This is a map of a river and its tributaries

Transcription

1 Notes prepared by Peter d Auvergne, Senior Project Geologist, Lihir Gold Ballarat, October

2 A hydrological map showing watercourses (blue indicafng that they are flowing). The slide is the start of building up a picture of what a floodplain or river system looks like in plan view (i.e., from directly overhead). This is a map of a river and its tributaries 2

3 A hydrological map showing watercourses (red indicafng that they are NOT flowing relict or dried up). The slide is second in showing a stage in the development of a floodplain or river system when the environment changes when the main watercourses find another pathway to flow, or water is scarce, or the climate is warmer (less humid ). This is the same map as in the first picture a river and its tributaries. But this 8me the rivers are coloured orange, not blue. The first map was a hydrological map (a map of water), this map is a geological map (a map of rocks), a map of the sands and gravels that were lea behind when the rivers no longer flowed. Both maps illustrate part of the natural history of the gold deposits of Ballarat one of the richest historical goldfields of its type in the world. 3

4 A side- on view of deposifon of sediments that will eventually form rock or consolidated material. This is a view of when there was a sea in the region. Let us explore why these rich gold deposits occur at Ballarat and what did they look like. 500 million years ago Ballarat was under the sea off the east coast of ancient Australia. Rivers flowing off Australia were eroding sand and mud from the land and deposi8ng it onto deltas and sub- marine flood plains off the coast of Australia. A ver8cal slice through the Ballarat area 500 million years ago would look like this a series of thin layers of sand (orange), silt (green) and purple mud) under the sea. 4

5 This is a plan view (directly overhead) of a sediment layer, flat and most likely with the coarse material (like sand) on the bopom and finer material (fine silt) on top. If we were to look at a geological map of Ballarat at the same age (a horizontal slice a view from above) this is what we would see. A flat layer which could be either sand, silt or mud depending upon at what level the slice is drawn. The sea has been removed to show only the rocks. 5

6 When those layers of sediment are then pushed and squeezed by tectonic forces, you get upliting like this mountain building. The red lines indicate faults or cracks in the strata which don t have the elasfcity to bend any further and which allow super- heated fluids which are rich in gold- bearing quartz to enter the system. This is the Fme gold is emplaced in Ballarat. With 8me the pile of sand, silt and mud became very thick. The weight of the sediments caused the sea water to be squashed out of the soa sediments and they gradually became harder and harder and eventually were changed into hard rocks. For a period of about 40 million years these rocks were squeezed from the east and the west as a chain of mountains started to grow along the east coast of ancient Australia. The layers of rocks were folded and bent. When the rocks could bend no more they started to crack. These cracks oaen occurred where the rocks had been folded. This cross- sec8on illustrates what the Ballarat geology might have looked like about 450 million years ago 6

7 As well as upliting, there is erosion which wears away the surface of the landscape and it is washed into the valleys and lower surfaces; any rocks containing gold are also weathered and eroded and tumble down the slope with gravity to the lowest areas. The mountain building con8nued for many millions of years. Molten rock (granite represented in red on this cross- sec8on) was pushed up from deep beneath the earths crust. Fluids escaped from these molten rocks and some8mes made their way into the fractures in the sandstones, siltstones and mudstones. Occasionally these fluids contained minerals, some8mes gold. The hot fluids from the granites cooked the Ballarat rocks slightly and the mudstones were changed into slates. These fluids con8nued to move around in the fractured rocks un8l a change in pressure, a change in temperature or a change in chemistry caused them to solidify, forming quartz veins which some8mes contained gold. 7

8 Over Fme, the mountains are reduced and new valleys, river courses and river plains are formed. You can see that the red in the bopom of the river beds is gold; and that one of the red gold- bearing quartz veins is now exposed at the surface waifng for a prospector to discover it and begin the process of imagining if there is more underground As the mountain chain was being built, the sea was pushed further eastwards. New rivers developed and started to erode the folded sandstone, siltstone, and slate layers. The granite, the quartz veins and the gold that was in the quartz were also eroded. The eroded material was deposited in valleys and river channels or washed out to the sea. At about 50 million years ago the once mountainous Ballarat area had been reduced to an almost flat plain with extensive areas of gravel and sand with livle islands of the older (500 million year old) rocks s8cking through. A cross sec8on through Ballarat about 65 million years ago might have looked like this. The red arrows show the erosion from the hills into the valleys. 8

9 A plan view (directly overhead) showing in white the hills that are sfll up above the level of the floodplain. Or, if we look at a map (birds- eye) view at 65 million years ago we see livle islands of old rock poking through a flat alluvial plain. 9

10 Same view with the rivers we saw previously added back in aiming to show the drainage paperns of where water would carry gold and other materials eroded from the hillsides. Water is always doing Nature s work, bapling the earth and it always wins. This is now starfng to like Ballarat s geology and hydrology. On this map we now have drawn the rivers which were draining this plain. Do you recognise this map? It is the first picture you looked at. 10

11 Now we have lava flows from volcanic acfvity coming on the western side. In the south and east, Mounts Warrenheip, Buninyong and others are formed. Between 65 and 3 million years ago volcanoes were developed along the eastern edge of Australia extending from Tasmania to Queensland. This is one of the last stages in mountain building processes that have occurred throughout the world at different 8mes in the earth s history. Volcanoes to the west of Ballarat poured out thick flows of molten lava (basalt) which covered the western part of Ballarat, burying the old rocks and the river channels of the Ballarat Plain. Towards the end of this period lava pushed up to the east of Ballarat to form the volcanic cones of Mount Buninyong and Warrenheip. The chemistry of these molten lavas is quite different to the chemistry of the granites. The basalt lavas do not contain much quartz and do not contain gold. 11

12 In plan view, you can see that the basalt from the lava flows has covered part of the floodplain and the leads beafng gold. In the East though, they are not covered by basalt. What does the geological plan of Ballarat look like now? The plain, the islands of ancient rock and the rivers are s8ll there, but they have been buried in the west by basalt lava (coloured mauve). 12

13 Under the basalt in Ballarat in the 1850s and 1860s, this was called going into the trap. Erosion and river development con8nued in the Ballarat area. Today the geological map looks like this. Compare this map with the previous map. A new river system (the Yarrowee River and its tributaries) covers the eastern part of Ballarat, the basalt lavas cover the western side of Ballarat. What has happened to the ancient river system that we have been looking at? It has been almost completely buried, either beneath the lavas in the west or the new sediments of the Yarrowee River plains (shown in green). If you look closely at the edges of the brown areas (the ancient rocks) you can see very short pale blue lines the upper ends (shallow leads) of the valleys of the ancient river system. 13

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15 Mechanical concentrafon means the acfon of water running water provides the energy needed to sit and sort the material and of course the gold sinks in a fluid mix. Gold is not the only mineral concentrated in this way in northern NSW, there are fields of Fn, sapphires, silver and gold in lead- type deposifons which are so shallow they are open- cut or were mined using placer deposit techniques hydraulic hosing of the earth to wash it into flumes and gravity separators. Running water provides the energy needed to sort the sediments and minerals that are being eroded from the rocks over which the water runs. The heavier minerals are concentrated at the base of the river beds while the lighter minerals are washed away. Gold is the heaviest material that the rivers have eroded from the rocks and is concentrated at the bovom of the river channels by the water ac8on. Gold is not the only mineral concentrated in this way in northern New South Wales there are river channels that contain 8n, sapphires, silver and gold. There are many other river channels throughout the world that contain concentra8ons of minerals. Such mineral deposits are known as leads. Leads are defined as two types shallow leads and deep leads 15

16 That is, you might sfll be able to see where it is going on the surface even though it is buried. A shallow lead is a lead that can be followed by looking at the land surface. Some of the 8n and sapphire leads in New South Wales are shallow leads and are mined using bulldozers, scrapers or even by washing the gravels with high pressure water jets. 16

17 No qualificafon of depth simply means that a lead can be a deep lead at any depth provided the condifon that you cannot tell where it is going is met. If the twists and turns of a lead cannot be predicted by looking at the present ground surface, no maver whether the lead is only a few metres or many metres below the ground surface, the lead is known as a deep lead. 17

18 This is showing the depth of the leads in East and West Ballarat fields. The pink in the west is volcanics basalt cover. This slide suggests that there was a sharp fall from East to West, and a consequent increase in the energy of the river systems in the West. Lots of gold because of that energy, but more water and tougher mining condifons, and a smaller area. This is the official map of the geology of Ballarat as published by the Geological Survey of Victoria. The deep leads are shown in orange (remember the second image you saw). This map is a livle bit special. Most geological maps show the geology at a par8cular depth, this map projects an image of the deep leads through the layers of younger rock to show where the deep leads lie beneath the streets of Ballarat. The black stars show the depth of the lead below the surface. The red stars show the loca8on of just two of the many hundreds of mines that once tunnelled beneath the City. The first gold in Ballarat was found in 1851 on the surface near the Canadian Lead close to the star showing a depth of 30 metres. The miners followed the lead northwards and then westwards reaching the edge of the basalt (coloured pink on this map) in A that 8me it was thought that the basalt dammed the ancient river (deep lead), that deep lead would stop and there would be no more gold. Shortly aaerwards a shaa near the present Town Hall was sunk through the basalt and found the con8nua8on of the Ballarat East deep lead and the Ballarat West goldfield had been discovered. Unlike the Ballarat East leads which followed a reasonably predictable course, the leads of Ballarat West twisted and turned and their course was highly unpredictable. This made mining very difficult and costly because a shaa could be a long way from the lead resul8ng in large distances of expensive tunnelling to reach the lead before mining could commence. In some mines the lead was discovered to have turned away from the company s claim and that, aaer the expenditure of large sums of money on mining, there was no gold to be found. Mining of the Ballarat East deep leads was virtually finished by 1856 but mining of the deep leads in Ballarat West con8nued into the early 1870 s by when all the deep leads had been worked out and there was no more gold lea. But the miners knew that there was gold in the quartz veins in the hills and also in the bedrock beneath the deep leads. While the deep leads in Ballarat East were being mined some quartz vein mining was being done on the hills nearby but this was not as profitable as deep lead mining. Mining speculators therefore preferred to invest their money in the deep lead mines. Once the deep leads of Ballarat West had been worked out investors turned their aven8on to the quartz mines. The quartz mines maintained a mining industry in Ballarat un8l There is s8ll gold in the quartz veins beneath Ballarat and there is currently a mine being developed to mine gold again. 18

19 Lava cover in the West, but not in the East. This is a cross- sec8on of the Geological Survey map showing the buried leads. 19

20 This shows how much gold was on the other side of the lava the red line is an indicator of what happened when you got under the basalt and the leads that were created by the much more energefc western flowing streams. I don t think we need this slide in the story 20

21 A map of the major lead systems in central Victoria. Although only a rela8vely small part of the deep lead system of Central Victoria, the Ballarat system (in the red circle) was very rich in gold by comparison with many of the other leads in the region. 21

22 Red lines showing worked out areas. The deep leads and the mines which tunnelled along the deep leads to extract the gold are shown on a recent aerial photograph of Ballarat. This map shows only some of the tunnels. Many tunnels, par8cularly those along the Canadian and Eureka leads, were never mapped or the mine records have been lost. These were the shallower and rela8vely small mines of the prospector or the small private mining companies in which the miners were the only shareholders in the company. Because the shareholders were the miners there was no need to report regulalry to the stock exchange. The very large public companies that were formed to mine deep beneath the basalts in the west were required by law to report regularly to their shareholders. This repor8ng resulted in good quality survey maps of the underground tunnels. 22

23 A close up view of the drives as mapped. An enlargement of part of the previous aerial photograph shows how extensive the tunnelling was. The red lines are the mine tunnels and the yellow areas are the deep leads. The pale green areas are old river terraces (known as reef wash) which are also part of the ancient river system. The reef washes also contained gold. The areas of the lead where no tunnels are shown was mined as extensively as the rest of the lead the mine records no longer exist. 23

24 A typical deep lead (covered by basalt in this case). The drit is the unconsolidated wet sand and mud and clay they had to dig trough to get to the guper. This is a cross- sec8on through a typical deep lead (river channel) of the Ballarat area. The leads oaen contained much water and the sediments were oaen poorly consolidated. When a mine was opened up in these unconsolidated water saturated sediments (dria) they could flow into the mine. The mine could collapse or be flooded with water or sand making it very dangerous for the miners. Before mining commenced holes were oaen drilled to find where the centre (guver) of the deep lead was and at what depth. The miners would then dig (sink) a shaa in the hard rock beside the lead un8l they were below the base of the guver and then they would dig a tunnel (the main or reef drive) through the bedrock un8l they were directly below the guver. 24

25 Panelling or blocking system to work out a deep lead and get all the gold in it. OTen tunnels or pillars are let to keep the roof up and then extracted working backward towards the shat. Important here to see that the drives were BELOW the guper to promote drainage and to allow gravity to do the work. Once the reef drive was completed drives were developed in the rock under the lead. Bore holes and rises were put up from these drives into the base of the lead to drain the water. The water flowed into the reef drive and back to a sump at the base of the shaa from where it was pumped to the surface. OAen this required very big pumps driven by very powerful steam engines. When the deep lead had been drained sufficiently wash drives were tunnelled along the base of the guver from the top of the rises that had been put in to drain the lead. 25

26 From the wash drives, tunnels (blocking drives) were driven across the base of the guver, dividing the wash into blocks ready for mining.

27 This image shows the reef drives, wash drives and blocking drives in a deep lead that has been completely blocked out. 27

28 To prevent the loose dria from falling into the wash drives and blocking drives the miners lined their tunnels with 8mber. This sec8on shows the 8mbering in a wash drive. The base of the drive is on bedrock, the wash- dirt is shown in the lower front of the tunnel and the sandy dria in the upper front and over the roof of the tunnel (the backs). If the dria was not supported it would almost certainly collapse and could crush the miners. The work was very wet, very dirty and very risky. Although great care was taken during 8mbering, many accidents occurred. Occasionally mines were flooded due to water burs8ng in of flows of very wet sand. Some8mes the walls or backs collapsed before the 8mbers were put in or the weight of water and dria caused 8mbers to break. Because of the shortage of 8mber due to the large number of mines having cut down the local trees either to 8mber their tunnels or to fuel their boilers, once an area in a mine was worked out the miners would try to remove the 8mbers to be used in a new part of the mine. This was highly risky and many miners lost their lives in accidents while recovering 8mbers.

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31 SFll quite a bit of luck and imaginafon as well a science in choosing the ground to work. This image could be lea out of the presenta8on 31

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33 Red line indicates the beginning of the basalt cover (or rather when serious work was going on underneath it you can see it was not as producfve as the East. Deep lead mines were also known as alluvial mines because they were mining gold from the alluvial deposits of the river channels. This graph shows how much gold was produced from the Ballarat deep leads in each year. The red doved line shows when the mines started to go under the basalt. Although alluvial mining lasted longer in Ballarat West, much more gold was won from Ballarat East (this includes the rich LiVle Bendigo goldfield along the Eureka lead to the northeast of Ballarat). The Eureka and Canadian Leads were the biggest tributaries of the Ballarat East deep lead system. 33

34 Showing gold producfon from alluvial sources (deep leads and shallow leads); Ballarat West hardrock and Ballarat East hardrock. The alluvial deposits were fabulous. The deep lead mines produced much more gold than the quartz (hard rock) mines which took over once the deep leads were worked out. The mines of Ballarat produced approximately 13 million ounces of gold, which at the December 2009 gold price of $1281 per ounce (Australian dollars), has a value of nearly 17 billion dollars. Of this, the alluvial (deep lead) mines contributed 7.6 million ounces valued at nearly 10 billion dollars today. This places the deep lead mines of Ballarat in the top five alluvial gold producers in the world. 34