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2021 SEG CODES Student Chapter Field Trip Report King Island, Tasmania, Australia

2021 SEG-CODES Student Chapter Field Trip Participants at Grassy Mine

Written and compiled by Carlos Díaz Castro, Nathaly Guerrero Ramirez, Max Hohl, Rhiannon Jones, Peter Berger, Thomas Schaap, Zebedee Zivkovic, Karla Morales, Xin Ni Seow, Tobias Staal, Alex Farrar and Hannah Moore

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Contents

Introduction 3 Trip Leaders and organisers 5 Day 1 – Grassy area, Dolphin open pit mine 6 Historical context 6 Skarn mineralization at King Island 7 SEG-CODES student chapter crew activities 9 Naracoopa 12 Day 2 – Stokes Point and Cape Wickham 15 Stokes Point 15 Cape Wickham 16 Day 3 – City of Melbourne Bay 18 City of Melbourne Bay south: volcanics and tillites 18 City of Melbourne Bay north: cap carbonate 20 Acknowledgements 23 References 23

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Introduction

When the Society of Economic Geologists UTAS student chapter met up at the start of 2021, the student chapter’s committee noted a general eagerness amongst the members to get out into the field. Indeed, many of the society’s postgraduate members had their research fieldwork postponed in 2020 due to travel and fieldwork restrictions established to reduce the spread of COVID-19. Discussions led to fundraising efforts, and the group secured sponsorship from the Geological Society of Australia and a travel grant from the Society of Economic Geologists to lead a field trip to King Island. From 14 to 18 April, 12 CODES postgraduates and four staff members were able to get a firsthand look at the unique geology of the island. Three full days were dedicated to the geology of King Island, allowing additional days for travel, introductory lectures, and a self- guided tour of the geology of the Currie area. This report compiles a series of student reports from each day of the trip.

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Figure 1. Regional Geology of King Island (from Calver 2007).

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Trip Leaders and organisers

Nicholas Direen Zebedee Zivkovic

UTAS Adjunct Staff PhD Candidate

Alex Farrar Hannah Moore

PhD Candidate PhD Candidate

Karla Morales

PhD Candidate

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Day 1 – Grassy area, Dolphin open pit mine

By Carlos Díaz Castro, Nathaly Guerrero Ramirez and Max Hohl Historical context The prospector Tom Farrell discovered scheelite (calcium tungstate, CaWO4) on the shore of Grassy Bay (Figure 1), in 1904, near the eastern end of what is now the abandoned open pit mine whilst he was prospecting the area for tin. The first mining activities were developed by the King Island Scheelite Development Company N.L. which took place from 1917 to 1920, but the operations were closed in mid-1920 following a collapse in tungsten prices. Mining activities resumed between 1938 and 1990 with high-production phases coupled with low- production times (https://monumentaustralia.org.au). The King Island Scheelite N.L. was formed in 1937 and in 1947 the company was reconstructed as King Island Scheelite (1947) Ltd. The operation required ore to be mined essentially by hand, but with the onset of World War 2 things changed due to tungsten’s significance to the war effort. The Korean War of the early 1950s helped to maintain the price of tungsten at healthy levels, but after the war and following the completion of building the stockpiles in the US and other countries, the price of tungsten declined, ultimately forcing the King Island mine onto a care and maintenance basis in August 1958. The mine reopened on a limited basis in early 1960, with again the Vietnam War helping to support prices during the 1960s. The mine was operated by King Island Scheelite (1947) Ltd until purchased by Geopeko Ltd in 1969. The main open pit area was developed between 1942 and 1974. Coupled with the open pit development, the miners found that the ore grades improved at depth in an easterly direction (under the sea), following the trend of the dipping carbonate layers, and the faults that acted as fluid migration pathways. Waste rock from the open pit was used to reclaim land in Grassy Bay, from where drilling was undertaken to prove up the Dolphin orebody seaward of the original shoreline. Mining was undertaken solely by open pit methods until October 1972, when underground mining commenced at the Bold Head mine, located approximately 3 km north of the Dolphin open pit mine, which was discovered from drilling on a soil geochemical anomaly in 1968. In June 1973 underground mining commenced at Dolphin, and in October 1974 production ceased from the Dolphin open pit mine. The ore from both mines (Dolphin and Bold Head) was milled and put through a gravity separation and flotation plant on the coast at Grassy. Scheelite concentrate was initially shipped from Currie, and after 1972, from the upgraded port of Grassy. Low tungsten prices led to closure of the Bold Head Mine in 1984 and the Dolphin Mine in November 1990, by which time the mines had produced a total of 60,000 tonnes of tungstate from 11.5 million tonnes of ore. Considerable amounts of ore remain in the area, comprising about 20 % of Australia’s total economic demonstrated resources of tungsten (https://www.kingislandscheelite.com.au/projects/dolphin/history). In May 2005, GTN Resources NL acquired the project, and subsequently changed its name to King Island Scheelite Limited. In 2009, as the price of tungsten was rising, the project was re-examined with underground mining proposed to extract high grade remnant scheelite ore at an average grade of over 1% WO3. The over 2 million tonne tailings resource was also examined for possible recovery of remaining scheelite values. Currently, the Dolphin and Bold Head deposits are being assessed for their potential to support a new mining operation, by King Island Scheelite Ltd. The Dolphin project is currently one of the world’s richest tungsten deposits, hosting a total indicated mineral resource estimate of 9.6Mt at 0.9% tungsten oxide,

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including mineral reserves of 3.14Mt grading at 0.73% tungsten oxide (https://smallcaps.com.au).

Skarn mineralization at King Island Skarn deposits form commonly at the contact between magmatic intrusions and carbonate rich wall rocks. They represent a special form of contact metasomatism, which causes the transfer of large amounts of Si, Al, Fe, and Mg and other elements. Characteristic minerals associated with the formation of a skarn deposit are grossular-andradite garnets, diopsides- hedenbergite, wollastonite as well as epidote. Characteristics of skarn deposits is the large crystal size of the silicates and the hardness of the resulting calc-silica rock. Proximal to the intrusion the formation temperature of skarn deposits range between 650-500 °C, and the salinity is high, with >50 wt.% NaCl. In more distal Skarns, fluids temperature drops to circa 400 °C. In skarns, economic elements are carried as complexes in hydrothermal fluids or H2O- rich vapor. They are mostly associated with plutons of granitic to granodioritic composition, which intrude in the continental upper crust at convergent plate boundaries. Host rock consisting of limestone form Ca-skarns, while metasomatism of dolomitic host rocks result in Mg-skarns. Economically mineralization occurs mostly in the former type. Skarn deposits contain various minerals including significant amounts of Au (Navachab, Namibia), Cu and Zn (Antamina, Peru) and W (King Island, Tasmania). Skarn mineralization at King Island has formed within the metamorphic aureole of the Bold Head and Grassy granodiorite plutons, part of the Sandblow granite emplaced about 351 million years ago, where they have come into contact with the calcareous sediments and carbonates of the Lower Grassy Group Cumberland Creek Dolostone (Figure 2). Both the Bold Head and Grassy mineralization is hosted in a similar stratigraphic sequence, although the carbonate units appear to be thicker in the Grassy area (Danielson, 1975,). The deposits formed over a 100-200m sequence of complex skarn mineralogy located in the lower part of the Grassy Group, with two main host horizons known as B and C lens hosted in carbonates of 10-30m thickness separated by a similar thickness of skarn altered volcanic sediments. Mineralization occurs predominantly as coarse scheelite in either garnet-hornfels, pyroxene garnet hornfels and sometimes the garnet-pyroxene altered banded footwall beds. Scheelite, one of the two main ore minerals for tungsten, is an inconspicuous translucent yellowish mineral, with the unusual property that it fluoresces bright blue under ultra-violet light (Figure 3). Tungsten is hard and steel-grey with the highest melting point of any metal. This metal is a key component in the manufacture of steel tools, lighting filaments and electronics, power engineering, coating and joining technologies. It is also used in microchip technology and liquid crystal displays. Most of the world’s tungsten reserves are in China, with the country supplying about 80% of the world’s demand (https://smallcaps.com.au).

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Figure 2. Dolphin and Bold Head Mines, prospect locations and simplified geology (from Callahan, 2011).

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Figure 3. Examples of scheelite in a vein (left) and disseminated (right) from the Dolphin open pit mine over UV light . SEG-CODES student chapter crew activities Throughout the fieldtrip, SEG-CODES student chapter participants were based in Currie, on the west of King Island, in the King Island Cabins which are located 5 minutes-drive north of the town centre. For our visit to the Dolphin open pit mine, we began our day with a 30 minute drive from Currie to the Grassy town at the east part of King Island. At Grassy, we met Tim, the geologist in charge of the Dolphin Mine project, to have a brief introduction to the geology of the mine (Figure 4A). After this short meeting, we went to the open pit mine by car, until stopped at a flat area in the northern part of the pit, to then do our way looking the geology of this tungsten mine. We walked around seeing the different rocks at the walls of the open pit and trying to collect the best photos from the skarn mineralization (like garnets and pyroxenes), the volcanic host rocks and especially the scheelite (Figure 4B).

The first rocks were related to the footwall skarns (Figure 4C), which were interlayered with sedimentary rocks that were altered (skarn) and showed garnet-rich skarn (brown layer) interbedded with pyroxene-rich skarn (green layer). As part of our exploration in the open pit walls, we found a striking example of massive molybdenite (Figure 4D) in a fractured volcanic rock in the west wall, and also saw a good example of an amazing-crystallized dyke that cut the rock sequence in the open pit (Figures 4E and 4F). The west wall of the open pit showed the pyroxene-rich skarn, but it also has well-crystallized garnets. The pyroxenes presented dark- green colour while the garnets presented light brown colour. It was well-noticed the skarn- alteration change from a more pyroxene-rich mineralization to a most garnet-rich mineralization near to the mine entrance (Figure 4F).

After that, we walked down to the east wall of the open pit, which was next to the underground mine entrance (Figure 5A). This area had the garnet-rich skarn, where the garnets showed dark brown colour while the pyroxenes showed lighter green colours (Figure 5B). In this area, it was possible to see beautiful crystals of garnets and a higher presence of scheelite disseminated that catch the attention of everyone there (Figure 5C). Many of the participants decided to collect as many rock samples as possible (as many as each one could handle in their backpacks) because the interesting scheelite crystals and skarn mineralization, especially well-crystallized garnet-bearing rocks.

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Figure 4. examples of the main activities conducted and rock samples observed during the visit to the Dolphin open pit. A: geology meeting at the grassy town, conducted by Tim. B: seg-codes student chapter participants walking into the open pit mine to see the skarn mineralization (photo looking south). C: skarn mineralization with well-defined pyroxene- and garnet-rick interbedded bands. d: molybdenite mineralization into a fracture. E: dyke sample from the west wall with well-developed crystals. f: panoramic view (looking southwest) of the open pit mine, with the pyroxene-rich skarn at the right and the garnet-rich skarn at the left.

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Figure 5. Bottom part near to the old underground mine entrance at the Dolphin open pit mine. A: SEG-CODES student chapter participants walking to the area near to the underground entrance at the east wall open pit mine (photo looking south). B: Example of well-crystallized garnet-bearing rock sample. C: Part of the SEG- CODES student chapter participants looking for scheelite under UV light.

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Our trip then continued to the waste area to see rejected rock samples from the old mine, which surprisingly presented a lot of scheelite mineralization on them. We took our lunch over this area and also spent some time performing a short game to see who was able to discover the best (more rounded, mineralized, coloured, complex, well-crystallized) rock sample while each one escaped from the tide and waves (Figure 6).

Figure 6. Beach near to the Dolphin open pit mine where it is possible to see big scheelite-mineralized rounded rocks, formed mostly by the waste material produced during the mine lifetime. A: Some SEG-CODES student chapter participants walking around, looking for good rounded mineralized rock samples. B: Examples of some interesting, rounded rock samples from the beach. C: Michael Roach taking photos from one of the best rock specimens found around the beach.

Naracoopa

By Rhiannon Jones and Peter Berger After our visit to the mine we headed to Naracoopa on the east coast of King Island. Here, exposed along the coastline, are sedimentary rocks of the Fraser Formation and volcanic rocks of the Grassy Group. The lower Grassy Group volcanic rocks are intruded by a dolerite sill. Rocks of the Fraser Formation include mudstones and siltstones, at Naracoopa we could see evidence of weak metamorphism by the presence of chlorite in the sedimentary rocks (Figure 7). We could also see evidence of laminations preserved in these siltstones, which had a darker colour.

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Figure 7. Weakly metamorphosed siltstones of the Fraser Formation, the light green colour indicates the abundance of chlorite. Walking east along the coastline there was a transition to igneous units, the rocks contain boulders of basaltic material and are part of the upper Grassy Group called the Shower Droplet Volcanics. Based on the stratigraphy, it has been interpreted that a fault has caused the lower Grassy volcanics to be missing at Naracoopa.

As we continued east along the coastline, we walked upwards through the volcanic succession. We could see a range of volcanic textures including vesicles which were infilled by chlorite (Figure 8).

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Figure 9. Toby and Hannah inspecting volcanic textures.

Figure 8. Chlorite infilled vesicles from the Grassy Group volcanics.

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Day 2 – Stokes Point and Cape Wickham Stokes Point

By Thomas Schaap and Zebedee Zivkovic The first half of day two featured a drive to the southernmost part of King Island to Stokes Point. This rugged peninsula, exposed to the wind and waves of the “Roaring 40’s”, was a picturesque backdrop for a morning stroll amongst the oldest known rocks in Tasmania (Figure 10). The geology of the Stokes Point area is dominated by the Mesoproterozoic Surprise Bay Formation, a sequence of psammites and pelites which have been dated to ~1350 Ma – making them some of the oldest rocks in King Island and Tasmania.

Figure 10. 1350 Ma metasedimentary rocks of the Surprise Bay Formation at Stokes Point. Tom Schaap for scale The upper greenschist to lower amphibolite regional metamorphism has produced some spectacular large porphyroblasts of andalusite and grossular garnet (Figure 11). Metamorphism is believed to have peaked ~1280 Ma during burial with temperatures reaching up to 580 degrees Celsius. Evidence for retrograde metamorphism can be seen with some andalusites having regressed to muscovite.

The deformation history of King Island is also captured in these rocks with all three major deformation events identifiable at Stokes Point.

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Figure 11. Large andalusite crystals in a garnet-rich matrix from Stokes Point.

Cape Wickham

By Karla Morales and Xin Ni Seow Regionally metamorphosed rocks of the Surprise Bay Formation are exposed at Cape Wickham in the north eastern part of King Island. The Surprise Bay Formation is a belt of schist and quartzite that have been subject to upper greenschist and amphibolite facies metamorphism (andalusite and garnet porphyroblasts).

These rocks were later intruded by ~760 Ma granite resulting in intense contact metamorphism, accompanied by multiphase deformation as evident by boudinage (Figure 12A) and intense folding in local rocks (Figure 12B). In proximity to the contact with the Surprise Bay Formation, the schists were crosscut by granitic dykes (migmatites) which is composed of interlocking coarse-grained quartz, feldspar, biotite, tourmaline and muscovite (Figure 12C and D). The age of this intrusion is much older and not comparable with other known intrusions elsewhere in Tasmania (~300 – 450 Ma).

Mafic intrusions both pre-date and post-date granite emplacement and associated metamorphism. The age of the basaltic intrusion is still under debate (Figure 12E)

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A B

C D

E

Figure 12. Outcrop photos from the Surprise Bay Formation and granite at Cape Wickam. (A): Boudinage structure of interlayered schists. (B): Intensely folded metasedimentary rocks. (C): A granitic dyke (migmatite) intruding the schist of the Surprise Bay Formation. (D): Close-up look of the granite comprising coarse-grained tourmaline, feldspar, quartz and muscovite. (E): A hand specimen of the plagioclase-phyric basalt.

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Day 3 – City of Melbourne Bay City of Melbourne Bay south: volcanics and tillites

By Hannah Moore and Tobias Staal The day started in the City of Melbourne Bay, where we headed south to find the reddish/purple Yarra Creek Shale (Figure 13). This shale was deposited by an accumulation of mud on the deep-sea floor over a substantial duration, about 60 Ma. This shale dips east and overlies the Cumberland Creek cap carbonate dolostone (636 Ma; see City of Melbourne Bay north) and Cottons Breccia. Walking a little further south, we soon came to some well- preserved volcanic breccias (Figure 14). The first lavas erupted through the soft water- saturated sediments on the seafloor, which instantly cooled and shattered to form the volcanic breccia.

Further to the east, which is also higher in the sequence, is a thick unit of pillow lava (Figure 15). Pillows form when the lava is erupted in seawater, and its surface instantly cools to form a skin; the pillow expands due to lava continually pouring into it. Once the pressure becomes too much, a breakout of lava will occur, and a new pillow will be formed. Many of the pillows contain vesicles (originally gas bubbles) which are now infilled by calcite or quartz. These volcanics form a sequence at least 2 km thick. The pillows appear greener in colour to the top of the sequence, a signature of picrite. Picrite is dense and rich in magnesium, often olivine- rich. The volcanics were erupted during a rifting event in the late Ediacaran Period, 570 Ma, potentially during the final episode in the break-up of the supercontinent Rodinia. Further south in Cotton’s Beach, the shale is exposed once again, and the lava appears to have mingled with the wet sediments to form peperite (Figure 16). The carbonates, shale and volcanics form the upper part of the Grassy Group.

Figure 14. Yarra Creek Shale Figure 13. Volcanic Breccia.

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a b

c

Figure 15. Pillow lavas of the City of Melbourne Bay Volcanics

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b

Figure 16. Peperites and volcanic breccias

City of Melbourne Bay north: cap carbonate

By Alex Farrar and Tobias Staal The northern end of the City of Melbourne Bay, on the east coast of King Island, hosts one of the scattered localities on Earth that preserves the Cryogenian – Ediacaran contact of the Snowball Earth global glaciation event of 635 Ma (Hoffman et al. 2017, Figure 17). On King Island, this corresponds to the contact between the Cottons Breccia and the overlying Cumberland Creek dolostone.

The Cottons Breccia is a non-graded, matrix supported, polymictic diamictite of glacial origin, approximately 150 m thick and conformably overlies the Robbins Creek Formation. Clasts within the Cottons Breccia are primarily carbonate and clast size is highly variable, ranging from the cm to multi-metre scale (Figure 18). There is no carbonate package on King Island that could be the source rock for the diamictite clasts, indicating they have been transported from another region. Numerous sub-metre scale lenses of finely laminated siltstone, often

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containing large carbonate clasts are distributed throughout the Cottons Breccia and are interpreted to represent glacial retreat and advance processes. Characteristic glacial striations were not observed on the carbonate clasts, nor were good examples of dropstones.

The Cumberland Creek dolostone (Figure 19), also known as the ‘cap carbonate’, overlies the Cottons breccia, and their contact mirrors laminations within the dolostone (Figure 20), indicating that if not conformable, they are temporally close in age, which is supported by geochronology. The dolostone is 10 m thick, pale yellow to pink, is finely laminated and contains patchy 1-2 % pyrite, which occurs parallel to laminations. The cap carbonate overlying the glacial diamictite is distinctive of the Cryogenian – Ediacaran Snowball Earth event and is seen at other locations around the world and represent the transition from glacial to post-glacial conditions.

Economic Geology implications

The most well-known example of mineralisation within this stratigraphic sequence is in the Sediment Hosted Copper province of the Central African Copperbelt. In the Copperbelt, the equivalent diamictite to the Cottons Breccia is called the Grand Conglomerate and it is also overlain by a cap carbonate. The Grand Conglomerate hosts one of the most significant copper discoveries made so far this century, the supergiant Kamoa-Kakula copper deposit (1.4 Bt @ 2.74 % Cu). Also hosted in the Grand Conglomerate, at the contact with the overlying cap carbonate, is the Lonshi deposit (18 Mt @ 3.6 % Cu, Figure 20). In the sediment hosted Cu model, the impermeable cap-carbonate acts as a fluid-flow barrier which traps oxidised, Cu-rich brines in the stratigraphic sequences below, in this case the diamictite, which contains reduced horizons, thus providing a redox site for the oxidised metal-bearing fluids. This stratigraphic sequence, with the impermeable cap and redox boundaries, is also a target for hydrocarbon explorers.

It is interesting to note that mineralisation associated with the Grassy tungsten mine is also hosted within this part of King Island stratigraphy. This relation may not be merely casual, given the impermeable cap, permeable diamictite and redox gradients within it.

Figure 17. From Hoffman et al (2017) showing distribution of Marinoan (635 - 645 Ma) glacial and peri-glacial deposits.

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Figure 18. Cottons Breccia Figure 19. Cumberland Creek dolostone

CB

CCD

Figure 21 Contact of Lonshi Conglomerate with Lonshi Figure 20. Contact of Cottons breccia (CB) with Dolomite. Massive chalcocite orebody at the contact, overlying Cumberland Creek dolostone (CCD). hydrothermal bleaching of Lonshi Conglomerate to clay. Equivalent units to Fig. 20.

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Acknowledgements

The trip was financially supported by a Stewart R. Wallace travel grant from the Society of Economic Geologists and sponsorship from the Geological Society of Australia. We gratefully acknowledge Nicholas Direen for donating his time and knowledge in order to prepare for, design and also lead the trip; Karen Huizing and Hannah Moore for their assistance in the organization of the trip; Zeb Zivkovic and Alex Farrar for designing the itinerary and driving us around King Island; Tim Callaghan for permitting access to the Grassy Mine; David Cooke and Lejun Zhang for their lectures on Skarn mineralisation; and all of the attendees for being so enthusiastic during the fieldtrip.

References Callahan T, 2011. Exploration potential of the Grassy mine area, King Island, Tasmania. Unpublished internal report for King Island Scheelite JV. Calver CR, 2007. Some Notes on the Geology of King Island. Tasmanian Geological Survey Record 2007/02. Danielson MJ, 1975. King Island Scheelite deposits. In Knight CL (editor), Economic Geology of Australia and Papua New Guinea. Monograph Serial Australian Institute of Mining and Metallurgy. https://www.kingislandscheelite.com.au/projects/dolphin/history/ https://monumentaustralia.org.au/themes/technology/industry/display/101169-grassy- scheelite-mine-memorial-cross https://smallcaps.com.au/king-island-scheelite-secures-offtake-deal-dolphin-tungsten-mine- restart/ Hoffman, P. F., Abbot, D. S., Ashkenazy, Y., Benn, D. I., Brocks, J. J., Cohen, P. A., Cox, G. M., Creveling, J. R., Donnadieu, Y., Erwin, D. H., Fairchild, I. J., Ferreira, D., Goodman, J. C., Halverson, G. P., Jansen, M. F., Le Hir, G., Love, G. D., Macdonald, F. A., Maloof, A. C., … Warren, S. G. (2017). Snowball Earth climate dynamics and Cryogenian geology-geobiology. Science Advances, 3(11). https://doi.org/10.1126/sciadv.1600983