Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 Insights to High Temperature Geothermal Systems (New Zealand) from Trace Metal Aqueous Chemistry Stuart F. Simmons and Kevin Brown Hot Solutions Ltd, PO Box 30-125 Devonport, Auckland, New Zealand GEOKEM P.O. Box 30-125, Barrington, Christchurch 8244,New Zealand stuart@hotsolutions.co.nz; kevin@geokem.co.nz Keywords: trace metals, aqueous geochemistry, New Zealand geothermal systems ABSTRACT In the period 2001-2005, we deployed a titanium down-hole sampler to obtain deep geothermal water samples for trace metal analyses from high temperature geothermal systems in the North Island, New Zealand. Samples were collected from production and monitor wells at ~1 km depth over the temperature range 200-330 C from Broadlands-Ohaaki, Kawerau, Mokai, Ngawha, Ngatamariki, Rotokawa, and Wairakei. Water samples and field blanks were analyzed for As, Ag, Au, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Te, Tl, W, and Zn by ICP-MS analysis at CSIRO, and the results show considerable variability. Most data are in the range of 0.1 to 1000 ppb, but Ag, As, Cu, Sb, Ni, Zn, and Te attain concentrations more than 1000 ppb in deep thermal waters from some systems. The concentrations of many metals range 1-2 orders of magnitude variation, but Ag and Te range 3 orders of magnitude concentration. The variability in metal concentrations between three systems, Ngawha, Rotokawa, and Wairakei, show a geospatial pattern that correlates to some extent with the concentrations of deep sourced constituents, Cl and B. The results show that metal supply is heterogeneous and controlled deep in the system, by a combination of intruding magmas and possibly local country rocks. 1. INTRODUCTION High temperature geothermal systems in New Zealand have long been known as sites of metal-transport and deposition (e.g., Brown, 1986; Krupp and Seward, 1987; Simmons and Browne, 2000; Reyes et al. 2002; Wilson et al., 2007). Because metal concentrations of reservoir waters tend to reduce due to boiling and cooling, during ascent through a geothermal well, we built a titanium sampler to acquire down-hole reservoir water samples for trace-metal analysis. Brown and Simmons (2003) and Simmons and Brown (2007) describe some of the results of this work, which mainly focused on gold and silver trends and fluxes in Taupo Volcanic Zone (TVZ) geothermal systems. In this paper, we present the full set of trace metal data that were analyzed on down-hole water samples acquired in the period 2001-2005. Samples were obtained from six geothermal systems in the TVZ, including Broadlands-Ohaaki, Kawerau, Mokai, Ngatamariki, Rotokawa, and Wairakei. In addition, we sampled the Ngawha geothermal system, which is affiliated with intra-plate mafic volcanism (Giggenbach et al., 1993; Smith et al., 1993; Simmons et al., 2005); it is the only high temperature geothermal system in New Zealand outside of the TVZ (Fig. 1). All of the wells sampled are located near the upflow zone of the system except for Nm4 at Ngatamariki, which is in a peripheral setting. 2. METHODS The procedures for down-hole trace-metal sampling were the same as those used during earlier investigations (Brown and Simmons, 2003; Simmons and Brown; 2006, 2007). The titanium sampler employs Viton o-rings and a non-return valve to seal the vessel once the fluid is sampled. We used copper seals to minimize leakage at sampling conditions 290 C, hence contamination may account for all of the copper measured at Broadlands-Ohaaki, 20% of the copper measured at Mokai, and 50% of the copper measured at Rotokawa. Before and after all the runs, the o-rings and seals were replaced and the sampler was cleaned with aqua regia, using BDH Aristar acids, and de-ionized water. After sampling, all of the fluid sample (550 800 ml of clear liquid with trace precipitates) plus 80 ml of aqua regia and 80 ml of de-ionized water, which were used to rinse the sampler of residual precipitates, were combined into a high density polyethylene plastic bottle before analysis by inductively coupled plasma mass spectrometry (ICP-MS) at CSIRO Energy Technology Centre for Advanced Analytical Chemistry in Lucas Heights, Australia. The method of standard addition was used to give analytical precision of 15% or better. Descriptions of the locations and samples are presented in Table 1, and the results are summarized in Figure 2. The restricted access to wells limited the number of down-hole samples and the number of replicate analyses obtained. Analyses of blanks at all geothermal fields, except Kawerau, wherein the sampler was partially filled with de-ionized water (500 625 ml), sealed, lowered to the sampling depth, retrieved and processed in the same manner as a deep fluid sample, support the reliability of our measurements. All of the deep hydrothermal waters studied are of meteoric origin, and they have been modified by deep circulation, the incorporation of magmatic gases, and subsequent fluid-mineral interactions (Sheppard, and Giggenbach, 1985; Giggenbach, 1995; 1997; Giggenbach et al., 1993). They are dilute (<2000 ppm Cl), near-neutral ph and reduced, and they are close to thermodynamic equilibrium with hydrothermal minerals that commonly occur in the deep altered volcanic host rocks, including albite, adularia, quartz, chlorite, illite, calcite, epidote and pyrite (Giggenbach, 1997; Simmons et al., 2005). Figure 3 shows the trends in chloride and boron, which generally represent deep sourced constituents (e.g., Giggenbach, 1995). 1
Figure 1: High-temperature geothermal systems in the North Island, New Zealand, and the localities studied. Table 1. Descriptions and locations of wells and fluid samples. 2
Figure 2: Trace metal results for deep thermal waters; concentrations in μg/kg (ppb). Figure 3: Relative proportions of chloride and boron in deep thermal waters. The highest Cl/B ratios are associated with western TVZ geothermal systems (Mokai, Ngatamariki, and Wairakei), and intermediate Cl/B ratios are associated with eastern TVZ geothermal systems (Broadlands-Ohaaki, Kawerau, Rotokawa). Within the TVZ, aqueous boron ranges from 10-35 mg/kg whereas aqueous chloride ranges from 500-2000 mg/kg. The lowest Cl/B ratio occurs at Ngawha, where aqueous chloride is ~1300 mg/kg and aqueous boron is ~950 mg/kg. 3
3. RESULTS Figure 2 shows considerable variability in the concentrations and ranges of trace metals. Cadmium, gold, mercury, molybdenum selenium, thallium and tungsten mostly have concentrations 100 ppb, and they range in value from 10 to 300 times. Arsenic, copper, manganese, nickel and zinc mostly have concentrations 100 ppb, and they range in value from 10 to 60 times. Antimony, lead, silver, and tellurium have concentrations that span 100 to >1000 times, bracketed by values of <1 to >1000 ppb. Arsenic, antimony, copper, nickel, silver, tellurium, and zinc attain the highest concentrations with values in some samples that exceed 1000 ppb. Only a few pairs of elements appear to show some degree of correlation (Fig. 4). For example, most of the TVZ samples have As/Sb ratios between 3 and 8, except Wairakei with As/Sb>50. For comparison, the average crustal abundance is As/Sb~7.6 (Hu and Gao, 2008). By contrast, the As/Sb ratio is ~0.2 at Ngawha, which is significantly lower. The Te/Ag ratio ranges from 0.7 to 15, compared to an average crustal rock value ~2 (Hu and Gao, 2008), and the Cu/Au ratio ranges from 700 to 25,000, compared to an average crustal rock value ~18,000 (Hu and Gao, 2008). When the relative proportions of these elements are plotted (Fig. 5), Ngawha, Rotokawa, and Wairakei occupy end member positions. In comparing these three systems with respect to absolute concentrations of metals, Rotokawa has the highest Cu, Pb, Zn, Ag, Au, and Te, Ngawha has the highest antimony and mercury, and Wairakei has the highest arsenic, noting that Wairakei and Rotokawa are only 10 km apart. Although not a focus of this paper, Figure 6 shows the effect of boiling on the trace metal concentrations of deep Rotokawa water. Evaporation of steam and exsolution of gases (CO 2, H 2 S, and H 2 ) during fluid ascent sharply reduces precious and base-metal solubilities, causing metal deposition in the two-phase pipeline (e.g., Brown, 1986; Reyes et al., 2002). 4. DISCUSSION AND CONCLUSIONS Evidence of metal-bearing mineral assemblages in potential source rocks is absent in all the systems studied. Hence, the heterogeneous nature of the trace metal data implies a localized deep control on the supply of metals in hydrothermal solutions. This is evident from examination of the six systems in the TVZ, which share a common geological setting and stratigraphy at <3km depth. The inclusion of Ngawha data further strengthens this argument. To illustrate this, we compare the trends in Figures 5 and 6, with Cl and B data in Figure 3. Within the TVZ, Cl/B values vary systematically from ~20-50 in the east to ~70-100 in the west. Giggenbach (1995), in conjunction with N 2 /He and helium isotope ratios, suggested these trends trace to intrusion of andesitic magma beneath eastern geothermal systems and intrusion of basaltic magma beneath western geothermal systems. On this basis, the differences in concentrations and proportions of metals between Rotokawa from Wairakei (Figs. 5 & 6) can be explained as being due to differences in the compositions of source intrusions. Comparison with the other four TVZ geothermal systems, however, shows significant metal variance, and this makes it is difficult to characterize a basaltic versus andesitic trace metal signature. That As/Sb ratios are similar for five of the six systems and close to the average crustal value highlight the possibility that these and some other metals have a deep country rock origin as well or instead. The B concentration, the Cl/B ratio, and the As/Sb ratio of the deep thermal water are highly anomalous at Ngawha. This system is hosted in sedimentary rocks, and fractured meta-greywacke makes up the reservoir rock, with an As/Sb ratio that ranges between 3 and 11. Evidence of deep magmatic intrusion comes from helium isotope ratios of 5-7 R/Ra (Giggenbach et al., 1993), and it is likely mafic in nature based on the extent of the adjacent basaltic Kerikeri volcanic field. None of these aspects, however, provide clues about the cause or deep sources of B, As, and Sb. Whether they derive from intruding magma or the country rock, it appears these fluid components have a unique deep origin. As an aside, Te is valued as a critical metal (Critical Metals Strategy, DOE, 2011), and its presence in significant concentrations in dilute thermal water at Rotokawa, complements the known occurrences of gold and silver as a potentially recoverable commodity. 5. ACKNOWLEDGMENTS We thank Contact Energy, Mighty River Power, Tauhara North No. 2 Trust, Top Energy, and Tuaropaki Power Company for access to geothermal wells, and we thank Century Resources for assistance with sampling. 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Hedenquist, J.W., Simmons, S.F., Giggenbach, W.F. and Eldridge, C.S.: White Island, New Zealand, volcanic hydrothermal system represents the geochemical environment of high sulfidation Cu and Au ore deposition, Geology, 21, (1993), 731-734. Hu, Z., and Gao, S.: Upper crustal abundances of trace elements: A revision and update: Chemical Geology, 253, (2008) 205-221. 4
Figure 4: Correlation plots of As vs Sb, Te vs Ag, and Au vs Cu for deep thermal waters. Figure 5: Relative proportions of As-Sb-Ag and Cu-Au-Te in deep thermal waters. Figure 6: Comparison of metal compositions of Ngawha, Rotokawa, and Wairakei. Grey vertical arrow indicates the effect of boiling determined from analysis of water samples taken from the two-phase pipeline at Rotokawa (Rk-5). 5
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