Staging Chamber Research Articles

Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma–mass spectrometry. These data, coupled with whole-rock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt–As and Pd–Bi–Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400–800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE tenors indicate that the ore-forming process may have differed slightly within the staging chambers that supplied the Platreef and GNPA member.

Geochemistry and Mineralogy of Platinum Group Element Mineralization in the River Valley Intrusion, Ontario, Canada: A Model for Early-Stage Sulfur Saturation and Multistage Emplacement and the Implications for "Contact-Type" Ni-Cu-PGE Sulfide Mineralization

The River Valley intrusion within the ~2.48 Ga East Bull Lake intrusive suite in Ontario, Canada, is an example of a mafic igneous intrusion with “contact-type” Ni-Cu-PGE sulfide mineralization along its base. Whereas many contact-type deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the River Valley intrusion, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The River Valley intrusion is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinum-group element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilized significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements (PGE), indicating the presence of a single, well homogenized, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered units above the mineralized units are not PGE depleted. Trace element data, including (Th/Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber; although some samples have S/Se ratios indicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of S loss. We propose a multistage model for the formation of the mineralization in the River Valley intrusion with a major contamination event at depth with the addition of S from local crustal rocks, inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit system with possible further upgrading of sulfide metal tenors (and reduction of S/Se ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid, eroded blocks of precrystallized and mineralized gabbro and footwall rocks, and emplaced an inclusion-bearing package as the lower 100 m or so of the River Valley intrusion. Later emplacement of main River Valley magma was from an S-undersaturated, PGE-fertile magma. The River Valley intrusion is thus an example whereby contact-type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization, rather than contamination triggering sulfide saturation in situ. In such cases, processes at depth determine the generation and subsequent tenor of the mineralization. In particular, dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/Se ratios, masking the signature of crustal contamination. In addition, a multistage emplacement such as this will not necessarily preserve the characteristic increase in Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical, and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal contamination and the presence of PGE mineralization.

Origin of PGE-Depleted Ni-Cu Sulfide Mineralization in the Triassic Hongqiling No. 7 Orthopyroxenite Intrusion, Central Asian Orogenic Belt, Northeastern China

The ~216 Ma Hongqiling No. 7 intrusion is located in the southern margin of the Central Asian orogenic belt, northeastern China. It was the second largest Ni producer after Jinchuan in China. The intrusion crops out in an area of ~0.013 km2 on the surface. It intruded gneisses and marbles of the Paleozoic Hulan Formation. The intrusion is mainly composed of orthopyroxenite (~95 vol %), with minor harzburgite and norite. Over 90 vol % of the intrusion contains disseminated, net-textured or massive sulfides. The bulk sulfide ores (recalculated to 100% sulfide) have low average PGE concentrations (5.6 ppb Os, 3.1 ppb Ir, 4.7 ppb Ru, 2.2 ppb Rh, 31 ppb Pt, and 11 ppb Pd) and high Cu/Pd ratios (3 × 105–3 × 107), consistent with the compositions of sulfide liquids segregated from PGE-depleted magma. Olivine composition (Fo <86 mol %) indicates a fractionated parental magma. Back calculation shows that the parental magma was possibly derived by olivine fractionation from a primary magma containing ~16 wt % MgO. Numerical modeling indicates that PGE depletion in the parental magma of the Hongqiling No. 7 intrusion was due to previous sulfide segregation in the staging chamber. Whole-rock data show light rare earth element (REE) enrichments, negative Nb-Ta anomalies, and positive ɛ Nd ( t = 216 Ma) values from 3 to 5, which can be explained by mixing of a mantle-derived magma with a granitic melt formed by magma underplating in the crust. The γ Os ( t = 216 Ma) values of sulfide ores vary from 50 to 187. The highly variable γ Os( t ) values coupled with similar ɛ Nd( t ) values indicate preferential assimilation of crustal sulfides during magma rock interaction. Based on the observations, we propose that the parental magma of the Hongqiling No. 7 intrusion formed by contamination of mantle-derived, S-undersaturated magma by crustally derived granitic melt, together with olivine crystallization, in a staging chamber. The contaminated magma became PGE depleted due to sulfide segregation in response to crustal contamination. The PGE-depleted magma became S undersaturated again during ascent due to the negative effect of pressure on the maximum solubility of sulfur in magma. Upon arrival near the present level of the Hongqiling No. 7 intrusion a second event of sulfide segregation took place due to addition of crustal sulfides from immediate country rocks. The immiscible sulfide liquids and orthopyroxene crystallizing from the magma were retained in a dynamic conduit to form the ore-bearing intrusion while much of the magma left the conduit to form some of the nearby sulfide-poor intrusions. We suggest future exploration in the region to focus on olivine-bearing and orthopyroxene-dominant intrusions with ages similar to that of the Hongqiling No. 7 intrusion.

PGE and Isotope (Hf-Sr-Nd-Pb) Constraints on the Origin of the Huangshandong Magmatic Ni-Cu Sulfide Deposit in the Central Asian Orogenic Belt, Northwestern China

Many magmatic Ni-Cu sulfide deposits have been found to be associated with Permian basaltic magmatism in the Central Asian orogenic belt, Xinjiang, China. The style of basaltic magmatism and the origin of associated Ni-Cu sulfide deposits in the region are highly debated. The Huangshandong mafic-ultramafic intrusion is at the center of the debate mainly because it hosts the largest magmatic Ni-Cu sulfide deposit in the region. In this paper we report integrated petrological and geochemical data for the Huangshandong Ni-Cu sulfide deposit. The host intrusion consists of a massive gabbronorite unit and a layered sequence, which is composed of two to three ultramafic layers overlain by gabbroic rocks with visible modal layering. Important sulfide mineralization in the intrusion is associated with the ultramafic rocks and massive gabbronorite. The ultramafic rocks and massive gabbronorite are characterized by moderate enrichments in light rare earth elements (LREE), pronounced negative Nb-Ta anomalies relative to Th and La, positive eNd (t = 274 Ma) values (6.6−8.3), low initial 87 Sr/ 86 Sr ratios (0.7031−0.7038), low initial 206 Pb/ 204 Pb ratios (17.7−18), and positive eHf values of zircon (14−17). These data are consistent with mixing between a depleted mantle-derived magma and a granitic melt formed by partial melting of a juvenile arc crust as a result of mafic magma underplating. Olivine crystals enclosed in large clinopyroxene and plagioclase crystals from the ultramafic rocks have Fo contents up to 83 mol %. The contents of Ni in the olivine crystals decrease rapidly with Fo contents, indicating sulfide segregation from magma during olivine crystallization. Numerical modeling using bulk sulfide compositions and constraints from olivine-liquid relationships reveals that the parental magma of the intrusion was significantly depleted in PGE and contained <7 wt % MgO. Depletion of PGE in the magma can be explained by previous sulfide segregation possibly due to magma mixing in a staging chamber. Fractional crystallization and addition of external sulfides from juvenile arc crust appear to have played a critical role in triggering the second event of sulfide segregation to form the deposit. We believe that the Huangshandong sulfide ore-bearing intrusion formed by basaltic magmatism related to postsubduction delamination and asthenosphere upwelling instead of a deep-seated mantle plume.

Mercury distribution amongst co-existing silicates within the Bushveld Complex

Thirty separates of plagioclase, orthopyroxene and clinopyroxene from the lower Main Zone of the Northern Limb of the Bushveld Complex were analysed for their mercury contents using combustion atomic absorption spectroscopy with gold amalgamation pre-concentration. The average mercury contents of plagioclase, orthopyroxene and clinopyroxene were found to be 0.9ppb, 1.2ppb and 1.1ppb, respectively. Mercury within the separates does not vary systematically with any of the major element oxides present in the minerals. Based on a positive 1:1 correlation between mercury in orthopyroxene and clinopyroxene, we estimate DOpxHg≈DCpxHg, and on this basis, can exclude the presence of significant Hg2+ within the melts from which these minerals crystallised. The lack of correlation between mercury in plagioclase and that in the mafic silicates may suggest diffusional loss of the element from the former during slow cooling under magmatic conditions and better retention of mercury by the mafic silicates under the same conditions. Alternatively and more likely, this lack of correlation may support earlier arguments based on distinct Sr-isotopic disequilibrium between co-existing plagioclase and mafic silicates, that plagioclase and the mafic silicates in the Northern Limb of the Bushveld Complex may have crystallised from different melts within a variably contaminated, sub-Bushveld staging chamber.

Primary origin of marginal Ni-Cu-(PGE) mineralization in layered intrusions: 33S evidence from The Platreef, Bushveld, South Africa

The margins of layered igneous intrusions can host ore deposits of nickel, copper, and platinum group elements (PGEs). These marginal deposits are characterized by a complex geologic history, which obscures direct evidence for the mineralization process. Mass-independent fractionation of sulfur isotopes is a distinguishing feature of Archean sedimentary rocks, and reflects processes operating solely in the Archean surface environment. As a result, mass-independent fractionation of sulfur isotopes is a chemically conservative tracer that indicates the involvement of crustal sulfur in marginal Ni-Cu-(PGE) deposits. In this study we use mass-independent fractionation of sulfur isotopes to evaluate the sulfur budget of the Platreef—the marginal PGE-bearing horizon of the northern limb of the Bushveld Complex, South Africa. Our multiple sulfur isotope measurements demonstrate that crustal sulfur in the Platreef originated from a restricted stratigraphic horizon (the Duitschland Formation) in the Platreef footwall. As this signature is spread out over the ≈100-km strike length of the Platreef, it does not reflect sulfur inputs from the local footwall. Nonlocal crustal sulfur isotope anomalies are unexpected given the high metal tenors in the Platreef. These features can be reconciled if a preformed Ni-Cu-(PGE)–rich sulfide melt assimilated sulfur in a staging chamber bounded completely by the Duitschland Formation, and then was emplaced into the Platreef. Such a model is supported by long-standing observations of lithologic, geochemical, and stratigraphic similarities between the Merensky Reef and the Platreef, as well as by recent suggestions that the northern limb represents an “escape structure” for the rest of the Bushveld Complex. Crustal sulfur assimilation is often thought to initiate the ore-forming process by producing an immiscible sulfide melt that can collect base metals and PGEs. In the Platreef, however, crustal sulfur assimilation apparently diluted a preexisting PGE-rich sulfide melt instead.

Mantle source, magma differentiation and sulfide saturation of the ∼637 Ma Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block, Central China

The sill-like Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block formed at 637±4Ma and consists of a lherzolite unit and an olivine gabbronorite unit, with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the Meso- to Neoproterozoic Zhujiashan Formation.Rocks from the lherzolite unit have 41.4–44.0wt.% SiO2 and 29.0–38.4wt.% MgO and contain olivine with Fo values ranging from 80 to 85mole%, indicating crystallization from a moderately evolved magma. They commonly have <0.1wt.% S and low, variable PGE (2–40ppb) and display flat primitive mantle-normalized chalcophile element patterns with relatively low Cu/Pd ratios (3900–427,415), indicating derivation possibly from S-undersaturated magmas. Rocks from the olivine gabbronorite unit contain 44.0–47.1wt.% SiO2 and 22.5–28.9wt.% MgO and contain olivine with relatively low Fo values (67–68mole%), indicating crystallization from a more evolved magma. They contain 0.03–0.9wt.% S and <15ppb PGE and have Cu/Pd ratios (462,500–10,915,500) much higher than those for the rocks from the lherzolite unit. Rocks from the olivine gabbronorite unit show relatively steep, trough-like patterns depleted in PGE relative to Ni and Cu, indicating that they formed from sulfide-saturated magmas. Rocks from the marginal zone have relatively high SiO2 (45.1–53.1wt.%) and low MgO (7.4–19.6wt.%) and have chemical composition varying between the olivine gabbronorite and the country rocks. They contain 0.57wt.% S and <10ppb PGE, with high Cu/Pd ratios (404,300) and chalcophile element patterns similar to the olivine gabbronorite unit. Rocks from the lherzolite unit have (Th/Yb)PM ratios (3.7–9.1) much lower than the upper crust (28.2) and the country rocks (41.3), indicating small degrees of crustal contamination. Their ɛNd(t) (−4.4 to +1.1) and γOs(t) (8–36) values indicate an EMI mantle source. Primary magma was PGE-poor because sulfide retained in the mantle source due to small degrees of partial melting of the enriched mantle source.We propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep-seated staging magma chamber, forming an olivine-laden crystal mush with minor sulfide droplets in the lower part and more evolved, PGE-depleted magmas in the upper part of the chamber. The evolved, PGE-depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit. The lherzolite unit formed from the emplacement of the olivine-laden crystal mush into the same chamber. Because of paucity of extensive crustal contamination, only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone. The Zhouan intrusion thus may not be a good target for economic Ni–Cu sulfide deposit.

THE UPPER ZONE OF THE BUSHVELD COMPLEX AT ROOSSENEKAL, SOUTH AFRICA: GEOCHEMICAL STRATIGRAPHY AND EVIDENCE OF MULTIPLE EPISODES OF MAGMA REPLENISHMENT

Drill core from a fence of five boreholes located in the vicinity of Roossenekal, in the eastern limb of the Bushveld Complex, South Africa, provides complete coverage of what is probably one of the thickest and most well-developed sections of the Upper Zone. The lowermost portions of the Rooiberg Group felsites that overlie the Upper Zone, and the uppermost portion of the Upper Main Zone, which forms the footwall, were also intersected. Field relationships from floor-attached domes (eastern limb) and gap areas (western limb) reveal a marked angular unconformity at the base of the Upper Zone. The Upper Zone represents a discrete intrusive event and should not be interpreted as a continuum, or the fractionated residue of the underlying components of the Rustenburg Layered Suite (RLS). A new subdivision of the Upper Zone into five subunits (A through E) is proposed on the basis of the cumulus mineralogy and whole-rock geochemistry, the latter based on samples at an average vertical spacing of 20 m throughout the column at Roossenekal. The separation of subzones A and B is based on our geochemical data, as is the identification of subzone E, but recognition of subzones C (olivine) and D (apatite) is broadly consistent with the detailed mapping and stratigraphic column presented by Von Gruenewaldt (1973). Subunits A through D reveal macro-layering for which the intrusion is so well known. More than 40 individual Ti-magnetite layers were identified in the drill-core, which together with the numerous anorthosite layers, are the most prominent feature of this part of the column. The principal lithology is a somewhat monotonous sequence of ferrogabbro or magnetite-olivine gabbro. The absence of Ti-magnetite layers and anorthosite from subzone E, which is dominated by diorite with <5 modal % Ti-magnetite is significant. The whole-rock geochemical data presented here is not consistent with a closed system fractionation hypothesis for subunits A through D, despite this being widely accepted in recent contributions. An upward differentiation trend is preserved only in subunit E. The incremental build up of cumulates from successive pulse of magma is not only consistent with our views on the underlying layered cumulates (periods of closed system differentiation are rare) but is also consistent with field relationships, including each of the principal limbs representing individual sill-like features. The Upper Zone crystallized from batches of iron-rich tholeiitic magma derived from a deep staging chamber. Magma was injected as near-lateral flows, probably in a southward direction in the eastern limb. The introduction of evolved magma into a shallow crustal environment resulted in the early crystallization of Fe-Ti oxides. Ti-magnetite layers crystallized from oxide-rich slurries intruded near the floor of the chamber. Fe-Ti oxide pipes formed from slurries forced downward in areas of structural weakness. The regular, albeit step-like, upward change in composition of the Ti-magnetite layers identified by Molyneux (1974) and others is ascribed to mixing of crystal-rich magma slurries with increasingly differentiated resident magma: the lowermost layers (and pipes) contain the highest concentrations of V2O5. The spatial association of Ti-magnetite layers and anorthosite is emphasized and we suggest the latter formed as a consequence of disequilibrium partial melting of earlier formed silicate cumulates due to heat associated with new pulses of magma.

Crustally Contaminated Komatiite: Primary Source of the Chromitites and Marginal, Lower, and Critical Zone Magmas in a Staging Chamber Beneath the Bushveld Complex

In the lowermost rocks of the Rustenburg Layered Suite (Bushveld Complex), levels of total Cr 2 O 3 are up to ten times higher than its putative solubility (0.16% Cr 2 O 3 ) in the magmas that many investigators have accepted as parental to the dunites, harzburgites, pyroxenites, and norites of the Lower, Lower Critical, and Upper Critical zones. Various existing models fail to explain this “chromium paradox” in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates. Simple mixing calculations and quantitative Adiabat_1ph runs support our model in which a heterogeneous (liquid + crystals) system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30% of crustal melts at depths appropriate to pressures between 10 and 4.5 kbars. We contend that crystal settling and the effects of elutriation during magma ascent had generated a high SiO 2 cap with modal olivine, orthopyroxene, and chromite increasing with depth in the staging chamber. The magma from the high SiO 2 cap of the staging chamber was first injected into the largely sedimentary Transvaal Supergroup, generating what is now termed the Marginal zone. Slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower Bushveld chamber to form the Lower and Critical zones. Chromite within the slurries introduced in this way could account for their high gross Cr content. Frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber. The lithologically diverse nature of the floors upon which the chromitite layers accumulated (peridotites, pyroxenites, norites, and anorthosites) was therefore more a function of the timing of emplacement of the Cr-rich slurries generated in the deeper reservoir than of specific evolutionary processes within the Bushveld chamber itself.

The Lower Main Zone in the Northern Limb of the Bushveld Complex—a &gt;1·3 km Thick Sequence of Intruded and Variably Contaminated Crystal Mushes

The lower Main Zone (LMZ) in the Northern Limb of the Bushveld Complex as intersected by the 1563·02 m deep Moordkopje (MO-1) drill hole shows very little large-scale differentiation as exemplified by parameters such as the An% of plagioclase, the Mg# of pyroxene and the modified differentiation index over a vertical interval in excess of 1·3 km. These features, coupled with the late entry of pigeonite (now inverted to orthopyroxene), are suggestive of the repeated intrusion of magma into the magma chamber of the Northern Limb during the early stages of the development of the Main Zone. Importantly, the lower Main Zone (LMZ) exhibits significant Sr isotopic disequilibrium between coexisting plagioclase (Sri ∼0·708) and orthopyroxene (Sri up to ∼0·711) in certain samples, a feature that has not previously been described for the Main Zone. Many of the features of the lower Main Zone (e.g. bulk compositions of LMZ cumulates unsuitable for the crystallization of plagioclase + two pyroxenes; poorly developed layering; non-cotectic proportions of plagioclase and pyroxene; decoupling of the differentiation trends of plagioclase and pyroxene over short vertical intervals) in the Northern Limb may be explained if it was intruded not as repeated influxes of aphyric magma, but instead by the repeated intrusion of crystal mushes from a deeper, sub-compartmentalized, crustal staging chamber. The model proposed is an alternative to previous models suggesting the loss of extrusive components from the Bushveld Complex, which were developed to explain compositional paradoxes within the Bushveld Complex.

Differentiation, crustal contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China

The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative eNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of crustal contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have eNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, crustal contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.

Mineralogical and Geochemical Controls on the Formation of the Woods Point Dike Swarm, Victoria, Australia: Evidence from the Morning Star Dike and Implications for Sourcing of Au Within Orogenic Gold Systems

The Woods Point dike swarm of eastern Victoria, Australia, has produced over 133 tonnes (t) of Au. Here, as in a number of other orogenic Au provinces, a close spatial relationship is observed between orogenic Au mineralization and intrusive rocks; this study focuses on the well-endowed Morning Star dike, a gabbro to gabbro-diorite dike that has produced ~28 t of hydrothermal quartz-carbonate-pyrite vein-hosted Au and a further 24 t of placer gold derived from primary rocks in the area immediately around Morning Star. The fractionated magma that formed the dikes within the Woods Point dike swarm was sourced from a staging magma chamber at a depth of 16 to 23 km; here, mantle-derived magmas were driven to S saturation by assimilation of crustal material, leading to the formation of Cu-Ni-platinum group element (PGE)-Au–bearing sulfides, most of which segregated and settled toward the bottom of the staging chamber. Emplacement of the dike initiated with tapping of the staging chamber and release of the magmas within the chamber, forming the Woods Point dike swarm; flow differentiation and the formation of dike bulges enabled the segregation of sulfide-bearing mafic units and intermediate sulfide-barren units within these dikes. This is exemplified by primitive amphibole-rich, sulfide-bearing gabbroic sections and more evolved sulfide-barren gabbro-diorite portions of the Morning Star dike. We suggest that the source of Au within the Morning Star dike as well as that in other members of the Woods Point dike swarm was the Cu-PGE-Au sulfides left at depth within the magma conduit system, for example within the Thomson River Copper Mine dike. Mass balance calculations undertaken suggest that ~104 million tonnes (Mt) of magmatic sulfide may exist within the magma conduit system; these magmatic sulfides, if they are the equivalent of the sulfides preserved in the Morning Star dike, probably contain around 3% Ni and 10% Cu. During postmagmatic metamorphism, we infer that hydrothermal fluids flowed along dikes, interacting with magmatic sulfides and leaching some of the Au, which was subsequently deposited in quartz-carbonate veins at higher levels in the system.

THE LENGSHUIQING Ni-Cu DEPOSIT, SICHUAN, SOUTHWESTERN CHINA: ORE CHARACTERISTICS AND GENESIS

Lengshuiqing is a Ni–Cu sulfide deposit related to the Neoproterozoic Panxi arc, in southwestern China. The mineralization is associated with five separate but genetically related intrusions. The magmatic rocks from Lengshuiqing range from peridotite and olivine pyroxenite to gabbro, diorite, quartz diorite, granodiorite and granite. The sulfide ore occurs in the ultramafic rocks and in four distinct facies: (1) disseminated to net-textured (the dominant type), (2) blebby (disseminated globular), (3) breccia matrix ore, and (4) rare veins of massive sulfides. We present the first geochemical data on the individual types of ore from Lengshuiqing. The disseminated to net-textured ore has Ni/Cu = 3, and average Ni tenor of 4.9%. The fractionation of monosulfide solid-solution induced the occurrence of high Ni tenors in the massive ore (6–8% Ni). The PGE tenors of all ore types are less than 1 ppm. The mantle-normalized values of the chalcophile elements show PGE depletion relative to Ni and Cu, which can be explained by a pre-emplacement equilibration of the magma in the presence of immiscible sulfides. The IPGE and Pt are depleted relative to Pd and Rh, suggesting the retention of some PGE as alloys in the mantle source. The formation of the ore is explained in conjunction with the conduit-related origin of the host intrusions. Sulfide immiscibility probably occurred in a staging chamber as a result of the assimilation of SiO 2 -rich crustal material. The segregated sulfide droplets, together with olivine crystals, were transported to the shallower intrusions at Lengshuiqing, which were likely to have been wide magmatic conduits. The decrease of magma velocity in the intrusions at Lengshuiqing determined the accumulation of sulfide melt and olivine crystals, and the formation of the disseminated to net-textured ore. Late pulses of volatile-rich magma produced breccias and led to the spatial redistribution of the sulfide melt, which generated breccia matrix ore.

Re-Os AND S ISOTOPE CONSTRAINTS ON TIMING AND SOURCE HETEROGENEITY OF PGE-Cu-Ni SULFIDE ORES: A CASE STUDY AT THE TALNAKH ORE JUNCTION, NORIL'SK PROVINCE, RUSSIA

The ultramafic–mafic Noril’sk-type intrusions in the northern part of Siberia, in Russia, host some of the world’s major economic platinum-group-element (PGE)–Cu–Ni sulfide deposits. We present the results of Re–Os and S isotope systematics of PGE–Cu–Ni sulfide ores associated with the economic Talnakh and Kharaelakh intrusions within the Talnakh ore junction, Noril’sk Province. At Kharaelakh, Re–Os isotope analyses of disseminated and massive PGE–Cu–Ni sulfide ores identify distinct sources of sulfide mineralization, with Re–Os isochron ages that cluster around 247 Ma. Different initial Os isotope compositions of massive (187Os/188Os = 0.1283 ± 0.0054, γOs(t) = 1.1) and disseminated (187Os/188Os = 0.1331 ± 0.0052, γOs(t) = 4.9) sulfide ores at Kharaelakh indicate either a less contaminated nature of the massive orebody with respect to sulfide disseminations or distinct isotope sources involved in their generation. At Talnakh, Re–Os isotope systematics of massive and disseminated ores show broadly similar patterns ( e.g., 187Os/188Os = 0.1366 ± 0.0037, γOs(t) = 7.6 and 187Os/188Os = 0.1350 ± 0.0036, γOs(t) = 6.4, respectively), indicating their similar sources, likely of mantle origin. Combined with new geochronological and isotope-geochemical data on zircon, the Re–Os isotope systematics of sulfide ores are consistent with protracted evolution of ultramafic–mafic magmas at deep-seated staging chambers in the Noril’sk region, which served as the favorable factor for accumulation of ores of unique scales and concentrations. Sulfur isotope values (δ34S) of disseminated and massive PGE–Cu–Ni sulfide ores at Kharaelakh (12.2 ± 0.5‰ and 12.7 ± 0.1‰, respectively) are notably different from those at Talnakh (10.8 ± 0.1‰ and 10.9 ± 0.1‰, respectively), indicative of distinct involvement of heavy sulfur during formation of PGE–Cu–Ni sulfide ores. The restricted range of δ34S values can be employed as useful fingerprint in the assessment of the potential of prospective ore deposits.

THE PETROGENESIS OF PLATINUM-GROUP ELEMENT REEFS IN THE UPPER MAIN ZONE OF THE NORTHERN LOBE OF THE BUSHVELD COMPLEX ON THE FARM MOORDDRIFT, SOUTH AFRICA

Boreholes have intersected the interval hosting the Pyroxenite Marker at the base of the Upper Main zone on the farm Moorddrift in the northern Bushveld Complex. The concentrations of the major and compatible trace elements in the rocks are in the range of other Pyroxenite Marker intervals elsewhere in the Bushveld Complex (MgO up to 22 wt %, Cr up to 300 ppm). Over a stratigraphic height of some 300 m the interval is characterized by a broad, but irregular reversal in Mg number and Cr/V, analogous to stratigraphically equivalent horizons elsewhere in the Bushveld Complex. The rocks have distinctly higher concentrations in incompatible trace elements, particularly large ion lithophile elements (LILE) and light rare earth elements (LREE), than other Main zone rocks and thus appear to contain an elevated crustal component. The interval hosts several thick (meters to 10s of meters), but mostly relatively low grade and laterally discontinous PGE reefs that have been delineated over a strike length of ca 1 km. PGE contents locally reach ~16 ppm Pt + Pd + Au in rocks containing up to ~4 to 5 percent sulfides. The PGE tenors of most sulfides are >100 ppm, Pt/Pd ratios are above unity, and Pd/Ir is >200. Based on well-defined positive correlations between Pt and Pd with Ir we interpret the mineralization as being largely of primary magmatic origin. Most previous models interpreted the Pyroxenite Marker to have formed from a major replenishment to the Bushveld magma chamber with relatively primitive magma. The new magma would have been emplaced on top of an ~4-km-thick cumulate package consisting of the ultramafic-mafic Lower and Critical zones and the lower to central portions of the gabronoritic Main zone. Here, we propose that the Upper Main zone including the Pyroxenite Marker did not result from a magma replenishment. Instead, it crystallized from the S-saturated residual magma of the upper Critical zone cumulates and originally directly overlaid the latter. The ~1-to 2-km-thick lower-central Main zone package formed from a crystal-charged magma emplaced at a relatively late stage in the solidification history of the Bushveld chamber, between the largely consolidated Upper Critical and Upper Main zones. We envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite, perhaps in response to the evacuation of a putative lower staging chamber from which the Bushveld Complex was fed. Semiconsolidated portions of the cumulate package sagged and slumped toward the center of the Complex, and the flowing crystal mushes underwent mechanical unmixing of light, relatively buoyant plagioclase from denser pyroxenes, oxides, and sulfides. This resulted in the formation of an interlayered sequence of relatively melanocratic sulfide-enriched with relatively leucocratic sulfide-poor layers. reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the Main zone magma.

The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ18O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ18O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ34S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ18O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ18O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.

A conduit-related genesis of the Lengshuiqing intrusive assemblage (Sichuan, SW China)

Lengshuiqing is part of the late Proterozoic igneous province from the western margin of the Yangtze craton. The Lengshuiqing area comprises five ultramafic–mafic intrusions, emplaced in the late Proterozoic Yanbian Group. The intrusions from Lengshuiqing contain cumulate ultramafic zones (peridotite + olivine pyroxenite), with cumulus olivine and Cr-spinel, and intercumulus pyroxenes, hornblende, phlogopite and plagioclase. Ni–Cu ore (pyrrhotite + pentlandite + chalcopyrite) is hosted in the ultramafic zones. Olivine-free diorite–quartz diorite ± gabbro and granite zones commonly occur above the ultramafic rocks. The genesis of the intrusions (conduit-related accumulation or differentiation from stagnant magma) was investigated. The amount of sulphides in the intrusions from Lengshuiqing is one order of magnitude bigger than the sulphides that can be dissolved by a volume of mafic magma similar with the volume of the intrusions. Most intrusions from Lengshuiqing have bulk composition (peridotite ± diorite ± granite) more magnesian (MgO = 21–22%; Mg# > 78) than the deduced composition of their parental magma (MgO = 9–11%; Mg# = 64–67). This indicates the accumulation of sulphide and mafic silicates from a volume of magma much bigger than the volume of the intrusions, which can be explained by the fractionation from magma ascending through the intrusions to shallower depths. A continuous supply and vent of magma is consistent with the lack of chilled margins, the melting of the wall rocks and the generation of high-temperature mineral assemblages (K-feldspar, diopside, and sillimanite) in the Yanbian Group. The intrusions from Lengshuiqing are seen as microchambers on conduits draining olivine-, Cr-spinel-, and sulphide-bearing mafic magma from a larger staging chamber.

PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis

The Merensky Reef is the consequence of several major influxes of magma into the Bushveld magma chamber that gave rise to at least two cyclic units, including the Pre-Merensky and Merensky cyclic units. Analysis of the S, Ni, Cu, platinum group element (PGE) and Au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the Reef to close to the UG-2 horizon of the Western Bushveld Complex provide important petrogenetic constraints on the origin of the Reef. Particular attention is paid to variations in the calculated PGE tenors and Cu/Pd ratios of sulfides, both vertically and laterally. All of the profiles show a decrease in Pt tenor and concomitant increase in Cu/Pd ratio upward across the pyroxenite of the Merensky unit and laterally away from certain centres that are interpreted as feeder zones. The data also document that more mafic horizons within the Merensky to UG-2 interval of the southwestern Bushveld are characterized by much lower Cu/Pd (<100–400) than the ambient ratio (∼4000). This is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there. The favoured interpretation for the Merensky Reef is that the Bushveld chamber(s) was occupied by a resident magma crystallizing orthopyroxene and plagioclase. Pre-Merensky and, subsequently, Merensky magmas entered the chamber(s) intermittently from localized feeder structures, mixing or mingling with the resident magma and giving rise to the more mafic cumulates. At specific horizons, including the Merensky Reef, the mixing or mingling caused sulfide immiscibility. In these cases, as the new magma pulses flowed laterally away from the entry zones they deposited sulfide, becoming depleted in PGE. On coming to rest, the magma continued to segregate sulfide and crystallize pyroxene, so that the sulfides became progressively depleted in PGE and acquired a higher Cu/Pd upward through the pyroxenite. This model requires that the Merensky magmas could dissolve up to two orders of magnitude more PGE than current measurements on dry diopside–anorthite melts have indicated. It is suggested that very PGE-rich magmas can develop as a result of sulfide-unsaturated magma reacting with early formed sulfide in a staging chamber, dissolving FeS and enriching the remaining sulfide in PGE. Later magmas equilibrating with this enriched sulfide will themselves become highly enriched in PGE.

A NEW GENETIC MODEL FOR THE GIANT Ni-Cu-PGE SULFIDE DEPOSITS ASSOCIATED WITH THE SIBERIAN FLOOD BASALTS

The Kharaelakh intrusion is one of several sill-like, multiphase gabbroic intrusions that host world-class Ni-Cu-PGE deposits in the Noril’sk-Talnakh region. The sulfide ores of the Kharaelakh intrusion are characterized by elevated δ34S values (10–12‰) and high PGE concentrations (e.g., up to10 ppm Pt). The δ34S values require addition of crustal S with elevated isotope ratios such as evaporite-bearing country rocks (δ34S, ~20‰) and R factors (magma/sulfide mass ratios) of 2,000) if Pt content in the magma is similar to the values in the coeval, most undepleted lavas (~10 ppb). Such a discrepancy can be explained by sulfide resorption by a new flux of mantle-derived magma into a deep staging chamber to form PGE-enriched magma. Interaction of the PGE-enriched magma with evaporite-bearing country rocks in the plumbing system at higher crustal levels could have produced sulfide liquids with high PGE contents as well as elevated δ34S values. Mass-balance calculations indicate that <0.9 wt percent anhydrite assimilation from evaporite-bearing country rocks is required to explain the elevated δ34S values of the sulfide ores. This model, although developed purely based on data from the Kharaelakh deposit, can also be applied to other coeval Ni-Cu-PGE deposits. For the coeval Noril’sk-I and Talnakh deposits different depths of anhydrite assimilation in the respective plumbing systems are required. This model differs from the previously proposed models in detail but reenforces the idea that chalcophile element depletion in continental flood basalts is a useful exploration tool for Ni-Cu-PGE deposits associated with coeval subvolcanic intrusions.

Assessing the potential involvement of an early magma staging chamber in the generation of the Platreef Ni–Cu–PGE deposit in the northern limb of the Bushveld Complex: a pilot study of the Lower Zone Complex at Zwartfontein

The Platreef of the northern limb of the Bushveld Complex is one of the world's most significant deposits of platinum-group elements (PGE) with associated Ni and Cu. The origin of the Platreef is controversial. Some workers suggest that it is a northern facies of the Merensky Reef or part of the Upper Critical Zone, while others have suggested that the Platreef formed by processes entirely contained within the northern limb, unrelated to mineralisation events elsewhere in the complex. The northern limb is separated from the rest of the complex by the Thabazimbi–Murchison Lineament (TML) and the effect that this structure had on the intrusion of Bushveld magmas is debated. The presence of chilled rocks and cross-cutting relationships between the Platreef and its hangingwall gabbronorites would seem to preclude the magma that formed the hangingwall also acting as a source of PGE to the Platreef. The base metal sulphides in the Platreef carry very high PGE tenors (comparable with the Merensky Reef) indicating that the PGE must have been concentrated from a large volume of magma, but the source of that magma has not been established. In order to solve this PGE mass balance paradox, McDonald and Holwell have suggested that the magmas that formed the (pre-Platreef) Lower Zone may have been the source of PGE. At present, other models do not involve any significant role for the Lower Zone magmas in forming the Platreef. The data presented in this pilot study of the Lower Zone intrusion at Zwartfontein test some of the predictions arising from the McDonald and Holwell model. They highlight some important first order differences between Lower Zone intrusions in the northern limb compared with the rest of the Bushveld Complex. The enrichment in Th and LREE that characterizes the Lower Zone rocks in the eastern and western Bushveld appears to be missing in the northern limb. The lithophile element signatures of the different types of Lower Zone are suggested to result from mafic magmas intruding north and south of the TML and being contaminated by these different types of crust. Most significantly, the study has also revealed strong depletion of chalcophile elements (Ni, Cu and PGE) in the Lower Zone intrusion at Zwartfontein. The results are consistent with the depleted products expected from the processing of pre-Platreef magma(s) by interactions with sulphides at a deeper level within the magmatic plumbing system. The results provide a positive first test of one of the predictions arising from the McDonald and Holwell Platreef model. The existence of a system capable of removing PGE and producing a large volume of depleted ultramafic cumulates, in close proximity to the most highly mineralised sector of the Platreef, is suggested to be highly significant.