Voyage Plans and Summaries

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Franklin Voyage Summary No. FR08/2001

Title

VAVE-2001 (Vanuatu and Australia Vents Expedition)
Seafloor Hydrothermal Activity in Arc and Backarc Settings, Vanuatu, SW Pacific.

Itinerary

Depart Noumea 0807hrs, Wednesday 5 September 2001
Port of Entry clearance at Tanna Island 1230hrs, Saturday 8 September 2001
Pick up day-visitors Norsup, Malakula Island 0820hrs, Wednesday 19 September 2001

Arrive Brisbane 0830hrs, Tuesday 25 September 2001

Principal Investigators

Dr Timothy F McConachy (Chief Scientist)
CSIRO Exploration and Mining
PO Box 136 North Ryde NSW 1670
Tel: 02 9490 8813
Fax: 02 9490 8921
Email: Tim.McConachy@csiro.au

Dr Ray Binns
CSIRO Exploration and Mining, Sydney

Scientific Objectives

The major scientific objective was to locate and study seafloor and sub-seafloor hydrothermal ore-forming activity in order to develop improved methods of exploring for ancient mineral deposits on land that originally formed by similar processes.

Specific objectives were:

1. To locate and sample sediment-hosted hydrothermal mineralisation, and gold-copper-zinc-rich massive sulfide deposits associated with submarine volcanic calderas in a back arc setting. Vanuatu has geological settings with excellent potential to host this range of hydrothermal ore systems.

2. To hunt for and locate "natural laboratories", and to collect data and samples that will be the subject of a range of laboratory investigations. These include assessments of tectonic setting, petrological and geochemical studies of volcanic rocks and sediments, examination of mineralisation and alteration phenomena, comparison with other submarine settings etc., all of which help build up actualistic models of hydrothermal processes and products in relation to their overall geological environment.

3. FR08/2001 will also contribute the definition of future ODP drilling targets to further clarify deep subsurface hydrothermal processes; our Manus Basin ODP Leg 193 in 2000/01 having lead the way.

Special attention was devoted to any locations that appeared during the cruise to have potential for sediment-hosted mineralisation (including within altered epiclastics). A discovery of the latter nature would open up a major new research field highly relevant to land-based mineral exploration.

Cruise Objectives

We aimed to survey selected sites in the Coriolis Troughs and their extensions, centred about 180km SE of Vila, using CTD-transmissometer profiling to detect zones of active venting, followed by bottom camera tows, grab sampling, dredging and sediment coring.

Our strategy was to work from south to north which optimised transit times and made the most efficient use of RV Franklin's time at sea. Decisions on which operations were conducted were made at sea, and depended on results.

The Coriolis Troughs located east of the Erromango, Tanna and Anatom Islands offered scope to investigate a number of volcanic-hosted hydrothermal fields. These troughs or basins had been surveyed by the Metals Mining Agency of Japan in 1994, and good quality seabeam bathymetry, magnetic and side scan sonar data are available over an area of 350 x 120km. We were also fortunate to be given a bathymetric compilation of Vanuatu waters by Dr Bernard Pelletier (Head of Laboratory, Geology/Geophysics Noumea, Institute de Reserche pour le Developpement (IRD)), and Dr Yves Lagabrielle (IFREMER, France). This compilation enabled us to survey more accurately targets outside the Coriolis Troughs and the area surveyed by the Japanese. We had also planned to survey a chain of seamounts to the east of the Coriolis Troughs. These were named the Lini Chain of Seamounts by our Vanuatu colleagues and ship-board participants, and 3 of the 7 features were surveyed by VAVE.

In addition we surveyed targets within submerged portions of the Vanuatu volcanic arc, including reputed sulfide-bearing altered lava from near the site of an 1453AD massive caldera eruption (Kuwae) near Epi Island in the central part of the arc, where a small post-caldera parasitic cone emerges occasionally.

An extra day was added to the original plan following inclusion of an on going joint research project with CSIRO Petroleum concerning inter relationships between hydrocarbon distribution and hydrothermal activity. The inclusion of a gas chromatograph and ultrasonic gas extraction system on board enabled methane gas levels to be measured in hydrothermal plumes and in sediments. A further 5 sediment coring operations were planned in the South Aoba Basin east of Malakula Island, but only four stations could be accessed safely by the Franklin.

Specific activities included:

• Dredging major features to "ground truth" them and if necessary upgrade their geological interpretation.

• CTD-transmissometer tow-yos and single dips to detect hydrothermal particulate plumes and locate the "eyes" of any plume discovered, to define hydrothermally active edifices and sites. Seawaters from plume "peaks" will be collected by Niskin for subsequent chemical analysis onshore.

• Use of an onboard gas chromatograph to establish methane levels in hydrothermal plumes as part of a research project on the inter-relationships between hydrocarbon distribution and hydrothermal activity.

• Bottom-tow camera-video traverses along promising features, looking for hydrothermal deposits (chimneys, mounds, crusts) and faunal concentrations.

• Precision dredging to sample any deposits found.

• Sediment coring and grabbing (Smith-McIntyre) for subsequent geochemical analysis in order to look for anomalies indicative of hydrothermal activity, and also identify sites to test for indications of subsurface sediment-hosted mineralisation, and possible association with hydrocarbons. The gas chromatograph was also used to establish methane levels in sediments.

Cruise Track

The overall cruise track in shown in Figure 1. Figures 2 and 3 show stations and operations, respectively.

Results

Highlights of VAVE-2001 were:

1. Discovery of a new and potentially large hydrothermal field, Nifonea Ridge, on a neovolcanic basalt ridge in the Vate Basin, SE of Port Vila. The initially detected weak light transmission and subsequent methane gas plume anomaly was mapped and the seabed comprehensively surveyed and sampled over a 48 hour period. At least three separate zones of hydrothermal activity were mapped using deep-tow video within an area measuring 2400m x 600m, with the largest field at around 600m x 400m. Extensive hydrothermal fauna, spire-like structures (some of which may be sulfide chimneys) and yellow-brown oxide crusts were photographed but dredging recovered only fauna and iron oxyhydroxides.

2. The recovery of sulfidic scoria breccia and agglomerate, and quenched native sulfur at Oscostar, an active submarine volcano, 100km south of Anatom.

3. Identifying a submarine dacite to rhyodacite volcano at Cioan volcano near Epi Island, which revealed a hydrothermal plume with high concentrations of methane gas, and recovery of iron rich hydrothermal material.

4. Sampling of the Kuwae caldera and identifying dacite rocks (less SiO₂ than Cioan) with minor basaltic andesite and a hydrothermal plume containing significant concentrations of methane gas.

5. Finding that the supposedly old and sediment-filled Coriolis Troughs (where we hoped to find sediment-hosted hydrothermal activity) were actually floored by very young basaltic lavas. Major revisions of tectonic concepts for the area are required. Our discovery of neovolcanic activity in the Coriolis Troughs is a major achievement of the VAVE cruise. Knowing now of its presence, there are clearly other sites bypassed by VAVE that deserve attention in the future.

6. The recovery of iron rich hydrothermal rocks from Temakons Seamount (formerly numbered 94SO1).

7. The serendipitous discovery of a large volcanic debris field centred about 10km equidistant from Gemini, Gemini North and Oscostar, 100km S of Anatom. Large blocks are strewn randomly over an area of at least 8km². Further echosounding is required to complete the picture of this significant debris field.

8. The discovery of a new Seamount, Roimata #B, where a 2000m deep is shown on published nautical charts.

9. Lastly, we confirmed our ability to be able to track down and sample new vent fields in a relatively short time using a variety of equipment and techniques. The exceptional turnaround time of on board methane analyses enabled quick decisions to be made of ‘where to next’ and was a perfect complement to the real time light transmission readouts from the CTD rosette.

Accordingly, the cruise fulfilled its main scientific objective which was to locate and study seafloor and sub-seafloor hydrothermal ore-forming activity in order to develop improved methods of exploring for ancient mineral deposits on land that originally formed by similar processes.

Other Objectives

VAVE-2001 was not in itself testing major hypotheses. Rather, it represented expeditionary "field work" to hunt for and locate "natural laboratories", and to collect data and samples that will be the subject of a range of laboratory investigations. These include assessments of tectonic setting, petrological and geochemical studies of volcanic rocks and sediments, examination of mineralisation and alteration phenomena, comparison with other submarine settings, all of which help build up actualistic models of hydrothermal processes and products in relation to their overall geological environment. To this end, VAVE-2001 was hugely successful.

Two secondary objectives of VAVE were to locate and sample (1) sediment-hosted hydrothermal mineralisation, and (2) gold-copper-zinc-rich massive sulfide deposits associated with submarine volcanic calderas in a back arc setting. Unfortunately, neither of these two objectives was achieved. In contrast, a number of unexpected results, particularly with regard to the tectonics of the area, provided surprising highlights (see below).

1. Sediment-hosted mineralisation

No evidence was found for hydrothermal activity within the sediment cores recovered, although the cores from the Futuna, Erromango and Vate Basins all have an unexpectedly dark brown massive, silty, hemipelagic clay. In places, we found mm- to cm-thick, very dark grey silty or sandy volcaniclastic layers. None contained any distinctive coloured horizons that were clear candidates for plume-derived metal enrichments. Post cruise research will test for more subtle effects and provide comparisons with hemipelagic sediments from other back arc basins to determine the cause of the dark brown colour, and also confirm or revise the interpretation of a multisource airborne ash origin for the dark grey volcaniclastic layers.

2. Massive sulfides in caldera settings

We did not recover any massive sulfide associated with submarine volcanic calderas in a back arc setting. A prime candidate for further work to achieve this objective was identified at Cioan volcano, where strong concentrations of methane gas were measured. However, we did photograph possible chimney structures, which resemble massive sulfides, in the new Nifonea hydrothermal vent field.

3. Tectonics and petrology

We examined two contrasting arc settings, and a number of back arc sites. The arc settings include the felsic Kuwae and Cioan eruptive centres in the north and basaltic Oscostar Seamount in the south.

In the back arc settings, basaltic rocks dominate, with some fractionation toward andesite. Many of the 35 targets studied are relatively old judging from weathering, manganese crusts, and shelly tops. These include the Chain of Lini Seamounts, Sineto and Nan Seamount.

Young, fresh volcanic rocks lacking weathered surfaces and manganese crusts were found at 5 locations within the Coriolis troughs. We have therefore demonstrated that the Coriolis Troughs have developed to the stage of ocean floor spreading. Given their relatively narrow width it is unlikely that spreading has been active for very long. In fact, they are likely among the most youthful of back arc basins, comparable with the Sumisu Rift (Izu-Bonin arc) and Okinawa Trough in Japan. We conclude that the rocks comprising the walls of the Coriolis Troughs are "pre-rift" Vanuatu arc basement, and note that the Nifonea Ridge is oriented normal to the extensional axis of the Troughs.

Other significant tectonic observations include:

• The eroded morphology of the Lini Seamounts and the apparently arc-related (basalt-andesite-dacite) character of their basement rocks is indicative of an important phase of arc volcanism, presumably preceding formation of the Coriolis Troughs. However, confirmation of the significance of these materials will depend on detailed analytical studies.

• The Masaga Seamount is a horst block composed of the same volcanic and sedimentary strata that form the walls of the Erromango Trough.

• We cannot confirm that the postulated offset of the Western Spreading Ridge exists in the vicinity of 17^o 20’S, or that Core Seamount is part of a westward-propogating rift system.

• The western rift of the Ambrym Volcano extends offshore.

4. Hydrothermal plumes

Of the 24 targets tested by the CTD-transmissometer package, 15 or just over 60% returned a positive indication of possible hydrothermal plumes. Three targets (Nifonea, Kuwae and Cioan) yielded light transmission and significant methane anomalies that are unambiguous plume signals. The other 10 targets yielded equivocal plume signals; varying from subtle to strong methane anomalies with no light transmission anomalies, to very weak light transmission anomalies with no methane anomalies.

The best results come from the Cioan, Kuwae and Nifonea areas. Nifonea is in a back arc position in the centre of the Vate Basin, and the other two are associated with arc volcanism.

The Erromango Basin appears to be devoid of current hydrothermal activity, except for one possible area.

The two seamounts the surveyed by the Japanese in 1994 (Temakons, and Sineto) recorded only possible plume signals. The Japanese photographed hydrothermal vents at Temakons but the plume signal from this area is weak, at best. The level of hydrothermal activity is therefore considered to be weak, and this is consistent with other results (e.g. dredging, grab) from this area.

The anomalously high concentrations of methane measured in the south of Vanutau at Gemini, Oscostar and Mystery Mounds are interesting, even though they are accompanied by no light transmission anomalies. Previous workers have tectonically interpreted this area as the southern extension of the Coriolis Troughs but it could be part of the volcanic arc, a direct extension from Erromango and Anatom. Such concentrations of methane would therefore be consistent with other arc-related volcanoes at Kuwae and Cioan in the north. These appear to be quite gaseous without strong light transmission anomalies.

5. Hydrocarbons and hydrothermal activity

An extra day was added to the original cruise plan for a ‘piggy back’ project concerning interrelationships between hydrocarbons and hydrothermal activity. Sediment and water samples from of 4 stations in the South Aoba Basin are subject to on-shore analyses.

6. Other Potential Resources

Echosounding over submarine volcanoes/seamounts at Gemini and Oscostar in southern Vanuatu and Roimata C near Efate identified possible fish schools above the summits. These could represent new fishing grounds for Vanuatu.

Cruise Narrative

A daily narrative is given in Appendix 1 and summary of stations is available from the Chief Scientist.

Summary

The FR08/2001 expedition to Vanuatu was extremely successful.

Not only did the expedition meet most of its scientific objectives but it also surpassed the number of planned operations, enabling collection of a large and comprehensive suite of water, sediment and rock samples, and video coverage for subsequent on shore laboratory research and analysis. We investigated 35 targets, conducted 208 operations and collected 474 rock and sediment samples, 251 water samples, and made 190 on-board methane gas analyses. Some totally unexpected results require significant revision of current interpretations of Vanuatu tectonics and geology. The expedition highlighted a number of areas for follow up research, including possible submersible dives sites and Ocean Drilling Programme targets, to clarify deep subsurface hydrothermal processes.

The success and high productivity of the cruise was a function of both the ship’s crew and scientific team who all worked well together, in a safe, professional, and diligent manner throughout the 21 days. In addition no operational time was lost to weather.

The RV Franklin proved once more to be an excellent vehicle for undertaking this type of work. For most situations, it maintained station to the appropriate degree of accuracy required. Problems were encountered with the SIMRAD EA500 echosounder dropping out every 10 minutes or so during the first half of the cruise. This was not only irritating but also caused loss of data at crucial times during operations. The problem was tracked successfully to not enough RAM in the current (spare) Master CPU card which was too slow, and it had not been upgraded from a faulty card during a recent replacement (FR07/01). We believe that the manufacturer is aware of this problem but find it unacceptable that they do not provide updates with service bulletins. With one dredge lost on this voyage, the ship is now down to 2 complete sets of weak links. The sheer bolt sets needs re-furbishing, and a third set of weak links is required as a back up for future dredging operations. The computers operated reliably during the voyage, with no major down-time being experienced.

About half a day of operation time was 'lost' as the ship had to visit Tanna for customs port-of-clearance. We learnt of the necessity for customs clearance only hours before sailing. Clearance therefore had to be arranged by the Chief Scientist during the initial stages of the cruise, and was facilitated by the good will of the Vanuatu Government and in particular, the Director, Ministry of Geology, Mines & Water Resources. Future cruises to Vanuatu should take customs port-of-clearances into account, arranged before hand, and incorporated into the cruise plan.

Scientific Personnel

*watch captain

VAVE-2001 Scientific Team

Left to right: Front Row: Bob Beattie, Toney Tevi, Richard Arculus, Brent McInnes
Back Row: Fernando Barriga, Stephen Sestak, Robina Sharpe, Chris Yeats, Brooks Rakau, Erik Madsen, Ray Binns, Tim McConachy

RV Franklin Personnel

Acknowledgments

The Master and crew of RV Franklin are thanked for their tireless and professional performance throughout the cruise which contributed greatly to the success of VAVE.

Mr Chris Ioan, Director, Ministry of Geology, Mines & Water Resources, Vanuatu is thanked for support of VAVE and for his contributions to logistics and arrangements, especially the port-of-clearance at Tanna and the day-visit near Malakula. Mr Ioan also kindly made available two of his senior officers to participate in the expedition, and contributed project funding from an AusAID grant.

The research on seafloor hydrothermal systems and applications to land-based mineral exploration by CSIRO Exploration and Mining would not be possible without the continuing support of industry sponsors, Normandy Mining Limited, Phelps Dodge Corporation and Rio Tinto Exploration Pty Limited, and the division's management, in particular Professor Neil Phillips and Dr Graham Carr.

Interwest (Vanuatu) LTD and President, Mr Floyd Cardinal are thanked for their support.

A travel grant from Research Centre Creminer and project Dream - Fundação para a Ciencia e Tecnologia - Portugal enabled Professor Fernando Barriga of Lisbon University to participate on VAVE. Dr Robina Sharpe acknowledges internal Centre for Ore Deposits Exploration SRC travel grant.

The Chief Scientist would like to acknowledge and extend his personal thanks to the scientific team who worked long hours and exceptionally hard to underwrite the success of VAVE. Finally, Dr Joanna Parr, CSIRO Exploration and Mining, Sydney, and VAVE's on-shore participant, is thanked for assistance in pre and post cruise organisation and planning.

Dr Timothy F McConachy
Chief Scientist

Figures

Figure 2 - Map of stations investigated.

Figure 3 - Map of operations.

Appendix - Daily narrative

Updated: 31/01/03