chapter 5 chemistry of the taemas vein swarm...chapter 5 chemistry of the taemas vein swarm 5.1...

Chapter 5

Chemistry of the Taemas VeinSwarm

5.1 Introduction

Structural observations of syntectonic veins (presented in Chapter 4) provide a ge-ometrical and temporal framework for understanding changes in stress states, fluidpressure regimes and fluid flow during crustal deformation. The isotopic and traceelement composition of veins reflect how fluid sources, fluid pathways, fluid-rockreaction and fluid composition vary with time and space. In this chapter, the chem-istry of the Taemas Vein Swarm and surrounding host rocks is examined.

As externally-sourced fluids react with rocks, they become progressively rock-buffered along fluid flow pathways (Fig. 5.1). Reactive transport modelling ofisotopic and trace-element profiles along flow paths has been used to constrain flowdirections and fluid fluxes. Several theoretical and analytical studies have beencarried out exploring fluid flow and fluid-rock reaction processes (e.g. Bickle andMcKenzie, 1987; Lassey and Blattner, 1988; Rye and Bradbury, 1988; Bowmanet al., 1994; Gerdes et al., 1995; McCaig et al., 1995; DePaolo and Getty, 1996;Abart and Sperb, 1997; Eppel and Abart, 1997; Steefel and Lichtner, 1998; Abartand Pozzorini, 2000; McCaig et al., 2000; Knoop et al., 2002; Badertscher et al.,2002; Matthaei, 2003; Steefel et al., 2005; DePaolo, 2006; Cox, 2007; Zack and John,2007).

During a continuous fluid flow situation, where externally derived fluid undergoesprogressive fluid-rock reaction, early formed veins will have essentially rock-bufferedvalues. Rock-buffering would be reflected in isotope systems such as O and Sr,where the δ18O or 87Sr/86Sr ratio would be the same in both rocks and veins. Asfluid-rock reaction proceeds, veins will gradually become progressively fluid-buffered(Fig. 5.1b). However, if fluid flow is discontinuous, due to dynamic permeabilitycreation and destruction during multiple fault rupture events (Sibson, 2001; Cox,2005), then multiple fluid ‘batches’ will migrate along variable flow pathways. Thiswill lead to different degrees of fluid-rock reaction during each fluid flow event dueto changes in reactive path length, and therefore variable δ18O and 87Sr/86Sr ratiosin different veins.

Isotopic variations in incrementally developed veins provide a record of changes in

87

88 5. Chemistry of the Taemas Vein Swarm

Host rock

FluidReservoir

δ18 O

of v

eins

δ18O rock

δ18O vein(in eqbm with

input fluid)

t0t1 t2 t3

Migrating isotopic front

δ18 O

of v

eins

δ18O rock


input fluid)

Time

Distance

(a)

(b)

δ18 O

of v

eins

δ18O rock


input fluid)

Low flow rate/rapid rctn rate

High flow rate/slow rctn rate

(c)

(d)

Distance

Distance

α

Low flow rate/rapid rctn rate

High flow rate/slow rctn rate

Arrival of fluidat point ‘α’

Figure 5.1: Schematic diagrams showing evolution of vein δ18O compositions withtime and space during progressive fluid-rock reaction after Cox (2007). (a) Fluid froma fluid reservoir with a depleted 18O composition advects through a rock sequencewith a less depleted 18O composition. (b) Evolution of vein δ18O compositions withtime (t0, t1, t2, t3) as low δ18O advect through the rock mass. (c) Illustration ofhow a vein at the same distance along a fluid flow pathway could have a markedlydifferent δ18O composition depending on the fluid flow and/or reaction rate betweenrock and fluid. (d) Evolution of the δ18O composition of a vein with time at a singlepoint ‘α’ in (a), depending on the fluid-rock reaction rate. At a fast reaction rate,rock carbonate becomes more rapidly buffered by the 18O-depleted fluid.

5.2. Methods 89

fluid chemistry during the evolution of a fluid flow system (Dietrich et al., 1983; Ryeand Bradbury, 1988; Elburg et al., 2002; Barker et al., 2006; Cox, 2007). In a fault-fracture system being infiltrated by an external fluid there will be (a) fluid containedin the host rock as interstitial pore fluid (‘matrix fluid’), which may be in equilibriumwith the host rock and (b) external fluid migrating through the fault-fracture system(‘fracture fluid’; DePaolo, 2006). Fluid moving through fractures interacts thermallyand chemically with the host rock via heat conduction, and diffusion of chemicalconstituents dissolved in the matrix and fracture fluid. The degree to which fluid-rock exchange affects the composition of the fracture fluid is dependent on the rateof exchange of chemical species between the host rock and fracture fluid, comparedto the fluid flow rate through the fracture. In many geological systems, wheretemperatures are < 500 ◦C, it is likely that fluid-rock reaction rates do not keeppace with fluid transport rates (DePaolo and Getty, 1996). Different elements maybe affected in different ways during fluid-rock reaction depending on the elementsolubility in solution, diffusion rate from the rock matrix into the fracture fluid, andthe surface reaction rates of minerals (Steefel and Lichtner, 1998; DePaolo, 2006;Zack and John, 2007).

Depending on the rock matrix, fluid composition and fluid source characteristics,different isotope systems (e.g. C, O, Rb-Sr, Sm-Nd, Pb-Pb) and trace elements willprovide different information about changing fluid source, and progressive fluid-rock reaction along flow pathways. Information captured in syntectonic veins mayinclude the migration of isotopic fronts through fault-fracture networks, changes influid source chemistry, and changes in fluid flow rates, reaction rates and the lengthof fluid flow pathways (DePaolo, 2006).

Cox (2007) examined the stable isotope (carbon and oxygen) signatures of hostrocks and calcite veins from the Taemas Vein Swarm. Cox noted a marked oxygenisotope alteration front, and a systematic increase in δ18O of vein calcite upwardsthrough the Murrumbidgee Group, which was attributed to the progressive bufferingof fluid compositions by fluid-rock reaction. Reactive transport modelling impliesfluid fluxes of ∼ 105 m3m−2 within fluid flow pathways (Cox, 2007). The study out-lined in this chapter builds on the study of Cox (2007) significantly by incorporating(a) additional stable isotope (carbon and oxygen) analyses of 104 individual veins,(b) Sr isotope (87Sr/86Sr) analyses of veins (108 samples) and host rock (10 samples)and (c) trace element analyses of veins (150 samples) and host rock (10 samples).Using the structural framework presented in Chapter 4, this study examines spatialvariations in vein chemistry throughout the Murrumbidgee Group.

5.2 Methods

5.2.1 Sample collection

Veins were collected from multiple localities on the Taemas Peninsula. Fault veins,bedding-parallel veins and extension veins were collected from outcrops at Shark’sMouth Peninsula (Localities 1,4 and 5; see Figure 4.1 for localities), Kangaroo Flat(Locality 2), and Tates Straight (Locality 3). The position and temporal relation-ships of veins was recorded in the field prior to sample collection.


Unaltered host rock (at distances of more than 50 m from any visible veins) wascollected from the Cavan Bluff, Spirifer yassensis, Currajong, Bloomfield and Re-ceptaculites Limestones. Characterising the isotopic and trace element compositionof a host rock is a challenging problem. For example, it is problematic to determinethe overall oxygen or strontium isotope composition of a calcareous shale becausesmall variations in the proportion of carbonate:silicate minerals may cause signifi-cant changes in Sr or O isotope ratios. In addition, it is not feasible to characteriseeach individual bed in an interbedded limestone-shale unit, and then produce an‘average’ isotopic composition. The majority of veins in the Taemas Vein Swarmare calcite, with minor quartz. Therefore, it was decided to characterise only thecarbonate component of host rocks via weak acid extraction of carbonate. Further-more, extracting only the Rb-poor carbonate component of host rocks means thateffectively no age correction is required to determine 87Sr/86Sr values at the time ofhydrothermal activity (Faure and Powell, 1972).

5.2.2 Stable isotope analyses

Oxygen and carbon isotope ratios for carbonates were measured on a FinniganMAT251 mass spectrometer. For each sample, 200 ± 20 µg of carbonate powderwas dissolved in 103% H3PO4 at 90

◦ in an automated carbonate (Kiel) device.Carbon isotope ratios are reported relative to Vienna Peedee Belemnite (VPDB).Oxygen isotope results are reported relative to VSMOW, and were converted fromVPDB values, where δ18OVSMOW=1.03091δ

18OVPDB + 30.91 (Coplen et al., 1983).The standard deviation (2σ) for the 50 replicate NBS-19 standards used during theanalysis of these samples was 0.02 h for δ13C and 0.06 h for δ18O.

Isotope results have been normalised on the VSMOW and VPDB scales so thatanalyses of:

NBS-19 δ18OVPDB = -2.20 h; δ18OSMOW = + 28.64 h and δ13CVPDB = +1.95 h

NBS-18 δ18OVPDB = -23.0 h; δ18OSMOW = + 7.2 h and δ13CVPDB = +5.0 h

Quartz oxygen isotope ratios were measured at the University of New Mexico, un-der the direction of Professor Zach Sharp and Dr. Viorel Atudorei. Quartz sampleswere separated from calcite (the calcite was stored for oxygen isotope analysis, asoutlined above), and the quartz chips were placed into a 1M HCl solution, until alleffervesence had ceased and calcite was removed.

The resulting quartz separates were examined using a binocular microscope, andclear quartz pieces were selected for analysis by laser fluorination (following themethod of Sharp, 1990). An internal laboratory standard (Lausanne-1) was analysedto determine the reproducibility of analyses (± 0.1h).

5.2.3 Strontium isotope analyses

Host rock

Host rock Sr isotope compositions were measured by thermal ionising mass spec-trometry (TIMS). Approximately 1 kg of each of two samples from each limestone

5.2. Methods 91

member (10 samples total) was crushed using a tungsten carbide swing mill. Ap-proximately 0.06 to 0.11 g of rock powder was placed into a clean teflon screw-capvial. To separate only the carbonate component of the limestones, 1 mL of distilled 1M acetic acid was added to each beaker, resulting in immediate, gentle effervescence.The beakers were allowed to rest for 2 hours at room temperature, and 1 further mLof acetic acid was added, after which effervescence ceased. The resulting mixtureof solution and sediment was centrifuged in clean tubes, and the liquid was drawnoff and dried on hot plates. The samples were then taken up in HNO3 for loadingonto cation-exchange columns, where Sr was separated from other matrix elements.Rubidium was not collected or analysed, as Rb concentrations in carbonate are verylow (Faure and Powell, 1972).

The purified Sr was loaded in H3PO4 on Ta filaments and analysed on a FinniganMat261 mass spectrometer. All filaments were outgassed for 30 minutes prior toloading. Sr isotope values were normalised to 86Sr/88Sr = 0.1194. The NIST SRM-987 Sr isotope standard analysed during this run had a 87Sr/86Sr value of 0.71023±0.00001.

Carbonate veins

Sr isotope compositions were analysed on the same samples on which trace ele-ment analyses were conducted. Analyses were made using in situ laser ablationmulti-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS),and results of vein analyses are reported in the appendix. Analyses were carried outusing a HelEx ArF excimer laser ablation system, interfaced to a Finnigan MATNeptune MC-ICP-MS (see Eggins et al., 1998; 2005, for details). For the purposesof this study, analyses have been performed using a single spot approach (with alaser spot diameter of 137–233 µm size). Laser pulse rates of 5 Hz in combinationwith a laser fluence of 5 J/cm2 , short laser wavelength (λ= 193 nm), and apertureimaging optics were used to attain controlled calcite ablation ( ∼1 µm per second)in a He ablation medium (Eggins et al., 1998).

The Finnigan MAT Neptune MC-ICP-MS is equipped with a moveable array of9 Faraday cups that were located to monitor all Sr isotopes and peak-strip key in-terfering species (Kr, Rb and REE++). Faraday cups were positioned to the highmass-side of peak-centres to minimize potential REE++ overlaps following the ap-proach of Ramos et al. (2004). The Finnigan Neptunes gas flow and electrostatic lenssettings were optimised for maximum Sr sensitivity and peak-shape while ablating amodern Tridachna clam shell which has a measured 87Sr/86Sr value of 0.709143±15(Woodhead et al., 2005).

Tridachna was additionally used to monitor instrument reproducibility and accu-racy. For 22 analyses of Tridachna, the average 87Sr/86Sr ratio was 0.709149 ± 38(2σ). All data were obtained using 1.024 s integration periods, for total analysistimes of 200 seconds. Data reduction involved subtraction of on-peak baselines,measured every 20-30 minutes (every 5-6 spot analyses), from raw peak intensi-ties. This was followed by correction of instrumental mass fractionation using anexponential law based on the measured 86Sr/88Sr ratio and a canonical 86Sr/88Srvalue of 0.1194. Removal of any 87Rb contribution to measured 87Sr was undertaken


using the measured 85Rb and mass fractionation factor measured for Sr. Correc-tions for 87Rb were negligible, as Rb concentrations are very low in carbonate veins.Krypton interference was accounted for via stripping of on-peak baselines. REE++

interferences were found to be negligible in all measurements.

5.2.4 Trace element analyses

Host rock

The trace element compositions of the carbonate component of limestone host rockswere measured by solution ICP-MS. Approximately 1 kg of each of two samples(10 samples total) from each limestone was crushed using a tungsten carbide swingmill. A known amount of approximately 50–60 mg of rock powder was placed intoa clean teflon screw-cap vial. To separate only the carbonate component of thelimestones, 1 mL of distilled 1 M acetic acid was added to each beaker resulting inimmediate, gentle effervescence. The beakers were allowed to rest for 2 hours at roomtemperature, and 1 further mL of acetic acid was added, after which effervescenceceased. The resulting mixture of solution and sediment was centrifuged in cleantubes, and the liquid was drawn off, dried on hot plates and taken up in around 100mL of 2% HNO3 for analysis.

Trace elements were measured using a quadropole ICP-MS (Agilent 7500s) runningin solution mode. Multiple major and trace elements (9Be, 23Na,25Mg, 29Si, 31P, 45Sc,49Mn, 57Fe, 75As, 85Rb, 86Sr, 89Y, 115In, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm,151Eu, 158Gd, 163Dy, 165Er, 166Er, 174Yb, 175Lu, 185Re, 208Pb, 209Bi, 232Th, 238U)were analysed, with multiple internal standards (Be, As, In, Re, Bi) to monitorinstrument performance and drift.

Carbonate veins

Veins were analysed for trace element compositions using a multiple spot analysis,laser ablation ICP-MS approach. Samples were analysed using a pulsed ArF Excimerlaser (λ=193 nm) and a quadrupole ICP-MS (Agilent 7500s Eggins et al., 1998).Samples were precleaned with ethanol. Every vein sample was analysed between 3to 10 times using a 70 µm laser spot, and an average composition was determined(results reported in the appendix). This approach has some drawbacks, particularlyregarding zoning of trace elements within individual crystals in carbonate veins.However, this approach was chosen to (a) avoid inclusions within veins (b) minimisesample preparation time and (c) allow veins with complex internal fabrics to bedocumented more completely.

Multiple major and trace elements (23Na,24Mg, 29Si, 43Ca, 44Ca, 45Sc, 49Mn, 57Fe,85Rb, 88Sr, 89Y, 138Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 158Gd, 163Dy, 166Er,174Yb, 208Pb, 232Th, 238U) were simultaneously analysed during laser sampling byrepeated, rapid sequential peak hopping, with a mass spectrometer cycle time of0.65 s. Data reduction followed established protocols for time-resolved analysis(Longerich et al., 1996), and 43Ca was used as an internal standard. Laser pulserates of 5 Hz in combination with a laser fluence of 5 J/cm2 , short laser wavelength

5.3. Results 93

(λ= 193 nm), and aperture imaging optics were used to attain controlled calciteablation ( ∼1 µm per second) in a He ablation medium (Eggins et al., 1998).

5.3 Results

In this section, trace element and isotopic variations in host rock and vein carbon-ate are described. Distal limestone samples (more than 50 metres from significantveining) have δ18O between 23–25 h and δ13C of − 0.5 and 3.0 h (Cox, 2007),which are consistent with values for oceanic carbonates deposited during the Devo-nian (Veizer et al., 1999). Distal limestone carbonates have 87Sr/86Sr ratios between0.70815 and 0.70828 (Table 5.1), which agree with seawater values for the middleDevonian (Veizer et al., 1999).

Table 5.1: Host rock carbonate Sr isotope ratios (and associated errors) as measuredby TIMS.

Sample 87Sr/86Sr ± 2σCavan Bluff Host Rock 1 0.70828 0.00003Cavan Bluff Host Rock 2 0.70828 0.00002Spirifer Yassensis Host Rock 1 0.70826 0.00003Spirifer Yassensis Host Rock 2 0.70826 0.00001Currajong Host Rock 1 0.70821 0.00002Currajong Host Rock 2 0.70815 0.00003Bloomfield Host Rock 1 0.70825 0.00002Bloomfield Host Rock 2 0.70826 0.00003Receptaculites Host Rock 1 0.70821 0.00002Receptaculites Host Rock 2 0.70819 0.00001

Host rocks measured within ∼ 1 mm to 1 cm of vein margins have a broaderrange of δ18O, varying between 14 − 24h. Depleted δ18O values are usually a fewcentimetres wide, except around a fault zone at the bottom of the Cavan BluffLimestone (Figure 4.11), where 18O depleted zones are a few metres wide (Cox,2007).

The majority of vein δ13C values are similar to host rocks (0−3h), with a few veins( < 10) showing significant 13C depletion (up to −11h). Vein carbonate δ18O valuesshow significant variation, from ∼ 0h to 25h. Vein carbonates 87Sr/86Sr varybetween 0.70808 and 0.70897, which are significantly greater than the range of hostrock carbonate 87Sr/86Sr values. Sr isotope ratios show no systematic relationshipto δ18O (Fig. 5.2) or δ13C values. The host rock, location, vein type, and isotoperatios (δ18O, δ13C, 87Sr/86Sr) for veins measured in this study may be found in theappendix. Oxygen isotope compositions from calcite and quartz inferred to havegrown at the same period of time suggest fluid temperatures between 100 and 250◦C (Table 5.2, Fig. 5.3), which were calculated using the thermometers of Claytonand Kieffer (1991) and Zheng (1993).


0

5

10

15

20

25

30

0.70800 0.70820 0.70840 0.70860 0.70880 0.70900 0.70920

Cavan Bluff

Spirifer yassensis

Currajong

BloomfieldIncr

ease

d ho

st c

arbo

nate

buf

ferin

g

Increased Sr scavenging

87Sr/86Sr

δ18 O

(‰

VS

MO

W)

Figure 5.2: Graph of Sr and oxygen isotope compositions for veins in differentstratigraphic units. Blue-green coloured box represents the isotopic composition ofhost rock carbonate.

10

15

20

25

30

15 20 25 30δ18Oqtz(‰)

300˚C

200˚C

δ18 O

cct(‰

)

150˚C

100˚C

Figure 5.3: Quartz-calcite oxygen isotope pairs (δ18O, relative to VSMOW) plottedon temperature contours derived from (Zheng, 1993). Black squares are data fromCox (2007), grey squares are data from this study.

5.3. Results 95

Table 5.2: Oxygen isotope ratios for coexisting quartz and calcite fibres. SampleCL-12 is a fibrous quartz-calcite vein, which is implied to have formed via a stretchedcrystal growth mechanism. Samples Cjcrk-C, Cjcrk-J and Cjcrk-K are from the veinshown in Fig. 4.23.

Sample Qtz δ18OVSMOW Cct δ18OVSMOWCL12-7 26.8 24.1CL12-5 27.1 23.8CL12-4 25.8 24.0

Cjcrk-C3 26.0 23.8Cjcrk-C4 26.1 23.8Cjcrk-C5 26.9 23.9Cjcrk-C6 27.1 23.8Cjcrk-C7 26.9 25.0Cjcrk-J1 27.1 23.9Cjcrk-J2 25.5 23.9Cjcrk-Ka 26.1 23.6Cjcrk-Kb 27.1 23.6

Host rock carbonates have variable Mg concentrations, particularly in the CavanBluff Limestone. In general, the Cavan Bluff Limestone has higher concentrationsof Mg, Mn, Sc, Y and the rare earth elements (REEs). Strontium concentrationsare relatively constant throughout the Murrumbidgee Group (1000–2000 ppm). Themassive limestones (Currajong and Receptaculites Limestones) generally have lowerconcentrations of most trace elements, in comparison to the interbedded limestone-shale members (Cavan Bluff, Spirifer yassensis and Bloomfield Limestones). Hostrock REE, Y and Sc concentrations, and REE patterns are similar to those measuredin a variety of cements and fossils in Devonian Limestones by Nothdurft et al. (2004).

Carbonate veins have Mn concentrations which are similar to host limestone car-bonate, while Sr concentrations are generally higher than host rock carbonates,particularly for veins within the CBF. Vein carbonate has highly variable Sc, Y andREE concentrations. In general, trace element concentrations in carbonate veinsdecrease with increasing stratigraphic height.

5.3.1 Vein chemistry changes with stratigraphic height

Unaltered limestones have an approximately uniform oxygen isotope compositionthroughout the Murrumbidgee Group (24 ± 1.2h; Cox, 2007). Figure 5.4 showsthe variation in distal host rock and vein carbonate δ18O with stratigraphic heightthrough the Murrumbidgee Group. Two predominant features exist in this dataset, (a) an increase in vein δ18O upwards through the Murrumbidgee Group and (b)significant scatter at any one stratigraphic level, with veins (exceptionally) having∼ 10h δ18O variation at the same stratigraphic level.Figure 5.5 shows host rock carbonate and vein carbonate 87Sr/86Sr compositions.

Veins in the Currajong Limestone have slightly lower 87Sr/86Sr ratios than veins


Table 5.3: Host rock carbonate trace element composition for selected trace elements(concentrations given in parts per million, ppm).

Element CBHR1 CBHR2 SP1 SP2 CJ1 CJ2 BL1 BL1 RCP1 RCP2Mg 57783 9767 7165 4931 3395 5821 2526 1921 2529 1848Sc 4.29 2.67 1.47 1.59 0.61 0.40 1.72 1.50 1.28 0.90Mn 1026 821 312 118 63 47 284 476 154 160Fe 1264 1805 2500 3500 2105 2014 2326 2183 1848 1593Rb 1.10 0.54 0.60 0.63 0.13 0.15 0.63 0.52 0.50 0.42Sr 1556 1478 1123 1647 1078 2182 1805 1808 1057 910Y 20.13 19.34 5.77 3.86 2.40 1.09 5.31 6.17 2.93 2.36Ba 39.21 38.13 10.21 8.18 6.10 6.61 9.51 8.70 6.81 5.81La 16.86 19.43 5.43 3.93 1.46 1.00 4.94 4.18 3.10 2.25Ce 37.63 42.39 12.33 8.95 3.33 1.98 11.17 10.56 6.71 4.93Pr 4.04 4.48 1.51 1.12 0.36 0.21 1.46 1.47 0.79 0.59Nd 16.95 18.21 6.14 4.59 1.47 0.86 6.18 6.72 3.14 2.38Nd 16.99 18.08 6.19 4.57 1.48 0.84 6.19 6.70 3.19 2.36Sm 4.02 3.94 1.32 0.91 0.33 0.19 1.27 1.50 0.62 0.47Eu 0.98 0.89 0.29 0.20 0.08 0.04 0.36 0.62 0.16 0.11Gd 4.17 4.01 1.26 0.83 0.38 0.18 1.16 1.38 0.58 0.45Tb 0.65 0.61 0.19 0.12 0.06 0.03 0.17 0.20 0.09 0.07Gd 4.12 3.93 1.22 0.79 0.38 0.18 1.11 1.33 0.57 0.44Dy 3.88 3.55 1.09 0.69 0.37 0.17 0.97 1.12 0.50 0.41Ho 0.77 0.70 0.21 0.13 0.08 0.04 0.20 0.21 0.10 0.08Er 2.03 1.83 0.54 0.35 0.21 0.09 0.49 0.51 0.27 0.21Yb 1.69 1.40 0.42 0.30 0.18 0.08 0.41 0.37 0.21 0.18Lu 0.23 0.20 0.06 0.04 0.03 0.01 0.06 0.05 0.03 0.02Pb 3.79 2.81 1.23 0.89 0.75 0.24 1.06 1.26 0.66 0.62Th 1.24 1.56 0.62 0.83 0.31 0.15 0.71 0.89 0.34 0.25U 0.38 0.64 1.99 1.38 1.51 0.75 0.49 0.24 0.62 1.12

in the interbedded limestone-shale units. Below the Cavan Bluff Limestone, veincarbonate 87Sr/86Sr is slightly lower than CBL 87Sr/86Sr host rock carbonate com-positions. In the CBL, vein carbonate is slightly higher, or in equilibrium with thehost rock carbonate with respect to 87Sr/86Sr. Many of the CBL analyses are fromthe Tates Straight fault-fold complex (Figure 4.11). Higher in the stratigraphic se-quence, within the Spirifer yassensis Limestone, veins have higher 87Sr/86Sr thanhost rock carbonate, and 87Sr/86Sr decreases with increasing stratigraphic heighttowards the Currajong Limestone. Within the Currajong Limestone, veins are gen-erally in equilibrium with host rock carbonate with respect to 87Sr/86Sr. However,immediately above the CJL, veins in the interbedded limestones and shales of theBloomfield Limestone show 87Sr/86Sr elevated above host rock values.

In the interbedded limestone-shale members, veins have the most variable trace el-ement concentrations, particularly in the Cavan Bluff Limestone (Figs. 5.6, 5.7, 5.8).

5.3. Results 97

This studyCox (2007)

Host carbonate (distal)

20

0

5

10

15

25

30

0 100 200 300 400 500 600 700 800Stratigraphic Height (m)

δ18 O

(‰

VS

MO

W)

Figure 5.4: Host rock carbonate (red data points) and vein carbonate (black sym-bols) δ18O as a function of stratigraphic height through the Murrumbidgee Group.Host rock and open symbols are from Cox (2007), filled symbols are from this study.

Stratigraphic Height (m)

0.7080

0.7082

0.7084

0.7086

0.7088

0.7090

0 100 200 300 400 500 600 700 800

Cav

an B

luff

Lim

esto

ne

Maj

urgo

ng F

orm

atio

n

Spi

rifer

Yas

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isLi

mes

tone

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rajo

ng L

imes

tone

Blo

omfie

ld L

imes

tone

Rec

ipta

culit

ies

Lim

esto

ne

87S

r/86

Sr

Figure 5.5: 87Sr/86Sr compositions of host rock and carbonate veins as a functionof stratigraphic height through the Murrumbidgee Group. Host rock is plotted as therange of the two analyses conducted for each limestone member, veins are plottedwith 2 SE bars.


Vein carbonates have been normalised to their respective host rock carbonate compo-sitions (Figure 5.9), allowing changes relative to host rock to be assessed. Generally,vein carbonate trace element concentrations decrease with increasing stratigraphicheight (excepting Mg). Manganese is in similar concentrations in veins and hostrock carbonate. Iron concentrations are higher in the Cavan Bluff Limestone, andare significantly higher than host carbonate Fe concentrations. However, Fe con-centrations decrease relative to host carbonate with increasing stratigraphic height(Figs. 5.6, 5.9). Lead concentrations in veins are consistently lower than host rockcarbonate.

Scandium, Y and heavy REE concentrations are generally similar, or lower thanhost rock carbonate. Light REE concentrations generally decrease with increasingstratigraphic height, while the heavy REEs show less variation. Europium concen-trations decrease with increasing stratigraphic height through the MurrumbidgeeGroup, and are generally higher than the surrounding host rock. Trace elementvariations are complex, and are discussed more fully below.

5.3.2 Vein chemistry within and between different outcrops

In this section, the chemistry of carbonate veins in different outcrops, with differentstructural settings is discussed. Veins were analysed on this scale to examine howfluid-rock reaction varies between veins in a similar host rock. Veins were alsoclassified into fault, bedding-parallel or extension veins to see if the vein type wasa controlling factor on chemical composition. The relative timing of vein formationwas discussed in Chapter 4, and it is noted that bedding-parallel laminated veinsformed relatively early in the deformation history. Extension veins related to foldlimb-parallel stretching and flexural flow folding occur relatively late compared tobedding-parallel slip veins, while bedding-discordant faults and associated extensionveins are also late in the deformation history.

In this chapter, nonparametric statistical tests (Mann-Whitney and Kruskal-Wallistests) were used to assess if the median value of data sets are significantly different.These tests do not make any assumption about the normality of data sets (as op-posed to the Student’s T-test), and allow significances between data sets to be morerobustly assessed, as outliers and skew in data sets are more effectively dealt with(Borgman, 1988; Rock, 1988).

Within-outcrop variations

Figure 5.10 shows the location, vein type and isotopic compositions (δ18O, 87Sr/86Sr)of different veins (fault, bedding-parallel and extension) from the fold-fault complexat Tates Straight (see also Fig. 4.11). Notable is that δ18O varies by ∼ 5hand 87Sr/86Sr varies between 0.70825 and 0.70846. A small extension vein isolatedwithin a single limestone bed (likely related to flexural flow) shows higher 87Sr/86Srand δ18O, while bedding-parallel and fault veins show some of the lowest O and Srisotope ratios.

This suggests that fluids which precipitated calcite in fault and bedding-parallelveins underwent less interaction with host rocks. This is likely related to the length

5.3. Results 99

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n (p

pm)

Figure 5.6: Mg, Mn, Fe, Sr, Sc and Y concentrations in host rock carbonate (colouredsquares), plotted as the range determined for the two samples analysed from eachlimestone member (see Table 5.3), and carbonate veins (red circles) with stratigraphicheight through the Murrumbidgee Group (see appendix B). Limestone formations andmembers are the same as Fig. 5.5.


0

10

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30

40

50

La

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Ce

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14

0 100 200 300 400 500 600 700 800Height (m)

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6

8

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0 100 200 300 400 500 600 700 800Height (m)

Sm Eu

Pr Nd

Con

cent

ratio

n (p

pm)

Con

cent

ratio

n (p

pm)

Con

cent

ratio

n (p

pm)

Figure 5.7: La, Ce, Pr, Nd, Sm and Eu concentrations in host rock carbonate(coloured squares), plotted as the range determined for the two samples analysedfrom each limestone member (see Table 5.3), and carbonate veins (red circles) withstratigraphic height through the Murrumbidgee Group (see appendix B). Limestoneformations and members are the same as Fig. 5.5.

5.3. Results 101

0

2

4

6

8

10

12

14

0

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6

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12

Dy

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1

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100

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250

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total REEs

Gd

Er

Height (m)

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3

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6

0 100 200 300 400 500 600 700 800

Pb

Yb

Con

cent

ratio

n (p

pm)

Con

cent

ratio

n (p

pm)

Con

cent

ratio

n (p

pm)

Height (m)

Figure 5.8: Gd, Dy, Er, Yb, Pb and total REE concentrations in host rock carbonate(coloured squares), plotted as the range determined for the two samples analysedfrom each limestone member (see Table 5.3), and carbonate veins (red circles) withstratigraphic height through the Murrumbidgee Group (see appendix B). Limestoneformations and members are the same as Fig. 5.5.


0.1

1

10

100

Mg Sc Mn Fe Sr Y Ba La Ce Pr Nd Sm Eu Gd Dy Er Yb Pb Th U

(a)

(b)

(c)

Con

c (h

ost r

ock

norm

)C

onc

(hos

t roc

k no

rm)

Con

c (h

ost r

ock

norm

)

Cavan Bluff

0.1

1

10

100

Mg Sc Mn Fe Sr Y Ba La Ce Pr Nd Sm Eu Gd Dy Er Yb Pb Th U

Bedding-parallel vein

Fault veinsExtension vein

Spirifer yassensis

0.1

1

10

100

Mg Sc Mn Fe Rb Sr Y Ba La Ce Pr Nd Sm Eu Gd Dy Er Yb Pb Th U

Bedding-parallel vein

Fault veinsExtension vein

Currajong

Figure 5.9: Vein carbonate trace element compositions normalised to host rockcomposition in the Cavan Bluff, Spirifer yassensis and Currajong Limestones. Fault,bedding-parallel and extension veins are plotted in different colours for the SYL andCJL.

5.3. Results 103

R2 = 0.7247

15

15.5

16

16.5

17

17.5

18

18.5

19

19.5

20

0.70820 0.70825 0.70830 0.70835 0.70840 0.70845

F

(2) δ18O=18.0‰87Sr/86Sr=0.70833

(4) δ18O=16.5‰87Sr/86Sr=0.70832

(5) δ18O=18.7‰87Sr/86Sr=0.70846

(3) δ18O=17.4‰87Sr/86Sr=0.70835

(1) δ18O=15.5‰87Sr/86Sr=0.70825

(7) δ18O=17.8‰87Sr/86Sr=0.70831

(6) δ18O=19.5‰87Sr/86Sr=0.70838

δ18 O

(‰

VS

MO

W) R2

= 0.7247

15

16

17

18

19

20

0.70820 0.70830 0.7084087Sr/86Sr

Figure 5.10: Faults and veins within the Cavan Bluff Limestone at a fault-foldcomplex in Tates Straight (see also Fig. 4.11). White lines mark bedding, yellowlines mark cleavage and red lines mark veins. A heavily qtz-cct mineralised faultzone is marked in orange. Shown are O and Sr isotope compositions for different veintypes (numbered below) contained within the outcrop: (1) bed-parallel Vein, (2) bed-parallel Vein, (3) saddle-reef, (4) hinge extn vein, (5) flex flow extn vein, (6) laminatedfault vein, (7) fault vein dilatant jog. Inset graph shows relationship between δ18Oand 87Sr/86Sr for different veins.

of fluid-flow pathways, with fault and bedding-parallel veins having ‘short circuits’,allowing fluids to undergo less reaction with host rock. These results emphasisethat veins within the same outcrop, or even within the same structure may havesignificantly different isotopic compositions (compare vein pair 1 and 3, and veinpair 6 and 7 in Fig. 5.10).

Vein-type variations

Approximately eighty different bedding-parallel, extension and fault veins were anal-ysed from the Spirifer yassensis and Currajong Limestones. Within the SYL, δ18Oin vein carbonate varies between 14.7 and 25.2 h, with the majority of veins havingδ18O values between 22 and 25 h. Strontium isotope values (87Sr/86Sr) show no re-lationship to the vein type (Fig. 5.11). Similarly, all veins within the CJL have δ18Oof 20 − 25h, with the majority of values clustering between 22 − 24h. However,87Sr/86Sr values systematically vary according to vein type, with extension veinshaving lower 87Sr/86Sr values than most fault or bedding-parallel veins (statisticallysignificant at the 95% confidence level, p-value for extension vs fault veins=0.006,Mann-Whitney test, Fig. 5.12).

Similar trends are observed for trace elements, with veins in the SYL having abroad range of trace element compositions, with no relationship observed between


0

5

10

15

20

25

30

0.70825 0.70830 0.70835 0.70840 0.70845 0.70850 0.70855 0.70860 0.70865

beddingextensionfault

Spirifer yassensis Limestone

87Sr/86Sr

δ18 O

(‰

VS

MO

W)

Figure 5.11: δ18O and 87Sr/86Sr isotope composition of extension, bedding-paralleland fault veins in the Spirifer yassensis Limestone. 2 SE error bars are shown for Srisotope analyses.

0

5

10

15

20

25

30

0.70810 0.70820 0.70830 0.70840 0.70850 0.70860 0.70870

beddingextensionfault

Currajong Limestone

87Sr/86Sr

δ18 O

(‰

VS

MO

W)

Figure 5.12: δ18O and 87Sr/86Sr isotope composition of extension, bedding-paralleland fault vein in the Currajong Limestone. 2 SE error bars are shown for Sr isotopeanalyses.

5.3. Results 105

vein type and trace element concentration (Fig. 5.13). In comparison, extensionveins within the CJL have lower concentrations of most trace elements compared tofault or bedding-parallel veins (Fig. 5.14). In particular, REE concentrations arelower in extension veins than in bedding-parallel or fault veins.

In general, extension veins in the Currajong Limestone have Sr isotope and traceelement compositions that are most similar to host rock carbonate compositionsas opposed to fault or bedding-parallel veins. In comparison, all veins in the SYLhave a broad range of Sr isotope ratios and trace element concentrations, which aregenerally elevated above host rock carbonate compositions. Many extension veins inthe CJL are related to fold limb-parallel stretching. In comparison, most extensionveins in the Spirifer yassensis Limestone are associated with bedding-discordant faultzones. The fluid which migrated through late extension veins in the CJL may haveundergone significantly more fluid-rock reaction with the local host rock than thefluid from which fault or bedding-parallel veins formed.

Inter-outcrop variations

Figures 5.15 and 5.16 illustrate the structural setting and Sr and O isotope ratios ofveins from two adjacent outcrops of folded Spirifer yassensis Limestone, at KangarooFlat (separated by approximately 50 m horizontally, Fig. 4.1). This is one of thefew locations in which the chemistry of veins from the same stratigraphic level, butdifferent structural settings may be compared (see red rectangles highlighted in Fig.4.4, which clearly show that the two outcrops in the SYL occur at almost exactlythe same stratigraphic height). One outcrop is an upright, close fold (hereaftercalled ‘close fold’) containing predominantly bedding-parallel veins, with severalsmall dilational jogs and late bedding-perpendicular extension veins related to limb-parallel stretching. The other outcrop consists of two open, asymmetric folds, whichwere described in Figure 4.7 (hereafter called ‘open folds’).

Within both outcrops, vein carbonate δ18O varies between 20 − 25h for mostveins, excepting a late strike-slip fault vein (14.7 h), and vein δ18O values forthe two outcrops are statistically indistinguishable at the 95% confidence level (p-value=0.63, Mann-Whitney test). However, the vein cluster at the ‘open folds’outcrop (Fig. 5.15) has consistently lower 87Sr/86Sr ratios than the vein cluster at theadjacent ‘close fold’ outcrop (Fig. 5.16; 87Sr/86Sr p-value=0.016; Fig. 5.17). Thesetwo different vein clusters also have significantly different concentrations of Mn, Fe,Sr and Eu. Veins in the ‘close fold’ have higher Mn, Fe and Eu concentrations, andlower Sr concentrations (p-values for Mann-Whitney test; Mn =< 0.0001, Fe =<0.0001, Sr = 0.0009, Eu = 0.0133; Fig. 5.18). For normalised REE patterns, bothoutcrops have LREE enriched patterns, with veins in the ‘close fold’ having a morepositive Eu anomaly than veins in the ‘open folds’.


100 150 200 250 300 350 400 450 0 5 10 15 20 25 301000

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Fe

(ppm

)

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pe R

EE

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Mn (ppm)

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8

Sc

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Bed Parallel

Extension

Fault

Figure 5.13: Trace element composition of extension (blue squares), bedding-parallel(red circles) and fault veins (green diamonds) in the Spirifer yassensis Limestone.

5.3. Results 107

0 0.5 1 1.5 2 2.5 3 3.5 40

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)

La (ppm)

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5000F

e (p

pm)

Mn (ppm)

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EE

Total REE (ppm)

0 5 10 15 20Y (ppm)

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4

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6

Sc

(ppm

)

Bed Parallel

Extension

Fault

Figure 5.14: Trace element composition of extension (blue squares), bedding-parallel(red circles) and fault veins (green diamonds) in the Currajong Limestone.


SW NE

Wing Crack, δ18O=22.5‰87Sr/86Sr=0.70847

Bed-parallel vein, δ18O=22.6‰87Sr/86Sr=0.70836


BP vein, δ18O=25.5‰87Sr/86Sr=0.70841

SS fault, δ18O=14.7‰87Sr/86Sr=0.70836

Figure 5.15: Outcrop sketch showing the location and isotopic compositions (δ18Oand 87Sr/86Sr) of vein cluster associated with two open folds in the Spirifer yassensisLimestone (see also Fig. 4.7).

Dilational jog, δ18O=22.4‰87Sr/86Sr=0.70852

Ext. Vein, δ18O=21.1‰87Sr/86Sr=0.70845

En echelon veins, δ18O=22.4‰87Sr/86Sr=0.70857



Large BP vein, δ18O=20.7‰87Sr/86Sr=0.70845

Figure 5.16: Strontium and oxygen isotope composition of vein cluster found withinthe ‘close fold’ situated at Kangaroo Flat, in the Spirifer yassensis Limestone.

5.3. Results 109

14

16

18

20

22

24

26

0.7083 0.7084 0.7085 0.7086

‘Close Fold’

‘Open Folds’

Strike-slip fault

87Sr/86Sr

δ18 O

(‰

VS

MO

W)

Figure 5.17: Comparison of δ18O and 87Sr/86Sr for two vein clusters at the samestratigraphic level in the Spirifer yassensis Limestone (see also Figs. 4.4, 5.15, 5.16).

0.0

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4500

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‘Close Fold’

‘Open Folds’

‘Close Fold’

‘Open Folds’M-W: Sr p-value=0.0009M-W: Eu p-value=0.0133

M-W: Fe p-value


5.4 Discussion

5.4.1 Fluid source

If it is assumed that calcite precipitated in equilibrium with the parent fluid, andthe fluid temperature can be estimated, then the δ18O of vein calcite may be used todetermine the isotopic composition of the parent fluid. Quartz-calcite oxygen isotopepairs (see Fig. 5.3, and Cox, 2007), suggest fluid temperatures of 100–250 ◦C, usingthe thermometers of Zheng (1993) and Zheng (1999), with the presence of illiteimplying temperatures of < 200 ◦C (Cox, 2007). Minimum δ18O values measuredin veins at the base of the Cavan Bluff Limestone are ∼ 0 to + 2h (Cox, 2007).At temperatures of 100–200 ◦C, this implies that fluid δ18O was between − 14hand − 8h (Zheng, 1999). Such low δ18O values are consistent with a meteoric fluidsource (Sheppard, 1986). To form a meteoric fluid with these low δ18O values, themeteoric fluid must have been sourced at either high latitude ( ∼ 50 − 60◦), or atsignificant altitude (2–4 km; Bowen and Wilkinson, 2002).

Several veins, or sections of individual veins have unusually elevated 87Sr/86Srand depleted δ13C values (e.g. sample C2a, see appendix). Barker et al. (2006)and Cox (2007) attributed localised negative-δ13C values to oxidation of organicmatter during fluid-rock reaction. The coupling of elevated 87Sr/86Sr and depletedδ13C values implies that a second fluid ‘source’ may have been present. This fluidmay have been a residual interstitial pore fluid, which underwent enhanced fluid-rock reaction with iron oxides and 87Sr-enriched minerals prior to mixing with theinvading meteoric fluid (Fig. 5.19).

Fra

ctur

e ne

twor

k

Ascending fluid

Lst

Microcracks

Ascending meteoric fluid

low-δ13C,interstital fluid

rock-buffered fluidmixes with external fluid

Resulting mixed fluid,lower low-δ13C, higherδ18O

Lst

Lst

Shale

Figure 5.19: Schematic diagram showing how low-δ18O fluids ascending through afracture network (left hand cartoon) could mix with interstitial pore fluid. This porefluid may have undergone enhanced reaction with wall rocks, leading to oxidation oforganic matter, and the generation of 13C-depleted fluids.

5.4. Discussion 111

5.4.2 Isotopic variations

Changing isotope ratios and varying trace and REE patterns in veins reveal infor-mation about how physiochemical conditions and fluid-rock reaction changed duringvein growth. Several key points may be extracted from the studies of Barker et al.(2006) and Cox (2007):

1. Depleted δ18O ratios in vein carbonate relative to host rock carbonate indicatethat fluid was derived externally to the host rocks.

2. Reactive transport modelling of oxygen isotope variations by Cox (2007) demon-strate that the systematic increase in vein δ18O, and marked O-isotope alter-ation front in the Taemas Vein Swarm was the result of the buffering of aninfiltrating low-δ18O fluid by progressive fluid-rock reaction.

3. O-isotope alteration haloes around veins (typically ∼mm wide, particularlystratigraphically higher in the vein swarm) indicate that no substantial lateralfluid flow occurred into vein sidewalls, and that upwards fluid advection wasmuch more rapid than infiltration of external fluid into wallrock, or interactionof fracture fluid and matrix fluid (Cox, 2007).

4. Analyses of δ18O and 87Sr/86Sr conducted during this study, and results re-ported in Barker et al. (2006) and Cox (2007) reveal that fluid flow pathways,path lengths and/or fluid flow and fluid-rock reaction rates changed dynam-ically during the growth of individual veins, and between different veins inthe same outcrop. This is likely related to the creation and destruction ofpermeability during multiple fracture opening and sealing events.

Assuming that calcite-deposition in veins was an equilibrium process, then theδ18O of vein carbonate changes in response to the δ18O of pore fluid, as pore fluidand wall rock undergo progressive reaction. Models by Cox (2007) utilising coupledadvective transport and kinetically-controlled O-isotope exchange between fluid androck suggests that individual fluid batches involved fluxes of ∼ 100 m3m−2 withinfluid pathways. In addition, from reactive transport models, Cox (2007) suggestedthat some fluid-rock reaction occurred prior to migration of fluids into and throughthe CBF. Fluid-rock reaction could have been accommodated by flow of fluids par-allel to the contact between the Black Range Group and Murrumbidgee Group priorto infiltration of fluids upwards into the Murrumbidgee Group (Cox, 2007).

Variations in 87Sr/86Sr are not consistent with fluids only reacting with host rockcarbonate. Sr isotope ratios in veins measured near the base of the Cavan BluffLimestone have 87Sr/86Sr ratios only slightly below host rock carbonate values, withmost veins in the CBL being in equilibrium with host rock carbonate with respectto 87Sr/86Sr. However, in the SYL, 87Sr/86Sr ratios are significantly higher thanhost rock values. This is attributed to reaction of fluids with 87Sr-rich mineralsin the Majurgong Formation, and shale-rich layers of the Spirifer yassensis Lime-stone. These minerals are likely to be potassium-rich, e.g. illite. With increasingstratigraphic height, 87Sr/86Sr decreases, and becomes closer to host rock carbonatevalues. By the top of the Currajong Limestone, vein 87Sr/86Sr ratios are identicalto host rock carbonate values.


The transition from the SYL to CJL is marked by a change between mixed shale-limestone lithology to only limestone. Carbonate contents of SYL limestone deter-mined in the laboratory (by loss of weight after acetic acid leaching) are 85–90%carbonate, while CJL limestones are 97–98% carbonate. The gradual decrease in87Sr/86Sr towards host rock carbonate values from the SYL to the CJL is interpretedas a result of decreasing amounts of shale available for fluid-rock reaction (i.e. in-creasing amounts of carbonate to buffer fluids). Carbonate veins in the BloomfieldLimestone (interbedded limestone-shale) have 87Sr/86Sr values elevated significantlyabove host rock carbonate, even as little as ∼ 10 m stratigraphically above theCurrajong Limestone. However, over this same region, there is no discernible vari-ation in δ18O values. This implies that fluids strip trace elements from the surfaceof clays and other minerals over short reactive path lengths during fluid flow.

5.4.3 Trace element variations

In this section, trace element variations between veins in different stratigraphic unitsare discussed. One obvious weakness of this study is the number of analyses of hostrock, compared to the number of veins analysed. In particular, shale analyses wereavoided during this study. This decision was made due to (a) the paucity of freshshale in outcrop and (b) logistical problems of characterising hundreds to thousandsof individual shale beds. The consistent isotopic and trace element compositionsof limestone carbonate throughout the stratigraphic sequence suggests that averagecarbonate compositions do not vary by a significant degree (except perhaps for Mg),particularly compared to the degree of variation displayed by veins. The study of Cox(2007) demonstrated that fluids dominantly interacted with carbonate with respectto δ18O. Thus, comparing only vein carbonate with only host rock carbonate allowsother controls on vein chemistry (such as interaction with other minerals, changesin physiochemical conditions) to be assessed.

Due to the multiple factors controlling trace element concentrations in fluids, andthe incorporation of trace elements into calcite, interpreting trace element variationsin hydrothermal veins is extremely difficult, and trace element concentrations maybe affected by several factors, including:

1. Spatial and temporal variations in the composition of infiltrating (source) fluid.

2. Interaction of source fluid with matrix fluid.

3. Dissolution of host rock minerals, releasing lattice-bound trace and minor el-ements i.e. host carbonate being dissolved.

4. Scavenging by fluids of trace elements loosely bound to mineral surfaces, par-ticularly for clays, releasing adsorbed elements.

5. Precipitation of hydrothermal minerals (e.g. calcite) leading to coprecipitationof other trace elements, removing those elements from solution.

6. Sorption of trace elements onto mineral surfaces during fluid migration, re-moving trace elements from fluids.

5.4. Discussion 113

7. Changes in physiochemical conditions (e.g. temperature, pressure, pH, com-plexing species), which may affect trace element incorporation and fractiona-tion from fluids.

An interesting aspect to this work is the variable information which different iso-tope systems provide in studies of fluid-rock reaction. For example, fluids infiltratingthe bottom of the Murrumbidgee Group precipitated veins with extremely depletedoxygen isotope values relative to host rock carbonate. However, these same fluidsprecipitated veins which are essentially host rock-buffered with respect to 87Sr/86Sr.These differences reflect the rate of different fluid-mineral reactions, and the massbalance between rock and fluid (see further discussion in §8.2.3).

Steefel and Lichtner (1998) and DePaolo (2006) provide models addressing thechemical interaction of host rock, ‘matrix fluid’ and ‘fracture fluid’ during fracture-controlled fluid flow. As fluid migrates through a fracture, diffusion of trace elementsoccurs between that fluid, and the rock matrix. The position of alteration fronts willbe dependent on the fluid flow rate, diffusion rate, kinetics of reactions, and reactivesurface area of minerals. Along any one fracture, these three factors will changedepending on the fracture aperture, fracture roughness and wall rock composition.For example, a small cataclastic fault zone, containing pulverised host rock with ahigh reactive surface area, will react in a different manner to an extension fracturecutting through the same host rock.

To understand causes of trace-element variations in an exhumed hydrothermalsystem, it is necessary to characterise the host rock mineralogy, the chemistry andsurface reactivity of those minerals, and the chemistry of both ‘matrix fluid’ and‘fracture fluid’. This may be possible in a modern hydrothermal system such asa mid-ocean ridge, with relatively simple, homogeneous host rock (basalt) and ho-mogeneous, well-characterised fluid (sea water). However, it is not tenable in anexhumed, ancient hydrothermal system with a diverse mineralogy and fluids withrelatively unconstrained chemistry. Hence, the discussions in this section are largelyqualitative, aimed towards understanding what processes may control trace elementfractionation within hydrothermal systems. An extension of this to a quantitativeexamination is not considered feasible in the timeframe of this study.

Trace element concentrations in vein carbonate are considerably higher than hostrock carbonate for most elements in the Cavan Bluff Limestone. This implies thatthe fluid which invaded the base of the Murrumbidgee Group was already enrichedin trace elements. Presumably, the trace element composition of fluids infiltratingthe base of the CBF reflects fluid-rock interaction which occurred with the under-lying rocks (i.e. volcanic sediments of the Black Range Group). Notable in someveins within the Cavan Bluff Limestone are extremely elevated Sr concentrations (in-deed, strontianite is present in at least one BPV), and veins with extremely positiveEu anomalies. Such chemical anomalies could be produced by dissolution of Ca-feldspar, which often has appreciable concentrations of both Sr and Eu (Schnetzlerand Philpotts, 1970).

In the Taemas Vein Swarm, the following chemical processes are considered mostlikely to have influenced the chemistry of carbonate deposited from hydrothermalfluid (see also discussion on potential influences of incorporation of trace elements


into calcite in §3.1):

1. Reaction of fluid with host rock carbonate.

2. Scavenging of trace elements from mineral surfaces.

3. Changes in calcite precipitation rate.

4. Changes in complexing species in solution.

5. Sorption of trace elements onto mineral surfaces.

When vein REE concentrations are normalised to chondrite values, it is apparentthat REE patterns in Cavan Bluff veins are generally LREE enriched, whereas veinshigher in stratigraphy in the SYL and CJL become gradually depleted in LREE(Fig. 5.20). In a solution dominated by CO2−3 complexation, changes in REE pat-terns could be the result of progressive precipitation of calcite and/or a progressivereduction in the degree of CO2−3 complexation in solution (see discussion in §3.2.1and Fig. 3.1; Bau, 1991; Bau and Moller, 1992). If this interpretation is correct,then Fig. 5.20 suggests that [CO2−3 ] decreased as fluids migrated upwards throughthe Murrumbidgee Group.

-30

-25

-20

-15

-10

-5

0

0 100 200 300 400 500 600 700 800Height (m)

RE

E S

lope

Figure 5.20: Graph showing stratigraphic height plotted against the slope of chon-drite normalised REE patterns in vein carbonate. This graph gives a measure of thedegree of LREE vs HREE enrichment, with more negative values indicating enrich-ment in LREE (see §3.2.1 and Fig. 3.1 for further information).

Correlating REE concentrations to one another reveals two interesting relation-ships. Adjacent REE (e.g. La and Ce, Dy and Er) concentrations show stronglinear relationships, with r2 values typically > 0.9. With increasing difference in

5.4. Discussion 115

Table 5.4: Correlation coefficients (r2) for the rare earth elements for all veins in theTVS.

La Ce Pr Nd Sm Eu Gd Dy Er YbLa 1.00Ce 0.90 1.00Pr 0.74 0.95 1.00Nd 0.61 0.85 0.97 1.00Sm 0.44 0.69 0.85 0.94 1.00Eu 0.21 0.21 0.23 0.25 0.29 1.00Gd 0.44 0.68 0.82 0.91 0.99 0.28 1.00Dy 0.43 0.65 0.78 0.85 0.91 0.20 0.95 1.00Er 0.40 0.61 0.73 0.78 0.80 0.13 0.85 0.96 1.00Yb 0.34 0.54 0.66 0.71 0.71 0.09 0.75 0.87 0.96 1.00

atomic number, the REEs become increasingly poorly correlated to one another.This suggests that some process causes increasing fractionation of the REEs fromone another (such as changes in REE complexation in solution). However, Eu showsvery poor correlations (r2 < 0.3) with all other REEs (Table 5.4). This implies thatEu concentrations were influenced by other fluid-rock reaction processes, which mayinclude changes in the valence state of Eu due to reduction and/or oxidation offluids, or variable dissolution of Eu-rich minerals (e.g. Ca-feldspar) along fluid flowpathways.

It is difficult to comment on whether fluid would have reacted more rapidly withcalcite or silicate minerals, as these reaction rates are highly dependent on pH andthe concentration of other chemical species (Beck et al., 1992; Dove, 1994). Asthe infiltrating fluid began to react with the limestone sequence, the fluid’s oxygenbudget would be affected by the rates of dissolution and precipitation of carbonateand silicate minerals. However, other elemental budgets could be more significantlyaffected by the scavenging of trace elements from mineral surfaces, and sorption oftrace elements onto mineral surfaces (which would have little affect on the oxygenbudget on the fluid). This may explain why 87Sr/86Sr isotope ratios and trace ele-ment concentrations are affected over scales of tens-of-metres, while oxygen isotoperatios are essentially unaffected by small variations in host rock lithology.

A key observation is that trace element concentrations vary to a much greaterextent than oxygen, carbon or strontium isotope ratios. Variations in oxygen iso-topes are related to progressive rock buffering by the Murrumbidgee Group rocksas ascending fluids underwent fluid rock reaction. Carbon isotopes are essentiallyrock-buffered throughout the TVS, because Crock >> Cfluid (Cox, 2007). Changesin 87Sr/86Sr are related to local rock buffering, and variable derivation of Sr fromcarbonate and other minerals. However, a number of dynamic processes influencetrace element concentrations in fluids, including (a) changing source fluid compo-sition, (b) fluctuating interaction of fluids with different minerals, (c) changes influid flow pathways influencing the degree of fluid-rock reaction (e.g. changing flowlength, surface area along flow path) and (d) variable removal of trace elements fromsolution during calcite precipitation.


Changes in mineral precipitation rate

With increasing stratigraphic height through the SYL and CJL, vein strontiumisotope ratios (87Sr/86Sr) become increasingly equilibrated with respect to host rockcarbonate within the Currajong Limestone. However, Sr concentrations increase inveins in the CJL compared to veins in the SYL, and are considerably higher than Srconcentrations in host rock carbonate. This is not consistent with the only controlon vein trace element composition being increased by fluid-rock reaction with hostrock carbonate. In addition, Fe concentrations are considerably lower in veins thanhost rock carbonate in the CJL. This suggests that another factor (rather thanjust the degree of reaction between fluid and host rock carbonate) influenced theincorporation of both Sr and Fe into vein carbonate. Faster calcite precipitation ratesare known to increase the partitioning of Sr2+ into calcite (Mucci and Morse, 1983;1990; Tesoriero and Pankow, 1996; Wasylenki et al., 2005; Gabitov and Watson,2006), and faster calcite precipitation rates are also known to lower the amount ofFe2+ incorporated into calcite (Dromgoole and Walter, 1990). Therefore, changesin trace element patterns in veins from the SYL to CJL are consistent with morerapid calcite precipitation rates for veins in the CJL. A potential cause for increasedprecipitation rate is a larger (or more rapid) change in fluid pressure, which couldbe related to fracture dilation rate, or changing fluid pressure in the bulk rock mass.

5.4.4 Fluid-rock reaction and fluid pathways

The analyses outlined here reveal that progressive fluid-rock reaction occurred dur-ing the ascent of externally-derived fluids through the limestone-shale sequence. Cox(2007) attributed δ18O variation between veins within the same outcrop to repeatedswitching of fractures between high and low permeability states, with consequentepisodic flow and variable reactive fluid flow path lengths, fluid flow rates and fluid-rock reaction rates.

This chapter offers several new, and significant insights into the dynamics of afault-fracture hydrothermal system. Firstly, veins within two outcrops of strati-graphically equivalent rock have systematically different chemical compositions. Sec-ondly, extension veins in the Currajong Limestone have rock-buffered compositionscompared to bedding-parallel and fault veins. In addition, trace elements revealinformation about changes in fluid-rock reaction and changes in physiochemical con-ditions during vein growth.

In §5.3.2, it was noted that bedding-parallel and fault veins within the TatesStraight fault-fold complex (illustrated in Fig. 5.10) which formed relatively earlyduring deformation, had lower 87Sr/86Sr ratios and lower δ18O (closer to fluid-buffered compositions) values than vein formed later in deformation (closer to rock-bufered compositions). These values are exactly the opposite to what would beexpected had these veins formed in a continuous fluid flow regime, where early veinsshould have rock-buffered compositions, and later veins should have more fluid-buffered compositions (see also discussion of ‘ordinary’ and ‘invasion’ percolationprocesses in §2.4.4 and §8.4.2).

As described above, two closely spaced outcrops (‘open folds’ and ‘close fold’)

5.4. Discussion 117

of Spirifer yassensis Limestone occur at equivalent stratigraphic heights but withdifferent structural settings. Veins in the ‘open folds’ have lower 87Sr/86Sr ratios,which are closer to host rock carbonate values. However, variations in trace elementconcentrations cannot be explained by an increased degree of fluid-rock reactionfor veins in the ‘open folds’, because in that case all isotope ratios and trace ele-ment concentrations should approach host rock values. The increased Mn and Fe,and decreased Sr for the ‘close fold’ (compared to the ‘open folds’) is consistentwith slower calcite precipitation rates for veins which formed in the ‘close fold’ (seeabove and Mucci and Morse, 1983; Dromgoole and Walter, 1990; Mucci and Morse,1990; Tesoriero and Pankow, 1996; Wasylenki et al., 2005; Gabitov and Watson,2006). The ‘close fold’ contains mainly small, narrow bedding-parallel and exten-sion veins, whereas the ‘open folds’ contain thicker bedding-parallel and fault veins.It is suggested that fault veins with larger apertures may have had faster calciteprecipitation rates. If thicker veins opened more rapidly than narrow veins, thenlarger fluid pressure changes would occur during vein opening, which could increasecalcite precipitation rates.

In a fault-fracture mesh (Hill, 1977), fluid ‘batches’ from a fluid source ascendthrough a ‘random’ series of faults and extension fractures. Each fluid batch wouldfollow a slightly different fluid flow pathway. Therefore, it is predicted that everyvein at the same stratigraphic level would undergo approximately the same amountof fluid-rock reaction. This would lead to veins having approximately the samechemical and isotopic composition, with some scatter depending on the exact fluidflow pathway, and fluid-flow rate (see Fig. 5.1c). While veins within outcrops showsmall variations in isotopic and chemical compositions, consistent with the migrationof fluids through a fault-fracture mesh, it is apparent that there are higher-ordercontrols on vein composition (i.e. causing the systematic differences between veinclusters in the ‘open’ and ‘close’ folds, above).

It is suggested that some larger faults act as long-lived higher permeability struc-tures, along which fluids are channelled. These faults may have been formed viathe amalgamation of smaller faults and fractures into dominant throughgoing faultzones (Sibson, 2001). Thus, veins in each outcrop may have formed from fluidswhich migrated along larger faults, or at different locations along the same fault(rather than ascending along a ‘random’ pathway along multiple small faults andfractures, Fig. 5.21). The throughgoing, bedding-discordant fault structure shownin Fig. 4.10 may represent one of these larger, high permeability fault structures.

Many extension veins in the Currajong Limestone formed during limb-parallelstretching, following fold limb lockup. Extension veins in the Currajong Lime-stone have compositions consistently closer to host rock carbonate than fault orbedding-parallel veins. This suggests that extension veins formed from fluid whichhad largely equilibrated with host rock carbonate. This could be a result of veinsforming from fluid which has spent considerable time equilibrating with the sur-rounding Currajong Limestone prior to vein dilation and precipitation (rather thanfluid ascent directly into veins). Alternatively, the ratio of ‘fracture’ fluid path-ways to ‘within-rock’ fluid pathways (i.e. flow along low-permability paths betweenmacroscopic fractures) may have been lower for extension veins in the CJL, resultingin enhanced fluid-rock reaction and a greater degree of rock buffering. In compar-


Overpressured fluid reservoir

Fault-fracturemesh

Large faults

Channelled fluid flow Channelled fluid flow

Figure 5.21: Schematic diagram illustrating a fault-fracture mesh. If overpressuredfluids cause failure of small faults, then they could travel along virtually any combina-tion of small faults and fractures, leading to variable degrees of fluid-rock reaction atany stratigraphic level. However, if a large fault acted as a high-permeability struc-ture which guided fluid flow, then fluids would undergo similar degrees of fluid-rockreaction.

ison, extension veins in the Spirifer yassensis Limestone are indistinguishable frombedding-parallel or fault veins, implying that extension veins in the SYL formedfrom fluids which did not have the opportunity to equilibrate with host rock car-bonate. This suggests that these veins formed from fluids which flowed dominantlyalong fracture-controlled pathways, rather than low-permeability ‘within-rock’ flowpathways.

5.5 Conclusions

The isotopic and trace element composition of syntectonic veins reflect different de-grees of fluid-rock reaction during hydrothermal fluid flow. The analyses presentedin this chapter reveal that the isotopic and trace element compositions of hydrother-mal veins is variable across stratigraphy, between outcrops, and within individualoutcrops.

A progressive increase in vein δ18O with increasing height through the Mur-rumbidgee Group is attributed to progressive buffering of 18O-depleted fluids byreaction with host rock carbonate. Vein δ18O values, at temperatures of 100− 200◦C, imply that the invading fluid had a δ18O of − 14h to − 8h, consistent with ameteoric fluid source.

Variability in O and Sr isotope ratios in veins is attributed to variable fluid-rockreaction along dynamically changing fluid flow pathways, during episodic fracturingand fluid flow events within a fault-fracture mesh. Systematic chemical variationsbetween veins in adjacent outcrops of stratigraphically equivalent limestone implythat larger-scale fault structures (extending over perhaps tens to hundreds of metres)acted as higher-permeability structures, which preferentially channelled fluid flow.

Strontium isotope ratios (87Sr/86Sr) indicate that fluids react with host rock car-

5.5. Conclusions 119

bonate, but also scavenge Sr from 87Sr-enriched minerals (e.g. illite), within shalebeds over short reactive path lengths. Trace element concentrations generally de-crease with increasing stratigraphic height. Variations in trace element concentra-tions and patterns are not consistent with a simple model of progressive bufferingof fluids via fluid-rock reaction (as indicated by differences in variation of δ18O and87Sr/86Sr at different stratigraphic levels). Trace element concentrations in veins,particularly Sr and Fe, may be affected by variable calcite precipitation rates. Rareearth element concentrations and patterns are influenced by progressive calcite pre-cipitation and sorption along fluid-flow pathways, and changes in CO2−3 complexa-tion in solution.

Trace element incorporation into hydrothermal calcite veins may be influenced bya variety of factors, including changes in the extent of fluid-rock reaction, changes incomplexing species and variations in calcite precipitation rate. By careful integrationwith Sr and O isotope measurements, differentiating some of these processes fromone another was shown to be possible. This study places first-order constraintson the dynamics of fluid pathways and chemical changes within a hydrothermalfluid-flow system. This study could be further developed by increasing the numberof host rocks analysed, and, in particular, characterising the trace element andisotope chemistry of individual mineral phases, particularly within shale layers. Thiswould be a challenging problem, especially for fine-grained phases. In particular,characterising the Sr isotope composition of Rb-bearing phases is complicated by arequirement to make an age correction to determine the Sr isotope composition ofdifferent minerals at the time of hydrothermal mineral deposition.

This chapter provides a macroscale view of spatial variations in C, O and Sr isotoperatios, and trace and rare earth element concentrations in the TVS hydrothermalsystem. In the following chapter, I examine how δ13C, δ18O, 87Sr/86Sr and traceand rare earth element concentrations evolve during the growth of individual veins,thus providing a record of temporal changes in fluid-rock reaction and fluid chem-istry. The interpretation of trace element variations in syntectonic veins is discussed,with particular regard for some of the factors which may influence calcite mineralchemistry that were explored in Chapter 3.

Chemistry of the Taemas Vein SwarmIntroductionMethodsSample collectionStable isotope analysesStrontium isotope analysesTrace element analyses

ResultsVein chemistry changes with stratigraphic heightVein chemistry within and between different outcrops

DiscussionFluid sourceIsotopic variationsTrace element variationsFluid-rock reaction and fluid pathways

Conclusions

chapter 5 chemistry of the taemas vein swarm...chapter 5 chemistry of the taemas vein swarm 5.1...

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