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Alluvial gold: A geological model (Part 2)

Philip Dunkerly (UK) In A geological model for the alluvial gold environment (Part 1), the first part of this article, I discussed how alluvial gold is found and suggested a geological model for alluvial gold deposits. (Readers are recommended to have another look at that part to remind them of the model.) In this second part, I now turn to the nature of the gold itself. Fig. 1. Gold bullion bars of 400 troy oz. Fig. 2. Sites from around the world. Gulch gold Gulch gold is the coarsest that exists in any part of a river system. If nuggets (pieces of gold weighing more than 0.1g) are present, they will mostly be found in gulches (narrow ravines), provided suitable traps are present, such as irregular bedrock. In gulch alluvium, the vast majority of the gold will be found on, or in crevices within, the bedrock. Gulch gold is often coarse and angular and may contain silicate debris, especially quartz. As examples, gold from Victoria Gulch on the Klondike was described as “sharply angular”. In the Ballarat gullies, some enormous nuggets were found and Canadian Gully yielded nuggets of 50.4, 34.7 and 31.4kg. At Bendigo, White Horse Gully, a 17.8kg nugget (including some quartz) was found. (Interestingly, of a list of 92 Victorian nuggets, 34 came from localities specifically named “gullies”.) Finally, in the Sierra Nevada of California, most of the gold is from gulches or minor streams close to croppings. Fig. 3. Old hydraulicking operation of terrace gravels, note … Read More

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Alluvial gold: A geological model (Part 1)

Philip Dunkerly (UK) Mankind almost certainly first found gold when a yellow, glint from the bottom of a stream bed attracted the attention of one of our ancestors in pre- historic Africa. Ever since, the allure of gold – its colour, improbable density, malleability and scarceness – meant it has been prized, and great efforts have been made to accumulate it. Most ancient peoples venerated and coveted gold and used it for decoration, and empires used gold as a store of value and a medium of exchange. The Egyptians are known to have used gold as early as about 5000 BC, followed by many others, including the Romans, the Incas, the Spaniards and, of course, the Anglo-Saxon invaders of North America, Africa, Australia and New Zealand. Fig. 1. Spectacular Roman paleogravel workings at Las Medulas, NW Spain, now a World Heritage site. The mouth of one of the tunnels through which water was released from a header tank is visible in the shadow. Fig. 2. Panoramic view of Las Medulas, worked by sluicing using water brought through canals up to 60km long. Though gold was won from hard-rock deposits in ancient times, most gold until perhaps 1900 was won from riverbeds, and was traditionally called alluvial or placer gold. Prospecting for alluvial gold required relatively little equipment and always attracted hardy pioneers willing to forego the comforts of society in the hope of ‘getting rich quick’. The gold they found – if they were lucky – could almost instantly be … Read More

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Geo Junkets: New Zealand, North Island (Part 3)

Jesse Garnett White (USA) Kohioawa Beach and Matatā Escarpment, Putauaki Volcano and the Kawerau Geothermal Field Kohioawa Beach and Matatā escarpment. Kohioawa Beach, an uninterrupted sweep of sandy beach, dunes and wetlands, is directly below the near vertical Matatā escarpment between the towns of Otamarakau and Matatā. The escarpment gradually gains elevation to its highest point behind Matatā. Infrequently cut by active and inactive canyons, flowing streams debouche across the beach into the Bay of Plenty. Atop the escarpment are rare, mature pohutukawa, puriri and manuka, and various types of scrub and grass. Part and parcel of the Whakatane Graben, the terraced escarpment is composed of Castlecliffian marine sediments, remnants of the Aranuian Interglacial period. On the western margin of the graben near Matatā, marine sandstone and siltstone outcrops contain bivalves, gastropods, crustaceans, sponge spicules and microfossils, overlain by tuffaceous sediments, ignimbrite gravels and conglomerate (Nairn and Beanland, 1989). The coastal dune and wetland areas of the Rangitaiki Plains and Tarawera River Valley near Matatā exhibit Holocene backswamp and floodplain deposits, including levees and meander sediments associated the Awatarariki, Waimea and Waitepuru stream catchments. The catchments rise from sea level to 370m elevation and drain into the Bay of Plenty. The Awatarariki and Waitepuru wetlands were destroyed in 2005 by large, storm-induced debris flows and associated floodwaters. Waimea was largely unaffected, retaining the majority of its pre-storm event character. Matatā township was severely impacted by debris flows, with over a hundred homes and properties damaged or destroyed. Prior to the … Read More

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Geo Junkets: New Zealand , North Island (Part 2)

Jesse Garnett White (USA) The Waipoua Forest and Parataiko Range I viewed the tilted volcanic outcrops of early Miocene Waipoua Basalt above the Waimamaku River (Fig. 1), then drove into the small town of Waimamaku. I saw a backpacker standing on the sidewalk thumbing a ride holding a paperback in her left hand. She had freckles, black-rimmed glasses, and long blonde dreadlocks. Unrolling my window as I pulled the vehicle over, she walked up to the car, “Can I put my stuff in the back seat and sit up front with you?”. “Absolutely!” I replied. She tossed her hefty pack inside, hopped into the front seat, looked me directly in the eyes, and asked, “Where are you heading?” Reaching out to shake her hand, I finally managed to reply, “I have no idea! My name is Jesse.” “My name is Clear. I am going into the Waipoua Kauri Forest and then hope to get south of Auckland.” And so it was that we decided to stick together until it was time to part and that is exactly what we did. Fig. 1. Early Miocene Waipoua Basalt above the Waimamaku River. We visited Tane Mahuta (The Giant Kauri Tree) in the Waipoua Forest. Joining a busload of German tourists, we walked through disinfecting stations (Fig. 2) where the soles of our shoes were scrubbed down and sprayed with disinfectant. Fig. 2. Waipoua Forest disinfecting stations. Tane Mahuta was immense (Fig. 3). The girth of the trunk, height and sprawling canopy signified … Read More

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Ancient weevil pupal cases: Trace fossils from Australia’s Pleistocene

Steven Wade Veatch (USA) Curious pupal cases made by prehistoric weevils, together with worm burrows, are found as trace fossils in rock exposures of the Upper Bridgewater Formation along the western coastline of the Eyre Peninsula in South Australia (Flint, 1992; Flint and Rankin, 1991; Rankin and Flint, 1992). According to Parker and Flint (2005), the Upper Bridgewater Formation is a middle to late Pleistocene aeolian calcarenite (a wind-blown, consolidated gritty calcareous sandstone). These trace fossils are found inland from the coast for a distance of about 40km. Microscopic analysis of these ancient pupal cases shows they are made of gritty sand and gravel that were cemented by calcite over thousands of years. Fig. 1. Fossil pupal cases from the Bridgewater Formation resemble small elongated eggs. The cases have a hole where the fossil organism exited. These trace fossils are characterized by their strong cementation and a hollow interior. Scale in mm. (Specimen from the S W Veatch collection. Photo by S W Veatch.) These cases are thought to have contained the pupae Leptopius duponti, a medium-size, soil-inhabiting weevil or snout beetle of the family Curculionidae. The Curculionidae are one of the largest families of organisms, with at least 44,000 described species (Grimaldi and Engel, 2005). Adults of most species of this family have a characteristic elongate snout or nostrum. At the end of this well-developed snout is a small pair of mandibles for biting and chewing food. Taxonomic Classification:KingdomAnimaliaPhylumArthropodaClassInsectaOrderColeopteraSuborderPolyphagaSuperfamilyCurculionoideaFamilyCurculionidaeSubfamilyLeptopiinaeGenusLeptopiusSpeciesdupontiThe adult female Leptopius duponti not only relishes the foliage of … Read More

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Hell and high water: The digs of Dinosaur Cove

Robyn Molan (Australia) Fig. 1. The location of the excavations. In an article in the Australian Age of Dinosaurs Journal (Issue 6, 2008) I dubbed the period between 1984 and 1994 ‘a decade of dedication’, thanks to the persistence of an American-Australian team headed by palaeontologists Tom Rich and his wife, Pat Vickers-Rich. (Tom wrote an article for Issue 16 of Deposits, entitled Tunnelling for dinosaurs in the High Arctic.) This was the decade that brought to the world the fascinating polar dinosaurs of south-eastern Australia and the eventual naming of three new dinosaur species – with a few other surprises along the way. Two hundred and twenty kilometres west of Melbourne, on the Otway Coast of Victoria, Australia, is a remote and little-known inlet. Set in a stretch of steep, rugged shoreline, this isolated cove is pounded by the Southern Ocean and blasted by Antarctic winds. Nearby, the world-renowned rocky sentinels, ‘The Twelve Apostles’ (see the cover of Issue 20 of Deposits), stand testament to the power of wave and wind, as they beckon tourists who travel the Great Ocean Road. Fig. 2. The rugged beauty of Dinosaur Cove. (Photo Ros Poole.) The excavation at Dinosaur Cove, as the inlet later became known, was the first major dinosaur dig conducted in Victoria. For several weeks each summer, the Rich family, and a crew of hardy volunteers, battled untold obstacles to wrestle fossils from the base of the cliff. It was gruelling, dirty and dangerous work, but subsequent scientific research … Read More

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Geo junkets: New Zealand, North Island (Part 1)

Jesse Garnet White (USA) Fig. 1. Legend/Key:1 = Sediments (Cretaceous and Cenozoic).2 = Greywacke (Permian and Triassic).3 = Schist (Carboniferous to Cretaceous).4 = Volcanic rocks (Cretaceous and Cenozoic).5 = Sediments and ophiolites (Northland and East Coast allochthon) (Cretaceous and Oligocene).6 = Pyroclastic rocks (Triassic and Jurassic).7 = Limestone, clastics and volcanic rocks (Central and Eastern sedimentary zone) (Cambrian to Devonian).8 = Granitoids (Paleozoic and Cretaceous).9 = West Fiordland metamorphic zone (Paleozoic and Cretaceous).10 = Ophiolites and pyroclastics (Permian).11 = Volcanic rocks (including pyroclastics) (Permian).12 = Mafic and ultramafic complexes (Paleozoic and Cretaceous).13 = Greywacke (Western sedimentary zone) (Cambrian to Ordovician). Auckland and the AVF In a thick brain fog, crusty eyed and yawning, I sat up in bed at 4:30 am. I was in Auckland, New Zealand. It was still dark outside when I drove to Mount Eden (Maungawhau), where I hiked up a narrow dirt trail lined by tall grass stippled with dew. Coming out of the verdure, my shoes, socks and shorts were soaked through. On top of the hill, a shadow-black grouping of trees blocked the creeping morning light from behind the Hanua Ranges. The burnt orange sunrise, obstructed by cumulous, lit up like a distant mountain wildfire. Auckland city centre was under puffy, lavender-white cirrus clouds, reflecting pastel colours across the harbour. Alone in the cool and crisp pre-dawn air, I viewed the various scoria cones in the Auckland Volcanic Field (AVF) bursting through the city neighbourhoods. Fig. 2. Map of New Zealand showing place names. … Read More

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Australia’s Polar Cretaceous mammals

Dr Thomas H Rich (Australia) The Cenozoic Era is commonly referred to as the ‘Age of Mammals’. That is certainly the time in the history of life when their fossils are most abundant and diverse. However, two-thirds of mammalian history was during the Mesozoic Era – and they appeared about the same time as the dinosaurs. All continents except Antarctica have some record of the early, Mesozoic mammals. Of those that do, Australia has the most meagre record of all. Despite this, with this landmass that today has the most distinctive terrestrial mammals on the planet, their Mesozoic origins are so enigmatic that it has motivated a major effort since 1984 to search for fossils of those mammals that lived alongside the dinosaurs on this now isolated continent. Fig. 1. A map of Australia showing the location of the four sites where Cretaceous mammals have been found on the continent. During the Cretaceous, Australia was much further south than at present. Shown here are the lines of latitude at that time on the continent: 50o south, 60o south and 70o south. The famous Lightning Ridge opal field has provided some of the answers – two different early Late Cretaceous egg-laying mammals (the monotremes), as well as a third mammal that may be a monotreme, have been discovered there. One thousand, three hundred kilometres to the south-southwest along shore platforms pounded by the waves of the Southern Ocean, which expose those rocks on south coast of the continent, are three sites … Read More

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Mineral collecting ‘Down Under’

Tony Forsyth (Australia) I got the collecting bug at about eight years of age, collecting (or ‘fossicking’ as it is called in Australia) fossilised sharks’ teeth and ancient whalebones eroding out of beach cliffs in my hometown of Melbourne, Australia. Some forty plus years later, I’ve still got the bug. However, nowadays, I tend to be more interested in collecting crystallised mineral treasures (although, my sharks’ teeth still hold pride of place in my showcase) and it is the mineral wealth of Australia that I will discuss in this article. Fig. 1. Map of Australia showing locations mentioned and an inset map comparing relative size of Australia and continental USA. In 1996, I started a website called The Australian Mineral Collector (www.mineral.org.au). After dabbling in web design, I saw the Internet as a great way to convey information about my hobby. Since that time, I have had countless thousands of visitors to the site, as well as countless emails from fellow collectors, many asking the same sort of question, for example, ‘Where can I go to find minerals in Australia? I’ve got three days to kill in Sydney and two days in Melbourne, and I want to find some nice specimens.’ Unfortunately, in many cases, I have had to tell them that it’s not at all easy. Fig. 2. The author and an old Cornish boiler at the Atlunga Gold Field, Central Australia.Firstly, let me draw you a picture of Australia. It is a very big country and, for those … Read More

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NZ Orphan mine site taken into care

Tanya Piejus (New Zealand) One of New Zealand’s most contaminated sites, the Tui Mine near Te Aroha, is to be cleaned up. The New Zealand budget for 2007 confirmed that NZ$9.88 million was available for the two-year project. The orphan mine site sits on the western flank of Mount Te Aroha, one of Waikato’s highest peaks. In Maori legend, the spirit of Te Mamoe, son of a Bay of Plenty chief, caused a stream of crystal water to flow from the heart of the mountain. However, that stream has become blighted by acid and metals from the abandoned mine workings and is now almost devoid of life. Fig. 1. Polluted water at Tui Mine Tui Mine’s story began in 1967. Norpac Mining Ltd opened it in order to extract metals, including copper, lead and zinc. The mine prospered and the company found several thousand ounces of gold and silver among the ore.  However, unacceptable levels of mercury in the ore soon proved to be the mine’s undoing. The company buying the ore pulled out in 1973 and, two years later, Norpac went into liquidation. Tui Mine was abandoned. Mining equipment was removed for reuse at other sites or was sold for scrap. Left behind was a large pile of larger ore pieces, along with sand-sized crushed ore (tailings). This was dammed to prevent it slipping down the mountainside but, through neglect, the tailings dam became unstable. In 1980, the Hauraki Catchment Board had to build a gravel embankment to stop … Read More

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Moeraki Boulders: The giant marbles of New Zealand

Tasman Walker (Australia) Scattered over Koekohe Beach on the South Island of New Zealand, dozens of huge spherical boulders look like the remains of a monster game of marbles. These were recently featured on the cover of Issue 22 of Deposits. The grey, stone balls are a fascinating tourist attraction, about 70km north of Dunedin, near Moeraki, a small town on the Otago Coast. Some boulders stand alone, but most sit in clusters, with the waves splashing over them at high tide. Many lie broken into segments on the sand. Fig. 1. Large, small and broken boulders. The boulders are spectacular examples of concretions, which form when a mineral precipitates and cements the loose grains of sediment into solid rock. As you walk down the steep bluff to the beach, you can see other enormous boulders still embedded in the uncemented mudstone, but being exposed as the ocean waves erode the loose embankment. They eventually fall onto the beach. Fig. 2. Popular with tourists. The boulders come in two distinct sizes: the diameters of the smaller ones range from 46cm to 92mm, but the larger ones are 137cm to 200cm in size. The largest ones weigh almost 20 tons. Most are spherical, but a few are slightly squashed in a direction parallel to the bedding of the mudstone in which they formed. Although fascinating, the boulders are by no means unique. In New Zealand, you can find similar ones on a beach just 12km south, and others on the North … Read More

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