Comparative Studies of Copper and Manganese Mineralisation in the Torlesse, Waipapa and Haast Schist Terranes, New Zealand

Abstract

Small stratabound bodies of massive sulphide and manganese oxide mineralisation occur within metabasite-coloured argillite-radiolarian chert horizons ("volcanic association") in the weakly metamorphosed sandstones and argillites ("greywacke suite") of the Permian – Lower Cretaceous Waipapa and Torlesse terranes, and also in more metamorphosed equivalents of the undifferentiated Haast Schist terrane. A small number of geologically similar deposits occur in Cretaceous rocks of low metamorphic grade elsewhere in New Zealand ("Cretaceous terrane"), and some aspects of these have been examined for comparative purposes. Field characteristics of the deposits and associated rock lithologies, and the petrography of host rocks, are briefly described. Contacts between mineralisation and the volcanic association are sharp, and stockwork mineralisation is lacking. Sheared contacts between the volcanic association and greywacke suite sediments are common, and primary sedimentary contacts are difficult to demonstrate. The emphasis of this work, however, is on the geochemistry of the host rock assemblages and their interrelations, comparative petrography of the massive copper mineralisation, and the geochemistry of the mineralisation itself. New major and trace element analyses of 81 metabasites show that they have affinities with oceanic basalts. Classical Harker variation diagrams show scatter indicative of extensive elemental redistribution. Systematic pseudoigneous variations occur between Ti, P, Y, Nb and Zr, however, suggesting that these elements have not been mobilised, and their systematics are used to determine magma type. Fifty-one samples are tholeiitic, 11 transitional and 19 alkaline. Examination of pillow core-rim variations, chemistry of four pillow matrix sediments, chemical variability with loss on ignition and comparison with the bulk chemistry of unaltered modern MORB shows that the main agent of element mobilisation is low-temperature seafloor alteration (halmyrolysis) though metabasites associated with mineralisation seem to have also been affected by hydrothermal alteration. Regional metamorphism has not altered bulk chemistry significantly. Tectonic setting of eruption has been determined using Ti-zr-Y, Ti-Zr' Ti-Cr, Th-Nb, Ti-V and Zr/Y-Zr relations, which show that both spreading ridge and intraplate types occur. With a single and ambiguous exception, significant mineralisation is associated only with tholeiitic metabasites. Ti, P, Zr and Nb contents, and Zr/Nb and K/Ba ratios show that Cu mineralisation is found with N-type metabasites from midocean ridges, where Mn-associated examples and many of those from barren sequences resemble intraplate tholeiites. XRF analysis of sandstones and argillites of the greywacke suite (63 samples) reveals systematic chemical variation. In a suite of samples from the Torlesse terrane of Wellington, Fe, K, Mg, Al, Ti and P show regular increase in abundance with gradual decrease in grain size from sandstone to argillite, while Si, Na and Ca decrease. The trace elements Rb, Cu, Ni, Zn, Th, Ga, Nb, Cr, V and Y also increase as grain size falls, and V, Zr, Pb, S and Mn show variable behaviour. Only Sr and Ba decrease from sandstone to argillite. These changes in chemistry are interpreted as a response to hydraulic mineralogical differentiation produced by turbidite deposition. Identical patterns are observed in Torlesse rocks elsewhere, and also in Haast Schist terrane equivalents. Similar trends, but of poorer definition, occur in Waipapa terrane greywacke suite rocks, and significant differences in si, K, Na, P, Rb, Cu and Cr content are seen. These are related to provenance. Application of an alkali ratio - silica discriminant plot developed here shows that the chemistry of the Waipapa sediments is consistent with deposition at a Pacific-type arc margin, whereas Torlesse and Haast Schist greywacke suite sediments were derived from an Andean margin or an evolved continental margin abutting active oceanic lithosphere. The development of the discriminant plot and application to other New Zealand sedimentary terranes is given in a text appendix. Sixty-seven new analyses of cherts, metacherts, piemontite schists, and red and green argillites of the volcanic association are presented. Coloured argillites in the Torlesse and Waipapa terranes are highly oxidised hemipelagic sediments with chemical affinities to respective greywacke suite argillites, but are significantly modified by contributions from hydrothermal, hydrogenous and siliceous biogenic sedimentation in the pelagic environment. Despite close association with metabasites, there is little evidence for a tuffaceous or fragmental basaltic volcanic component. Silica:SiO2,/A12O3 systematics and Fe oxidation ratios of Torlesse and Haast Schist terrane siliceous volcanic association sediments indicate that these rocks are simple mixtures of coloured argillite detrital material and silica (probably biogenic). Strong positive correlations of Ti, K, Na, Mg, P and Ga with A12O3 in the ratio found in average coloured argillite support this conclusion. Most other elements (especially Fe, Mn, Zr, Y, Zn, Ni) also show linear covariation, but are substantially enriched with respect to coloured argillite ratio, and for these elements additional sources are required. Fe/Ti-AL/(Al+Fe+Mn) patterns demonstrate that for most samples, a significant metalliferous component (hydrothermal + hydrogenous) is present, in the ratio of 15:95 with detrital material. Samples close to massive mineralisation and metabasites are more metalliferous, A modelled mixture of 85% coloured argillite and 15% unmodfied proximal East Pacific Rise hydrothermal sediment successfully accounts for the observed anomalies in total Fe, Fe3+, Zr, Y, and Zn, and also for most excess Mn and Ni. Residual anomalies of the latter two elements are likely due to minor hydrogenous enrichment. Cu values are anomalously low in samples other than those close to mineralisation, and nay be affected by loss during diagenesis, a process that lowers Cr, V and perhaps Rb contents below predicted levels. The model also accounts for most excess Ca, Pb and Sr, but positive residual anomalies remain. Strong correlation of Pb and Sr residuals with those of Ca suggest additional contributions of these elements from a minor calcareous biogenous source. Similar SiO2,/Al2O3 and Fe/Ti - Al./(Al+Fe+Mn) relations are exhibited by Waipapa cherts, except that metalliferous contents relative to detrital are higher (25:75) than those in the Torlesse-Haast Schist terranes. Linear covariation of elements with Al2O3 occurs also, and similar enrichment anomalies with respect to average Waipapa coloured argillite ratios are resolved by a modelled 25:75 hydrothermal:detrital mix. The bulk chemistry of the siliceous sediments of the volcanic association in the three terranes is thus a product of three primary sediment sources: biogenous, detrital and hydrothermal. Hydrogenous sedimentation and diagenetic effects are minor, and the hydrothermal component is inferred to originate from active spreading centres as a buoyant floc. By comparison with chemical indicies of modern marine sediments, it is apparent that the volcanic association sediments were deposited over 1000 kilometres from land in a deep ocean environment. Higher metalliferous contents of those in the Waipapa terrane may mean they were deposited further from land than their Torlesse and Haast Schist counterparts. Primary Mn mineralisation consists of stratabound pods, lamellae and lenticules, and usually occurs in red argillite. Secondary supergene mineralisation is common and often extensive. Manganese deposits mineralogy consists mostly of complex oxide minerals, and is briefly reviewed. Copper mineralisation is typically massive, and compositional banding is infrequent. Pyrite is the major primary sulphide, with lesser chalcopyrite and minor but persistent sphalerite. Pyrrhotite is a major phase in Haast Schist deposits in place of pyrite, but otherwise is found only in minor or trace quantity. Small or accessory amounts of bornite, marcasite, carrollite, tetrahedrite and idaite occasionally occur, and secondary mineral assemblages are varied but poorly developed. Sulphide textures record progressive textural change in response to metamorphism, and at each grade are consistent with host rock recrystallisation. Although frequently brecciated, primary precipitation textures such as collomorphic and spheroidal aggregates are well preserved in Cretaceous terrane deposits. These features are rare in prehnite-pumpellyite facies Torlesse and Waipapa terrane deposits, where a representative texture consists of intensely cataclastic pyrite with moderately recrystallised interstitial chalcopyrite. In the greenschist facies deposits of the Haast Schist terrane textures are almost totally metamorphic. Pyrrhotite of metamorphic derivation, chalcopyrite and minor sphalerite occur in strongly annealed equant or foliated duplex aggregates. Pyrite is found as unzoned, rounded relics and partially recrystallised aggregates. Deformational textures are fewer and of different style than in the lower grade terranes. A quantitative treatment of textural adjustment is made by measurement of triple point angles in monomineralic aggregates of quartz, chalcopyrite and pyrite. Results show that the degree of recrystallisation correlates well with intensity of metamorphism, and at a given grade a closer approach to textural equilibrium is exhibited by aggregates in the order quartz > chalcopyrite > pyrite. Degree of textural equilibration is similar with that of comparably metamorphosed massive sulphide deposits elsewhere. Major element and Cu, Ni, Zn and Pb data are given for 27 samples of primary and secondary manganese mineralisation. Linear Ti-Al correlation in primary samples indicates the presence of a significant detrital component, and from modelling using coloured argillite normalising sediments it is deduced that this is usually 2-10 weight per cent. Largest residuals are for Mn, Si, Fe and Ca. Comparison with Si-Al and Si-Fe-Mn relations in modern and ancient Mn accumulations indicates that the nondetrital component is hydrothermal and biogenous. Hydrothermal input is verified by Fe/Mn-(Cu+Ni) relations and low Pb and Zn contents, which also show that hydrogenous enrichment is not significant. Partial analyses (Cu, Zn, Pb, Ni and S) of 51 samples of copper mineralisation show that it is of Cu>Zn type and contains only trace quantities of Pb and Ni, consistent with observed mineralogy. Cu-Pb-Zn relations are comparable with modern EPR massive sulphide deposits and Cyprus- and Besshi-type mineralisation, and distinct from deposits formed at convergent margins. Sulphur isotope ratios of 80 sulphide mineral separates cluster between 0 and +6 per mil, similar to those exhibited by midocean ridge basalts, and EPR, Cyprus & Besshi massive sulphide deposits. Sulphur isotope geothermometry gives temperatures ranging to >900°C, but these are considered invalid, as fractionations in coexisting mineral pairs are highly variable and frequently negative, indicative of isotopic disequilibrium. Closer approach to equilibrium is exhibited by isotopic pairs from the greenschist facies deposits, suggestive of partial isotopic reequilibration during regional metamorphism. A number of models for the source of sulphur in the deposits and the derivation of observed isotopic ratios are examined. It is concluded that the isotope systematics are best explained by derivation of reduced sulphur from the underlying volcanic pile by circulating high-temperature hydrothermal fluid at low water/rock ratios, with only minor contribution from inorganically reduced seawater sulphate. Isotopic disequilibriun is caused by mixing of the high-temperature fluid with ambient seawater at or near the sediment seawater interface. Sulphur isotope ratios of several separates of authigenic pyrite from Torlesse terrerne argillite range from -3.7 to -5.6 per mil, and are probably produced by reduction of coeval seawater sulphate in closed system wet sediments. The copper and manganese mineralisation studied represents rapidly precipitated proximal hydrothermal deposits formed at oceanic hydrothermal centres. Mn deposits are of intraplate seamount association, and were probably formed from cool, highly diluted solutions. Deposition from high-temperature solutions in a spreading ridge environment is indicated for the copper mineralisation by the geochemistry of associated metabasites and the petrography, mineralogy and geochemistry of the mineralisation itself. Comparisons are made with active and recent East Pacific Rise massive sulphide deposits, and ancient Cyprus- and Besshi-type analogues. Both the copper and manganese deposits, and the volcanic association sequences that contain them, have been incorporated into the greywacke suite sediments in an accretionary prism at a subduction margin. The deposits examined are uneconomic, are likely to remain so, and potential for discovery of commercially viable deposits seems low. Some geochemical indices useful for exploration are given, however, and these may be applicable in accretionary terranes elsewhere.