Las Bambas Cu-skarn, Peru : implications of whole rock, mineral, and isotope geochemistry for ore genesis and exploration

The Las Bambas district in Southern Peru contains three major Eocene-Oligocene skarn deposits, Ferrobamba (1,257 Mt @ 0.57% Cu), Chalcobamba (325 Mt @ 0.55% Cu) and Sulfobamba (230 Mt @ 0.55% Cu), together with several skarn prospects. Southern Peru had a complex tectonic setting during the Eocene. Multiple compressional pulses related to subduction of the Farallon oceanic plate beneath the South American continental crust produced the Andean Orogeny along an active magmatic arc. In this region, the continent was bent at the Arica Elbow, and the Andes Cordillera was offset 200 km along the Abancay deflection due to differential shortening that induced block rotations and regional strike-slip displacements during the compressive Incaic tectonic phase. To the south of the Abancay deflection, the Eocene-Oligocene Andahuaylas-Yauri Batholith intruded marine sedimentary sequences, producing a wealth of porphyry and skarn Cu ¬¨¬± Mo ¬¨¬± Au deposits that constitute the 300 km long Andahuaylas-Yauri Belt. Las Bambas district is the richest Cu district in this belt. Intrusions in the Las Bambas district were emplaced over an ~11.1 m.y. time period in the late Eocene to early Oligocene. Zircon U-Pb geochronology, whole rock geochemistry, and spatial associations with mineralisation have allowed four magmatic stages to be defined, premineralisation (41.86 ¬¨¬± 0.60 to 38.10 ¬¨¬± 1.60 Ma; gabbros, diorites, and granodiorites), earlymineralisation (36.54 ¬¨¬± 0.62 to 35.06 ¬¨¬± 0.54 Ma; diorites, quartz diorites, quartz monzodiorites and monzonites), syn-mineralisation (34.36 ¬¨¬± 0.40 to 32.95 ¬¨¬± 0.28 Ma; monzodiorites, quartz monzodiorites, quartz monzonites, and granodiorites), and late-mineralisation (33.21 ¬¨¬± 0.55 to 31.87 ¬¨¬± 0.46 Ma; quartz monzodiorites, monzonites, and monzogranites). Whole rock trace element ratios (Sr/Y and V/Sc) and zircon geochemistry (Eu anomaly and Dy/Yb) indicate that all magmatic stages were hydrous, oxidized, and prospective for mineralisation. Lower Sr/Y and V/Sc ratios from pre-mineralisation intrusions indicate that these were the least hydrous. The early-mineralisation magmatic stage has strong indications of magma hydration and fertility (high Sr/Y and V/Sc ratios), despite a lack of any known Cu mineralisation. The high MREE/HREE ratios of early-mineralisation intrusions imply a garnetstable deeper-seated magmatic source related to a period of thickened crust, which appears to have been exhumed and partially eroded in the late Eocene, potentially resulting in the loss of any shallow level early formed Cu mineralisation. Progressive fractionation of Las Bambas magmatism was disrupted by mafic magma injection during syn-mineralisation magmatism, reflected both in elevated whole rock Fe\\(_2\\)O\\(_3\\) and high Ni and Cr contents in disseminated magnetite. Mafic magma underplating of the felsic magma chamber is interpreted to have triggered Cu mineralisation. Tha late-mineralisation stage saw a return to a conventional fractionation trend, with only minor skarn mineralisation formed. Syn-mineralisation intrusions from Las Bambas produced weak porphyry copperstyle mineralisation with early K-feldspar ‚Äö- biotite and later quartz ‚Äö- magnetite ‚Äö- Cu-sulfide vein stockworks. Intrusions in contact with limestone produced skarn alteration defined by garnet (35.0 ¬¨¬± 1.1 to 33.4 ¬¨¬± 0.6 Ma; U-Pb dates) and banded pyroxene ‚Äö- magnetite skarns. Retrograde skarn alteration produced massive magnetite after garnet and pyroxene skarn, epidote after grandite skarns, massive epidote endoskarns ¬¨¬± chalcopyrite, and quartz ‚Äö- calcite ‚Äö- epidote ‚Äö- specularite ‚Äö- Cu-sulfide mineralisation filling skarn voids, where the bulk of high grade mineralisation is located. Epidote endoskarn U-Pb dates (32.7 ¬¨¬± 1.0 to 31.0 ¬¨¬± 1.8 Ma) are younger than the intrusive protolith, with the time between emplacement of the intrusion and epidote endoskarn alteration related to the volume of the magma intruded, producing complexities in the retrograde paragenesis. Stable isotopic analyses show that magnetite (˜í¬•\\(^{18}\\)O: 6.6 to 13.2 ‚ÄövÑ‚àû) and epidote (˜í¬•\\(^{18}\\)O: 3.3 to 8.3 ‚ÄövÑ‚àû) precipitated from magmatic hydrothermal fluids with minimal meteoric water involvement. Epidote has heterogeneous compositions at the grain scale, but there are systematic variations in epidote chemistry at the deposit scale. At Ferrobamba and Chalcobamba, the Fe/ Al ratios of epidotes decrease with distance outward from the centre of mineralisation. Fe-Al substitutions affect the epidote crystal lattice lengths, and have resulted in systematic changes in the SWIR 1550 nm absorption peak position across Las Bambas (Q1 = 1543.2 nm; Q3 = 1545.2 nm) with regards distance to the centres of mineralisation, providing a field exploration tool. Propylitic epidote from syn-mineralisation dykes at Chalcobamba shows systematic spatial variations in Pb, Mg, Sr, Ni, and Cr contents with distance. A 2-D Monte Carlo optimisation of multi-variable linear regression of Pb, Mg, Sr, Ni, and Cr contents for Chalcobamba epidote predicts its centre of mineralisation to be at 786681 mE, 8444007 mN. The Mn and Ga contents of hydrothermal magnetites vary systematically with respect to the centre of Ferrobamba and Chalcobamba, and a Ga proximitor based on Ferrobamba and Chalcobamba magnetite compositions predicted the hydrothermal centre of Sulfobamba to be at 781250 mE, 8443600 mN, 4200 m a.s.l. Carbonate fluorescence and isotopic fractionation show systematic variations with distance to the mineralised centres at Las Bambas. Red-pink fluorescent calcite occurs proximal to the deposits and is related with high water/rock ratios. Decreasing water-rock interaction outboard of the mineralised centre produced a 3 x 5 km ˜í¬•\\(^{18}O\\) depletion halo in the carbonate host rocks around Ferrobamba. Epidote and magnetite compositions provide fertility indicators for skarn mineralisation at Las Bambas. High As and Sb contents in epidote are favourable signs of prospectivity, as are elevated Ni and Cr contents in disseminated magnetite. The Sallahue stock, a prospect located 3 km northeast of Chalcobamba, has high As and Sb concentrations in epidote and high Ni and Cr contents in disseminated magnetite, and is recommended as a priority for exploration.