The Xinyuan intrusion of the Eastern Junggar terrane is one of the mafic–ultramafic intrusions located in the Northern Xinjiang region and is associated with the southern part of the Central Asian Orogenic Belt (CAOB). Based on the secondary ion mass spectrometry (SIMS) zircon U–Pb dating, the intrusion is 326.2 ± 1.1 Ma old. Positive zircon ɛHf(t) values (+12.4 to +15.7) and mantle-like δ18O values (5.0–5.5‰) suggest that the parental magma was derived from a depleted mantle source with a low degree of crustal contamination. Significant negative Nb–Ta anomaly and large iron lithophile element (LILE) enrichment of the Xinyuan intrusion are identical to those of the coeval basalt–andesite rocks in the study area. Temporal, spatial, and geochemical evidence suggest that the Xinyuan intrusion and coeval basalt–andesite rocks are associated with the northward subduction of the Kelameili Ocean. This, together with the ages of alkaline granites and sedimentary record in the Eastern Junggar, suggests that closure of the Kelameili Ocean was probably between 326 Ma and 310 Ma. Fractional crystallization modelling indicates that the parental magma of the Xinyuan intrusion has experienced significant fractional crystallization at depth. The Xinyuan parental magma was estimated to contain ~20 ppm Ni, which is significantly lower than the coeval basaltic magma (~200 ppm). Magma that is depleted in Ni has potentially undergone extensive fractional crystallization, during which most of the Ni has been sequestered into olivine in the early stages. The Xinyuan intrusion has Hf isotope and trace element characteristics similar to the Permian sulphide-bearing mafic–ultramafic intrusions. However, the low degree of crustal contamination apparent in δ18O data is more characteristic for the pre-Permian sulphide-poor intrusions in the area. Extensive fractional crystallization and the low degree of crustal contamination in the parental magma of Xinyuan intrusion experienced are suggestive of a low-magmatic Ni–Cu mineralization potential.
Small intrusions dominated by olivine- and pyroxene-rich cumulates are well known to be favourable hosts to magmatic Ni-Cu-(Platinum Group Element - PGE) sulfide mineralization. Such intrusions are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides; these tend to be of conduit or chonolith style. If prospectivity could be discriminated from sparse sampling at early exploration stages, then the discovery rate for deposits of this type could be improved. To this end, a number of pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits have been investigated including the Noril'sk-Talnakh camp in Siberia, the Kotalahti nickel belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen of Australia, Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, samples from unmineralised intrusions in four of these regions were also investigated along with four mafic intrusions from other localities that are not associated with any known economic sulfide mineralisation. Using fine-scale (<5 μm/pixel) chemical imaging on the Australian Synchrotron, complex zoning in chromium was found in cumulate and poikilitic pyroxenes within the strongly mineralised intrusions. The zoning patterns can be separated into three distinct types: 1) abrupt zoning: a single change in trace element concentration with a sharp boundary; 2) sector zoning: hourglass style zonation; and 3) oscillatory zoning: small scale oscillations that are usually cyclic. Zoning of all three types can be present in a single grain. The presence of cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. This combination of zoning patterns is postulated to be an indication of high magma flux and fluctuating cooling rates that accompany wall rock assimilation in dynamic conduits where sulphide liquid forms and accumulates. The distinctive zoning patterns reported here can, in many cases, be easily imaged using desktop microbeam XRF mapping techniques and may provide a useful fertility indicator for the exploration of new magmatic Ni-Cu-(PGE) deposits.
Abstract The Nova-Bollinger Ni-Cu-platinum group element (PGE) deposit in the Fraser zone of the Albany-Fraser orogen consists of two main orebodies, Nova and Bollinger, hosted by the same tube-shaped intrusion but having distinctly different Ni tenors of around 6.5 and 4.8 wt %, respectively. Nova is also higher in Pd, but Cu and Pt tenors are similar. Both deposits have very low PGE tenors, with average Pd concentrations of 110 ppb in massive sulfide at Bollinger and 136 ppb at Nova. The Nova and Bollinger orebodies show relatively little internal differentiation overall on deposit scale but show strong differentiation into chalcopyrite-rich and chalcopyrite-poor regions at a meter scale. This differentiation is more prevalent at Nova, where massive sulfide-filled vein arrays are more extensively developed, and in massive ores, particularly veins, than in net-textured ores. Net-textured and disseminated ores have on average Ni and Cu grades and tenors similar to those of massive, semimassive, and breccia ores in the same orebody but a smaller range of variation, largely due to a more limited extent of sulfide liquid fractionation and higher average concentrations of Pt and Pd than adjacent massive ores. Unusually for differentiated magmatic sulfides, there is no systematic positive correlation between Pt, Pd, and Cu. A partial explanation for the lack of a Pd-Cu correlation is that Pd was partitioned into peritectic pentlandite in the middle stages of sulfide liquid solidification. This explanation is not applicable to Pt, as Pt characteristically forms its own phases rather than residing in base metal sulfides. PGE tenors are very low in both orebodies, very similar to those observed in other Ni-Cu-Co sulfide ores in orogenic settings, notably the Savannah and Savannah North orebodies. This depletion is attributed to sulfide retention in the mantle source of the parent magmas rather than to previous fractional extraction of sulfide liquid in staging chambers or feeder networks. The higher Ni and Pd tenors at Nova are attributed to reworking and upgrading of precursor sulfide liquid originally deposited upstream at the Bollinger site. Replicate analyses of multiple jaw-crusher splits returned highly variable Pt and Au assays but much smaller relative errors in the other PGEs. The poor Pt and Au reproducibilities are attributed to nugget effects, explicable by much of the Pt and Au in the samples being present in sparse Pt- and Au-rich grains. This is principally true for Pt in massive rather than disseminated ores, accounting for a strong contrast in the distribution of Pt/Pd ratios between the two ore types. Numerical simulation suggests that Pt is predominantly resident in Pt-rich platinum group minerals with grain diameters of 100 µm or more and that at the low (<100 ppb) concentrations in these ores, this results in most assays significantly underreporting Pt. This is likely to be true in other low-PGE ores, such that apparent negative Pt anomalies in massive ores may in such cases be attributable to sampling artifacts.
Abstract Nickel contents of olivine have been widely used as petrogenetic indicators and as fertility indicators for magmatic sulfide potential of mafic-ultramafic intrusions, on the assumption that olivines crystallized from magmas that had equilibrated with sulfide liquid should be relatively depleted in Ni compared with a sulfide-free baseline. This has given rise to a large accumulation of data that is brought together here, along with data on volcanic olivines, to critically evaluate the effectiveness of the approach. We identify multiple sources of variance in Ni content of olivine at a given Fo content, including variability in mantle melt composition due to depth, water content (and possibly source), subsequent fractional crystallization with and without sulfide, recharge and magma mixing, batch equilibration between olivine and sulfide at variable silicate-sulfide ratio (R), and olivine/liquid ratio; and subsequent equilibration during trapped liquid crystallization in orthocumulates. Baselines for Ni in olivine in relation to Fo content are somewhat lower in orogenic belt settings relative to intrusions in continental large igneous provinces (LIPs). This is probably related to differences in initial parent magma compositions, with plume magmas generally forming deeper and at higher temperatures. No clear, universal discrimination is evident in Ni in olivine between ore-bearing, weakly mineralized, and barren intrusions, even when tectonic setting is taken into account. However, sulfide-related signals can be identified at the intrusion scale in many cases. Low-R factor and low-tenor sulfides are associated with low-Ni olivines in several examples, and these cases stand out clearly. Anomalously high-Ni olivines are a feature of some mineralized intrusions, in part due to trapped liquid reaction effects. However, in some cases, this mechanism cannot account for the magnitude of enrichment. In these cases, enrichment may be due to re-entrainment of “primitive” Ni-rich sulfide by a more evolved Fe-rich magma, driving the olivine to become Ni-enriched due to Fe-Ni exchange reaction between sulfide and olivine during transport. An extreme case of this process may account for ultra-Ni enriched olivine at Kevitsa (Finland), but more subtle signals elsewhere could be positive indicators. A lack of clear mineralized/barren distinction in specific groups of related intrusions, e.g., the deposits of NW China or the Kotalahti Belt in Finland, may well be due to “false negatives” where undiscovered mineralization exists in specific intrusions or in their feeder systems, or may also be due to a multiplicity of confounding factors. Wide variability of both Fo and Ni between related intrusions at regional scale may be a useful regional prospectivity indicator, more than an intrusion-scale discriminant, and is certainly informative as a petrogenetic indicator. In general, the use of Ni-olivine as a fertility tool is more likely to generate false negatives than false positives, but both are possible, and the technique should be used as part of a broader weight-of-evidence approach.
Research Article| August 01, 2018 S, O, and Re-Os Isotope Studies of the Tamarack Igneous Complex: Melt-Rock Interaction During the Early Stage of Midcontinent Rift Development V. Taranovic; V. Taranovic 1 Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana 47405 †Corresponding author: e-mail, valentina.taranovic@csiro.au Search for other works by this author on: GSW Google Scholar E. M. Ripley; E. M. Ripley 1 Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana 47405 Search for other works by this author on: GSW Google Scholar C. Li; C. Li 1 Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana 47405 Search for other works by this author on: GSW Google Scholar S. B. Shirey S. B. Shirey 2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015 Search for other works by this author on: GSW Google Scholar Economic Geology (2018) 113 (5): 1161–1179. https://doi.org/10.5382/econgeo.2018.4585 Article history accepted: 22 Apr 2018 first online: 31 Jul 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation V. Taranovic, E. M. Ripley, C. Li, S. B. Shirey; S, O, and Re-Os Isotope Studies of the Tamarack Igneous Complex: Melt-Rock Interaction During the Early Stage of Midcontinent Rift Development. Economic Geology 2018;; 113 (5): 1161–1179. doi: https://doi.org/10.5382/econgeo.2018.4585 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Tamarack Intrusive Complex (1105 ± 1.2 Ma), in northeastern Minnesota, occurs within the Midcontinent rift system and hosts potentially economic Ni-Cu-(PGE) mineralization. The system represents "conduit-style" mineralization and with 1.3 wt % Ni, 0.7 wt % Cu, 0.3 ppm Pt, and 0.25 ppm Pd, is similar in many aspects to the Eagle deposit in Michigan. Sulfur, O, and Os isotopes have been used to evaluate the role of crustal contamination in promoting sulfide liquid saturation. All of the types of mineralization in the Tamarack Intrusive Complex are characterized by δ34S values between –0.2 and 2.8‰, values that are not strongly anomalous relative to uncontaminated mantle values near 0‰. The values are very similar to those from the Eagle deposit, but contrast sharply with values of disseminated sulfides in intrusions of the Duluth Complex and Crystal Lake Gabbro, which may be as elevated as 17‰. Initial 187Os/188Os ratios in the Tamarack Intrusive Complex are between 4 and 44% higher than the same ratio of the undepleted primitive mantle at 1105 Ma and correspond to gamma Os values for all magmatic sulfide types from the Tamarack Intrusive Complex ranging from 10 to 92. These values are consistent with crustal contamination but for S, the isotopic ratios are remarkably lower than those from mineralization in the Duluth Complex, where initial 187Os/188Os ratios are more than 110% higher than that of primitive mantle and γOs values may be in excess of 1,000. Olivine from an unmineralized but sparsely serpentinized portion of the Tamarack Intrusive Complex has O isotope compositions from 5.2 to 5.5‰, indicating a fraction of a percent crustal contamination of the parental magma.The striking contrast between Os-S isotope systematics in conduit-type deposits associated with the intrusions that formed during the Main stage in the history of the rift is a primary characteristic of these ore systems and as such is important to explain by a geologic model. High Re and Os contents are found in sulfide minerals and organic matter in the sedimentary country rocks. Selective contamination via partial melting and devolatilization involving these phases could have been responsible for S and Re-Os transfer from country rocks to the sulfides without affecting the silicate magma. Therefore, the contamination of the system may have been an order of magnitude greater than that recorded by the silicate portion of the system. Alternatively, the rather low degrees of crustal contamination indicated by the Re-Os and S systems in the Eagle intrusion and the Tamarack Intrusive Complex may also be related to isotopic exchange between contaminated and pristine magmas in the conduit system during the Early stage of the rift development. The high Ni grades of the complex are in part related to the dynamic conduit environment and contamination of picritic magmas promoting sulfide saturation before large quantities of Ni were sequestered by olivine. You do not currently have access to this article.
The C zone, located in Bannockburn Township approximately 30 km W of Matachewan, Ontario, is one of several komatiite-associated Ni-Cu-(PGE) deposits within the 2710 to 2704 Ma Tisdale volcanic episode of the Abitibi greenstone belt. The sulfides, host rocks, and country rocks are superbly exposed over a strike length of 150 m in glacially polished and hydraulically stripped outcrops and have been intersected in 53 diamond drill holes. The mineralized zone is up to 2.5 m thick and comprises (from base to top) massive to semimassive, net-textured, and disseminated sulfide facies characterized by a pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage. The massive sulfide zone contains an anastomosing network of dextral and sinistral shears that are oriented broadly subparallel to the contacts with the footwall dacite and hanging-wall andesite. Some contacts with underlying dacites are sheared, but others are scalloped and bordered by skeletal-euhedral Fe-rich chromites and appear to be primary magmatic features. The footwall rocks grade from chloritized dacites within 20 to 30 cm of the contact into massive and brecciated plagioclase-phyric dacites farther away from the contact. The host unit is up to 8 m thick and comprises massive olivine porphyritic and ortho- to mesocumulate komatiite in the eastern and central parts and a texturally heterolithic komatiite breccia in the western part. The breccia is composed of subrounded to subangular clasts 1 to 30 cm in length that exhibit mainly fine ( 80 m). The komatiitic sequence at the C zone is much thinner than most other sequences within the Tisdale volcanic episode and is oriented oblique to the NW trend of the regional stratigraphy, but all contacts appear conformable and all younging indicators are uniformly to the SSE, suggesting that the local sequence is intact. The C zone is similar in many respects to other type I (Kambalda-type) komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt but differs by not being confined within a well-developed footwall embayment and in being partly hosted by komatiitic breccias. The C zone Ni-Cu-(PGE) mineralization appears to be hosted in the eastern part of the stripped exposure by a thin lava pathway and in the western part by breccias formed via lateral breakout and roof collapse.
Abstract The Nova-Bollinger Ni-Cu sulfide deposit is associated with a small chonolith (tube-shaped) intrusion emplaced at lower crustal depths into granulite facies migmatite gneisses. The deposit comprises disseminated and net-textured ores within the intrusions and a high proportion of massive, semimassive, and breccia exocontact ores within the underlying country rocks. Internally disposed endocontact ores show typical magmatic textures including conventional net texture, leopard net texture characterized by the presence of centimeter-sized clots of olivine and intercumulus phases, and globular ores. Some of the globular ores show an association of sulfide blebs with clinopyroxene-carbonate intergrowths that may represent infilling of original CO2-rich vapor bubbles. The exocontact ores have an assemblage of textures indicative of emplacement into hot, soft country rocks at a large-scale melting-infiltration front. Characteristic features range from hard-walled extensional vein arrays to complex infiltrations of disseminated sulfide within chaotically folded paragneiss. Sulfide infiltration was accompanied by partial melting of the country rock, producing felsic leucosomes, some of them strongly enriched in garnet, mainly occupying vein walls and interpreted as the result of counterflow of displaced silicate partial melt. Coarse-grained pentlandite-chalcopyrite-pyrrhotite loop textures are characteristic of all ore types, down to the scale of the infiltrating sulfides within the gneisses, and are regarded as diagnostically magmatic textures generated by sulfide liquid fractionation and growth of high-temperature pentlandite by peritectic reaction between fractionated sulfide melt and early crystallized monosulfide solid solution. The highly distinctive features of the Nova-Bollinger ores are a consequence of their emplacement in the mid to lower crust under peak granulite facies conditions. Under these unusual conditions the timescales for cooling between the silicate solidus and sulfide solidus temperatures were of the order of millions of years, being controlled by the temperature-time path for the exhumation of the orogen as a whole. Sulfides solidified over a time period three orders of magnitude greater than the thousand-year timescale for the solidification of the host silicate magmas. Furthermore, timescales for deformation matched those for cooling and solidification, allowing the country rocks to undergo deformation during ore emplacement. Fluctuating strain rates during and after initial emplacement of the carrier magmas into the host intrusion caused episodes of brittle extension, allowing unusually efficient penetration of partially molten sulfide into heterogeneous, partially molten silicate country rock, resulting in an unusually extensive thermomechanical aureole compared with other mafic intrusion-hosted nickel systems globally.
Abstract Pentlandite is the dominant Ni-hosting ore mineral in most magmatic sulfide deposits and has conventionally been interpreted as being entirely generated by solid-state exsolution from the high-temperature monosulfide solid solution (MSS) (Fe,Ni)1–xS. This process gives rise to the development of loops of pentlandite surrounding pyrrhotite grains. Recently it has been recognized that not all pentlandite forms by exsolution. Some may form as the result of peritectic reaction between early formed MSS and residual Ni-Cu–rich sulfide liquid during differentiation of the sulfide melt, such that at least some loop textures may be genuinely magmatic in origin. Testing this hypothesis involved microbeam X-ray fluorescence mapping to image pentlandite-pyrrhotite-chalcopyrite intergrowths from a range of different deposits. These deposits exemplify slowly cooled magmatic environments (Nova, Western Australia; Sudbury, Canada), globular ores from shallow-level intrusions (Norilsk, Siberia), extrusive komatiite-hosted ores from low and high metamorphic-grade terranes, and a number of other deposits. Our approach was complemented by laser ablation-inductively coupled plasma-mass spectrometry analysis of palladium in varying textural types of pentlandite within these deposits. Pentlandite forming coarse granular aggregates, together with loop-textured pentlandite where chalcopyrite also forms part of the loop framework, consistently has the highest Pd content compared with pentlandite clearly exsolved as lamellae from MSS or pyrrhotite. This is consistent with much of granular and loop pentlandite being formed by peritectic reaction between Pd-rich residual sulfide liquid and early crystallized MSS, rather than forming entirely by subsolidus grain boundary exsolution from MSS, as has hitherto been assumed. The wide range of Pd contents in pentlandite in individual samples reflects a continuum of processes between peritectic reaction and grain boundary exsolution. Textures in metamorphically recrystallized ores are distinctly different from loop-textured ores, implying that loop textures cannot be regenerated (except in special circumstances) by metamorphic recrystallization of original magmatic-textured ores. The presence of loop textures can therefore be taken as evidence of a lack of penetrative deformation and remobilization at submagmatic temperatures, a conclusion of particular significance to the interpretation of the Nova deposit as having formed synchronously with the peak of regional deformation at temperatures within the sulfide melting range.
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