©2011 Society of Economic Geologists, Inc. Economic Geology, v. 106, pp. 653–666
The Origin and Evolution of Mineralizing Fluids in a Sediment-Hosted Orogenic-Gold Deposit, Ballarat East, Southeastern Australia* A. M. FAIRMAID,† M. A. KENDRICK, D. PHILLIPS, AND FU B School of Earth Sciences, University of Melbourne, Victoria, Australia 3010
Abstract The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (~4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 × 10–3 and 2.9 × 10–3 and I/Cl values between 150 × 10–6 and 500 × 10–6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have 40Ar/36Ar ratios ranging from 322 (close to the atmospheric 40Ar/36Ar ratio of ~296) up to a maximum of 4503. 40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 × 10-4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm 40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3–2.7 ppb). Fluid inclusion 84Kr/36Ar and 130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma.
Introduction THE WESTERN SEGMENT of the Lachlan orogen in central Victoria is richly endowed with “gold-only” ore deposits, which are classified as orogenic gold deposits (Fig. 1, Phillips and Hughes, 1996; Ramsay et al., 1998; Phillips et al., 2003). These deposits form a historically significant gold province with over 2,500 metric tonnes (t) of gold produced (80 Moz; Phillips and Hughes, 1996) largely from the well-known Ballarat (408 t Au), Bendigo (697 t Au), and Stawell (82 t Au) gold fields. As with many orogenic gold provinces, considerable controversy exists as to the origin of gold in the western Lachlan orogen deposits. Suggested sources include the underlying Cambrian volcanic rocks (Sandiford and Keays, 1986; Gulson et al., 1988; Willman et al., 2010), surrounding Paleozoic sediments (Wood and Large, 2007), fragments of Proterozoic(?) continental crust that may underlie some parts of the region (e.g., Cayley et al., 2002; Miller et al., 2005) and magmatic intrusions (Bierlein and McKnight, 2005). Both the sediments and volcanic rocks are reported to contain sufficient gold such that either lithology could arguably produce the total endowment suggested for the western Lachlan orogen (Keays, 1987; Cox, 1991b; Wood and Large, 2007; Large et al., 2009). † Corresponding author: e-mail, [email protected]
*A digital supplement to this paper is available at
Despite the uncertainty surrounding the source of gold, most authors agree that metamorphic fluids were responsible for scavenging the gold and transporting it to higher levels in the crust during the Ordovician-Silurian. The Ballarat deposits exhibit features that are typical of orogenic gold deposits worldwide: (1) gold mineralization formed during or shortly after regional metamorphism/deformation; (2) gold mineralization and alteration are strongly controlled by local structures, with gold sited in second- or third-order fold hinges, stockwork veins, and breccia zones (Groves et al., 1998, 2003); (3) quartz veins are associated with carbonate, chlorite, white mica, and albite in carbonate and potassic alteration zones that occupy large bleached haloes, tens of meters in width; (4) gold is sited in quartz veins associated with pyrite and/or arsenopyrite; and (5) quartz veins contain low salinity H2O-CO2 fluid inclusions. One notable feature of the Ballarat deposits is that the latest phase of gold mineralization is associated with sphalerite and galena, indicating the involvement of base metal (Pb-Zn)-rich fluids, possibly derived from regional sedimentary basins (e.g., Carpenter et al., 1974; Hitchon, 2006). The Ballarat East deposit (the largest of three deposits within the Ballarat gold fields) represents an ideal location to investigate the origin and evolution of hydrothermal fluids associated with orogenic gold mineralization because it is relatively young (Ordovician-Devonian), lacks significant overprinting by later metamorphic events, and fresh samples are
Submitted: August 19, 2010 Accepted: February 24, 2011
FAIRMAID ET AL.
BENDIGO ZONE AF
Ballarat Goldfield (Fig. 2)
Lachlan Fold Belt
Mantle Continental crust - Australian Craton Continental crust - Selwyn Block Interlayered mafic and sedimentary rocks Proterozoic-Cambrian volcanic rocks Cambrian volcanics
Moornambool Metamorphic complex Cambrian turbidites Ordovician turbidites (Bendigo Zone) Ordovician-Devonian turbidites (Melbourne Zone) Early Devonian granites Late Devonian granites Cenozoic basalt outline (Newer Volcanic Group)
Gold deposits Epizone/anchizone boundary MF - Moyston Fault CB - Coongee Fault LF - Landsborough Fault PF - Percydale Fault AF - Avoca Fault CaF - Campbelltown Fault MuF - Muckleford Fault WF Whitelaw Fault HFZ - Heathcote Fault Zone
FIG. 1. Geological map and cross section of the western Lachlan orogen of southeast Australia showing the major gold fields, including the Ballarat East deposit. Modified from Miller et al. (2006) and Cayley et al. (in press). The Ballarat gold field is projected onto the cross section but is situated south of the actual seismic transect, which is defined by A-A' and B-B'.
available from underground mining. Constraining the origin and evolution of gold-related hydrothermal fluids is fundamental to gold exploration models because the various metamorphic, sedimentary-syngenetic, and magmatic models proposed suggest different vectors for targeting high grade gold deposits (e.g., Goldfarb et al., 2001; Groves et al., 2003; Large et al., 2009). The current study uses noble gases and halogens as fluid tracers to reevaluate the origin and evolution of gold-related 0361-0128/98/000/000-00 $6.00
fluids in the Ballarat gold fields. The noble gases (40Ar, 36Ar, 84Kr, 130Xe) and halogens (Cl, Br, I) can constrain the source of fluids because they are transported with the major volatiles (H2O and CO2) and have elemental or isotopic ratios that vary by orders of magnitude between different reservoirs. Meteoric fluids and seawater have 40Ar/36Ar ratios equal to the atmospheric value of ~296, whereas ancient basement rocks and mantle-derived fluids have 40Ar/36Ar values of >10,000 to 40,000 (Kendrick et al., 2001a, Ballentine et al., 2002; Ozima
ORIGIN AND EVOLUTION OF MINERALIZING FLUIDS
and Podosek, 2002). Organic-rich sediments can have high concentrations of atmospheric noble gas as well as the organophilic halogens Br and I (Ozima and Podosek, 2002; Muramatsu, 2007). As a result, fluid-rock interactions can be monitored via increases in fluid inclusion noble gas concentrations, changes in Cl/36Ar ratios (Kendrick et al., 2006a, in press) and/or Br and I released from organic-rich shales during metamorphism (Muramatsu and Wedepohl, 1998). Finally, fractionation of noble gas elemental ratios (e.g., Kr/Ar or Xe/Ar) or 40ArE/Cl1 values is expected during phase separation and could potentially provide information on fluid processes such as H2O-CO2 immiscibility that are sometimes suggested as a trigger for gold mineralization (e.g., Wilkinson and Johnston, 1996). Regional and Local Geology The western Lachlan orogen is dominated by a thick package of greenschist-facies, metasedimentary shales and sandstones, which have a postdeformational thickness of ~10 to 15 km. These sequences were deposited as turbidites in the Early Ordovician (490–450 Ma; Cas and Vandenberg, 1988) onto Cambrian ocean floor volcanics and subsequently deformed and metamorphosed during orogenic events in the Late Ordovician to Devonian. The western Lachlan orogen is subdivided into three structural zones; the Stawell, Bendigo, and Melbourne zones (Fig. 1), which are bounded by major north-south–trending, west-dipping reverse faults (e.g., Cox et al., 1991a; Gray and Willman, 1991; Fergusson and Coney, 1992; Foster et al., 1996; Gray, 1997; Gray and Foster, 1998). These major fault zones contain exhumed fragments of the Cambrian volcanic sequences, and their presence in the basement beneath the region is also inferred from seismic data (Cayley et al., in press). Granitic plutons, intruding the sedimentary rocks, have Early to Late Devonian ages (400–360 Ma; Richards and Singleton, 1981; Gray, 1988) and outcrop across the entire western Lachlan orogen. The Ballarat East gold field (Fig. 2) is located in the southern portion of the Bendigo structural zone of the Western Lachlan orogen in central-western Victoria (Fig. 1) and shares many similarities with the well-known Bendigo gold field. In common with gold fields elsewhere in the region, the Ballarat gold field occurs in close proximity to a major west-dipping fault (the Avoca fault), which separates the Stawell and Bendigo zones (Fig. 1; Gray, 1988). The host rocks are more tightly folded than at Bendigo, and the folds are upright to overturned and asymmetric, with steeply west dipping axial surfaces (Boucher et al., 2008). Gold has a coarse nuggetty form and is hosted by large fault-related quartz veins. The gold-bearing quartz veins (known as reefs or spurs and historically termed “Leather Jackets”) are stacked in arrays associated with west-dipping reverse faults (Baragwanath, 1953) and tension veins showing crack-seal textures (Forde and Bell, 1994). The primary mineralization target is the intersection of vertical shear zones (which develop in slate beds or on the margins of large rigid sandstone bodies) with reverse faults.
1 40Ar is defined as excess 40Ar not derived from the atmosphere or by in E situ radiogenic decay of 40K.
Gold mineralization at Ballarat East can be classified into three main paragenetic stages: (1) early quartz-veins associated with large pyrite-arsenopyrite haloes surrounding the orebody (Bierlein et al., 1999); (2) quartz-carbonate veins that contain sphalerite, galena, and chalcopyrite; and (3) quartz veins with fractures and stylolites associated with reactivation of reverse faults (Osborne, 2008) that contain sphalerite, galena, and pyrite. Posttectonic granitic rocks that outcrop less than 20 km from the Ballarat area include the Mount Egerton and Gong Gong granodiorites. These granodiorites are classified as metaluminous, I-type granitic rocks (Chappell et al., 1988; Taylor et al., 1996), and have K-Ar ages of ca. 360 Ma (Richards and Singleton, 1981) indicating that they postdate the bulk of gold mineralization. In addition, felsic porphyritic dykes intrude the gold fields at the nearby Ballarat West deposit and have been dated at ca. 370 Ma (Arne et al., 1998; Bierlein et al., 1999). Previous 40Ar/39Ar studies (Foster et al., 1996; Arne et al., 1998; Foster et al., 1998; Bierlein et al., 2001) have suggested that gold mineralization occurred in the Bendigo zone at ~440 Ma. The age of mineralization at the Ballarat East deposit itself, however, is less well constrained with 40Ar/39Ar data suggesting that gold mineralization occurred episodically between 455 and 375 Ma (Bierlein et al., 1999, 2001). However, structural relationships within the Ballarat East deposit are consistent with a 440 Ma age for the main phase of gold mineralization (D. Osborne, pers. commun., 2010), which is supported by more recent 40Ar/39Ar age data (Fairmaid et al., in prep.). Sample Descriptions and Fluid Inclusion Characteristics Twelve samples of ore-related hydrothermal quartz and two carbonate samples were selected from various depths and locations within the Ballarat East mine (Table 1, Fig. 2). The Woah Hawp (WH) and Prince (PRD) declines both intersect high-grade ores containing up to 5 and 17 g/t Au, respectively (D. Osborne, pers. commun., 2010), in which the gold is situated in structurally controlled quartz veins predominantly related to movement on reverse faults. The samples were selected to cover a broad range in quartz vein type and sulfide mineralogy with respect to the stages defined above (Table 1). Doubly polished fluid inclusion wafers (120–200 µm thick) were prepared and characterized in detail prior to noble gas and halogen analysis. The quartz preserves isolated fluid inclusions with a primary origin. Dynamic recrystallization (e.g., subgrain formation) is limited to grain boundaries and is only present in a few of the samples. Individual fluid inclusions were analyzed using a Linkham FTIR 600 heating-cooling stage coupled with a Nikon transmitted light microscope. Microthermometric data is presented for quartz only because fluid inclusions in carbonate (from samples WH189S/09 and WH218S/17), although abundant, were too small to obtain exact measurements. Most fluid inclusions are either aqueous (H2O) or mixed (H2O-CO2), but rare CO2-only fluid inclusions are present in some samples (Table 1). The abundant aqueous inclusions are similar to gold-related fluid inclusions across the entire region (e.g., Cox et al., 1991b; Gao and Kwak, 1995; Changkakoti et al., 1996; Mernagh, 2001), and are typical of orogenic gold
FAIRMAID ET AL. 750 000 mE
BALLARAT WEST BALLARAT EAST GOLDFIELD GOLDFIELD
Quaternary Lower Ordovician
Newer Volcanic Group Castlemaine Supergroup ‘Big sandstone’
‘Little sandstone’ ‘Amalgamated sandstone’ Urban development
Scand inavia n Antic line
ells U Bign
F orse Pack H
ive F Beeh
th F rF ie p a harles N
rms F lian A Austr a
F le ha W ue Bl
quartz ‘reef’ ‘leather jacket’
First C hance Anticli ne
AT ANT IC
N AN ALBIO
TICLI NORI UM
Old hard-rock mine Fault Sample locality Mine Access Drive
WH189S WH218S PRD385 PRD405 0
km FIG. 2. Geological map of the Ballarat East region showing the Ballarat East and Ballarat West gold fields. The cross section A-A' shows the mine decline with sample localities (courtesy of Lihir Gold Limited). The prefixes “WH” and “PRD” indicate the Woah Hawp and Prince Decline followed by the approximate depth from surface in meters. A representative cross section (B-B') shows the observed relationships between the main stratigraphic units, major faults, and quartz veining (modified from Boucher et al., 2008).
deposits elsewhere (e.g., Ridley and Diamond, 2000; Groves et al., 2003). The microthermometric properties of the fluid inclusions associated with different stages of vein growth, alteration style, and mine location are similar. Therefore, the fluid inclusion characteristics are representative of the timeaveraged compositions of fluids associated with gold mineralization and alteration throughout the mine. Two-phase aqueous fluid inclusions were identified in all quartz samples and occur as isolated groups of primary and/or 0361-0128/98/000/000-00 $6.00
pseudosecondary fluid inclusions. Secondary fluid inclusions (trail-bound) are also present, but are extremely small (