JCU SEG Student Chapter
New Zealand, North Island Field Trip 2015 Field trip report
Rotorua (www.flyingkiwi.com)
JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Table of Contents Introduction by the JCU SEG Student Chapter Committee .............................................. p. 2 Itinerary and Map ............................................................................................................. p. 3
Wednesday, Nov. 11th – Rangitoto Volcano Hike ............................................................. p. 5 Thursday, Nov. 12th – Glenbrook Steel Mill.......................................................................... p. 9 Friday, Nov. 13th – Martha Mine and Karangahake Gorge ............................................... p. 13 Saturday, Nov. 14th – White Island Volcano ..................................................................... p. 17 Sunday, Nov. 15th – Wai-O-Tapu geothermal field ........................................................... p. 19 Monday, Nov. 16th – Waimangu geothermal field ........................................................... p. 27 Tuesday, Nov. 17th – Wairakei GNS Research Center and Wairakei Power Plant ............ p. 32 Wednesday, Nov. 18th - Tongariro Alpine Crossing .......................................................... p. 36 Thursday, Nov. 19th – Mount Ruapehu ............................................................................. p. 38
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Introduction by the JCU SEG Student Chapter Committee The James Cook University (JCU) SEG Student Chapter has recently organized a geological fieldtrip to the North Island of New Zealand. It took place from the 10th to the 20th of November, 2015 and brought together an international group of geologists from Argentina, Chile, France, Germany, Mongolia, Switzerland, the United States of America, and Australia. Eleven participants, 3 industry representatives and 8 postgraduate students (7 from JCU), met in Auckland on November 10th for the start of an unforgettable journey across the North Island of New Zealand. The aim of this field trip was to traverse from Auckland to Wellington and learn about ore forming processes in an active arc setting covering aspects of mineral exploration, mining, metallurgy, volcanology, and geothermal processes. The itinerary provided the participants with a broad exposure to economic geology on New Zealand’s North Island. This field trip report, written by the student participants, is a day-by-day overview of the fascinating and spectacular geological features witnessed during the trip, complemented by background information and field observations. Some of the highlights of field trip include a day hike on the Rangitoto Volcano, a visit to the Glenbrook steel mill, to the low-sulfidation epithermal Martha gold mine, to White Island, to the Wai-O-tapu and Waimangu geothermal fields and a hike through the Tongariro Alpine Crossing. This trip would not have been possible without the generous support of our sponsors and industry participants whose contributions enabled us to offer a reduced student participation fee. Sincere gratitude is extended to the Society of Economic Geologists (Stewart R. Wallace fund), AMIRA International, Economic Geology Research Centre (EGRU) at JCU, Australian Institute of Geoscientists (AIG), James Cook University Student Association (JCUSA), and Dr. Pat Williams (JCU). We thank and acknowledge Bunnings Warehouse, Coles, and Woolworths for their contributions of food and supplies to our monthly BBQ fundraisers. We would like to thank our student volunteers, including many who are not able to join us on this field trip, who worked up to 25 hours each to raise funds for the JCU SEGSC. The field trip was a great success, filled with memorable and inspiring experiences and where participants gained new geological knowledge of one of the most beautiful regions in the world.
The JCU SEG Student Chapter Executive Committee
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Rangitoto Volcano (geoffbilling.co.nz)
White Island (infogeonet.org.nz)
Ruapehu Volcano (visitruapehu.com) Waiotapu (madtravelshop.com)
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Wednesday, November 11th
Rangitoto Volcano Hike, Auckland Volcanic Zone – Matthew Horsfall The Auckland Volcanic Field (AVF; Fig. 1), is an area of volcanism with a predominately basaltic composition. It forms part of the Northland Auckland Peninsula and is situated 350-400 km west of the Australian-Pacific plate boundary. The rocks from the AVF are geochemically different from convergent and divergent plate boundary rocks and are interpreted as intraplate volcanic rocks. There are three occurrences of intraplate volcanism in the Auckland area, the AVF being the youngest and furthest north, the Southern Auckland Volcanic Field and the Ngatutura Volcanic Field (University of Tasmania, 1998).
Figure 1: Geological sketch map of the Auckland Volcanic Field (AVF) showing the location of the 48 volcanic centres. Inset shows the position of the AVF in the North Island of New Zealand (Beddington and Cronin, 2009).
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The volcanoes that stud the AVF are numerous, small and range in age from sixty thousand years to less than 800 years. Each volcano was geologically active for a short period of time ranging from days to years. Volcanoes of this nature are termed “monogenetic” (University of Tasmania, 1998). The field contains 7 cubic kilometres of lava and the concentration of volcanoes in the area is very high with 48 volcanoes found in a 400 square kilometre area. The majority of basalts found are nepheline normative (basanite) and alkali basalt. Minor transitional and tholeiitic (hypersthene normative) basalts are also found (University of Tasmania, 1998).
Rangitoto Rangitoto Island is the largest volcano in the AVF. It is 6 km across, rises 260 metres above sea level and has a dense rock volume of 1.78 cubic kms. The central scoria cone is bordered to the north and south by remnant scoria mounds and ridges from older cones. The central and south cones erupted sub-alkali basalts while the northern cone erupted alkali basalt. The volcano flanks are gently dipping pahoehoe and aa sub-alkali basalt lava fields. Pahoehoe lava can be described with a smooth billowy or ropy surface, while aa has a rough, rubbly or clinkery surface made of broken blocks called clinker. The pyroclastics of the Rangitoto eruptions are poorly preserved with scattered exposures found on the adjacent Motutapu Island (Shane et al., 2013). The geochronology studies of Rangitoto suggests it represents nearly 1000 years of activity in the one spot lasting from 1498 BP to 504 BP. This is in distinct contrast to the other volcanoes of the AVF with life spans of less than a year (Shane et al., 2013).
Field observations The flanks of the Rangitoto are gently sloping. Aa lava covers much of the higher flanks with minor amounts of pahoehoe lava (Fig. 2A). Lower down and by the shore, pahoehoe is the dominant lava type (Fig. 2B).
A
B
Figure 2: A. Aa lava on the gentle slopes of the Rangitoto volcano. B. Pahoehoe lava near the shore.
The Aa lava observed is typical with vesicular and lose clinkers (Fig. 3A). In these clinkers, iddingsite, a weathering product of olivine can be found as well as olivine. Fe oxides are also present on weathered surfaces.
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On the hike to the summit, lava tubes are present and many are large enough to walk through (Fig. 3B).
Figure 3: A. Hand sample of the aa lava from the slops of the Rangitoto volcano. B. One of the larger lava tubes on the island. Photos from Otgonbayer Ochirbat.
At the summit, no scoria was found nor along any of the walking tracks. Ridges and mounds can be seen (Fig. 4A, B) which may be the remnants of older cones as referred to in Shane et al. (2013).
A
B
Figure 4: A., B. Ridges and mounds on the slopes of Rangitoto that may be remnants of older cones.
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Bebbington, M., and Cronin, S., 2010. Spatio-temporal hazard estimation in the Auckland Volcanic Field, New Zealand, with a new event-order model. Bulletin of Volcanology, v. 73(1), p. 55-72. Shane, P., Gehrels, M., Zawalna-Geer, A., Augustinus, P., Lindsay, J., and Chaillou, I., 2013. Longevity of a small shield volcano revealed by crypto-tephra studies (Rangitoto volcano, New Zealand): Change in eruptive behaviour of a basaltic field. Journal Of Volcanology And Geothermal Research, v. 257, p. 174-183. University of Tasmania, 1998. Geology 3 Field Excursion: North Island, New Zealand. Hobart.
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Thursday, November 12th
Glenbrook Steel Mill - Stephanie Mrozek Departing from Auckland, we traveled 60 km south to New Zealand Steel’s Glenbrook Mill for a site tour and overview of the steelmaking process. The Glenbrook Steel Mill (established 1970) is a unique operation that was specifically designed for processing ironsand into steel. From 10 am – 2 pm we received an introduction to ironsand geology and mining, then took a guided tour of the mill and observed four processes: iron making, steel making, hot rolling, and cold rolling (Figs. 1 and 2). Following the mill tour, we took a scenic lunch break at Karioitahi Beach, a nearby ironsand beach (Fig. 3).
Figure 1: Glenbrook Steel Mill with the slabmaking plant in the background. Back row (L-R): Alister, Helge, Gautier, Stephanie, Jaime, Walter. Front row (L-R): Michael, Natalie, Matthew, Nick, Oodoo.
Geology Quaternary ironsand beaches occur nearly continuously along the west coast of the North Island. These beaches host significant concentrations of heavy minerals, such as titanomagnetite and ilmenite, derived from erosion of volcanic rocks from Mt. Taranaki and the Taupo Volcanic Zone. Longshore currents transport the sands northward along the coast where they are continuously reworked and concentrated by shallow marine and aeolian processes. Deposits form in natural depressions and dune fields. The Taharoa and Waikato North Head (WNH) deposits are two of the largest ironsand deposits in New Zealand, both owned and operated by New Zealand Steel; their large sizes are attributed to the paleo-basins in which they formed. Compositionally, both
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deposits contain up to 59% Fe-magnetics, plus variable amounts of pumiceous sand, clay, and tephra (Mauk et al., 2006). Deposit resource estimates are published on the New Zealand Steel website and are summarized as follows. The Taharoa deposit covers approximately 1,300 hectares and contains an estimated 488 Mt of ironsand at an average grade of 55.8% Fe-magnetics. The WNH deposit contains over 150 Mt of ironsand at an average grade of 59% Fe-magnetics. Approximately 1.2 Mt of ironsand are mined at the WNH mine each year; at this rate, mining could continue for another 80 years.
Mining and Pre-Processing Ironsand mining is conducted using a dredge (Taharoa) or a bucket wheel excavator (WNH). The WNH mine supplies 100% of the ironsand concentrate that is processed into steel at the Glenbrook Mill, while Taharoa concentrates are loaded directly onto ships for export. At WNH, the sands are pre-processed on site using gravitational settling and magnetic separation methods. The heavy mineral concentrate is mixed with water to create a slurry (50% ore + 50% water), which is transported to the Glenbrook Mill via an 18 km underground pipeline. At the mill, the concentrate is dewatered and stockpiled for steelmaking.
Milling and Processing This section provides a summary of the presentations and processes we observed on the mill tour. Chemical reactions have been added to clarify the written explanations and are modified from a handout provided during the tour, written by Ure (2000).
Iron making Ironsand concentrate is combined with low-grade coal (imported) and limestone (mined near Waitomo, NZ) in a multi-hearth furnace where moisture and volatiles are driven off. The mixture is then fed into a reduction kiln where carbon from coal reduces iron to up to 92% solid metallic iron (Fe) through two reactions: Fe2O3(solid) + CO(g) 2FeO(solid) + CO2(g) FeO(solid) + CO(g) Fe(melt) + CO2(g) The next step involves melting and purification of the concentrate. Limestone added in the first step of the process acts as a flux to lower the melting point of the overall mixture. Once melted, the slag forms a low density molten layer that floats atop the molten iron. All remaining low-density impurities in the molten iron (such as SiO2 or Al2O3) rise into the slag layer. Slag is poured off, quenched, and crushed for industrial uses (i.e., aggregate), and molten iron is transferred to the steel making furnace.
Steel making The concentrate is purified once more to remove V, Mn, and Ti, which are known to reduce the strength and hardness of finished steel. To facilitate the reaction, FeO scale (recovered from the process) is added to the melt and undesirable metals are oxidized into a high-grade slag byproduct, while Fe (melt) is preserved as follows: 2V(melt) + 3FeO(scale) V2O3(slag) + 3Fe(melt) Ti(melt) + 2FeO(scale) TiO2(slag) + 2Fe(melt) Mn(melt) + FeO(scale) MnO(slag) + Fe(melt)
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Polymetallic slag is poured off and molten iron is cast into steel slabs in preparation for rolling and sheetmaking.
Figure 2: Highlights from the Glenbrook Steel Mill tour. No personal photography was permitted on site. Photos taken from New Zealand Steel website (NZS, 2015). A. Aerial photo of the Glenbrook Mill site. B. Bucket wheel excavator mining ironsand at the Waikato North Head mine. C. A coil of incandescent rolled steel in the hot rolling mill. D. Stacks of coiled steel sheets in the cold rolling mill.
Hot and Cold Rolling Mills The highlight of our tour was the hot rolling mill, where we observed cooled steel slabs (up to 210 mm thick, weighing over 10 tonnes) reheated to an incandescent orange then rolled into a 25 mm thick sheet. Once the desired thickness is achieved and the sheet is still glowing hot, the sheet is cooled with water, trimmed, coiled, and secured for transport to the cold rolling mill. All products from the Glenbrook Mill are custom made to the specifications of the client. At the cold rolling mill the steel sheets are oiled and rolled to their final thickness and durability in preparation for finishing (such as galvanization, painting, or pipemaking), which we did not observe but are also done on site.
Karioitahi Beach Following our tour of the Glenbrook Mill, we had lunch at Karioitahi Beach overlooking the Tasman Sea. Here, we had a first-hand look at ironsands in the beach environment (Fig. 3).
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Figure 3: Lunch at Karioitahi black sand beach with a backdrop of eroded dune formations of the Pleistocene-Holocene Karioitahi Group.
Mauk, J.L., Macorison, K., and Dingley, J., 2006. Geology of the Waikato North Head and Taharoa Ironsand Deposits in Geology and Exploration of New Zealand Mineral Deposits, Monograph 25 (eds: A.B. Christie and R.L. Brathwaite), The Australasian Institute of Mining and Metallurgy: Melbourne, p. 231-234. NZS (New Zealand Steel): The Story of Steel. (n.d.). Retrieved October 13, 2015 from http://www.nzsteel.co.nz/newzealand-steel/the-story-of-steel Ure, C., 2000. Alternative Ironmaking at New Zealand Steel in ISS Electric Furnace Conference Proceedings, v. 58.
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Friday, November 13th
Martha Mine, Waihi – Jaime Poblete The group arrived at the Administration Office of OceanaGold, in Waihi on the morning of the 13 of November. OceanaGold had recently acquired the Waihi operation from Newmont Mining Corporation in October 2015. After a brief introductory video, the Martha Mine chief geologist, Lorrance Torckler, gave an insightful talk about the discovery, geology and exploration methods of the mine. The main challenges for exploration faced by OceanaGold are, among others: the location of the open pit in the middle of the town of Waihi (Fig. 1), government regulations, post-mineral cover (Fig. 2) and soils affected by anthropogenic activity. However, oriented core has been important to improve the geological model. th
Figure 1: Location of the Martha open pit with respect to the town of Waihi. The closeness to Waihi is one of the challenges that OceanaGold faces for brownfield exploration.
As explained by the mine geologists, mineralization is more developed and higher grades are found when veins encounter quartz-andesite. This rock type has favourable rheology for mineralization. The best grades are also in steeply dipping, wider veins. The Martha and Welcome lodes are the main structures, which are connected by secondary quartz veining (Fig. 2). After the talk the group had the opportunity to observe high-grade core intercepts and key hand-specimens (Fig. 3). This gave the group the opportunity the study the quartz textures associated with gold mineralization. After observing mineralized drill core intercepts, the group moved to the open pit view point for an arm-waving explanation of the general geology and an overview of the main mineralized veins (Figs. 2 and 4). 13
JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Figure 2: Panoramic view of the Martha Mine open pit. Looking east, shotcrete is the post-mineral cover, Martha and Welcome lodes encounter at depth.
Figure 3: Slabby ‘oatmeal’ texture with vugs containing fine crystalline quartz.
Figure 4: Group photo at Martha open pit mine. Back row (L-R): Matthew, Stephanie, Alister, Walter, Helge, Nick, Michael. Front row (L-R): Natalie, Oodoo, Gautier, Jaime.
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Karangahake Gorge – Michael Calder Following our visit to Martha Mine, the group made a stop to visit the nearby Karangahake Gorge, situated approximately 9 km from Waihi. The gorge is home to some of the earliest mining and industrial sites in New Zealand, where gold mining occurred from the 1870’s through to the 1950’s. Our tour through the gorge included a hike through historical mine workings and tunnels as well as a look at the diverse historical mining equipment on display (Fig 4A and B). The mining tunnels run through the gorge walls and ‘windows’ through the cliff faces provided light. Many of the tunnels are also home to numerous glow-worms. The historical mining method was to blast the gold bearing quartz veins with explosives to shatter the rock and load the ore into horse-drawn trolleys running on tram rails (Fig. 4A). The ore would be transported to the nearby Woodstock or Talisman batteries for processing which as of 1889 was done using the McArthur-Forrest cyanide process (Karangahake Gorge gold mining history, 2006).
A
B
Figure 4: A. Tramway and cart used to transport ore out of the Karangahake mines. B. Tunnel into the gold mines of the Karangahake Gorge.
Mineralization at the Karangahake Gorge (29,425 kg of Au, 97,290 kg of Ag) is hosted by andesite and it shares many of the characteristics of other epithermal gold deposits of the Hauraki Goldfield, Coromandel Peninsula (Christie et al., 2007 and references therein). Gold is predominantly found in quartz stockwork veining (Fig 5A) or vein filling high angle fractures with widths ranging from 5 to 30 cm across (Fig 5B). The hydrothermal alteration observed enveloping the quartz veins includes argillic alteration consisting of clays (smectite, illite, kaolinite?), quartz and pyrite (Fig 6). Proximal alteration has been recoded as intense with plagioclase partially or completely replaced by illite or adularia and illite and distal alteration as propylitic alteration including quartz, chlorite, illite, pyrite, adularia, albite, calcite, and epidote (Simpson and Mauk, 2001).
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Figure 5: A. Stockwork of thin quartz veins. B. Comb-euhedral quartz growing into an open fracture.
1 meter Figure 6: Outcrop of argillic alteration proximal to the entrance of the Karangahake mine.
Christie, A. B., Simpson, M. P., Brathwaite, R. L., Mauk, J. L., and Simmons, S. F., 2007. Epithermal Au-Ag and related deposits of the Hauraki goldfield, Coromandel volcanic zone, New Zealand. Economic Geology, v. 102(5), p. 785-816. Karagangahake Gorge gold mining history, 2006. Retrieved December 20, 2015 from http://www.doc.govt.nz/Documents/about-doc/concessions-and-permits/conservation-revealed/karangahakegorge-lowres.pdf Simpson, M.P. and Mauk, J.L., 2011. Hydrothermal alteration and veins at the epithermal Au-Ag deposits and prospects of the Waitekauri area, Hauraki goldfield, New Zealand. Economic Geology, v. 106(6), p.945-973.
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Saturday, November 14th
White Island Volcano – Helge Behnsen The fourth day of our trip was spent on White Island, the most active, andesite-dacite composite stratovolcano of New Zealand, located in the Bay of Plenty (Fig. 1A-D). Volcanic activity is constantly surveilled by GNS Science, who operate three cameras installed along the crater rim. The webcams take pictures in 15 to 30 min intervals and can be observed at: http://www.geonet.org.nz/volcano/info/whiteisland/cameras. The boat ride to White Island was organized by White Island Tours (www.whiteisland.com) located in Whakatane. Weather conditions in the Bay of Plenty are unstable and determine if scheduled tours happen or not. Luckily, we had favorable and sunny weather and could do the tour on our scheduled date. After a 1hr 30min boat ride out into the bay, we got on-shore at White Island.
A tour guide led the group through to the center for the volcano, stopping to observe hydrothermal features (Fig. 2A-C). Features in figures 2A and B include the native sulphur field forming today. This sulphur was mined in the late 19th, early 20th century and remnants of the old processing plant still exist on White Island. 17
JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Figure 2C shows a mudpool from an area of active vents along the margin of the central subcrater. An additional highlight to look out for is shown in figure 2D. It shows an outcrop in Shark Bay, only visible from the boat while taking a tour around the island before heading back to the harbor. This outcrop shows oxidized veins following fractures around massive blocks of andesite/dacite (?) lava. The whitish color of the rocks at the top of the outcrop show the advanced argillic alteration.
Figure 2: A. Native sulphur field which was mined in the late 19th century. B. Massive native sulphur crystal. C. Mudpool along the margins of the central subcrater (area of active vents). D. Shark Bay outcrop (only observable from boat) showing oxidised veins following fractures in andesite/dacite (?) host rock.
To be right in the middle of and experience epithermal processes in action on White Island was truly an amazing experience (Fig. 3).
Figure 3: Dramatic scenery at White Island volcano. Back row (L-R): Matthew, Michael, Gautier, Nick, Helge, Jaime, Alister, Walter. Front row (L-R): Oodoo, Stephanie, Natalie.
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Sunday, November 15th
Wai-O-Tapu, ‘Sacred Waters’ – Natalie McIver The Wai-O-Tapu geothermal field is one of the largest in New Zealand. It’s geothermal evolution and existence has favoured it as an important resource for academic research, mineral exploration and power production. Wai-O-Tapu extends over 17 km² in the centre of the Taupo Volcanic Zone (Fig. 1) and approximately a 20 minute drive north of Taupo. With reference to Map 1 (see pg. 27), the key geothermal features observed during the group’s visit will be discussed below, with field photographs and figures where appropriate.
Figure 1: Map of the area studied by Wood (1994) to give geographic and geologic context.
The mud pools visited at Wai-O-Tapu (1, Fig. 2A) are the result of steam and gas from fumaroles at depth interacting with the overlying soil to form mud. The composition and volatility of the mud depends upon the soil composition and gas type respectively. The degree of volatility in a mud pool decreases with the increase of H₂O(g) from the fumaroles beneath (Houghton & Scott, 2002). The viscosity of the mud varies seasonally, resulting in the development of ‘mud volcanoes’ in particularly dry periods. Mud volcanoes can also develop on the circumference of the pools due to the proximity of the fumarole vents (GSNZ, 2001). The Wai-O-Tapu mud pools are constantly active, boiling due to the dominance of CO₂ from the fumarole vents (Cody, 2015). Current research 19
JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
estimates that eruptive activity peaks within the Wai-O-Tapu mud pools every 5 to 7 minutes (Simpson, 2015). Another smaller example of mud pools within the park are the Devil’s Ink Pots (6, Figure 2B), where 60-70°C sulphurous black mud ‘boils’ due to the release of sulphur dioxide gas (WaiO-Tapu guide, 2015).
2A
2B
Figure 2: A. Erupting mud at the Wai-O-Tapu mud pools (3, Map 1), and B. ‘’boiling’’ black mud within the Devil’s Ink Pots (6, Map 1).
The Lady Knox Geyser (2) is named after Lady Constance Knox, the daughter of the 15th Governor of New Zealand, Uchter Knox. In 1901, Wai-O-Tapu was host to a prison. Prisoners found the clearing where the geyser is located and discovered that it erupted with the addition of soap after they washed their clothes in the hot water. As a result, the eruption of the Lady Knox Geyser is now triggered by the addition of soap to the vent. It is currently thought that two chambers supply the geyser, one below the other in an hourglass-like shape. The top chamber is estimated to be between 70 and 80°C, and the bottom chamber is thought to be between 120 and 130°C. The soap alters the surface tension of the fluid, allowing the hotter reservoir to mix with the colder chamber above, catalysing the eruption (Simpson, 2015). During eruption, the vent can exert water to heights of 20 metres. Over time, the precipitation of silica has increased the size of the initial cone, which was constructed anthropogenically to emphasise the eruption event (Fig. 3) (Wai-O-Tapu, 2015).
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Figure 3: Eruption of Lady Knox Geyser at Wai-O-Tapu geothermal park, 2015.
The Devil’s Home (3, Fig. 4), Rainbow (4), Thunder (5) Inferno (18), Bird’s Nest (19) and Devil’s Bath (20) Craters are all collapse craters. The collapse chambers form due to corrosive sulfuric acid produced from the disproportionation of geothermal fluid during subsurface boiling. The sulfuric acid dissolves the surrounding country rock, eventually resulting in collapse (Simpson, 2015). Some of the collapse craters also coincide with the location of historic geysers and hot springs, with abandoned sinter deposits at their circumferences (Houghton & Scott, 2002). Coincidentally, the crystallization of sulphur is common at these locations, and some still contain boiling mud or water at their bases. Spectacular examples of sulphur crystallization from gas released can also be seen within several caves (S) around the park. The Devil’s Bath is a particularly striking feature, as it contains fluorescent green water at its base. The colouration is due to the interaction of sulphur and ferrous salts from surrounding features with the water within the crater (Fig. 5) (Wai-O-Tapu guide, 2015).
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Figure 5 (Top): Fluorescent water within Devil’s Bath, an example of a collapse crater at Wai-O-Tapu geothermal park, 2015. Figure 4 (Left): Devil’s Home, an example of a collapse crater at Wai-O-Tapu geothermal park, 2015. Image courtesy of Ashley Cody, 2005.
Fumarole Flat (F) is a field of multiple fumaroles, resulting in extensive steam and gas production and alteration of the surrounding soil. The soil colour surrounding geothermal features (such as the craters and on the Fumarole Flat) is indicative of chemical alteration that it has undergone. Yellow soil indicates sulphuric alteration, red soil indications iron oxide alteration and purple colouration is indicative of magnesium oxide alteration (Wai-O-Tapu guide, 2015). The Champagne Pool (9), Frying Pan Flat (13) and Lake Ngakoro (15) are all hydrothermal eruption craters which have now developed into springs. The Artist’s Palette (7) has developed adjacent to the Champagne Pool due to overflowing fluid. Fluid from the Champagne Pool has also overflowed to the south (12), filling a crater thought to be formed by a hydrothermal eruption. The Champagne Pool is a particularly famous feature because it is the largest in the district. It is 65 metres in diameter and maintains an alkaline water temperature of 74°C (Houghton & Scott, 2002). It is of economic interest, with 80g/t Au recently measured within the silica precipitate. Antimony oxide precipitates can be seen encrusted on one side of the pool rim, suggesting evidence for tilting of the pool (Fig. 6). It is hypothesised that the tilting of the pool is caused by regional tectonic activity. Material from the previous hydrothermal eruptions has been preserved in the stratigraphy surrounding the Pool. Scientists have correlated the rock fragment types to the stratigraphy beneath the Champagne Pool to calculate the depth of the source material, with some recent eruption material sourced from up to 70m below the surface (Simpson, 2015).
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Figure 6: Antimony oxide rims one side of the Champagne pool at Wai-O-Tapu geothermal park, 2015. Image courtesy of H. Behnsen, 2015.
The fluid within Frying Pan Flat occurs from the condensation of hydrothermal steam. The location of the fumaroles at depth can be seen by localized occurrences of bubbling at the surface. The springs in this area are host to several extremophile species, making the area of particular interest for extraterrestrial research (Wai-O-Tapu guide, 2015). The Artists Palette is named for its myriad of colours. The colouration is derived from varied chemical enrichments; orange from antimony, yellow and grey from sulphur and green from ferrous iron (Houghton and Scott, 2002). Lake Ngakoro or ‘Grandfather Lake’ has a distinctive emerald green colouration due to algal inundation, in contrast to the predominantly element-induced colouration of nearby pools (Fig. 7) (Simpson, 2015).
Figure 7: Lake Ngakoro or ‘Grandfather Lake’ at Wai-O-Tapu geothermal park, 2015.
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
There are also several naturally occurring sulphurous springs at Wai-O-Tapu. Some of these include the Opal Pool (8) and the Oyster Pool (named for its distinct shape) (14). In contrast to Lake Ngakoro, these pools obtain their colour from element enrichment, silica-saturated turquoise blue is most common (Simpson, 2015). The precipitation of silica from the hydrothermal fluid creates world renowned sinter terraces at Wai-O-Tapu. Specific examples of these include the rim of the Wai-O-Tapu geyser (10), Bridal Veil Falls (16) and the Primrose Sinter Terraces (17). Geyserite, a sinter texture formed by splattering silica rich fluid, occurs particularly around the Wai-O-Tapu geyser (Simpson, 2015). Eruption of the Geyser has ceased lately, potentially due to recent subterranean activity (Wai-O-Tapu guide, 2015). Bridal Veil Falls obtains its name from the spectacular cascading nature of the sinter, formed from overflowing fluid from the Artist’s Palette and the Primrose Terraces (Fig. 8). The Primrose Terraces (Fig. 9) cover an area of 3 acres, and are now the largest in New Zealand following the destruction of the Pink and White Terraces at Waimangu during the Mt Tarawera eruption in 1886 (Wai-O-Tapu guide, 2015). The terraces are only 700 years old, and it is estimated that more than 50 cm of silica sinter have been deposited since their formation (Houghton & Scott, 2002). Each terrace scarp has a width of 225oC) geothermal systems in Taupo Volcanic Zone. Seven of the fields have been developed and accounts for ~17% of national electricity supply. The geothermal activity manifested on the Earth’s surface is often restricted in size and distribution. One way of determining the extent of geothermal systems is by measuring the ground’s electrical resistivity (waikatoregion.govt.nz, n.d.). Map 1 shows the geothermal systems in the Taupo Volcanic Zone as they are defined by electrical resistivity. Resistivity is a measurement of the ground’s ability to conduct electricity. Generally, geothermal water has lower resistance (conducts electricity better) than normal ground water. • Low resistivity (red) indicates the presence of geothermal water. • High resistivity (purple) indicates normal ground water, soil and rock.
Compared to geothermal fields in Philippines and Japan, those in New Zealand are more easily utilized due to low topographic relief of the area. Identification of clay minerals in the geothermal system is important as they could provide useful information such as temperature (epidote ~250oC), and pH of the system. Geothermal power plants require a minimum of 175oC of heat to be able to run with a profit. Knowing the pH of the system can help prevent corrosion of pipes. Furthermore, knowing what type of clay mineral is present in the system gives drillers an idea about the swell factor of clays which can help prevent potential future drilling challenges. Besides having economic values, the geothermal fields of the TVZ exhibit a wide range of geothermal phenomena including hot and boiling springs and streams, geysers, silica sinter deposits, mudpools, fumaroles, hot and steaming ground, altered ground and hydrothermal eruption craters (Cody, 2007). The Champaign Pool in Wai-O-Tapu is a spectacular example of an eruption crater in the TVZ (Picture 3).
Picture 3: Champaign pool in Waiotapu geothermal area (Photo: Alex Pokrovsky)
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
The Magmatic-Hydrothermal Transition of TVZ This talk was given by Isabelle Chambefort, a geothermal geologist at GNS. The TVZ extends from White Island to Ruapehu. Magmatism at the TVZ started at about 2 million years ago as a magmatic arc within the Coromandel peninsula and migrated toward the east. The speaker discussed extensively about porphyry systems and the magmatic and hydrothermal transition at the TVZ. A comparison was made with current mineralization and alteration models of porphyry systems.
Picture 4: Mark Simpson during & after Geothermal Systems of TVZ lecture at GNS Wairakei Research Centre
In the afternoon, the group drove to National Park village in Tongariro National Park. On the way to National Park village, we made a quick stop at Waikato River, and Tokaanu Thermal Walk. Tokaanu Thermal Walk took about 20 min and we looked at more geothermal features like hydrothermal crater pools, and mud volcanoes. Cody, A. D., 2007. Geodiversity of geothermal fields in the Taupo Volcanic Zone. Retrieved from Science & Technical Publishing Department of Conservation website: http://www.doc.govt.nz/Documents/science-andtechnical/drds281entire.pdf GNS Science, 2015. Retrieved from http://www.gns.cri.nz/Home/About-Us NZ Geothermal Association, 2009. Introduction to Wairakei - New Zealand Geothermal Association. Retrieved from http://www.nzgeothermal.org.nz/education/wairakei.html NZ Geothermal Fields, 2013. NZ Geothermal Fields - New Zealand Geothermal Association. Retrieved from http://www.nzgeothermal.org.nz/nz_geo_fields.html Waikatoregion.govt.nz. (n.d.). Geothermal systems in the Taupo volcanic zone. Retrieved from http://www.waikatoregion.govt.nz/Environment/Natural-resources/Geothermal-resources/Classifying-geothermalsystems/Geothermal-systems-in-the-Taupo-volcanic-zone/
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Wednesday, November 18th
Tongariro Alpine Crossing, Tongariro National Park Otgonbayar “Oodoo” Ochirbat Early morning, our group headed to the trailhead of Tongariro Alpine Crossing and started hiking at around 7 am. The Tongariro Alpine Crossing is a tramping track in the Tongariro National Park and is one of the most famous day hikes in New Zealand. Crossing passes over the volcanic terrain of the Mount Tongariro, passing the eastern base of Mount Ngauruhoe. The full distance of the track is approximately 19.4 km (Department of Conservation NZ. n.d.). Red Crater (in Picture 5) and Mt Ngauruhoe (in Picture 6) are the most recently formed features on the Tongariro Alpine Crossing. All volcanics in the Tongariro National Park are andesitic. Red Crater (Pictures 5 & 7) was formed about 3000 years ago. It lies within a scoria cone which rests on top of the older Tongariro lava flows. Mt Ngauruhoe is the youngest volcano in the area having begun to form about 2,500 years ago. It is the most active vent in the Tongariro area with its last eruption recorded in 1975. The most recent flows from Mt Ngauruhoe are easily visible on the way to the South Crater.
Pictures 5-8: A selection of photographs from the hike through the Tongariro Crossing.
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Erfurt-Cooper, P., Keys, H., and Williams, K., 2014. Geotourism in Volcanic and Geothermal Environments: Playing with Fire? Geoheritage, v. 155. McArthur, J. L., and Shepherd, M. J., 1990. Late Quaternary glaciation of Mt Ruapehu, North Island, New Zealand. Journal of the Royal Society of New Zealand, v. 20, p. 287–296.
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Thursday, November 19th
Mount Ruapehu - Otgonbayar “Oodoo” Ochirbat On this day, we spent most of the time at Mount Ruapehu (Picture 8). By the time we got a ski lift up to the trailhead, it started to snow and our planned Crater Lake rim hike was cancelled for safety concerns (Picture 9 and 10). During the last ice age, Mt Ruapehu and the Tongariro massif carried glaciers extending down to as low as 1100 m elevation (McArthur and Shepherd, 1990). These glaciers and those of the previous two glacial maxima altered the shape of the existing volcanoes by excavating deep valleys, damming and interacting with lava flows and other volcanic material, and depositing material including prominent moraines (Erfurt-Cooper, 2014). The team (Picture 12) stayed one more night in National Park Village.
Picture 9 and 10: Mt Ruapehu in snow. Picture 11: Glacial valley on Mt Ruapehu near ski lift.
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JCU SEG Student Chapter New Zealand, North Island Field Trip 2015
Picture 12: JCU SEG Chapter New Zealand trip participants (from left: Jaime, Matthew, Walter, Stephanie, Oodoo Ochirbat, Michael, Natalie, Gautier, Nick, Alister, and Helge) Department of Conservation NZ. (n.d.). Tongariro Alpine Crossing [factsheet]. Retrieved from http://www.doc.govt.nz/Documents/parks-and-recreation/tracks-and-walks/tongariro-taupo/tongariro-alpinecrossing-factsheet.pdf
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