Elsevier

Chemical Geology

Volume 185, Issues 1–2, 15 April 2002, Pages 71-91
Chemical Geology

Stable isotope geochemistry of cold CO2-bearing mineral spring waters, Daylesford, Victoria, Australia: sources of gas and water and links with waning volcanism

https://doi.org/10.1016/S0009-2541(01)00397-7Get rights and content

Abstract

Mineral springs in the Victorian Central Highlands, Australia, have high CO2 contents and naturally effervesce. δ13C values of CO2 gas and dissolved inorganic carbon are −10.6‰ to −7.0‰ and −5.9‰ to −0.1‰, respectively, with a net δ13C of −8‰ to −3‰. The carbon in these waters was derived from a mantle source associated with local Pliocene to Recent basaltic Newer Volcanic Province rocks. Previously reported 3He/4He (1.2–3.1 relative to air) and high He/Ne ratios are also consistent with the presence of magmatic volatiles. Silica contents imply that the waters were never heated above 130 °C and that the system is not hydrothermal. The occurrence of carbonated mineral springs in a relatively small region of the Newer Volcanics Province where volcanic activity ceased several thousands of years ago may be due to the presence of late intrusions combined with deep circulation of water through deeply weathered and fractured Ordovician basement. A region of low seismic velocity under the Daylesford area potentially images those intrusions. Most spring waters have δ18O and δ2H values of −8‰ to −6‰ and −45‰ to −35‰, respectively, and lie to the left of the local and global meteoric water lines. The anomalously low δ18O values results from CO2 exsolution at low temperatures which strongly partitions 18O into the gas. The lack of waters lying to the right of the local meteoric water line also implies that water–rock interaction at elevated temperatures did not occur. The δ2H values are lower than contemporary meteoric water, suggesting that the waters may have recharged under colder climate conditions several thousand years ago. The local Ordovician rocks are gold bearing. The present spring system is cold and would not efficiently transport Au. However, volcanic is waning and the spring systems at the time of volcanism may have been hotter and able to redistribute Au.

Introduction

Understanding the systematics of mineral springs is important for a wide range of geological processes. In regions where there are little data from monitoring wells, the geochemistry of springs together with their distribution and discharge rates and temperatures can provide valuable information on sources of groundwater, groundwater flow patterns, water–rock interaction, timing of recharge and mixing of distinct groundwater bodies. Mineral springs are commonly used as a source of bottled water or as a recreational resource (tourism, spa facilities, etc.) and a sound knowledge of their hydrogeology is required for sustainable usage.

Mineral springs may be hot (e.g. Sheppard, 1986, Parry and Bowman, 1990, Krupp and Seward, 1990, Larsen et al., 2001), defined by Mazor (1991) as having a temperature of >6 °C above that of mean annual surface temperature, or cold (e.g. Harris et al., 1997, Céron et al., 1998, Larsen et al., 2001). Understanding the distribution of temperatures in mineral springs is also important as hot springs are characteristic of active hydrothermal areas with elevated geotherms while cold springs imply less or no hydrothermal activity. There will be a transition from cold to hot spring systems as volcanic activity increases, and a transition back to cold spring systems as volcanic activity wanes. Thus, springs may also constrain the stage of the volcanic cycle in a particular region.

Gas (typically CO2, N2, CH4 and/or H2S, together with important minor components such as He and Ar) commonly accompanies water discharge in spring areas. CO2 in mineral springs may be derived from a variety of sources, including metamorphic devolatilisation, magmatic degassing, oxidation of organic matter and interaction of water with sedimentary carbonates (e.g. Schoell, 1983, Chivas et al., 1983, Chivas et al., 1987, Griesshaber et al., 1992, Giggenbach, 1992, Giggenbach and Corrales-Soto, 1992, Harris et al., 1997, Céron et al., 1998). Determining the origin(s) of these volatiles is critical. The presence of mantle-derived or magmatic carbon attests to the presence of current or very recent volcanic activity that may not be apparent from the surface geology. Variations in the proportions of mantle- to crustally derived volatiles allows the spatial extent of volcanism to be delineated.

Finally, many mineral springs systems contain water with relatively high total dissolved solids. Such high solute loads require either high-temperature water–rock interaction or a protracted period of low-temperature water–rock interaction. Thus, mineral springs (especially in nonhydrothermal systems) are likely to contain palaeowaters that may preserve evidence of former climatic conditions.

Here, we examine the stable isotope geochemistry of mineral springs from the central Victorian Highlands, Australia. We show that the waters are palaeowaters and that the spring system is not hydrothermal. Further, we show that the CO2 is derived from a mantle source and that there consequently is current, albeit waning, igneous activity at depth that is not immediately apparent from the surface geology.

The Daylesford region in the Central Highlands of Victoria (Fig. 1) lies some 600–700 m above sea level contains more than 100 mineral springs (Fig. 2). The springs lie between 2 and 45 km from the crest of the Great Dividing Range (a range of hills some 50–60 m above general elevation levels that forms a major watershed in southeast Australia), mainly in the valleys of the Loddon and Campaspe rivers and their tributaries (e.g. Wishart and Wishart, 1990, Shugg and Knight, 1994, Shugg, 1996). The region mainly comprises a deeply dissected plateau of Quaternary basalts (the Newer Volcanics) overlying Ordovician slates, shales and sandstones of the Lachlan Fold Belt. The Newer Volcanics are largely extrusive intraplate igneous rocks that cover over 15 000 km2 of Victoria and South Australia (Fig. 1) with over 400 known eruption points (Fig. 2). The comprise mainly basalts with lesser volumes of trachytes and phonolites Coulson, 1954, Price et al., 1988. Fig. 2 shows the location of eruptive centres in the Newer Volcanics Province. Eruptions in the Newer Volcanics Province commenced at ∼4.6 Ma and peaked at ∼2.6 Ma, with activity in the southwest of the province continuing to as late as 5 ka Gill and Gibbons, 1969, Price et al., 1988. In the Daylesford area, a hawaiitic basalt from the Mount Franklin scoria cone was dated using K/Ar at 470 ka (Nicholls et al., 1993). The Ordovician rocks belong to the Lancefeldian and Bendigonian units, which are part of a ∼4500-m thick sequence of turbidites (Cas and VandenBerg, 1988). The Ordovician rocks were folded and deformed under greeschist-facies conditions during a series of tectonic episodes in the Late Ordovician to Middle Devonian (Gray, 1997), and economically significant vein-hosted gold deposits were also formed at that time (Gray and Foster, 1998). Kilometre-scale granitic batholiths were emplaced into the region in the Devonian White et al., 1988, Gray, 1997, and there are several outcrops of the Harcourt Granite in the Daylesford region (Fig. 2). Minor rock units cropping out in the Daylesford Region include thin (<30 m) Permian gravels and clays and Pliocene to Recent alluvial sediments.

Except for a few sandstone units that contain an intergranular porosity, the Ordovician rocks form a fractured aquifer (the Ordovician Bedrock Aquifer) through which mineral water flows in fissures and joints Lawrence, 1969, Laing, 1977, Shugg, 1996. The area is transected by a dominant set of west-dipping thrust faults with less common conjugate east-dipping structures (Gray and Willman, 1991) that form major conduits for large-scale groundwater flow (Shugg and Knight, 1994). Mining records show that active mineral water flows were often encountered in gold mines at depths of up to 450 m, indicating that fracture-hosted fluid flow occurs to considerable depth (Shugg and Knight, 1994). Recharge of the mineral waters was assumed by Shugg and Knight (1994) and Shugg (1996) to mainly occur on the crest of the Great Dividing Range. Near-surface (<50 m) fluid flow also occurs though the weathered zone of the Ordovician bedrock. Water in this shallow Ordovician aquifer is typically brackish to saline due to evapotranspiration and dissolution of cyclic salts in the weathered Ordovician rocks.

The mineral springs typically occur in stream beds or banks where the water table intersects the ground surface. In their natural state, spring discharges are marked by seeps of brown water and gas discharges through stream water. However, most mineral springs in the Daylesford Region have been developed by installing hand pumps over the spring eyes, driving metal pipes into the stream bank, or digging culverts into weathered rock and inserting metal pipes (Wishart and Wishart, 1990). In most cases, development also involved stopping flow from surrounding spring eyes to increase flow from one or two points. Here we describe the stable isotope geochemistry of spring waters and gases from 19 localities in the Daylesford region (Fig. 2). Descriptions of the springs are provided by Wishart and Wishart (1990), and are summarised in Table 1. At all sites, CO2 and minor mineral water discharges were observed in the adjacent creeks and rivers for up to several metres from the springs, especially at times of low flow rates. The springs cover a wide area both south and north of the Great Dividing Range and include sources in basalt and the basement Ordovician turbidites.

Spring water in the Daylesford region of Victoria, southeast Australia, has significantly different chemistry compared with much Australian groundwater. Notably, the spring waters have high bicarbonate, Ca and Mg contents Wishart and Wishart, 1990, Weaver et al., 2002 rather than being Na- and Cl-dominated (Table 2; Fig. 3). The waters naturally effervesce (Laing, 1977), and gas discharge is commonly observed in rivers adjacent to the springs. Although the springs have been exploited as a source of drinking water and for spa facilities for over a century, their hydrogeology is still relatively poorly understood. This paper presents the first comprehensive stable isotope study of the mineral springs in the Daylesford region and discusses their origins, location and gas–water–rock interaction. A companion paper (Weaver et al., submitted for publication) discusses the hydrogeology and chemistry of the springs.

Several models for the origin of the CO2 in the minerals springs have been suggested. McLaughlin and Macumber (1968) suggested that the carbon was derived from oxidation of local carbonaceous shales by groundwater. Due to the occurrence of the springs in the area of Newer Volcanics, Lawrence (1969) proposed that the CO2 had an igneous source. Similar conclusions were reached by Chivas et al. (1983) on the basis of limited C and He isotope data of gasses from unnamed springs in the Daylesford region. A magnetic source for liquid CO2 from a deep well near Mount Gambier in the far west of the Newer Volcanics province (Fig. 1) was proposed by Chivas et al. (1987) again from C and He isotopes. By contrast, Shugg and Knight (1994) proposed that the CO2 in the Daylesford region was derived from carbonate via reactions that breakdown kaolinite to form Na-Beidelite. These reactions liberate H+ that may cause dissolution of carbonates that are locally present in joints. This model is similar to those of Blake (1989) and Herczeg et al. (1991) for the formation of bicarbonate-rich groundwater in the Otway and Great Artesian Basins. However, while the origin of the CO2 has been a source of contention, there has been relatively little stable isotope work carried out on the mineral waters and their associated gases. Additionally, the limited carbon isotope data are for gas, with no data for the dissolved inorganic carbon (DIC) that represents a substantial reservoir of the carbon in the system.

Section snippets

Sampling and analytical techniques

Several parameters (pH, Eh, EC, alkalinity, temperature, and CO2) were measured in the field. Major cations were analysed on filtered and acidified samples by ICP-OES and anions were analysed on unfiltered and unacidified samples by ion chromatography. Representative chemical analyses and field parameters are shown in Table 2, a full discussion of water chemistry is presented in Weaver et al. (submitted for publication). Water and gas samples for stable isotopes were collected in 50-ml glass

Physical and geochemical characteristics

Representative physical and geochemical characteristics of the waters are shown in Table 2. The waters have high C contents (up to 2516 mg/l CO2 and 2648 mg/l HCO3) reflecting the abundance of CO2 in the springs system. The spring waters have high Mg (up to 215 mg/l) and Ca (up to 250 mg/l) contents, and Si contents of up to 124 mg/l (as SiO2). EC values are in the range 1120–4700 μS/cm and pH values vary from 5.8 to 6.9. Typical discharge temperatures are between 7 and 16 °C, with winter

Temporal and spatial variations

The springs show little temporal variation in geochemistry. In particular, there is little difference between the δ18O and δ2H values of the samples collected in May or April after prolonged dry periods and those collected in the wetter winter months (July–September). The exception to this are the Taradale, Turpins Falls and Kyneton springs in the north of the area. For Kyneton and Taradale, water samples collected in August lay on the LMWL while for Turpins Falls, the June sample lay on the

Origin of carbon

Mantle-derived carbon has a δ13C ∼−8‰ to −5‰ Deines, 1970, Kyser, 1986, Sheppard, 1986. At igneous temperatures, the 13C fractionation between carbon contained in a magma and CO2 is ∼2‰ (Sheppard, 1986). Hence, CO2 derived from a magmatic source is predicted to have a δ13C value of −6‰ to −3‰. This δ13C value is close to the net δ13C value of the spring waters. However, the δ13C value of mantle CO2 overlaps with that of bulk crustal carbon (−7.0‰ to −5.5‰: Ohmoto and Rye, 1979), and unambiguous

Conclusions

Mineral water from the Daylesford region is comprised of components from different sources. The water itself is predominantly locally recharged meteoric water that is several thousand years old. Other components (principally C and He) are mantle-derived and associated with recent basaltic volcanic activity. These components were initially transported as a gas that mixed with deeply circulating meteoric water in a fractured aquifer system. Many of the other solutes originated from the

Acknowledgements

This study was supported by grants from the Australian Research Council and the Victorian Mineral Waters Commission. We thank M. Yanni for help with the stable isotope analyses. Comments by L. Hoke and Y. Taran helped improve the manuscript.

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