MINERALOGICAL MAGAZINE, DECEMBER 1983, VOL. 47, PP. 453-63 Pyrrhotine-pentlandite ore textures: a mechanistic approach D. P. KELLY AND D. J. VAUGHAN Department of Geological Sciences, University of Aston in Birmingham, Birmingham B4 7ET ABSTRACT. The kinetics and mechanisms of pentlandite mental and theoretical study of ore textures, [(Fe,Ni)9Ss] exsolution from the monosulphide solid especially those which involve exsolution processes. solution [or MSS (Fe,Ni)l_xS ] have been studied by Potentially, such new approaches may lead not synthesis and annealing experiments in part of the only to a better understanding of the origins of the Fe-Ni-S system particularly relevant to sulphide nickel textures observed in many ores, but also to further orebodies. This experimental work has been combined with the examination of the compositions and textures of applications of textural studies in estimating rates pyrrhotine-pentlanditeintergrowths from a variety of ore of cooling and of diffusion within and throughout deposits. Isothermal annealing of MSS compositions with orebodies. The aim of this paper is to outline the varying M:S and Fe:Ni ratios at 400 ~ shows the methods and the concepts we have used in experi- formation of a sequence of textures which depend for mental studies of the Fe-Ni-S system, a system their full development chiefly on initial M:S ratio and relevant to the formation of the pentlandite- annealing time. The full sequence of textures starts with pyrrhotine ore textures which are characteristic heterogeneous nucleation of pentlandite at MSS grain of the so-called 'sulphide nickel' deposits. After boundaries, the growth of these blebs to form 'rims', describing some initial results on the kinetics nucleation of finer bladed particles arranged 'en 6chelon' along fractures, and, finally, homogeneous nucleation and mechanisms of pentlandite exsolution in the of fine orientated pentlandite lameUae. Using the Fe-Ni-S system, these results will be used to information from the annealing experiments, the inter- provide a model for the interpretation of textures growths produced by cooling in the natural ores are observed in the natural ores. interpreted using a simplified Time-Temperature- Examples of sulphide nickel ore bodies occur in Transformation Model. a variety of geological environments and include: the Sudbury Basin, Ontario, Canada (Hawley and THE importance of an understanding of ore Haw, 1957, Hawley, 1962); the Noril'sk-Talnakh textures has been recognized in numerous studies deposits of Northern Siberia, USSR (Smirnov, of the textures observed in individual ore deposits 1977); the Kotalahti-Hitura deposits, Finland at the macroscopic and microscopic levels as (Papunen, 1970); the Insizwa deposit, Southern summarized in various texts and reference works Africa (Tischler et al., 1981); the Pikwe-Selebi (e.g. Edwards, 1947; Ramdohr 1969, 1981). The deposits, Botswana (Lear, 1979); and the Kambalda more recent texts have emphasized the importance deposits of Western Australia (Gresham and Loftus- of studies of the phase equilibria in relevant Hills, 1981). Suggested classification schemes for systems to textural interpretation (e.g. Craig and these deposits are given by Naldrett and Cabri Vaughan, 1981). However, although phase equilibria (1976), Naldrett (1979), and Ross and Travis (1981). have been studied in a wide variety of ore mineral Most workers consider these ores to have formed systems and these studies have helped in the overall by the separation of an immiscible sulphide liquid problem of textural interpretation, their application from a parent marie or ultramafic silicate magma to the understanding of textures does have severe with subsequent crystallization of sulphides from limitations. This is largely because phase equili- the melt. Mechanisms of sulphide separation and brium studies provide a very incomplete and partitioning of elements between sulphide and idealized picture of mineral behaviour, characteri- silicate liquid are discussed in Duke and Naldrett zing only the final equilibrium state. In natural ore (1978), Rajamani and Naldrett (1978), and Boctor mineral systems, the mechanisms and rates of (1981). In deposits such as those of the Sudbury reactions will have a very important influence on Basin, the textures resulting from the crystallization the textures which result in any particular case. of the immiscible sulphide liquid and the subsolidus New approaches are therefore needed in the experi- transformations which occurred on further cooling Copyright the Mineralogical Society 454 D.P. KELLY AND D. J. VAUGHAN appear to have been preserved. In contrast, deposits Fel-xS to Nil_~S termed the monosulphide solid such as those at Kambalda have been subjected to solution or MSS (see fig. 1). Pentlandite (Fe,Ni)9S s dynamothermal metamorphism up to amphibolite becomes stable at 610 ~ in a compositional range facies and this has led to extensive recrystallization where no liquid is present. Below ~ 270 ~ of the ore minerals and to textural changes (Barrett the MSS breaks down and eventually disappears et al., 1977; Ostwald and Lusk, 1978; Marston and completely; reports of the temperature at which Kay, 1980). this occurs vary, illustrating the difficulty of The theoretical concepts used in describing and obtaining equilibrium below ~ 300 ~ interpreting the synthetic and natural textures will The bulk compositions of the sulphide fraction of not be discussed here in any detail. They can be many sulphide nickel orebodies plot within the found in reviews such as Chadwick (1972), Martin MSS, generally towards more Fe-rich compositions and Doherty (1976), Nicholson (1970), Greenwood (< 20 at ~), when plotted in terms of the Fe-Ni-S (1969), Manning (1973), and Gjostein (1973). Texts system (Naldrett et al., 1967). This indicates that and articles with a greater mineralogical emphasis the bulk of the sulphides in these ores initially include Yund and McCallister (1970), Yund and crystallized as MSS, which remained homogeneous Hall (1970), McConnell (1975), Champness and on cooling to temperatures in the region of 500- Lorimer (1976), and, in particular, Putnis and 600 ~ The subsolidus process which there- McConnell (1980). after produces pentlandite can be illustrated by considering fig. 2. Here the iron-rich and sulphur- Experimental studies of the Fe-Ni-S system poor portion of the MSS is shown in terms of atomic percent S plotted against atomic percent Ni Previous work. The phase relations in the Fe-Ni-S (the compositional field in which pentlandite occurs system have been studied by a number of authors, is shifted by 10 atomic percent to the left--a more recently including Kullerud (1963), Naldrett necessary convenience for drafting). In addition to et al. (1967), Shewman and Clark (1970), Misra and the sulphur-poor limits of the MSS (data from Fleet (1973a), Craig (1973), and through compila- Naldrett et al., 1967) for 600, 400, and 300 ~ the tions of Barker (pers. comm. 1981). plot shows a series of MSS compositions which From 900 ~ down to 270 ~ the system is were synthesized in this work (as discussed below). dominated by a complete solid solution from From this it can be seen that, as the temperature ~ 500~162 n ,~ p~ " ~ vs % * Naldrelt et al (1967) Misra & Fleet (19730)-bulk 4- Naidmtt etal (1967) o Mism & Fleet ( ) -single phase A Shewman & clark (1970) FIG. i. Phase relations in the central portion of the Fe-Ni-S system at 500 ~ (compiled by Barker (pers. comm. 1980) from published data). PYRRHOTINE-PENTLANDITE ORE TEXTURES 455 ot%Ni mosaics showing 120 ~ triple junctions (fig. 5a). Each 0 2 4 6 8 10 12 14 16 18 synthesized MSS composition was subdivided into 52~a~ 300~ batches which were annealed in isothermal runs at 400 ~ varying in duration from 1 h to 3 months. This enabled both the rate of reaction and the evolution of the textures produced to be monitored. Although only data obtained from the 400 ~ runs S 49 will be reported here, further work is being under- 48 taken at lower temperatures. Pentlandite-MSS exsolution textures in the Fe-Ni-S system at%Ni --p Composition5 in the iron - richMSS and 10entlandite fields plotted as S at~ vs Ni at% The compositions of the MSS phases are very (lhe pentlandite field is shifted 10 at% Ni towards the Fe-S join) different from that of pentlandite and so are the m Initial MSS cam/x~ifions syathesiseclat 600~ z, Coexisllng MSS*Pn compositions at 400~ (E.M.RA.) respective crystal structures, the former having r Misra and Fleets[ 1973koexisfing MSS +Pn compositions,400~ the NiAs-type structure (Francis, 1974; Misra --MSS sulphur poor limits, NaldmttCraig&Kullernd(1967) and Fleet 1973b) and the latter an unusual FIG. 2. Compositions in the Fe-rich MSS and pentlandite cubic structure with most of the metals in tetra- (pn) fields plotted as S at % vs. Ni at %. hedral coordination (Rajamani and Prewitt, 1973; Vaughan and Craig, 1978). Because of this, exsolu- tion involves a nucleation event with the agglo- decreases below 600 ~ the MSS field retreats meration and stabilization of small clusters of to more sulphur-rich compositions. Hence, when atoms with the pentlandite structure occurring samples synthesized in this work were annealed at within the MSS matrix. Clearly, this also involves 400 ~ the originally homogenous MSS composi- the diffusion of Fe and Ni and, to a lesser extent, S tions exsolved a more Ni-rich pentlandite and atoms through the MSS lattice. In order for the moved towards more Ni-poor and Fe- and S-rich formation of pentlandite within the MSS to occur, compositions. Phase equilibria studies indicate, there must be a driving force for the nucleation therefore, that on cooling of sulphide nickel ores event. below 600 ~ the originally homogenous MSS Fig. 3 shows a T-X section along a tie-line will exsolve a more Ni-rich pentlandite. As cooling linking two of the coexisting MSS compositions in and exsolution proceeds this will leave the residual fig.