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Basin inversion and supercontinent assembly as drivers of sediment-hosted Pb-Zn

mineralisation in the Mount Isa region, northern Australia

George M. Gibson 1,2, Laurie J. Hutton 3, & Josef Holzschuh 2

1Research School of Earth Sciences, Australian National University, Canberra, ACT 2601,

Australia

2Geoscience Australia, Symonston, Canberra, ACT 2604, Australia

3Geological Survey of Queensland, Brisbane, Queensland 4002, Australia

Abstract: Deep seismic reflection imaging combined with geochronological, paleomagnetic and

stratigraphic data indicate that the world class sediment-hosted Pb-Zn deposits of northern Australia

are preferentially concentrated in the post-extensional syn-inversion fraction of the Calvert and Isa

superbasins. These fractions were deposited from 1654-1640 Ma and 1615-1575 Ma respectively,

and overlap the age of known orogenic events in both Australia and western Laurentia, pointing to a

common origin linked to crustal shortening and supercontinent assembly. Crustal shortening

resulted in thrust faulting and reactivation of earlier-formed extensional faults leading to upward

expulsion of mineralising fluids at or close to the seafloor while basin inversion and sedimentation

were still in progress. This is contrary to most existing models for Pb-Zn ore genesis in northern

Australia where fluid flow and mineralisation have been attributed to syn-extensional processes

accompanying continental breakup or thermal sag. Instead, mineralisation post-dates passive margin

formation and more likely occurred in a foreland basin or convergent plate tectonic setting involving

one or more collisional events. The terminal continent-continent collision between Australia and

Laurentia resulted in the 1600 Ma Isan and Racklan orogenies and assembly or reassembly of the

Nuna supercontinent.

Introduction

The global and temporal distribution of sediment-hosted Pb-Zn deposits is far from uniform.

Instead, a disproportionate number of deposits formed in the latest Paleoproterozoic-earliest

Mesoproterozoic (1700-1600 Ma), mid-Mesoproterozoic (1100-1050 Ma) and late

Precambrian-early Paleozoic (600-300 Ma), inviting comparisons with the supercontinent

cycle (Fig. 1) whose peaks overlap these same three time intervals (Barley and Groves, 1992;

Groves and Bierlein, 2007; Leach et al., 2010; Leach et al., 2005; Pehrsson et al., 2016). A

few researchers have gone even further and advanced the idea that supercontinent assembly

and fragmentation not only determined the timing and distribution of Pb-Zn mineralisation

but influenced its character during successive stages of the tectonic cycle (Cawood and

Hawkesworth, 2013; Groves and Bierlein, 2007; Huston et al., 2006). Thus, carbonate-hosted

Pb-Zn deposits of Mississippi Valley-type (MVT) are posited to have formed during the

continental aggregation stage (Bradley and Leach, 2003; Leach et al., 2001) when collisional

Journal of the Geological Society 174, 2017 http://dx.doi.org/10.1144/jgs2016-105 © 2017 Commonwealth of Australia (Geoscience Australia). Published by The Geological Society of Londonhttps://www.geolsoc.org.uk/Publications/Lyell-Collection/Using-the-Lyell-Collection/Copyright-Permissions-and-Terms-of-Use/Copyright-Policy-and-Terms-of-Use-for-Authors (Publisher journal website 6/8/2019)

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processes are at their height and tectonic conditions favour the formation of foreland basins

over other depositional environments in which SEDEX or clastic-dominated Mt Isa-type

deposits are more likely to develop (Leach et al., 2010; Leach et al., 2005). Mt Isa-type

deposits are thought to form later in the supercontinent cycle when plate convergence has

largely ceased and continental assembly has given way to rifting followed by continental

breakup and thermal subsidence (Cawood and Hawkesworth, 2013). However, while a back-

arc basin or passive margin setting is commonly inferred for this type of mineral deposit

(Betts et al., 2003; Gibson et al., 2012; Goodfellow, 2007; Goodfellow et al., 1993; Huston et

al., 2006; Large et al., 2005; Leach et al., 2005; Reynolds et al., 2015), most are hosted by

sedimentary rocks or basins that have since been deformed so that the evidence in support of

a syn-extensional as opposed to later origin for mineralisation is not always immediately

obvious or conclusive. Moreover, even where Pb isotopic data are available to better

constrain the age of mineralisation as is the case with many of the Mount Isa deposits (Carr et

al., 2004), doubts persist as to its timing and relationship to basin formation, particularly in

younger parts of the succession where the evidence for mineralisation during active rifting is

least convincing. Indeed, some researchers have rejected a syn-extensional origin altogether,

arguing instead that the bulk of mineralisation was coeval with the basin sag phase (Betts et

al., 2003; Betts et al., 2004) or occurred later still during an episode of crustal shortening that

either completely post-dates basin formation (Broadbent et al., 1998; Groves and Bierlein,

2007; Perkins, 1998; Zhang et al., 2006) or occurred during its closing stages whilst

sedimentation was still in progress (Gibson et al., 2016; McConachie and Dunster, 1996).

The depositional and tectonic setting of clastic-dominated Mt Isa-type mineralisation is thus

far from resolved and comparisons continue to be made with MVT deposits with which they

share many similarities (Huston et al., 2006; Leach et al., 2005), raising questions not only

about the relative timing of one deposit type against the other but the very relationship of Pb-

Zn mineralisation to the supercontinent cycle. Here, we review the depositional and tectonic

setting of sediment-hosted Pb-Zn mineralisation in the Mount Isa region with a view to better

constraining its timing and relationship to global events.

Pb-Zn mineralisation and basin evolution

The Mount Isa terrane (Fig. 2) lies wholly within the north Australian craton and is the

world’s largest single repository of Pb-Zn metal (Huston et al., 2006; Southgate et al.,

2006). Although widely deformed and metamorphosed from 1620-1500 Ma during the Isan

Orogeny and younger events, nappe-like structures and large-scale thrust displacements are

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for the most part absent or confined to the eastern part of the terrane (Beardsmore et al.,

1988; Betts et al., 2006; Blenkinsop et al., 2008; O'Dea et al., 2006; Rubenach et al., 2008;

Withnall and Hutton, 2013). Consequently, much of the pre-orogenic crustal architecture is

still largely intact and dominated by three vertically stacked basins (Jackson et al., 2000;

Southgate et al., 2000a), preserving a 200 Myr record of continental rifting, crustal thinning

and bimodal magmatism linked at mid-crustal levels to low-angle extensional faulting and

the emplacement of metamorphic core complexes (Gibson et al., 2008; Holcombe et al.,

1991; Passchier and Williams, 1989).

The lowermost and oldest of the three basins (1790-1740 Ma Leichhardt Superbasin)

developed in response to ENE-WSW intracontinental rifting (Fig. 3) and comprises a

network of variably deformed asymmetric basins and half-graben mainly filled with

continental tholeiites, felsic volcanics and fluviatile-lacustrine sediments (Eriksson et al.,

1983; Gibson et al., 2008; Jackson et al., 2000; Withnall and Hutton, 2013). Its volcanic

rocks were erupted as part of a large igneous province (LIP) and date back to the time that

northern Australia is thought to have formed part of the Nuna supercontinent and lain

opposite western Laurentia in a pre-Rodinia SWEAT-like configuration (Betts et al., 2016;

Betts et al., 2008; Furlanetto et al., 2016; Gibson et al., 2012; Lambeck et al., 2012; Payne

et al., 2009; Pehrsson et al., 2016; Thorkelson and Laughton, 2016; Thorkelson et al., 2001;

Zhang et al., 2012). Basaltic rocks of the 1785 Ma Eastern Creek Volcanics make up 5-8 km

of basin fill in the Leichhardt River Fault Trough (Bain et al., 1992; Gibson et al., 2008)

whereas farther east basaltic rocks are much less common and basin fill is dominated by

quartzite and felsic volcanics of 1780 Ma and 1760 Ma age (Neumann et al., 2009).

Bimodal volcanism was accompanied by the intrusion of granite and gabbro at depth and

continued through to 1740 Ma in the east (Neumann et al., 2009) by which time the first of

the metamorphic core complexes were being extensionally unroofed (Gibson et al., 2008;

Holcombe et al., 1991) but lithospheric thinning had not yet progressed to the point of

continental rupture and breakup. Shortly after 1740 Ma, bimodal magmatism and rifting

temporarily arrested (Fig. 3), followed by cooling and burial of the basin beneath a post-rift

blanket of shallow marine quartzite and carbonate rocks (Jackson et al., 2000).

Rifting resumed after 1730 Ma with formation of the Calvert Superbasin (1730-1640 Ma;

Fig. 3). Faults and half-graben associated with phase of basin development more typically

trend NW-SE or NE-SW and cut across the earlier-formed basin architecture at high angles

(Betts et al., 2006; Derrick, 1982; Gibson et al., 2008; O'Dea et al., 1997). Early basin fill is

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dominated by non-marine red-beds, fanglomerates and subsidiary amounts of basaltic lava

but after 1700 Ma the basin rapidly deepened and shallow marine conditions spread across

the Lawn Hill Platform whereas turbidites (Kuridala and Soldiers Cap groups; Fig. 3) began

to accumulate in a deep marine basin developing off the continental shelf to the east

(Gibson et al., 2012; Southgate et al., 2013; Withnall and Hutton, 2013). Turbidite

deposition was accompanied by the intrusion of dolerite dykes and sills (Neumann et al.,

2009; Rubenach et al., 2008; Withnall and Hutton, 2013) and an elevated thermal gradient

consistent with ongoing lithospheric extension and crustal thinning. Crustal thinning and

basaltic magmatism peaked around 1655 Ma by which time the dykes and sills had evolved

towards more Fe-rich compositions and increasingly resembled oceanic tholeiites emplaced

at or close to the point of continental breakup and rupture (Baker et al., 2010; Gibson et al.,

2012). Subsequent to 1655 Ma, but possibly commencing much earlier, the continental

margin began to rapidly subside and the turbidites along with their shallow water

equivalents on the Lawn Hill Platform were buried, together with the underlying rift

architecture, beneath of a transgressive sequence of marine carbonates, siltstones and black

finely-laminated carbonaceous siltstones dated to between 1670 and 1650 Ma (Neumann et

al., 2006; Page et al., 2000). This transgressive sequence hosts the Mount Isa Pb-Zn deposit

as well as several other world class ore bodies (Hilton-George Fisher, Lady Loretta) and is

separated from the underlying rift-related rocks by the Gun Unconformity (Southgate et al.,

2000a). Based on published seismic images (Gibson et al., 2016), total cumulative basin

thickness then exceeded 10-12 km and the depositional environment bore more resemblance

to a passive margin than intracontinental rift (Fig. 4). Extensional faulting and basaltic

magmatism by this time had all but ceased and this part of northern Australia faced an open

ocean or marginal sea (Gibson et al., 2012; Gibson et al., 2008; Lambeck et al., 2012). This

margin not only included the McArthur Basin but extended eastward into the Georgetown

province (Fig. 4) where syn-rift sediments and basaltic rocks of Calvert-age (Neumann and

Fraser, 2007) are similarly overlain by a transgressive sequence of thinly laminated

carbonaceous shales and siltstones of marine origin (Jackson et al., 2000; Lambeck et al.,

2012; Withnall and Hutton, 2013). Sedimentation in the Calvert Superbasin concluded no

later than 1640 Ma (Fig. 3) by which time the basin had been inverted (Fig. 3), leading to

emergent conditions on the Lawn Hill Platform and the replacement of platform carbonates

by an upward-coarsening and increasingly siliciclastic-dominated sedimentary sequence

(Loretta Supersequence; Krassey et al., 2000a).

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The Isa Superbasin is largely confined to the Lawn Hill Platform (Fig. 2) and incorporates a

5-6 km thick sequence of rhythmically-bedded turbidites, carbonaceous shales and dolomitic

siltstone deposited in a shallow to deep water marine environment (Hutton and Sweet, 1982;

Krassay et al., 2000a; Krassay et al., 2000b). Although variously interpreted as a foreland

basin (Broadbent et al., 1998; McConachie and Dunster, 1996), strike-slip basin (Feltrin et

al., 2009; Southgate et al., 2000a) or thermal sag phase (Betts et al., 2003; Betts and Lister,

2001), seismic reflection images for this part of the Lawn Hill Platform have since revealed a

far more complicated basin history involving a return to extensional faulting and half-graben

formation during River and Term time (Fig. 3). Thermal sag sediments (uppermost Term)

constitute only a small fraction of the basin fill (Fig. 3) and are subordinate in volume and

thickness to the overlying syn-inversion package. Deposition of this syn-inversion package

commenced no later than ca. 1615 Ma with onset of the Isan orogeny (Gibson et al., 2016)

and continued through to at least 1575 Ma by which time (Fig. 3) sedimentation in the Isa

Superbasin ceased and the last of the major Pb-Zn deposits had been emplaced at Century

(Southgate et al., 2000a). Its 1595 Ma host rocks (Wide Supersequence) postdate the onset of

crustal shortening by 20 Myr, precluding all but a syn-orogenic origin for this particular

mineral deposit (Broadbent et al., 1998; Feltrin et al., 2009; Gibson et al., 2016). In this

respect, Century bears more similarity to MVT deposits and has already been described as

such (Broadbent et al., 1998; Huston et al., 2006). Yet in many other respects its host rocks

are little different to the even more richly endowed Gun Supersequence in the Calvert

Superbasin whose finely laminated carbonaceous to dolomitic siltstones are not only

compositionally and lithologically similar to the Wide Supersequence but occupy much the

same stratigraphic position towards the top of the basin (Fig. 3).

Timing of Pb-Zn mineralization in Calvert Superbasin

Except for Dugald River and Cannington (Figs. 2 & 4) dated at 1655 Ma and 1665 Ma

respectively (Carr et al., 2004), all other Pb-Zn mineral deposits of any size in the Calvert

Superbasin occur on the Lawn Hill Platform (e.g. Lady Loretta) or along the western margin

of the immediately adjacent Leichhardt River Fault Trough (Mount Isa, Hilton-George

Fisher) where they are hosted by the 1670-1640 Ma Gun and Loretta supersequences

(Neumann et al., 2006; Page et al., 2000). Their Pb model ages range from 1654 Ma at

Mount Isa/Hilton-George Fisher to 1648 Ma at Lady Loretta (Carr et al., 2004) and, like

many other deposits, have long been known to plot at a prominent bend or inflection point

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(Fig. 1b) in the apparent polar wander path for northern Australia (Idnurm, 2000). Some

form of external tectonic control is evident but, unlike the 1575 Ma Century and 1640 Ma

McArthur River Pb-Zn deposits for which there is a corresponding and long recognized

episode of crustal shortening and/or transpresssion (Gibson et al., 2008; Hinman, 1995;

Scott et al., 2000; Southgate et al., 2000b; Withnall and Hutton, 2013), no matching

compressional event at ca. 1650 Ma has ever been identified in the Leichhardt River Fault

Trough. Rather, the conclusion till now has been that mineralization at 1650 Ma was more

likely controlled by transient extensional processes, including further normal faulting, in a

tectonic regime otherwise dominated by strike-slip faulting (Southgate et al., 2000b) or

thermal sag (Betts et al., 2003).

Yet mapped faults in the Leichhardt River Fault Trough, including many structures of

unequivocal extensional origin, typically terminate in lowermost Moondarra Siltstone at the

Gun Unconformity and do not extend upward into the Gun Supersequence or immediately

overlying Loretta Supersequence (Fig. 5). These faults are instead confined to the syn-rift

component of the Calvert Superbasin or older rocks of the Leichhardt Superbasin (Fig. 5). A

syn-extensional origin for Pb-Zn mineralisation and its host rocks in this part of the Mount

Isa region is therefore deemed unlikely. Moreover, even though contacts between the

Loretta and Gun supersequences in this part of the Leichhardt River Fault Trough are

conformable with little or no direct evidence that one or both units were undergoing

deformation at the time they were being deposited, sedimentation patterns nevertheless

change quite dramatically across this depositional boundary. Indeed, beginning at the very

top of the Gun Supersequence, and continuing through Loretta into the overlying River

Supersequence, bedding thickness, grain size and the amount of quartz sand entering the

basin all increased (Domagala et al., 2000). Growing tectonic instability and the emergence

of a new source region have been invoked as the most likely explanation for these changes

(Southgate et al., 2000b) and find support in observations made farther north the Lawn Hill

Platform and Murphy Ridge (Fig. 2) where uplift and erosion have removed as much as

1700 m of sedimentary section from beneath the Loretta Supersequence (Bradshaw et al.,

2000). This includes all of the Gun Supersequence and most of the underlying syn-rift

fraction of the Calvert Superbasin (Bradshaw et al., 2000). As with the Isa Superbasin, basin

inversion in the Calvert Superbasin evidently commenced late in the basin cycle and before

the last of the sedimentary units had been deposited, an interpretation for which there is

independent support in recently published seismic reflection data (Gibson et al., 2016).

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Seismic evidence for basin inversion at 1600 Ma and 1650 Ma

Revised and slightly modified interpretations of two previously published seismic reflection

profiles (06GA-M1 and 06GA-M2) are presented in Figure 6. Both pertain to the Lawn Hill

Platform and were acquired with a view to better understanding basin architecture in the

vicinity of the Century deposit (Fig. 2b). All three superbasins have been imaged but the

focus here is on the upper parts of the Calvert and Isa superbasins where stratal geometries

more obviously developed in response to uplift and erosion accompanying crustal shortening.

Sedimentary units making up these parts of the basins typically thin over the crests of

antiformal folds developed in the hangingwalls of reactivated normal faults or their

associated footwall shortcut thrusts (Gibson et al., 2016). In the case of the Isa Superbasin,

such thinning is particularly well developed in the 1615 Ma Lawn and 1595 Ma Wide

supersequences (Fig. 3), both of which thin through onlap onto the flanks of folds developed

in the underlying Term Supersequence (Fig. 6a). Deposition of these two units occurred

while folding and thrusting were still in progress.

In marked contrast, the syn-extensional fraction of basin fill in this superbasin (River and

lowermost Term supersequences) retains its original wedge-like geometry and thickens into

the Riversleigh Fault (Fig. 6a) as is expected of sediments deposited in a half-graben or

asymmetric basin. Overlying this syn-extensional component is an even thinner post-rift

fraction that post-dates normal faulting and makes up the uppermost part of the Term

supersequence. The Doom Supersequence (Fig. 3), for which a syn-orogenic 1585 Ma

depositional has been obtained (Page et al., 2000; Page and Sweet, 1998), is also taken to be

part of the syn-inversion sedimentary package although this cannot be determined from the

seismic data alone.

Basin inversion and anticlinal folding are no less evident in the hangingwalls of several other

reactivated extensional structures, including the Termite Range Fault which directly underlies

the Century deposit (Fig. 6a) and may have served as a fluid conduit during mineralisation.

However, unlike the Riversleigh structure, the Termite Range Fault breeches the surface and

does not terminate in the post-rift fraction of the Isa Superbasin (Fig. 6a). It exhibits a

component of reverse-slip (or oblique-slip) displacement and continued to be active during

deposition of the Lawn and Wide supersequences. The Wide Supersequence hosts the bulk of

Pb-Zn mineralisation at Century and was deposited subsequent to the onset of the Isan

Orogeny at 1615 Ma. Mineralization at Century thus not only post-dates the onset of crustal

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shortening as originally proposed (Broadbent et al., 1998) but was coeval with thrust faulting

and basin inversion in its host rocks.

Where imaged in the seismic data along line 06GA-M1 (Fig. 6b), the much older River

Supersequence at the base of the Isa Superbasin (Fig. 3) visibly thins through onlap onto the

flanks of asymmetric folds that must have already existed or grown while the unit was being

deposited (Gibson et al., 2016). Several such folds have been imaged at depth beneath Mount

Caroline (Fig. 2b) but none of them nor any of the thrust faults developed in their overturned

limbs continue upwards for any distance into the overlying stratigraphic units (Fig. 6b).

Instead, these structures rapidly die out or are abruptly truncated at the base of the overlying

Term Supersequence (Fig. 6b), indicating that basin inversion had not only concluded by this

time but cannot be any younger than the 1635 Ma depositional age usually assigned to this

unit (Fig. 3). In this region, the base of the Term Supersequence is an angular unconformity

that later deformation during the Isan Orogeny failed to mask (Fig. 6b). This unconformity

has been deformed into a broad arch but is much less intensely folded than rocks making up

the thicker and more strongly inverted core of the Calvert Superbasin. In contrast, the angular

unconformity with the Leichhardt Superbasin at the base of this inverted sequence exhibits

few signs of basin inversion and has a near-zero apparent dip in the seismic line (Fig. 6b)

owing to an orientation that is subparallel to the main basin-forming structures (Fig. 2).

Directly south of Mount Caroline, but still east of the Termite Range Fault, contact relations

between the River and Loretta supersequences along line 06GA-M2 are more difficult to

decipher owing to a confused set of reflections arising out of a bend in the seismic line. Any

thinning of units in this region is confined to the Loretta Supersequence (Fig. 6a). The River

Supersequence, on the other hand, rests unconformably on a truncated Loretta Supersequence

and maintains constant thickness all the way westwards to the Termite Range Fault before

undergoing a progressive but rapid thickening into the Riversleigh Fault (Fig. 6a). This is

more consistent with an extensional origin for most, if not all, of the River Supersequence.

However, as beneath Mount Caroline, onlap of the River Supersequence onto a more steeply

dipping Calvert Superbasin is plainly discernible just east of the Riversleigh Fault (Fig. 6a),

leaving open the possibility that deformation may not have concluded during deposition of

the Loretta Supersequence but continued through into early River time. In either event, there

is little evidence to suggest that basin inversion lasted much beyond 1640 Ma by which time

the depositional environment dramatically changed and shallow-water coarse clastic

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sediments were no longer being deposited (Shady Bore Quartzite). Instead, there was a return

to deep water conditions and the deposition of siltstone. The episode of basin inversion that

commenced with the aforementioned influx of quartz sand into the Leichhardt River Fault

Trough during late Gun or early Loretta time at ca. 1654 Ma evidently concluded around this

same time and lasted more than 10 Myr, encompassing the entire time interval over which

Pb-Zn mineralisation in the Mount Isa/George Fisher/Hilton ore bodies occurred. Southgate

et al (2000b) attributed the change in depositional patterns to the onset of strike-slip faulting

and concluded that this was in turn linked to plate convergence and orogenesis along the

southern margin of the north Australian craton (see also Scott et al., 2000; Betts et al., 2003).

Another possibility, and the one explored in more detail here, is that these and other changes

towards the end of the Calvert Superbasin were driven by subduction-related events to the

east of the cratonic margin and involved one or more episodes of arc-continent collision (Fig.

7). A similar train of events has been invoked to explain basin evolution along the continental

margin of ancestral North America, including in the equally well-endowed but much younger

late Neoproterozoic-early Paleozoic Selwyn Basin with which the rocks of Mount Isa might

be usefully compared (Fig. 4).

Selwyn Basin as a Paleozoic analogue for Calvert and Isa superbasins

After northern Australia, the second largest concentration of sediment-hosted Pb-Zn

deposits in the world occurs in western North America (Leach et al., 2010), accounting for a

significant proportion of the global peak in mineralization from 600-300 Ma (Fig. 1a).

These deposits are mainly hosted by epicratonic basins formed during and subsequent to

Rodinia breakup in the late Neoproterozoic to early Paleozoic, and stretch from southern

Canada to the Brooks Range in Alaska (Dickinson, 2006; Goodfellow, 2007; Goodfellow et

al., 1993; Leach et al., 2010; Leach et al., 2005; Nelson et al., 2002; Slack et al., 2004;

Young, 2004). The Selwyn Basin is one of the largest and best studied of these basins (Fig.

4) and incorporates a variably deformed continental margin sequence in which turbidites

and other deep water sedimentary facies have been thrust eastwards over a shallow-water

carbonate-dominated platform and outer shelf sequence (Goodfellow et al., 1993; Mair et

al., 2006; McClay et al., 1989; Nesbitt and Muehlenbachs, 1994). As with the Mount Isa

basins, Pb-Zn deposits occur at more than one stratigraphic level but are particularly

common along the interface between the platform and deeper water sediments (Fig. 4b). No

less importantly, the Selwyn Basin shares a very similar evolution to the basins in northern

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Australia and comprises not one but three separate unconformity-bounded successions

(Goodfellow, 2007; Goodfellow et al., 1993).

The oldest of these three successions includes basaltic rocks with continental affinities but

for the most part its basin fill is dominated by shallow-water sediments deposited in an

intracontinental setting from late Neoproterozoic- early Cambrian time. It bears some

similarity to the Leichhardt Superbasin in terms of both its history and evolution, and was

followed in the Cambrian-Devonian by a transition to deeper water sedimentation and

passive margin conditions before onset of the Late Devonian-Carboniferous Antler Orogeny

(Mair et al., 2006; Nelson et al., 2002). The youngest and uppermost succession (Devonian-

Carboniferous) in the Selwyn Basin is dominated by deeper water slope to basinal

sedimentary facies bearing a striking resemblance to the thinly laminated dolosiltstones and

black carbonaceous shales of the Gun Supersequence. This succession contains MacMillan

Pass as well as several other clastic-dominated (SEDEX) Pb-Zn deposits (Fig. 4b) for which

a syn-sedimentary, syn-extensional, Late Devonian-Carboniferous age is widely accepted

(Goodfellow, 2007; Goodfellow et al., 1993; Leach et al., 2005).

MVT deposits have a more restricted distribution in the basin and only occur in the flanking

carbonate platform rocks (Goodfellow, 2007; Leach et al., 2001; Leach et al., 2010; Nesbitt

and Muehlenbachs, 1994). They include the Robb Lake deposit (Fig. 4b) dated at 362 ± 9

Ma (Nelson et al., 2002). It exhibits many of the attributes expected of a typical MVT

deposit (Nesbitt and Muehlenbachs, 1994) but paradoxically is no different in age to some

of the SEDEX deposits at MacMillan Pass and elsewhere in the basin for which a syn-

extensional origin has been proposed (Goodfellow, 2007; Leach et al., 2010; Leach et al.,

2005). This has prompted suggestions (Nelson et al., 2002) that Robb Lake and other MVT

deposits like it in the Selwyn Basin do not conform to type. More specifically, a syn-

orogenic for these deposits was rejected in favour of an alternative interpretation in which

these deposits were not only coeval with their SEDEX counterparts but formed during the

same extensional event. It was further suggested that this extensional event was driven by

subduction roll-back along the ancestral continental margin of North America, resulting in

formation of a back-arc basin in which both MVT and SEDEX mineralisation took place

(Nelson et al., 2002).

Other researchers have taken the view that these same MVT deposits originated during

crustal shortening (Nesbitt and Muehlenbachs, 1994), an interpretation seemingly at odds

with a syn-extensional origin for the coeval SEDEX deposits but consistent with the

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observation that the Antler Orogeny shares the same late Devonian-Carboniferous age.

Elsewhere along the western Cordillera, this event was accompanied by basin inversion and

the shedding of sedimentary detritus eastwards into a foreland basin (Nesbitt and

Muehlenbachs, 1994; Smith et al., 1993; Trexler et al., 2004). This would suggest that

existing models for Pb-Zn mineralisation based around the contemporaneity of

mineralisation with extension in the Selwyn Basin are not without their problems and still

open to debate (Dickinson, 2006). More importantly, given the many parallels between this

basin and those in northern Australia, there is a high probability that a model developed for

Pb-Zn mineralisation in one basin is equally applicable to the others and rests on a better

understanding of the tectonic environment in which the basins evolved despite being

separated in time and space by more than one billion years. By virtue of its younger age and

location behind an extant and still evolving rifted continental margin, the plate tectonic

setting of the Selwyn Basin from the time of its inception until cratonisation is far more

amenable to reconstruction and thus affords important clues as to the likely tectonic drivers

and processes involved at successive stages of basin development and Pb-Zn mineralisation

not only in western North America but northern Australia.

Discussion

Geodynamic setting and evolution of the east Australian rifted margin

As with other basins developed along the Pacific margin of ancestral North America

following Rodinia breakup, the Selwyn Basin has been subjected to successive episodes of

extension and contraction from the time it first formed in the late Neoproterozoic-early

Paleozoic until the last of the terranes in the immediately adjacent Intermontane Belt was

accreted during the Laramide Orogeny (Mair et al., 2006; Nelson et al., 2002). This

episodicity was driven by far-field tectonic stresses arising out of subduction advance and

retreat along the margin and brought about periods of plate convergence and orogenesis

separated by extensional collapse, arc magmatism and back-arc basin formation (Mair et al.,

2006). The Antler Orogeny is considered to be the earliest of the contractional events and

resulted from arc-continent collision (Dickinson, 2006; Smith et al., 1993). Orogenic

loading associated with such collisional events not only offers an effective mechanism for

driving regional-scale fluid flow of the type implicated in the formation of MVT deposits

(Bradley and Leach, 2003; Dickinson, 2006; Garven, 1985; Leach et al., 2001) but obviates

the need to have separate tectonic triggers for deposits of different type but identical ages in

a single basin. Such an event therefore has considerable appeal as a means of forming

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mineral deposits in basins like the Selwyn and Calvert. However, whereas subduction

polarity and the sense of plate convergence at different times are reasonably well known for

western North America, no such certainty pertains to the older basins of northern Australia

and tectonic models based on both north- and west-dipping subduction have been proposed.

Most researchers subscribe to the former and have the Leichhardt and Calvert superbasins

developing in a back-arc setting located above a north-dipping, but southward-retreating,

subduction zone that lay along the southern margin of the north Australian craton (Betts et

al., 2003; Giles et al., 2002; Huston et al., 2012; Scott et al., 2000) or was located even

farther afield on the southern side of the still connected south Australian and east Antarctic

cratons (Mawsonland) from whence it continued along strike into southern Laurentia (Betts

et al., 2016; Betts et al., 2008; Giles et al., 2004). In either case, the 1800-1600 Ma interior

basins of northern Australia and Laurentia share a common origin and geodynamic setting,

and lie inboard of an active continental margin that faced an open ocean to the south. Far

field effects arising out of changes in the rate or configuration of subduction along this

margin are thought to have driven orogenesis and crustal shortening in the interiors of both

continents, deforming the basins along with their underlying continental basement rocks

(Betts et al., 2016; Betts et al., 2008).

Best known of these deformed elements in central Australia is the Warumpi terrane whose

accretion and /or collision with the north Australian craton during the 1640-1635 Ma Liebig

Orogeny (Scrimgeour et al., 2005) overlaps the age of basin inversion and mineralisation

(McArthur River) in the Calvert Superbasin (Fig.1). A causal relationship between crustal

shortening in central Australia and sediment-hosted Pb-Zn mineralisation in the Calvert

Superbasin therefore seems likely as has been suggested many times before (Betts et al.,

2016; Betts et al., 2008; Gibson et al., 2016; Giles et al., 2004; Huston et al., 2012). Despite

any such connection, it is still far from certain that successive episodes of deformation and

mineralisation in central and northern Australia at 1650 Ma or earlier were predominantly

driven by subduction rollback and the repeated opening and closing of ocean basins to the

south of a still growing north Australian craton the other side of Antarctica. Deformed

basinal sequences and terranes of late Paleoproterozoic age in the Gawler, Curnamona and

north Australian cratons all share a common provenance and Nd isotopic signature

(Barovich and Hand, 2008; Payne et al., 2006) and would not appear to have been far

removed from each other at any time between 1800 Ma and 1640 Ma (Payne et al., 2009)

when the last of Australia’s world class sediment-hosted Pb-Zn mineral deposits formed at

13

McArthur River. Other interpretations of basin formation and crustal shortening in central

Australia cannot be ruled out, including intraplate deformation accompanying the opening

and closing of an ocean basin to the east (Gibson et al., 2008). Sedimentary facies, fault

orientations and kinematic indicators in exhumed metamorphic core complexes, along with

compositional changes in basaltic magmatism (Baker et al., 2010; Gibson et al., 2008;

Jackson et al., 2000; Southgate et al., 2013), are all consistent with crustal thinning and

opening of such a basin in a easterly or northeasterly direction from 1800 Ma onward but

cannot easily discriminate between a passive margin formed through breakup of the Nuna

supercontinent as opposed to one developed on the inboard side of a back-arc basin or

marginal sea.

Gibson et al (2008) argued for the latter and proposed that the Leichardt and Calvert

superbasins both evolved behind an east-facing magmatic arc that initially rifted off the

Australian margin along with its underlying continental basement during Gun time before

being reunited with the passive margin no later than 1640 Ma (Fig. 7). Paleomagnetic data

pointing to an episode of ocean basin closure between 1720 Ma and 1650 Ma (Pisarevsky et

al., 2014) are consistent with such an interpretation but cannot completely rule out other

tectonic models in which continental breakup and passive margin conditions were

established earlier and predate deposition of most if not all of the Calvert Superbasin.

Notwithstanding the possibility of more complicated trajectories for basin evolution, by

1720 Ma or shortly after, continental rifting and crustal extension had become well

established along the eastern margins of both the north and south Australian cratons, and

sedimentary basins of Calvert age extended in a north-south direction (present-day

coordinates) all the way south from Mount Isa and Georgetown through the eastern

Curnamona and Gawler cratons into east Antarctica (Gibson et al., 2008; Peucat et al., 1999;

Rutherford et al., 2006; Szpunar et al., 2011), cutting across and isolating the Warumpi

terrane and other deformed basinal sequences of late Paleoproterozoic age in south-central

Australia from their inferred counterparts along strike in southern Laurentia. In this

interpretation, rifting and crustal extension are preferentially concentrated along the eastern

margin of conjoined Australian and Antarctic continents (Gibson et al., 2008; Payne et al.,

2009) and there is no need for these two events to be paired with either plate convergence or

subduction roll-back along a more distal margin on the opposite side of Antarctica or the

south Australian craton (Betts et al., 2016; Betts et al., 2008; Giles et al., 2004). Quite apart

from the substantial volume of intervening continental crust and difficulties of linking

14

interior back-arc basin development to such a distant margin (but see Betts et al., 2016 for a

possible solution), this interpretation predicts an overall progression of sedimentary basins

and their depocentres southward in tandem with the changing position of the north-dipping

subduction zone. Instead, the opposite is observed in the Leichhardt and Calvert superbasins

whose volcanic and sedimentary depocentres not only migrated eastwards during successive

stages of basin development but progressively deepened in the same direction (Gibson et al.,

2012; Gibson et al., 2016; Southgate et al., 2013; Withnall and Hutton, 2013).

Pb-Zn mineralisation and its tectonic drivers in northern Australia

Ore genesis models for clastic-dominated Pb-Zn mineralisation in the basins of northern

Australia and western North America often assume contemporaneity of mineral deposits

with extensional faulting or thermal sag in a rift or passive margin setting (Betts et al., 2003;

Goodfellow, 2007; Huston et al., 2006; Leach et al., 2010; Leach et al., 2005; Reynolds et

al., 2015; Slack et al., 2015). As argued here, the case for such models is open to question

and a majority of deposits in the Calvert and Isa superbasins more likely formed subsequent

to crustal extension during times of basin inversion (Fig. 6). Moreover, even though the

conditions for basin inversion and mineralisation in these two basins were only occasionally

met, these were almost certainly externally imposed and driven by crustal-scale orogenic

processes, involving crustal shortening and deep burial in more proximal parts of the orogen

as evidenced by Barrovian metamorphism beginning around 1655 Ma in the Georgetown

province and 1650-1640 Ma elsewhere (Cihan et al., 2006; Rubenach et al., 2008). Crustal

shortening during the Isan Orogeny has long been recognized as one of the drivers for

mineralisation in the Century deposit at the top of the Isa Superbasin (Broadbent et al.,

1998; Gibson et al., 2016) but, with the exception of McArthur River at 1640 Ma (Hinman,

1995), the primary trigger for fluid flow and attendant Pb-Zn mineralisation elsewhere in

the Mount Isa region is usually considered to be crustal extension not shortening.

Accordingly, Century has been classified as an MVT deposit and thought to be the sole

example of this deposit type in the Mount Isa region (Huston et al., 2006). In the analysis

presented here no such distinction is made and all sediment-hosted Pb-Zn deposits in the

Calvert and Isa superbasins are thought to share a common tectonic origin, irrespective of

age and whether they are located in the Leichhardt River Fault Trough or neighbouring parts

of the Lawn Hill Platform.

Conversely, there is no need to abandon elements of existing ore genesis models in which

mineralisation is observed to form proximal to extensional faults and be syn-sedimentary in

15

origin (Goodfellow, 2007; Goodfellow et al., 1993; Huston et al., 2006; Large et al., 2005;

Leach et al., 2010; Leach et al., 2005). As determined here for the Mount Isa deposits, basin

inversion was accompanied by reactivation of former extensional faults and took place

while sedimentation in the Calvert and Isa superbasins was still in progress (Fig. 6).

Consequently, these faults and their associated footwall shortcut thrusts, more than likely

acted as conduits along which mineralizing fluids would have migrated during inversion

before being expelled at or near the seafloor. Given the thinly laminated character of the

marine sediments hosting these deposits, it seems equally likely that this process initially

took place in deep water and was entirely submarine in nature. Sediment loading and crustal

shortening during basin inversion then combined to over-pressure the basin, driving fluids

upward along the faults all the way to the seafloor or sediment-water interface where

mineralisation actually took place. As deformation progressed and folds grew in amplitude

on the seafloor, it might be further surmised that this led to restrictions in bottom water

circulation and possibly even anoxic conditions as parts of the basin became ever more

isolated, creating ideal conditions for sediment-hosted mineralisation. Breaching of regional

seals may have assisted fluid flow but, unlike many existing models for SEDEX or Mount

Isa-type deposits that combine extensional faulting with convecting fluid cells to drive

mineralisation (Goodfellow, 2007; Leach et al., 2010), there is no need to appeal to elevated

heat flow. In the Mount Isa region, fluid flow is principally driven by post-extensional

crustal shortening and occurs at a stage in basin evolution when any residual heat or thermal

anomaly resulting from earlier rift-related magmatism and lithospheric thinning is already at

a low ebb and losing its potential to promote vigorous hydrothermal activity.

Supercontinent assembly as driver for Pb-Zn mineralisation in northern Australia

While it is increasingly agreed that northern Australia and western Laurentia were once

proximal to each other and formed part of the Nuna supercontinent (Betts et al., 2016; Betts

et al., 2008; Furlanetto et al., 2016; Gibson et al., 2012; Lambeck et al., 2012; Medig et al.,

2016; Thorkelson and Laughton, 2016; Thorkelson et al., 2001; Zhang et al., 2012), there is

still no consensus about the exact timing and manner of supercontinent assembly, let alone

its subsequent breakup when the majority of Pb-Zn mineral deposits supposedly formed

(Cawood and Hawkesworth, 2015). Some researchers have proposed that crustal extension

and rifting in western Laurentia may have commenced as early as 1840 Ma (Davidson,

2008; Rainbird et al., 2003) and proceeded to continental breakup no later than 1800 Ma by

which time passive margin conditions were established and Proterozoic North America

16

faced an open ocean to the west (Cook et al., 2005). The Leichhardt and Calvert Basins both

formed later and so cannot possibly be conjugate to any passive margin of 1800 Ma or older

age in western Laurentia. SWEAT-like reconstructions of Nuna in which northern Australia

was originally connected to northwest Canada (Betts et al., 2016; Betts et al., 2008; Gibson

et al., 2012) still remain valid but only if the east Australian margin underwent further

crustal extension and rifting after breakup at 1800 Ma and then evolved independently of its

Laurentian counterpart behind an east-facing magmatic arc until ca. 1650 Ma. This is the

interpretation adopted here and elsewhere for basin evolution in the Leichhardt and Calvert

superbasins (Gibson et al., 2012; Gibson et al., 2008) and the one presented in Figure 7.

Paleomagnetic data are consistent with this tectonic model and require only that Australia

and Laurentia be widely separated at 1720 Ma before being brought together at ca. 1650 Ma

(Pisarevsky et al., 2014). Bends or inflection points at 1710 Ma and 1653 Ma in the

apparent polar wander path for northern Australia (Fig. 1) would further indicate that these

times overlapped or coincided with periods of tectonic instability during the course of which

the external stress field or plate tectonic setting changed (Fig 7a-c). Even more conspicuous

is the hairpin bend at 1640 Ma (Fig. 1). It has long been interpreted as evidence of a major

plate reorganization at 1640 Ma (Idnurm, 2000; Southgate et al., 2000b) and occurs around

the same time that basin inversion and crustal shortening concluded in the Calvert

Superbasin, indicating that the changes if not reversals in plate motion recorded between

1650 Ma and 1640 Ma, particularly if accompanied by collision (Fig. 7d), may have been

the single most important factor in creating the conditions necessary for regional-scale fluid

flow and Pb-Zn mineralisation in this basin. Most, if not all, of the major clastic-dominated

Pb-Zn deposits in the Leichhardt River Fault Trough and neighbouring parts of the Lawn

Hill Platform formed during this period and it is clear from the seismic reflection data that

this mineralisation was linked to basin inversion. By 1650 Ma (Fig. 7d), crustal shortening

was already underway across much of the Mount Isa region and ever increasing quantities

of quartz sand were being transported southward from a source region on the northern Lawn

Hill Platform undergoing uplift and erosion in response to northeast-southwest compression

(Bradshaw et al., 2000; Southgate et al., 2000b).

No less importantly, this deformation involved a significant component of crustal thickening

as evidenced by contemporaneous medium-pressure Barrovian-style metamorphism,

garnetiferous relicts of which date back to 1655 Ma or 1650 Ma and occur widely

throughout the Georgetown Province and more easterly parts of the Mount Isa region

17

(Cihan et al., 2006; Rubenach et al., 2008). Coincidently, clastic sedimentary sequences

during late Gun time, and continuing through to deposition of Shady Bore Quartzite at the

very top of the Loretta Supersequence, began to coarsen and shallow upwards (Krassay et

al., 2000a). Infilling of a foreland basin is inferred here (Fig. 7d) in keeping with

observations that sedimentary transport during the closing stages of the Calvert Superbasin

was consistently directed southward away from areas in the east and northeast that had been

subjected to higher metamorphic grades and deeper burial during Barrovian metamorphism.

Crustal thickening and tectonic loading evidently increased from west to east during late

Calvert time consistent with a collisional event whose origins lay east of Georgetown and

most likely involved plate convergence along the eastern margin of Proterozoic Australia

(Fig. 7d). At the conclusion of this deformational event, large tracts of northern Australia

had been subjected to regional-scale fluid flow and hydrothermal potassic alteration which

occurred no later than 1640 Ma (Cooke et al., 1998; Gibson et al., 2016) and facilitated Pb-

Zn mineralisation at McArthur River (Fig. 2) during close of the Calvert Superbasin (Fig.

3).

Gibson et al (2008) argued that deformation at the end of Calvert time were driven by arc-

continent collision but were unable to locate the associated magmatic arc, speculating that it

may have subsequently rifted away during Rodinia breakup and now resided in western

North America where it makes up part of the Mojave terrane. More recently, Canadian

researchers have identified fragments of a magmatic arc caught up in breccias at the top of

the 1649 ± 14 Ma Wernecke Supergroup in the Canadian Cordillera (Furlanetto et al., 2009;

Furlanetto et al., 2016; Thorkelson and Laughton, 2016; Thorkelson et al., 2001). These

fragments date back to 1720 Ma (Furlanetto et al., 2013) and are thought to have formed

part of an east-facing magmatic arc (Bonnetia) that originated along or close to the east

Australian margin prior to its obduction and emplacement over the west Laurentian margin

during the ≤ 1663 Ma Forward and Racklan orogenies (Furlanetto et al., 2016; Thorkelson

and Laughton, 2016). Obduction followed closure of an ocean basin lying to the east of the

arc and was accomplished through subduction of older intervening oceanic crust westwards

from 1720 Ma (Furlanetto et al., 2016; Thorkelson and Laughton, 2016), mirroring in many

respects the tectonic model proposed by Gibson et al (2008), particularly in regard to the

existence of an older ocean basin that opened and closed ahead of arc-continent collision.

The arcs identified in both models would appear to be one and the same, and are interpreted

as such in Figure 7. Accordingly, the Canadian name Bonnetia (Furlanetto et al., 2013;

18

Furlanetto et al., 2016; Thorkelson and Laughton, 2016) has been adopted for this magmatic

arc (Fig. 7).

Important differences between the two interpretations nevertheless remain, particularly in

relation to the interpreted ca. 1600 Ma age of arc-continent collision in northwest Canada as

opposed to an older 1650 Ma age for northern Australia (Fig. 7). Paleomagnetic data

(Pisarevsky et al., 2014) lend strong support to the idea that Proterozoic Australia and

Laurentia were already proximal to each other by 1650 Ma. An older age for collision is

also supported by the observation that more distal parts of the Calvert Superbasin had

already been subjected to crustal thickening and Barrovian metamorphism before the

overlying and significantly younger ≤ 1640 Ma Isa Superbasin had even been deposited let

alone deformed during the 1620-1575 Ma Isan orogeny. The Wernecke Supergroup

overlaps the Isa Superbasin in age so that any equivalent collisional event in northwest

Canada might be expected to be of comparable age and predate deposition of this

supergroup. Instead, Bonnetia is interpreted to have been obducted and emplaced over the

west Laurentian margin about 1600 Ma and therefore about the same time that the Isan

Orogeny was underway in northern Australia (Furlanetto et al., 2016; Thorkelson and

Laughton, 2016; Thorkelson et al., 2001). Either there has been more than one arc-continent

collision or Bonnetia first collided with the Australian margin at 1650 Ma before being

obducted over the opposing Laurentian margin at 1600 Ma. The second interpretation is

thought more likely here (Fig. 7) and matches the 1650-1640 Ma north Australian events

with the less well known ≤ 1663 Ma Forward Orogeny, best known from seismic images

and whose early stages (Phase One) were similarly accompanied by basin inversion and the

deposition of syn-tectonic sediments (Hornby Group) (Cook and MacLean, 1995; MacLean

and Cook, 2004). An angular unconformity separates this older inverted sequence from

rocks not much different in age to the Isa Superbasin and Wernecke Supergroup (Cook and

MacLean, 1995). Such similarities in basin history are unlikely to be coincidental,

indicating that the change in plate tectonic regime or external stress field identified at 1650

Ma for northern Australia may also have had an impact in northwest Canada and further

supporting the view that Australia and Laurentia were probably proximal to each other by

this time but perhaps not yet engaged in continent-continent collision. In any event, by 1650

Ma, plate boundary conditions had changed sufficiently to induce deformation along both

continental margins but more importantly bring about collapse of the back-arc basin that

had been developing along the east Australian margin since 1800 Ma (Fig. 7). Deformation

19

and crustal shortening were presumably initially partitioned into this back-arc basin because

the Calvert Superbasin had only just ceased rifting and was in a thermally weakened state

following basaltic magmatism at 1655 Ma. More speculatively, deformation at 1650 Ma in

the Calvert Superbasin may have had its origins in the changing nature of subduction in the

ocean basin to the east of the arc which had either arrested its eastward retreat following

entry of thinned Laurentian crust into the subduction channel or simply begun to advance in

a manner akin to that which drove basin inversion and collapse in the younger Paleozoic

Selwyn Basin (Dickinson, 2006; Mair et al., 2006).

In either case, by 1640 Ma, northern Australia was again undergoing extension

accompanied by deposition of the Isa Superbasin at about the same time that the Wernecke

Superbasin was being laid down in northwest Laurentia. The depositional setting and reason

for this switch in tectonic regime in northern Australia are not yet well constrained but may

have involved either subduction retreat or extensional collapse of crust over-thickened

during the earlier arc-continent collision. Irrespective of cause, both continental margins

were again converging and undergoing crustal shortening from around 1620 Ma or 1600

Ma, culminating in continent-continent collision and orogenesis (Fig. 7e). The Racklan and

Isan orogenies both date from this time interval and both verge inland away from their

respective former continental margins, indicating that they are probably not only mirror

images of each other but manifestations of one and the same doubly-divergent collisional

event (Fig. 7e).

No less importantly, these two orogenic events appear to have been instrumental in

formation of the Century MVT deposit in northern Australia as well as a raft of SEDEX and

MVT deposits throughout northwest Canada (Thorkelson, 2000). Pb-Zn deposits older than

1600 Ma in northwest Canada (Thorkelson, 2000) better match those in the Calvert

Superbasin. Thus, contrary to most prevailing petrogenetic models for sediment-hosted Pb-

Zn mineralisation in northern Australia and elsewhere, these deposits not only formed long

after continental breakup but in a depositional and tectonic environment where crustal

shortening not crustal extension was the predominant driver of fluid flow. Crustal extension

still remains important in the genesis of such deposits but only insofar as it gave rise to the

basinal sequences that provided the future source of both fluids and metals. It is equally

possible that the dearth of major Pb-Zn deposits in northern Australia after Century (Fig. 1)

is in part be due to the fact that Australia and Laurentia remained together until Rodinia

breakup in the late Neoproterozoic, inhibiting a repeat of the depositional and tectonic

20

conditions that produced this style of mineralisation. Until that time, northern Australia

remained adjacent to western Canada, shedding sediment and detrital zircons of 1550 Ma or

younger age eastwards into the post-1600 Ma sedimentary basins of western North America

(Medig et al., 2016). Recent suggestions that sediment-hosted Pb-Zn deposits are more

likely to form during the disaggregation rather than aggregation stage of the supercontinent

cycle are therefore open to question and need to be modified in light of the analysis

presented here.

Conclusions

Sediment-hosted Pb-Zn mineral deposits in northern Australia are not part of a passive

margin sequence or sag phase as is commonly assumed but occur towards the top of the

Calvert and Isa superbasins in their respective syn-tectonic, syn-inversion sedimentary

fractions. During these stages of basin evolution, extensional faulting all but ceased and the

depositional environment resembled a foreland basin in which basin fill was controlled by

uplift and erosion of an emergent fold and thrust belt driven by far-field stresses or collision

at a continental margin distal to the Mount Isa region. Collision-related basin inversion

events are recognized at ca. 1650-1640 and 1615-1575 Ma, broadly coinciding with times of

Pb-Zn mineralisation and marking critical stages in the progression towards assembly of the

Nuna supercontinent at the end of the Isa Superbasin. Proterozoic Australia and ancestral

western North America collided during the second and terminal event giving rise to the

1600 Ma Isan and Racklan orogenies. All major sediment-hosted Pb-Zn deposits in the

Mount Isa region predate this last collision indicating that mineralisation occurred during

rather than after supercontinent assembly as is sometimes suggested. The late

Neoproterozoic-early Paleozoic Selwyn Basin in Canada shares many similarities with the

basins of northern Australia, including a history of extension interrupted by basin-inversion

forming events that were similarly driven by continental margin processes. A linkage

between late Paleoproterozoic-early Mesoproterozoic basin formation and subduction-

related events along the more distal southern margin of Laurentia (Betts et al., 2016; Betts et

al., 2008) is not precluded by this interpretation but suggests it may have been of secondary

importance to those occurring between eastern Australia and western Laurentia.

Acknowledgements

Paul Donchak (GSQ) and Karol Czarnota (GA) kindly provided comments on an earlier

draft of the paper. The seismic data for the Lawn Hill Platform were acquired as part of the

21

Predictive Mineral Discovery Co-operative Research Centre and jointly funded by the

Geoscience Australia, Geological Survey of Queensland and Zinifex (now Mines and

Metals Group). This is a contribution to IGCP 648: supercontinent cycles and global

geodynamics.

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Figure 1. (a) Temporal and global distribution of sediment-hosted Pb-Zn deposits (Leach et al., 2010) versus

supercontinent cycle (Bleeker, 2003). Note predominance of deposits in late Paleoproterozoic-earliest

Mesoproterozoic and early Paleozoic time and their overlap with times of continental aggregation; (b) Apparent

polar wander path for northern Australia showing coincidence in timing between bends in path and major Pb-Zn

deposits (Idnurm, 2000; Loutit et al., 1995).

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Figure 2. (a) Simplified geological map for northern Australia showing principal tectono-morphological

subdivisions and Pb-Zn mineral deposits (after Jackson et al., 2000). (b) Position of seismic reflection lines

06GA-M1 and 06GA-M2 in relation to main structures and outcropping parts of Calvert and Isa superbasins on

northern Lawn Hill Platform around Century mine.

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Figure 3. Simplified stratigraphic column for Mount Isa region showing three-fold subdivision into Leichhardt,

Calvert and Isa superbasins but different interpretations of basin development and tectonic evolution (Betts et

al., 2003; Betts and Lister, 2001; Gibson et al., 2016; Southgate et al., 2000a).

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Figure 4. (a) Schematic representation of paleogeography in Calvert Superbasin at ca. 1655 Ma with platform

carbonates (Upper Gun Supersequence) passing laterally into deep marine turbidites which together form part of

a passive margin sequence facing a marginal sea or open ocean basin to east. (b) Analogous passive margin

sequences for Paleozoic Selwyn Basin, along with distribution of main Pb-Zn mineral deposits.

Figure 5. (a) Part of Leichhardt River Fault Trough in Lake Moondarra area showing limited disruption of post-

rift Moondarra Siltstone (Gun Supersequence) by extensional faults compared to underlying older units in

Calvert and Leichhardt superbasins. Host rocks (Urquhardt Shale) to the Mount Isa and Hilton-George Fisher

deposits are identified as a separate unit within the Calvert Superbasin. (b) Cross-section showing extensional

faults terminating at Gun Unconformity.

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Figure 6. Seismic reflection profiles orthogonal (06GA-M2) and sub-parallel (06GA-M1) to main basin-forming faults in vicinity of Century Mine. (a) Partially inverted half-

graben in rocks of Calvert and Isa age with syn-inversion Lawn and Wide supersequences thinning over antiformal folds developed in hangingwalls of a northeast-dipping

reactivated extensional fault (Riversleigh Fault) and associated footwall shortcut thrust (see inset for more detail). Century Mine is underlain by a reactivated northeast-

dipping extensional fault or footwall shortcut thrust (Termite Range Fault). (b) Longitudinal section (06GA-M1) through same inverted basins illustrating along-strike

geometry of hangingwall antiforms in Calvert and Isa superbasins beneath Mount Caroline and Ploughed Mountain; fold amplitude is greatest where basin fill is thicker and

more strongly inverted. Owing to low obliquity of section to regional strike, both the Riversleigh and Termite Range faults exhibit shallow apparent dips.

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Figure 7. Interpreted geodynamic setting of Australian and opposing Laurentian margins from 1840-1600 Ma.

(a) Continental breakup and formation of conjugate rift margins between 1840 Ma and 1800 Ma; (b) Leichhardt

and Calvert superbasins develop in back-arc setting behind an east-facing magmatic arc (Bonnetia) and west-

dipping subduction zone. Continued slab roll-back and eastward retreat of this subduction zone will eventually

lead to plate convergence between Australia and Laurentia and closure of the original intervening ocean basin

by 1650 Ma; (c) By 1655 Ma, inboard side of back-arc basin has evolved into a passive margin with carbonate

platform in west and deep water turbidite sequence extending eastwards as far as Georgetown and beyond; (d)

Arc-continent collision with east Australian margin being overridden by Bonnetia. Thermally weaker back-arc-

basin collapses in response to subduction advance as leading edge of Laurentia encroaches upon subduction

zone; (e) Following a short interval of renewed crustal extension and deposition of the Isa Superbasin and

Wernecke Supergroup, Australia and Laurenta collide and an already deformed and fragmented Bonnetia is

obducted in the opposite direction over the Laurentian margin. Collision results in the doubly divergent Isan and

Racklan/Forward orogenic belts.