1 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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 2 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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 3 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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. 4 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ Figure 1. Regional Geology of King Island (from Calver 2007). 5 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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 6 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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, 7 CODES SEG STUDENT CHAPTER, KING ISLAND FIELDTRIP 2021 __________________________________________________________________________________ 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