Introduction

Figure 1 Surface geology of the Six Mile Hill and Roopena areas. (Simplified from Krapf et al. 2016)

During the Mineral Systems Drilling Program (MSDP) in 2015, four diamond holes totalling ~2,788 m were drilled in the Six Mile Hill area, on the southeast margin of the Gawler Ranges (Fig. 1; Fabris et al. 2017). Three of these drillholes intersected a succession of mafic and felsic volcanics of the lower Gawler Range Volcanics in excess of 1,100 m thick, which is poorly preserved at the surface where it is overlain by Meso- to Neoproterozoic and Cenozoic cover sequences (Krapf et al. 2016). This volcanic succession contrasts quite dramatically with that recorded at Roopena, only 25 km to the south of Six Mile Hill, which is significantly thinner (<250 m), and comprises mafic volcanics interlayered with volcaniclastic lacustrine sediments (McAvaney and Wade 2015).

The volcanic facies of MSDP01 (SA Geodata drillhole number 287995), MSDP03 (287997) and MSDP04 (287998) have been described in three reports (Simpson 2017a, 2017b, 2017c), based on detailed core logging and thin section petrography. A fourth report discusses the correlation of individual units between drillholes and presents a model for the depositional environment of the Gawler Range Volcanics in the Six Mile Hill area (Simpson 2017d). This article summarises the key features of MSDP01, which reached a depth of 1,116.8 m and intersected the thickest sequence of volcanics, making it a critical stratigraphic drillhole for unravelling the stratigraphy of the Gawler Range Volcanics in this area.

Geological setting

The Six Mile Hill area is located on the northeastern Eyre Peninsula, in the southern Gawler Craton. Basement to the Gawler Range Volcanics is not exposed in the Six Mile Hill area, but is thought to be similar to basement in the Roopena area, ~25 km to the south, which has a north–south-orientated structural trend. The region is positioned across two Paleoproterozoic basement domains, separated by the Roopena Fault (McAvaney et al. 2016), a zone of highly strained rocks which developed during the 1740–1690 Ma Kimban Orogeny, and is interpreted to have been reactivated as an extensional structure during the Mesoproterozoic (McAvaney and Wade 2015) and Cenozoic (Crone et al. 2003). To the east of the Roopena Fault, basement comprises volcaniclastic sediments of the c. 1755 Ma Moonabie Formation, intruded by the c. 1740 Ma Gluepot Granite (McAvaney et al. 2016). To the west of the Roopena Fault, basement comprises the c. 1790 Ma Broadview Schist, intruded by the c. 1770 Ma Tip Top Granite and Wire Dam Dolerite (McAvaney et al. 2016).

Paleoproterozoic basement is unconformably overlain by the c. 1590 Ma Gawler Range Volcanics. In the Roopena area, the Gawler Range Volcanics comprise the Roopena Basalt, which is up to ~250 m thick and interlayered with volcaniclastic fluviolacustrine sediments of the Fresh Well Formation, which is up to 225 m thick (McAvaney and Wade 2015). Outcrop of the Gawler Range Volcanics in the Six Mile Hill area is dominated by the Spearfelt Rhyodacite (Fig. 1), an extensive feldspar-phyric evenly porphyritic unit. It is underlain by a number of different volcanic units, including a quartz-phyric rhyolitic ignimbrite near Junction Dam, mafic volcanics near Princefoot Dam, and a vitriclastic ash tuff at Nine Mile Dam (McAvaney et al. 2016). The Gawler Range Volcanics are overlain by flat-lying Neoproterozoic sediments of the Stuart Shelf, Tertiary silcrete, and Quaternary alluvial and colluvial sediments.

Volcanic facies intersected in MSDP01

Figure 2 Lithology log of MSDP01.

MSDP01 intersected six units of the Gawler Range Volcanics (Fig. 2), comprising:

• upper felsic volcanic unit, 30.5–66.46 m
• upper mafic volcanic unit, 66.46–368.97 m
• middle felsic volcanic unit, 368.97–665.15 m
• lower mafic volcanic unit, 665.15–964.2 m
• quartz- and feldspar-phyric felsic unit, 964.2–1,095.95 m
• lower felsic volcanic unit, 1,095–1,116.8 m.

The upper felsic volcanic unit is a massive, homogeneous dacite. It is moderately crystal-rich (~20 modal %) and contains abundant plagioclase and minor ex-ferromagnesian mineral phenocrysts and up to 1 modal % quartz phenocrysts, set in an originally glassy groundmass, which retains a micropoikilitic texture. A flow foliation is faint in the lower half of the unit and the base is brecciated, amygdaloidal, perlitic (Fig. 3a) and contains small fine-grained sediment rafts that have undergone minor peperitic mixing with the host dacite (Fig. 3b). The dacite has an irregular lower contact with a 1.5 cm wide band of laminated siltstone, and the dacite fills a crack within the siltstone (Fig. 3c), suggesting it was at least semi-lithified prior to emplacement of the dacite. The unit lacks evidence of a fragmental origin and is interpreted as a coherent body, emplaced either as a lava over-riding wet sediment or a shallow sill intruding them.

The upper mafic volcanic unit consists of 27 basalt flow units, ranging in thickness from 2 to 25 m, but also contains a small volume of autoclastic rocks and volcaniclastic sedimentary rocks toward the base. The flow units are characterised by a conspicuously amygdaloidal and oxidised top and some display a patchy vesicular texture that may indicate a flow-top breccia. Four packages of flow units were observed (from top to bottom):

• 6 aphyric to very sparsely plagioclase-phyric flow units
• 5 moderately plagioclase-phyric units which also contain scattered plagioclase megacrysts (Fig. 3d)
• 9 almost aphyric flow units, several of which contain convincing pillow texture (Fig. 3e)
• 7 aphyric basalt flow units which are intercalated towards the base with more voluminous hyaloclastite (Fig. 3f), volcaniclastic breccia and minor normally graded sandstone intervals (Fig. 4a).

The middle felsic volcanic unit is a homogeneous, coarse-grained, relatively crystal-rich (25–30 modal %) dacite. This unit contains abundant phenocrysts of plagioclase and minor replaced ferromagnesian phenocrysts with inconspicuous trace quartz phenocrysts. The groundmass has a micropoikilitic texture indicative of its original glassy texture. A faint flow texture in the groundmass is mainly observed close to the base. The upper contact of the unit is sharp and planar with tiny flames of the mafic material extending into the dacite at millimetre scale; and the lower contact with the lower mafic volcanic sequence is sharp. The base of the dacite is finely amygdaloidal (Fig. 4b), perlitic and spherulitic and contains slightly more conspicuous quartz as well as a small amount of alkali feldspar phenocrysts.

The lower mafic volcanic unit comprises a sequence of 15 flow units, ranging in thickness from almost 5 to 43 m, and all either aphyric (mainly upper half) or very sparsely plagioclase-phyric (Fig. 4c; mainly lower half). At the top of the highest basalt flow unit, the basalt has a quenched texture (Fig. 4d) and includes small (submillimetre) peperitic lenses of fine, quartz-bearing sediment that was unconsolidated when incorporated. Apart from these tiny sediment lenses, there is only a few metres of volcaniclastic sandstone in this thick sequence and one of these was found to contain conspicuous quartz and felsic lithic grains. The majority of the basalt flow units display the same systematic internal structure, which involves an amygdaloidal top, in places with flow-top breccia texture and minor incorporated sediment, and a massive, weakly amygdaloidal interior. Probable pillow margins and hyaloclastite (Fig. 4e) are less common than in the upper interval of mafic volcanic rocks in this drillhole and mainly occur in the first two flow units below the crystal-rich dacite.

The quartz- and feldspar-phyric unit is a massive, crystal-rich (~30 modal %) rhyolite (Fig. 4f), which is distinguished from all other logged units in the three MSDP holes by the presence of relatively abundant quartz phenocrysts (12–15 modal %). The feldspar phenocrysts include both alkali feldspar > plagioclase and there is a small number of highly altered probable ferromagnesian phenocrysts. There is sporadic development of a diffuse flow-banding and a 4 m wide band of brecciated rhyolite is either an autobreccia separating two units of the same rhyolite or, more likely, a zone of brittle deformation. The phenocrysts in the rhyolite are unbroken and the groundmass was originally glassy. Based on the observed core and thin section textures, the rhyolite is considered to be a coherent lava rather than a densely welded ignimbrite.

The lower felsic volcanic unit is an altered dacite which has a slightly oblique, possibly erosional, contact with the overlying rhyolite. This unit is finer grained than the overlying rhyolite and is also distinguished by a moderately crystal-rich (20–25 modal %), plagioclase-rich, quartz-poor composition, presence of ex-biotite phenocrysts and a lenticular groundmass foliation. The intense sericite–quartz–hematite alteration prevents distinction of this unit as a coherent lava or ignimbrite.