Idaite, "Cu5.5FeS6.5" and related phases: their physico-chemical features, occurrence and genesis in ore deposits

Abstract

Idaite and "Cu5.5FeS6.5" are separate phases distinguished by various optical, physical and crystal chemical features. Their data are summarised as follows:Idaita: pseudocubic, a0=5.26Å; P or F lattice, m3m? (cf. Wang, 1976 a0=10.50Å c0=10.72Å); superstructure: 8A; cell content: Cu3FeS4; habit: parquet-mimetic twins – epitaxial chalcopyrite; cleavage: (111)? (124)? (001)?; density: 4.28; hardness: >"Cu5.5FeS6.5", <chalcopyrite; bireflection: R0 – brown yellow, RE – light green yellow (Air); anisotropism: -45° - bright green; thermal stability: 380-400°C (DTA/TGA), 280°C + (evacuated tube); ΔH°f298.15°K = -70335 cal. mole-1. "Cu5.5FeS6.5": hexagonal, a0 = 3.77Å c0 = 11.17Å (cf. Yund, 1963); P63/MMC? or P6/MMN?; superstructure: 3C?; cell content: Cu3.38Fe0.62S4.00 – Cu3.33Fe0.66S4.00; habit: pinacoid, pyramid, spiral growth twins; cleavage: 0001; density: 4.55±0.05; hardness: <idaite, >covellite; bireflection: R0 – pink brown, RE – light brown grey; anisotropism: - 45° - grey green; thermal stability: "Cu5.5FeS6.5" + pyrite - 501°C (Yund and Kullerud, 1966); ΔH°f298.15 = -96122 cal. mole-1. Natural occurrences of idaite and "Cu5.5FeS6.5" are discussed as follows: (i) Yauricocha, Peru; idaite and "Cu5.5FeS6.5" parageneses are described in the context of petrographic-petrogenetic investigation into this 'subvolcanic-porphyry' pipe deposit, the presence of idaite is attributed to hydrothermal incongruent dissolution of bornite; pervasive idaitization is accompanied by an anisotropic "advanced argillic" sulphide paragenesis: covellite (stoichiometric)-"Cu5.5FeS6.5"-digenite-enargite-luzonite-tennantite-'argentian tetrahedrite'-mawsonite-'zincian stannoidite'-'colusite' (lazarevicite?)-goldfieldite-chalcopyrite-sphalerite-bismuthinite-wittichenite-emplectite?-klaprothite?-pyrite-barite-marcasite quartz; idaite-chalcopyrite pseudomorph bornite euhedra in covellite-enargite-pyrite veins, covellite and "Cu5.5FeS6.5" further pseudomorph incongruent dissolution products, enigmatic associations are noted in areas adjacent to a 'arsenian goldfieldite'-tennantide-enargite-covellite-'non stoichiometric chalcopyrite-hubnerite assemblage'. The hydrothermal incongruent dissolution concept is corroborated by petrogenetic investigations detailed as follows: fluid inclusion and isotope data; sphalerite geothermometry: sphalerite (FeS)-pyrite equilibria in terms of ΔG°r =-63107.86 + 58.339895 T°K ; predicted hydrothermal phase equilibria for the Cu2O-FeO-As2O3-H2SO4-H2S-H2O system, 'mainline' "advanced argillic" aqueous equilibria (high aH+ quotient regions) are consistent with bornite undersaturation ; covellite + pyrite + "Cu5.5FeS6.5" solubility, computed according to mutual solubility constraints for the Cu2O-FeO-H2SO4-H2S-HCl-NaCl-H2O system (entropy/heat capacity models – fitted to published experimental solubility for the cp + bn + py triple point). (ii) Petrographic, analytical and paragenetic data for various other occurrences of idaite and "Cu5.5 FeS6.5" highlight the unusual nature of the Yauricocha paragenesis. The phases of interest have been described from the following locations: Ok Tedi, Panguna (PNG); Pilou, Fernhill (New Caledonia); Cucuruhuasi, Triunfo (Peru); Cerro Huemul (Argentina); Motueka River, Pupuke, Te Kumi, Kawau Island, N. Great Barrier Island (New Zealand); Butte (USA); Mt Isa (Australia); and Nukundamu (Fiji). In general low temperature parageneses are described but the Nukundamu "Cu5.5FeS6.5" assemblages are similar to those described from Yauricocha. New Zealand occurrences of idaite are introduced according to the occurrence of bornite in the NORTHLAND TRIPARTITE METALLOTECT – a three tier consanguineous 'subvolcanic-porphyry'-'volcanogenic stratabound' strip or metalliferous regional tectonic domain(s); in this respect comparative metallogeny for the Northland Submarine Stratabound Volcanogenic Metallotect, magmatic-hydrothermal C.F.S.E. concepts and magmatic sulphide assemblages are also discussed. The closing chapter is in essence a summary of points which arise from this research: general paragenetic considerations; comparative subvolcanic hydrothermal sulphide zonation; paragenetic and zonation models for "Cu5.5FeS6.5" based on predicted hydrothermal phase equilibria: particularly important in this context are factors responsible for abundance and occurrence, idaite- depending on previous bornite saturation and post-bornite "advanced argillic" or supergene conditions; "Cu5.5FeS6.5" - occurrence constrained by it's activity space locus in the Cu2O-FeO-As2O3-H2SO4-H2S-H2O system, arsenite activity constraints severely limit the occurrence of this phase in hydrothermal ore deposits; at 300°C, aAsOH4-.aH+ = 10-11, aH2S = 10-2 a 'quintuple' point is predicted for "Cu5.5FeS6.5" + enargite + tennantite + pyrite + bornite; at lower arsenite activities the "Cu5.5FeS6.5" activity space is bounded by three triple points involving pyrite, covellite and bornite; at AsOH4-.aH+ = 10-10 the "Cu5.5FeS6.5" space is usurped by enargite-tennantite; oxidative-H+ metasomatic processes limit the preservation of "Cu5.5FeS6.5", preferential covellite-enargite replacement is predicted. At lower temperatures (high fO2) covellite-chalcopyrite-"Cu5.5FeS6.5" colinearity is expected. Idaite is considered a metastable phase on the basis of thermochemical data, its 'residuality' and it's positioning in the Cu-Fe-S system ; adjusted (to cp + bn + py triple point solubility, experimental data, Crerar and Barnes, 1976) covellite + "Cu5.5FeS6.5" + pyrite solubility integrated with late stage Yauricocha mineralogic, isotopic and fluid inclusion data; the chalcopyrite-"Cu5.5FeS6.5", bornite-pyrite ties and the predicted region of permissive metastability ; predicted low temperature phase equilibria; comparative features of synthetic hydrometallurgical leaching products and low temperature natural incongruent dissolution phase assemblages ; the questions posed at the beginning of the thesis are addressed in respect of the research findings presented.