonite, and associated copper oxide minerals. The porphyry-related skarn mineralisation, at Las Bambas was oxidised to form gossan zones as oxidation products of magnetite and massive sulphides (Perelló et al., 2003).
Recent glaciation has removed any secondary enrichment, if it existed, from all three deposits and only thin zones of oxidation are preserved.
Published resource figures include (X-Strata Annual Report, 2006), using a 0.5% Cu cut-off grade:
Total indicated + inferred resource - 508 Mt @ 1.14% Cu, 0.0220% Mo, 0.11 g/t Au,
Ferrobamba indicated + inferred resource - 377 Mt @ 1.27% Cu, 0.0214% Mo, 0.13 g/t Au; including
Skarn mineralisation - 239 Mt @ 1.36% Cu, 0.0220% Mo, 0.14 g/t Au.
Published resource figures at 31 December, 2011 include (X-Strata Annual Report, 2012) using a 0.2% Total Cu cut-off grade:
Total measured +indicated resource - 1210 Mt @ 0.66% Cu, 0.0173% Mo, 3.3 g/t Ag, 0.05 g/t Au,
Total Inferred resource - 500 Mt @ 0.50% Cu, 0.0149% Mo, 2.4 g/t Ag, 0.03 g/t Au.
Published JORC compliant ore reserves and mineral resources at 30 June, 2015, include (MMG Mineral Resources and Ore Reserves Statement, 2015) using a 0.2% Total Cu cut-off grade:
Ferrobamba Oxide Copper
Indicated resource - 21 Mt @ 1.9% Cu;
Inferred resource - 6 Mt @ 1.7% Cu;
Sub-total resource - 27 Mt @ 1.8% Cu.
Ferrobamba Hypogene Copper
Measured resource - 388 Mt @ 0.8% Cu;
Indicated resource - 490 Mt @ 0.6% Cu;
Inferred resource - 452 Mt @ 0.6% Cu;
Sub-total resource - 1330 Mt @ 0.7% Cu; includes proved + probable reserves of 784 Mt @ 0.7% Cu.
Ferrobamba TOTAL resource - 1.357 Gt @ 0.7% Cu.
Chalcobamba Oxide Copper
Indicated resource - 5.9 Mt @ 1.4% Cu;
Inferred resource - 0.5 Mt @ 1.5% Cu;
Sub-total resource - 6.4 Mt @ 1.4% Cu.
Chalcobamba Hypogene Copper
Measured resource - 96 Mt @ 0.4% Cu;
Indicated resource - 190 Mt @ 0.6% Cu;
Inferred resource - 41 Mt @ 0.5% Cu;
Sub-total resource - 327 Mt @ 0.5% Cu; includes proved + probable reserves of 227 Mt @ 0.6% Cu.
Chalcobamba TOTAL resource - 0.334 Gt @ 0.5% Cu.
Sulfobamba Oxide Copper
Inferred resource - 0.02 Mt @ 2.8% Cu;
Sub-total resource - 0.02 Mt @ 2.8% Cu.
Sulfobamba Hypogene Copper
Indicated resource - 102 Mt @ 0.6% Cu;
Inferred resource - 214 Mt @ 0.4% Cu;
Sub-total resource - 0.315 Gt @ 0.5% Cu; includes proved + probable reserves of 68 Mt @ 0.8% Cu.
Las Bambas TOTAL resource - 2.007 Gt @ 0.6% Cu; includes reserves of 1.079 Gt @ 0.7% Cu.
The outlines of the three deposits above merged, and interpreted from diagrams contained in an X-Strata presentation by P Forrestal (2005), from Perelló et al. (2003) and Kelley et al. (2016).
The most recent source geological information used to prepare this summary was dated: 2016.
Record last updated: 1/12/2016
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.
References to this deposit in the PGC Literature Collection:
Kelley, D., Wise, J. and Shannon, J., 2016 - The Las Bambas porphyry cluster in the Andahuaylas-Yauri Batholith, southern Peru: in Denver Region Exploration Geologists Society, Technical Presentation, October 3, 2016, Abstract. Abstracts 2p.|
Perello, J., Carlotto, V., Zarate, A., Ramos, P., Posso, H., Neyra, C., Caballero, A., Fuster, N. and Muhr, R., 2003 - Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru: in Econ. Geol. v.98., pp. 1575-1605.|
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