Carbonate_spots_understanding_the_relati.pdf

Miner Deposita (2009) 44:205–219 DOI 10.1007/s00126-008-0209-z

ARTICLE

Carbonate spots: understanding the relationship to gold mineralization in Central Victoria, southeastern Australia Allison L. Dugdale & Christopher J. L. Wilson & Lawrence D. Leader & Jamie A. Robinson & L. Jonathon Dugdale

Received: 27 August 2007 / Accepted: 2 September 2008 / Published online: 8 October 2008 # Springer-Verlag 2008

Abstract Historically, carbonate spots have been identified as an indicator of gold mineralization throughout central Victoria, Australia. However, the exact timing relationships between the growth of carbonates, development of deformation fabrics, and the introduction of gold has only been determined in more recent times through isolated studies on individual gold deposits. Detailed examination of the evolution of hydrothermal alteration associated with the Magdala gold deposit at Stawell recognized the fact that there were at least two generations of carbonate growth, an early rounded ankerite phase that predated gold mineralization and a later euhedral siderite phase coincident with gold mineralization. This pattern of carbonate growth is repeated in the majority of significant gold deposits, including Bendigo and Ballarat, throughout central Victoria. Timing relationships within the carbonates suggest that

a fluid was introduced along bedding planes and early deformation fabrics prior to the main upright folding events that significantly modified the original sedimentary basin. It is suggested that the early rounded carbonates may have formed as a result of anaerobic oxidation of methane, derived from the sediments and advected along normal growth faults within the sedimentary basin, through interaction with downward diffusing seawater sulfate. Although the growth of the early carbonates is not related to gold mineralization, the change in the speciation of the carbonate during the later carbonate event is critical and can be tracked using a simple geochemical index that can be used not only in areas of outcrop but also in conjunction with exploration undercover. Keywords Hydrothermal alteration . Carbonate . Gold . Central Victoria

Editorial handling: A. Boyce A. L. Dugdale (*) Ballarat Goldfields Pty Ltd, 10 Woolshed Gully Drive, Ballarat, Victoria 3350, Australia e-mail: [email protected] A. L. Dugdale : C. J. L. Wilson : L. D. Leader School of Earth Sciences, The University of Melbourne, Melbourne, Victoria 3010, Australia J. A. Robinson CSIRO Exploration and Mining, Australian Resources Centre, Bentley, Western Australia 6102, Australia L. J. Dugdale Lion Selection Group, Queen St, Melbourne 3000, Australia

Introduction The Victorian gold fields (Fig. 1) are one of Australia’s most important gold mining regions with over 2,500 t of gold produced since 1851 (Phillips and Hughes 1996). Historically, exploration has followed traditional methods in regions where Cambrian and Palaeozoic ‘turbidite’ or ‘slate belt’ rocks host confirmed orogenic gold deposits. However, exploration for new deposits under cover in central Victoria is difficult, although the prize for persistence can be great (e.g., Bendigo ∼1,900 kg Au produced; Willman and Wilkinson 1992). The majority of known deposits are structurally controlled with dilation associated with reverse faults, extensional fracture arrays and saddle reefs particularly adjacent to anticlines (Cox et al. 1991).

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New Zealand, Craw 2002) and is recognized distal to mineralization. This is particularly true within greenschist facies metamorphic terranes, where the deposits share numerous characteristics (e.g., elemental associations, alteration assemblages, and ore fluid properties) and are likely to originate from a common source. Kerrich and Cassidy (1994) argued that many orogenic gold provinces are formed close in space and time within accretion and metamorphism of the host terranes. Similarly, Kerrich and Ludden (2000) argue that, in such a metallogenic province, there will be brines rich in CH4 that penetrate the basement during an extensional phase. An examination of modern deep sea basins (e.g., Derugin Basin and Santa Barbara Basin) shows that carbonates commonly occur proximal to cold seep sites where they form acicular botryoids (Greinert et al. 2002; Eichhubl et al. 2000). The mechanism of formation for these authigenic carbonates is thought to involve the interaction between methane-bearing fluid, either biogenic or thermogenic in origin, that is advected along fault structures with downward diffusing of seawater sulfate, which results in the anaerobic oxidation of the methane and the release of bicarbonate and sulfide into the pore water (Irwin et al. 1977; Welhan 1988; Hinrichs et al. 1999; Whiticar 1999; Boetitus et al. 2000). Is it possible that some of the carbonate spots spatially associated with gold mineralization in central Victoria are the result of cold seep methane advection during the rapid deposition of turbidite sediments in the Ordovician? If so, how can we distinguish between these carbonates and identify which carbonate phase is directly related to gold mineralization? Could the

The use of geographical information systems (GIS) software in gold exploration and spatial modeling techniques (Miller et al. 2006; Rawling et al. 2006) provide relatively new approaches for assessing the spatial relationship between known deposits and individual parameters of a deposit model (e.g., favorable lithology, alteration, and tracing exposed structural trends under cover using aeromagnetic and gravity imagery). The target area using this approach will be large enough to encompass an entire goldfield. However, the surficial projections of the majority of individual deposits (Fig. 1) within a field have dimensions of less than 2 km2. Therefore, more localized indicators to mineralization are necessary to reduce the target size. Historically, carbonaceous “indicator” beds were thought to indicate the presence of gold (e.g., Whitelaw 1911; Baragwanath 1923), although more recent research has shown that these indicators do not necessarily point to the presence of economic gold (Bierlein et al. 2001a). So-called carbonate porphyroblasts or poikiloblasts, commonly known as spots, are ubiquitously associated with central Victorian gold deposits (Sandiford and Keays 1986; Binns and Eames 1989; Cox et al. 1991; Forde 1991; Phillips and Hughes 1996; Bierlein et al. 1998, 2000; Dugdale et al. 2006). The occurrence of early carbonate spots is well known in other turbidite-hosted orogenic gold metallogenic provinces (e.g., Dolaucothi goldfield, Wales, United Kingdom, Annels and Roberts 1989; Hill End goldfield, NSW, Australia, Windh 1995; Moose River goldfield, Meguma Terrane, Nova Scotia, Canada, Ryan and Smith 1998; Reefton goldfield, Buller Terrane, New Zealand, Christie and Brathwaite 2003; Macraes goldfield,

Fig. 1 Distribution and location of principal orogenic gold deposits with carbonate spots within Victoria

1 2 3 4 5 6 7 8 9

Avoca Ballarat Beufort Bendigo Clunes Creswick Drummond-Lauriston-Taradale-Malmsbury Dunolly Fosterville

20

10

18 7

6 2

Bendigo Zone

Melbourne Zone

lt

Stawell Zone 3

5

au rF

11

no ve

12 1

14 4 9

Go

16

17

Heath

8

cote

19 15

Moyston Fault

Maldon Maryborough Percydale Pitfield Raywood St Arnaud Stawell Tarnagulla Wattle Gully Wederburn Kewell

Fault

Avoca Fault

Coongee Fault

10 11 12 13 14 15 16 17 18 19 20

Melbourne Extent of outcrop

13

100 Km

N

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distinction between these carbonates provide a simple tool to aid in the exploration for turbidite-hosted gold deposits under cover? The purpose of this paper is to address these questions through the examination of structural timing relationships and geochemistry of spots, using current research from the Stawell and Bendigo Zones and previous published research on other deposits within central Victoria.

Regional setting of the gold deposits The majority of gold deposits in central Victoria are located in the Bendigo Zone (Fig. 1), within the Lachlan Orogen, and are hosted by the Ordovician Castlemaine Group that comprises mudstone, siltstone, sandstone deposited as a turbiditic sequence (Gray et al. 2003). Gold deposits in the Stawell Zone are hosted by Cambrian St Arnaud Group turbiditic sedimentary rocks similar to the Castlemaine Group in the Bendigo Zone. However, deposits west of the Coongee Fault (e.g., Stawell; Fig. 1) are hosted by similar sedimentary rocks that have been geochemically modified in response to seawater interaction with underlying hot basaltic edifices (Squire and Wilson 2005; Dugdale et al. 2006). The Cambrian aged sediments (Squire and Wilson 2005) within the Stawell Zone have been deformed by both the Delamerian and the Lachlan orogenies (Miller et al. 2005). The Delamerian Orogeny resulted in the main ductile (D1– D3) and initial brittle (D4a–b) deformation (Miller et al. 2006), and the Lachlan Orogeny is manifested by brittle deformation (D4c–D5; Miller et al. 2006). Regional metamorphism in the Stawell Zone ranges from lower greenschist in the east to upper amphibolite facies on the western margin abutting the Moyston Fault (Fig. 1) and is pre- to syn-D2 (Miller et al. 2005). An early foliation (S1) that is subparallel to bedding is pervasive in the Ordovician sequences of the Lachlan Orogen and has been identified by numerous researchers, e.g., Powell and Rickard (1985); Wilson and de Hedouville (1985); Wilson et al. (1992); Forde and Bell (1994); Schaubs and Wilson (2002); Gray et al. 2003; and Willman (2007). Subsequent to this, the main deformation event (D2) in the Bendigo Zone (Fig. 2) occurred between 455 and 425 Ma (Bucher et al. 1996; Foster et al. 1996) and produced the main folding (F2), faulting and cleavage (S2) development and associated subgreenschist to lower greenschist facies metamorphism (Offler et al. 1998; Fig. 2). The mid-Devonian (385–380 Ma; Foster et al. 1999) Tabberabberan Orogeny produced kink folds and minor crenulation cleavage (S3) throughout the Bendigo Zone (VandenBerg et al. 2000). Gold mineralization in deposits of Bendigo and eastern Stawell Zone is primarily restricted to quartz veins of which

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there are five main types: subhorizontal (spurs); fault reefs (leather jacket or legs); saddle reefs; laminated veins and breccia veins, all of which postdate the D2 cleavage development (Cox 1987, Cox et al. 1991; Willman and Wilkinson 1992). In the western portion of the Stawell Zone, gold occurs in tensional quartz vein arrays, reactivated laminated quartz veins, and disseminated in association with sulfides (Miller and Wilson 2002; Dugdale et al. 2006). Mineralization in the Stawell Zone also post-dates D3 ductile deformation and is related to the Lachlan Orogeny. Bierlein et al. (2001b) suggested that there were three periods of significant gold mineralization within the Bendigo and Stawell Zone, although the youngest event ∼370 Ma is attributed to gold remobilization (Fig. 2). The earliest gold event ∼440 Ma resulted in the formation of the larger gold deposits (e.g., Bendigo, Ballarat, Stawell) and the second and third events 425–400 and 370 Ma respectively, produced generally smaller tonnage mineralization (e.g., Wonga, Tarnagulla, Percydale, and Fosterville). The sedimentary sequences and their metamorphic derivatives in the Stawell Zone are dominated by the quartz-rich turbidites of the Cambrian St Arnaud Group west of the Avoca Fault that were deposited in a sedimentary basin developed by rifting of the Gondwana craton and its margin in the Early Cambrian (Squire and Wilson 2005). To the west and east of these basins, major faults are inferred to flank major continental basement blocks, as imaged by geophysical data sets (Murphy et al. 2006), and there appears a spatial coincidence in the depositional style of the turbidite sediments that may be a product of the variable basin-margin configuration and its subsequent reactivation (Miller et al. 2006). Convergence during the Delamerian Orogeny along part of the western continental margin in the Late Cambrian resulted in the inversion of preexisting normal faults and the development of a fold and thrust belt verging toward the craton (Flöttmann and James 1997; Miller et al. 2005). On the eastern margin of the Bendigo Zone, the Lachlan orogenesis is characterized by east vergent structures that are probably related to the reactivation of the Heathcote Fault (Miller et al. 2005). The total thickness of the St Arnaud Group is >2,000 m thick, whereas, the total thickness of the Ordovician turbidite succession in the Bendigo Zone is in excess of >3,000 m (Squire et al. 2006). These sedimentary basins are underlain by basalts that can be related to an extensional event that affected the entire proto-Pacific margin of East Gondwana and was accompanied by rapid deposition of a massive volume of quartz-rich detritus (Squire et al. 2006). The onset of the ∼440 Ma east–west compression in the Lachlan Orogen shortened the Cambrian–Ordovician sedimentary basins and their basement between the Selwyn Block and the Australian Craton to the order of 70% (Cayley et al. 2002).

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STAWELL ZONE Fiddlers Reef & St Arnaud

FAULT

Au

Au

St Arnaud

Beds

460

Au Au

M

Au M

M

M

M

470

Castlemaine 480

DELARMERIAN

Au

WHITELAW

Au

Au

450

ORDOVICIAN

Au

Au

430

440

LACHLAN

x x

Au

410

420

Tarnagulla Fosterville

FAULT

x x

Ballarat East Ballarat West Bendigo & & Wattle Gully Clunes

FAULT

400

COONGEE

SILURIAN

DEVONIAN

390

AVOCA

TABBERABBERAN

Stawell

BENDIGO ZONE

Group

490

500

x x

Au

M

M

Granite intrusions Gold mineralization Carbonate spots

Commencement of ductile deformation

M

Regional metamorphism Brittle deformation

Fig. 2 Schematic summary of stratigraphy, deformation, mineralization and hydrothermal alteration for the Stawell, St Arnaud, Percydale, Bendigo, Ballarat East and West, Clunes, Tarnagulla, and Fosterville orogenic gold deposits

Distribution, timing, and composition of carbonate spots In the majority of the central Victorian gold deposits, carbonate spots extend less than 50 m into the surrounding country rock. However, in a minority of cases (e.g., Percydale and Bendigo), carbonate spots can define a 1-km wide envelope surrounding mineralization. Documented timing relationships and composition of carbonate spots from various gold deposits within the Stawell and Bendigo Zone are presented to establish the relationship to gold mineralization.

Stawell zone Stawell Carbonate spots occur in the fine-grained metasandstone and mudstone of the Albion Formation (Squire and Wilson 2005) in a 50- to 70-m wide zone in the hanging-

wall to the main reactivated laminated quartz vein (Central Lode, Fig. 3). At least two generations of carbonate spots occur at Stawell, which comprise: (1) rounded grains, which accumulate along bedding (S0) and microfractures and also occur parallel to the S2 cleavage but are rotated in the S3 cleavage; and (2) euhedral crystals that occur as isolated rhombohedra and as strain shadows to the rounded grains. The initial development of the Central Lode structure during D2 is interpreted to have been the conduit for the hydrothermal fluid that promoted the growth of the carbonates within essentially carbonate poor turbiditic sediments (Dugdale et al. 2006). The rounded grains and euhedral crystals distal to Central Lode both comprise ankerite. However, within 30 m of Central Lode, carbonates display compositional zonation and are pseudomorphed by siderite (Dugdale et al. 2006). Formation of the main Central Lode quartz vein occurred during D3, and reactivation during

Miner Deposita (2009) 44:205–219

209

t ou ing n t t o gi sp ottin e t p na te s a rbo ca rbon ca

Leviathan Formation (pervasively muscovite altered)

Albion Formation (pervasively muscovite altered)

N

t ou te ori l n h i c rite chlo

Ce

ntr

al

lod

e

Stawell Facies

ta

on

C alt

Stawell Facies

de

lo ct

s

Ba

SD607 SD606W2

ne

50 m

Albion Formation

Magdala Basalt

m no

ilp

st

110

SD598CW1

a el

SD598CW2

basalt SD 606

Fig. 3 A cross-section through the Stawell gold deposit showing the location of the main gold-bearing structures (Central Lode and Basalt Contact Lode), diamond drill hole traces, interpreted geology, the distribution of carbonate spots and the distribution of alteration of the hydrothermally altered turbidites (Stawell Facies) and the muscovite alteration of the overlying Albion and Leviathan Formations (after Dugdale et al. 2006)

Kewell Basalt

< 110

< 110

Projection of resource

D4 coincided with introduction of a gold-bearing fluid (Miller et al. 2006). Siderite overgrows the S3 cleavage and hence is related to gold mineralization with the reactivation of Central Lode during D4 (Fig. 2; Dugdale et al. 2006).

A

A'

Leviathan Formation

Kewell The Kewell prospect is located approximately 100 km north of Stawell on the northern extension of the Stawell stratigraphy under the Tertiary Murray Basin sediments (Fig. 1). The geology of the Kewell area is similar to Stawell in terms of lithologies, structure, and timing of gold mineralization and contains a scoped resource that is estimated to be 500,000 T at 6 g Au/t for 3,000 kg (Fig. 4). Rounded carbonate spots predominantly occur within the hydrothermally altered sedimentary rocks (Stawell Facies) adjacent to the Kewell basalt, although these spots are invariably replaced by siderite in association with stilpnomelane and pyrrhotite. Spots however, do extend up to 10 m within the mudstone-dominated upper Albion Formation, and the carbonate spots in this vicinity are elongated within the S2 slaty cleavage (Grewar 2004). Hence, the timing of carbonate spots within the Albion Formation is pre-S2. Percydale The occurrence of carbonate spots in the Percydale gold field forms an extensive envelope to known mineralization (Fig. 5). Sandiford and Keays (1986) noted that spots extended up to 1 km across strike from mineralization at Fiddlers Reef mine. Carbonate spots also occur proximal to

120

500 m

Fig. 4 A plan view of the interpreted geology of the Kewell Prospect, derived from aircore and diamond drill holes beneath the Tertiary Murray Basin sediments (the depth of which is contoured in 10 m intervals) and showing the location of cross-section A-A’ (Fig. 12)

the Poverty and Fiddlers Faults (Fig. 5). Two generations of carbonate occur within the metasedimentary host rocks and comprise rhombohedral ankerite spots that become overgrown by Fe- and Mg-rich carbonate which contributes to a rounded appearance, proximal to mineralization. Marek (1997) shows that even though the rounded spots are elongated within the S2 cleavage, the calcium-rich cores preserve the S1 cleavage. Figure 2f in Bierlein et al. (2000) shows a carbonate spot that is interpreted to have been rotated and is elongated parallel to external cleavage (S2, NNW–SSE), which in turn has been crenulated by a nonpervasive cleavage S3 (N–S). An internal NE–SW

210

Miner Deposita (2009) 44:205–219

ult Fa

le da rcy Pe

N

N

Laminated siltstone + mudstone

10 30 40 30 10

extent of carbonate spots

10 10

Fiddlers Reef Mine

B-B' 10 30

10 40

30 40

Sandstone

40

40

ult

rty

ve Po

Fa

40

rs

ult

dle

Fid

Fa

Mudstone

40

40 40

2 km

30

mine

10

Glenfine Mine 10 30

Fig. 5 Geological plan of Percydale gold field showing location of gold workings, principal faults, and the extent of carbonate spots (after Marek 1997)

A-A' 30

Cambro-Ordovician sedimentary rocks

30

asalt

40

drill holes 40

Pitfield B

cleavage is preserved within the rounded Ca-rich core. Sandiford and Keays (1986) suggested that the carbonate spot development accompanied reef formation. However, main quartz vein development in the Fiddlers Reef mine postdates S2 (Marek 1997) and clearly postdates the development of the carbonate spots; hence, carbonate spots developed prior to gold mineralization (Fig. 2). St Arnaud Two generations of carbonate spots have been identified spatially associated with mineralization in the St Arnaud gold field and comprise euhedral to subhedral carbonate cores that preserve an internal cleavage. These are overgrown by a poikiloblastic generation of carbonate (Krokowski De Vickerod et al. 1997). Motton (1990) noted that the internal cleavage within the early cores was discordant with the main regional S2 cleavage that may suggest that the cores had been rotated (Fig. 2). Krokowski De Vickerod et al. (1997) interpreted that the carbonate spots precipitated from metamorphic fluids accompanying the growth of D3 and D4 faults, during formation of the quartz reefs.

Fig. 6 Geological plan of the Pitfield area, derived from aircore and diamond drill holes beneath the Tertiary basalt, the depth of which is contoured, showing the location of previous gold workings including Glenfine and cross-section traverses A-A’ (Fig. 8) and B-B’ (Fig. 13)

Pitfield Gold mineralization at Pitfield is hosted by CambroOrdovician sedimentary rocks, similar to the Albion Formation at Stawell, adjacent to Cambrian tholeiitic basalt under a blanket of Tertiary basalt, of variable thickness, and recent colluvium (Fig. 6; Morand et al. 1995). Deformation in the

Pitfield area comprises at least two coaxial ductile events and the preservation of early mesoscopic folds within transposed layering (Fig. 7). Hydrothermal alteration associated with mineralization at the Glenfine Prospect (Fig. 6) comprises

60 60

500 m

Miner Deposita (2009) 44:205–219

211

Bendigo Zone

S2a/S2b

Quartz vein folded into F2b

5 cm

Fig. 7 Photograph and sketch of diamond drill-core from the Glenfine area showing transposition of folded quartz veins into the S2a/S2b foliation

pervasive muscovite, the development of carbonate spots and wisps up to 50 m from mineralization and chlorite growth proximal to mineralization (Fig. 8). Mineralization is hosted solely within bedding parallel laminated and discordant tensional quartz veins that postdate S2b. Carbonate spots at Glenfine overgrow the crenulated and transposed S2a fabric but are wrapped by the S2b fabric (Fig. 9a,b).

A

A'

PFD004

PFD021 0

PFD005 PFD020 PFD010 0

0

0

0

0.6 m @ 5.43 gAu/t mRL100

1m@ 13.9 gAu/t

mRL0

0.3 m @ 11.1 gAu/t

0.6 m @ 1.57 gAu/t

50 m

Tertiary basalt

Chlorite

Cambro-Ordovician sedimentary rocks

Graphite

Cambrian basalt

Carbonate wisps

Carbonate spots

Ballarat Carbonate spots occur up to ∼60 m from quartz vein hosted mineralization at Ballarat East and comprise two generations of carbonate. The early generations are rounded Fe- and Mg-rich carbonate elongated in the S2 cleavage with cores of quartz, pyrite, and kaolinite (Besanko 1996). The second generation are rhombohedrons of calcite to dolomite composition that exhibit preferential growth along the S2 cleavage without any elongation (Besanko 1996). Gold mineralization at Ballarat East is associated with faults that postdate D2 and D3 and (Taylor et al. 1996), therefore, postdate the development of the carbonate spots (Fig. 2). At Ballarat West, carbonate spots are strongly associated with pervasive sericite alteration, and their intensity increases with decreasing distance to the lode (Bierlein et al. 1998). Two generations of carbonate are present: (1) dolomite that is elongated within the pervasive S2 cleavage and, (2) randomly oriented siderite that overprints, rims and traverses the dolomite and forms as wisps along the S2 cleavage (Bierlein et al. 1998). Gold mineralization at Ballarat West clearly overprints the cleavage (S2), and hence, the timing of the early carbonate spotting clearly predates gold mineralization (Fig. 2). Wattle Gully Carbonate spots at Wattle Gully occur up to 50 m away from mineralization (Bierlein et al. 1998). Two generations of carbonate have been identified: (1) dolomite (plus minor calcite) spots, that are deformed and elongated along the main penetrative slaty cleavage (S2), and (2) ankerite rims the dolomite spots and overgrows the slaty cleavage (Bierlein et al. 1998). Gold mineralization at Wattle Gully postdates the development of the slaty cleavage and is localized in dilational sites along a network of reverse faults that traverse the chevron folds (Fig. 2; Cox 1995). Tarnagulla The lateral extent of carbonate spots at Tarnagulla is variable from 1 m up to 50 m from the main Poverty Reef (Cooke 1997). Only one generation of carbonate has been identified and comprises a magnesite–siderite composition (Molloy 1994). The carbonate spots are elongated along the main axial planar slaty cleavage (S2) and are overprinted by the spaced S3 cleavage (Molloy 1994). Mineralization at Tarnagulla is hosted within laminated veins formed during sinistral strike–slip reactivation on the Poverty Fault that clearly post dates D2 (Cuffley et al. 1995).

Quartz reefs

Fig. 8 Diamond drill-hole cross-section through the Glenfine area (Fig. 6) showing interpreted geology and the distribution of hydrothermal alteration about mineralized quartz reefs

Clunes Two generations of carbonate spots occur: (1) ankerite elongated within the pervasive slaty S2 cleavage and, (2) siderite, which rims the ankerite spots with atoll textures and overprints the S2 cleavage (Binns and Eames 1989; Taylor

212 Fig. 9 Photomicrographs from Pitfield (a–b) and Fosterville (c–e) with: a Carbonate spots, ankerite–siderite in composition, overprint the crenulated S2a cleavage but are traversed by the S2b cleavage, cross-polarized light (PFD5 133 m); b carbonate spots, ankerite–siderite in composition, dislocated by strong S2b cleavage, plane polarized light (PFD5 133 m); c Ankerite spots traversed by the main penetrative cleavage, plane polarized (DD32); d Ankerite spots overprint the early bedding parallel cleavage and dismembered by main penetrative cleavage, plane polarized (DD18B); and e Ankerite also occurs within fractures that parallel and cross-cut the main penetrative cleavage, plane polarized (DD09)

Miner Deposita (2009) 44:205–219 ankerite spots ankerite spots

S2b S2b S2a

a

0.30 mm

0.30 mm

b

S2 ankerite spots

ankerite spots

c

S2

e et al. 2000). Mineralization at Clunes transgresses mesoscale folds, associated axial planar cleavage (S2), and locally developed crenulation cleavage (S3; Taylor et al. 2000). Bendigo At least two generations of carbonate spots have been documented at Bendigo: (1) zoned subrounded and (2) euhedral zoned crystals (Swann 2002). The subrounded spots overprint the penetrative S2 slaty cleavage and are elongated within this cleavage, therefore the timing of carbonate precipitation is pre- to syn-S2. Compositional analysis of these carbonates by Swann (2002) revealed cores of ankerite–dolomite with rims of siderite. The euhedral zoned carbonate crystals only occur proximal to mineralization and clearly overprint the S2 cleavage; therefore, the timing of this carbonate deposition is postS2. Swann (2002) showed that the composition of these carbonates is siderite which is identical to that within the rims of the rounded spots. This is however, the reverse of the timing relationship suggested by Li et al. (1998); even though Fig. 11 of Li et al. (1998) shows siderite, associated with gold mineralization, concentrated in the core of the anticline at Nell Gywnne, rimmed by siderite with ankerite as the outermost carbonate.

S1 0.25 mm

d

S2 0.25 mm

ankerite/siderite filled fractures

0.25 mm

Fosterville Carbonate spots at Fosterville are limited to the finer grained lithologies and comprise spots up to 75 μm in diameter, which increase in abundance toward mineralization and persist to a distance of 25–50 m from mineralization (Kwak and Roberts 1996; Bierlein et al. 2000). Contrary to the observations of Bierlein et al. (2000), there is a cleavage that predates the upright penetrative S2 cleavage (Fig. 9c). Rounded carbonate spots postdate the early cleavage but predate the main penetrative cleavage (Fig. 9d). A second generation of carbonate occurs in fractures and parallel to the main penetrative cleavage (Fig. 9e). Quantitative SEM analyses of the carbonate spots indicates compositions from ferroan dolomite to dolomite, although zoned individual spots shows a trend toward more ferroan compositions from core to rim (Arne et al. 2000).

Geochemistry Visible alteration halos to the central Victorian turbiditehosted ore deposits are narrow and often defined by arsenopyrite and pyrite, both confined to an area <10 m

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213

from mineralization, and carbonate spotting, which can occur up to 70 m from mineralization (e.g., Stawell). An effective measure of the extent of carbonate development beyond the visible alteration halo is the analysis of CO2. The CO2 halo at Fosterville extends to 73 m from mineralization; at Bendigo, it extends to 45 m; at Fiddlers Reef, it extends >49 m; and at Ballarat East, it extends >60 m (Arne et al. 2000; Bierlein et al. 2000). Samples of background sediment, in general, have CO2 values of ≪1%, although exceptions to this include calc-silicate inclusions developed in the contact aureoles of Devonian granites (Morand 1994) and cone-in-cone limestone that occurs in rare thin beds conformable with the enclosing turbidites in the Bendigo area (Willman and Wilkinson 1992). However, samples adjacent to mineralization can contain up to 8% CO2 (Stüwe et al. 1988; Binns and Eames 1989; Bierlein et al. 2000). A plot of the range in CO2 values for several of the deposits within central Victoria (Fig. 10) combined with the total gold production shows that the larger deposits have average CO2 values in excess of 1%. The carbonate spots in most deposits displays a systematic change from Ca-rich to Ca-poor or Fe-rich carbonates toward the area of mineralization. A simple geochemical index that tracks changes in the carbonate speciation is a molar ratio of CO2/CaO developed by Davies et al. (1990). This index was originally defined for basalt, although it can be equally applied to turbiditic sediments that typically have minimal carbonate inclusions. An index value of ≤1 indicates the presence of calcite, and a value of between 1 and 2 is indicative of ankerite and/or dolomite. Index values >2 suggest the change from calcium-bearing to non-calcium-bearing carbonates, e.g., siderite and magnesite. Bierlein et al. (2000) provided detailed whole-rock geochemical data for many of the larger gold deposits in central Victoria. The carbonate

speciation index for Bendigo, Ballarat East, and Tarnagulla plotted against arsenic and low-level (ppb) gold is shown in Fig. 11. These figures show that areas distal to mineralization have calcite as the dominant carbonate with a gradual increase in the carbonate speciation index toward mineralization with values >2 coincident with elevated As and Au. This index has been applied in the exploration for gold deposits beneath Tertiary cover sequences in northwestern (Kewell) and western (Pitfield) Victoria. Drill samples (e.g., aircore or reverse circulation chips) were collected from fresh rock beneath the base of oxidation. Whole-rock samples from the bottom of aircore holes at Kewell and Pitfield were analyzed for a range of multi-elements including Ca via induced coupled plasma mass spectrometer (ICPMS) and CO2 via Leco furnace. Plots of the 10000

Au As CO2/CaO

Ballarat East

1000

30 25 20

100 15

10 10

1

5

0.1

0 120

100

80

60

40

20

0

Distance to mineralization (m) 4

10000

Ballarat West

3.5

1000

3 2.5

100

2

10

1.5 1

1

0.5 0

5

10

15

20

25

Distance to mineralization (m)

684 t* 0.025 t*

10

10000 56 t*

8

CO2 %

0 30

0.1

12

1.5 t*

16

Bendigo

14

1000

12 6

100

27 t *

10

10

4

8

62 t 17 t

2

6

1

4

31.5 t*

0.1 0

Ballarat West

Ballarat Bendigo East

Stawell Fosterville Fiddlers Tarnagulla Clunes Reef

2

0.01 0

20

40

60

80

100

120

140

160

180

0 200

Distance to mineralization (m)

Fig. 10 Chart illustrating the range and average (horizontal line) value of CO2% for Ballarat East and West, Bendigo, Stawell, Fosterville, Fiddlers Reef, Tarnagulla, and Clunes plus the total gold production for each of respective mines. Asterisk indicates historic production only

Fig. 11 Graphs of data (logarithmic scale) from Ballarat East, Ballarat West, and Bendigo showing the variation in carbonate index, Au ppb and As ppm with respect to distance to mineralization (m). The two horizontal light dashed lines highlight carbonate index values of 1 and 2

214

Miner Deposita (2009) 44:205–219

carbonate index across key aircore traverses, which were later tested with diamond drilling, showed significant anomalies without significant supporting Au or As in the metasediment >100 m above mineralization (Figs. 12, 13). In addition, an aircore hole above the up-dip projection of the Stawell Facies at Kewell shows coincident anomalous carbonate index, Au and As.

Discussion Timing of the carbonate spots Bierlein et al. (1998) recognized four different types of carbonate spots comprising genuine carbonate, quartz–carbonate aggregates, mica– carbonate, and pyrite–carbonate spots. However, analysis of the structural timing relationships between fabric development and carbonate spot growth from numerous previous studies shows that only two main carbonate events occurred. In the Stawell Zone, the early carbonate event occurred coincidentally with the main upright folding event (D2), whereas in the Bendigo Zone, the early carbonate event occurred prior to D2 (Fig. 2). The later carbonate event in both the Stawell and Bendigo Zones is coincident with gold mineralization (Fig. 2). The time gap between the formation of the early carbonates and gold mineralization in the Stawell Zone is in excess of 50 million years, whereas in the Bendigo Zone, this time gap is reduced to 20 million years. Therefore, the occurrence of the early carbonates is not related to gold mineralization apart for the coincidental

spatial association. However, the later carbonates that form as overgrowths and pseudomorphs of the early carbonates are directly related to gold mineralization. Formation of the carbonate spots The early carbonate spots typically comprise either rounded aggregates or euhedral crystals of calcite or ankerite. The rounded morphology of these spots is unusual for calcite or ankerite, which have a trigonal crystal lattice that will form rhombohedral crystals in an unstressed environment (Reeder 1983). This combined with the aggregated nature of these spots suggests that the calcite or ankerite has replaced a precursor mineral. The most likely precursor mineral is aragonite, which has an orthorhombic crystal lattice that forms acicular radiating crystals (Lippmann 1973). Aragonite commonly occurs proximal to cold seep sites in rapidly forming deep sea basins (e.g., Derugin Basin and Santa Barbara Basin) where it can form acicular botryoids (Agar 1990; Greinert et al. 2002; Eichhubl et al. 2000). The mechanism of formation for these authigenic carbonates is thought to involve the interaction between methane-bearing fluid that is advected along fault structures with downward diffusing seawater sulfate, which results in the anaerobic oxidation of the methane and the release of bicarbonate and sulfide into the pore water (Boetitus et al. 2000; Hinrichs et al. 1999). It is generally accepted throughout the literature that formation of the carbonate spots in central Victoria is related to an influx of CO2–H2O-bearing hydrothermal fluid, which caused the breakdown of metamorphic chlorite

100

10

1

Au ppb As ppm CO2/CaO

0.1 A 100 m RL

A'

Murray Basin Sediments saprolite

0 m RL

base of oxidation

-100 m RL

Albion Formation

1.35 m @ 5.88 Au g/t Stawell 0.75 m @ 3.38 Au g/t Facies

3 m @ 1.62 Au g/t

2

KD

05

KD

Leviathan Formation -200 m RL

016

2 02 KD 5 02 KD 51 0 D K

1.1 m @ 4.7 Au g/t -300 m RL

Kewell Basalt

Fig. 12 A geological cross-section through the Kewell Prospect (Fig. 4), showing aircore and diamond drill hole traces and key intersections of mineralization, together with a graph (logarithmic scale) showing values

200 m

for carbonate index, Au ppb and As ppm for bottom of hole samples from the aircore holes. The two horizontal light dashed lines highlight carbonate index values of 1 and 2

Miner Deposita (2009) 44:205–219

215

100 10 1 .01 .001 Au ppb As ppm CO2/CaO

.0001 .00001

B

Colluvium 0

B'

0 0

Clay

0 Clay

0

Clay Clay

Tertiary Basalt

TerB

TerB

pre

c

TerB

Grav

0.02

Qrtz

0.02

Sapr

0.02

Grav

0.04

0.00

TerB

0.01

base of oxidation

Sapr

0.00

Silt

0.02

0.01

0.04 0.02

Sapk

0.00

0.03 0.01 0.00 Sapr 50

Bas

0.02

a

0.00

PeCb

0.01

0.00

0.02

0.00 50

saprolite Grav

0.00

v Gra t Los

0.01 0.00 0.04

k Sap t Los b PeC t Los

PePb

50 PePb

50

0.08

Sapk

0.01

50

0.01

0.10

Qrtz

b PeC

0.29

Sapr

0.00

Sapk

0.00 0.01

t Los bt PeC Los

0.01

PeCb

b o ShZ b PeC

Sapk

0.01

0.01

0.00

0.00

Basa

0.01

0.01

PeC

PePb 66

b

0.02

0.01

0.02

0.01

66

PFA111

PeP

66

PFA109

PFA110

PeCb

0.01

Psam

0.01

0.01

mRL150

Sapk

b PeC

100

0.00

Peli b PeC t Los

0.07 b PeC

Lost

0.00 0.00 b 0.00 PeC 0.00 0.00 t 0.00 Qrtz t 0.00 Los Qrtz Los b 0.00 0.00 PeC 0.00 0.00 Qrtz bb 0.00 PeC o 0.00 PeP 0.00 ShZ 0.00 0.00 b 0.00 PeC 0.00

Psam

0.40

1.92

PFA112

95

Qrtz b PeC

Mudstone

0.00 0.00 0.01 0.05 0.00

Peli bb PeC PeP

PeC

b

Peli

b PeC

150 b

PeP

b PeC b PeP o ShZ

PeP

b

b PeC

b PeP

200

Peli

mRL50

Fine-grained sandstone Laminated fineand mediumgrained sandstone and mudstone b

PeP

0 0.0 0 0.0

0 0.0 0 0.0

z Qrt b PeP z b zb Qrt PeC z Qrt PeC Qrt

00 0.0 0.0 0 0.0 0 10 0.0 00 0.0 0.0 0 0.0 0.0 0.0 0 0.0 0 0.0 0 0.0 0 0.0

b PeP

3 0.0 40 0.0 0.0

Qrt

z

b PeP

250

PsP

e b

0 0.0 00 0.0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0

PeP PsP

e

b PeP

quartz veins

0 0.0 00 0.0 0.0 0 0.0 0 0.0 00 0.0 00 0.0 0.0 0 0.0 0.0 01 0.0 0.0 00 0.0 0 0 0.0 0.0 0.0 0 0.0 0 0.0

iz Pel Qrt

b PeP

ib Pel PeP i Pel

b PeP

0 0.0 0 0.0 0.0 b PeC bz PeP Qrt e b PsP b PeP o PeC ShZ

00 0 0.0 0.0 2 0.0 0.0

36 75 0.0 0.7 0.5 0 0.0

b PeP

0 0.0 0 0.0 00 0.0 0.0 6 0.0 2 0.0 6 0.0 5 0.0 2 0.0 0 0.0 0 0.0 4 0.0 76 1.4 0.0 0 0 0.0 0.0 m Psa z b Qrt PeP

300

Pel

07 0.0 0.0 2 0.0

i

b PeP

6 0.1 2 0.1 03 0.0 0.0 0 4 0.0 0 0.2 3 0.0 0 0.0 0.0 02 0 0.0 0.0 0.0 001 0.0 66 0.0 59 0.3 0.2 0.0 4.9 0.3 0 0.0 0 e 0.0 00 PsP z e 0.0 4 ze Qrt 0.0 0 PsP 0.1 Qrt PsP m 0.0 7 0.0

0.4 m @ 4.95 Au g/t 0.6 m @ 12.3 Au g/t e PsP i b Pel PeP

Psa e PsP mz Psa Qrt e PsP

Pel

i

Qrt

z

0 0.0 0 0.0 00 0.0 0 0.0 0.0 0 0 0.0 5 0.0 0 0.3 0.0 0 0.0 0 0.0 90 0.0 0.0 000 0.0 0.0 0.0 0 0 e 0.0 0.0 0 0 PsP z e 0.0 0 Qrt 0.0 PsP z 0.0 02 Qrt 730 0.0 0.0 e 0.0 12. 8 5 PsP 0.1 3 0.0 z 0.0 0 Qrt ez e 0.0 0 5 PsP z Qrt 0.0 0 PsP 0.0 0 Qrt 0.0 m 0.0

i Pel

Psa

e PsP

mRL-50

0 0.0

m Psa

0 0.0 0 0 0.0 0.0 0 0.0 0 0.0 44 5.10.8 4 1.6 8 0.1 1 350 0.2 0 0.0 0 0.0 0 0.0 0 0 0.0 0.0 1 0.1 2 m 0.0 0 0.0

e PsP

0.5 m @ 5.14 Au g/t

0 0.0 0 0.0

Psa

e PsP i Pel

0 0.0 0 0.0

00 0.00.0 0 0.0

m Psa

e PsP

m

2 0.0 0 0.0 0 0.0 0

0.0 0 0 e 0.00.0 0 PsP t m 0.0 0 Los 0.0 0 Psa e 0.0 0 PsP m Psa e o 0.0 PsP ShZ m Psa e PsP m Psa e

PsP

m Psa

e PsP

Qrt PsP Qrt ez z

PsP

e

0.4 m @ 7.38 Au g/t

0.3 0 0.0 6 0.2 0.3 9 88 4.9 0.1 92 400 0.10.0 6 2 0.00.01 6 0.0 PsP 0.2 0 0 0.0 e 0.31.310 1 0.1 Psa 0.0 Qrt 36 m 0.21.0 2 PsP z 0.0 0 8 0.0 6 e 0.0 0 0.0 0 0.0 0.0 Psa 0.0 000 m 0.0 0 Qrt 0.0 0 z 0.0 0 PsP 0.0 0 e 0.2 0 Qrt 0.0 4 z 0.0 0.0 2 0.0 0 0 0.0 0 0.0 0.0 00 0.0 0.0 0 PsP 0.0 0 0 e 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 Qrt 0.0 0 Psa 0.0 Qrt z PsP 0.0 0 m Qrt Psa 0.0 1 0 ez 0.0 PsP mz 0.7 1.6 10 0.1 40 Psa e 0.0 0.0 m 0.0 13 0.1 0.7 31 0.1 PsP 0.0 70 e 0.0 9 0.0 1 2 0.0 0 Psa 0.0 0.0 0 PsP m 0.0 0 Qrt e 0.0 0 0 z 0.0 0 PsP 0.0 0 e 0.0 0 0.0 0.0 20 Psa 0.0 0.0 03 PsP m 0.0 0 Qrt PsP e 0.0 0 0.0 Qrt ez 0 0.0 PsP z 0.0 0 0 4500.0 Psa e 0.6 0 0 PsP m 0.4 7 0.0 Psa e 0.1 73 0.0 PsP m 0.0 0 5 e 0.0 0 Psa 0.0 0 0.0 m 0.0 00 0.0 0 0.0 0 0.0 0 460 0.0 0 0

0.0 1 PeC 0.0 0 PsP Pel e h 0.0 0 Qrt 0.0 0 PsP i Qrt z 0.0 0 PsP e 0.0 0 Qrt e z PsP Qrt 0.00.00 0 z PsP Qrt ze 0.01.02 0 0.0 PsP ze 0.4 9 0.1 Qrt e 0.1 05 PsP 0.4 Qrt 0.6 z 03 0.2 PsP z e 7.3 99 0.7 Qrt 0.0 81 e PsP 0.1 95 Psa ez 1.80.6 5 m

0.0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0

0.0 0.0 0 0.0 0 1

Psa

0.4 m @ 4.99 Au g/t

PFD36

0.7 m @ 1.64 Au g/t 25 m

PF

D0

36

Metres

E730300

E730250

E730200

E730150

E730100

E730050

Fig. 13 A geological cross-section through the northern portion of the Pitfield Prospect (Fig. 6), showing aircore and diamond drill hole traces and key intersections of mineralization, together with a graph (logarithmic scale) showing values for carbonate index, Au ppb and As ppm for bottom of hole samples from the aircore holes. Two light dashed lines highlight carbonate index values of 1 and 2

and resulted in the growth of muscovite (phengite) and carbonates in the vicinity of gold deposits in Central Victoria (Binns and Eames 1989; Gao and Kwak 1997; Bierlein et al. 1998). This hypothesis relies on the presence of sufficient chlorite within these sediments in order to produce the rounded early carbonates’ postpeak metamorphism, although the structural timing relationships of these early carbonates shows that they clearly formed pre- to synpeak metamorphism. However, this hypothesis may have been responsible for the precipitation of siderite during the later carbonate event associated with the gold mineralization, which produced isolated euhedral crystals or overgrowths on early carbonates.

The composition of the fluid that produced the early carbonate event is unknown, although if these carbonates formed from anaerobic oxidation processes (Boetitus et al. 2000; Hinrichs et al. 1999), it is likely that the fluid would have contained methane. The two principle sources of methane are either biogenic, created by microbial process in low temperature anaerobic environments, or thermogenic, where high temperatures (>100°C) are sufficient to cause thermal maturation of organic matter (Irwin et al. 1977; Welhan 1988; Whiticar 1999). Another by-product of this oxidation process is the reduction of seawater sulfate to sulfide, which would accumulate in the sediment surrounding the vent. A signature of this sulfur, in terms of sulfur isotopes, should theoretically be preserved in the disseminated pyrite that is associated with many of these deposits. Disseminated pyrite samples from gold deposits within the Bendigo Zone contain positive sulfur isotope values with a range of 0–20‰, the exception being sulfides from Ballarat East that have values of −5 to 0‰ . Pyrite and pyrrhotite from the hydrothermally altered turbiditic sedimentary rocks at Stawell have negative values ranging from −20 to 0‰ (Dugdale et al. 2006). Sulfides produced either by inorganic reduction of seawater sulfate to sulfide have positive sulfur isotope values (δ34S ∼0–15 ‰), and sulfides formed by the bacterial reduction of seawater sulfate have strongly negative values, whereas sulfides precipitated from mantle or metamorphic sulfur have isotope values of 0±3‰ (Chaussidon and Lorand 1990; Chambers 1982; Coleman 1977). Therefore, the sulfur in the disseminated pyrites from the Bendigo Zone is most likely to have been derived from inorganic reduction of seawater sulfate which supports the anaerobic oxidation of methane in the production of carbonates. Sulfur isotopes of sulfides from Ballarat East indicate additional contribution from mantle or metamorphic sourced sulfur, while sulfides from Stawell are most likely to be derived from bacterial reduction of seawater sulfate (Dugdale et al. 2006). The second phase of carbonate growth is associated with gold mineralization and fluid inclusion studies at Stawell, Bendigo, Chewton, Fosterville all record the presence of CO2– H2O–CH4 in inclusions from quartz veins associated with gold mineralization with homogenization temperatures of ∼300°C (Cox 1995; Mapani and Wilson 1998; Jia et al. 2000; Mernagh 2001). Bierlein et al. (1998) postulated that the source for the gold-bearing hydrothermal fluids was metamorphic devolatilization with fluid injection occurring over a prolonged period of time from 455 to 440 Ma. However, a mantle source similar to that described by Sherwood Lollar et al. (1997) cannot be discounted. Squire and Miller (2003) proposed that a major magmatic and hydrothermal event throughout the Lachlan Orogen may be associated with upwelling of the asthenosphere in response to slab rollback at the margin of East Gondwana. Whatever the source of the fluid at this time, the breakdown of metamorphic chlorite in

216

Miner Deposita (2009) 44:205–219

the presence of a CO2-bearing fluid led to the precipitation of phengite and siderite proximal to mineralization. Fluid pathways The basin into which the Cambrian-aged sediments in the Stawell Zone were deposited was filled and deformed in less than ten million years (Miller et al. 2005). The Ordovician–Silurian basin of the Lachlan Orogen is interpreted by Gray et al. (2006) to be a back-arc basin that was filled with >3,000 m of turbiditic sediments in 20–30 million years and subsequently deformed by thin-skinned tectonics with up to 60–70% shortening. The depth of the sediments and the timescale of filling suggest rapid subsidence of the basin. A recent seismic survey across the Stawell and Bendigo Zones conducted by GeoScience Australia in cooperation with the Geological Survey of Victoria and the pmd*CRC revealed the depth of the sedimentary pile to be in excess of 10 km, which would equate to >4,000 m originally (Korsch et al. 2008). The seismic survey also revealed the lack of a décollement zone as predicted by Gray et al. (2006) if the basin was deformed by thin-skinned tectonics. Instead, deep rooted faults are observed, some of which extend to the Moho, which is indicative of thick-skinned tectonics. A feature indicative of thick-skinned tectonics is the inversion of former extensional faults during orogenesis (Butler and Mazzoli 2006; Gray et al. 2006). The fact that the early carbonate spots form parallel to earliest fabric would imply their formation in the very early stages of the basement detachment with fluid advection along normal faults and possible thrusts (Fig. 14). The localization of the carbonate spotting to specific regions of the crust may, in fact, be a direct consequence of the

geometry and intensity of the extensional incision of the depositional basins margins. During the contractional Lachlan Orogeny, the deeply incised east-dipping Moyston Fault on the western margin (Fig. 14) was transpressionally reactivated (Miller et al. 2006; Murphy et al. 2006). In contrast, the faults in the central section of the depositional basin, which probably correspond to growth faults during the initial phase of extensional opening (Squire et al. 2006) are more or less normal to the principal displacement vector as is the west-dipping Heathcote Fault on the eastern margin. This scenario is common along plate boundaries and marine basins, e.g., Shimanto Belt of SW Japan, Derugin Basin, and Santa Barbara Basin (Agar 1990; Greinert et al. 2002; Eichhubl et al. 2000). The uneven distribution of the early carbonate spots along possible early faults may reflect changes in strike along these faults where dilation occurred permitting the flow of methane with the resultant precipitation of aragonite, calcite, and ankerite. Conversion of aragonite to calcite would occur during the early stages of subsequent deformation and metamorphism. Reactivation along early faults would be modified by the change in competency between the carbonated and noncarbonated sediment along the trace of the existing fault line. This modification, or ground preparation, provided sites for later dilation and brittle failure during the introduction of hot CO2 and gold-bearing fluid and the precipitation of iron-enriched carbonate that commonly either mantles or pseudomorphs earlier carbonate spots. Implications for exploration The recognition and understanding of the timing relationships between the formation

STAWELL ZONE

BENDIGO ZONE E

CF

MF

AF

Castlemaine Group deposited from ca 490 to 455 Ma

PF

W

permeation of seawater sulfate

Ordovican Castlemaine Group sediments

AF

Early to mid-Cambrian sediments

CH4 biogenic or theromogenic

zone of anaerobic oxidation of CH4.

Cambrian volcanic rocks/oceanic substrate Continental substrate

Fig. 14 Schematic diagram illustrating the structural architecture of the Stawell and Bendigo zones during basin formation and deposition of the Castlemaine Group between ca 490 to 455 Ma, together with fluid flow including both percolation of seawater sulfate and advection of CH4

(either biogenic or thermogenic in origin) along normal growth faults and the zone of anaerobic oxidation of CH4 to produce bicarbonate and sulfides (modified after Miller et al. 2006). MF Moyston Fault, CF Coongee Fault, PF Percydale Fault, AF Avoca Fault

Miner Deposita (2009) 44:205–219

of carbonate spots, deformation, metamorphism, and mineralization provides exploration geologists with a powerful targeting tool for finding buried turbidite-hosted orogenic gold deposits. At Kewell and Pitfield, reconnaissance aircore drill holes penetrated unoxidised Ordovician metasedimentary rocks beneath 60 m of Tertiary Murray Basin sediments and noxidised Cambro–Ordovician metasedimentary rocks beneath 60 m of Tertiary basalt, respectively. Routine analysis of the bottom of hole samples for multielements including Ca and CO2, enabled the calculation of the carbonate index, which showed anomalies within the metasedimentary rocks >100 m above mineralization (Figs. 12, 13). Therefore, geochemical analysis of aircore, reverse circulation chips, or diamond core for CO2 will provide a broad anomaly (>50 m from mineralization) that will not only express the lateral extent of hydrothermal system but also provide information as to the longevity of the existing structures. In general, the larger gold deposits have CO2 levels in excess of 1%. In addition, Ca should be analyzed to calculate the carbonate index that will enable the mapping of the extent of overprinting gold-related carbonate modification and provide a vector to gold mineralization.

Conclusions Carbonate spots are ubiquitously associated with gold mineralization in the central Victorian gold fields and they provide the most obvious visual sign of hydrothermal alteration. However, the absolute timing of the growth and change in composition of these spots is not so obvious. What is identified in this paper are two main carbonate events that are separated by up to 50 million years in the Stawell Zone and up to 20 million years in the Bendigo Zone. The early calcite to ankerite event is characterized by rounded carbonates that formed either pre- to syn- the main upright folding event in both the Stawell and Bendigo Zones. Fluids that resulted in this early carbonate event may not have contained significant CO2 (low XCO2 assemblages) but probably contained methane that was advected along normal growth faults during the early stages of basement detachment. The later carbonate event relates directly to gold mineralization and is marked by a systematic change in the composition of the carbonates from calcite or ankerite to siderite with decreasing distance to mineralization. The fluid responsible for this carbonate event contained CO2 and was probably derived either from devolatilization processes or from the mantle related to upwelling of the asthenosphere. Therefore, the rounded carbonate spots are not directly associated with gold mineralization, but the later carbonates are, and it is the recognition of this difference in timing and

217

geochemistry and the relationship between these carbonate events that can provide a simple geochemical tool to aid in the discovery of giant gold deposits in the central Victorian gold province. Acknowledgments This paper evolved from ARC-Linkage Grant (LP 0211491) and subsequent Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) project T6 with Leviathan Resources Ltd and project T7 with Perseverance Corporation Ltd. Comments from an anonymous reviewer and D. Craw were very much appreciated. We are also grateful to Larry Meinert for the editorial comments.

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