EN EL ESCENARIO PESIMISTA:

Oikocrystic harzburgite occurs above the main dunite sequence and below the chromitites of the Black Label deposit and occasionally within the Black Thor sequence.

Figure 3.7. Thin section showing serpentinized olivine in dunite with very fine grained disseminated chromite. Sample 486008 is from 117 m in DDH

BT-08-10.

Figure 3.8. The serpentine is frosted with ultrafine magnetite. Note the chain-like network of chromite interstitial to cumulus olivine. Sample 486165 is from 155.8 m in DDH BT-09-31.

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Figure 3.9. Coarse grained massive grey pyroxene at right replaces medium grained olivine. Sample is from 306.38 m in DDH BT-11-197. Diameter of core is 46.7 mm.

Figure 3.10. Closeup of coarse grey pyroxene (out of focus) replacing cumulus dark green olivine (in focus) as in Figure 3.1. Sample is from 306.38 m in DDH BT-11-197. Diameter of core is 46.7 mm.

Metasomaticpyroxene Olivine

Metasomaticpyroxene Olivine

Figure 3.11. Patchy grey coarse grained pyroxene at right replaces pale grey-white talc- altered cumulus olivine. There is very fine interstitial chromite. Sample is from 381.57 m in DDH FW-11-87. Diameter of core is 46.7 mm.

Figure 3.12. Harzburgite containing 35 modal % 1 cm oval to rounded tremolitized oikocrysts of pyroxene in serpentinized fine grained dark cumulus olivine groundmass. Sample is from 143.6 m in DDH BT-09-31.

Metasomaticpyroxene

The oikocrystic harzburgite is distinguished from the above harzburgite by the presence of round phases of greater portions of intercumulus pyroxene within groundmass olivine. Serpentinized, fine cumulus olivine is enveloped by coarse, up to 1.5 cm diameter, pyroxene oikocrysts that account for up to 35 modal % of the rock (Fig. 3.12). There could be other secondary minerals such as brucite, which has not been identified. Complete petrographic study on the hydrous phases to the chromites, the subject of further study. Oikocrystic harzburgite commonly occurs in dunite-peridotite sequences above pyroxenite. This relationship is well documented in several cycles in DDH FW- 09-34. The occurrence of common intercumulus minerals suggests the presence of larger amounts of pore liquid material surrounding cumulus olivine. Observations by Jackson (1961), in his observation of the Stillwater Intrusion reported by Wager and Brown (1967), noted that in areas where the amount of pore material was greatest, there was a higher rate of crystal accumulation in the basin. The intercumulus textures of oikocrystic harzburgite suggest the Black Label lithologies and chromitites formed in a more dynamic system under faster rates of crystallization due to greater nucleation in the pore fluid melt content. Like dunite, oikocrystic harzburgite occasionally contains interstitial very fine cumulus chromite.

In thin section, serpentinized olivine is enveloped by talc and tremolite-replaced pyroxene. Incipient replacement by highly birefringent tremolite occurs along the cleavage planes of the pyroxenes (Fig. 3.13). Electron microprobe analysis of the tremolite identifies both Cr-bearing and non Cr-bearing tremolite. The rarer Cr-bearing tremolite indicates accompanying chromite replacement to magnetite during hydration supplying Cr content to the tremolite. The oikocrystic harzburgite does contain rare primary olivine, pyroxene and amphibole. Amphibole is identified by its brown colour in plane light, two oblique cleavages and high relief indicative of hornblende. The amphibole is suggested to be igneous rather than metamorphic (see discussion chapter). The few primary olivine, orthopyroxene and amphiboles microprobed yield Fo #s of 83- 90 for cumulus olivine and Mg #s of 0.87-0.99 for cumulus orthopyroxene (Fig. 3.14). 3.5 Disseminated chromite

In the chromite mineralized dunite, fine grained cumulus olivine is bordered by very fine, interstitial, cumulus chromite (Fig. 3.15). There is typically 5 to 10 modal %

Figure 3.13. Incipient to pervasive replacement by highly birefringent tremolite along the cleavage planes of the pyroxene. Note low order serpentinized olivine in contrast to higher order tremolitic pyroxene. Sample 486153 is from 151.8 m in DDH BT-09-31.

Figure 3.14. Relict micro-fractured olivine occurring with larger, highly birefringent amphibole in a talcose groundmass. Also shown are very finely disseminated euhedral chromite. Sample 486163 is from 155 m in DDH BT-09-31.

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Figure 3.15. Up to 20 modal % very fine interstitial chromite hosted in light green fine grained serpentinized olivine, forming olivine-chromite heteradcumulate. Sample is from 59 m in DDH BT-08-10.

Figure 3.16. Minute smaller cumulus chromite surrounding coarser grained serpentinized cumulus olivine clearly demonstrating chromite after olivine crystallization. Sample 486009 is from 117.7 m in DDH BT-08-10.

disseminated chromite that occurs as interstitial chromite and thin bands. Up to 15 modal % light grey intercumulus pyroxene accompanies the olivine. The interstitial chromite commonly concentrates to form mm-scale layers. In thin section, minute equant intercumulus chromites rim larger serpentinized cumulus olivine, thereby showing that it is second to crystallize after olivine (Fig. 3.16). Intercumulus chromite is ofter associated with brown platy chlorite grains after pyroxene (Fig. 3.17). Very rarely chromite included in olivine, but commonly interstitial chromite is included within intercumulus pyroxene indicating pyroxene crystallized post chromite.

The textural relations clearly demonstrate chromite crystallized before pyroxene (Fig. 3.18). The concentric growth of fine cumulus chromite around olivine clearly demonstrates crystallization next after olivine and prior the pyroxene. Pyroxene crystallized as oikocrysts around the chromite that rims the olivine. The order of crystallization for both dunite and oikocrystic harzburgite is: cumulus olivine Æ cumulus chromite Æ intercumulus pyroxene. This order of crystallization has been demonstrated in other stratiform chromite deposits such as Bushveld, the Great Dyke, the Stillwater Complex and the Burakovsksy intrusion (Schulte et al., 2012).

Some of the intercumulus pyroxene has been hydrated to chlorite. This chlorite has been confirmed by electron microprobe to be either chromian chlorite or kaemmererite (with over 7 wt. % Cr2O3; Fig. 3.19). Pyroxene has also been hydrated to

talc as many of the olivines are surrounded by talc rims (Fig. 3.20). The talc was originally primary intercumulus pyroxene.