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Applications of stable and radiogenic isotopes to magmatic cu ni-pge deposits examples and cautions

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  • 1. EARTH SCIENCE FRONTIERSVolume 14, Issue 5, September 2007Online English edition of the Chinese language journalCite this article as: Earth Science Frontiers, 2007, 14(5):124–132. RESEARCH PAPERApplications of Stable and Radiogenic Isotopes toMagmatic Cu-Ni-PGE Deposits: Examples and CautionsEdward M. RIPLEY1,2,∗, Chusi LI1,21 Department of Geological Sciences, Indiana University, Bloomington, IN 47401, USA2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Abstract: Measurements of S, O, and radiogenic isotope ratios are all potentially powerful tracers of magma interaction with country rocks and the importance of assimilation processes in the genesis of magmatic Ni-Cu-PGE deposits. Sulfur isotope measurements of deposits such as those in the 1.1 Ga Duluth Complex, the Permo-Triassic intrusions of the Noril’sk area, and the 1.4 Ga Kabanga intrusions provide evidence for the derivation of S from both sulfide-and sulfate-bearing country rocks. The 1.3 Ga Voisey’s Bay deposit provides an example where δ34S values of the ores commonly fall within the accepted mantle range of 0 ± 2‰, but detailed studies of Proterozoic metasedimentary country rocks show that their weighted average δ34S value is also within this range. A thorough knowledge of the isotopic compositions of potential contaminants is essential for a proper evaluation of the role of country rock derived S in ore formation. When O and radiogenic isotopic measurements are employed as tracers to evaluate magma-country rock interaction, it is essential to provide evidence that open system processes have not perturbed the isotopic systematics. Low-temperature hydrothermal processes can mask evidence of high-temperature processes in the oxygen isotope system and in radiogenic systems such as Re/Os and Pb where involved elements may be mobile under hydrothermal conditions, or host phases may close to exchange and uptake at different temperatures. Careful petrographic observation and analyses of individual minerals may be required before the models involving the contamination of magmas by country rocks can be meaningfully applied. Key Words: S isotopes; O isotopes; radiogenic isotopes; magma-country rock interaction importance of S assimilation in promoting sulfide saturation in1 Introduction a mafic magma[6,7]. This review is intended to highlight some of the Isotopic analyses are a powerful tool for evaluating the fundamental applications of isotopic measurements to thepotential interaction between mantle-derived magmas and assessment of magma contamination as applied to the studiescrustal rocks. Both stable and radiogenic isotopic systems of magmatic ore genesis and cautionary notes that need to behave been applied to the studies of magma contamination andorigin. It has become apparent that the contamination of mafic considered to avoid potential pitfalls in the interpretation ofmagma by crust plays a significant role in the generation of isotopic data. Examples from well-known Cu-Ni-PGElarge, sulfide-rich magmatic Cu-Ni-PGE deposits[1,2]. deposits will be utilized to illustrate key concepts.Interaction between siliceous country rocks and mafic magmahas also been suggested as an important prerequisite for the 2 Sulfur isotopesdevelopment of chromite deposits in layered intrusions[3,4] andsulfide deposits in some intrusions[5]. Experimental studies Owing to the relatively large difference in δ34S valuesthat emphasize the role of pressure as a control of the between mantle-derived magma (MORB as the principalsolubility of sulfur in mafic magmas have also stressed the example) and either sulfide or sulfate minerals in sedimentaryReceived date: 2007-08-20; Accepted date: 2007-09-11.* Corresponding author. E-mail: ripley@indiana.eduFoundation item: Supported by the Ministry of Education of China project (111-B07011); the National Natural Science Foundation of China (40534020); theNational Science Foundation of the United States (EAR-0710910).Copyright © 2007, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.
  • 2. Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132rocks, sulfur isotopic studies have provided important insights Tanzania[17]. Mica schists and graphitic schists containinto the processes of magma contamination. Mantle sulfur is pyrrhotite with δ34S values ranging from -2‰ to 24‰. Sulfidecommonly taken as having δ34S values between -2‰ and 2‰. mineralization in mafic to ultramafic intrusive rocks isThis range is defined largely from the analyses of sulfide characterized by a range of δ34S values between 10‰ andminerals in meteorites and sulfide-bearing basalts (see 24‰, again lending strong evidence to a genetic model thatRipley[8] for a review of early S isotope studies). It should involves magma contamination by country rocks.always be kept in mind, however, that sulfide minerals indiamonds from kimberlites are characterized by a wide rangeof δ34S values from -11‰ to +14‰[9,10], suggesting that thedeep mantle may be far more heterogeneous in terms of δ34Svalues than the shallow mantle appears to be. Seawater δ34Shas varied between ~10‰ and 30‰ from the Neoproterozoicto the present[11,12]. Sulfide minerals are produced fromseawater sulfate owing to reduction that is often related to thelife cycles of sulfate-reducing bacteria. Depending on thevariables related primarily to the rate of sulfate reduction, thesulfide produced may be characterized by positive or negativeδ34S values. Pyrite that forms owing to the reaction ofproduced sulfide and reactive iron may show a wide range inδ34S values, typically from values close to the source sulfate(e.g. ~ 20‰), to values as low as -50‰, or occasionally evenless. Sulfate minerals that form via evaporative processesshow little isotopic fractionation relative to the source sulfate,and hence are generally characterized by δ34S values between~10‰ and 30‰. It is clear that because of the potentially large S isotopicdifference between sedimentary S-bearing minerals andmantle-derived magmas, assimilation processes may beelucidated using S isotope measurements. Three examples willserve to illustrate the basic principles.2.1 Duluth Complex, Minnesota Cu-Ni sulfide mineralization in the 1.1 Ga MidcontinentRift-related Duluth Complex represents one of the best-knownexamples of major sulfur derivation from country rocks. TheProterozoic (~1.85 Ga) Virginia Formation is a peliticsequence that contains layers that are sulfidic andcarbonaceous, and are characterized by pyrite δ34S valuesranging from 0 to 30‰ (Fig. 1)[13–15]. Metamorphism of thesulfide-bearing sedimentary rocks has led to the production ofpyrrhotite and the generation of an H2S-bearing, carbonic fluid.Magma that formed the Duluth Complex interacted with –15]various levels of the sedimentary sequence, assimilated the Fig. 1 Sulfur isotope data[13 from sulfide mineralization in thesedimentary-derived H2S, and crystallized the sulfide minerals Duluth Complex and Virginia Formation country rocksthat were characterized by elevated δ34S values ranging from~0 to 20‰ (Fig. 1). Various mixing models involving external 2.2 Noril’sk, SiberiaS and mantle S can be derived[16], but the importance ofexternally derived S is highlighted by the δ34S values and the Another well-known example where δ34S values of sulfideinternally consistent models of fluid production in the ores strongly suggest that assimilation has been operative ismetamorphic rocks and localized partial melting. the world famous mineralization at Noril’sk, in the A similar geologic setting in terms of mafic intrusions Permo-Triassic Siberian flood basalt province. Early sulfurfound hosted by pelitic and graphitic metasedimentary rocks isotope studies[18–20] suggested that assimilation of evaporiteshas been described for the large 1.4 Ga Kabanga Ni-deposit in that are common in the stratigraphic sequence occurred, or
  • 3. Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132that sulfur may have been derived from pyritic shales and envisioned, or hydrothermal leaching of sulfate followed bycoals, or H2S derived from the coals. Li and his partial reduction, or a process involving the dissolution, ratherco-workers[21,22] developed slightly different models to explain than melting, of sulfate in mafic magma. The observation ofthe anomalous S isotopic signatures (Fig. 2); however, the abundant wollastonite in contact aureole rocks suggests thatprobable involvement of sulfate as a contaminant is reactions similar to CaSO4 + SiO2 + H2O = CaSiO3 + H2S +noteworthy. The assimilation of sulfate via partial melting by a 2O2 occurred, and that sulfate may have been reduced tobasatic magma is difficult to conceive owing to the high sulfide prior to incorporation into the magma. The importantmelting points of sulfate minerals (typically ranging between point here is that sulfate minerals may serve as S sources, but1360 and 1450°C). Therefore, either a fluxing mechanism is the mechanism of S assimilation must be carefully evaluated. Fig. 2 Sulfur isotope data[21,22] from the Noril’sk area process, and positive values indicating a relatively rapid rate2.3 Voisey’s Bay of sulfate reduction, possibly under closed system conditions. A third example where the interpretation of δ34S values is The weighted average δ34S value of the sulfide assemblages incritical is the 1.3 Ga Voisey’s Bay Ni-Co-Cu deposit in the Tasiuyak Gneiss is -1.3‰. This value is very similar to theLabrador, Canada. The deposit is associated with the Nain average value of the sulfide ores and illustrates a veryPlutonic Suite, which was emplaced along a boundary important principle. It is important to precisely know the δ34Sbetween crustal blocks, one comprised predominantly of values of the proposed contaminant before concluding. In theArchean rocks and the other dominated by Proterozoic rocks. case of Voisey’s Bay, even though the δ34S values are close toSulfide mineralization is found in troctolitic to gabbroic rocks. 0‰, the additional study clearly showed that S could haveCountry rocks include the Proterozoic Tasiuyak Gneiss, which been derived from the country rocks, in agreement with theis locally sulfidic and graphitic. Several investigators[23,24] widespread geological evidence, which suggests that Sproposed that large amounts of S in the mineralization may assimilation occurred.have been derived from the Tasiuyak Gneiss. Initial S isotopic Voisey’s Bay is also an excellent example where supportingstudies[25] determined that the bulk of the sulfide radiogenic isotopic data was extremely valuable. The Re/Osmineralization was characterized by δ34S values ranging isotopic data (see below)[27] clearly indicated that the magmabetween 2‰ and -4‰ (Fig. 3). The δ34S values in the sulfide from which the sulfides were derived assimilated crustalores were significantly depleted only locally near the contact material. Carbon isotopic data also indicated that assimilationwith the Tasiuyak Gneiss, which contained pyrrhotite with of sedimentary country rocks had occurred[26], as did the S/Senegative δ34S values. However, additional studies[26] ratios of the sulfide mineralization. The supporting datadetermined that pyrrhotite in the Tasiuyak Gneiss was strengthens the promise that even though the δ34S values ofcharacterized by a wide range in δ34S values, including both the ores are close to those of mantle sulfides, a largenegative values suggesting an open system sulfate reduction component of the S was derived from country rocks.
  • 4. Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132 derived in part from underlying sedimentary rocks. The anomalous isotopic signature illustrated in the δ34S-δ33S plot strongly suggests that the interpretation was correct. Fig. 4 δ33S versus δ34S for sulfide samples from the Stillwater Complex, Voisey’s Bay, and Alexo IAEA S-1 is an international standard. Note the anomalous [26]Fig. 3 Sulfur isotope data from sulfide mineralization at character of the Alexo samples, strongly supporting the premise Voisey’s Bay and sulfide minerals in the Tasiuyak Gneiss that sulfur from Archean-aged metasedimentary country rocks has country rocks been involved in ore genesis. A final point with respect to the use of S isotopes as 3 Oxygen isotopesindicators of crustal contamination regards Archean-agedsulfide deposits. Ripley[8] discussed that during the Archean Mantle-derived mafic magmas are generally characterizedbacterial sulfate reduction, then if operative, appears to have by δ18O values in the range of 5‰ to 7‰[31–33]. Sedimentarygenerated considerably smaller ranges in δ34S values of rocks and crustally derived granitic rocks are normallyreduced sulfur species, and values close to 0‰. For this characterized by considerably higher δ18O values, typically inreason, the determination of externally derived S in Archean the range of 8‰ to 30‰. Owing to these large differences,deposits, or in younger deposits that may have interacted with oxygen isotopes may also be sensitive indications of countryArchean rocks, may be difficult using δ34S values. However, a rock assimilation. General features of how oxygen isotopesprocess known as mass-independent fractionation (MIF) of S may be applied in the study of magmatic ore deposits haveisotopes has led to variations between δ33S and δ34S, which are been discussed by Ripley[8] and will not be repeated here.distinct from most terrestrial processes that fractionate S However, common difficulties with a straight-forwardisotopes and are referred to as mass-dependent fractionation application of δ18O values to problems of assimilation byprocesses. Farguhar and Wing[28] discussed the observation mafic magmas must be highlighted.that sedimentary pyrite of Archean age is characterized by It is often assumed that elevated δ18O values in maficδ33S-δ34S relations that are indicative of MIF. The process has igneous rocks (e.g. >7‰) are indicative of contamination andbeen attributed to low oxygen levels in the Archean assimilation of high-18O country rocks. Although this mayatmosphere and photochemical reactions involving SO2, but indeed be the case, careful petrographic examination of thethe controversy with respect to the origin of the process rocks, along with an evaluation of the magnitude of the δ18Ocontinues. The record of MIF may be present in magmas that shift, should be undertaken. For example, consider a mafichave assimilated S of the Archean age[29,30], and future studies rock with bulk δ18O value of 9‰, a contaminant with a δ18Ousing the technique may be able to distinguish the importance value of 16‰, and a presumed magma initial value of 5.5‰.of S derived from Archean rocks in ore genesis. Figure 4 is a Assuming that the concentrations of O in the contaminant andδ33S-δ34S plot showing sulfide samples from Voisey’s Bag (1.3 the magma are similar, a value of 9‰ in the mafic rock will beGa) and Alexo (a komatiite-associated, ~2.7 Ga deposit). suggested in ~35‰ bulk contamination. This degree ofNaldrett[1] proposed that S in the Alexo deposit may have been contamination is unreasonable in most situations because of
  • 5. Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132the amount of heat required to achieve such a large geosciences. Several excellent text books and review articlescontaminant/magma ratio[34]. Unless xenoliths provide address these methodologies[41–44]. Here we wish to emphasizeevidence for such high degrees of contamination (the presence that care must be taken to assure that closed-system conditionsof refractory oxides such as corundum and spinel), other prevailed before conclusions regarding contamination historymethods that can explain the 18O-elevation must be explored. can be made. For example, the Re/Os isotopic data fromObviously knowledge of the δ18O values of potential sulfide minerals in the Voisey’s Bay deposit[27] mentionedcontaminants is extremely important. The assimilation of very above produces an isochron, which is consistent with the agehigh-18O siliceous rocks, carbonates, or hydrothermally altered determined via U-Pb geochronology Fig. 5. The elevatedfelsic rocks can be responsible for elevated δ18O values in initial 187Os/188Os ratio clearly indicates that crustalmafic igneous rocks and can satisfy reasonable thermal contamination occurred, the sulfide system was well-mixed,constraints. and that closed system conditions have generally prevailed. In The most common cause of elevated δ18O values in mafic contrast, in situations where a reasonable isochron is notrocks that is often overlooked is low-temperature produced, an interpretation involving crustal contaminationhydrothermal alteration and isotopic exchange. Low- must proceed with caution. Figure 6 illustrates a case where antemperature isotopic exchange involving a fluid has been initially chondritic system interacts with a fluid afterdiscussed by several investigators[35–37]; one pertinent example crystallization. Loss of rhenium during the interaction maywill be presented here. At temperatures below ~200°C, the significantly lower the 187Re/188Os ratio. Continued decay mayoxygen isotopic fractionation between plagioclase and water is then produce samples with 187Os/188Os ratios that will generategreater than 6‰[38]. For a situation where the fluid-rock anomalous γOs values, assuming that an age for the rocks isinteraction occurs below 200°C with a large excess of fluid known from a system such as U/Pb. The elevated γOs valueswith a δ18O value of, say 0‰, a plagioclase δ18O value in in this case will not be a signal of crustal contamination, butexcess of 6‰ can result. Alteration minerals that may form at will have been produced as a result of the interaction with alow temperatures (clays, zeolites, oxides, carbonates) are also hydrothermal fluid. A scattered population of Re/Os datacharacterized by large mineral-water fractionation factors, and points may offer little insight into the contamination processestherefore, elevated whole rock δ18O values will be expected, if an age is unavailable from other isotopic systems. Even ifeven if the fluid involved in alteration and isotopic exchange an age is available, the potential effects of hydrothermalwas relatively low-18O meteoric water. Elevated δ18O values processes must be carefully evaluated.should be evaluated in concert with trace elements and otherisotopic systems, but often careful petrographic examination issufficient to detect the involvement of hydrothermal fluids.The measurement of mineral separates is also encouraged.Where contaminated magma is envisioned, all primaryminerals should be characterized by anomalous δ18O values.Owing to the kinetic exchanged effects at low temperature, allminerals may not illustrate the same extent of exchange withthe hydrothermal fluid. Large degrees of fractional melting of country rocks maylead to the development of small xenoliths in mafic rocks thatcontain refractory minerals such as corundum and spinel[39,40].In this case, elevated and widespread δ18O values in maficrocks may be related to unusually high degrees of assimilation.However, such degrees of assimilation are frequently relatedto repeated magma passage in a conduct system, and no singlemagma pulse attains an anomalously high δ18O value.Elevated δ18O values may also be related to diffusive oxygenisotopic exchange near high-18O xenoliths, but such exchange Fig. 5 Re-Os isotopic data[27] from sulfide minerals in thezones tend to be confined to contacts with xenoliths and are Voisey’s Bay deposit The sulfide samples define a very reasonable isochron and thenot spatially widespread[8]. initial ratio strongly indicates that assimilation of radiogenic crust has occurred.4 Radiogenic isotope systems Recent studies of the Bushveld Complex by Mathez and Radiogenic isotopes and trace elements have been the coworkers[45,46] have shown the importance of analyzingstandard methods of assessing contamination problems in the individual minerals. These suggest that the variability in Pb
  • 6. Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132isotopic ratios result from the introduction of at least one other minerals are open. When whole rocks are analyzed, thefluid characterized by a different Pb isotopic composition at a isotopic signal is that of the mixed populations andtime when feldspars are closed to Pb addition but sulfide meaningful interpretations of the data may be impossible. Fig. 6 Theoretical example of the open system behavior for a sulfide deposit formed at 1.1 Ga with a chondritic Os isotopic ratio Hydrothermal perturbation to the system during decay since 1.1 Ga may lead to a lowering of 187 Re/188Os ratios. In this example, hydrothermal alteration at 0.25 Ga has led to a low 187 Re/188Os ratio and a γOs value, if computed for 1.1 Ga, which will be highly anomalous. An interpretation of crustal assimilation at 1.1 Ga will be erroneous; isochronous samples similar to those at Voisey’s Bay (Fig. 5) will be required to substantiate such an interpretation. A similar situation has been described by Griffin and his evaluation of possible perturbations to isotopic systems as aco-workers[47,48], who noted the presence of several distinct result of hydrothermal alteration and exchange should beRe/Os populations associated with different generations of undertaken before an interpretation involving magmasulfide minerals in mantle peridotites. A whole rock approach contamination by country rocks is invoked.in this case renders data that are not only difficult to interpret,but essentially meaningless. Technological advances now Acknowledgementspermit the determination of isotopic ratios with high precisionin individual minerals using either laser ablation or ion This invited review paper is a contribution to the Ministrymicroprobe methods. When spot analyses are not practical, of Education of China Project 111-B07011 awarded to Chinamineral separates must be utilized if possible. University of Geosciences (Beijing) where the junior author is appointed as a Concurrent Professor. The senior author is5 Conclusions proud to be a Guest Professor at Lanzhou University. Research in magmatic deposits at Indiana University is Both stable and radiogenic isotopic measurements can currently funded by grants from the National Scienceprovide significant information in the evaluation of country Foundation of China (40534020) and from the Nationalrock contamination in the genesis of magmatic Cu-Ni-PGE Science Foundation of the United States (EAR-0710910).deposits. Sulfur isotopes provide direct evidence for theinvolvement of externally derived S in ore genesis. Even in Referencessystems where the δ34S values are close to the MORB-mantlevalue of 0 ± 2‰, care must be taken to ensure that potential [1] Naldrett A J. The rule of sulphurization in the genesis ofcontaminants are not characterized by δ34S values close to 0‰ iron-nickel sulphide deposits of the Porcupine District, Ontario.before dismissing processes of country rock assimilation. Can. Inst. Min. Bull., 1966, 4: 489−497.Oxygen and radiogenic isotopes are also powerful tracers of [2] Li C, Naldrett A J, Ripely E M. Critical factors for themagma-country rock interaction but of the interaction of mafic formation of a nickel-copper deposit in an evolved magmaigneous rocks with low-temperature processes. Careful system: lessons from a comparison of the Pants Lake and
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