Structural and geochemical evolution of the Rosario copper-molybdenum porphyry deposit and related copper-silver veins, Collahuasi District, Northern Chile

摘要

The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northernudChile. It comprises high-grade Cu-Ag-(Au) epithermal veins, superimposed onto theudcore of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 Mt at 1.03%udCu. An additional 1,022 Mt at 0.93% Cu occurs in the district at the Ujina andudQuebrada Blanca porphyry deposits. The Collahuasi district constitutes theudnorthernmost known occurrence of late Eocene-early Oligocene porphyry deposits inudChile.udThe Rosario deposit is hosted within Early Permian basement comprising audsequence of northeast dipping volcanic and sedimentary rocks. This package isudcharacterised by basal submarine andesitic and upper subaerial dacitic to rhyodaciticud(U/Pb age = 293 ± 14 Ma) facies. Graded feldspathic sandstones are interbedded withudthe andesites and dacites. Limestone lenses, interfingered with the dacite, areudconsistent with emergence of the submarine sequence, and a shallow subaqueous toudsubaerial volcanic arc depositional setting is inferred for the Permian host rocks. Theudvolcanic sequence was intruded by Early Triassic granodiorite (U/Pb age = 245 ± 12udMa) and late Eocene porphyritic quartz-monzonite (40 Ar/30 cooling age of primaryudbiotite = 34.4 ± 0.4 Ma). udHigh-grade copper ore is hosted by the Rosario fault system, a series ofudnorthwest-trending faults that dip 50° southwest. Detailed structural mapping indicatesudthat the main mineralising events at Rosario coincided with normal movement on thisudfault system. The main faults were reactivated as dextral wrench faults after the periodudof copper deposition. At Cerro La Grande, similar high-grade copper veins are hostedudin north-northeast-trending, sinistral wrench faults. Normal movement on the Rosarioudfault system is inferred to have been synchronous with sinistral strike-slip deformationudat La Grande. Stress modelling indicates that this brittle deformation event occurredudunder southeast-directed compression.udThe Permian basement has been uplifted, relative to Mesozoic rocks west ofudCollahuasi, along the north-striking Domeyko Fault System. This 2000 km long, arcparalleludfault system consists of a series of fault segments, which are interpreted to beudinverted basin-margin normal and strike-slip faults with highly varied movementudsenses. The orientation of the principal compressive stress axes in post-Eocene strike- slip deformation at Rosario is consistent with that inferred for sinistral movement,udduring the same period, on the Domeyko fault system. However, this local paloestressudregime is inconsistent with the regional stress expected for north-northeast-directedudconvergence during the late Eocene-early Oligocene, and implies that upper and lowerudcrustal stresses were decoupled. The calc-alkaline Rosario Porphyry is characterised by concave upwards REEudprofiles, and high La/Yb (>20) and Sr/Y (>20) values. Lead isotope data define audlimited range bracketed by values of the Paleozoic host rocks. Initial 87 Sr/86Sr andudI43 Nd/144 Nd values range from 0.7050 to 0.7053 and 0.5124 to 0.5126, respectively.udAlthough high Sr/Y values are a characteristic of adakites, the radiogenic isotopicudsignatures and high La/Yb values, indicate the ore-related magmas were derived fromudgarnet-fractionated MASH zones at the mantle-crust interface, rather than from partialudmelting of subducted oceanic crust. Although high Sr/Y values are a characteristic of adakites, the radiogenic isotopicudsignatures and high La/Yb values, indicate the ore-related magmas were derived fromudgarnet-fractionated MASH zones at the mantle-crust interface, rather than from partialudmelting of subducted oceanic crust. Hydrothermal alteration at Rosario is characterised by a K-feldspar altered core,udfocused on the Rosario Porphyry, that grades out to a secondary biotite-albitemagnetiteudassemblage. Paragenetic relations indicate that magnetite was the earliestformedudalteration product, but has been replaced by biotite-albite. Vein cross-cuttingudrelationships indicate that K-feldspar formed during and after biotite-albite alteration.udChalcopyrite and bornite were deposited in quartz veins associated with both Kfeldsparudand biotite-albite assemblages. The hydrothermal fluid responsible for earlyudalteration and mineralisation was a hypersaline brine (40 to 45 wt. % NaC1) thatudcoexisted with vapour between 400° and >600°C. Weakly mineralised illite-chloriteud(intermediate argillic) alteration of the early K- and Na-silicate assemblages wasudcaused by moderate temperature (2500 to 350°C), moderate salinity brines (10 to 15udwt.% NaCl). Molybdenite was precipitated in quartz veins during the transitionudbetween potassic and intermediate argillic alteration. These fluids were 350° to 400°Cudwith salinities between 10 and 15 wt. % NaCI.