Itisanexcellentexampleofanactivehotspringssystem. Most pictures shown were taken on a PACRIM field trip in 1995(unless otherwise referenced).

Transcription

1 White Island, New Zealand lies along the Taupo Rift zone, which runs through the North Island of New Zealand and extends out to Fiji. This is an active andesitic volcano with eruptionsthroughthe20 th century. Itisanexcellentexampleofanactivehotspringssystem. Most pictures shown were taken on a PACRIM field trip in 1995(unless otherwise referenced). 1

2 Tectonic map of the South Pacific-New Zealand area (modified from a GNS(NZ) production): In contrast to the other circum-pacific arcs which form a discontinuous series of convex-outward andesitic arcs, the sector from the New Britain Arc east and SS west to the Indo-Pacific Rise is more complex. From New Britain, through the Solomons, New Hebrides, the arc volcanic chains have a trench on both side with subduction taking placebothtothenneandtothessw.asinglelineartrench extends SSW from Samoa for 1600 nm lying east of the Tonga-Kermadec volcanic islands and fading out near East Cape, New Zealand. The TK arc continues through White Island off the coast of NZ, through Mt Edgecumbe, Mt Tarawera, the Rotorua thermal area and its many collpase calderas, the Okataina Centre, through Rainbow Mtn, the Maroa Centre, Mt Tauhara and the Taupo Centre, the Pihanga Group, the Tongariro Group, Mt Ngauruhoe and finally the 9000ft Mt Ruapehu. 2

3 This slide summarizes the characteristics of White Island. White Island is an active andesitic stratovolcano. Periodic eruptions have produced both lava flows and explosive eruptions of ash. At most times the volcanic activity is limited to steaming fumaroles and boiling mud. 3

4 White Island lies along the Taupo Volcanic Zone on a line with the Tongariro National Park. The TVZ is highly active volcanic area in the North Island that is approximately 350 kilometres long by 50 kilometres wide. There are numerous volcanic vents and geothermal fields in the zone, with Mount Ruapehu, Mount Ngauruhoe and White Island erupting most frequently. 4

5 Thismapshowsa closerlookatthetaupovolcaniczoneshowing the different volcanic rock compositions. 5

6 Model of a modern hot springs environment: - Thermal contours are shown. Yellow areas will contain higher temperature alteration assemblages these assemblages will be more acidic near the conduit. - Note how the change from acid to neutral to basic further from the heat source. 6

7 From Hedenquist et al(1993): The White Island volcanic-hydrothermal system, New Zealand, is thought to closely represent the chemical conditions that lead to the formation of highsulfidationcu-auoredeposits.theamountsofcuandauproducedovera 10 ka period of activity, largely from degassing magma, are calculated to be 10 6 and45t,respectively.alteredandesiteblocksejectedfromrecentvents contain alunite, anhydrite, and pyrite. Their S isotopic composition indicates veinfillingat 380 C.Atthistemperature,CuandAuarehighlysolublein acidsolutions,whichmayexplainthedepletionofcuandabsenceofauin the ejecta. Mass-balance calculations, however, suggest that Cu and Au are precipitated in cooler zones before the acid solutions discharge at the surface 7

8 Attempts were made in the late 1800 s and early 1900 s to mine sulphur from White Island, but ended in 1914 when part of the western crater rim collapsed, creating a lahar which killed all workers. Someyearslaterin1923miningwasagainattempted but ended in 1930 s. Remnants of old concrete buildings, damaged in an eruption, canbeseenalongthebay. 8

9 Photos from White Island (it was raining the day these pictures were taken and very slippery.) The layered material is very glassy. Wewerenotabletosamplethehotpotsbecausetheywere toohotwithwetsteameruptingfromthem. 9

10 The rock shown, which is obviously andesitic, was collected from one of the acid streamlets similar to the one shown in the picture. The yellow is alunite; white is gypsum. The spectra are labeled relative to major constituents, which include alunite, silica and gypsum. The silica is probably opaline Another rock collected showed kaolinite. This is the classic suite for an acid sulfate environment. 10

11 A fumarole emits steam and gases such as carbon dioxide, sulfu dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from the ground. Many fumaroles are present onwhite Island as shown in the pics here. The fumerole is an exciting example of a dynamic system. Thesulfure(yellow)aroundtheventsformswhenH 2 Sgasis expelled from the vent (composed of opaline silica). It hits the air and oxidizes. Sulfur precipitates and hydrogen volatilizes as a cloud above the vent. 11

12 Examples of fossil fumarole systems: These photos show rocks from Chile and Goldfield, NV. The one rock from Goldfield was formed in a very gaseous environmentwhiletheotherissimilartothepascuarock a sulfur and silica breccia. The Pascua sample also contains blue veins of alunite. 12

13 These pictures show a comparison of man and volcano! 13

14 Pictures of the vent showing the precipitates on the walls most likely sulfates. ThelakehaspHof ~1. 14

15 This chart shows the dominant water chemistry and mineralogy present. Note: we would also expect jarosite if there is pyrite. The anhydrite is actually gypsum - when the anydrite is exposed to air, since it is hygroscopic, it starts changing to gypsum. 15

16 Champagne Pool is a prominent geothermal feature within the Waiotapu geothermal area (North Island of New Zealand). Like White Island, it is also contained also within the Taupo Volcanic Zone and is another excellent example of active epithermal processes. The name Champagne Pool is derived from the abundant efflux of carbon dioxide (CO2) similar to gas bubbles in a glass of bubbling champagne. The hot spring was formed 900 years ago by a hydrothermal eruption. Its crater is around 65m in diameter with a maximum depth of approximately 62m and is filled with an estimated volume of 50,000m 3 ofgeothermalfluid. The pool is most famous for the active precipitation of gold, which is contained in the orange arsenide material in the pool. 16

17 Photo showing the bubbles at the surface of the Champagne Pool. 17

18 These maps show the location of the Champagne Pool within the Taupo Volcanic Zone and the features surrounding it. 18

19 This model represents the major components found in a solfatara or hot springs environment. Note the temperature gradients and how the waters circulate and mix. 19

20 Data from Giggenbach et al. (1994); Jones et al. (2001); Mountain et al.(2003): The deep geothermal water below Champagne Pool is ~260 C but water temperature within the pool is maintained at73 Cto75 Cbylosingheattotheatmosphere. The ph of 5.5 is relatively constant due to buffering by the flux of CO 2. Gases are mainly CO 2, but to less extent nitrogen (N 2 ), methane (CH 4 ), hydrogen (H 2 ), hydrogen sulfide (H 2 S) and traces of oxygen (O 2 ). The siliceous geothermal fluid is oversaturated with metalloid compounds such as orpiment (As 2 S 3 ) and stibnite (Sb 2 S 3 ) which precipitate and form orange subaqueous deposits. The colorful deposits are in sharp contrast to the grey-white silica sinter surrounding Champagne Pool. 20

21 The Champagne Pool is well-known for actively precipitating gold in colorful deposits around its edges. According to Pope et al., 2005: Champagne Pool has measured gold concentrations of 109 ngl 1 dissolved and 362 ngl 1 total. This spring is slightly undersaturated with gold in solution, but the total concentration is higher than the solubility of Au(s). Undersaturation with respect to Au(s) is consistent with deposition of gold by adsorption and concentration within an As- and Sb-sulfide precipitate that forms around Champagne Pool and is similar in magnitude to that expected from modeling of gold adsorption onto As- and Sb-sulfide precipitates. 21

22 As noted previously, the orange precipitate along the pool edge is an arsenide and contains gold. This is an acid environment. The white material in all the pictures is opaline silica (i.e., above the water table). 22

23 Thewhitematerialattheedgeofthepoolisopalinesilica. Itisabout6-9inchestothewater. The water is about 4-5 inches deep - to the orange precipitate. ph here is about

24 Several samples were collected from the edge of the Champagne Pool and were analyzed using a PIMA. The orange arrows show where each sample was collected. The first 3 spectra show an acid opaline silica. It is less intense in spectrum 3, which is on top of the orange precipitate, where the gold occurs. Below the orange precipitate, the composition of the silica changes and it becomes a neutral chalcedonic quartz brown and sugary. This continues further into the pool. 24

25 Diagrammatic sketch of the Champagne Pool showing temperature gradients and mineral distribution. 25

26 This chart summarizes the types of minerals that are typically associated with different geothermal systems. Examples of all of these can be found in the New Zealand White Island and Waiotapu areas they are an excellent modern analogue to epithermal Au-Ag deposits. 26

27 Asummaryofepithermalprocesses asacomparisontothoseseeninthe New Zealand geothermal systems(source: B. Taylor, GSC): This is a schematic cross-section illustrating the general geological and hydrological settings high- and low- suflidation deposits (from Taylor, 1996; partially adapted from Henley and Ellis, 1983, and Rye et al., 1992). Characteristics shown evolve with time; all features illustrated are not implied to be synchronous. Local environments and examples of low-sulphidation deposits include: (A) basin margin faults;(b) disseminated ore in sedimentary rocks;(c) veins in degassing, CO2-rich, low sulphide content, low-sulphidation systems; (E) porphyry-associated vein-stockwork, sulphide-rich (intermediate sulphidation) and sulphide-poor stages; and (H) disseminated replacement associated with porphyry-type and stockwork deposits, involving seawater. Examples of high-sulphidation environments include: (D and G) steamheated advanced argillic alteration (quartz-kaolinite-alunite) zone; (F) magmatic-hydrothermal, high-sulphidation vuggy quartz zone (± aluminoslicates, corundum, alunite) 27