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1.
The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration.
This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base
of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions
of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef
was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes
of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the
northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as
the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky
Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic
stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef
and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences
in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth
elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of
the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in
the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture
of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma
rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks
preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the
resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with
elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and
Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and
the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility
that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events
involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone. 相似文献
2.
R. G. Cawthorn 《Mineralium Deposita》2005,40(2):231-235
The Merensky Reef and the underlying Upper Group 2 chromitite layer, in the Critical Zone of the Bushveld Complex, host much
of the world’s platinum-group element (PGE) mineralization. The genesis is still debated. A number of features of the Merensky
Reef are not consistent with the hypotheses involving mixing of magmas. Uniform mixing between two magmas over an area of
150 by 300 km and a thickness of 3–30 km seems implausible. The Merensky Reef occurs at the interval where Main Zone magma
is added, but the relative proportions of the PGE in the Merensky Reef are comparable to those of the Critical Zone magma.
Mineral and isotopic evidence in certain profiles through the Merensky Unit suggest either mixing of minerals, not magmas,
and in one case, the lack of any chemical evidence for the presence of the second magma. The absence of cumulus sulphides
immediately above the Merensky Reef is not predicted by this model. An alternative model is proposed here that depends upon
pressure changes, not chemical processes, to produce the mineralization in chromite-rich and sulphide-rich reefs. Magma was
added at these levels, but did not mix. This addition caused a temporary increase in the pressure in the extant Critical Zone
magma. Immiscible sulphide liquid and/or chromite formed. Sinking sulphide liquid and/or chromite scavenged PGE (as clusters,
nanoparticles or platinum-group minerals) from the magma and accumulated at the floor. Rupturing of the roof resulted in a
pressure decrease and a return to sulphur-undersaturation of the magma. 相似文献
3.
F. Johan Kruger 《Mineralium Deposita》2005,40(5):451-472
“His mind was like a soup dish—wide and shallow; ...” - Irving Stone on William Jennings BryanA compilation of the Sr-isotopic stratigraphy of the Bushveld Complex, shows that the evolution of the magma chamber occurred in two major stages. During the lower open-system Integration Stage (Lower, Critical and Lower Main Zone), there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing, crystallisation and deposition of cumulates. Larger influxes correspond to the boundaries of the zones and sub-zones and are marked by sustained isotopic shifts, major changes in mineral assemblages and development of unconformities. During the upper, closed system Differentiation Stage (Upper Main Zone and Upper Zone), there were no major magma additions (other than that which initiated the Upper Zone), and the thick magma layers evolved by fractional crystallisation. The Lower and Lower Critical Zones are restricted to a belt that runs from Steelpoort and Burgersfort in the northeast, to Rustenburg and Northam in the west and an outlier of the Lower and Lower Critical Zone, up to the LG4 chromitite layer, in the far western extension north of Zeerust. It is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the Lower Critical Zone occur. These chromitites include the economically important c. 1 m thick LG6 and MG1 layers exposed around both the Eastern and Western lobes of the Bushveld Complex. The Upper Critical Zone has a greater lateral extent than the Lower Critical Zone and overlies but also onlaps the floor-rocks to the south of the Steelpoort area . The source of the magmas also appears to have been towards the south as the MG chromitite layers degrade and thin northward whereas the LG layers are very well represented in the North and degrade southward. Sr and Os isotope data indicate that the major chromitite layers including the LG6, MG1 and UG2 originated in a similar way. Extremely abrupt and stratigraphically restricted increases in the Sr isotope ratio imply that there was massive contamination of intruding melt which “hit the roof” of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite (Kruger 1999; Kinnaird et al. 2002). Therefore, each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor. Nevertheless, this was achieved by in situ contamination by roof-rock melt of the intruding Critical Zone liquids that had an orthopyroxenitic to noritic lineage. The Main Zone is present in the Eastern and Western lobes of the Bushveld Complex where it overlies the Critical Zone, and onlaps the floor-rocks to the south, and the north where it is also the basal zone in the Northern lobe. The new magma first intruded the Northern lobe north of the Thabazimbi–Murchison Lineament, interacted with the floor-rocks, incorporated sulphur and precipitated the “Platreef” along the floor-rock contact before flowing south into the main chamber. This exceptionally large influx of new magma then eroded an unconformity on the Critical Zone cumulate pile, and initiated the Main Zone in the main chamber by precipitating the Merensky Reef on the unconformity. The Upper Zone magma flowed into the chamber from the southern “Bethal” lobe as well as the TML. This gigantic influx eroded the Main Zone rocks and caused very large-scale unconformable relationships, clearly evident as the “Gap” areas in the Western Bushveld Complex. The base of this influx, which is also coincident with the Pyroxenite Marker and a troctolitic layer in the Northern lobe, is the petrological and stratigraphic base of the Upper Zone. Sr-isotope data show that all the PGE rich ores (including chromitites) are related to influxes of magma, and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion; with associated transgressive disconformities onto the floor-rocks coincident with major zone changes. These positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped. The outcrop patterns and the concordance of geochemical, isotopic and mineralogical stratigraphy, indicate that during crystallisation, the Bushveld Complex was a wide and shallow, lobate, sill-like sheet, and the rock-strata and mineral deposits are quasi-continuous over the whole intrusion.
| F. Johan KrugerEmail: |
4.
Judith A. Kinnaird D. Hutchinson L. Schurmann P. A. M. Nex Renee de Lange 《Mineralium Deposita》2005,40(5):576-597
The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb
of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly
mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main
Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the
limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation
occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike
is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have
been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on
the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors
further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in
the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of
the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture
was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long,
are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones
whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections.
These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional
range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides
are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are
disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and
minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the
margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides,
bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite
grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE
grade may be decoupled from S and base metal abundance. 相似文献
5.
Northwest of Pretoria, the UG2-Merensky Reef interval overlies a Critical Zone-Lower Zone sequence that contains numerous
large blocks of floor material. Nevertheless, individual layers can be correlated with equivalent units at Crocodile River
mine, the Rustenburg, Impala, Union, and Amandelbult sections. Concentrations of platinum-group elements in two borehole intersections
of the UG2 chromitite are 4 ppm over 1.2 m and 2.4 ppm over 2.2 m. Therefore, bulk PGE levels appear to be only moderately
lower than those at Western Platinum mine. This renders models explaining PGE enrichment by upward percolating melt or fluids
problematic. The Merensky Reef, although containing sulphides, is only weakly mineralized with PGE (0.6 ppm). The UG2 pyroxenite
is separated from the UG2 chromitite by a 15 m noritic layer. The introduction of feldspathic cumulates between two units
that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts
of the chamber distally located with regard to a major feeder zone at Union Section. It also suggests that the UG2 unit is
a multiple rather than a single cyclic unit. 相似文献
6.
The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies
with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant
in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though
thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation,
a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not
strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of
its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where
Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast,
Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this
is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations
on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals,
including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims
of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were
observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage
felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals.
We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation
of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd
formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where
they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides
and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination
were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and
blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite. 相似文献
7.
Trace-element study and age dating of zircon from chromitites of the Bushveld Complex (South Africa)
Marina Yudovskaya Judith Kinnaird Anthony J. Naldrett Nickolay Rodionov Anton Antonov Sergey Simakin Dmitry Kuzmin 《Mineralogy and Petrology》2013,107(6):915-942
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051?±?9 Ma (2σ, MSWD?=?0.1) and 2,056?±?5 Ma (2σ, MSWD?=?0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity. 相似文献
8.
Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish
the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly
different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at
Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized
by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky
Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein,
where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase
in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel,
the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks,
but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is
interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation
of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction
of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the
Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different
Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich
sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation,
the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt
ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides
that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas
with the crustal rocks that underlie the Complex. 相似文献
9.
Crustal Contamination and Fluid-Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa 总被引：4，自引：0，他引：4
The Platreef is the main platinum group element (PGE)-bearingfacies of the northern limb of the Bushveld complex, but unlikethe Merensky Reef of the eastern and western limbs, it is indirect contact with the country rock. Mineral separate 相似文献
10.
Platinum-group element (PGE) mineralisation within the Platreef at Overysel is controlled by the presence of base metal sulphides (BMS). The floor rocks at Overysel are Archean basement gneisses, and unlike other localities along the strike of the Platreef where the floor is comprised of Transvaal Supergroup sediments, the intimate PGE–BMS relationship holds strong into the footwall rocks. Decoupling of PGE from BMS is rare and the BMS and platinum-group mineral assemblages in the Platreef and the footwall are almost identical. There is minimal overprinting by hydrothermal fluids; therefore, the mineralisation style present at Overysel may represent the most ‘primary’ style of Platreef mineralisation preserved anywhere along the strike. Chondrite-normalised PGE profiles reveal a progressive fractionation of the PGE with depth into the footwall, with Ir, Ru and Rh dramatically depleted with depth compared to Pt, Pd and Au. This feature is not observed at Sandsloot and Zwartfontein, to the south of Overysel, where the footwall rocks are carbonates. There is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the Platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place. Textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable, inter-granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the Platreef magma. PGE were originally concentrated within a sulphide liquid in the Platreef magma, and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the IPGE and Rh from it whilst still within the mafic Platreef. Transport of PGE into the gneisses, via downward migration of the residual sulphide liquid, fractionated out the remaining IPGE and Rh in the upper parts of the gneisses leaving a ‘slick’ of disseminated sulphides in the gneiss, with the residual liquid becoming progressively more depleted in these elements relative to Pt, Pd and Au. Highly sulphide-rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall. This study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of PGE from the Platreef into the footwall rocks. Where the floor rocks are sediments, fluid activity related to metamorphism, assimilation and later serpentinisation has decoupled PGE from BMS in places, and transport of PGE into the footwall is via hydrothermal fluids. In contrast, where the floor is comprised of anhydrous gneiss, such as at Overysel, there is limited fluid activity and PGE behaviour is controlled by the behaviour of sulphide liquids, producing an intimate PGE–BMS association. Xenoliths and irregular bands of chromitite within the Platreef are described in detail for the first time. These are rich in the IPGE and Rh, and evidence from laurite inclusions indicates they must have crystallised from a PGE-saturated magma. The disturbed and xenolithic nature of the chromitites would suggest they are rip-up clasts, either disturbed by later pulses of Platreef magma in a multi-phase emplacement or transported into the Platreef from a pre-existing source in a deeper staging chamber or conduit. 相似文献
11.
The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation 总被引：1，自引：0，他引：1
Potholes represent areas where the normally planar PGE-rich Merensky Reef of the upper Critical Zone of the Bushveld Complex
transgresses its footwall, such geometric relationships being unusual in layered intrusions. The recognition of vertical dykes
of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tensional deformation due to the loading effects of major new
magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement. The Merensky Reef and its footwall have distinct initial Sr-isotope ratios (R
0 > 0.7066 and <0.7066, respectively), which may be used to constrain these processes related to pothole formation. Merensky
Reef in potholes (R
0 = 0.7069−0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discordant
replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall.
The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R
0 = 0.7066), and immediately adjacent stratiform lithologies are slightly modified towards footwall values. In contrast, Neptunian
pyroxenitic (Merensky) dykes cross-cutting the footwall lithologies, with a large surface area to volume ratio, and low Sr
content, do not display footwall-like Sr-isotope initial-ratios (R
0 = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement
pyroxenite (“replacement pegmatoid”), at the base of the Merensky Reef within potholes, has a high initial-ratio (R
0 > 0.7071), and so models of pervasive metasomatism by footwall material are not applicable. This isotopic evidence indicates
that there was no active interaction of footwall material with the overlying magma during, or after, the formation of Merensky
Reef potholes, a basic tenet of existing pothole formation hypotheses involving footwall mass-transfer. In contrast, the isotopic
data are entirely consistent with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating
laterally to fill extensional zones in the footwall layers.
Received: 11 October 1997 / Accepted: 21 December 1998 相似文献
12.
The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and
along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb
and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main
chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at
approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic
harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses
reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the
highest Cr2O3 content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the
influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated
magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature,
but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence
of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone
magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports
an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of
chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs
of the Bushveld Complex. 相似文献
13.
M. D. Roberts D. L. Reid J. A. Miller I. J. Basson M. Roberts D. Smith 《Mineralium Deposita》2007,42(3):271-292
The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic
melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies
of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to
the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two
sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into
the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types
are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2
reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic
platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole
rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the
base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections
of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression
by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate
footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation
of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine
and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the
Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates
in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an
immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the
concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic
units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted
footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed
by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic
PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the
Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted
footwall. 相似文献
14.
W. D. Maier L. de Klerk J. Blaine T. Manyeruke S.-J. Barnes M. V. A. Stevens J. A. Mavrogenes 《Mineralium Deposita》2008,43(3):255-280
In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of
strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the
farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable
variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized
sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites
are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the
sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal
% and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with
the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform
into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca.
400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites,
and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation
of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than
elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics.
At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with
minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly
defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates
that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings
is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic
units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated
with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination
with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas,
notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which
precipitated during cooling and crystallization. 相似文献
15.
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. 相似文献
16.
Geochemical exploration for platinum-group elements in the Bushveld Complex, South Africa 总被引：2，自引：0，他引：2
H. J. Wilhelm H. Zhang F. L. Chen J. H. Elsenbroek M. Lombard D. de Bruin 《Mineralium Deposita》1997,32(4):349-361
Analyses of stream sediment and soil samples from the Bushveld Complex, South Africa have revealed enhanced precious metal
concentrations, which can be related both to mining activities and the presence of hidden concentrations of platinum-group
elements (PGEs) and gold. The economically important PGE deposits hosted by the Upper Critical Zone of the Rustenburg Layered
Suite are revealed by a high PGE and Au content in the overlying soils. A second zone of elevated precious metal concentrations
straddles the boundary between the Main and Upper Zones and has to date been traced for more than 100 km. This zone follows
the igneous layering of the Rustenburg Layered Suite and is offset by the Brits Graben. It is therefore thought to be the
reflection of a magmatic PGE-Au mineralisation.
Received: 31 May 1996 / Accepted: 7 January 1997 相似文献
17.
Inga Osbahr Reiner Klemd Thomas Oberthür Helene Brätz Robert Schouwstra 《Mineralium Deposita》2013,48(2):211-232
Base-metal sulfides in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite, and pyrite were determined in samples from the mineralized portion of four Merensky Reef intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for trace elements (PGE, Ag, and Au). Whole rock trace element analyses were performed on representative samples to obtain mineralogical balances. In Merensky Reef samples from the western Bushveld, both Pt and Pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns. In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite only contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores. Palladium and Rh concentrations in pentlandite reach up to 700 and 130 ppm, respectively, in the samples from the eastern Bushveld, and up to 1,750 ppm Pd and up to 1,000 ppm Rh in samples from the western Bushveld. Only traces of Pt are present in the base-metal sulfides (BMS). Pyrrhotite contains significant though generally low amounts of Ru, Os, and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries only extremely low PGE concentrations. Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10–40 %) of the Os, Ir, and Ru. Chalcopyrite and pyrrhotite usually contain less than 10 % of the whole rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of discrete platinum-group minerals such as cooperite/braggite, sperrylite, moncheite, and isoferroplatinum. Distribution patterns of whole rock Cu, Ni, and S versus whole rock Pd and Pt show commonly distinct offsets. The general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, is Pd in pentlandite?→?Pd in whole rock?→?(Cu, Ni, and S). The relationship is not that straightforward in general; some of the reef sequences studied only partially show similar trends or are more complex. In general, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation. 相似文献
18.
In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized
zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change
from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly
perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope
system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of
2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave.
−7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local
contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the
Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef
has generally higher plagioclase and pyroxene δ
18O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic
evidence for sub-solidus interaction with fluids, and the Δ
plagioclase–pyroxene values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope
system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative
δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ
plagioclase–pyroxene in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO2-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction
is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale.
The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination
played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO2 produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation. 相似文献
19.
Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa 总被引：5，自引：0，他引：5
The Merensky Reef of the Bushveld Complex is one of the world'slargest resources of platinum group elements (PGE); however,mechanisms for its formation remain poorly understood, and manycontradictory theories have been proposed. We present precisecompositional data [major elements, trace elements, and platinumgroup elements (PGE)] for 370 samples from four borehole coresections of the Merensky Reef in one area of the western BushveldComplex. Trace element patterns (incompatible elements and rareearth elements) exhibit systematic variations, including small-scalecyclic changes indicative of the presence of cumulus crystalsand intercumulus liquid derived from different magmas. Ratiosof highly incompatible elements for the different sections areintermediate to those of the proposed parental magmas (CriticalZone and Main Zone types) that gave rise to the Bushveld Complex.Mingling, but not complete mixing of different magmas is suggestedto have occurred during the formation of the Merensky Reef.The trace element patterns are indicative of transient associationsbetween distinct magma layers. The porosity of the cumulatesis shown to affect significantly the distribution of sulphidesand PGE. A genetic link is made between the thickness of theMerensky pyroxenite, the total PGE and sulphide content, petrologicaland textural features, and the trace element signatures in thesections studied. The rare earth elements reveal the importantrole of plagioclase in the formation of the Merensky pyroxenite,and the distribution of sulphide. KEY WORDS: Merensky Reef; platinum group elements; trace elements 相似文献
20.
The concentrations of platinum-group elements (PGE), Co, Re,Au and Ag have been determined in the base-metal sulphide (BMS)of a section of the Merensky Reef. In addition we performeddetailed image analysis of the platinum-group minerals (PGM).The aims of the study were to establish: (1) whether the BMSare the principal host of these elements; (2) whether individualelements preferentially partition into a specific BMS; (3) whetherthe concentration of the elements varies with stratigraphy orlithology; (4) what is the proportion of PGE hosted by PGM;(5) whether the PGM and the PGE found in BMS could account forthe complete PGE budget of the whole-rocks. In all lithologies,most of the PGE (65 up to 85%) are hosted by PGM (essentiallyPt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride).Lesser amounts of PGE occur in solid solution within the BMS.In most cases, the PGM occur at the contact between the BMSand silicates or oxides, or are included within the BMS. Pentlanditeis the principal BMS host of all of the PGE, except Pt, andcontains up to 600 ppm combined PGE. It is preferentially enrichedin Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, butexcludes both Pt and Pd. Chalcopyrite contains very little ofthe PGE, but does concentrate Ag and Cd. Platinum and Au donot partition into any of the BMS. Instead, they occur in theform of PGM and electrum. In the chromitite layers the whole-rockconcentrations of all the PGE except Pd are enriched by a factorof five relative to S, Ni, Cu and Au. This enrichment couldbe attributed to BMS in these layers being richer in PGE thanthe BMS in the silicate layers. However, the PGE content inthe BMS varies only slightly as a function of the stratigraphy.The BMS in the chromitites contain twice as much PGE as theBMS in the silicate rocks, but this is not sufficient to explainthe strong enrichment of PGE in the chromitites. In the lightof our results, we propose that the collection of the PGE occurredin two steps in the chromitites: some PGM formed before sulphidesaturation during chromitite layer formation. The remainingPGE were collected by an immiscible sulphide liquid that percolateddownward until it encountered the chromitite layers. In thesilicate rocks, PGE were collected by only the sulphide liquid. KEY WORDS: Merensky Reef; Rustenburg Platinum Mine; sulphide; platinum-group elements; image analysis; laser ablation ICP-MS 相似文献