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The massive mudstone and host rock of the Rosebery Mine are overlain by the White Spur Formation (Fig. 3.28: Corbett and Lees, 1987; Allen, 1991; Gifkins and Allen, 2001; Corbett et al., 2014. The White Spur Formation consists of quartz-feldspar-phyric pumice breccias with minor beds of black mudstone (WSF 1), and facies association (WSF 2) comprising of three facies: basaltic andesite volcaniclastic breccia interbedded with minor 5-10m black mudstone (WSF 2A), polymictic volcaniclastic breccia (WSF 2B) and rhyolitic breccia (WSF 2C: Fig. 3.28). Detailed descriptions of the facies are given below.

Facies WSF 1: Quartz-feldspar-phyric pumice breccia

WSF 1 overlies the thick mudstone which in turn overlies the host rock (Fig. 3.36). It is 100-120 m in thickness and consists of multiple weakly normally graded beds of 5-20 m. Beds are composed largely of pumice-fiamme fragments, with medium to coarse grained quartz and euhedral feldspar crystal fragments and subordinate volcaniclastic siltstone and mudstone lithic clasts at the base (Fig. 3.36 A-D). Interbeds of black mudstone and volcaniclastic siltstone, 1-4 m in thickness, are common. Siltstone layers may contain outsized feldspar-phyric-fiamme in their upper parts.

As the pumice breccia facies was very texturally homogeneous in hand specimen, two samples were taken from the quartz-feldspar-phyric pumice breccia at 1211 m and at 1135 m of drill hole 337R (Fig. 3.36). The sample taken from the lower part of the stratigraphy at 1211 m is

79 dominantly composed of pumice-fiamme clasts (1-5 cm, 60-70 %), angular quartz crystal fragments (0.25-0.50 mm, 15-20 %), euhedral feldspar crystal fragments (0.25-0.50 mm, 2-3%), glass shards (2- 3%), and mudstone lithic clasts (1-2%) in sericite altered matrix (Fig. 3.36 A-B). The quartz: feldspar ratio is estimated to be 90:10. The upper part of the stratigraphy at 1135 m is also dominantly composed of pumice-fiamme clasts (1-5 cm, 60-80%), angular quartz crystal fragments, often fractured and embayed (0.5 mm, 5-10%), euhedral feldspar crystals (10-15%) and mudstone lithic clasts (1-2%) in sericite altered matrix (Fig. 3.37 C-D). The estimated quartz: feldspar proportion is 40:60.

Interpretation

The quartz-feldspar-phyric pumice breccia is primarily composed of juvenile pumice clasts, former glass shards and angular crystal fragments that were produced by explosive felsic eruptions (Dimroth and Yamagishi, 1987; Branney and Kokelaar, 1992; Gifkins, 2001). The thick 10-20 m scale bedding, grading from coarse pumice and other lithic clasts at the base to variably crystal-rich pumiceous sandstone is consistent with density sorting. This interpretation combined with the dominant pumice component suggests deposition from high concentration density currents during or soon after eruption (Fig.3.36 A-D: Branney and Kokelaar, 1992; Gifkins, 2001)

The mudstone and siltstone intraclasts at the base of the beds are interpreted to be eroded from the underlying unconsolidated mudstone substrate suggesting that the density currents were highly erosive and its transport in a submarine setting. The pumice-rich sandstone and siltstone ash tops are interpreted to be from water settled fallout suspension sourced either directly from the eruption plumes or from the trailing of ash cloud associated with the high concentration flows (Gifkins and Allen, 2001). The outsized pumice fiamme within the stratified volcanogenic siltstone and mudstone tops of most of the beds are interpreted to represent pumice clasts that were initially buoyant but became water logged and settled from suspension in the water column (Gifkins,

80 2001). The deposition of the narrow intervals of black mudstones suggests periods of volcanic quiescence in between the eruption pulses.

Figure 3.36. Stratigraphy of the White Spur Formation showing (A) photograph of a quartz-feldspar phyric pumice breccia (Pbx) with mudstone (MDST) lithic clasts. (B) Photomicrograph of qtz-fsp-phyric pumice breccia. (C) Photograph of qtz-fsp-phyric pumice (P) breccia. (D) Photomicrograph showing fractured quartz (Qtz) and feldspar (Fsp) from the upper part of the pumice breccia (cross-polarized light).

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Facies WSF 2A: Basaltic andesite volcaniclastic breccia

The basaltic andesite volcaniclastic breccia overlies the quartz-phyric pumice breccia of WSF 1. It is about 50-70 m in thickness in total with 10-40 m thick beds that grade from very coarse breccia bases to volcaniclastic sandstone tops, in places separated by 3-10 m thick black and pyritic mudstone beds (Fig. 3.36).

It is mainly composed of angular to sub-rounded, feldspar-phyric basaltic andesite clasts (~10 cm diameter, 40-70%), euhedral feldspar crystal fragments (0.5-1.0 mm, 15%) and minor quartz (0.25 mm, 1-2%) in volcaniclastic sandstone matrix that lack pumice clasts (Figs. 3.37, 3.38). The matrix of WSF2A has quartz to feldspar proportion of 90:10, which distinguishes it from other facies.

Figure 3.37. Photograph of a slab of WSF 2A showing clasts of feldspar-phyric basaltic andesite of about 10 cm diameter. Drill hole 397R: 1356.80 m.

Figure 3.38. Photomicrograph of crystal-rich volcaniclastic breccia of WSF 2A showing feldspar (Fsp)–phyric basaltic andesite (Bas-And) clasts and minor quartz (Qtz). Drill hole 337R: 1106.3 m. Image taken under XPL

Interpretation of basaltic andesite volcaniclastic breccia (WSF 2A)

The thick (10-40 m) graded beds of facies 2A suggest rapid deposition from high concentration mass- flow current. The normally grading from very coarse grained base of basaltic andesite clasts and mudstone lithic clasts that fines to volcaniclastic sandstone is consistent with density sorting (Gifkins, 2001). The presence of mudstone lithic clasts indicates that the mass-flow currents were

82 erosive. The angular to sub-rounded clast morphology indicates minimal reworking and the dominance of basaltic andesite clasts indicates an intermediate source. The facies is characterized by dense clasts that lack pumice suggesting it was a product of effusive volcanism.

The thinly stratified pyritic, black mudstones and siltstone beds interbedded with the graded pumice-rich beds suggests below wave base sedimentation either from water-settled fallout or secondary turbidite currents.

Facies WSF 2B: Polymictic volcaniclastic breccia

WSF 2B overlies WSF 2A and varies between 40-80 m in thickness laterally across the observed drill holes (Fig. 3.42). It is pale grey to greenish, massive to weakly foliated, and individual bed thicknesses range from 10 m in hole 250R to a maximum of 20 m in hole 337R and is absent in hole 411R-D1 (Fig. 3.42). Individual beds generally grade from very coarse clast-rich base to a ~1 m volcaniclastic sandstone top.

It is composed of sub-rounded to angular clasts with the following components: feldspar- phyric and aphanitic basaltic andesite, minor amygdaloidal basalt (2.0-2.5 mm, 5-7%), aphanitic and quartz-phyric rhyolite (1-2 mm, 1-2%), in addition to euhedral feldspar (3-5 %), angular quartz crystal fragments (0.5 mm, 3-5%) in a sericite altered matrix (Fig. 3.39-3.40).

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Figure 3.39. Photograph of WSF 2B showing Fsp- phyric and aphanitic basaltic andesite clasts from hole 337R at 1013 m. Scale shown is in centimetres.

Figure 3.40. Photomicrograph of WSF 2B showing quartz (Qtz), feldspar (Fsp) crystals and basaltic andesite (L) rich volcaniclastic breccia from hole 337R at 1013 m. Image taken under XPL

Interpretation of polymictic volcaniclastic breccia (WSF 2B)

WSF 2B has thick 10-20 m beds that are graded with ~ 1m thick volcaniclastic sandstone tops suggesting transport by gravity currents. The polymictic mixed clast composition of both felsic and mafic clasts indicates erosion and incorporation of multiple volcanic products into the gravity currents. The sub-rounded to angular clast morphology indicates moderate reworking from the source area. Black mudstone beds are absent in this facies suggesting there was a continuous supply of detritus into the basin.

WSF 2C: Rhyolitic volcaniclastic breccia

WSF 2C overlies WSF 2B and is generally about 40-50 m in thickness with a maximum of ~200 m in drill hole 411R-D1 (Fig. 3.42). Individual bed thicknesses range from 10-40 m and grade from a coarse base dominated by sub-rounded to sub-angular 1-3 cm diameter clasts of grey, aphanitic (40- 60%) and (2-5 cm, 5%) quartz-phyric clasts set in volcaniclastic sandstone. The matrix is dominated by angular quartz crystal fragments (0.25-0.5 mm, 10-15%), euhedral feldspars (<0.25 mm, 1-2%) and minor quartz-phyric clasts (0.5 mm, 2%: Figs. 3.41). The facies is distinguished from the

84 underlying facies as it lacks basaltic andesite clasts and the matrix has an estimated quartz to feldspar proportion of 85:15.

Figure 3.41. Photomicrograph of quartz crystal fragment-rich volcaniclastic breccia from hole 397R at 1298.60 m. Image taken under XPL

Interpretation of rhyolitic volcaniclastic breccia (WSF 2C)

The WSF 2C is comprised of thick 10-40 m beds that are poorly to moderately sorted, that grade to siltstone suggesting a possible transport from high concentration density currents or gravity currents. The clast composition is dominated by aphanitic and quartz-phyric clasts and quartz crystal fragments that indicate a felsic to intermediate dominated provenance. The angular, fractured and embayed quartz crystal morphology indicates minimal reworking derived from a proximal unconsolidated crystal rich rhyolitic source.

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