Thesis title: Characterizing Mineralization Phenomena in Active Geothermal Systems. Ryan Libbey. Supervisor: Dr. A. E. Williams-Jones.

Transcription

1 Thesis title: Characterizing Mineralization Phenomena in Active Geothermal Systems Ryan Libbey Supervisor: Dr. A. E. Williams-Jones March 1, 2011 McGill University Department of Earth and Planetary Sciences 3450 University St, Montréal Québec, Canada H3A 2A7

2 1. Introduction Active geothermal systems provide an analog for understanding ancient epithermal metallic mineral deposits. The steep geothermal gradients intrinsically present in the regions containing these systems activate subsurface convection of hydrothermal fluids, a process that entrains and concentrates available metals. Understanding the systematics behind this transport and precipitation of metals increases our ability to model extinct environments that may be of economic interest. Additionally, related hydrothermal processes provide valuable information pertaining to the development of active geothermal resources for electricity generation. My thesis is aimed at the study of mineralization phenomena in active geothermal systems. This project involves the detailed characterization of downhole geology and geochemistry through analysis of drill cuttings obtained from wells that intersect deep geothermal reservoirs. The overarching goal of the project is to model the interaction between deep hydrothermal fluids and the (previously unaltered) host rock to provide an explanation for the present-day mineralogy. The chosen field areas for this project are the Reykjanes Geothermal Field in Iceland and the Rotokawa Geothermal Field in New Zealand. The Reykjanes Geothermal field is a seawater-dominated hydrothermal system hosted within the basaltic lithology and dilative tectonics of the Reykjanes Ridge. The Rotokawa Geothermal field, on the other hand, is a meteoric-water-dominated geothermal system within the andesitic backarc/sudbuction-related volcanics of the Taupo Volcanic Zone. The varied lithology, hydrology, and tectonics between these localities provide me with the opportunity to assess how these variables affect observable mineralization phenomena. In both of these geothermal fields, the deep reservoir fluid has been directly sampled and analyzed in past studies. Reservoir fluid parameters provide valuable information regarding metal saturation and physicochemical conditions at depth, factors which control the dissolution and precipitation of minerals throughout the hydrothermal system. The physical and compositional information provided by this past research provides a unique opportunity to constrain the chemistry of the reacting fluids in the model developed by this thesis - a variable which otherwise requires much extrapolation. This study will include detailed petrology; the study of stable isotope systematics in downhole mineral assemblages; fluid inclusion studies to assess the composition of the mineralizing fluid; Scanning Electron Microscopy to characterize micro-textures and phase relationships; X-Ray Diffraction to assist in mineral identification and provide crystallographic information; Electron Probe Micro-Analysis to quantify elemental compositions; and computer modeling to explain mineralogical changes in terms of fluidrock interactions, and to assess the role of these processes in concentrating metals.

3 2. Background 2.1 Low Sulfidation Epithermal Systems Low-sulfidation epithermal (geothermal) systems are developed in magmaticallyheated upper crustal environments, distal from volcanic centers. Convecting fluids in these systems can penetrate to a depth of 5 km and are typically near-neutral in ph and meteoric in origin. However, notable magmatic components are displayed in some systems, and it is generally maintained that gaseous components in geothermal fluids (dominantly CO 2 and H 2 S) are products of magmatic exsolvation. Phase formation in these systems is determined by temperature, pressure, fluid composition, host rock composition, duration of activity, and water/rock ratios (Pirajno, 2010; Browne, 1978), the latter commonly being a function of permeability. Of these often intimately related parameters, temperature, fluid composition, and water/rock ratios are considered the dominant controls of mineralization in geothermal systems (Browne, 1978); however, the significance of host rock composition increases as fluid/rock ratios decrease. Convective flow in geothermal systems is generally vertical or sub-vertical, and is primarily controlled by faults and fractures, but lithological control can also be a significant factor in directing fluid pathways. Fluids that reach the surface generally discharge as hot, near-neutral, chloride-rich waters or acidic, sulfate-rich solutions that can have a ph of 2 or less (Pirajno, 2010). The latter is produced by separated vapour condensing near the surface, oxidizing H 2 S to H 2 SO 4 and forming a zone of acidic alteration (Vadose zone), analogous to conditions present in high-sulfidation epithermal systems. Terrestrial geothermal systems can endure for a few thousand to ~2 million years (Pirajno, 2010). Over this lifespan, a significant concentration of ore minerals can accumulate as hot fluids leach the surrounding wall rocks of metals, which are then transported via ligand (i.e. S or Cl) complexes. The stability of these complexes becomes comprised with decreases in temperature or pressure (boiling), reduced activity of ligand species, or changes in redox conditions of the transporting fluid, resulting in the precipitation of metallic phases. Ore minerals in low sulfidation epithermal systems are dominantly precipitated within open veins, breccias and stockworks between temperatures of C, at depths of m (White and Hedenquist, 1995). 2.2 Reykjanes Geothermal Field, Iceland Situated at the Southwest tip of Iceland (within the North Atlantic Igneous Province), the Reykjanes Geothermal Field is a basaltic seawater-dominated geothermal system resulting from the onshore expression of the oblique dilatational tectonics of the Reykjanes Ridge. Geothermal systems along the Reykjanes peninsula are the result of sheeted dike complexes rather than pooling magma chambers (Freedman et al., 2009); no central volcanoes are present along the peninsula. Surface hydrothermal manifestations at the Reykjanes Geothermal Field span an area of approximately 1 km 2, however, the thermal anomaly measured at a depth of 800 m stretches to ~2 km 2.

4 Wellheads and pipe infrastructure associated with the Reykjanes Geothermal Power Plant are situated at an elevation of <20 m from the current sea level (Hardardóttir et al., 2010). Seawater enters the hydrothermal system at a depth of about 1.5 km, where it rapidly heats and rises to the surface through steep-angled fractures (Marks et al., 2010). Geothermal fluids in the Reykjanes system represents heated seawater with a salinity of ~3.2 wt.% NaCl, a calculated ph of ~5-6, and temperatures of 270 to 315 C at reservoir depths of >1000 m (Hardardóttir et al., 2010; Freedman et al., 2009). The fluids in this system are 2 ph units more acidic than those found in other Icelandic geothermal fields. Boiling zones in the Reykjanes Geothermal System typically occur at approximately 1200 m depth (Freedman et al., 2009). The host rocks that characterize the Reykjanes peninsula are primarily basaltic in origin and consist of highly porous tholeiitic basalt lavas, basaltic hyaloclastite, and tuffaceous sediments (Marks et al., 2010; Kristmannsdóttir, 1984). The upper 1000 m of stratigraphy is characterized by hyaloclastites and volcaniclastic/marine sediments, while deeper lithologies are dominantly lavas and diabase intrusives (Marks et al., 2010). Host lithologies in the Reykjanes Geothermal Field have been subjected to extensive hydrothermal alteration. This alteration style is typical of low water/rock hydrothermal systems and progrades with increasing depth through smectite-zeolite, chlorite, chloriteepidote, and finally epidote-actinolite (>300 C) zones (Marks et al., 2010; Freedman et al., 2009). Albite is the most common secondary replacement of primary plagioclase in the Reykjanes geothermal system (Lonker et al., 1993) and is ubiquitous where temperatures are in excess of 150 C (Freedman et al., 2009). It is postulated that >1.5 km of ice blanketed the Reykjanes Peninsula during the Pleistocene glaciation. This glacial overburden is believed to have increased hydrostatic pressure in the hydrothermal system, raising the boiling point curve, and accounting for the disequilibrated high-temperature hydrothermal minerals which are present at shallow depths in the Reykjanes system (Fridleifsson et al., 2005). Additional evidence of glacial influence to this hydrothermal system is provided by fluid inclusion studies of hydrothermal quartz, plagioclase, and calcite, which indicate the paleo-presence of a fresh water lens in the upper section of the Reykjanes Geothermal Field (Franzon et al., 2002). Deep reservoir liquid in wells RN-12, RN-19, and RN-21 (Fig. 1) were sampled by Hardardóttir et al. (2009). When compared with boiled surface discharge fluids, it was realized that the deep liquids contained metal concentrations that were three to four times the magnitude of the surface samples. Hardardóttir et al. (2009) reasoned that the deposition of metal-sulfides during adiabatic ascent and boiling accounts for this contrast in metal budgets a process evidenced by the presence of metal-rich scales lining the geothermal pipelines. Sulfide scales are primarily composed of Zn (59 wt%), Cu (31 wt%), Fe (23 wt%), and Pb (17 wt%), with Ag and Au concentrations reaching 2.3 wt% and 600 ppm, respectively (Hardardóttir et al., 2009; 2010).

5 Fig. 1. Map of the Reykjanes Peninsula showing major geological features, surface alteration and location of geothermal wells (Marks et al., 2010). 2.3 Rotokawa Geothermal Field, New Zealand The Taupo Volcanic Zone (TVZ) is a km wide, SSW-NNE trending area of active rifting within the North Island of New Zealand with a NW-SE extension rate ranging from 2 to 16 mm per year; values increase northwards towards the Bay of Plenty (Harvey and Browne, 1991; Harrison and White, 2006). There is also notable NE-SW oblique extension fabricated by rift segment interactions (Rosenberg et al., 2009). The TVZ is a continental expression of the Tonga-Kermadec oceanic backarc spreading center, resulting from the westward sudbuction of the Pacific Plate (Stratford and Stern, 2006). As a result, the North Island of New Zealand has experienced rhyolitic volcanism for approximately 12 million years, with active andesitic and rhyolitic volcanism now confined to the TVZ (Stratford and Stern, 2006). The TVZ is a tectonically vibrant region that extends for 200 km between the andesitic volcanoes of Ruapehu to the south, and White Island to the north in the Bay of Plenty (Harvey and Browne, 1991). Giggenbach (1995) used stable isotopic and chemical compositions of gaseous and liquid geothermal discharge to group the hydrothermal systems of the TVZ into arctype and rift-type classifications. It was found that arc-type systems are concentrated along the eastern boundary of the TVZ (along with the active andesitic stratovolcanoes of the North Island). Metal-rich scales are found associated with these volatile-rich, andesitic geothermal systems. Westward of these arc-related systems are geothermal areas fueled by the region s dilational back-arc tectonics and magmatism (Fig. 2).

6 Fig. 2. West-East cross-section through the TVZ. Group A (arc-type) geothermal systems include Rotokawa, Broadlands-Ohaaki and Kawerau. Group B (rift-type) systems include Wairakei and Mokai (Giggenbach, 1995). Rotokawa Geothermal Field, categorized as an arc-type system, is located along the eastern boundary of the TVZ. It is dated at <20,000 years old, and is developed in a sequence of Pleistocene and Holocene volcanics overlying greywackes and argillites of the Torlesse supergroup. Geothermal fluids at Rotokawa adiabatically ascend along subvertical faults, creating boiling horizons between 1500 and 700 m depth. These fluids are dominantly meteoric in origin, bearing measured temperatures as high as 320 C (Krupp, 1987). Reservoir fluids sampled at a depth of 1300 m by Simmons and Brown (2007) contained maximum Au and Ag concentrations of 23 ppb and 2.4 ppm, respectively. From this, it was calculated that gold and silver fluxes in Rotokawa could supply enough metal to equal the largest known terrestrial hydrothermal deposits in ~50,000 years or less. Other signatures of the high metal fluxes in the Rotokawa system are surface deposits of sulfide-rich mud, containing ore grades of gold within arsenic and antimony complexes, and the metal-rich scales found in geothermal pipelines (Krupp, 1987; Reyes et al., 2002). These scales, produced as extracted geothermal fluids are flashed at the surface, contain weight percent concentrations of Ag (4.0 wt%), Cu (5.6 wt%), Te (3.0 wt%), Zn (1.5 wt%), Pb (0.6 wt%), and Au (0.2 wt%).

7 3. Methodology A) Determine mineralogical characteristics of hydrothermally-altered rock Optical microscopy is being utilized to describe mineralogy, alteration intensity and phase relationships in the supplied drill cuttings; Scanning Electron Microscopy (SEM) of rock fragments will supplement the study of phase relationships. Energy Dispersive Spectrometry (EDS) and Wavelength Dispersive Spectrometry (WDS) of polished thin sections are being used to assist in phase identification and to assess trace and major element compositions. Basally-oriented preparations of the <2 µm fraction, isolated by means of centrifugation and decantation, are being utilized to describe clay and zeolite mineralogy via X-ray diffraction (XRD). Preliminary results from microprobe and optical microscope analysis have already confirmed the presence of unpublished phases in the Reykjanes geothermal system (see Appendix). B) Determine mineralogical characteristics of unaltered rock Fieldwork is required to retrieve representative samples of unaltered Reykjanes basalt/hyaloclastite, and Rotokawa rhyolite/tuffaceous breccia. Optical microscopy, and EDS/WDS will be used to delineate and quantify the mineralogy of these samples. These methods, along with XRD of the <2 µm fraction will also be used to confirm the unweathered/unaltered nature of the rocks. If necessary, SEM of unprocessed rock fragments will be used to provide textural information pertaining to phase origins and relationships. C) Determine physical-chemical conditions of reacting fluid Downhole temperature measurements and reservoir fluids sampled and analyzed by Hardardóttir et al. (2009) and Simmons and Brown (2007) provide valuable information pertaining to the physicochemical conditions of the subsurface fluid. It is appreciated, however, that these values may not represent the paleo-conditions present during the mineralization of these systems. To assess paleo-fluid temperatures and paleofluid compositions this study will utilize mineral-pair stable isotope geothermometry, fluid inclusion analyses and inverse modeling techniques (using calculated compositions for unaltered and altered host rocks). D) Develop a model to explain interaction of fluid with rock GEM-Selektor and HCh modeling software will be utilized to create a model for the studied geothermal reservoirs. The premise of these models will be to react the calculated geothermal fluid with the calculated unaltered host rock to derive a geochemical explanation for the altered/mineralized rock in the present-day subsurface.

8 4. Expected Contributions The results of this study are expected to contribute to a better understanding of mineralization in deep geothermal reservoirs, which is important for the exploration and assessment of fossil low-sulfidation epithermal metal deposits, and to the development of geothermal energy resources and the mitigation of mineralized fluid pathways. The results of this study may also provide insight into the relative roles of magmatic fluids versus host rocks in supplying metal budgets to hydrothermal systems. 5. Project Timeline March 2011-October 2011 Optical microscopy work. Microprobe work. XRD of the <2 µm fraction (University of Western Ontario). Stable isotope analysis of mineral pairs. Fieldwork. October 2011-March 2012 Fluid inclusion studies. Remaining microprobe work. Develop thermodynamic models in GEM-Selektor/HCh. March 2012-June 2012 Write manuscript. Supplement studies if necessary. 6. References Brown, P.R.L., 1978, Hydrothermal alteration in active geothermal fields, Ann. Rev. Earth Planet. Sci, n. 6, p Franzson, H., Thordarson, S., Bjornsson, G., Gudlaugsson, S., Ritcher, B., Fridleifsson, G.O., and Thorhallsson, S., 2002, Reykjanes high-temperature field, SW-Iceland. Geology and hydrothermal alteration of well RN-10, Proc. Twenty-Seventh Workshop on Geothermal Reservoir Engineering, Freedman, A.J.E., Bird, D., Arnorsson, S., Fridriksson, T., Elders, W.A., Fridleifsson, G.O., 2009, Hydrothermal minerals record CO2 partial pressures in the Rykjanes geothermal field, Iceland, American Journal of Science, v. 309, p Fridleifsson, G., Blischke, A., Kristjánsson, B., Richter, B., Einarsson, G., Jónasson, H., Franzson, H., Sigurdsson, O., Danielsen, P., Jónsson, S., Thordarson, S., Thórhallsson, S., Hardardóttir, V., and Egilson, T., 2005, Reykjanes well report RN-17 & RN-17ST. Technical Report ISOR-2005/007, ISOR, 284 pp.

9 Giggenbach, W.F., 1995, Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand, Journal of Volcanology and Geothermal Research, v. 68, p Hardardottir, V., Hedenquist, J.W., Hannington, M.D., Brown, K., Fridriksson, TH., and Thorhallsson, S., 2010, Proc. World Geothermal Congress 2010, Bali, Indonesia. Hardardóttir, V., Hannington, M., Hedenquist, J., Kjarsgaard, I., and Hoal, K., 2010, Cu-rich scales in the Reykjanes Geothermal System, Iceland, Economic Geology, v. 105, p Hardardóttir, V., Hannington, M.D., Hedenquist, J.W., and Kjarsgaard, I., 2007, Quenched blacksmoker fluids: Evidence from bornite scales precipitated from seawater-dominated geothermal fluids on the Reykjanes peninsula, Iceland: Geological Society of America Abstracts with Programs, v. 39, n. 6, paper no Harrison, A., and White, R., 2006, Lithospheric structure of an active backarc basin: the Taupo Volcanic Zone, New Zealand, GJI Volcanology, Geothermics, Fluids and Socks, v. 167, p Harvey, C., and Browne, P., 1991, The application of mixed-layer clays as mineral geothermometers in the Te Mihi sector of Wairakei Geothermal Field, New Zealand, Proc 13 th New Zealand Geothermal Workshop, v. 13, p Kristmannsdóttir, H., 1984, Chemical evidence from Icelandic geothermal systems as compared to submarine geothermal systems, NATO Conference Series, IV Marine Science, v. 12, p Krupp, R.E., and Seward, T.M., 1987, The Rotokawa Geothermal System, New Zealand: An active epithermal gold-depositing environment, Economic Geology, v. 82, pp Lonker, S.W., Franzson, H., and Kristmannsdóttir, H., 1993 Mineral-fluid interactions in the Reykjanes and Svartsengi geothermal systems, Iceland, American Journal of Science, v. 293, p Marks, N., Schiffman, P., Zierenberg, R.A., Franzson, H., Fridleifsson, G.O., 2010, Hydrothermal alteration in the Reykjanes geothermal system: insights from Iceland deep drilling program well RN- 17, Journal of Volcanology and Geothermal Research, v. 189, p Pirajno, F., 2010, Hydrothermal Processes and Mineral Systems, Springer. Reyes, A. G., Trompetter, W. J., Britten, K., Searle, J., 2002, Mineral deposits in the Rotokawa geothermal pipelines, Journal of Volcanology and Geothermal Research, v. 119, p Rosenberg, M., Bignall, G., and Rae, A., 2009, Geological framework of the Wairakei-Tauhara Geothermal System, New Zealand, Geothermics, v. 38, p Simmons, S., and Brown, K., 2007, The flux of gold and related metals through a volcanic arc, Taupo Volcanic Zone, New Zealand, Geology, v. 35, no. 12, p Stratford, W., and Stern, A., 2006, Crust and upper mantle structure of a continental backarc: central North Island, New Zealand, GJI Volcanology, geothermics, fluids and rocks, v. 166, p White, N.C., and Hedenquist, J.W., 1995, Epithermal gold deposits: styles, characteristics and exploration, SEG Newsletter, no. 23, pp. 1, p

10 7. Appendix Figure showing selected preliminary optical microscopy and microprobe data from Reykjanes wells RN-10 and RN-17 (Libbey and Williams-Jones, unpubl.). A) BSE image of unconfirmed Cu-Zn vein mineral phase. B) BSE image of fractured euhedral chromite and chalcopyrite. C) BSE image of ilmenite mineralization surrounding coarse chlorite rimming calcite. D) EDS Al-map of zoned euhedral epidote. E) EDS Fe-map of zoned euhedral epidote. F) Skeletal ilmenite, reflected light. G) Subhedral chalcopyrite and chromite, reflected light. H) Unconfirmed Cu-Zn vein mineral phase, reflected light. I) Radial epidote overprinting massive replacement epidote phase, XPL. J) Resorbed coarse pyrite with stable chalcopyrite. K) EDS spectrum of unconfirmed Cu-Zn vein mineral phase.