8.4 Petrogenesis and ore genesis

As one of the PGE, osmium is uniquely suited as a tracer of petrogenetic and ore-forming processes of noble metal deposits. PGE deposits are generally associated with major mafic complexes, and have been attributed to mixing process between mantle-derived and crustal components (e.g. Naldrett, 1989). The strong fractionation between Re and Os in crust-forming processes, which generates very radiogenic osmium in the crust relative to the mantle, makes osmium a powerful tracer for such studies. Several of the world’s largest basic–ultrabasic intrusions have been subjected to Re)Os analysis; however, the complexities of their chemistry have typically required several studies to gain a reasonable understanding of their genesis.

8.4.1 The Bushveld Complex

The Bushveld Complex is the world’s largest layered mafic intrusion and principal PGE producer. Historically, most of these PGEs came from the famous Merensky Reef, but the UG1 and UG2 chromite layers (chromitites) are now also major sources. Hart and Kinloch (1989) made an ion-microprobe study of PGE sulphides from the Merensky Reef on the western lobe of the intrusion. They found consistent initial ¹⁸⁷Os/¹⁸⁸Os ratios around 0.175 (¹⁸⁷Os/¹⁸⁶Os = 1.45) for grains of laurite (RuS₂) from the Rustenburg, Union and Amandelbult mining areas. These values are far above the mantle growth line for osmium at the time of intrusion of the Bushveld (2.05 Byr ago), indicating a large crustal component in the ore. However, two grains of erlichmanite (OsS₂) from Rustenburg and Union gave low ¹⁸⁷Os/¹⁸⁶Os ratios, lying on the chondritic evolution line (¹⁸⁷Os/¹⁸⁸Os = 0.112).

The Bushveld laurites can be interpreted as the products of crustally contaminated magmas, as has been proposed to explain Sr isotope data for the Bushveld Complex (Sharpe, 1985). However, the erlichmanite results pose a major problem for this interpretation. They cannot be attributed to open-system perturbation of the Re–Os system, since these minerals contain no rhenium. Hence, if the osmium isotope variations in the laurites are attributed to magmatic processes, then the erlichmanites must represent mantle-derived PGE phases which were somehow carried into the intrusion (which seems unlikely). Alternatively osmium isotope variations in the laurites must be hydrothermal in origin, and some component of the Merensky Reef mineralisation must therefore be attributed to hydrothermal introduction of PGE.

In an attempt to solve this conundrum, Schoenberg et al. (1999) analysed separated minerals from the ‘Critical Zone’ underlying the Merensky Reef and the ‘Bastard Unit’ of the Main Zone above it (Fig. 8.22). Powerful evidence against the ‘metasomatic contamination model’ came from four analyses of the Bastard Unit. Four poikilitic pyroxenites, collected from 4 – 20 m above the Reef gave a perfect Re–Os isochron with an age of 2043 " 11 Myr (MSWD = 0.7). When we take into account the uncertainty of around 1% in the Re decay constant, this is in excellent agreement with an intrusive age of 2059 " 1 Myr from U–Pb dating (Buick et al., 2001). Schoenberg et al. argued that such a good isochron would have been very unlikely if pervasive osmium introduction by metasomatism had occurred in the vicinity of the Merensky Reef.

Fig. 8.22. Plot of initial osmium isotope ratios in the Bushveld Complex against stratigraphic height relative to the Merensky Reef. ( " ) = pyroxene; ( Q ) = sulphide; ( ! ) = chromite. After Schoenberg et al. (1999).

Support for the alternative ‘magma mixing model’ came from analysis of chromitities of the Critical Zone (Schoenberg et al., 1999; McCandless et al., 1999). These chromitities were found to have variable initial ratios (Fig. 8.22), including some unradiogenic values near the chondritic evolution line that confirmed the erlichmanite analyses of Hart and Kinloch (1989). Since chromite crystallisation is attributed to magma mixing, the variable initial ratios of the Critical Zone chromitites suggest that two magmas mixed several times in the history of the magma chamber, in different proportions. The same magma mixing process which pushed the mixed magma into the field of chromite crystallisation probably also caused the separation of immiscible sulphide droplets. The chromite grains and sulphide droplets would then have settled at the same time, forming cumulate chromitites with interstitial sulphide. However, the composition of the sulphide droplets thus formed might have been quite variable, as they scavenged PGE from different pockets of an isotopically heterogeneous magma. Hence the data of Hart and Kinloch (1989) are reasonably explained within the overall framework of the magma mixing model.

8.4.2 The Stillwater Complex

The Stillwater Complex in Montana has similarities to the Bushveld Complex, and has also produced some puzzling Re–Os results which have only recently been resolved. As in the Bushveld case, two distinct magmas have been proposed to explain the petrology and chemistry of the pluton. An ultramafic liquid apparently gave rise to the lower Ultramafic Series (UMS), whereas a magma similar to high-Al basalt formed most of the overlying Banded Series. The PGE-bearing J)M reef is located near the stratigraphic boundary between these two liquids, whose mixing may have promoted the segregation of a PGE-bearing sulphide liquid to form the reef. However, chromite-rich layers scattered through the ultramafic series may also reflect small influxes of a high-Al basaltic liquid into a magma chamber crystallising an ultramafic liquid. These chromitites are identified by letters (A, B, G etc).

Despite the initial ratio heterogeneity of the complex, Lambert et al. (1989) obtained a Re)Os isochron from four Re-rich whole-rock samples, comprising two sulphide-rich cumulates, a bronzite pegmatite, and the K-seam chromitite band. The isochron had a low MSWD of 0.03, although this was achieved at the expense of throwing out a fifth data point from a hydrothermally altered harzburgite (Fig. 8.23a). Using a decay constant of 1.64 H 10^!11 yr^!1, the isochron gave an age of 2.66 " 0.08 Byr, but recalibration of the spike (Lambert et al., 1994) increased this to 2.74. Nevertheless, the age is revised back down to 2.69 " 0.08 Byr using the new decay constant of 1.666 H 10^!11 yr^!1. This result is in good agreement with U)Pb and Sm)Nd ages for the intrusion (section 4.1.2), and suggests that fresh whole-rock samples from the Stillwater Complex generally remained as closed systems for Re and Os during a thermal event which re-set Rb)Sr mineral systems in the complex.

Fig. 8.23. Re)Os isochron diagrams for whole-rock samples from the Stillwater Complex. a) Rhenium-rich samples used to construct an isochron. b) Re-poor samples, including the J)M reef ( " ) , chromitite bands A to K, and un-named chromitites ( ! ). NOTE: ages on reference lines should be 2.69 Byr. After Lambert et al. (1989).

Analysis of whole-rock samples with low Re/Os ratios by Lambert et al. revealed a degree of initial ratio heterogeneity similar to that of the Bushveld Complex (Fig. 8.18b). Chromitite bands from the ultramafic series had initial ¹⁸⁷Os/¹⁸⁸Os compositions ranging from a late Archean chondritic value of 0.109 to a maximum initial ratio of 0.145 in the J)M reef. Similar results were also obtained on a smaller suite of samples by Martin (1989). Lambert et al. attributed the variable initial Os ratios in the chromitites to crustal contamination of mantle-derived magmas by an enriched crustal component, whereas Martin (1989) attributed isotopic variation in the reefs to variable mixing between chromite cumulates and contaminated intercumulus liquid.

Further study of fresh Stillwater chromites by Marcantonio et al. (1993) gave puzzling results. Chromite separates and chromitite whole-rocks from four horizons had initial ¹⁸⁷Os/¹⁸⁸Os ratios within error of the ‘chondritic’ mantle ratio of 0.109 at the time of intrusion. On the other hand, samples from the fifth horizon (G chromitite) gave very variable initial ratios, ranging from below the chondritic evolution line to well above it (initial ¹⁸⁷Os/¹⁸⁸Os = 0.79) in a molybdenite-bearing ample. The simplest explanation for the latter sample would be later disturbance of the Re–Os system, but the molybdenite from this sample gave a Re)Os age of 2.74 " 0.08 Byr, suggesting that osmium re-distribution occurred soon after emplacement of the complex. Therefore, Marcantonio et al. attributed the elevated initial ratios in this and other chromitite samples to hydrothermal introduction of radiogenic Os immediately after crystallisation. They also suggested that the Sm–Nd system might have been similarly upset by hydrothermal re-mobilisation, but this seems extremely unlikely.

Subsequent work by Lambert et al. (1994) and Horan et al. (2001) helped to resolve some of these problems. For example, analysis of additional chromitite samples gave a more coherent picture in which initial osmium isotope ratios of massive chromitites gradually decreased upwards through the ultramafic series (Fig. 8.24). Horan et al. attributed this effect to the mixing of two magmas in changing proportions; an osmium-rich melt with a chondritic isotope signature that was probably derived from a mantle plume, and a second magma probably contaminated by upper crustal rocks. According to this model, the fraction of the second (contaminated) magma must have decreased with time. However, the lack of a complete Nd data set on all samples analysed for Os makes the model speculative.

Fig. 8.24. Plot of initial Os isotope ratios (( Os) for massive chromite layers of the Stillwater Complex against stratigraphic height in the ultramafic series (UMS). After Horan et al. (2001).

The J–M reef itself represents a reversal of this trend which is difficult to explain (Lambert et al., 1994). However, as in the Bushveld Complex, it seems most likely that a much larger influx of crustally contaminated melt, mixing with the plume-derived magma, caused major precipitation of immiscible sulphide, which then took on the relatively radiogenic osmium signature of the mixed liquid. Additional Os and Nd analyses on the same samples are needed in order to test and clarify these models.

8.4.3 The Sudbury Igneous Complex

The Sudbury Igneous Complex (SIC) in Ontario, Canada, is a large mafic body which also hosts the world’s largest nickel reserves. However, isotopic evidence indicates a unique origin for the magmas which gave rise to this mineralisation. This can be attributed to the genesis of the Sudbury structure in a meteorite impact, a model first proposed by Dietz (1964) and now confirmed by numerous lines of evidence.

Nd isotope data for the silicate rocks of the SIC were presented by Faggart et al. (1985) and Naldrett et al. (1986). Both groups showed that the silicate rocks had a remarkably strong crustal signature, with , Nd at 1.85 Byr averaging about !7.5, although Naldrett et al. found a range of , values from !5 to !9. Faggart et al. argued that their data could be explained by an exclusively crustal origin for the SIC, whereas Naldrett et al. preferred a model involving gross crustal contamination of a mantle-derived magma. However, because Nd is a lithophile element, this evidence cannot reliably be extrapolated to deduce such an origin for the nickel-bearing sulphide ores of the complex. Furthermore, the enrichment of Nd in crustal relative to mantle-derived melts makes it an insensitive tracer for a small mantle-derived source component, which would tend to be swamped by crustal Nd. In this situation, Os data may be more diagnostic.

Walker et al. (1991) demonstrated approximate agreement between Re)Os isochron ages for sulphide ores and the 1.85 Byr U)Pb age of the silicate rocks (Krogh et al., 1984). This substantiated previous geochemical evidence that the sulphide and silicate melts were co-genetic. However, age correction of measured isotope ratios in ores from the Levack West, Falconbridge and Strathcona mines gave rise to variable initial Os isotope ratios 1.85 Byr ago. This was attributed to a heterogeneous magma body, formed by variable mixing between mantle-derived osmium and radiogenic crustal osmium.

In contrast to these results, Dickin et al. (1992) observed relatively good homogeneity of initial osmium isotope ratios for sulphide ores from the Creighton, Falconbridge and Levack West mines. Tails to lower initial ratios in two of these mines, and the large scatter of initial ratios from the Strathcona mine, were attributed to post-intrusive open-system behaviour of the Re)Os system, possibly in response to the Grenville orogeny. The consensus of initial ratios for Sudbury mines falls within the range of estimated crustal compositions at 1.85 Byr, and was attributed by Dickin et al. to an entirely crustal source for osmium in the Sudbury ores. This is consistent with an origin of the SIC as an impact melt sheet (Dietz, 1964). However, no evidence of material contribution from the meteorite itself is seen.

Further work by Dickin et al. (1999), Cohen et al. (2000) and Morgan et al. (2002) has resulted in some convergence between the previous positions. It is now recognised that the complex probably had a 100% crustal origin, but it is also recognised that there is considerable Os isotope heterogeneity in the complex due to incomplete mixing between the melted target rocks. The degree of osmium heterogeneity in the complex is demonstrated by the compilation of data in Fig. 8.25. This histogram shows good agreement between the initial ratios of Creighton and Falconbridge ores, but much larger variations in mineralised ‘inclusions’ from the Whistle mine. These ‘inclusions’ are mafic–ultramafic xenoliths whose elemental chemistry is indicative of an origin as cumulates from the SIC magma. However, their isotopic signatures are indicative of an origin from locally melted crustal rocks, at least some being of basaltic composition. Hence it appears that the SIC magma was very poorly homogenised at an early stage in its evolution when these cumulates were formed, but became much better homogenised as it cooled and differentiated (Dickin et al., 1999).

Fig. 8.25. Histogram of initial Os isotopic ratios (( Os) for Sudbury ores 1.85 Byr ago. Some mines display homogeneous osmium, while others (e.g. Whistle) are very heterogeneous. After Cohen et al. (2000).

The evidence from the Whistle mine for impact melting of mafic as well as felsic crustal rocks is supported by ¹⁹⁰Pt–¹⁸⁶Os isotope evidence (Morgan et al., 2002). Comparison of initial ¹⁸⁶Os and ¹⁸⁷Os abundances in three mines revealed a rough inverse correlation, consistent with the mixing of distinct lithologies. Thus, Strathcona samples had radiogenic ¹⁸⁷Os but unradiogenic ¹⁸⁶Os, indicative of a large component of melted felsic rocks with high Re/Pt ratios. On the other hand, samples from Falconbridge and McCreedy West had less radiogenic ¹⁸⁷Os, but more radiogenic ¹⁸⁶Os, indicative of a component of melted basic rocks with lower Re/Pt ratios. This mixing model is supported by Sr isotope evidence (e.g. Dickin et al., 1999). However, Pb isotope evidence provides a different slant, by revealing distinct contributions to the impact melt sheet from different crustal depths (Dickin et al., 1996, 1999). Thus, most Pb in North Range ores came from Archean crust, whereas most Pb in South Range ores came from Huronian supracrustals.

Hence it is concluded that the melt sheet formed by the Sudbury meteorite impact was a complex mixture of shock-melted crustal rocks. Mixing of mafic and felsic lithologies probably caused the melt to enter the field of immiscibility between silicate and sulphide melts (Naldrett et al., 1986). This sulphide melt was of crustal origin and probably did not originally contain high levels of PGE. However, the melt must have been in intimate contact with the pool of fused crustal material for considerable time. During this time, PGE were partitioned from the bulk crustal melt into the sulphide phase in a process analogous to the nickel sulphide fire assay method (e.g. Hofmann et al., 1978). Hence, Sudbury represents ‘nature’s largest fire assay’.

8.4.4 Flood basalt provinces

A combination of Os and Nd isotope data has been used to study mixing between source components of the picritic Karoo flood basalts from southern Africa (Ellam et al., 1992). Initial Os)Nd isotope data for these samples (at 190 Myr) are shown relative to possible source reservoirs in Fig. 8.26. These data reveal some of the potential advantages of Os isotope data, relative to other tracers such as Nd, in discriminating between components involved in basalt genesis. Nd isotope compositions in the continental crust and sub-continental lithosphere may be similar, making these components hard to resolve. In addition, the greater incompatible-element inventories of lithospheric reservoirs, relative to asthenospheric melts, make it hard to quantify the contamination processes which may occur during magma ascent through the lithosphere. On the other hand, Os isotope data can resolve these components because the Re-depleted sub-continental lithosphere typically has an unradiogenic signature relative to plume sources and crustal units (Walker et al., 1989a). The model mixing lines in Fig. 8.26 suggest that there were variable contributions of material to magma genesis from asthenospheric and lithospheric mantle (see section 7.2.4).

Fig. 8.26. Plot of , Nd against ( Os for Karoo basalts and possible source reservoirs. Mixing lines between plume and lithospheric components are shown. After Ellam et al. (1992).

Another flood basalt province from which osmium isotope data have been obtained is the Deccan trap province of southern India (Allegre et al., 1999). Samples collected from a wide range of stratigraphic heights in the lava pile nevertheless had very homogeneous initial ratios close to the chondritic evolution line. This is in contrast to lithophile isotope tracers, which record significant effects of crustal contamination. This contrast in behaviour reflects the known enrichment of osmium in primitive mantle-derived magmas relative to continental crust.

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