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         M.D. Roberts1, I.J. Basson2, J.A. Miller3, D.L. Reid2, M. Roberts4, and D.S. Smith5
         Department of Earth Sciences, University of Southern California, Los Angeles, USA
      Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa
             Geology Department, University of Stellenbosch, Stellenbosch, South Africa
                               NovaGold Resources, Vancouver, Canada
                        Northam Platinum Limited, Thabazimbi, South Africa

Introduction: At Northam Platinum Mine, on            over a distance of 20 cm. PGMs identified by
the Western Limb of the Bushveld Complex,             electron microprobe include moncheite, laurite,
South Africa, distinctive mineral assemblages         niggliite, cooperite and sperrylite and are mostly
and textures in the immediate footwall (up to 1.2     associated with the sulphide clusters
m) to the Merensky Cyclic Unit mark the effect            Merensky chromitites (Normal, NP2 and P2
of “footwall reconstitution” associated with the      Reefs). The texture and mineralogy of the Meren-
influx of the Merensky magma in both the Nor-         sky chromitite in each reef type consists of fine-
mal Merensky Reef and Regional Pothole reef           to medium-grained, variably annealed chromite
types (NP2, P2, FWP2). The nature of footwall         within a matrix of plagioclase ± orthopyroxene,
reconstitution varies primarily due to the original   minor biotite and interstial pentlandite, pyrrhotite
bulk composition and mineralogy of the affected       and chalcopyrite (the NP2 and P2 Reefs also con-
footwall protolith at various levels in the pot-      tain minor pyrite). PGMs include cooperite, brag-
holed stratigraphy. In particular, harzburgites and   gite, laurite and moncheite associated with sul-
melanorites in the footwall to the MCU in Nor-        phide clusters and laurite, cooperite, braggite and
mal and P2 reef types display coarse pegmatitic       niggliite along the contacts between chromite and
textures, whereas anorthosites in the NP2 reef        matrix silicates.
type exhibit the growth of post-cumulus olivine           Footwall pegmatites (Normal and P2 Reefs).
(“troctolization”) and spatially associated sul-      Despite significant variation in absolute grain
phide and PGE mineralization. Our recent geo-         sizes and modal mineralogy, pegmatitic pyrox-
chemical and petrological investigation of these      enites, melanorites and harzburgites in the foot-
features indicate that footwall reconstitution re-    wall to the Merensky chromitite in the Normal
flects thermal and some minor chemical disequi-       and P2 Reef share similar microscopic textures.
librium associated with the emplacement of the        Locally coarser grain sizes (up to 5 cm in diame-
Merensky magma (i.e. static recrystallization of      ter) reflect the compaction, static coarsening and
footwall cumulates and minor silicate-sulphide        recrystallization of clusters of fine- to medium-
melt infiltration), rather than fluid-related proc-   grained (presumably) cumulus orthopyroxene.
esses (i.e. incongruent melting of orthopyroxene      Hence, pegmatitic textures are dissimilar to those
to olivine in the formation of NP2 troctolites).      displayed by fluidized pegmatites [c.f. 1, 2] and
This led us to hypothesize that; 1) thermo-           most likely record thermal requilibration with the
mechanical erosion, rather than significant fluid     overlying Merensky magma. PGMs in pegmatites
streaming, was responsible for Normal Reef thin-      of both reef types include laurite and moncheite,
ning and the formation of the Regional Pothole        although the Normal Reef also contains cooperite
sub-facies, and 2) the Merensky Cyclic Unit and       and rustenburgite while the P2 Reef contains
associated PGE mineralization occurred as a ho-       braggite and niggliite. All PGMs are typically
mogenous “drape” from Normal Reef sub-facies          associated with interesitial sulphide clusters.
through to the Regional Pothole sub-facies. To            Troctolites (NP2 Reef). Troctolites in the NP2
test these hypotheses, we investigated the de-        Reef are the lateral equivalent of leuconorites
tailed PGE geochemistry and PGM mineralogy of         from the Footwall Marker in Normal Reef sec-
units associated with Normal and Regional Pot-        tions (i.e. unpotholed). Here, troctolization, de-
hole reef types (NP2 and P2).                         fined by the growth of anhedral olivine over cu-
    Rock Textures and PGMs:                           mulus plagioclase, occurs up to 1.2 m below the
    Hanging wall melanorite (Normal, NP2 and          Merensky chromitite. In this zone, troctolization
P2 Reefs). Melanorites in the hangingwall to the      is spatially associated with sulphide and PGE
Merensky chromitite in all reef types comprise        mineralization, and hence a coupled process is
cumulus othopyroxene, interestial plagioclase,        inferred. The major element chemistry and tex-
occasional clinopyroxene oikocrysts, minor bio-       tures displayed by troctolites relative to unaf-
tite and clusters of intergrown pentlandite, pyr-     fected leuconorites suggest that troctolization
rhotite and chalcopyrite. Sulphide mineralization     occurred where leuconorites, in erosional contact
is greatest at the base of the melanorite (~5-8%)     with the Merensky magma chamber, were infil-
and systematically decreases upwards to 0-1%          trated by a primitive silicate melt and a coexisitng
sulphide melt. Moncheite, cooperite, braggite and     are characterized by high Pd contents, similar to
laurite are associated with sulphides.                that of chromitites.
     PGE Geochemistry: Whole-rock Cu, Ni, Au              Discussion: Our initial microprobe investiga-
and PGE results (NiS fire assay followed by ICP-      tion appears to suggest that there is no significant
MS) for rock units that comprise the Normal,          or systematic difference in the types of PGM that
NP2 and P2 reef types reveal remarkable simi-         occur in the Normal Reef compared to the NP2
larities in the absolute abundances of PGEs and       and P2 reef types within the Regional Pothole
the shape of mantle-normalized PGE profiles           sub-facies at Northam. In contrast, [4] found that
(Fig. 1). Chromitites in each reef type display the   small-scale potholes at various locations in the
arched profile typical of the Merensky chromitite     Bushveld Complex typically contain Pt-Fe alloys
elsewhere [3]. Hangingwall melanorites from all       at their centres and Pt-Pd-Te along their edges.
three reef types also have similar PGE abun-          These authors attributed these variations to the
dances and profiles. However, while these             influence of magmatic volatiles channeled by
melanorites (which occur 10 cm above the chro-        potholes.
mitites) have similar Cu, Ni and Au abundances            The remarkable similarity in the absolute
relative to the chromitites (and hence similar sul-   PGE abundances and mantle-normalized profiles
phide contents), PGEs are one to two orders of        are also testament to the apparent similarity in
magnitude less abundant. This signature has been      PGE mineralization in Normal and Pothole
interpreted to reflect PGE-depleted sulphides         Reefs. This is particularly striking in the hang-
from a magma chamber that has already had sig-        ingwall melanorites and the Merensky chromit-
nificant PGE collected by an earlier sulphide melt    ites from all three reef types. The similarity of
[3].                                                  these profiles and the occurrence of PGE-
                                                      depleted sulphides in the hangingwall to the Mer-
                                                      ensky chromitite is consistent with a magmatic
                                                      model of PGE collection and deposition [e.g. 3]
                                                      and suggests that this event occurred as a ho-
                                                      mogenous drape over a variably eroded footwall
                                                      surface. The absence of a mineralogical or geo-
                                                      chemical signature that may indicate a high fluid
                                                      activity (in the form of variable PGMs or PGE
                                                      abundances remobilized by fluids), supports our
                                                      hypothesis that regional pothole formation was
                                                      associated with thermo-mechanical erosion rather
                                                      than significant fluid streaming. Variable PGE
                                                      mineralization in the footwall to the Merensky
                                                      chromitite may be the result of local metal mobil-
                                                      ity during reconstitution (pegmatite formation),
                                                      variable infiltration of PGE-bearing sulphides
                                                      (during pegmatite formation and troctolization of
                                                      leuconorites), or overprinting of a previous epi-
                                                      sode of sulphide collection related to an earlier
                                                      magmatic cycle (at the base of pre-existing cyclic
                                                      units in the Normal and P2 Reefs).
                                                          References: [1] Cawthorn R. G. and Barry S.
                                                      D. (1992) 6th Inter. Plat. Symp, 39, 263-276. [2]
                                                      Barnes S-J. and Maier W. D. (2002) J. Petrol.,
Fig. 1. Primitive mantle-normalized metal pat-        42, 103-128. [3] Barnes S-J. and Maier W. D.
terns for rock types from Normal, NP2 and P2          (2002) CIMMP, 54, 431-458. [4] Kinloch E. D.
reef types at Northam.                                and Peyerl W. (1990) Econ. Geol., 85, 537-555.

PGE abundances for pegmatitic rocks in the foot-
wall to the chromitite (Normal and P2 Reefs) are
slightly lower and their profiles are more vari-
able. Trotolites have similar PGE abundances and

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