The protracted hydrothermal evolution of the Mount Isa Eastern Succession: A review and tectonic implications

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Precambrian Research 163 (2008) 108–130

The protracted hydrothermal evolution of the Mount Isa Eastern Succession: A review and tectonic implications Nicholas H.S. Oliver a,∗ , Kristen M. Butera a , Michael J. Rubenach a , Lucas J. Marshall a,1 , James S. Cleverley a,b , Geordie Mark c , Frank Tullemans d , David Esser d a

Predictive Mineral Discovery Cooperative Research Centre, Economic Geology Research Unit, School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia b Predictive Mineral Discovery Cooperative Research Centre, CSIRO Exploration and Mining, Perth, Australia c Predictive Mineral Discovery Cooperative Research Centre, School of Earth Sciences, Monash University, Clayton, Victoria, Australia d Barrick Gold of Australia Limited, Level 10/2 Mill Street, Perth, WA 6000, USA Received 29 September 2006; received in revised form 27 April 2007; accepted 13 August 2007

Abstract Protracted metal and sulfur contributions to the Eastern Succession iron-oxide–Cu–Au (IOCG) province of the Proterozoic Mount Isa Block occurred primarily as a consequence of long-lived fluid fluxes, stimulated by repeated emplacement of voluminous magmas during rifting and thinskinned convergence cycles. Although there is a direct role for felsic intrusions of the ca. 1530 Ma Williams–Naraku Batholith in hydrothermal ore genesis, these intrusions came at the culmination of protracted metal reorganization in the crust, not as the sole cause, as indicated by geochronology, mineral paragenesis, and the shapes of some orebodies relative to pre-1530 Ma structures. Spatial and geochemical data on mafic rocks suggests that the concentration of copper and gold into some of the mineral deposits involved a significant component of m- to 1000 m-scale remobilization and reworking of early enrichments, formed during basin evolution and initial inversion, by later regional metamorphic and magmatic–hydrothermal fluids. Osborne (eastern domain) and Eloise-type ores (or ore precursors) initially formed during or before the 1600 Ma regional metamorphic peak, by interaction of basinal or early metamorphic fluids with mafic rocks and ironstones, whereas younger oxidised brines released by the Williams/Naraku intrusions at ∼1530 Ma overprinted magnetite ± sulfides at Osborne (western domain) and Starra to produce the presently mined hematite–chalcopyrite ores. CO2 with mantle-like stable isotope character is abundant at all stages of the hydrothermal evolution and is present in high concentrations even in felsic magmas. We thus infer that CO2 was released directly from enriched mantle, or indirectly from mafic magmas, contaminating the process of volatile release from the top of felsic magma chambers and contributing to production of carbonate gangue in orebodies. Ernest Henry, the largest IOCG deposit in the district, remains the best candidate for a true syn-granite magmatic–hydrothermal orebody. We infer that ore deposition occurred when mantle- or mafic-derived H–C–O–S fluid mixed with saline, oxidised brine derived from the Williams/Naraku Batholith, stripping some ore components (Fe, Sr, Cu) from the local wallrocks, in particular mafic rocks. The protracted hydrothermal evolution is reminiscent of modern back-arcs but the position of the arc during the post-1800 Ma history was hundreds of kilometres east. We propose that mantle enrichment in volatiles occurred around a pre-1840 Ma plate boundary leaving the Kalkadoon–Leichhardt belt as a magmatic arc remnant. This metasomatised mantle was subsequently re-tapped during prolonged distal back-arc spreading and periodic shortening accompanying ongoing magmatism. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper; Mafic rocks; Mineralization; Hydrothermal; Extension; Back arc

1. Introduction

∗

Corresponding author. Tel.: +61 7 47815049; fax: +61 7 47814020. E-mail address: [email protected] (N.H.S. Oliver). 1 Present address: Teck Cominco Limited, #600-200 Burrard Street, Vancouver, British Columbia V6C 3L9, Canada. 0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.08.019

Ore genesis models developed in recent years for Proterozoic iron-oxide–Cu–Au (IOCG) deposits of the Cloncurry District have focussed attention mostly on the role of volatile phase separation from the 1550–1500 Ma Williams and Naraku Batholiths (‘Williams–Naraku Batholith’) as the most likely source of metals (e.g., Perring et al., 2000; Mark et al., 2006a). This paradigm

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Fig. 1. Summary geology of the Eastern Succession of the Mount Isa Block, showing the location of key regions, mineral deposits, and the area of Fig. 3. Inset shows the location of the Eastern Succession within the Mount Isa Inlier in northern Australia.

has not particularly helped mineral explorers because of the irregular spatial relationships between the intrusions and known ore deposits, several deposits being horizontally separated from major intrusions by a few or more kilometres (Fig. 1). Mustard et al. (2005), Ford and Blenkinsop (2007), and McLellan and

Oliver (2008) have demonstrated strong spatial relationships between mid-Proterozoic faults and mineral deposits. Williams (1994), Oliver et al. (2004), Williams et al. (2005) and Mark et al. (2005a) have also proposed that several elements present in the IOCG deposits of the district (in particular Fe, K, Rb, Sr, REE)

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were derived from the local and/or broader host-rock package, rather than an external source such as magmatic–hydrothermal fluids (see below). Consequently, we need to consider possibilities that the migration of fluids back and forth between fault zones and host rocks may have played a role in ore genesis, potentially independently of the role of magmatic–hydrothermal fluids. Finally, geochronology on the second largest deposit of the district (Osborne) has shown that it was formed during or before the peak of regional metamorphism at ca. 1600 Ma, well before the emplacement of the Williams–Naraku Batholith. Earlier authors also proposed some of the IOCG deposits and/or iron-oxide-rich rocks were formed during sedimentation and volcanism (Davidson et al., 1989). Irrespective of the specific timing of initial metal introduction, the geochronological results and our recently gathered paragenetic data (see below) suggest that some metal accumulation occurred, at least locally, before the emplacement of the Williams–Naraku Batholith. A non-magmatic model for copper derivation through regional circulation of basinal fluids has already been proposed in the Western Succession of the Mount Isa Block. The giant Mount Isa copper orebody was emplaced between 1550 and 1500 Ma in a syntectonic brecciation event (Perkins, 1984; Waring et al., 1998). Heinrich et al. (1995) proposed that the source for copper in this world-class deposit was from regional leaching by basinal fluids of the underlying metabasalt sequence, the Eastern Creek Volcanics. Gregory (2006) analyzed geochronology, whole rock and mineral geochemistry of Eastern Creek Volcanics in the broader district around Mount Isa, and proposed one or more phases of copper pre-enrichment prior to formation of the Mount Isa copper deposit, supporting the general principles proposed by Heinrich et al. (1995). Although the formation of the Mount Isa copper deposit apparently shares ages with the emplacement of the Williams–Naraku Batholith in the Eastern Succession, there are no major intrusions of this age in the Western Succession. Matth¨ai et al. (2004) proposed that it formed by forced convection of basinal fluids in response to juxtaposition of older heat-producing intrusive rocks and the younger ore sequence across a reverse fault (see also McLaren et al., 1999). There are several models for the tectonic setting of the Eastern Succession, but they all share a common uncertainty regarding the presence, timing and proximity of a Proterozoic plate boundary. The ca. 1650 Ma lead–zinc deposits of the Mount Isa Western Succession have been placed into a ‘far-field back-arc’ tectonic setting by Betts et al. (2003, 2006) and their tectonic assessment has been used to refine and re-define the Mesoproterozoic evolution of much of Australia (Giles et al., 2004). However, there are noteable differences between the evolution and isotopic and geochemical signals of the Western Succession and Eastern Succession (e.g., Griffin et al., 2006; Murgulov et al., 2007; Mark et al., 2005a), with the latter showing interesting parallels and differences with Proterozoic rocks of the Georgetown and nearby Inliers, now exposed some 500–1000 km east of the Eastern Succession. A tectonic reappraisal of the Eastern Succession is needed in order to place the hydrothermal evolution into perspective and to clarify the relationship of the Eastern Succession to the Western Succession and other Proterozoic blocks.

Here we present some new data concerning mafic rock geochemistry, the spatial distribution of copper deposits in the Eastern Succession, and some observations of orebody paragenesis pertinent to an appreciation of the protracted hydrothermal evolution. This is combined with data from recent publications, and reports from the Predictive Mineral Discovery Cooperative Research Centre, in order to demonstrate a protracted history of contribution of metals and sulfur to this terrain. We also attempt to draw parallels between the hydrothermal evolution and the possible tectonic evolution, by examining different tectonic models for protracted crustal heating, magma addition and volatile release. In doing so we attempt to develop a more holistic model that rationalizes the role of the Williams–Naraku Batholith in the 1750–1500 Ma hydrothermal evolution of the Eastern Succession. 2. Williams–Naraku Batholith Intrusions of the Williams–Naraku Batholith were emplaced between 1550 and 1500 Ma (Page and Sun, 1998), some 50 m.y. after the peak of regional metamorphism at 1600–1580 Ma (Giles and Nutman, 2002; Rubenach et al., 2008). The release of fluid from crystallization of the Williams Batholith has been related to widespread and intense sodic–calcic alteration and related brecciation from 1550 to 1500 Ma (Mark and Foster, 2000; Perring et al., 2000; Pollard, 2001; Mark et al., 2004; Oliver et al., 2004, 2006; Marshall et al., 2006). Coincidence of U–Pb (zircon) ages of the intrusions with Ar–Ar dating of mica and amphibole in many of the IOCG deposits has been used as evidence for a genetic connection between the intrusions and the deposits (Page and Sun, 1998; Perkins and Wyborn, 1998; Baker et al., 2001). Perring et al. (2000) and Pollard (2001), in consideration of the association of a large body of hydrothermal magnetite in the roof zone of the ca. 1530 Ma Squirrel Hills intrusion, proposed that release of saline, iron-rich fluids from the intrusion resulted in phase separation and simultaneous sodic–calcic alteration and magnetite deposition, and may also have liberated copper for incorporation into magma-distal IOCG deposits. Mark and Foster (2000) and Mark et al. (2004) observed clear textural evidence for a magmatic–hydrothermal transition in the Mount Angelay Granite, in which sodic–calcic veins and alteration are intimately associated with aplite, pegmatite and ‘balloon-textured’ rocks. Recent re-analysis of fluid inclusions from the Lightning Creek prospect and Mount Angelay Granite reveals the presence of chalcopyrite as a solid inclusion in the most saline varieties (Mustard et al., 2005). Despite an absence of chalcopyrite in the magnetite mineralization, this suggests that some of these granites did transport and release copper upon crystallization. Mark et al. (2006b) dated mineralization at Ernest Henry using U–Pb on titanite intergrown with pre-ore hydrothermal minerals, strengthening the apparent 1530 Ma connection between intrusions and the largest of the IOCG hydrothermal systems of the district. Oliver et al. (2004) demonstrated a plausible connection between the release of saline fluids from the intrusions and the regional extraction of iron from the country rocks to subsequently form the iron-oxide dominant gangue of most of the deposits. A direct connection

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between 1530 Ma intrusions, brecciation and Na–Ca alteration has recently been clarified by observations of Na–Ca altered breccia pipes emanating from contact aureoles. The pipes contain hydrothermal magnetite and local sulfides and are possibly spatially connected to some ore deposits, particularly Ernest Henry (Oliver et al., 2006). Many of these issues were considered in the review of global IOCG deposits by Williams et al. (2005), but such apparently direct connections to crystallizing batholiths are not necessarily clear elsewhere in the world in the genesis of this family of deposits. Some models for generation of IOCG deposits favour regional leaching by saline basinal fluids of iron, copper and potentially sulfur from the broader rock package, with or without the aid of intrusions as a heat or fluid source (e.g., Barton and Johnson, 1996). However, there are many IOCG deposits where direct magmatic involvement is considered pivotal (e.g., Marschik and Fontbot´e, 2001). Hybrid models, where magmatic fluids have interacted with country rocks to collectively provide Fe, K, Rb and Ba for IOCG deposits, have also been proposed in the Eastern Succession (Williams, 1994, 1998; Oliver et al., 2004). There is abundant evidence that several hydrothermal systems operated during the protracted 250 m.y. evolution of the entire Eastern Succession, not just the Mary Kathleen Fold Belt in the west (Oliver, 1995). Despite the strong evidence for the role of the Williams–Naraku Batholith in hydrothermal systems between 1550 and 1500 Ma, there are several reasons these granites may not have been the sole cause of all the IOCG deposits in the Eastern Succession:

Fig. 2. Stable isotope data for calcite extracted from regional vein sets (open symbols) and mineral deposits (shaded) from the Eastern Succession, adapted from Marshall et al. (2006), expressed with reference to Standard Mean Ocean Water in ‰ using conventional terminology. Although temperature and fluid compositional effects varied, fluids in equilibrium with these relatively hightemperature calcites (mostly >400 ◦ C) would have been within 1 or 2‰ of the indicated calcite compositions with the exception of ␦13 C for methane-rich fluids. The majority of the IOCG deposits are contained within the circled area (heavy solid line) for which corresponding fluid compositions at 400 ◦ C are ␦13 C −3 to −7‰ and ␦18 O 9–11‰ for reasonable ranges of H2 O–CO2 ratios (heavy dashed circle), inferred to reflect mantle or basalt carbon, and mafic to felsic magmatic oxygen sources (see text). Lines 1 and 2 show the relationship between carbonate-rich and black shale host rocks (respectively) and the core of the vein and mineralized systems, with the stable isotope values being dominated by the external fluid over m- to 100 m-scales as the hydrothermal systems are approached. The remaining lines show inferred sources for carbon and oxygen that did not involve influx of magmatic or mantle-derived fluid, including shale and carbonate hosts at the Dugald River Pb–Zn deposit (3), and the iron formation hosts at Starra and Osborne (4).

• Sodic–calcic alteration related to calcite veins in the Mary Kathleen Fold Belt has the same ages (1550–1525 Ma) and similar style (Oliver et al., 2004) as albite alteration in the Cloncurry District. However, no intrusions of Williams–Naraku Batholith age are exposed in the Mary Kathleen Fold Belt. Furthermore, U-REE deposits (Mary Kathleen) formed or were strongly remobilized at this time (e.g., Page, 1983a). Further west, at the same time, the giant Mount Isa copper orebodies formed, without any obvious involvement of magmatic fluids (Heinrich et al., 1995). • The stable isotopic data for barren and mineralized calcitebearing vein systems of all ages (from 1740 to 1500 Ma) converge upon magmatic or mantle-like values, with outliers clearly related to admixture with Corella marine carbonates, or Soldiers Cap black shales (lines 1 and 2, Fig. 2). Although the Corella Formation contains abundant marine carbonates, the variation in C and O isotopes is not a function of metamorphism or diagenesis of marine precursors, rather it is a function of proximity to ore systems or size of veins (Oliver et al., 1993; Marshall et al., 2006). Rocks containing abundant secondary calcite such as the core of large (m- to 10 mwide) veins or the carbonate-rich gangue to many ore deposits show the most depleted C and O isotopes, characteristically with ␦13 C −2 to −8‰ and ␦18 O +9 to +10‰. As the vein networks become less intense, or individual veins become thinner, or the rocks show less alteration moving away from the ore deposits, then the original and diverse marine iso-

tope signals (pelite, black shale, carbonate, BIF) begin to dominate (see also Kendrick et al., 2008). The convergence cannot be related only to the influence of Williams–Narakuage magmatic fluids because it includes pre-1530 Ma veins (e.g., Osborne mine) and abundant veins sampled from belts where these intrusions are not exposed (e.g., Mary Kathleen Fold Belt). The mass of carbon with mantle- or mafic-like signals in these carbonate-rich alteration systems also cannot readily be explained by release of “normal” hydrous, saline fluids from crystallizing granites (see below). A source of CO2 from other than the Williams–Naraku Batholith is implied for veins and orebodies hosted in these older rocks. Some deposits (lines 3 and 4, Fig. 2) show evidence for little or no involvement of magmatic fluids during carbonate precipitation. • IOCG deposits apparently formed at different times (Mark et al., 2006a) share many metallogenic characteristics, i.e., Osborne at 1600–1590 Ma and Ernest Henry and Mt Elliott at 1530–1510 Ma, despite apparent absence of voluminous felsic intrusions at 1600 Ma (Williams, 1998; Rubenach et al., 2001; Mark et al., 2006a,b). • Sulfides with ␦34 S values around 0‰ are found in the deposits (Davidson and Dixon, 1992; Mark et al., 2006a,b), including deposits that formed earlier (e.g., Osborne) than the ca. 1530 Williams Batholith. There is no obvious correlation between the apparent age of the deposits and the ␦34 S values of the sulfides. Davidson and Dixon (1992) identified

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two sulfur sources in the deposits of the district, noting that the second, subordinate variety related to local rocks (e.g., black shales, pelites) whereas the predominant variety in most deposits was apparently magmatic. Although it is possible that reduction of Proterozoic seawater (␦34 S ∼ 20‰) could give apparent sulfur isotope signals around 0‰, lead–zinc deposits of syngenetic or diagenetic origin in the Western Succession show a very broad range of ␦34 S values (−10 to +30‰), consistent with variable reduction of seawater sulfate and/or interaction of two distinctive sulfur sources of widely varying oxidation state (Large et al., 2005). In contrast, the relatively tight clustering of Eastern Succession deposit data around 0‰ suggests a predominant, single and reduced sulfur source, with the modest variations away from this representing contributions of black shales or carbonates in the local rock package (Davidson and Dixon, 1992). Spatially, both negative (<0‰) and positive (>4‰) values may locally be found close to the deposits, but the values converge upon 0‰ as the core of the deposits is approached (e.g., Mark et al., 2006b). Because such sulfur isotope ratios are also found in pre-Williams–Naraku Batholith deposits, either earlier igneous rocks or some other mechanism(s) contributed to the predominance of apparently igneous S isotope values in the older deposits (see below). Another feature noted by Williams (1998), Williams et al. (2005) and Mark et al. (2006b) is a strong mafic minor element association for many of the ores and alteration systems, in particular enrichment in Co and Ni (±Zn, Pb, V). This association may relate in part to a role for fluid released by mafic magmas emplaced synchronously with the Williams–Naraku Batholith. However, the local mafic rocks may have contributed some of these elements to the ore deposits via leaching. Mark et al. (2005a) note that, for both the Osborne and Ernest Henry deposits, Sr and Nd isotope ratios of ore minerals closely reflect their host rocks, indicating local derivation of REE.

ent input parameters. Weights of Evidence measures the strength of a spatial relationship of a set of ‘training data points’, i.e., mineral deposits, with lithological or structural units. The basis for the calculation is the measured chance of a mineral deposit falling within an area surrounding the studied unit (i.e., within a buffer of a certain radius around that unit), over the chance that it does not sit within that area. The resulting parameter of the strength of spatial relationship of a deposit to the studied area is termed the Contrast Value, while the statistical strength of the Contrast Value is the Confidence. Typically, a Contrast Value >0.5 is considered a good spatial association, although we have chosen a more strict definition here (>1). This study has applied the WofE test to iron-oxide Cu–Au deposits, Cu deposits, all (metalliferous) deposits, large deposits (>500 t metal), and Au-only deposits in the Mount Angelay and Selwyn 1:100 K Geological Sheet Areas (Fig. 3 ), based on the deposit and prospect classification and fault size categories outlined in the North-West Queensland Mineral Province Report (Queensland Department of Mines and Energy, 2000), and also including the Wimberu, Mount Dore, Gin Creek, Mount Cobalt, Mount Angelay, Granite, Yellow Waterhole, Saxby, Cowie, and Marumungee Granites (Figs. 1 and 3). Only 8 of the ∼240 deposits shown in Fig. 3 are dominated by Pb–Zn–Ag without significant Cu or Au (including Cannington Ag–Pb–Zn deposit). Conditional independence tests, to measure the effects of stray data, or smaller sets of data, were performed in order to check the statistical viability of the results. The results were obtained using the MI-

3. Mafic rocks—temporal and spatial associations and geochemistry Table 1 summarizes the magmatic and thermal events that affected the Eastern Succession (see also Foster and Austin, 2008; Rubenach et al., 2008). Extrusive and/or intrusive mafic rocks, predominantly tholeiitic, were emplaced synchronously with every thermal event identified to date (Bultitude and Wyborn, 1982; Ellis and Wyborn, 1984; Betts et al., 2006) although the detail of the igneous history between 1700 and 1650 Ma is poorly understood (cf. Rubenach et al., 2008). 3.1. Weights of evidence Mustard et al. (2005) and Ford (2005) recently applied Weights-of-Evidence (WofE) methods to measure the spatial relationships of geological features to copper and related mineralization in the Eastern Succession, and we contribute to that analysis here by an independent appraisal of a smaller area (Mt Angelay and Selwyn 1:100,000 sheet areas) using some differ-

Fig. 3. Distribution of mineral deposits from the Mount Angelay and Selwyn 1:100,000 Geological Sheet Areas, based on the deposit and prospect classification and fault size categories outlined in the North-West Queensland Mineral Province Report (QLD DME, 2000; REF). Mafic rocks are shown with heavy shading, and faults connected to mafic rocks (at 100,000 scale) are also indicated. The position of granites is shown in Fig. 1. Open squares: undifferentiated Cu, Au or Pb–Zn deposits, circles with stars: IOCG deposits. There is a strong apparent spatial association between deposits and mafic rocks, borne out by our Weights-of-Evidence analysis (see text and Table 2).

Table 1 Summary of thermal events affecting the Eastern Succession, see also Betts et al. (2006) and Rubenach et al. (this issue) Rocks

Age

Distribution

Alteration and mineralization

Other key references

Williams–Naraku Batholith

Granitoids, fewer mafic and intermediate intrusions

1550–1490 Ma, (Page and Sun, 1998; Wyborn et al., 1988)

Widespread in eastern half, 10 km-scale intrusions

Widespread Na–Ca alteration, Ernest Henry IOCG (U–Pb titanite), other IOCG Ar–Ar ages

Peak of Isan Orogeny

Mafic dykes, locally derived (anatectic) pegmatites

Early sodic alteration

Pelitic schists, mafic rocks

Pegmatites in south (Osborne, Cannington), mafic dykes axial planar to Snake Ck anticline Widespread throughout schists of the Soldiers Cap Group

Loss of Cu, Co, Ni and S from mafic rocks, local albitization associated with partial melting Albitites, ?related early accumulation of Cu, Au, Fe

Early intrusion

Tonalite, dolerite, gabbro

1600–1580 Ma (Gauthier et al., 2001; Giles and Nutman, 2002; Rubenach et al., 2001, 2008) 1670–1630 Ma (Rubenach et al., this issue; Foster and Austin, 2008) 1686–1676 Ma (Foster and Austin, 2008)

Perkins and Wyborn (1998), Pollard et al. (1998), Mark (1999), Perring et al. (2000), Pollard (2001), Oliver et al. (2004), Williams et al. (2005), Mark et al. (2005b) Rubenach and Barker (1998), Rubenach (2005)

Ag–Pb–Zn enrichment, possible ore formation at Cannington Ag–Pb–Zn deposit, ?circulation of basinal fluids

Volcanism

Basalts

ca. 1700–1660 Ma, (Page and Sun, 1998; Foster and Austin, 2008)

Wonga Intrusions

Granitoids, gabbro, dolerite

1750–1730 Ma (Page, 1983a,b)

Intruded into pelitic schists of the Llewellyn Ck Formation in the south of the Snake Creek Anticline, ore hosts at Cannington Ag–Pb–Zn mine Toole Creek Volcanics of Soldiers Cap Group, older basalts in package intruded by Tonalite Mary Kathleen Fold Belt, Gin Creek Granite west of Starra

Marraba and Argylla Volcanics

Basalt, rhyolite

1785–1780 Ma, (Page, 1983b)

Mary Kathleen Fold Belt, Kalkadoon–Leichhardt Block, Duck Creek Antiform

?Syn-volcanic Cu enrichment

Rubenach and Lewthwaite (2002), Butera et al. (2005), Rubenach (2005) Williams, 1998

Possible syngenetic enrichment in Cu, Au, S (Osborne, Starra)

Davidson et al. (1989)

Skarns, U-REE enrichment, Na–Ca alteration

Holcombe et al. (1991), Pearson et al. (1992), Cartwright and Oliver (1994), Oliver (1995), Oliver et al. (1994, 1999)

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Table 2 Summary of Weights-of-Evidence analysis of the spatial relationships between mineral deposits, granites, mafic rocks and major faults, for the Mount Angelay and Selwyn sheet areas shown in Fig. 3 All deposits

Larger deposits

Iron-oxide–Cu–Au

Granites Distance Contrast Confidence Deposits

3.25–3.5 km 1.41 5.28 15

<1 <1 <1 <1

Mafic rocks Distance Contrast Confidence Deposits

0–250 m 1.99 14.2 71

0–250 m 1.77 4.09 7

250–500 m 1.71 3.48 5

0–250 m 1.7 8.2 30

0–250 m 1.2 2.5 5

Major faults Distance Contrast Confidence Deposits

0–100 m 1.23 5.62 23

0–100 m 3.03 7.92 11

0–100 m 2.18 4.76 6

0–100 m 1.23 4.06 12

0–100 m 2.21 5.2 7

<1 <1 <1 <1

Cu deposits 1.25–1.5 km 1.19 3.45 9

Au deposits 1.25–1.5 2.29 5.07 6

The results demonstrate a closer spatial relationship between deposits and both mafic rocks and faults, relative to relationships between deposits and granites.

SDM add-in from the Mapinfo software package. Buffers were created at various scales (0–5 km) around mafic rocks, granites and different fault types in order to evaluate the spatial relationships between these units and the different mineral deposit types. Fig. 3 shows the spatial relationships between mafic rocks mapped on the 1:100,000 sheet areas (75 km × 85 km) and the metalliferous deposits (IOCGs and others; Queensland Department of Mines and Energy, 2000). Also indicated are faults that connect to mafic rocks within a 1 km buffer around the faults. Table 2 summarizes the results of the WofE analysis; details of the full analysis are available from the authors on request. The results show the strongest spatial association between all deposits and mafic rocks and faults connected to mafic rocks (in plan view) within a 1 km buffer. The spatial correlation between granites and deposits is poorer than that between mafic rocks and deposits, and faults and deposits. The poorer correlation between granites and deposits does not mean they

are genetically unrelated. For example, fluids emanating from granites may be too hot or too sulfur poor to precipitate sulfides (e.g., Perring et al., 2000). However, because of the strong correlation between mafic rocks and deposits, there is a good basis for questioning what specific role the mafic rocks may have played in ore genesis. Mustard et al. (2005) considered several other features in their more regional analysis that are not considered here, including fault bend and intersection features, the details of the sedimentary rock lithology, and the proximity to major geological contacts. Individually, none of these features showed a stronger correlation than mafic rocks to all deposits, although sets of fault geometries and fault intersections were also considered particularly important controls, allowing Mustard et al. (2005) to identify several areas of previously lightly prospected rocks to the north of our study area, one of which in particular has proven economic (Rocklands, near Chumvale station).

Fig. 4. (a) preservation of igneous textures occurs in some mafic rocks such as this ∼1600 Ma dolerite from the Snake Creek Anticline; here the original pyroxenes have been replaced pseudomorphously by amphibole leaving rare relict pyroxene cores, but the sub-ophitic primary texture is preserved, as are some sulfides and oxides (indicated); (b) loss of igneous textures is widespread as shown in this figure. Felted, weakly foliated arrays of actinolitic amphibole are intergrown with oligoclase to andesine, with secondary titanite, and loss of primary oxides and sulfides, compared with the igneous textured rocks. The textural changes from (a) to (b) correspond to substantial geochemical losses of Cu, Au and S from the original igneous rocks during fluid flow accompanying 1600 Ma regional metamorphism and deformation. Abbreviations: am, calcic amphibole; px, pyroxene; pl, plagioclase.

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3.2. The geochemical effect of metamorphic and metasomatic fluids on mafic rocks Mafic rocks exposed in the Eastern Succession show a variable degree of preservation of igneous textures. Approximately 20–60% of individual km-scale mafic bodies (where we have observed them) show abundant amphibole that is aligned into planar, linear, felted or foam textures developed during the ∼1600 Ma regional metamorphism, although some mafic rocks also preserve igneous textures but show complete pseudomorphism of pyroxenes by amphibole in thin section (Fig. 4). Narrow corridors of intense post-metamorphic Na ± Ca alteration (albitization) surround many fault zones in the district (e.g., deJong and Williams, 1995; Oliver et al., 2004), and some granite contact aureoles (Mark and Foster, 2000; Pollard, 2001; Oliver et al., 2006). These zones, in which mafic rocks show complete conversion to albite-actinolite altered equivalents for up to 10% of their exposure (deJong and Williams, 1995; Oliver et al., 2004), are easier to detect than zones of amphibolitization. Other rock types (particularly calcsilicate rocks) display more widespread albitization than mafic rocks. The geochemical effects of regional metamorphism are well displayed by 10–50 m wide mafic dykes that were emplaced, pre- or syn-metamorphism, parallel to the axial plane foliation of the Snake Creek Anticline (Fig. 1; Table 3). Within several metres of their margins, they develop a felted or foliated, amphibolitic texture, whereas the cores of the dykes preserve igneous textures (Fig. 4). Representative geochemical changes for the conversion of dolerite to amphibolite during the 1600–1580 Ma regional metamorphism are indicated in Table 3 and Fig. 5a—syn-metamorphic depletions of S, Cu, Co and Ni are prominent. Given that the isocon (after Grant, 1986) shows a slope near 1 indicating little or no total mass change during metamorphism, then raw geochemical data may be used to assess relative changes during metamorphism. Table 3 shows the prominent depletion in Cu, Au and S during amphibolitization of the two main suites of mafic rocks (∼1680 and 1600 Ma), such changes being superimposed on the primary igneous fractionation trends (see below). Fluid–rock interaction during progressive amphibolitization of igneous textured mafic rocks to completely recrystallized equivalents resulted in loss of 90% of the sulfur, ∼65% of initial copper and 90% of the gold, on average. Because the amphibolitization of mafic rocks is spatially irregular, it is difficult to quantify the amount of S, Cu and Au leached from mafic rocks and then potentially contributed to ca. 1600 Ma ore systems. It is also difficult to directly measure elemental changes in the near mine environment due to the overprinting and remobilising effects of ore-related alteration (see also Oliver et al., 2004; Mark et al., 2006b)—some near-mine amphibolites are enriched in Cu, Au and S (e.g., Baker, 1998). An approximate mass balance calculation can be made for the theoretical assumption of transfer of copper from mafic rocks depleted during regional metamorphism to syn-metamorphic mineral deposits, if such depletion affected 20% of the exposed mafic rocks. Assuming complete efficiency from leaching to subsequent precipitation, then only 400 m depth of mafic rocks

Fig. 5. Isocon diagrams (after Grant, 1986) showing the geochemical changes in selected elements during metamorphism and alteration of mafic rocks from the Eastern Succession. Major element oxides are in percent, minor elements in ppm, multiplied or divided by the factor indicated. The two lines represent visually selected isocons that straddle the likely immobile elements (open symbols). Elements showing losses are shown with black symbols. (a) Dykes inferred to be emplaced near the peak of the Isan Orogeny at ∼1590 Ma from the Snake Creek Anticline. Average data with 2σ standard deviation compares six igneous-textured mafic rocks from the core of the dykes with seven amphibolitic equivalents from the dyke margins. Full geochemical data is available from the authors on request. (b) Dykes inferred to be emplaced between 1740 and 1670 Ma from the Mary Kathleen Fold Belt (Knobby Quarry) that underwent sodic alteration at 1550–1525 Ma. The diagram shows the average of three least altered samples compared with five moderately to strongly albitized equivalents, full geochemistry for some of these samples was presented by Oliver et al. (2004). Both metamorphism and later alteration have depleted the mafic rocks in Cu, S, Zn, Ni and other similar elements to a greater or lesser degree (Cr, V, Co), raising the possibility that some IOCG deposits may have derived at least some of these elements from local leaching and reprecipitation over mto km-scales.

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Table 3 Whole rock geochemical results for mafic rocks (both intrusive and extrusive) of the early (ca. 1686 Ma) and syn-Isan Orogeny (Syn-D2 ); full analyses are available from the authors on request Texture

Sample

Age

S

Cu

Au

Igneous

T15 T14A T12B M9 T12A M8 T1 T2 M13 M1 M2 M14

Syn-D2 Syn-D2 Syn-D2 Syn-D2 Syn-D2 Syn-D2 1686 Ma 1686 Ma 1686 Ma 1686 Ma 1686 Ma 1686 Ma

1420 1420 1530 1480 1370 430 945 1540 1320 390 321 720

128 137 101 166 141 121 138 201 234 211 132 230

b.d. 0.007 b.d. 0.013 0.007 0.008 0.017 0.027 0.011 0.01 0.009 0.009

1074 483

162 46

0.01 0.007

Average S.D. Partly recrystallized or pseudo-morphous

T14B

Syn-D2

85

103

b.d.

T10 T8 M12 M7

Syn-D2 Syn-D2 1686 Ma 1686 Ma

110 950 440 285

84 145 101 92

0.007 b.d. 0.011 0.008

374 36

105 24

0.007 0.005

3.3. Primary fractionation trends in mafic rocks We compiled our own data and data from the Queensland Government Mineral Occurrence database (http:// www.nrw.qld.gov.au/science/geoscience/products/digital/geo mineral occur.html) database to assess the possible connection between primary igneous fractionation trends and Cu, Au and S. Fig. 6 shows data for the older 1686–1660 Ma mafic rocks of the district, which also includes some metamorphosed-textured rocks that we did not analyse ourselves, but no albitized rocks. Albitization affects iron contents of the mafic rocks, whereas regional metamorphism does not (Fig. 5 and Oliver et al., 2004), so although we cannot directly exclude the effects of regional metamorphism on copper from this data, the iron contents most likely reflect the primary igneous values. The mafic rocks are very strongly Fe-enriched (10–20% Fe2 O3 as total Fe), relative to MORB (Herzberg, 2004). The rocks are also enriched in Cu and S at relatively low degrees of fractionation; at 10–13% Fe2 O3 (as total Fe), S contents of the rocks range between 1000 and 1500 ppm, and Cu contents range between 150 and 250 ppm. Because the sulfides in these rocks are present as late stage interstitial phases (Fig. 4a), we propose they represent the product of volatile release during magmatic crystallization, as these concentrations lie well below the expected magmatic sulfide saturation levels for mafic rocks emplaced at 2 kb (Mavrogenes and O’Neil, 1999). At greater iron contents, the rocks show lower amounts of Cu and S (e.g., 10–500 ppm S and 5–50 ppm Cu at 18–20% Fe2 O3 , Fig. 6). This same trend was observed by Stanton (1994) at Broken Hill and elsewhere, who argued that decreasing Cu with increasing Fe and Si (SiO2 > 52%) in very Fe-rich tholeiites was due to loss of Cu into the volatile phase. It is possible that the data is strongly affected by regional metamorphic copper and sulfur depletions,

Average S.D. Recrystallized, foliated or lineated

T17

Syn-D2

b.d.

152

0.003

T9 T16 T11 T3

Syn-D2 Syn-D2 Syn-D2 1686 Ma

b.d. b.d. b.d. 60

31 24 26 b.d.

b.d. b.d. b.d. b.d.

60 n.a.

47 60

0.001 0.001

50

5

0.001

Average S.D. Detection limit (ppm)

albitization. However, only a few samples of mafic rocks were considered in that study. The albitized rocks show enrichment in Na ± Ca, with depletions in Fe, Ba, Rb, K, Mn, Ni and Zn and inconsistent trends for Cu, on the basis of immobility of Ti, Y, Al and Zr. Albitized mafic rocks showed the biggest depletions in Cu, Co and Ni. Additional data for mafic rocks presented here suggests that 1550–1530 Ma albitization away from known IOCG deposits shows prominent Cu and S depletion (Fig. 5b) whereas similar rocks near to IOCG deposits show no obvious Cu depletion and rare Cu enrichment (e.g., Oliver et al., 2004; Mark et al., 2006b). Na–Ca alteration developed in other rock types (calc-silicate rocks, pelites) in the 1550–1500 Ma range did not clearly strip Cu from the rocks (Oliver et al., 2004). Collectively, the data for metamorphosed or albitized mafic rocks indicate that both regional metamorphic and magmatic–hydrothermal fluids have leached a suite of elements that includes Cu, S, and several siderophile and chalcophile elements (Co, Ni, Zn ± V, Pb) that are also found in several of the IOCG orebodies. Relative to pelitic and calc-silicatebearing rocks, mafic rocks appear to have been particularly susceptible to loss of Cu, Au and S during metamorphism and alteration.

The results show depletions in Cu, Au and S for mafic rocks that were affected by deformation accompanying regional metamorphism, relative to samples preserving igneous textures.

across the total exposure area indicated in Fig. 3 would provide approximately all the requirements for copper relative to the amount presently contained in the known copper sulfide deposits (ca. 350 Mt of ore at 1% Cu). Sodic–calcic alteration emanating out from, and genetically linked with the Williams–Naraku Batholith emplacement also resulted in geochemical changes to mafic rocks. Williams (1994) and Oliver et al. (2004) demonstrated that these metasomatic fluids leached Fe, K, Pb, Zn, Rb and Sr from pelitic and calcsilicate-rich metasedimentary rocks in the Eastern Succession during sodic ± calcic alteration synchronous with emplacement of the Williams–Naraku Batholith. Oliver et al. (2004) presented data for the sodic–calcic alteration (albitization) of metasedimentary and mafic rocks affected by Williams Batholith-related metasomatism, and constrained the ages of this alteration to 1550–1520 Ma using single crystal U–Pb TIMS analysis of titanite formed by biotite and amphibole breakdown during

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Fig. 6. Total Fe (as Fe2 O3 T) versus selected other elements for mafic rocks of the 1686–1660 Ma suite in the Eastern Succession, including Toole Creek Volcanics of the Soldiers Cap Group and a major un-named intrusive and extrusive suite on the Mt Angelay and Selwyn 1:100,000 sheet areas. The data exclude obviously altered samples and includes analyses by the authors (conducted at the Advanced Analytical Centre at JCU) and also a compilation from the Geoscience Australia Ozchem database. The plots of Y, Zr and Ti against Fe confirm that increasing fractionation enriched iron linearly with an increase in the HFSE. In contrast, Cu and S show an interpreted change from enrichment to eventual depletion with increasing Fe, suggesting that they were not progressively accumulated in the melt with ongoing fractionation. Although metamorphic fluids may have depleted some mafic rocks in Cu and S (Fig. 5), the overall trend is inferred to represent loss of these elements by volatile release during crystallization (see also Stanton, 1994), as the metamorphic fluids did not affect the majority of the mafic rock samples (see text).

particularly for those data lying well below the maxima described in Fig. 6. However, this would appear to require that particularly iron-rich rocks were preferentially more susceptible to loss of these elements during metamorphism and alteration, whereas we observed no correlation between iron content of mafic rocks and degree of amphibolitization in the field. In summary, our data are consistent with the release of substantial amounts of Cu, S (and possibly Au) into the surrounds during crystallization of the widespread 1686–1660 Ma mafic rocks.

4. Orebody paragenesis and geochemistry 4.1. Evidence for reworking or multi-stage evolution of IOCG deposits In the Mary Kathleen Fold Belt in the western-most part of the Eastern Succession, Oliver (1995) demonstrated a protracted hydrothermal history with at least three major phases of fluid–rock interaction, only one of those being clearly related to intrusive activity, and at a time (ca. 1740 Ma) well before

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the emplacement of the Williams–Naraku Batholith further east. This protracted hydrothermal evolution was crucial in the assembly of the 1550–1500 Ma Mary Kathleen U-REE orebody. U–Pb and Sm–Nd results show clear evidence for several stages of concentration of uranium and rare earths (Page, 1983a; Maas et al., 1988). The source region for the U-REE in the orebody was most likely within several kilometres of the current ca. 1530 Ma deposit, because U-REE-rich skarns associated with the 1750–1730 Ma Wonga Batholith and related Burstall Granite are restricted in their distribution and host numerous U-REE prospects localized in ca. 1530 Ma structural sites (Oliver et al., 1999). Because elevated uranium concentrations are spatially rather distinctive in the Mount Isa context, it is relatively straightforward to suggest that the coincidence of both “old” and “new” uranium developed by chemical and physical remobilization of older uranium by cycles of deformation-driven dissolution and reprecipitation, the remaining question being whether or not the “old” uranium enrichments were high grade or disseminated (Page, 1983a; Oliver et al., 1999). There are textural, geochemical and geochronological arguments that suggest that many IOCG deposits in the Eastern Succession may have had a protracted history of metal accumulation similar to the style portrayed by Mary Kathleen, a common pattern in Proterozoic orebodies in Australia and worldwide (e.g., Marshall and Gilligan, 1993; Oliver et al., 1998; Cartwright and Oliver, 2000). Despite numerous Ar–Ar ages for micas related to alteration in the range 1540–1490 Ma for the IOCG deposits (Perkins and Wyborn, 1998), and abundant evidence for paragenetically late sulfides and related alteration (e.g., Adshead et al., 1998; Baker, 1998), U–Pb and Re–Os data for some of these deposits point to inherited components. The understanding of the age of peak regional metamorphism at the time of earlier studies (Adshead et al., 1998; Baker, 1998; Rotherham et al., 1998; Baker et al., 2001) was focussed around the 1550 Ma age inferred from the Western Succession (Page and Bell, 1986), such that the Williams Batholith was regarded as immediately post-peak metamorphic and evolving metamorphic–magmatic fluid systems were seen as viable for ore deposition. More recent recognition of 1600–1580 Ma regional metamorphism (Table 1) has been derived from U–Pb (zircon) (Giles and Nutman, 2002), and U–Pb (monazite) (Rubenach et al., 2008). Along with earlier hydrothermal events at 1660–1630 Ma (Rubenach et al., 2008) and possible syngenetic metal transport (Davidson et al., 1989), these new data require a reinterpretation of the absolute timing of paragenetic stages determined by the earlier work. Calcite associated with the paragenetically earliest, finegrained and apparently bedded magnetite at Osborne has highly distinctive carbon and oxygen isotope signals (␦13 C −4 to −16‰, ␦18 O 22–28‰) (Marshall et al., 2006) with unusually 18 O-enriched oxygen suggestive of a sedimentary origin for the magnetite. This is consistent with earlier studies by Davidson et al. (1989) and Davidson and Large (1994) on syngenetic origins for at least the ironstone components of some of the IOCGs, which therefore probably formed at 1680–1700 Ma, the likely age of the host metasedimentary rocks (Foster and Austin, 2008). At Osborne, Rubenach et al. (2001) determined

a 1595 Ma age for albitization associated with peak metamorphic (S-type) pegmatites pre-dating Cu–Au mineralization, and Gauthier et al. (2001) determined a 1600 Ma Re–Os age for ore-related molybdenite. Previous 40 Ar–39 Ar analysis of amphibole and biotite yielded ages as old as 1595–1568 Ma, and as young as 1540 Ma (Perkins and Wyborn, 1998). Preservation of syn-peak metamorphic 40 Ar–39 Ar ages in amphibole suggests that the subsequent effects of thermal events associated with the post-metamorphic Williams–Naraku Batholith were insufficient to completely reset the 40 Ar–39 Ar systematics of high temperature alteration minerals. The association of 1540 Ma amphibole with the sulfides found in the orebody (Adshead et al., 1998) is apparently at odds with the 1600 Ma Re–Os age of ore-related molybdenite (Gauthier et al., 2001), and suggests resetting of the 40 Ar–39 Ar systems with protracted cooling. Many of the orebodies display paragenetic features that are consistent with the broad range of age dates obtained at the Osborne deposit (Figs. 7 and 8). Silica alteration forming the proximal envelope to the eastern domain at Osborne is locally affected by a strong foliation, subparallel to the edges of the silicification and related ironstones, and this foliation is folded by folds which are correlated with regional D2 (Fig. 7). We interpret that the regional position of Osborne relative to the main deformation features, and the sigmoidal structure of the overall deposit and its siliceous alteration envelope, relate to asymmetric folding on the flank of a regional D2 antiform. Mineralization was formed, or deformed, in the hinge of the D2 minor folds. The earliest iron oxides in this deposit also are clearly pre-D2 (Fig. 8), being overprinted by coarse magnetite and chalcopyrite that was apparently introduced (or remobilized) during and after D2 . Similar observations can be made at Eloise, where at least one of the orebodies contains abundant folded and foliated sulfides and local durchbewegung texture (Fig. 8). Baker (1998) originally inferred a progression from peak metamorphism to syn-granite ore genesis at Eloise, with significant alteration and potential mineralization occurring during the metamorphic stage. By this reasoning, with the recent geochronology, these stages would likely be separated by 50 m.y. or more. Despite the bulk of the ore at Starra being dominated by coarse chalcopyrite and hematite which transgresses foliations, we have observed rocks in the open cuts containing magnetite and chalcopyrite in which the chalcopyrite forms irregularly spaced bands that look like bedding, and all are folded by D2 folds (i.e., similar to Osborne, Fig. 8b). At both Osborne and Starra, some or even all of the later paragenesis of hematite–chalcopyrite may have overprinted or remobilized earlier magnetite that already contained chalcopyrite, pyrite and/or pyrrhotite. The original premise of Davidson et al. (1989) concluding that ironstones and some sulfides pre-date peak metamorphism is also supported by our stable isotope data which support a clear, nonmagmatic origin for early magnetite–carbonate associations at both Osborne and Starra (Fig. 2). Although it could be argued that the geochronological data indicating 1595 Ma and older for some components of the Osborne ore system could simply relate to inheritance from the surrounding host rocks, the textural evidence requires explanation of the origin of “old” molybden-

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Fig. 7. Osborne mine and surrounds, adapted from Placer Exploration cross sections produced in 2001. (a) Regional interpretive cross section showing Osborne on the flanks of a regional D2 fold developed during the 1600–1580 Ma Isan Orogeny, lying in the plane of either S0 or an earlier tectonic fabric (S1 ) possibly developed during one or more early extensional phases soon after basin formation (see text); (b) mine-scale cross section showing apparent thickest ore intersections in the hinge of D2 folds. One interpretation of this pattern is that this reflects migration of pre-D2 stratigraphy-parallel ore into fold hinges during deformation, which is supported also by folding of the adjacent silica-rich alteration envelope which overprints earlier, probable sedimentary ironstone (see Davidson et al., 1989); (c) sketch from field notebook of silica alteration immediately in the hangingwall of ore in the eastern domain at Osborne, showing a well developed S1 foliation overprinting that silicification and also folded by F2 folds, strengthening the case for hydrothermal alteration commencing before the peak of the Isan Orogeny.

ite in the ore, preserved 1595 Ma Ar–Ar ages in amphibole, and a folded silica alteration envelope to the deposit. We infer that several of the mined IOCG deposits, including Osborne, Eloise and Starra, had an early stage of mineralization during or prior to regional metamorphism at ca. 1600 Ma. This may have taken the form of stratiform lenses or disseminations, which were then chemically and physically redistributed during both regional metamorphism and subsequent hydrothermal activity related to the Williams–Naraku Batholith (Fig. 9). Such an interpretation would be consistent with the spatial association of many of the deposits with mafic rocks and faults, the mafic rocks having contributed Cu, Au and S during emplacement and subsequent regional metamorphism, faulting and alteration.

4.2. Ernest Henry The pipe-like Ernest Henry IOCG deposit is dominated by physically and chemically abraded clasts of potassic-altered metavolcanic clasts in a matrix comprising magnetite, calcite, pyrite, chalcopyrite and a host of accessory minerals including locally abundant titanite. In this regard it is dissimilar to the afore-mentioned deposits which tend to form elongate, massive to disseminated masses in ductile shear zones or fold limbs. Magnetite, chalcopyrite and titanite are intimately associated texturally and paragenetically, and Mark et al. (2006b) obtained new U–Pb (titanite) ages of 1529 + 11/-8 Ma and 1514 ± 24 Ma, overlapping with previously established Ar–Ar ages for biotite and amphibole in the range 1526–1504 Ma

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Fig. 8. Pictures of rock slabs and drill core from Osborne and Eloise mines showing evidence for early (probably pre-1600–1580 Ma) hydrothermal mineral precipitation and subsequent remobilization during the Isan Orogeny. (a) Levuka shear zone sample from Eloise showing bands of deformed pyrrhotite (darker) and calcite (pale) lying in the fabric of an inferred Isan-age shear zone, suggesting that these minerals grew during or before development of this fabric. These are overprinted by calcite-pyrrhotite veins which have mimicked the mineralogy of the adjacent sheared rock, suggesting local mass transfer produced this vein and (by inference) some of the other sulfides in this shear zone. Smaller pyrrhotite and pyrite veins, possibly Isan or post-Isan in timing, may also have derived metal and sulfur from previous local accumulations; (b) core sample from the centre of the main mineralized zone in the Osborne eastern domain, showing folded layers of magnetite adjacent to layers with disseminated pyrite and chalcopyrite, cut by axial planar zones of second-generation magnetite and sulfides. The axial planar sulfides and iron oxides could plausibly have been derived from centimetre-scale dissolution and re-precipitation of the minerals found in the folded layers. We infer that fine grained sulfides and folded magnetite in this sample are pre-Isan Orogeny in timing (and potentially syngenetic), and that the coarse-grained sulfides and axial planar magnetite represent remobilization during D2 of the Isan Orogeny; (c) classic durchbewegung structure in sulfide ore from the main ore zone at Eloise, showing shredded fragments of inferred Isan-age (D2 ) folds (dark: biotite, pale: silica alteration) surrounded by intensely deformed and reworked sulfides (pyrrhotite, pyrite, chalcopyrite). Such textures are very common in massive VHMS deposits that have been subject to medium- to high-grade metamorphism (Marshall and Gilligan, 1993), and imply that the silica alteration and the sulfides predate the ductile deformation event but the sulfides were strongly physically and chemically modified during the deformation. Abbreviations: cc, calcite; cpy, chalcopyrite; mt, magnetite; po, pyrrhotite; py, pyrite; qz, quartz.

(Perkins and Wyborn, 1998) and a U–Pb (rutile) age of 1538 ± 37 Ma (Gunton, 1999, unpublished Hons thesis). The nearest felsic intrusion to Ernest Henry, the Mt Margaret Granite, was emplaced at 1530–1528 Ma (Pollard and McNaughton, 1997; Page and Sun, 1998). Even though there are hints of inheritance from preliminary Pb–Pb step-leaching of chalcopyrite (Bassano et al., 2006), the errors are large (1591 ± 46 Ma) and associated magnetite, despite a large range of 206 Pb/204 Pb ratios, yielded an age of 1044 ± 450 Ma. Our U–Pb titanite ages at Ernest Henry are more than 60 m.y. younger than those for Osborne. There was a geologically rapid progression from crystallization of nearby felsic magma (1530 Ma), precipitation of high temperature U-bearing minerals in the

ore (1540–1515 Ma), and lower temperature potassium-bearing minerals in the proximal alteration (1525–1505 Ma). The orebody is also notable for significant enrichment in fluorine (in biotite, fluorite, and apatite (Cleverley and Oliver, 2005a; Mark et al., 2006b)). F is also enriched in the skarn-like Mt Elliott deposit (Wang and Williams, 2001) and in breccias emanating directly off the Mt Angelay intrusion (Cleverley and Oliver, 2005b). In addition, the Ernest Henry breccia has very similar internal characteristics (clast spacing, roundness, roughness and particle size distribution) to the unmineralized but magnetiteenriched breccias near the Mt Angelay Granite (Fig. 10), and may share a common physical origin (comminution, chemical corrosion and abrasion in a fluidised breccia pipe or chamber,

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been substantially accumulated into crustal rocks at the time of emplacement of the Williams Batholith, in either disseminated or concentrated form (Fig. 9). Our speculation follows the lines of reasoning taken by Cartwright and Oliver (2000) and Tomkins and Mavrogenes (2003); that is, that rocks undergoing deformation and regional metamorphism or other thermal overprinting that already contained concentrated accumulations of metals would be prone to re-concentrate the metals into younger structural sites (Fig. 9). Using this logic, we here attempt to correlate the likely metal accumulation processes with the new temporal framework of thermal events. 5.1. 1750–1730 Ma

Fig. 9. Schematic cartoon showing the basic concepts of remobilization of (left) previous orebodies, or (right) dispersed metal concentrations, based on the principles proposed by Marshall and Gilligan (1993). Applied to the Eastern Succession, the shaded box is intended to indicate mafic rocks with pre-Isan Orogeny metal accumulations, either concentrated or dispersed. The throughgoing fluid represents either metamorphic fluids during the Isan Orogeny, or later magmatic–hydrothermal fluids released by the Williams–Naraku Batholith. Derivation of iron and REE by the mechanism shown on the right has been demonstrated by Oliver et al. (2004) (see also Fig. 5); we propose here that some orebodies lying in younger structures (schematic faults at top) may have been derived by the mechanism shown on the left, e.g., Starra or Osborne.

Oliver et al., 2006). These milled breccia textures in ore are not found at Osborne, Eloise or Starra. Mark et al. (2006b) describe a range of features at Ernest Henry that indicate large fluctuations in fluid composition, and they inferred these fluctuations were a consequence of fluid mixing. One of the fluids was inferred to be related to volatile release from the Williams–Naraku Batholith, but the origin of the other fluid remains problematic. It was most likely a S-bearing fluid because pyrite zoned in arsenic and the presence of paragenetically late barite in the deposit suggests that the sulfur source for the deposit was most likely not the same as the iron and barium source. S isotopes for the deposit are strongly clustered around ␦34 S 2‰, also suggesting a magmatic sulfur source. 5. Discussion—a revised hydrothermal evolution and tectonic implications We have not been able to identify clear evidence for the accumulation of ore grade Cu–Au sulfides during volcanism and sedimentation in the Eastern Succession. Rather, we have presented data and observations that suggest that metals had already

Prior to Soldiers Cap deposition, the end of the rifting and sag cycle that produced the Corella Fm and equivalents culminated in extensional deformation and the development of upper-crustal hydrothermal systems in which granite-gabbro bodies triggered circulation of basinal and magmatic–hydrothermal fluids (Oliver et al., 1994). Widespread dolerites and gabbros were emplaced into the Corella Fm and probably reflect the first major injection of Cu into the Eastern Succession. U-REE and probably gold (Tick Hill) were added in subhorizontal shear zones and skarns (Oliver, 1995), despite preponderance of younger geochronological results for uranium mineralization (Page, 1983a). This event most likely affected rocks well to the east of the Mary Kathleen Fold Belt because several granites of similar timing have been identified (Gin Creek Granite, Levian Granite, Dipvale Granite, Pollard and McNaughton, 1997; Page and Sun, 1998). Constraints on the oxidation state and sulfur and metal content of fluids responsible for the U-REE and gold enrichments are scant; however, widespread scapolitization of dolerites at this time, and some granites, points to the circulation of salty, CO2 bearing basinal brines probably derived by evaporate dissolution (Oliver et al., 1994). 5.2. 1700–1620 Ma The 1700–1660 Ma part of this time period involved extension, widespread mafic volcanism, intrusion, and sedimentation in the Eastern Succession. We infer a contribution of significant syngenetic or diagenetic base metals via exsolution of a CO2 –H2 O–S fluid late during fractional crystallization of mafic rocks, either distributed through mafic rocks and surrounding sediments, or potentially reaching ore grades (Cannington Ag–Pb–Zn, Maronan Pb–Zn, Pegmont Pb–Zn, Osborne Cu–Au, Starra Au–Cu). A strong contribution from mafic rocks to the bulk metal budget at this time is implied from our results, and the Cannington Ag–Pb–Zn deposit shows a distinctive mantlelike isotopic character, similar to Broken Hill (Carr et al., 2004). Circulation of basinal brines may also have been important, potentially in stripping and redistributing metals originally sourced from mafic rocks. At similar times in the Mt Isa area of the Western Succession, widespread circulation of basinal fluids was responsible for localization of syn-sedimentary and diagenetic Pb–Zn and possibly Cu enrichments in the ca. 1650 Ma Mt Isa Group (Large et al., 2005).

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Fig. 10. Typical examples of inferred syn-Williams–Naraku Batholith barren and mineralized breccias from the Cloncurry area and Ernest Henry, showing dissimilar textures to those of the apparently reworked ores shown in Fig. 8. (a) Barren discordant breccia emanating from the contact aureole of the Saxby Granite, in the south-western Snake Creek area (Fig. 1), with clasts of albitized calc-silicate rock in a matrix comprising abraded albitized micro-clasts and infill of actinolite, magnetite, calcite and apatite (see also Cleverley and Oliver, 2005a,b; Oliver et al., 2006); (b) similar textures are observed in the main orebody at Ernest Henry, the difference being in the mineralogy which comprises K-feldspar–hematite (previously albitized) clasts in a matrix of magnetite, chalcopyrite, calcite, and pyrite. The textural similarity of both implies similar processes of brecciation, which along with other evidence from mineral chemistry at Ernest Henry (Mark et al., 2006b) indicates an important role for volatile release and derivation of at least some ore components from the Williams–Naraku Batholith.

Later in this time period, shortening and a phase of metamorphism commenced at ca. 1640 Ma (Rubenach et al., 2008), probably shutting down the prior extension-related events, but also developing the first of a long history of sodic alteration systems in Soldiers Cap Group rocks. This pre-peak Isan metamorphism activity may have involved circulation of evaporate-derived fluids from overthrust Corella Fm, into the Soldiers Cap Group, driven by deformation and/or topography into the core of the newly developing orogenic belt (Rubenach and Oliver, 2005). The impact of this system on Cu–Au distribution is uncertain, but was probably similar, although more localized, than the effects of the main phase of the Isan Orogeny. It may have leached a large volume of copper previously disseminated or concentrated around Osborne, where pre-metamorphic sodic alteration is prominent (Rubenach and Oliver, 2005). 5.3. 1600–1580 Ma The main phase of the Isan Orogeny liberated metamorphic H2 O and CO2 from the Corella Fm and equivalents as well as significant quantities of NaCl as suggested by scapolite mineral–fluid equilibria (Oliver et al., 1992), and H2 O from the Soldiers Cap Group (Rubenach and Barker, 1998; Rubenach and Lewthwaite, 2002). However, stable isotope data from carbonate veins hosted in Soldiers Cap Group rocks require a mixed CO2 -source, from both Corella Fm carbonates (by dissolution or devolatilization), but also from a magmatic or mantle source (Fig. 2; see also Kendrick et al., 2008). As felsic magmas were absent at this time except for pegmatites at Osborne and Cannington (Giles and Nutman, 2002; Mark et al., 2005b), mafic

rocks or the mantle are implicated in the metamorphic fluid budget (Rubenach et al., 2001; Rubenach, 2005). Mafic dykes emplaced into the core of the Snake Creek Anticline at this time share geochemical characteristics with earlier mafic rocks emplaced during rifting (e.g., Table 3), implying that melt source regions were capable of being tapped during mid- to upper crustal rifting, or shortening. In situ or proximal partial melting of near-granulite facies metasedimentary rocks produced localized pegmatites at Osborne (Rubenach et al., 2001; Rubenach and Oliver, 2005; Mark et al., 2005b). The abundance of pyrrhotite preserved in the eastern domain at Osborne suggests that metamorphic or other fluids at this time were relatively reduced, as reflected in the local presence of methane and nitrogen in the fluid inclusions there (Fu et al., 2003). Osborne methane-bearing brines contain elevated Cu concentrations, and this may reflect the capacity of reduced S-poor fluids to carry Cu as species other than sulfate, i.e., various Cu chlorides (Fisher et al., 2005). Elsewhere, metamorphic fluids leached sulfides from mafic rocks (Table 3). The 1600–1590 Ma Re–Os and U–Pb ages at Osborne may represent the major time for metal accumulation, in which case a cycle of leaching by and reprecipitation from metamorphic and/or mafic-derived fluids may explain this deposit. However, if we consider that the paragenetic evidence indicates iron oxide accumulation during sedimentation and volcanism, and possible Cu and Au mineralization (e.g., similar in timing to metal introduction at Cannington; Walters and Bailey, 1998), then the subsequent metamorphism both imparted the radiogenic isotope signal and redistributed sulfides (both chemically and mechanically) into favourable D2 structures. Skarn-like

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rocks found at Cannington Ag–Pb–Zn deposit (Chapman and Williams, 1998) show syn- to post-metamorphic paragenesis and may relate to the overprinting of syngenetic metal accumulations by later metamorphic and magmatic fluids (e.g., Roache et al., 2005). Other IOCG deposits showing apparently pre- to synpeak metamorphic sulfides may also have accumulated these metals to ore grade during early basin evolution (e.g., Eloise, Starra; Davidson et al., 1989). Fluids at this time included a population of high temperature, high salinity brines, abundant CO2 , and the presence of methane and nitrogen in some fluid inclusions (Fu et al., 2003; Fisher et al., 2005; Mustard et al., 2005). 5.4. 1550–1500 Ma Examination of alteration systems in close proximity to the Williams–Naraku Batholith gives the best idea of how these intrusions may have contributed to the IOCG deposits (Fig. 9). Mark (1998) first documented the complexity of alteration around the top of intrusions at Mt Angelay, speculating on the exsolution of hypersaline, CO2 -bearing brines as a cause of albite alteration that affected the granite carapace and surrounds. Perring et al. (2000) and Pollard (2001) documented the co-occurrence of sodic alteration and voluminous magnetite at Lightning Creek, inferring an origin for this alteration by unmixing of complex brines upon release from the crystallizing granite-gabbro sill complex. Oliver et al. (2004) built on this work to propose that granite-derived fluids moving through metasedimentary rocks attained elevated Fe- and K-contents by wallrock interaction, prior to their involvement in Ernest Henrytype IOCG genesis. Notable in all of these systems, however, is the widespread occurrence of primary CO2 -rich inclusions, either unmixed from complex NaCl–H2 O–CaCl2 –CO2 fluids (Perring et al., 2000; Pollard, 2001; Fu et al., 2003), or derived directly from degassing of crystallizing mafic intrusions. The solubility of CO2 in some granitoids is sufficient that CO2 is sometimes observed in fluid populations with an unequivocal magmatic origin (Baker, 2002); however, such CO2 is not commonly associated with potassic granites except in cases where some involvement of mafic magmas or other mantle-derived melts is demonstrated (Mungall, 2002). The large number of CO2 -rich fluid inclusions in this district, both in the intrusions of the Williams–Naraku Batholith and in the regional vein sets, suggests that volatile release from the mantle, or mantle-derived melts in the mid-crust, was a powerful fluxing agent for metasomatic processes. Simultaneous intrusion and magmatic mingling of CO2 - and possibly Cu-bearing mafic magmas may have triggered release of Cu and CO2 during quenching of the mafic rocks (e.g., Wada et al., 2004), which in turn forced exsolution of the brine from the granitoids. In the Snake Creek area, breccia pipes emanating from contact aureoles of 1530 Ma granitoids are dominated by albite, magnetite, hematite and actinolite with minor apatite (Cleverley and Oliver, 2005b), similar to the mineralogy of veins and breccias found in the carapace of the Mount Angelay Granite (Mark and Foster, 2000). Primary fluid inclusions found in these magmatic–hydrothermal transition rocks include saline brines (locally bearing hematite) and CO2 (Pollard, 2001; Fu

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et al., 2003; Mark et al., 2004; Oliver et al., 2004), and Oliver et al. (2006) proposed that the mechanism of forceful breccia emplacement above the granites was via release of overpressured CO2 -rich fluid. These fluids (oxidized brine plus CO2 ) may have assisted ore deposition in several different ways, these differences potentially explaining the diversity of detail of iron oxide and ore associations in the district (Mark et al., 2006a), as follows: 1. Without necessarily adding significant copper, such oxidised fluids may have remobilized earlier IOCGs and produced hematite-ore associations that reworked or possibly concentrated earlier copper produced during or before peak-metamorphism. 2. The fluids may have added copper derived from the granites to earlier reduced iron oxide ± sulfide assemblages, for example in the reaction of oxidised copper chloride brine with previous pyrrhotite − pyrite ± magnetite rocks (“ironstones”) to produce hematite and chalcopyrite (see also Gow et al., 1994; Skirrow and Walshe, 2002). 3. They may have provided oxidized, copper-bearing fluids to locations where mixing with one or more different fluids triggered ore deposition, such as has been proposed for Ernest Henry (see below). One or two of these relationships may explain the spatial relationships between hematite-bearing and pyrrhotite-bearing ores at Osborne, which apparently require either mixing of two fluids with very different redox capacity, or interaction of oxidized fluid with reduced rock (or vice versa). Some of the smaller deposits in the district show evidence for local sulfur derivation because of the incorporation of sedimentary sulfur isotopes in the ores, particularly for deposits hosted in black shales (e.g., Davidson and Dixon, 1992). However, as we have argued, the predominance of ␦34 S values for ore sulfides around 0‰, and of ␦13 C values for gangue carbonates between −2 and −8‰, implies that most carbon and sulfur for the deposits was derived either from mafic rocks within the broader host rock package, or from distal sources with mantlelike character, i.e., mantle degassing. Fluids that produced the widespread calcite veins of the Mary Kathleen Fold Belt are similar in mineralogy and isotopic character to gangue at Ernest Henry (Marshall et al., 2006). These fluids contained sulfur (inferred from local pyrrhotite and pyrite in the veins), abundant CO2 , and Ca- and Na-chlorides (Fu et al., 2003). Given that there are no Williams-age felsic intrusions in the MKFB, then speculation of a mantle source to these fluids is warranted. Further to the east, the abundance of CO2 in potassic phases of the Williams Batholith may be related to interaction with coemplaced mafic rocks, which then released mantle-derived CO2 during crystallization (see below). 5.5. Ernest Henry fluid mixing and sources Unlike Osborne or Cannington, the Ernest Henry orebody shows little or no physical attributes that can clearly be related to pre-Williams–Naraku hydrothermal events: it appears to be

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a genuine 1530 Ma orebody (Mark et al., 2006b). Furthermore, despite the growing evidence for inheritance and protracted copper addition to the crust over 250 m.y., Ernest Henry contains about half of all of the currently mineable or mined Cu ore yet discovered in the Eastern Succession, implying that this event was potentially the most important of all, at least for exploration. Although inherited source components are present (Bassano et al., 2006), some of which require local leaching of iron and REE (Oliver et al., 2004; Mark et al., 2005a), we remain uncertain as to the extent to which local (≤1 km scales) pre-1530 Ma concentrations of Cu–Au ore, sulfides, or ironstones provided mass for the present orebody. The distinctive K-feldspar–hematite alteration of the host volcanic rocks associated with ore deposition was probably caused by reaction of initially granite-derived fluids, modified by albitization, with the host metavolcanic rock, as suggested by geochemical models (Oliver et al., 2004; Cleverley and Oliver, 2005a). Barian Kfeldspar associated with this ore-related alteration reflects likely absence of sulfur in one of the fluids, whereas the presence of barite late in the ore paragenesis is a classic hallmark of fluid mixing (Mark et al., 2006b). We regard the key ore-forming fluid ingredients at Ernest Henry (Fig. 11) as: • oxidised Fe- and K-rich fluid derived by CO2 -stimulated brine release from the Williams Batholith during violent brecciation events, modified by subsequent wallrock reaction along the transport and deposition sites, and potentially carrying copper derived from magma mingling with mafic bodies at 1530 Ma, mixing with • H–C–O–S fluid derived either directly from the mantle, by leaching of pre-existing mafic rocks by mantle-derived fluids, or by release of fluids from crystallising Williams-age gabbros. Rocks encountered during fluid–rock reactions along the transport path of these fluids may have provided iron, Ca and a host of alkali earth elements to the orebody (Oliver et al., 2004; Marshall and Oliver, this volume), and REE are also likely to have been derived locally (Mark et al., 2005a). Interaction with the proximal altered volcanic host rocks also contributed to the specific nature of alteration zones developed around the orebody (Cleverley and Oliver, 2005a; Mark et al., 2006b). Outflow of ‘spent’ hydrothermal fluids may have been transported upwards into lower temperature environments, and Mark et al. (2005c) have speculated that some of the lower temperature style zinc deposits in the district may have been derived this way. 6. Tectonic setting of metal transfer Some of the most remarkable features about the Eastern Succession in a global and temporal context are the extraordinary volume and duration of hydrothermal alteration, with two world class mineral deposits (Cannington and Ernest Henry) and evidence for hydrothermal activity during all phases of the >250 m.y. tectonic evolution. Such observations can contribute to an understanding of the hitherto disputed tectonic setting by comparison with modern and ancient analogues.

Wilson (1978) proposed, on the basis of the overall asymmetry of the sedimentary-volcanic packages, that the eastern edge of the Mt Isa Inlier was close to a plate boundary. Part of the evidence included an appreciation of the Soldiers Cap Group as relatively deep water, high energy turbidites in comparison to possible time equivalents in the centre and west of the Inlier. Subsequently, a continent-scale model for intracratonic rifting and limited thickening was developed (Etheridge et al., 1987), supported by concepts of bimodal igneous geochemistry, lack of andesites and blueschists, and recognition of apparent temporal similarity between packages of rocks in the Western and Eastern Successions. The Eastern Succession is currently underlain by felsic and mafic continental crust which contributed zircons and detritus into the post-1700 Ma intrusions and basins, implying proximal continental sources to these basins from this time onwards (Griffin et al., 2006) and a strong component of vertical accretion by magma underplating (Mark et al., 2005b). However, the global distribution of IOCGs is also not restricted to rifted continental interiors, with several examples lying in proximal arcs or back-arcs in the Cainozoic tectonic context, particularly in the Andes (Williams et al., 2005). A feature of arc-related hydrothermal systems worldwide is the capacity of dewatering subducted oceanic slabs to act over a protracted period, to liberate fluids and incompatible elements directly by devolatilization, and to trigger mantle metasomatism, upper mantle partial melting and lower crustal melting, all of which subsequently can lead to further volatile release via emplacement of magmas into the crust (Peacock, 1993; Peacock et al., 1994; Mungall, 2002; Manning, 2004; Tornos and Chiaradia, 2004; Ducea et al., 2005; Rutherford et al., 2006). In modern convergent systems, porphyry copper deposits and island-arc related epithermal systems are a product of this type of process in fore-arcs, and such deposits are not apparently found at Mt Isa. However, back-arc extension systems can produce Cyprus-style (magnetite–chalcopyrite-dominant) and Kurokotype (Cu–Pb–Zn) VHMS deposits, IOCGs, and sediment-hosted deposits (Barley et al., 1989; Hitzman et al., 1992; Large, 1992; Bradley and Leach, 2003; Leach et al., 2001, 2005; Large et al., 2005), from the initial volcanism and rift-related sedimentation, through to basin reactivation and local shortening triggered by subduction of oceanic plateaus or continent scale shift in plate vectors (e.g., Tornos and Chiaradia, 2004). In such an environment, the genesis of metal-laden magmas, both mafic and felsic, may contribute substantially to the total metal and volatile flux. Mark et al. (2005b) propose that genesis of the Williams–Naraku granites may have occurred by back-arc mantle upwelling driven by episodic slab rollback, with emplacement of the granites occurring during compression when rollback ceased. However, they proposed that these granites developed in response to underplating of magmas and consequent vertical accretion in reworked continental crust. They also drew attention to the problems of arc proximity because of the contribution of continental crust older than 1800 Ma to the inheritance of the post-1700 Ma intrusions, and the specific interpretation of the far-field position of the plate boundary (east of Georgetown) at 1600–1500 Ma (Giles et al., 2002; Betts et al., 2006).

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Fig. 11. Cartoon showing the inferred relationships between magmas, previous ore accumulations, and the genesis of 1530 Ma IOCG deposits. CO2 released from mid-crustal mafic magmas (open circles) is thought to have driven explosive release of saline fluids from the top of the Williams–Naraku Batholith, possibly by mingling and mixing of the magmas near the top of the crystallizing plutons. These fluids were transported upwards in breccia pipes, most likely directly contributing copper to Ernest Henry-style orebodies. Other deeper-seated fluid sources, possibly from upper mantle melts, may have carried more primitive CO2 to make unmineralized veins regionally but also carbonate gangue at Ernest Henry, triggering ore deposition by fluid mixing. Release of saline fluid (filled circles) from the granites may also have driven cycles of dissolution of sulfur and/or metals from previous copper deposits or disseminated accumulations in mafic rocks, and subsequent reprecipitation during faulting or by reaction with chemically favourable host rocks (Fig. 9). Such a process would explain some of the paragenetic and geochronological features of Starra- and Osborne-style orebodies which also show evidence for pre-1530 Ma metal accumulations.

Some geochemical and isotopic indicators have been recognised that suggest that a former suture or plate boundary may have been present, approximately at the present location of the Pilgrim Fault Zone, early in the evolution of the Mount Isa Block. Mafic rocks show different geochemistry from west to east, reflecting either different crustal thickness relative to the melt source regions, or the presence of a terrane boundary (e.g., Page and Sun, 1998). Both Mark et al. (2005a) and Griffin et al. (2006) recognise a spatial transition in the radiogenic isotope signals of both intrusions and volcano-sedimentary basins from the Western to Eastern Succession. From around 1800–1700 Ma onwards Murgulov et al. (2007) propose that the Georgetown and Eastern Succession rocks showed geochemical and isotopic evidence for separate evolution, but a common evolution prior to that. Deposition of the Corella Formation and equivalents as a blanket across the proposed boundary at ca. 1760 Ma places some constraints on the timing and position of a possible former suture. The ∼1900–1850 Ma Barramundi Orogeny that affected much of northern Australia most likely occurred at a plate boundary in the Mount Isa region, manifest now by the Kalkadoon–Leichhardt Block on the western edge of the Eastern Succession (Fig. 1). Easterly migration of such a pre-1800 Ma plate boundary to an inferred position east of Georgetown in 1500 Ma reconstructions (Fig. 12) may have provided appropri-

ate conditions for repeated injection of mafic magmas in a distal back-arc setting throughout the Mesoproterozoic, although Giles et al. (2002) and Betts et al. (2003, 2006) have proposed that the main influence was from retreat of the southern margin of the northern Australian plate. MacCready et al. (2006) proposed that the high velocity slabs inferred from an E–W seismic section were most likely underplated mafic rocks developed by east–west extension between 1800 and 1600 Ma and thrust back into the crust during the 1600–1500 Ma shortening events, but did not rule out the possibility that it represented formerly subducted oceanic crust. Isotopic analysis of detrital zircon suites suggest a major phase of rifting of the eastern edge of this Kalkadoon–Leichhardt Block at around 1820 Ma (Griffin et al., 2006), and major crustal extension from 1800 to 1600 Ma, which may have been a response to rollback to the east or south. Subsequent east–west directed collision at 1600–1500 Ma east of the Georgetown Block, possibly involving Laurentia (Giles et al., 2002, 2004; Betts et al., 2006), propagated westwards to the Eastern Succession, assisted by the inferred weakness of the previously thinned crust and thermal weakening by heat-producing granitoids (McLaren et al., 2005) (Fig. 12b). The total shortening during the Isan Orogeny was limited, however, possibly because the lower crustal architecture was already established (Blenkinsop et al., 2008).

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Fig. 12. Inferred tectonic environment that can explain the protracted hydrothermal history and aspects of the mafic magma geochemistry in the context of proximal and distal back-arc settings (e.g., Betts et al., 2006). (a) Early in the history of the Eastern Succession, a plate boundary may have existed immediately east of the Kalkadoon–Leichhardt Belt, prior to 1800 Ma—alternately this lay east or south of the current Mount Isa Block exposures (Giles et al., 2002), but the relationship to Georgetown at this time is uncertain. We infer that subduction was proximal, with slab melting, volatile enrichment of mantle, and the first influx of back-arc mafic magmas and related metals in the 1800–1700 Ma period. Some oerbodies formed at this time (black). Blenkinsop et al. (2008), however, propose that upper crustal inversion during the Isan Orogeny was essentially para-autochthonous, reducing the possibility that a plate margin and oceanic crust did develop after 1800 Ma within the exposed parts of the Eastern Succession. (b) By 1500 Ma, there is no evidence for a proximal arc, the Georgetown Block and Eastern Succession both being influenced by orogenesis at a similar time (1600–1500 Ma) probably driven by collision with Laurentia (see Betts et al., 2006). The repeated injection of volatile-rich melts, and the large amounts of CO2 liberated into the hydrothermal systems and during ore genesis (orebodies—black), suggest that volatile-laden mantle was still able to contribute to the fluid budget even though the arc was distant, a situation that we envisage occurring by melting of mantle enriched in volatiles through previous interaction with the pre-1800 Ma arc. Protracted extension, with periodic contraction (as shown here), would have favoured ongoing elevation of the mafic solidus (dashed line) into the root zone of the Mount Isa Block, because of the previous geochemical changes to the mantle lithosphere. Thermal erosion of the mantle lithosphere before and during convergence cycles may also have assisted ongoing mantle melting and volatile release.

Extra heat sources derived from subcontinental plumes were envisaged by Oliver et al. (1991) for Mount Isa, and Baker et al. (1998) for Yemen. Such a hypothetical plume does not explain the geochemistry of the mafic rocks at Mount Isa and is not needed if thickening accompanying shortening was capable of re-melting the previously enriched upper mantle, or if rift-related melting (e.g., by rollback) was punctuated by periods of shortening in which melts were injected upwards into the crust (see also Mark et al., 2005b). We suggest that protracted extension would allow preferential partial melting underneath the Mount Isa Block because of the combination of elevated crustal geotherms due to prior enrichment in U- and K-rich granites (McLaren et al., 2005) and also because extension in the mantle lithosphere would permit repeated upwards migration of the solidus for mafic magmas, either by thinning and/or because of the prior metasomatism during the Barramundi Orogeny plate boundary event (Fig. 12b). The highly complex P-T-t paths shown in the Eastern Succession during the 1650–1500 Ma period reflect multiple cycles of extension and shortening, and heating by repeated advective heat transfer into the mid crust by magmas (Foster and

Rubenach, 2006; Mark et al., 2005b; Rubenach et al., 2008), with a probable contribution from radiogenic heat production (e.g., McLaren et al., 1999). 6.1. Source of fluids Given that there was a protracted fluid evolution in the Eastern Succession, a mechanism must be sought for repeated fluid streaming that does not require a proximal arc. We have summarized here evidence for CO2 fluid inclusions, carbonate vein systems and carbonate gangue to IOCG ores that have distinctive magmatic or mantle-like isotopic signatures. Some authors have speculated on a carbonatite connection with IOCGs (e.g., Groves and Vielreicher, 2001). Carbonate vein systems in the Eastern Succession show superficial similarities to some carbonatites (calcite-dominant, minor pyrrhotite, actinolite, clinopyroxene, apatite), but appear to lack the distinctive REE-mineral suite, HFSE-enrichment and related mica-dominant alteration (e.g., fenitization). Furthermore, carbonatites are generated by low% partial melt of metasomatized mantle (e.g., Kamenetsky et al.,

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2004), and the geological record of the Eastern Succession rather suggests repeated rift-related melting, probably at high %partial melt in the upper lithospheric mantle, to produce voluminous tholeiitic basalts. Newton and Manning (2002) have suggested that calcite-bearing rocks may be partially melted under upper mantle lower crustal conditions (6–14 kbar) in the presence of NaCl-bearing fluids, and that this would provide potentially large fluxes of CO2 . However, the initial incorporation of sedimentary carbonate into mantle source regions by subduction of arc sediments may still leave distinctive marine C- and O-isotope signatures (Ducea et al., 2005), so partial melting of Corella Fm rocks (for example) can probably be excluded. If the upper lithospheric mantle is sufficiently volatileenriched to provide voluminous CO2 upon crystallization of basalts derived by partial melting, the implication is that the mantle was at some time fluxed by arc-derived fluids or was depleted in non-volatile components by arc-related melting (e.g., Mungall, 2002). We suggest that during or before the development of the ca. 1800 Ma plate boundary inferred for the central Mount Isa Block, thin, volatile enriched lithospheric mantle developed beneath the Eastern Succession in a near-arc environment (Fig. 12a). Subsequent (?rapid) trench retreat to the east may have left behind a substantial enriched pocket of lithosphere previously involved in arc-proximal processes, such that later distal back-arc extension triggered release of volatile-laden mafic magmas from this source, and collisional events that drove the Isan Orogeny and later granite genesis involved partial melting of lower crust and enriched upper mantle lithosphere. Direct parallels may be drawn for the Paleoproterozoic evolution of the Curnamona Province in southern Australia (Rutherford et al., 2006), and parts of Greenland (Goodenough et al., 2002) and the Canadian Shield (Cousens et al., 2001), in which mantle lithosphere enriched by arc-derived fluids was preserved subsequently to release volatile-laden magmas much later in the geological history.

Acknowledgements We thank Tim Baker, Tom Blenkinsop, Mick Carew, Dave Cooke, Richard Crookes, Damien Foster, Michel Gauthier, Kathryn Lewthwaite, Glen Little, John McLellan, Roger Mustard, Peter Pollard, Rick Valenta, Pat Williams, Dugi Wilson and Bruce Yardley for discussions. Bill Collins and Tony Kemp are particularly thanked for discussions on arc tectonics and geochemistry. Josh Bryant, Perry Collyer, Dan Johnson and Glen Little are thanked for collaboration and access at Ernest Henry, Ian Hodkinson is thanked for his support at Eloise, and Ian Cartwright, Garry Davidson, Sue Golding and Jodie Miller contributed to the stable isotope data at various stages. This report is a product of the collaboration between the F1-2 and I2-3 team members of the Predictive Mineral Discovery CRC, but includes concepts and data developed prior to that during research sponsored by the Australian Research Council, Placer (now Barrack) Osborne Mine, and Mount Isa Mines Exploration (now Xstrata Copper Exploration), particularly at Ernest Henry.

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References Adshead, N.D., Voulgaris, P., Muscio, V.N., 1998. Osborne copper-gold deposit. In: Berkman, D.A., Mackenzie, D.H. (Eds.), Geology of Australian and Papua New Guinean Mineral Deposits. Australasian Institute of Mining and Metallurgy, pp. 793–799. Baker, J., Chazot, G., Menzies, M., Thirlwall, M., 1998. Metasomatism of the shallow mantle beneath Yemen by the Afar Plume; implications for mantle plumes, flood volcanism, and intraplate volcanism. Geology 26, 431– 434. Baker, T., 1998. Alteration, mineralization and fluid evolution at the Eloise Cu–Au deposit. Econ. Geol. 93, 1213–1236. Baker, T., 2002. Emplacement depth and carbon dioxide-rich fluid inclusions in intrusion-related gold deposits. Econ. Geol. 97, 1111–1117. Baker, T., Perkins, C., Blake, K.L., Williams, P.J., 2001. Radiogenic and stable isotope constraints on the genesis of the Eloise Cu–Au deposit, Cloncurry District, northwest Queensland. Econ. Geol. 96, 723–742. Barley, M.E., Eisenlohr, B.N., Groves, D.I., Perring, C.S., Vearncombe, J.R., 1989. Late Archaean convergent margin tectonics and gold mineralization: a new look at the Norseman-Wiluna belt. Geology 17, 826–829. Barton, M.D., Johnson, D.A., 1996. Evaporitic source model for igneous-related Fe oxide-(REE-CAu-U) mineralization. Geology 24, 259–262. Bassano, K.A., Hergt, J., Maas, R., Woodhead, J., 2006. A feasibility study of the application of the Pb–Pb isotope step-leaching technique to ore minerals. In: Denham, D. (Ed.), Australian Earth Sciences Convention 2006. RESolutions Resource and Energy Services Pty Ltd., Melbourne, pp. 102–103. Betts, P.G., Giles, D., Lister, G.S., 2003. Tectonic environment of shale-hosted massive sulfide Pb–Zn–Ag deposits of Proterozoic northeastern Australia. Econ. Geol. 98, 557–576. Betts, P.G., Giles, D., Mark, G., Lister, G.S., Goleby, B.R., Ailleres, L., 2006. Synthesis of the Proterozoic evolution of the Mt Isa Inlier. Aust. J. Earth Sci. 53, 187–211. Blenkinsop, T.G., Huddlestone-Holmes, C.R., Foster, D.R.W., Edmiston, M.A., Lepong, P., Mark, G., Austin, J.R., Murphy, F.C., Ford, A., Rubenach, M.J., 2008. The crustal scale architecture of the Eastern Succession, Mount Isa: the influence of inversion. Precambrian Res. 163, 31–49. Bradley, D.C., Leach, D.L., 2003. Tectonic controls of Mississippi Valleytype lead–zinc mineralization in orogenic forelands. Miner. Dep. 38, 652–667. Bultitude, R.J., Wyborn, L.A.I., 1982. Distribution and geochemistry of volcanic rocks in the Duchess-Urandangi region, Queensland. B.M.R. J. Austr. Geol. Geophys. 7, 99–112. Butera, K., Oliver, N.H.S., Rubenach, M.J., Collins, W.J., Cleverley, J.S., 2005. Multiple generations of metal and sulphur contributions to the IOCG budget of the Mount Isa Eastern Succession. In: Blenkinsop, T.G. (Ed.), Total Systems Analysis of the Mount Isa Eastern Succession. Predictive Minerals Discovery Cooperative Research Centre, I 2+3 Final Report, pp. 325–342. www.pmdcrc.com.au/final reports projectI2.html. Carr, G.R., Denton, G.J., Parr, J., Sun, S., Korsch, M.J., Bodon, S.B., 2004. Lightning does strike twice; multiple ore events in major mineralised systems in northern Australia. In: Muhling, J. (Ed.), SEG 2004—Predictive Mineral Discovery Under Cover, vol. 33. Centre for Global Metallogeny, University of Western Australia, pp. 332–335. Cartwright, I., Oliver, N.H.S., 1994. Up- and down-temperature fluid flow during contact metamorphism, Mary Kathleen, Queensland, Australia. J. Petrol. 35, 1493–1520. Cartwright, I., Oliver, N.H.S., 2000. Metamorphic fluids and their relationship to the formation of metamorphosed and metamorphogenic ore deposits. In: Marshall, B., Vokes, F. (Eds.), Metamorphosed and Metamorphogenic Ore Deposits. Reviews in Economic Geology, pp. 81–96. Chapman, L.H., Williams, P.J., 1998. Evolution of pyroxene–pyroxenoid–garnet alteration in the southern zone of the Cannington Ag–Pb–Zn deposit, Cloncurry District, Queensland, Australia. Econ. Geol. 93, 1390–1405. Cleverley, J.S., Oliver, N.H.S., 2005a. Comparing closed system, flow-through and fluid infiltration geochemical modelling: examples from K-alteration in the Ernest Henry Fe-oxide–Cu–Au system. Geofluids 5, 289–307. Cleverley, J.S., Oliver, N.H.S., 2005b. Crustal scale volatile transfer: melts and breccias in the Mount Isa Eastern Fold Belt. In: Hancock, H. (Ed.), Structure,

128

N.H.S. Oliver et al. / Precambrian Research 163 (2008) 108–130

Tectonics and Ore Mineralisation Processes. Economic Geology Research Unit, James Cook University, Townsville, p. 26. Cousens, B.L., Aspler, L.B., Chiarenzelli, J.R., Donaldson, J.A., Sandeman, H., Peterson, T.D., LeCheminant, A.N., 2001. Enriched Archean lithospheric mantle beneath western Churchill Province tapped during Paleoproterozoic orogenesis. Geology 29, 827–830. Davidson, G.J., Dixon, G.H., 1992. Two sulphur isotope provinces deduced from ores in the Mount Isa eastern succession, Australia. Miner. Dep. 27, 30– 41. Davidson, G.J., Large, R.R., 1994. Gold metallogeny and the copper-gold association of the Australian Proterozoic. Miner. Dep. 29, 208–223. Davidson, G.J., Large, R.R., Kary, G.L., Osborne, R., 1989. The BIF-hosted Starra and Trough Tank Au–Cu mineralization: a new stratiform association from the Proterozoic Eastern succession of Mt. Isa, Australia. Econ. Geol. Monogr. 6, 135–150. deJong, G., Williams, P.J., 1995. Giant metasomatic system formed during exhumation of mid-crustal Proterozoic rocks in the vicinity of the Cloncurry Fault, northwest Queensland. Austr. J. Eth. Sci. 42, 281– 290. Ducea, M.N., Saleeby, J., Morrison, J., Valencia, V.A., 2005. Subducted carbonates, metasomatism of mantle wedges, and possible connections to diamond formation: an example from California. Am. Mineral. 90, 864–870. Ellis, D.J., Wyborn, L.A.I., 1984. Petrology and geochemistry of Proterozoic dolerites from the Mount Isa Inlier. B.M.R. J. Austr. Geol. Geophys. 9, 19–32. Etheridge, M.A., Rutland, R.W.R., Wyborn, L.A.I., 1987. Orogenesis and tectonic processes in the Early to Middle Proterozoic of northern Australia. Am. Geophys. Union, Geodyn. Ser. 17, 131–147. Fisher, L.A., Kendrick, M.A., Mustard, R., 2005. Noble gas and halogen evidence for the source of mineralizing fluids in the Osborne IOCG deposit, Mt Isa Inlier, Australia. In: Hancock, H. (Ed.), Structure, Tectonics and Ore Mineralisation Processes. Economic Geology Research Unit, James Cook University, Townsville, p. 49. Ford, A., 2005. Fractal distribution of mineral deposits for exploration. In: Blenkinsop, T.G. (Ed.), Total Systems Analysis of the Mount Isa Eastern Succession. Predictive Minerals Discovery Cooperative Research Centre, I 2 + 3 Final Report, pp. 429–434. www.pmdcrc.com.au/final reports projectI2.html. Ford, A., Blenkinsop, T.G., 2007. Combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization: Application to copper deposits of the Mount Isa Inlier, NW Queensland, Australia. Ore Geol. Rev., doi:10.1016/j.oregeorev.2007.01.004. Foster, D.R.G., Rubenach, M.J., 2006. Isograd pattern and regional low pressure, high temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east-west section through the Mt Isa Inlier. Austr. J. Earth Sci. 53, 167–186. Foster, D.R.G., Austin, J.R., 2008. The 1800 to 1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent paleoproterozoic terranes. Precambrian Res. 163, 7–30. Fu, B., Williams, P.J., Oliver, N.H.S., Dong, G., Pollard, P.J., Mark, G., 2003. Fluid mixing versus unmixing as an ore-forming process in the Cloncurry Fe-oxide–Cu–Au District, NW Queensland, Australia: evidence from fluid inclusions. J. Geochem. Exp. 78–79, 617–622. Gauthier, L., Hall, G., Stein, H., Schaltegger, U., 2001. The Osborne deposit, Cloncurry District: a 1595 Ma Cu–Au skarn deposit. In: Williams, P.J. (Ed.), A Hydrothermal Odyssey: Extended Conference Abstracts. Economic Geology Research Unit, James Cook University, Townsville, pp. 58–59. Giles, D., Nutman, A.P., 2002. SHRIMP U–Pb monazite dating of 1600–1580 Ma amphibolite facies metamorphism in the southeastern Mt. Isa Block, Australia. Aust. J. Earth Sci. 49, 455–466. Giles, D., Betts, P.G., Lister, G.S., 2002. Far-field continental backarc setting for the 1.80–1.67 Ga basins of northeastern Australia. Geology 30, 823– 826. Giles, D., Betts, P.G., Lister, G.S., 2004. 1.8–1.5-Ga links between the North and South Australian Cratons and the Early–Middle Proterozoic configuration of Australia. Tectonophysics 380, 27–41. Goodenough, K.M., Upton, B.G.J., Ellam, R.M., 2002. Long-term memory of subduction processes in the lithospheric mantle: evidence from the geochem-

istry of basic dykes in the Gardar Province of South Greenland. J. Geol. Soc. 159, 705–714. Gow, P.A., Wall, V.J., Oliver, N.H.S., Valenta, R.K., 1994. Proterozoic iron oxide (Cu-U-Au-REE) deposits: further evidence of hydrothermal origins. Geology 22, 633–636. Grant, J.A., 1986. The isocon diagram – a simple solution to Gresens equation for metasomatic alteration. Econ. Geol. 81, 1976–1982. Gregory, M., 2006. Copper mobility in the Eastern Creek Volcanics, Mount Isa, Australia: evaluation from laser ablation ICP-MS of iron-titanium oxides. Miner. Dep. 41, 691–711. Griffin, W.L., Belousova, E.A., Walters, S.G., O’Reilly, S.Y., 2006. Archaean and Proterozoic crustal evolution in the Eastern Succession of the Mt Isa district, Australia: U–Pb and Hf-isotope studies of detrital zircons. Austr. J. Earth Sci. 53, 125–150. Groves, D.I., Vielreicher, N.M., 2001. The Phalabowra (Palabora) carbonatitehosted magnetite-copper sulfide deposit, South Africa; an end-member of the iron-oxide copper–gold-rare earth element deposit group. Miner. Dep. 36, 189–194. Heinrich, C.A., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., Waring, C.L., 1995. Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Econ. Geol. 90, 705–730. Herzberg, C., 2004. Partial crystallization of mid-ocean ridge basalts in the crust and mantle. J. Petrol. 45, 2389–2405. Hitzman, M.W., Oreskes, N., Einaudi, M.T., 1992. Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits. Precambrian Res. 58, 241–287. Holcombe, R.J., Pearson, P.J., Oliver, N.H.S., 1991. Geometry of a middle Proterozoic extensional detachment surface. Tectonophysics 191, 255–274. Kamenetsky, M.B., Sobolev, A.V., Kamenetsky, V.S., Maas, R., Danyushevsky, L.V., Thomas, R., Pokhilenko, N.P., Sobolev, N.V., 2004. Kimberlite melts rich in alkali chlorides and carbonates: a potent metasomatic agent in the mantle. Geology 32, 845–848. Kendrick, M.A., Baker, T., Fu, B., Phillips, D., 2008. Noble gas and halogen constraints on regionally extensive Na–Ca alteration, Eastern Mount Isa, Block Australia: implications for CO2 sources and metalogenesis. Precambrian Res. 163, 131–150. Large, R.R., 1992. Australian volcanic-hosted massive sulphide deposits: features, styles, and genetic models. Econ. Geol. 87, 471–510. Large, R.R., Bull, S.W., McGoldrick, P.J., Derrick, G.M., Carr, G.R., Walters, S., 2005. Stratiform and stratabound Zn–Pb–Ag deposits of the Proterozoic sedimentary basins of northern Australia. In: 100th Anniversary Volume, Society of Economic Geologists, Boulder, pp. 931–963. Leach, D.L., Bradley, D., Lewchuk, M.T., Symons, D.T.A., deMarsily, G., Brannon, J., 2001. Mississippi Valley-type lead–zinc deposits through geological time: implications from recent age-dating research. Miner. Dep. 36, 711–740. Leach, D.L., Sangster, D.F., Kelley, K.D., Large, R.R., Garven, G., Allan, C.R., Gutzmer, J., Walters, S.K., 2005. Sediment-hosted lead–zinc deposits: a global perspective. In: 100th Anniversary Volume, Society of Economic Geologists, Boulder, pp. 561–607. Maas, R., McCulloch, M.T., Campbell, I.H., 1988. Sm–Nd isotope systematics in uranium rare-earth element mineralization at Mary Kathleen uranium mine, Queensland. Econ. Geol. 82, 1805–1826. MacCready, T., Goleby, B.R., Goncharov, A., Drummond, B.J., Lister, G.S., 2006. Shifts in the locus of crustal thickening during Mesoproterozoic orogenesis in the Mt Isa Terrane. Aust. J. Earth Sci. 53, 27–40. Manning, C.E., 2004. The chemistry of subduction-zone fluids. Earth Plan. Sci. Lett. 223, 1–16. Mark, G., 1998. Albitite formation by selective pervasive sodic alteration of tonalite plutons in the Cloncurry district, NW Queensland. Aust. J. Earth Sci. 45, 765–774. Mark, G., 1999. Petrogenesis of Mesoproterozoic K-rich granitoids, southern Mt Angelay igneous complex, Cloncurry district, northwest Queensland. Aust. J. Earth Sci. 46, 933–950. Mark, G., Foster, D.R.W., 2000. Magmatic–hydrothermal albite-actinoliteapatite-rich rocks from the Cloncurry district, NW Queensland, Australia. Lithos 51, 223–245.

N.H.S. Oliver et al. / Precambrian Research 163 (2008) 108–130 Mark, G., Foster, D.R.W., Pollard, P.J., Williams, P.J., Tolman, J., Darvall, M., 2004. Magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide–Cu–Au district, NW Queensland, Australia. Terra Nova 16, 54–61. Mark, G., Pollard, P.J., Foster, D.R.W., McNaughton, N., Mustard, R., 2005a. Episodic syn-tectonic magmatism in the Eastern Succession, Mount Isa Block, Australia: implications for the origin, derivation and tectonic setting of potassic ‘A-type’ magmas. In: Blenkinsop, T.G. (Ed.), Final Report, Total Systems Analysis of the Mt Isa Eastern Succession, Predictive Mineral Discovery CRC, pp. 51–74. Mark, G., Foster, D.R.W., Mustard, R., Pollard, P.J., 2005b. Sr–Nd isotopic constraints on the crustal architecture and evolution of the Eastern Succession, Mount Isa Block, Australia. Precambrian Res. In: Blenkinsop, T.G. (Ed.) Final Report, Total Systems Analysis of the Mt Isa Eastern Succession, Predictive Mineral Discovery CRC, pp. 75–98. Mark, G., Wilde, A.R., Oliver, N.H.S., Williams, P.J., 2005c. Predicting alteration patterns in the outflow zones of hydrothermal ore systems: a case study using the “spent” fluids from the Ernest Henry Fe-oxide–Cu–Au deposit. J. Geochem. Expl. 85, 31–46. Mark, G., Oliver, N.H.S., Carew, M.J., 2006a. Insights into the genesis and diversity of epigenetic Cu-Au mineralisation in the Cloncurry District, Mt Isa Inlier, northwest Queensland. Austr. J. Earth Sci. 59, 109–124. Mark, G., Oliver, N.H.S., Williams, P.J., 2006b. Mineralogical and chemical evolution of the Ernest Henry Fe oxide-Cu-Au ore system, Cloncurry district, northwest Queensland, Australia. Miner. Dep. 40, 769–801. Marschik, R., Fontbot´e, L., 2001. The Candelaria-Punta del Cobre iron oxide Cu–Au(–Zn–Ag) deposits, Chile. Econ. Geol. 96, 1799–1826. Marshall, B., Gilligan, L.B., 1993. Remobilization, syn-tectonic processes and massive sulphide deposits. Ore Geol. Rev. 8, 39–64. Marshall, L.J., Oliver, N.H.S. Constraints on hydrothermal fluid pathways within Mary Kathleen Group stratigraphy of the Cloncurry iron-oxide–copper–gold district, Australia. Precambrian Res., this volume. Marshall, L.J., Oliver, N.H.S., Davidson, G.J., 2006. Carbon and oxygen isotope constraints on fluid sources and fluidwallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia. Miner. Dep. 41, 429–452. Matth¨ai, S.K., Heinrich, C.A., Driesner, T., 2004. Is the Mount Isa copper deposit the product of forced brine convection in the footwall of a major reverse fault? Geology 32, 357–360. Mavrogenes, J., O’Neil, J.R., 1999. The relative effects of pressure, temperature and oxygen fugacity on the solubility of sulfide in mafic magmas. Geochim. Cosmochim. Acta 63, 1173–1180. McLaren, S., Sandiford, M., Hand, M., 1999. High radiogenic heat-producing granites and metamorphism; an example from the western Mount Isa Inlier, Australia. Geology 27, 679–682. McLaren, S., Sandiford, M., Powell, R., 2005. Contrasting styles of Proterozoic crustal evolution: a hot-plate tectonic model for Australian terranes. Geology 33, 673–676. McLellan, J.G., Oliver, N.H.S., 2008. Discrete element modelling applied to copper prospectivity in the eastern Mount Isa Inlier. Precambrian Res. 163, 174–188. Mungall, J.E., 2002. Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits. Geology 30, 915–918. Murgulov, V., Beyer, E., Griffin, W.L., O’Reilly, S.Y., Walters, S.G., Stephens, D., 2007. Crustal evolution in the Georgetown Inlier, North Queensland, Australia: a detrital zircon study. Chem. Geol. 245, 198–215. Mustard, R., Blenkinsop, T.G., Foster, D.R.G., Mark, G., McKeagney, C., Huddlestone-Holmes, C., Partington, G., Higham, M., 2005. Critical ingredients in Cu–Au–iron oxide deposits, NW Queensland: an evaluation of our current understanding using GIS spatial data modeling. In: Blenkinsop, T.G. (Ed.), Total systems analysis of the Mount Isa Eastern Succession. Predictive Minerals Discovery Cooperative Research Centre, I 2+3 Final Report, pp. 291–324. www.pmdcrc.com.au/final reports projectI2.html. Newton, R.C., Manning, C.E., 2002. Experimental determination of calcite solubility in H2 O–NaCl solutions at deep crust/upper mantle pressures and temperatures: implications for metasomatic processes in shear zones. Am. Mineral. 87, 1401–1409.

129

Oliver, N.H.S., 1995. The hydrothermal history of the Mary Kathleen Fold Belt, Mount Isa Block, Queensland, Australia. Aust. J. Earth Sci. 42, 267–280. Oliver, N.H.S., Cartwright, I., Wall, V.J., Golding, S.D., 1993. The stable isotopic signature of large-scale fracture-hosted metamorphic fluid pathways, Mary Kathleen, Australia. J. Metamorphic Geol. 11, 705–720. Oliver, N.H.S., Cleverley, J.S., Mark, G., Pollard, P.J., Fu, B., Marshall, L.J., Rubenach, M.J., Williams, P.J., Baker, T., 2004. Modeling the role of sodic alteration in the genesis of iron oxide–copper–gold deposits; eastern Mt. Isa Block, Australia. Econ. Geol. 99, 1145–1176. Oliver, N.H.S., Holcombe, R.J., Hill, E.J., Pearson, P.J., 1991. Tectonometamorphic evolution of the Mary Kathleen Fold Belt, northwest Queensland: a reflection of mantle plume processes? Aust. J. Earth Sci. 38, 425–456. Oliver, N.H.S., Pearson, P.J., Holcombe, R.J., Ord, A., 1999. Mary Kathleen metamorphic–hydrothermal uranium-rare-earth deposit: ore genesis and a numerical model of coupled deformation and fluid flow. Aust. J. Earth Sci. 46, 467–484. Oliver, N.H.S., Rawling, T.R., Cartwright, I., Pearson, P.J., 1994. High temperature fluid–rock interaction and scapolitization in a large extension-related hydrothermal system, Mary Kathleen, Australia. J. Petrol. 35, 1455–1491. Oliver, N.H.S., Rubenach, M.J., Valenta, R.K., 1998. Precambrian metamorphism, fluid flow and metallogeny of Australia. In: Wyborn, L.A.I. (Ed.), Ore deposits of Australia and their tectonic and geochemical setting, vol. 17. AGSO J. Austr. Geol. Geophys., pp. 31–53. Oliver, N.H.S., Rubenach, M.J., Fu, B., Baker, T., Blenkinsop, T.G., Cleverley, J.S., Marshall, L.J., Ridd, P.J., 2006. Granite-related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry district, Australia. Geofluids 6, 346–358. Oliver, N.H.S., Wall, V.J., Cartwright, I., 1992. Internal control of fluid compositions in amphibolite-facies scapolitic calc-silicates, Mary Kathleen, Australia. Contrib. Mineral. Petrol. 111, 94–112. Page, R.W., Bell, T.H., 1986. Isotopic and structural response of granite to successive deformation and metamorphism. J. Geol. 94, 365–379. Page, R.W., Sun, S.-S., 1998. Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier. Aust. J. Earth Sci. 45, 343–362. Page, R.W., 1983a. Chronology of magmatism, skarn formation and uranium mineralisation, Mary Kathleen, Queensland, Australia. Econ. Geol. 78, 838–853. Page, R.W., 1983b. Timing of superposed volcanism in the Proterozoic Mt. Isa Inlier, Australia. Precambrian Res. 21, 223–245. Peacock, S.M., 1993. Large-scale dehydration of the lithosphere above subducting slabs. Chem. Geol. 108, 49–59. Peacock, S.M., Rushmer, T., Thompson, A.B., 1994. Partial melting of subducting oceanic crust. Earth Plan. Sci. Lett. 121, 227–243. Pearson, P.J., Holcombe, R.J., Page, R.W., 1992. Synkinematic emplacement of the Middle Proterozoic Wonga Batholith into a mid-crustal extensional shear zone, Mount Isa Inlier, Queensland, Australia. In: Stewart, A.J., Blake, D.H. (Eds.), Detailed Studies of the Mount Isa Inlier. Austr. Geol. Surv. Org., Canberra, pp. 289–328. Perkins, C., Wyborn, L.A.I., 1998. Age of Cu–Au mineralisation, Cloncurry district, Mount Isa Inlier, as determined by 40 Ar/39 Ar dating. Aust. J. Earth Sci. 45, 233–246. Perkins, W.G., 1984. Mount Isa silica-dolomite and copper orebodies: the result of a syntectonic hydrothermal alteration system. Econ. Geol. 79, 601–637. Perring, C.S., Pollard, P.J., Dong, G., Nunn, A.J., Blake, K.L., 2000. The Lightning Creek sill complex, Cloncurry District, northwest Queensland: a source of fluids for Fe oxide Cu–Au mineralization and sodic–calcic alteration. Econ. Geol. 95, 1067–1089. Pollard, P.J., McNaughton, N., 1997. U–Pb geochronology and Sm/Nd isotope characteristics of Proterozoic intrusive rocks in the Cloncurry district, Mount Isa Inlier, Australia. In: Pollard, P.J. (Ed.), AMIRA P438 Final Report: Cloncurry Base Metals and Gold, p. 19. Pollard, P.J., 2001. Sodic(-calcic) alteration associated with Fe-oxide–Cu–Au deposits: an origin via unmixing of magmatic-derived H2 O–CO2 –salt fluids. Miner. Dep. 36, 93–100. Pollard, P.J., Mark, G., Mitchell, L.M., 1998. Geochemistry of post-1540 Ma granites in the Cloncurry district, northwest Queensland. Econ. Geol. 93, 1330–1344.

130

N.H.S. Oliver et al. / Precambrian Research 163 (2008) 108–130

Queensland Department of Mines and Energy, Taylor Wall & Associates, SRK Consulting Pty Ltd. & ESRI Australia, 2000. North-west Queensland Mineral Province Report, Queensland Department of Mines and Energy, Brisbane. Roache, A.J., Williams, P.J., Richmond, J.M., Chapman, L.H., 2005. Vein and skarn formation at the Cannington Ag–Pg–Zn deposit, northeastern Australia. Can. Miner. 43, 241–262. Rotherham, J.F., Blake, K.L., Cartwright, I., Williams, P.J., 1998. Stable isotope evidence for the origin of the Starra Au–Cu deposit, Cloncurry district. Econ. Geol. 93, 1435–1449. Rubenach, M.J., Barker, A.J., 1998. Metamorphic and metasomatic evolution of the Snake Creek Anticline, Eastern Succession, Mount Isa Inlier. Aust. J. Earth Sci. 45, 363–372. Rubenach, M.J., Lewthwaite, K.J., 2002. Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, northeastern Australia: microstructures and P-T-t paths. J. Metamorph. Geol. 20, 191–202. Rubenach, M.J., Oliver, N.H.S., 2005. Geochemistry of albitites and related metasomatic rocks, Eastern Succession of the Mount Isa Block. In: Blenkinsop, T.G. (Ed.), Final Report, Total systems analysis of the Mt Isa Eastern Succession. Predictive Mineral Discovery Cooperative Research Centre, Melbourne, pp. 343–358. Rubenach, M.J., 2005. Relative timing of albitization and chlorine enrichment in Proterozoic schists, Snake Creek Anticline, Mount Isa Inlier, northeastern Australia. Can. Miner. 43, 349–366. Rubenach, M.J., Adshead, N.D., Oliver, N.H.S., Tullemans, F., Esser, D., Stein, H., 2001. The Osborne Cu-Au deposit: geochronology and genesis of mineralization in relation to host albitites and ironstones. In: Williams, P.J. (Ed.), A Hydrothermal Odyssey. Extended Conference Abstracts. Economic Geology Research Unit, James Cook University, Townsville, pp. 172– 173. Rubenach, M.J., Foster, D.R.G., Evins, P.M., Blake, K.L., Fanning, C.M., 2008. Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier. Precambrian Res. 163, 81–107. Rutherford, L., Barovich, K., Hand, M., Foden, J., 2006. Metatholeiites in the Curnamona Province, Australia: a record of melting of a heterogeneous, subduction-modified lithospheric mantle. Aust. J. Earth. Sci. 53, 501–519.

Skirrow, R.G., Walshe, J.L., 2002. Reduced and oxidized Au–Cu–Bi iron oxide deposits of the Tennant Creek Inlier, Australia: an intregrated geologic and chemical model. Econ. Geol. 97, 1167–1202. Stanton, R.L., 1994. Ore elements in arc lavas. Oxford Monographs Geol. Geophys., vol. 29, 391 pp. Tomkins, A.G., Mavrogenes, J.A., 2003. Generation of metal-rich felsic magmas during crustal anatexis. Geology 31, 765–768. Tornos, F., Chiaradia, M., 2004. Plumbotectonic evolution of the Ossa Morena zone, Iberian Peninsula: tracing the influence of mantle–crust interaction in ore-forming processes. Econ. Geol. 99, 965–985. Wada, H., Harayama, S., Yamaguchi, Y., 2004. Mafic enclaves densely concentrated in the upper part of a vertically zoned felsic magma chamber: the Kurobegawa granitic pluton, Hida Mountain Range, central Japan. Geol. Soc. Am. Bull. 116, 788–801. Walters, S., Bailey, A., 1998. Geology and mineralization of the Cannington Ag–Pb–Zn deposit; an example of Broken Hill-type mineralization in the eastern succession, Mount Isa Inlier, Australia. Econ. Geol. 93, 1307–1329. Wang, S., Williams, P.J., 2001. Geochemistry and origin of Proterozoic skarns at the Mount Elliott Cu–Au(–Co–Ni) deposit, Cloncurry District, NW Queensland, Australia. Miner. Dep. 36, 109–124. Waring, C.L., Heinrich, C.A., Wall, V.J., 1998. Proterozoic metamorphic copper deposits. AGSO J. Austr. Geol. Geophys. 17, 239–246. Williams, P.J., 1994. Iron mobility during synmetamorphic alteration in the Selwyn Range area, NW Queensland: implications for the origin of ironstone-hosted Au–Cu deposits. Miner. Dep. 29, 250–260. Williams, P.J., 1998. Metalliferous economic geology of the Mt Isa Eastern Succession, Queensland. Aust. J. Earth Sci. 45, 329–341. Williams, P.J., Barton, M.D., Fontbot´e, L., deHaller, A., Johnson, D.A., Mark, G., Oliver, N.H.S., Marschik, R., 2005. Iron oxide–copper–gold deposits: geology, space-time distribution and possible modes of origin. In: 100th Anniversary Volume, Society of Economic Geologists, Boulder, pp. 371–405. Wilson, I.H., 1978. Volcanism on a Proterozoic continental margin in northwestern Queensland. Precambrian Res. 7, 205–235. Wyborn, L.A.I., Page, R.W., McCulloch, M.T., 1988. Petrology, geochronology and isotope geochemistry of the post-1820 Ma granites of the Mount Isa Inlier: mechanisms for the generation of Proterozoic anorogenic granites. Precambrian Res. 40/41, 509–541.