Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185intrusions in the world[2,3] show that this simple comparison K = KD (γNiS/γFeS)sulfide / (γNiO/γFeO)olivine (3)can lead to errors. It is important to compare natural data with where, γ refers to the activity coefficient. If Ni and Femodel curves that have been generated for and reflect the substitutions into olivine are ideal, as suggested by Campbellcumulus mineralogy of each intrusion in question. Using this and Roeder, i.e., both γNiO and γFeO are equal to 1, theapproach, the natural data can be closely duplicated by model exchange partition coefficient for the reaction will be equal tocurves, which in some cases, place additional constraints on the equilibrium constant multiplied by the ratio of the activitypossible petrologic interpretations. The importance of this coefficients of the two sulfide components in the sulfide liquid.aspect in evaluating any specific intrusion as a host for Brenan found that although it is relatively insensitive tomagmatic sulfide ore is self-evident. temperature, KD varies with ƒO2, and the Ni content of the sulfide liquid decreases as NiS decreases. Thus the ratio of2 Controls on olivine composition NiS/FeS is not constant, but is considerably greater than unity. However, at constant ƒO2 and sulfide liquid composition, KD is Controls on olivine composition include parental magma constant, and equation (2) predicts that when a suite ofcomposition, fractional crystallization, reaction with olivine crystals with varying FeO contents are immersed in ainterstitial silicate and sulfide liquids, and hydrothermal sulfide liquid, after reaction with the sulfide liquid, the olivinealteration at elevated temperatures. Nickel and magnesium are with higher FeO content will contain more Ni than the olivinecompatible in early crystallizing olivine and thus decrease in with lower FeO content, producing an inverse Ni-Fo relation of olivine. This relation can be used to evaluate the sulfideabundance as crystallization proceeds. In contrast, Fe is less liquid composition using olivine from sulfide-bearing rockcompatible in comparison with Mg; thus, the FeO/MgO ratios samples. An example of such application to the Jinchuanof a magma and of olivine crystallizing from it increase and Ni-Cu sulfide deposit in western China is given in Li et al.provide an index to the fractional crystallization experienced Finally, metamorphism/alteration may also affect theby the magma. Since nickel is present as a trace element in olivine composition. Olivine in mafic and ultramafic rocks ismagmas, its behavior during crystallization of olivine tends to commonly partially altered to serpentine along grain marginsfollow Henry’s law, and its concentration in olivine provides a and micro-fractures. The available data indicate thatguide to the concentration in the magma. The same is not true serpentinization does not affect the composition of residualfor major elements such as Mg and Fe, but Roeder and olivine. However, olivine may be converted to a more Fe-richEmslie have shown that in the case of olivine crystallizing variant during actinolite alteration, as noted in the Jinchuanfrom basaltic magma, the ratio (FeO/MgO)olivine/(FeO/MgO)magma intrusion. Re-crystallized olivine during metamorphism/is constant with a value of 0.3±0.03. Thus, the Fo (forsterite) alteration should be avoided in attempts to interpret thecontent of olivine indicates the FeO/MgO ratio of the magma composition of parental magma.from which it crystallized. The compositions of early cumulus olivine can be modified 3 Approach to modelingby subsolidus re-equilibration with trapped silicate liquid inintrusive rocks. Olivine crystallizing from the trapped silicate The crystallization of solid phases in magma chambers andliquid, nucleating on a core of original cumulus olivine, will sub-volcanic conduits can occur in a number of ways. Forbecome poorer in the Fo component as the trapped silicate example, solids can form in a magma, remain in suspension,liquid fractionates further. Diffusion of Fe and Mg within the and thus remain in equilibrium with the magma asolivine structure will result in olivine with a composition that crystallization proceeds. This results in solid phases ofis poorer in Fo than the original cumulus material. Barnes uniform composition, and is referred to as equilibriumreferred to this effect as “trapped silicate liquid shift”. Discrete crystallization. Alternatively, the above process can continue,and poikilitic olivine crystals are inevitably subjected to such until, at some stage, all the existing solids are removed at once,compositional modification, resulting in lower Fo contents and the magma then continues crystallizing out of contact withthan their original compositions. the early formed material. This is referred to as bulk When olivine is immersed in a sulfide liquid, the crystal and equilibration. Phenocrysts that are presented in a magma as itliquid will exchange Ni and Fe according to the following intrudes, and which are then settled out once flow stops beforereaction: reaching the surface, are also an example of bulk NiOolivine + FeSsulfide = NiSsulfide + FeOolivine (1) equilibration. The exchange partition coefficient KD for this reaction is Perfect fractional crystallization occurs if crystals aredefined as: removed from a magma as soon as they form. This gives rise KD = (NiS/FeS)sulfide / (NiO/FeO)olivine (2) to the maximum change in the composition of the remaining where, the concentrations are expressed in mole fractions. magma. In intrusive rocks, some magma will remain trappedKD is related to the equilibrium constant, K, by the expression: between accumulating crystal phases. This has two results (1)
Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185the fractional crystallization of the main body of magma is initial liquids, respectively, F is the proportion of remainingless extreme than that due to perfect fractional crystallization, liquid, and D is the bulk partition coefficient of Ni between alland (2) the trapped liquid reacts with early forming phases, solid phases and the coexisting magma. When the proportionsmodifying their compositions. In some cases, a liquid that is of crystallizing phases change, new values of CO, F, and bulkout of equilibrium with the cumulus phases percolates through D must be computed and used in the new steps ofthe cumulus pile, perhaps because it is denser than the trapped fractionation. The proportions of crystallizing phases and theintercumulus liquid, reacting and modifying the composition partition coefficient of Ni between each of the crystallizingof the cumulates. phases and the magma are required for the calculation of the Crystallization of magma is clearly not a simple process. bulk DNi value. The DNi value between a solid and magma,Since this review article is concerned with identifying particularly between olivine and magma (see a recent reviewmagmatic systems in which chalcophile element depletion due by Bédard 2005), is related to the melt composition andto reaction with sulfide liquid has occurred, we have chosen to temperature. Li and his co-workers[2,3] suggested anmodel perfect fractional crystallization of silicates, since this olivine/melt DNi value of 7 for S-bearing basaltic magmaswill produce the maximum compositional change. If the based on the results of MORB samples. In komatiiticobserved change is greater than this, then one must look to magma, the olivine/melt DNi is between 3 and 5. Based onother factors such as segregation of immiscible sulfide liquid. the available experimental data, we believe that the DNi values There are two ways to model the variation of Fo in olivine of 1 and zero for clinopyroxene and plagioclase, respectively,during fractional crystallization, depending on whether or not are reasonable for basaltic systems.the full composition (i.e., all major element concentrations) of In situations where in the segregation of liquid sulfidethe parental magma is known. The compositions of associated occurs during silicate crystallization, the DNi value of 500lavas, dykes, and chilled marginal rocks may be used to between the sulfide liquid and magma from Peach andrepresent the parental magmas of associated intrusions. If only Mathez has been used by several people. The Ni content ofthe FeO/MgO ratio of the parental magma is known, then one the sulfide can be calculated by applying this coefficient to thecan use a numerical approach in which a very small fraction Ni content of the magma. The Fe removed as sulfide is then(generally 0.1 wt%) of magma is removed as olivine, subtracted as FeO from the FeO content of the magma, alongolivine+plagioclase, olivine+plagioclase+clinopyroxene, and with the FeO involved in the silicate phases removed at eachthe composition of the magma remaining after the subtraction step.is recalculated. The proportions of the silicate phases that are The “trapped liquid shift” can be modeled using theremoved at each step should be based on the cumulus proportions of silicate phases by equilibrium crystallization.mineralogy of the igneous bodies in question. Crystallization The proportions of silicate phases crystallized from theof plagioclase does not change the FeO/MgO ratio in the trapped liquid can be calculated using the MELTS program ofmagma, and therefore, does not affect the content of Fo in Ghiorso and Sack if all major components of the liquid areolivine. However, it affects the bulk solid/magma DNi value known. If only FeO and MgO in the liquid are known,that is required for modeling the fractionation of trace assumption has to be made for the proportions of silicateelements such as Ni. Crystallization of clinopyroxene affects phases in the liquid based on the available experimentalboth the bulk DNi value and the FeO/MgO ratio in the magma. results of similar systems. The amounts of “trapped liquid” inIn the case of the modeling of olivine and clinopyroxene cumulate rocks can be estimated using the concentrations offractionation, the (FeO/MgO)olivine/(FeO/MgO)magma ratio of incompatible trace elements from whole rock analyses.0.3 from Roeder and Emslie (1970) and the(FeO/MgO)clinopyroxene/(FeO/MgO)magma ratio of 0.26 are most 4 Examples from Voisey’s Bayappropriate for basaltic systems based on the availableexperimental data. The Voisey’s Bay deposit was discovered in 1994 by If all major components of the parental magma are known, prospectors working for Diamond Fields Resources Ltd. Itfractional crystallization of the magma can be simulated using was acquired by INCO Ltd. in August 1996 through theirthe MELTS program of Ghiorso and Sack. The updated subsidiary, Voisey’s Bay Nickel Company. Numerous articlesversion of the MELTS program is currently available online at describing the geology of the Voisey’s Bay deposit have beenhttp://melts.of mresearch.org. published[3,15–21]. The deposit occurs in and at the base of the In either case, after the relation between the Fo content of bodies of 1.334 Ga troctolite (Fig. 1) that occur close to theolivine and the degree of crystallization is obtained from the 1.85 Ga collisional boundary between Archean Nain Provinceabove calculations, the fractionation of Ni can be calculated to the east and the Proterozoic Churchill Province to the west.using the following equation for trace elements: The most current understanding of the genesis is that mafic CL = CO F(D-1), (4) magma rose to intrude sulfide- and graphite-bearing pelitic where, CL and CO are the concentrations in the residual and gneisses. During its ascent, it reacted with mid-crustal rocks
Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185and developed a mid-crustal trace element and isotopic km-long sheet-like conduit to spread out as a higher levelsignature. Further fractionation and reaction occurred at the intrusion. The fresh, undepleted magma upgraded the sulfideslevel of the sulfidic-graphitic gneisses, peridotitic and that had lodged in the conduit and at the mouth of the conduitmela-troctolitic (olivine+intercumulus plagioclase) cumulates with the base of the higher level intrusion. It also disrupteddeveloped, an additional trace element signature related to the cumulates within the lower intrusion, giving rise to theenclosing gneisses was imposed on the magma, and sulfides peridotitic and mela-troctolitic inclusions, which are thesegregated, depleting the magma in chalcophile elements. A record of this early phase of crystallization. The bulk of thesecond wave of fresh magma entered the system, forcing the troctolite that has resulted from the second wave of magma isdepleted magma, along with its contained sulfides, upward a 1 a plagioclase-olivine cumulate. Fig. 1 A 3D view of the Voisey’s Bay Ni-Cu-Co sulfide deposit in Labrador, Canada Plots of Ni versus Fo content in olivine are shown in Fig. Deeps chamber and the feeder do not follow the same model2A and 3B. Olivines in the melatroctolite inclusions observed fractionation curve but fall below it. Their behavior isin the feeder show a steep trend of decreasing Ni content with explicable if the magma underwent a period fractionationdecreasing Fo, in which Ni decreases from 3100 ppm at about involving the removal of sulfide and silicate in a ratio of 1:133Fo 80 to 1500 ppm at Fo 72 (Fig. 2A). Olivines in the (red line in Fig. 2A), followed by further sulfide freeLeucotroctolite of the Reid Brook chamber, and some fractionation of the Ni-depleted magma (purple line in Fig.Troctolites and Olivine Gabbro of the Eastern Deeps chamber 2A). Li and Naldrett proposed that the interaction betweenand Feeder Olivine Gabbro show a more gentle trend with Ni the troctolite magma and the surrounding sulfide-bearingdecreasing from 1500 ppm at Fo 75 to as low as 200 ppm at Tasiuyak gneiss had caused this sulfide saturation. TheFo 41 (Fig. 2A). The model curves shown in the figure have sulfide/silicate ratio used in the model (red line in Fig. 2A) isbeen derived using the MELTS program for the fractional similar to the cotectic ratio calculated based on the results ofcrystallization of a mafic magma with composition similar to crystallization simulation and the empirical equation of sulfurthe average compositions of two high-MgO and low-TiO2 solubility at sulfide saturation by Li and Ripley. Anchilled marginal rock samples from the feeder dyke. example of calculating the cotectic ratio of sulfide/silicate The olivines of the melatroctolite inclusions follow the during sulfide segregation from sulfide-saturated magma ismodel curve for fractional crystallization of the assumed given in Barnes.initial magma. Olivines from sulfide-free troctolites from the Olivines in the Normal Troctolite and Varied-texturedEastern Deeps chamber and Leucotroctolites of the Reid Troctolite (Fig. 2B), both of these rock types contain minor toBrook chamber, along with olivine gabbros from the Eastern significant amounts of sulfide and show no systematic trend
Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185Fig. 2 Plot of nickel (Ni) content of olivine versus forsterite (Fo) content for (A) rocks related to theinitial introduction of magma; and for (B) rocks related to the later introduction(s) at Voisey’s BayThe field of Simkin and Smith is shown in (B) for comparison.
Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185but form a “cloud” with Fo varying between 73 and 52 and Ni Smith is not a satisfactory method of identifying chalcophilevarying between 2000 and 700 ppm. The cloud extends from depletion (Fig. 2B). Olivines from a single rock unit such asthe model curve for depleted magma up to and above the the troctolite of the Eastern Deeps chamber or thecurve for undepleted magma. Li and Naldrett showed that leucotroctolite of the Reid Brook chamber plot within andolivines that are in contact with sulfide react with the sulfide, below the field of Simkin and Smith. A simple comparisonexchanging Ni and Fe. Figure 3 illustrates the difference in the can lead to the conclusion that both Ni-undepleted andNi-Fo co-variation in olivines from the Normal and Varied- Ni-depleted magmas were involved in the formation of thesetextured Troctolites at Voisey’s Bay between those in contact rock units. However, the results of our numerical modelingwith sulfide and those that do not show such contact. Olivines have shown that only an Ni-depleted magma was involved.in contact with sulfide are distinctly poorer in Ni than thosethat occur as inclusions in plagioclase and are insulated from Acknowledgementssulfide. This is evident since at Voisey’s Bay, the reactionbetween sulfide and olivine has depleted the olivine in Ni, and It is our great pleasure to contribute this invited reviewtherefore, the Ni contents shown for the Normal and Varied-textured paper to Earch Science Frontiers, an important geologicalTroctolites at Voisey’s Bay are minimum values, whereas the journal in China. Professor Yu-sheng Zhai, Editor-in-Chief oforiginal contents were likely higher. Geologic relationships this journal, is a former supervisor of the senior author and aindicate that the Normal and Varied-textured Troctolites close friend of the second author since 1992. This is apostdate the other intrusive phases, thus providing evidence contribution to the Ministry of Education of China Projectfor the passage of a later, undepleted magma through the 111-B07011. Research in magmatic sulfide deposits at Indianasystem. University is currently funded by grants from the National Science Foundation of China (40534020) and from the National Science Foundation of the United States (EAR-0710910). References  Simkin T, Smith J V. Minor element distribution in olivine. Journal of Geology, 1970, 78: 304–325.  Li C, Ripley E M, Naldrett A J. Compositional variations of olivine and sulfur isotopes in the Noril’sk and Talnakh intrusions, Siberia: implications for ore forming processes in dynamic magma conduits. Economic Geology, 2003, 98: 69–86.  Naldrett A J, Li C. The Voisey’s Bay deposit, Labrador, Canada. In: Goodfellow W D (ed.), Mineral Resources of Canada: A Synthesis of Major Deposit-types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Fig. 3 The contrasting nickel (Ni) contents of olivine in contact Geological Survey of Canada and Geological Association of with sulfide and those that occur as inclusions in plagioclase and Canada, 2007, 387–408. are therefore out of contact with sulfide in the Voisey’s Bay  Roeder P L, Emslie R F. Olivine-liquid equilibrium. Intrusion Contributions to Mineralogy and Petrology, 1970, 29: 275–289.5 Concluding statement  Barnes S. The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. It has been seen that by the use of reasonable estimates of Contributions to Mineralogy and Petrology, 1986, 93:partition coefficients, and reasonable starting compositions for 524–531.magma, model curves of Ni versus Fo based on Rayleigh-type  Campbell F E, Roeder P P. The solubility of olivine andfractionation of olivine, plagioclase, and pyroxene in the pyroxene in the Ni-Mg-Si-O system. American Mineralogist,proportions observed in the cumulus mineralogy ± sulfide can 1968, 53: 257–258.account for the variations observed in mafic intrusions such as  Brenan J M. Effects of fO2, fS2 temperature, and meltthe Voisey’s Bay intrusion in Labrador. One lesson that composition on Fe-Ni exchange between olivine and sulfideemerges from our analysis is that the comparison of liquid: implications for natural olivine-sulfide assemblages.observational data with the field defined by Simkin and Geochimica et Cosmochimica Acta, 2003, 67: 2663–2681.
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