Reinterpretation of the Palaeoproterozoic Mooloogool and Tooloo Groups, Western Australia from mapping the Yadgimurrin Member, Yelma Formation

The Palaeoproterozoic Capricorn Orogen of Western Australia records the assembly of the Yilgarn and Pilbara Cratons at c. 2200-1700 Ma. The volcanic and sedimentary basins of the eastern Capricorn orogeny are an important source of gold, copper and highly prospective for base metals, cobalt and iron.

Regional Geology

The eastern Capricorn Orogen is made up of the Yerrida Basin, Bryah-Padbury Basins and overlying Eararheedy Basin (Gee, 1987). The geology of the Bryah-Padbury basins is described by Pirajno & Occhipinti (1998), the Yerrida basin is described by Pirajno et al. (2009) and the geology of the Earaheedy basin is described by Pirajno et al. (2008).

Figure lifted from Occipinti et al. 2017

The basins were first studied by Gee (1987) who considered the Bryah, Padbury and Yerrida Basins to be part of the Glengarry Basin. The Glengarry Basin was divided into the Bryah, Padbury and Yerrida Basins by Pirajno et al (2000) based on inter-fingering relationships and relatively limited geochronology at that time.

More recently, additional geochronology from the Frere Formation and Yelma Formation in the Eararheedy basin (Shepard et al. 2016) has allowed a refinement of the ages and stratigraphic associations of the Yerrida and Eararheedy basins, with the stratigraphy reorganised by Occhipinti et al. (2017).

Quick Stratigraphic Refresher

The Yerrida Basin is the oldest, with basal Windplain Group shales and sandstones. New constraints correlate the Karalundi Fm into the Windplain Group, which is now present in the basal parts of the Bryah basin.

The remainder of the Bryah basin is composed of the Narracoota Formation (host to VHMS mineralisation and lode gold), Ravelstone Fm and Horseshoe Formation.

The upper Yerrida Basin is composed of the Mooloogool Group, split into the Doolgunna and THaduna Formations (and equivalents in the Bryah), and the Maralouu and Killara Formations, which are not well understood at this time.

The Eararheedy basin no longer includes the Yelma Formation and Sweetwaters Well dolomite (host to world class Pb-Zn MVT mineralisation), which have been moved out into the Tooloo Group. The Frere Formation granular iron formation is now understood to be within its own separate stratigraphic sequence, bounded by unconformities (Rasmussen et al. 2012, Atkins et al 2013).

The following stratigraphic arrangement of the basins is taken from Dentith et al (2014), modified after Sheppard et al (2016) and Occhipinti et al. (2017).

Figure 2. Stratigraphic columns of the Bryah, Padbury, Yerrida and Eararheedy Basins

New Stratigraphic Understanding

Recent work and geochronology has allowed a better understanding of the inter-repationships of the major Groups within the basins, and related the basins to one another; this has partially reconstructed the earlier 'Glengarry Basin' of Gee (1987) who saw them all as inter-fingering parts of the same basin.

The current understanding is that this is true of various chronologically defined units, with the existing stratigraphy rearranged into several Groups based on stratigraphic relationships aided by geochronology. For example, the time-equivalent units of the Windplain Group are present in the Bryah and Yerrida basins.

There are still some blind spots in the geochronological coverage and stratigraphic understandings, notably the Maraloou Formation, Killara Formation and the Mooloogool Group relationship to others Groups and units.

Work in 2012 and recently, and mapping by myself and others (J Rodgers, 1996; P Spitalny and C Cook), has highlighted that the obscure Yadgimurrin Member of the Yelma Formation is widespread within the Mooloogool and Tooloo Groups and may provide direct geological evidence that can resolve the relationships between these Groups, and the Killara and Maraloou Formations.

Yelma Formation

The Yelma Formation is described in various sources (Pirajno & Adamides 2000, Pirajno 2003) as consisting of dolomite, sandstone, breccia, shale and siltstone. The Formation was divided (Adamides et al 2000a, 200b) into the Sweetwaters Well member, consisting of stromatolitic dolomite, the Yadgimurrin Member consisting of breccia, and undivided Yelma Formation, dominated by sandstones, minor conglomerates and argillaceous shales and silts.

Sheppard et al. (2016) demonstrated that micro-tuffs within the Sweetwaters Well Member of the Yelma Formation contained zircons which were isotopically dated to c. 1950 Ma, and in concert with the upper bounding unconformity with the overlying Frere Formation, the Sweetwaters Well Member was upgraded to Formation status (Sheppard et al, 2016, Occhipinti et al. 2017).

The maximum age of deposition of the Sweetwaters Well Formation is provided by zircon Pb-Pb dating of micro-tuffs at 1960 +/- 11 Ma (Sheppard et al. 2016) with the minimum age of 1949 +/- 10 Ma, providing a range from 1982 to 1939 Ma.

The Yadgimurrin Member

The Yadgimurrin Member (Adamides et al 2000a, 200b) is a restricted conglomerate fan facies of the Yelma Formation recognised from scattered outcrops on the Fairbairn and Marymia sheets. Mapping by Lodestar Resources, Giralia and others (P Spitalny pers. comm.; this author) have extended the known outcrops attributable to this unit and correlated them across the basin within the lower part of the Yelma Formation.

In the north west of the Yerrida Basin, sedimentary breccia (diamictite and immature conglomerate) is present between the underlying Windplain Group (Juderina Formation and Johnson Cairn Formation), and the overlying Mooloogool Group (Thaduna Formation) folded into the Thaduna Syncline fold belt. Exposures of sedimentary breccia outcrop on the eastern margin of the Yerrida Basin around MacDonald Well, No 5 Bore, and the range of hills at the north-western edge of Lake Gregory adjacent to Pelican Pool.

At Little Well (MGA50 780300E, 7177500N), megabreccia and diamictite grades laterally southwest into dolomitic gritstone and immature sandstone assigned to the Yelma Formation.

Figure 3. Diamictite exposure at Little Well. A: ~2.5m sized chert boulder in megabreccia. B: Trapezoidal chert clast with deformed vein

Further south, in the Gregory Synform, the breccia deposits in the east at Pelican Pool (MGA50 780,600E, 7163700N) overlie crystalline dolomite nominally mapped as part of the Sweetwaters Well Dolomite, and shales of the Johnson Cairn or Maraloou Formations, which in this area appear relatively indistinguishable. The diamictite contains abundant clasts of the Windplain Group sediments, and large crystals of quartz.

At Combine Well (MGA51, 192500mE, 7156000mN) conglomeratic dolomitic breccia overlies dolomite which is possibly Sweetwaters Well Dolomite, but is similar to that at Pelican Pool. Here the breccia and dolomite interfingers and overlies basalts and dolerites of the Killara Formation, and shales of the Maraloou Fomration.

Figure 4. Outcrops of Yadgimurrin breccia (RGC, WAMEX report A50015)

These outcrops, plus others north of No 13 Well and along the margin of the Marymia Inlier and Goodin Inlier as far south as Doolgunna where diamictite inter-fingers with the Doolgunna Formation, show that rocks of this breccia facies occur in a large portion of the northern Yerria Basin.

The same facies and lithology overlies the Magellan lead mine, where dolomitic grits and conglomerates with abundant crystalline quartz contain odd 70mm rounded quartz pebbles, as noted by various workers (eg; Pirajno's Monograph, 2005). This lithotype is present at Pelican Pool and throughout the Gregory Synform, mapped erroneously as silcrete due to pronounced surface silicification.

Interpretations

The Yadgimurrin Formation appears to occupy the lower sections of the Yelma Formation and is generally overlain by finer-grained immature sandstones in most outcrops. A few exposures of the Yadgimurrin Member (eg, Pelican Pool, Combine Well) potentially overlie the Sweetwaters Well Dolomite.

The diamictite within the Doolgunna Formation may represent an earlier development of the same local sedimentary facies as the unit within the Yelma Formation.

The Yadgimurrin Member and Yelma Formation, which are facies equivalents, show the onset of the Glenburgh Orogeny was characterised by horst and graben faulting, with fans of diamictite and sheet wash, and immature fluviatile deposits, flanking the fault scarps, grading up into immature sandstones, dolarenites. The sequence finishes with the Sweetwaters Well Dolomite, a stromalolitic unit, in shallow marine to inshore lagoonal facies. 

Correlation of diamictite in the Tooloo and Mooloogool Groups as being of similar association implies that the development of the Tooloo Group and Mooloogool Group was at least partly synchronous, which has been implied by Sheppard et al (2016).

The last development of the Yadgimurrin overlaps with the Imbin Porphyry and Sweetwaters Well Dolomite, which implies that the Mooloogool Group and Tooloo Group represent a coherent related package of rocks representing back-arc rifting in a continental margin, culminating in shallow evaporitic and lagoonal conditions at c. 1960 +/- 11 Ma.

As suggested in Sheppard et al. (2016) and Occhipinti et al (2017) the Tooloo Group, including the Yadgimurrin Member, records the onset of the Glenburgh orogeny. The Yadgimurrin Member age is unknown, but it is probably synchronous with back-arc spreading during the Glenburgh orogeny, representing development of grabens and shedding of talus, rubble and immature rudaceous sediment into restricted terrestrial valleys and playa lake environments.

This study, which interprets the Yadgimurrin member as a facies within the Mooloogool and Tooloo Groups, provides further constraints on the Glenburgh orogeny and the sequence of events associated with the closure of the Bryah Basin aulacogen.

This implies that a predominantly extensional tectonic regime was present within the current boundaries of the Yerrida basin until approximately1950Ma, with the eruption of tuffs from the Imbin Porphyry.

The stratigraphy of the Windplain, Bryah, Mooloogool and Tooloo Groups in relation to the Yadgimurrin member facies is shown below, illustrating its temporal association of diamictite units with the Glenburgh Orogeny between ~1980 Ma and ~1950 Ma.

Implications

The implications are fourfold;

1. Prospectivity for MVT associated mineralisation is associated with the Tooloo Group is widespread and may exist in areas considered to be 'silcrete' or unprospective 'breccia' according to current mapping
2. Under this interpretation the Mooloogool Group age is better constrained by reference to the Yelma Formation, as the two groups share similar derivation from extensional tectonism, albeit with differing local sedimentology
3. The age of the Maraloou shale (underlying the Yelma and Yadgimurrin) is better constrained and should be considered equivalent to the Johnson Cairn FM, and moved out of the Mooloogool Group
4. The inter-fingering and partial conformable relationships between the Windplain and Mooloogool-Tooloo Groups can be explained by observable field relationships

References

ABEYSINGHE, P. B., 2005, Mineral occurrences and exploration potential of the Earaheedy area, Western Australia: Western Australia Geological Survey, Report 96, 82p.

Akin, S.J., 2014. Sedimentology and stratigraphy of the Paleoproterozoic Frere Formation, Western Australia: implications for the evolution of the Precambrian ocean. Geological Survey of Western Australia, Report 130 133p.

Dentith, MC, Johnson, SP, Evans, S, Aitken, ARA, Joly, A, Thiel, S and Tyler, IM 2014, A magnetotelluric traverse across the eastern part of the Capricorn Orogen: Geological Survey of Western Australia, Report 135, 49p.

GEE, R. D., 1987, Peak Hill, W.A. (2nd edition): Western Australia Geological Survey, 1:250 000 Geological Series Explanatory Notes, 24p.

Occhipinti S., Hocking R., Lindsay M., Aitken A., Copp I., Jones J., Sheppard S., Pirajno F., Metelkaa V., October 2017.  Precambrian Research 300: pp, 121-140 Paleoproterozoic basin development on the northern Yilgarn Craton, Western Australia

PIRAJNO, F., and ADAMIDES, N. G., 2000, Geology and mineralization of the Palaeoproterozoic Yerrida Basin, Western Australia: Western Australia Geological Survey, Report 60, 43p.

PIRAJNO, F., OCCHIPINTI, S. A., and SWAGER, C. P., 2000, Geology and mineralization of the Palaeoproterozoic Bryah and Padbury Basins, Western Australia: Western Australia Geological Survey, Report 59, 52p.

RASMUSSEN B., Fletcher I.R., Bekker A., Muhling J.R, Gregory C.J., & Alan M. Thorne A.M., 2012. Nature 484: pp, 498–501. Deposition of 1.88-billion-year-old iron formations as a consequence of rapid crustal growth

Rogers, J. 1996. Annual Report for the period 18/07/1996 to 31/11/1996, Freshwater Project, C130/1996. WAMEX Report no A50015 (unpublished)

SHEPPARD, S. and Fletcher, I. and Rasmussen, B. and Zi, J. and Muhling, J. and Occhipinti, S. and Wingate, M. et al. 2016. A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs. Precambrian Research. 286: pp. 1-19