Etapa I: Definición de la infraestructura

alteration to similar looking material (Fig. 3.6D). This same very fine grained acicular amphibole material fills randomly orientated, mm-scale fracture networks within the cumulus plagioclase (Fig. 3.6E). This texture suggests that the

71 anorthositic rocks of the East Bull Lake suite formed as cumulates of mostly plagioclase (and minor orthopyroxene) set in an intercumulus matrix of clinopyroxene, before being intruded by a second pulse of mafic magma which form the pyroxenitic (now altered to amphibole) vein material.

Fig. 3.6. Photomicrographs of the East Bull Lake suite anorthositic rocks A) XPL view of cumulus plagioclase; B) XPL view of typical alteration of plagioclase to fine grained sericite and quartz; C) XPL view of overgrowth of tremolite across plagioclase crystal boundaries; D) XPL view of typical alteration of orthopyroxene to fine grained (often acicular) amphibole; E) XPL view of fracture networks of amphibole within cumulus plagioclase.

Inclusion-Bearing rocks

The matrix of the Inclusion-Bearing rocks is gabbronoritic-leucogabbronoritic in composition and contains varying amounts of plagioclase, clinopyroxene and orthopyroxene (Fig. 3.7A). The matrix is largely equigranular and ranges in size from medium-coarse grained. In the majority of samples studied, the dominant

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72 mineral is plagioclase (up to ~70% in some samples) which forms euhedral, elongate, inter-locking prisms with adcumulate texture (Fig. 3.7B). The plagioclase is moderately well preserved in the matrix with alteration limited to very fine grained, disseminated replacements of quartz and sericite. Clinopyroxene and orthopyroxene occur in approximately equal proportions and form subhedral crystals which are heavily altered to fine grained, occasionally acicular amphibole (Fig. 3.7C). The sulphide content of the gabbronoritic matrix of the inclusion-bearing rocks is highly variable and in the samples studied ranges from non-existent up to ~5%. In sulphide-bearing samples the sulphides form very fine grained, anhedral crystals which are evenly disseminated through the matrix (Fig. 3.7D).

The pyroxenite pods in the Inclusion-Bearing zone are primarily composed of large, equant megacrysts of orthopyroxene set within a finer grained groundmass of amphibole, orthopyroxene and clinopyroxene (Fig. 3.7E). In hand specimen, the orthopyroxene megacrysts can be up to ~5 cm in length and form elongate, euhedral prisms which account for ~50% of the rock (Fig. 3.7F). In thin section, the orthopyroxene megacrysts are generally well preserved, although embayments along the grain boundaries are filled with fine grained amphibole suggesting that alteration has affected the margins of the orthopyroxene (Fig. 3.7G). The interiors of the orthopyroxene megacrysts are generally well preserved, although, minor alteration to amphibole is manifested as fine grained amorphous blebs which are disseminated through the crystals, or as elongate replacements concentrated along cleavage planes (Fig. 3.7H). The orthopyroxene megacrysts also contain very fine grained inclusions of opaque material which ranges in habit from anhedral blebs to euhedral, equant crystals with a rectangular-diamond shaped cross section (Fig. 3.7I). Amphibole constitutes approximately 60% of the groundmass of the pyroxenite pods and occurs in two distinct forms. The most abundant form of amphibole is preserved as euhedral, elongate laths with equant, diamond shaped cross-sections (Fig. 3.7J). The second form of amphibole occurs as anhedral masses, characterised by a pervasive and randomly aligned opaque fracture network (Fig. 3.7K and L) which does not penetrate into the other type of amphibole or the orthopyroxene megacrysts.

Occasionally, this ‘fractured’ type of amphibole is observed to replace remnants of clinopyroxene (Fig. 3.7M) which suggests that this type of amphibole is secondary.

73

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Fig. 3.7. Photomicrographs of the East Bull Lake suite Inclusion-Bearing unit: A) XPL view of typical texture and composition of gabbronoritic matrix of inclusion-bearing rocks; B) XPL view of adcumulate texture in plagioclase from the gabbronoritic matrix; C) XPL view of fine-grained acicular replacements of amphibole after clinopyroxene in the gabbronoritic matrix; D) PPL view of gabbronoritic matrix showing the anhedral and disseminated nature of the sulphides (opaques); E) XPL view of typical texture and composition of pyroxene inclusions; F) Photograph of pyroxene inclusion in hand-sample. Reflective laths are cleavage-plane sections of orthopyroxene; G) XPL view of alteration of orthopyroxene megacrysts to fine grained amphibole along crystal margins; H) XPL view of fine grained alteration of orthopyroxene within the megacrysts; I) XPL view of very fine grained inclusions of opaque minerals within orthpyroxene megacrysts; J) XPL view of typical texture and composition of the matrix of the pyroxenite inclusions; K) PPL view of ‘fractured’ amphibole; L) XPL view of K; M) XPL view showing alteration of clinopyroxene to ‘fractured’ amphibole; N) PPL view of typical texture and composition of the granitic inclusions; O) XPL view of N; P) XPL view of orthoclase phenocryst showing both Carlsbad and polysynthetic twinning; Q) XPL view of very fine grained sericite and quartz alteration of orthoclase; R) PPL view of chlorite crystal with irregular margins and inclusions of biotite; S) XPL view showing alteration of amphibole to fine grained clay minerals.

The granitic xenoliths are predominantly composed of orthoclase, quartz, chlorite, amphibole and biotite (Fig. 3.7N and O). The granitic inclusions have an inequigranular texture with orthoclase phenocrysts forming subhedral, elongate prisms (<5mm in length) which display both Carlsbad and polysynthetic twins (Fig.

3.7P). These orthoclase phenocrysts are altered to fine grained sericite, concentrated within crystal interiors (Fig. 3.7Q). The finer groundmass is composed of subhedral, equant orthoclase which shows similar twins and alteration as the phenocrysts. Fine grained orthoclase makes up ~60% of the groundmass. Quartz accounts for ~20%

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76 and forms anhedral crystals. Chlorite comprises ~15% of the groundmass and forms elongate laths which have irregular crystal margins with adjacent orthoclase crystals (Fig. 3.7R). Amphibole accounts for ~5% of the groundmass and displays strong, green pleochroism. The amphibole is significantly altered to fine grained clay and quartz (Fig. 3.7S). Biotite occurs in trace amounts in the granitic inclusions and forms, thin elongate crystals which are partially altered to chlorite.

Gabbroic and anorthositic xenoliths are also present (see Fig. 3.4J and 3.4K). In thin section, these inclusions are indistinguishable from similar lithologies higher up.

3.4. Blue Draw Metagabbro