Halogen Geochemistry of the Stillwater and Bushveld ...

Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids by A. E. BOUDREAU, E. A. MATHEZ AND I. S. McCALLUM Department of Geological Sciences, AJ-20, University of Washington, Seattle, Washington 98195

ABSTRACT Compositional data on apatite, phlogopite, and amphibole indicate that the high-temperature hydrothermalfluidswhich affected the lower portions of the StiUwater and Bushveld Complexes were Cl-rich. Apatites from the platinum-group element (PGE) ore zones from both complexes are enriched in Cl relative to other cumulus and noncumulus apatites in these intrusions and to apatites from the Skaergaard and Kiglapait Intrusions and the Great Dyke. Apatites from all five intrusions can be grouped into three distinct compositionalfields:(a) Cumulus apatites are essentially fluorapatites with molar C1/(C1 + OH + F) < O03; (b) noncumulus apatites, with the exception of those from the PGE ore zones of the Stillwater and Bushveld Complexes, have C1/(C1 + OH + F) < 0-20; (c) Cl-rich apatites associated with PGE-rich zones have C1/(C1 + OH + F) between 0-45 and 10. The REE content of noncumulus and Cl-rich apatites also show a positive correlation with Cl concentration. It is argued that because Cl is less soluble in silicate melts than F and because melts with extremely high Cl/F ratios are unknown, the Cl-rich apatites equilibrated with Cl-rich hydrothermal fluids exsolved during solidification of the cumulate sequence. The Cl, F, and OH contents of phlogopites and amphiboles are more variable. Compositional heterogeneity is due to crystal-chemical controls on halogen contents, variation in the halogen content of the original melt/fluid phase and subsolidus re-equilibration during cooling with both surrounding mineral phases and low temperaturefluids.However, both the Stillwater and Bushveld phlogopites are enriched in Cl compared to those from the Skaergaard and Kiglapait Intrusions. The compositions of coexisting minerals from the platinum deposit of Oli vine-Bearing Subzone I of the Stillwater Complex are used to compute a fluid composition. Thefluidisrichin alkalis and iron as well as HC1, and the solution composition is consistent with fluid compositions deduced for the PGE-bearing secondary hortonolite pipes of the Bushveld Complex. The high (Pt + Pd)/Ir ratios of these deposits are also consistent with a hydrothermal origin, as both Pt and Pd are more soluble in Cl-complexing fluids than Ir. INTRODUCTION The debate over the genesis of the stratiform deposits of the platinum-group elements (PGE) in the Bushveld and Stillwater Complexes has focused on whether concentration mechanisms are dominated by stratabound accumulations of PGE-rich immiscible sulfide liquids separating from the magmas (e.g., Campbell et al., 1983), or whether volatile-rich fluids migrating through partially to completely crystallized cumulus sequences were also important. The presence of pegmatoids and volatile-bearing phases in the stratiform PGE-bearing zones of both intrusions suggests that fluids have played some role in petrogenesis (e.g., Todd et al., 1982; Ballhaus & Stumpfl, 1985a). Furthermore, studies on the hortonolite pipes in the eastern Bushveld (Cameron & Desborough, 1964; Peyerl, 1982; Schiffries, 1982; Stumpfl & Rucklidge, 1982) demonstrate that the PGE can be concentrated {Journal ofPttrologj,

VoL 27, Part 4, pp. 967-986, 1986]

C Oxford Unjvenity Prcn 1986

Downloaded from http://petrology.oxfordjournals.org/ at Duke University on May 29, 2012

(Received 21 August 1985; revised typescript accepted 4 February 1986)

968

A. E. BOUDREAU

ETAL.

SAMPLES Samples from the Stillwater Complex are from the Ultramafic Series, Olivine-Bearing Zone I (OBZ I) and Anorthosite Zone II (AN II) (stratigraphic subdivisions of McCallum et al., 1980). The Ultramafic Series samples were collected from both the Mountain View and Chrome Mountain areas (Raedeke & McCallum, 1984). The OBZ I samples are mostly from the Minneapolis adit (Bow et al., 1982) but include several from the Frog Pond and West Fork adits (Todd et al., 1982). The AN II samples were collected from the Contact Mountain and Picket Pin Mountain areas (McCallum et al., 1980; Boudreau & McCallum, 1986). The Bushveld samples were collected during the fieldtrip associated with the 1981 Third International Platinum Symposium. Most of those from the Critical Zone are from the Rustenburg Platinum Mine and Western Platinum Mine in the western Bushveld Complex, but several samples from localities in the northern and eastern limbs of the Complex are also included. All the data reported for the Merensky Reef are from samples of the 'normal' reef. More detailed information on individual samples is included in Tables 1, 2, and 3. One sample from the Great Dyke, an apatite-bearing PGE-enriched pyroxenite supplied by M. Prendergast, was also studied. APATITE Occurrence In the Stillwater Complex, apatite is rare and occurs exclusively as an accessory interstitial mineral. It is present in amounts up to 0 3 per cent in quartz-bearing and incompatible element-enriched parts of the Anorthosite subzones (AN I and AN II) of the Middle Banded Zone (Salpas et al., 1983) (Fig. 1 A), including the PGE- and sulfide-bearing anorthosite at the top of AN II (Boudreau & McCallum, 1986). Apatite is also associated with the economic platinum deposit of OBZ I (the J-M, or Howland Reef; Bow et al., 1982; Todd et al., 1982). Within OBZ I, up to 0 5 per cent apatite is commonly associated with coarse-grained,


by subsolidus hydrothermal fluids. In addition, volcanic gases are now known to contain unexpectedly high concentrations of Ir and other noble elements (Zoller et al., 1983). Therefore, the importance of hydrothermal fluids, as either directly responsible for or causing substantial modifications to the stratabound ore zones, cannot be ruled out. The few data available that bear directly on the hydrothermal environments of the Stillwater and Bushveld Complexes indicate that Cl was an important component. Schiffries (1982) found Cl-rich amphiboles in the vicinity of the Driekop pipe, and Cl- and F-rich phlogopites (Ballhaus & Stumpfl, 1985a; Johan & Watkinson, 1985) and hypersaline fluid inclusions (Ballhaus & Stumpfl, 1985b; SchifTries, 1985) have been reported from the Merensky Reef and associated rocks of the western Bushveld. In addition, there are brief references to Cl-rich minerals elsewhere in the Bushveld and Stillwater Complexes e.g., Kinlock (1982) and Volborth & Housley (1984). Apatites, phJogopites, and calcic amphiboles in the Stillwater and Bushveld Complexes associated with the ore zones are shown to be particularly Cl-rich compared to the same minerals from other parts of these intrusions and other layered intrusions. We argue that Cl-rich hydrothermal fluids exsolved during solidification of the cumulate sequence and acted as transport agents for the PGE and REE. Thermochemical data on mineral-fluid systems are used to calculate compositional characteristics of the high temperature Cl-bearing hydrothermal fluids.

THE STILLWATER AND BUSHVELD COMPLEXES

0.2 5mm

olivine-rich rocks containing abundant interstitial phlogopite. In some cases, these apatite grains are several millimeters in diameter and contain inclusions of phlogopite, monazite, and pargasitic hornblende (Fig. IB). Negative crystal faces in the host apatite indicate that these inclusions grew from trapped, volatile-rich silicate melt. Large (~ 1 cm long) apatite also occurs in norite pegmatoids associated with massive PGE-sulfide mineralization in the Minneapolis adit (A. Cridell, pers. comm.). Apart from these occurrences, apatite and the other magmatic halogen-bearing minerals are relatively rare. Apatite is an abundant cumulus mineral in the evolved diorites of subzone C of the Upper Zone of the Bushveld Complex (von Gruenewaldt, 1973). It is also present as an interstitial phase in three of the ten UG-2 and Merensky Reef samples included in this study (Fig. 1C). We are unaware of other reports of intercumulus apatite in the Bushveld Complex but presume, based on analogy with other intrusions, that it exists as a rare accessory mineral throughout the complex. As in the case of apatite in the Stillwater ore zone, the apatite in the Critical Zone rocks is typically associated with phlogopite. In several Bushveld samples,


FIG. 1. Photomicrographs of apatites, phlogopites and amphiboles from the Stillwater and Bushveld Complexes. (A) Euhedral apatites (a) associated with interstitial clinopyroxene (with exsolution lamellae) in the upper part of Anorthosite subzone II, Stillwater Complex. Opaque mineral is an Fe-Ti oxide. (B) Apatite and phlogopite in a partially serpentinized dunite from the Minneapolis adit, Olivine-bearing subzone I of the Stillwater Complex. The light mineral surrounding the phlogopite (ph) and apatite is orthopyroxene. The inclusions (i) in the apatite consist of pargasitic hornblende and phlogopite. A probe traverse across this apatite grain is shown in Fig. 4. ( Q Euhedral apatites (a) with plagioclase (l'gh'X bronzite (dark), and hornblende (hb) in the Merensky Reef, Western Platinum Mine, Bushveld Complex. The sample is from approximately 10 cm above the footwall norite in normal reef. (D) An apatite- (a) and phlogopite-bearing vein in the UG-2 Chromitite, Western Platinum Mine. The vein is interpreted to have been a fluid + melt filled fracture (see text).

A. E. BOUDREAU ET AL.

970

apatite and phlogopite form isolated grains arranged in ~ 01 mm wide semi-continuous, planar, zones (Fig. 1D). These zones traverse and in some cases offset the cumulate fabric and are loci along which the fabric is deformed and recrystallized. They may be related to the macroscopic veins of coarse-grained plagioclase-phlogopite in the Critical Zone. Such veins, termed 'flame structures' by the mine geologists at Rustenburg, are also present in the Minneapolis adit in the Stillwater Complex. The assemblages in these macroscopic and microscopic vein-type structures suggest that they were at one time fluid-)-melt-filled microfractures in the hot and possibly partially molten cumulate rocks.

TABLE 1

CaO P2O5 F

aH

2O' SiO 2 MnO FeO 2 MgO SrO Na 2 O Y2O3 U2O3 Ce 2 O 3 total O = F, Cl total

1

2

3

4

5

6

7

8

9

54-00 5O07 0-30 4-86 0-34 0-33 O06 0-17 0-06 O02 0-20 0-18 0-28 0-59 101-46 1-22 100-70

54-59 41-64 1-60 093 0-70 O09 0-11 0-07 nd nd 004 na 003 009 99-89 0-89 99-00

54-72 41-08 1-76 046 074 013 008 O20 nd nd 001 na 003 O10 99-30 085 98-45

5515 42-76 2-46 039 051 016 005 006 004 nd 006 nd 015 Oil 101-90 111 10O79

53-34 4043 O12 6-85 OOO 049 na 028 na 004 na 007 027 056 102-45 1-60 10O85

5415 4O90 096 3-40 084 049 na 049 na 005 na 013 021 047 102O9 117 I0O92

53-63 4086 OOO 6-61 O09 034 na 007 na 005 na 013 007 O30 10215 1-49 10O66

52-42 39-82 029 617 OO9 061 na 017 na O03 na O07 039 082 10088 1-51 99-37

55-30 41-42 2-55 036 O95 na na O90 na nd na 006 002 0-08 101-64 116 10048

Notes: 1. H 2 O calculated on basis Cl + OH + F = IO in structural formulae. 2. All Fe calculated as FcO. Abbreviations for this and following tables: na = not analyzed; nd = not detected; tr = trace amounts noted. Analytical conditions: 15 kV acceleration potential, 20-50 nA sample current. Standards included natural chlor- and fluorapatite, Q-bearing scapolite and fluorite. REE were referred to synthetic glasses. Data were corrected using a Bence-Albee correction procedure (Bence & Albee, 1968). Samples: Stillwater: (1) 1 mm diameter apatite associated with phlogopite (Table 2, analysis 1), from a serpcntinizcd dunite, OBZ I, Minneapolis adit, sample 5104E-X (Fig. IB). (2) Core and (3) rim of intercumulus apatite from strongly altered rock from the Picket Pin ore zone (Boudreau & McCallum, 1985), top of AN II, Contact Mountain area, sample WD 13. (4) Intercumulus apatite, AN II, unaltered sample. Bushveld: (5) and (6) Two grains from a sample of the Merensky Reef, Western Platinum Mine, Western Bushveld (Fig. 1C). (7) UG-2 chromitite, Western Platinum Mine (Fig. ID). (8) UG-2 chromitite, bronzdtite pegmatoid, Hackney area, eastern Bushveld. (9) Cumulate apatite from diorite, subzone C of Upper zone, Steynsdrift, eastern Bushveld.

Composition Representative analyses of Stillwater and Bushveld apatites are presented in Table 1, and the halogen and REE contents of these and apatites from other layered intrusions are compared in Figs. 2 and 3, respectively. The apatites in the Stillwater and Bushveld Complexes, and Skaergaard and Kiglapait Intrusions and the Great Dyke fall into three compositional groups.


Electron microprobe analyses of apatites from the Stillwater and Bushveld Complexes


971

CUMULUS: A Skaergaard (Nash. 1976, Brown & Peckett, 1977) O Kiglapait (Huntington. 1979) O Bushveld (Grobler & Whitfleld. 1970. this study)

NONCUMULUS: A Skaergaard (Nash, 1976, Brown & Peckett, 1977) 3 Kiglapait (Huntington, 1979) O Stillwater AN II (thi. study) *7 Great Dike Pt Zone (this study)

•

Stillwater OB I

this study

Bushveld Critical Zone

APATITE MOLE96 FIG. 2. A comparison of the compositions of cumulus, noncumulus, and Cl-rich ore zone apatites from the Stillwater and Bushveld Complexes, the Kiglapait and Skaergaard Intrusions and the Great Dyke.

Cumulus apatite As can be seen in Fig. 2, all the cumulus apatite plots near the fluorapatite apex along the F-OH join and contains less than 3 mol per cent chlorapatite. Although cumulus apatite from the Bushveld Complex has relatively low concentrations of the REE (i.e. La 2 O 3 + Ce 2 O 3 abundances are usually < 0-2 wt. per cent), that from highly evolved rocks of other intrusions, such as the Sandwich Horizon in the Skaergaard Intrusion, may be more REE-rich. Noncumulus apatite Noncumulus apatite is part of the intercumulus assemblage. Such apatite is present in the Stillwater anorthosites, throughout the Skaergaard Intrusion (including the chilled margin) (Nash, 1976; Brown & Peckett, 1977) and in the Outer Border Zone of the Kiglapait Intrusion (Huntington, 1979). Brown & Peckett (1977) have noted that noncumulus apatite of the Skaergaard Intrusion contains lower F and higher OH and Cl concentrations than cumulus apatite. This relationship holds for apatite from other intrusions as well. The REE contents of noncumulus apatite are variable but similar to those in cumulus apatite (Fig. 3). Cl-rich apatite Interstitial apatite and vein apatite from the PGE ore zones in the Bushveld and Stillwater Complexes are characterized by high Cl contents. In fact, in some samples, the halogen site is occupied with Cl to the maximum extent possible. Such Cl-rich apatite has not been reported from other intrusions or elsewhere in the Bushveld or Stillwater Complexes. The REE contents of the Cl-rich apatite are variable but typically higher than those of cumulus apatite.


CI-RICH:

972


ro O CVJ

cvj OH » Cl (Stormer & Carmichael, 1971; Korzhinskiy, 1982). Experimental data on the partitioning of the halogens between silicate melt and aqueous fluid indicate that Cl is preferentially partitioned into thefluidwhereas F is retained in the melt, the fluid/melt partition coefficients for these elements being on the order of 10 and 01, respectively, for melts of granitic composition (e.g., Kilinc & Burnham, 1972; Hards, 1976; Candela, 1985); i.e. the distribution behavior of halogens among vapor (v), melt (m) and apatite (ap) is such that (Cl/F),p < (Cl/FL < (Cl/F). Thus, the combined processes of degassing of magma and fractional crystallization of non-halogen bearing minerals will result in a decrease in the Cl/F ratio of the magma with progressive fractionation. The F-rich cumulus apatite is consistent with this distribution behavior. Second, the moderate Cl/F ratios observed in both natural glasses (e.g., Devine et al., 1984) and volcanic gases (e.g., Oskarsson, 1981) mitigate against magma compositions with Cl/F ratios significantly greater than one. In addition, there is no evidence from the apatites from the chilled margins of either the Skaergaard Intrusion (Nash, 1976) or the Outer Border Zone of the Kiglapait Intrusion (Huntington, 1979) that the parental magmas of these intrusions were particularly Cl-rich. Indeed, the chilled margin Skaergaard apatite contains only 21


ro O


973

REE geochemistry In addition to being Cl-rich, the ore zone apatites possess relatively high concentrations of REE (Table 1). These contents are comparable to those in apatites from REE-rich alkaline rocks, such as the Shonkin Sag (Nash, 1972) and the Gremyakha-Vyrmes Intrusion (Tikhonenkova & Udod, 1984). Apatites from these two intrusions exhibit regular increases in REE contents with stratigraphic height, a relationship which exists neither for the Skaergaard apatite (Nash, 1976) nor for the Bushveld and Stillwater ones. However, the noncumulus apatites do exhibit a broad correlation between their Cl and REE contents (Fig. 3), with the Cl-rich apatites as a group containing as much or more of the REE as the cumulus apatites from the more evolved rocks of other intrusions. For example, apatite from the Sandwich horizon of the Skaergaard Intrusion contains only ~ 05 wt. percent La 2 O 3 + Ce2O3 (Nash, 1976). In view of the experimental evidence that Cl-rich fluids may transport significant amounts of REE (Flynn & Burnham, 1978; Webster & Holloway, 1980; Candela, 1984, 1985), this correlation suggests that the ore zone apatites could have become enriched


mol per cent chlorapatite component and is lower in Cl than intercumulus apatite from the same intrusion (Nash, 1976). Thus, the Cl-rich apatites of the Stillwater and Bushveld ore zones cannot be reasonably explained by crystallization of a trapped melt but must have involved the addition of Cl. It is clear that simple crystallization of apatite from a closed fluid-melt system should produce crystals which have relatively low Cl/F ratios. How, then, were the Cl-rich apatites of the ore zones generated? It is proposed that Cl-rich fluids evolved during solidification of the intercumulus melt in the lower parts of the Stillwater and Bushveld Complexes and that these fluids migrated to and were trapped in the ore zones by redissolution in vapor-undersaturated interstitial melt. These fluids were added to the crystal/melt assemblages (with low initial Cl/F ratios) in amounts such that thefluidmasses dominated that of the intercumulus melts and thus significantly elevated the bulk Cl/F ratios of the ore zone assemblages. That the ore zones were enriched in fluids is supported by the characteristic coarse-grained to pegmatitic nature of the rocks and the relatively high modal abundance of halogen-bearing or incompatible element-rich minerals. The presence of high temperature fluids in the Merensky Reef and associated rocks has been hypothesized by a number of workers, including Ballhaus & Stumpfl (19856) from observations of fluid inclusions, Johan & Watkinson (1985) from data from coexisting chromites and micas, Elliot et al. (1982) and Buntin et al. (1985) from redox relationships in rocks in the vicinity of potholes, and Kruger & Marsh (1985) from textural and mineralogical observations. Because of the partition behavior noted above, the dissolution of fluid into hotter, vapor-undersaturated intercumulus melt can increase the Cl/F ratio of the melt by several orders of magnitude, depending on the amount of vapor added. When, on cooling, this enriched melt reaches vapor saturation, the separating fluid would be richer in Cl than the original fluid. In this respect, fluids may become progressively enriched in Cl by a process akin to zone refining as a fluid saturation front migrates through the crystal pile during solidification. The lower but variable Cl contents and intermediate Cl/F ratios of the stratigraphically higher noncumulus apatite from the Stillwater anorthosites might be explained by one of two mechanisms: (1) the apatite may have crystallized from interstitial melts to which relatively small and variable amounts of Cl-rich fluid were added; and (2) it equilibrated with fluid exsolved from the more evolved magma. Such magma would have been poorer in Cl and richer in F than more primitive magma if fluid was lost by degassing as crystallization proceeded.

974

A. E. BOUDREAU ET AL

in the REE by addition of Cl-bearing hydrothermal fluids. If the apatites equilibrated with trapped intercumulus melts in a closed system, however, then such melts would have had to have contained approximately 2000 to 5000 p.p.m. total REE, based on the partitioning data of Watson & Green (1981). The addition of REE to the ore zones is also consistent with the variable REE contents of OBZI plagioclase separates from the Stillwater Complex reported by Lambert & Simmons (1983), who found order of magnitude variations in samples separated by distances of less than a meter. Also, Cameron (1978) reported the presence of REE-rich Ti-Cr oxides in the Merensky and several of the chromitite horizons of the Critical Zone in the eastern Bushveld. Low temperature re-equilibration

FIG. 4. Electron microprobc traverse across a large apatite grain from OBZ I, Stillwater Complex, showing variation in Cl and F. The grain (inset) is that shown in Fig. 1B. Erratic highs in Cl content are not obviously related to fractures and defects in the crystal, although the apatite does contain numerous micron-size inclusions which may have affected the analyses.


Because the Stillwater Complex was subjected to greenschist facies metamorphic conditions subsequent to emplacement, it is appropriate to evaluate the possibility that the apatite compositions were acquired during metamorphism rather than during crystallization. The data of Korzhinskiy (1982) show that at 500 °C (the lowest reported experimental temperatures), an apatite in which C1/(C1 + F + OH) > 0-7 could only coexist with a fluid in which/ HC1 // HF > 103 and aHCI > 10" 2 (for a standard state activity referred to a hypothetical 1 molal solution concentration). However, there is no evidence for the existence of such extreme fluid compositions from the secondary tremolites, which are essentially Cl-free (see below). Instead, loss of Cl by exchange with Cl-poor metamorphic fluids appears to have occurred along the rims of apatite in rocks in which the surrounding original igneous silicate assemblage has been replaced by secondary minerals (compare core/rim analyses, analyses 2 and 3, Table 1). This is not seen in apatites in which the surrounding minerals are largely unaltered (Fig. 4).


975

PHLOGOPITE Occurrence

Composition Compositional data are summarized in Figs. 5 and 6, and representative analyses are presented in Table 2. The Stillwater and Bushveld phlogopites are clearly enriched in Cl compared to those from other layered intrusions (Fig. 5). The most Cl-rich phlogopite reported by Nash (1976) from the Skaergaard rocks contains 028 wt. per cent Cl, whereas concentrations of more than 0-7 wt. per cent are observed in some of the Bushveld phlogopites. Similarly Cl-rich phlogopite is reported in Bushveld rocks by Ballhaus & Stumpfl (1985a) and Johan & Wilkinson (1985). The Bushveld and Stillwater phlogopites are also much more Cl-rich and possess higher Cl/F ratios than those from alkalic basalts, kimberlites, carbonatites, and mantle xenoliths (e.g., Boettcher & O'Neil, 1980, Dawson & Fuge, 1980, Delaney et al, 1980). Unlike the apatites, the Stillwater and Bushveld phlogopites exhibit considerable compositional variability, even within individual probe mounts (Fig. 5). There is a strong crystal-chemical control on the halogen content of micas (e.g., Jacobs & Parry, 1979; Munoz & Swenson, 1981; Volfinger et al., 1985). The amount of Cl that can be incorporated into mica (or amphibole) increases with an increase in either small tetrahedral or large octahedral cations (i.e. higher Si/Al ratio or Fe 2+ /Mg ratio, respectively). There is a tendency for Cl contents to be higher in the more Fe-rich phlogopites of both intrusions (Fig. 6A). In addition, the Bushveld phlogopites tend to be more siliceous than those from the Stillwater


Phlogopite is a common accessory mineral in the lower third of the Stillwater Complex. It is usually present in the olivine-bearing rocks of the Ultramafic Series and Lower Banded Series. It also occurs in pegmatoids throughout the lower part of the complex and, rarely, as in interstitial mineral in orthopyroxenites from the Ultramafic Series. Except for OBZ I, phlogopite is rare or absent in the cumulate rocks of the rest of the Banded Zone. Within OBZ I, phlogopite occurs as an interstitial mineral in olivine-rich rocks, where it may form large (several centimeters) oikocrysts enclosing olivine and comprising up to 10 per cent of the rock. Phlogopite also occurs as round inclusions in chromite and apatite (Fig. IB) of OBZ I, together with pargasitic hornblende, orthopyroxene, and serpentinized olivine (cf., Barnes, 1983). Whereas most phlogopites show strong reddish colors associated with high Ti contents, phlogopite in inclusions within apatite is unusually Ti-poor (Table 2, analysis 3). Phlogopite is present as an accessory mineral throughout the Bushveld Complex. It is most common as interstitial grains associated with chromite. It also occurs as a replacement of pyroxene, usually with quartz. The textures and assemblages suggest that the relevant reaction was melt + vapor + orthopyroxene => phlogopite + quartz. Finally, phlogopite occurs with apatite in microfractures (Fig. ID). As with the Stillwater rocks, the Bushveld rocks that contain > 1 per cent modal phlogopite also contain trace amounts of other incompatible element-rich minerals, i.e. apatite, zircon, sphene, and carbonates. Textural observations suggest that phlogopite and chromite represent a reaction pair. In OBZ I of the Stillwater Complex phlogopite forms rims on chromite, chromite grains have embayed margins suggestive of resorption and samples with abundant phlogopite do not contain chromite. A similar relationship between biotite and magnetite in the Upper zone of the Bushveld Complex is indicated by well-developed phlogopite rims on cumulus magnetites.

976

A E. B O U D R E A U ET

AL.

TABLE 2

Electron microprobe analyses of phlogopite from the Stillwater and Bushveld Complexes 10

11

39-66 5 79 1383 1 28 3-98 21-38 058 961 023 067 0-14 388

39 14 5 19 13-15 1 50 8 58 18 22 009 9-46 021 016 037 3-97

2

3

4

5

6

7

8

9

37 23 5-61 14-25 008 888 nd 013 18 55 nd 080 074 8-66 016 023 3-92 99-24

38-68 010 1503 nd 9-32 na na 21-96 014 005 042 851 023 023 3-90 98 57

35 63 4 79 14-46 1-20 5 76 006 015 22-07 029 0O5 1 39 517 012 003 3 95 95-12

38-65 380 1509 085 402 008 016 22 49 026 nd 014 914 008 009 409 99-20

37-65 5-00 14-27 500 6-22 na na 2O19 nd 036 0-45 9-75 051 030 3-71 99-44

39-03 3 22 13 16 1-08 6-41 na na 20-75 nd 009 Oil 1031 Oil 054 3-92 98-73

38 93 5 81 14 16 1-52 2 85 na na 22-27 na 022 054 9-59 045 032 3-95 100-6

41-58 075 1360 003 7-28 na na 20-90 na 005 020 9-10 093 053 3-57 98-79

SiO 2 TiO 2 AI 2 O 3 Cr 2 O 3 FeO MgO BaO K2O Na 2 O F Cl

H2O2 total

37 98 5-67 1315 003 1051 nd 016 17-29 001 029 076 8 74 016 023 388 9902

O - F, Cl total

012 98 90

012 99-12

015 98 42

006 95-06

005 99-15

028 99 16

017 98-56

026 10O3

051 98 28

O - F, Cl 031 100-7 total

SiOj

TiO 2 A1 2 O 3 Cr2O, FeO 1 MnO NiO MgO CaO BaO Na2O K2O F

a

Si Ti

Al Cr Fe Mg Na K F

aOH

2 808 0351 1146 0001 O650 1907 0108 0825 0037 0029 1-934

2 723 O202 1 232 0023 0545 2038 0105 0810 0036 0028 1936

2-807 0248 1 199 0005 0574 2O54 0100 0817 0039 0O32 1929

2 643 0-274 1-265 0-070 0357 2451 O200 0-488 0027 0004 1-969

Ions per 12 (0, OH, 2-734 2 759 0-273 0204 1 221 1269 0059 0048 0-377 0240 2-197 2401 0064 0019 O902 0856 0017 0118 O037 0011 1-845 1-972

F, Cl) 2851 0177 1133 0062 O392 2 270 0016 0960 0-025 0O66 1-909

2 750 0309 1 179 0085 O169 2 345 0074 0864 0-101 0038 1-861

3011 0041 1 160 0018 0441 2-256 0028 0840 0212 0065 1-723

HjO

total

Si Ti

Al Cr Fe Mg Na K F

a

OH

1010

2-801 0308 1 151 O072 O235 2-250 0O32 0866 0151 0017 1-832

100O 015 99-87

2 839 0-283 1-124 0086 O520 1 969 0029 0875 0036 0O46 1-918

Notes 1. Total Fe expressed as FeO 2 H 2 O based on (OH + F + Cl) — 2-0 in structural formulae. Analytical conditions as for Table 1 Major elements referred to natural phlogopites, other elements as for Table I Samples: Stillwater Complex. (1) Interstitial phlogopite by apatite (Table 1, analysis 1), OBZ I, sample 5104-X (2) Interstitial phlogopite by sulfide, OBZ I, sample SMA 144. (3) Phlogopite inclusion in apatite (Table 1, analysis 1), OBZ I, sample 5104-X (4) Inclusion in chromite, OBZ I, sample SMA 206 (5) SmaJl grain on chromilc, OBZ I, sample SMA 206 (6) Interstitial phlogopite in dunile, Ullramafic Series, sample 248. (7) Interstitial phlogopite in orthopyroxenite, Ultramafic Series, sample 622.

Complex, and this may explain the generally lower Cl content of the latter (Fig. 6B). Similarly, among the Stillwater phlogopites, those with the highest Cl contents occur in orthopyroxenites rather than in olivine-bearing rocks. It should be clear from the scatter of data in Fig. 6 that factors other than the crystal-chemical ones influenced the halogen content of phlogopite. Some of the compositional heterogeneities observed on the thin section scale may be attributed to small-scale variations of fluid compositions and/or re-equilibration with low temperature pore fluids, particularly in those samples with minor phlogopite. Volfinger et al. (1985) have noted that partitioning of Cl between fluid and biotite is strongly dependent on fluid acidity; thus, annites equilibrated with HCl may contain ten times as much Cl as those equilibrated with KC1 solutions of equivalent Cl concentration (cf., Munoz & Swenson, 1981). Also, Stormer & Carmichael (1971) have shown that halogens in biotite may readily re-equilibrate on cooling, and Nash (1976) noted a correlation between the F content of biotites from the Skaergaard and oxygen isotopic systematics, which suggests that F was lost by exchange with meteoric water. In this context, it is of interest to note that for the Bushveld samples no systematic differences exist in halogen contents of phlogopites either from different textural types or


/


977

T A B L E 2 (cont.)

SiO 3 TiOj

A12O3 Cr2O3 FeO 1 MgO BaO K2O Na 2 O F

a total total Si Ti

AJ CT

Fe Mg Na K F

a

OH

F,

a

13

14

15

16

n

18

19

20

21

22

23

39-95 4-72 13 79 1-23 4-59 21-82 0-30 9-87 0-23 060 0-35 3-85 101-3

39-78 2-93 13-79 0-55 7-76 20-31 0-17 9-12 0-20 0-46 0-39 3-82 99-28

39-35 4-30 14-06 1 33 4-70 21-58 0-40 9-18 0-38 0-49 0-20 3-92 99-89

39-83 0-26 13-94 0-19 9-42 20^4 002 916 0-11 0-21 0-45 3-86 98O9

38-72 3-47 1316 0-44 10-26 18-40 0-10 9-27 008 0-11 0-61 384 98 46

39-57 348 15-33 1-56 2-99 23-23 0-31 817 1 12 002 0-21 4-22 100-2

39H)3 214 14-82 169 400 2310 0O8 9-83 022 Oil 029 407 99-37

39 34 4 72 14-30 1-53 2-22 22-99 037 9O8 O61 O31 012 4O6 99-49

39-98 3-80 13-41 1-36 7O9 19-74 014 9-91 Oil 058 036 3-80 10O3

37-90 3-44 14-43 002 12-99 16-22 O40 9-43 022 042 035 3-74 99-56

38-04 5-25 13-70 1 62 9-32 17-36 065 8-43 085 029 048 3 83 9^65

38-88 1-77 12-83 006 9-89 20-44 O22 7-88 O10 Oil O10 3-93 96-20

0-33 101-0

O28 99-00

0-25 99-64

0-19 97-90

0-19 98-27

006 100-2

Oil 99-26

016 99-33

033 99-95

O26 99-30

018 99-47

007 96-13

2-820 0-251 1148 0069 0-271 2 296 O032 0-889 0-135 0O42 1-823

2-882 0-160 1117 0032 0-470 2193 0029 O843 0-106 0O49 1-845

2-810 O231 1183 0O75 0-281 2-297 0053 0-836 0-112 0024 1-865

Ions 2-930 0O14 1-209 0011 0-580 2-264 0016 0-860 0O50 0-057 1893

per 12 (0, OH, F, C/) 2-869 2-792 2-775 0O14 0-115 0-184 1-209 1-250 1-267 0026 0096 0087 0-636 0240 0-175 2-032 2-463 2-428 0O12 0030 0153 O897 0-876 0731 0026 0025 0005 0077 O035 0O19 1-897 1-940 1-976

2-738 0251 1 192 O086 O131 2-424 0-084 0820 O070 0015 1-915

2-878 0217 1138 0078 O427 2118 O015 O910 0131 0045 1824

2-818 O193 1-264 0-002 O808 1 798 0032 O895 O100 O044 1-856

2 785 O289 1183 0094 0571 1895 0122 0788 0067 0O60 1-873

2-908 O100 1130 0004 O619 2-279 O015 O751 0026 O013 1-961

Bushveld Complex. (8) UG-2, Rustenburg, sample B2. (9) Pyroxene porphyry, Rustenburg, sample B3. (10) Pyroxene porphyry, Rustenburg, sample B3. (11) and (12) Mercnsky Reef, Rustenburg, two separate grains from same sample B5C. (13) and (14) Mercnsky Reet, two separate grains, Western Platinum Mine, sample B7. (15) and (16) Mercnsky Reef, two different grains from different locality than above, Western Platinum Mine, sample B11. (17) UG-2, Western Platinum Mine, sample B14. (18) Harzburgite subzone of Lower Zone, Oliphants River, sample B2Z (19) Steclpoort seam, Oliphants River, sample B26. (20) UG2 chromititc, pegmatoida! broruritite, Hackney, sample B31. (21) Magnetite gabbro, Magnetite Heights, sample B37. (22) Vlakfontein dunite pipe, sample B6a. (23) Onverwacht pipe, sample B30

from different parts of the intrusion, despite systematic major and minor element variations (noted below). For example, in Fig. 5 it can be seen that the biotites in one sample each of an Upper Zone magnetite-bearing gabbro, Vlakfontein dunite and Onverwacht hortonolite fall within the compositional range defined by those from a single sample from the Merensky Reef. The presence of small-scale heterogeneities in combination with the lack of any systematic large-scale variations in the halogen contents of the Bushveld samples may result from continued local re-equilibration on the microscopic scale with pore fluids as the intrusion cooled. In contrast, the compositions of the large poikilitic grains from several locations within the Minneapolis adit of the Stillwater Complex exhibit much less variability than the fine-grained phlogopites in other Stillwater or Bushveld samples. These large grains apparently grew from a more chemically uniform environment or were less affected by exchange with a low temperature assemblage. The major and minor element contents of phlogopite reflect equilibration with adjacent mineral phases. For example, the Fe-Mg exchange equilibria between phlogopite and olivine in the Stillwater Complex indicate equilibration temperatures in the range of 545-700 °C (Fig. 7). These temperatures are in contrast to a minimum temperature of 925 °C obtained by two pyroxene geothermometry (Barnes, 1983). Furthermore, within the Minneapolis adit suite, phlogopites that exist as inclusions in or rims on chromite are more magnesian and richer in Cr than are the large, poikilitic phlogopites which equilibrated with olivine only.


o-

12


978

Range of Stillwater &. Bushveld phlogopites

PHLOGOPITE 1.0

D

Sample B5, Merensky Reef, Rustenburg

•

Poikilitic phlogopite, OB I, Minneapolis adit

O

Skaergaard & Kigl&pait phlogopites

0.8

~

0.6

o

0.2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

F (wt. %) FIG. 5. Concentrations of Cl and F in phlogopites from the Bushvcld and Stillwater Complexes and other layered intrusions. V, O, and MH indicate the ranges of compositions observed in samples from the Vlakfontein dunite pipe, the Onvenvacht hortonolite pipe and the Upper Zone magnetite gabbro from Magnetite Heights, respectively. Sources for the Kiglapait and Skaergaard data as in Fig. 2.

Interstitial phlogopite associated with chromite in the Bushveld rocks is significantly enriched in Ti and Cr and depleted in Fe relative to phlogopite which replaces orthopyroxene (Table 2, analyses 11 and 12). The former are also much richer in Ba than the latter. A similar distinction was noted by Jacobs & Parry (1979) between Ba-rich phlogopite phenocrysts and Ba-poor late igneous and secondary biotites from a granodiorite porphyry. To summarize, the small scale variations in phlogopite compositions probably reflect: (1) heterogeneities in the compositions of the intercumulus melts and/or Cl-rich fluids from which the phlogopite crystallized; and (2) subsolidus re-equilibration with adjacent minerals and lower temperature pore fluids. Despite the chemical complexities introduced by the late magmatic and lower temperature hydrothermal processes, there are some large-scale compositional trends that should be noted. In particular, both Stillwater and Bushveld phlogopites exhibit the expected compositional trends as functions of their stratigraphic position in the intrusions. Thus, for Stillwater phlogopite, that in OBZ I is more Fe-rich than that from the Ultramafic Series (Fig. 7). In the Bushveld Complex the phlogopite of the magnetite-bearing gabbro in the Upper Zone is more Fe-rich and Cr-poor than the Critical Zone phlogopite, whereas that from the Lower Zone is more magnesian (Table 2, analyses 18 and 21). AMPHIBOLES AND SERPENTINE At least two separate generations of amphiboles are evident in several of the Stillwater and Bushveld samples. First, pargasitic hornblende occurs in reaction relationship with pyroxene, as inclusions in chromite and apatite, and intimately associated with phlogopite


0.4

979


A.

•

1

i

1

1

1

r~

1—

0.20 0.16 S 0.12

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oo 0

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(p

o

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8008

%oc^o

CfcCGD

o 1

cri£> WJB

JB OO O *O

r> 3D

o o •

&

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0

M • oa° 6b

-

• 10 20 Fe/(Fe+Mg+Ti), mole %

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• Stillwater o Buahveld o

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0.16 0

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