| Exploration |
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The Process of Exploration
The giant high grade Charters Towers ore bodies are rare in the world. Citigold Corporation is in gold production with major gold mining infrastructure and has re-activated this major high-grade goldfield.
Citigold is a leading exploration corporation with well established infrastructure and highly skilled mining personnel based in Charters Towers, along with current defined Inferred Mineral Resources of 10 million ounces of gold (23 million tonnes @14 g/t Au). There is considerable potential for Citigold to expand its gold resources and reserves with an upside potential target of 50 million ounces of gold, based on undrilled repeats of known mineralised veins. This process will require a great deal of hard work, extensive drilling and application of the best technology and practices.
Citigold holds exploration permits and other tenure covering in excess of 600 square kilometers at the heart of the goldfield. With some 80 known gold prospects within Citigold’s exploration permits, this presents substantial opportunities for an increase in the company’s future resources, production levels and its overall gold asset value. In addition to the major mines planned there is enormous potential for additional satellite mines to feed the central plant.
Achieving the greatest results in realising Citigold’s exploration potential requires a dedicated team of leading experts including geologists, geophysicists, geochemists, surveyors, field hands, and drillers as well as external contractors and consultants who apply their skill base at various stages of the project.
Our aim is to further unlock the vast exploration potential of the bonanza grade Charters Towers goldfield for its growing base of shareholders. Citigold has 5 diamond core drill rigs that are focused on growing gold reserves and resources. The term 'Bonanza' gold deposits usually applies to gold deposits over 5 million ounces with a grade of 10 g/t or more. The Charters Towers field is such a bonanza deposit.
Identifying the main prospect areas with the best potential for new gold and mineralisation requires a solid understanding of the formation of gold deposits, the geology, past production and the application of geophysics. The identification process involves extensive geological mapping, geochemical sampling, geophysical surveys, assaying of rock-chip samples, and drilling to enable predictions to be made on the nature, size, geological continuity and shape of potential ore bodies.
Compilation of the various data then enables final resource estimations to be analysed which determines the final delineation of mineral deposits. Evaluation of the economic and financial costs involved in the project then follows and ultimately leads to the final development stage of the gold mines.
Deep Hole Drilling
Of the 1,559 significant drill intersections used in the 2005 Inferred Mineral Resource estimate, there are 22 drill intersections deeper than 1,000 metres, of which 19 are deeper than 1,100 metres and 10 deeper than 1,200 metres, with a maximum gold grade of 20.54 g/t Au. The deepest intersection in 2005 was 1,300.1 metres which returned 2.3 metre-grams per tonne.
In 2008, a 2,000 metre deep hole was drilled. This hole produced the following highlights:
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Gold mineralisation extended 700 metres vertically to 1,900 metres. |
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Continuity of mineralisation confirmed. |
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Potential to significantly increase total gold resource. |
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Geological model of deep gold structures proven. |
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Record depth for Atlas Copco Diamac U8 APC drill. |
This is believed to be a world record depth hole drilled with the Atlas Copco U8 APC diamond core drill and the deepest diamond drill hole in northeast Queensland. The 2 kilometres of BQ drill rods in the ground weighed nearly 13 tonnes with this powerful 'mighty midget' drill rig completed the hole and pulled the rods successfully.
The final piece of drill core retrieved by the rig was from 2001.1 metres. The hole was drilled at a steep angle from surface of 78 degrees and stayed remarkably straight.
Citigold Corporation’s Managing Director and CEO Mr Mark Lynch said “The results from the deep hole are awesome and prove beyond doubt that the Charters Towers mineralised system extends to great depths, way beyond the limits previously mined and remains open at depth and to the east”
The hole intersected additional separate mineralised structures of 0.50 metres length at 1,615 and 1,778 metres plus a smaller 0.02 metres at 1,982 metre depth. The results confirm the presence of gold related alteration and mineralisation and demonstrate that the mineralised structures persist to at least 1,900 m depth on the eastern side of the gold field.
UPSIDE - 50 Million ounce (1,600 tonnes) Gold Potential
Exploration to date has yielded the current 10 million ounce gold mineral resource. Growing the gold resources and reserves is a core business for Citigold.
Citigold’s exploration and studies to date have led to the conclusion that the overall Charters Towers goldfield, currently held by Citigold, has the exploration potential to contain up to 50 million ounces of gold. This conclusion is well founded and based on the following facts –
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The goldfield has already yielded 6.6 million ounces of gold to depths of 400 m to 980 m; |
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Citigold has already defined an additional 10 million ounces of mineral resources from part of the goldfield down to a depth of 1200 metres; |
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The structures containing the gold mineralisation are considered to extend to at least 3,000 metres in depth. Significant mining to date has generally remained above 700 metres depth. This makes the area a 16 million ounce field down to 1200m depth, the field averaging 13,000 oz per vertical metre. The main Day Dawn- Brilliant structure averaged 4,000 oz per vertical metre (4 million ounces down to 1,000 m). |
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At least four additional major parallel structures remain unexplored. The structures have been mapped, are gold mineralised and have previously been partly mined. These parallel structures could contain four times the known mineralisation. Four structures averaging 4,000 oz/vertical metre down to 3000 m would contain 48 million ounces. |
Citigold’s exploration strategy is focused around and adjacent to its mining operations at Charters Towers. This is referred to as “brownfields” exploration, exploring in and around known mineralised areas, rather than exploring “greenfields” or unknown areas. The Company holds over 360 square kilometres of exploration tenements covering approximately 80 prospects that have potential for economic mineralisation.
The Geological Model – Tech Talk
This is an extract of Chapter 2 of the 100 page document "Report on the Inferred Mineral Resources for the Charters Towers Gold Project, May 2005". For further information on the goldfield please see the full report. Click HERE
A key part of accurate mineral resource and ore reserve estimation is a clear understanding of the geological model or models of a mineralised body or system — the shape of the mineralised bodies, their orientation and location, the nature, chemistry and origin of the gold-bearing fluids, the fluid pathways, the control mechanisms on metal deposition and the continuity of the mineralised bodies.
The model must be robust and proven by testing. This information can be also used to define future exploration targets.
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Research |
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Since 1980, there has been extensive research conducted on the Charters Towers and adjacent areas by over 20 government, industry and university researchers and presented in peer-reviewed publications and public domain documents.
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Recent research since 1997 was based on new drill core, underground openings and open pits not available to previous workers. This research provided sound evidence that mineralisation is reasonably considered by Citigold to persist to at least 3,000 metres, although Mineral Resources have been estimated to only 1,200 metres, limited by the deepest drilling at 1,270m. Key points on which Citigold's geological models are based are listed below:
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Mapping and petrological research shows the mineralised system is very large, over 40km across. Mineralisation at the Charters Towers and the Rishton-¬Hadleigh Castle mines was isotope dated and found to be the same age within an indistinguishable range, indicating synchronous formation of auriferous veins dated at 404-408 million years (Late Silurian to Early Devonian geological age) and spread across a significant segment of the Ravenswood Batholith host ( Kreuzer 2003, p.B-41, D-32, D-45).
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Nitrogen isotope data indicates that the granitoid-hosted gold mineralisation is derived from deep-seated granitic plutons or metamorphics and has risen through the crust to its present position uncontaminated by near-surface water (Kreuzer 2003, p.D-58).
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Fluid inclusion studies on vein samples from the Brilliant, Day Dawn and Queen Reefs using petrography, microthermometry and laser Raman spectroscopy indicate formation pressures of' the gold-bearing veins equivalent to depths of 5 to 14 km. Mineralogical studies on gangue, alteration and metamorphic minerals support this range. The preferred depth range of formation is 5km ± 2km. (Kreuzer 2003, p. D-3 1; Peters & Golding 1989).
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Oxygen and hydrogen isotope fractionation data indicate a formation temperature ranging from 170° C to 360° C with a preferred value of 310° C. This temperature range is supported by studies of fluid inclusions, textures and wall-rock alteration mineralogy (Peters & Golding, 1989; Kreuzer 2003, pp. C 1, C-51, D-30).
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The low-permeability intrusions of' the Ravenswood Batholith restricted and focused the ascending fluids rising from deep in the Earth's crust. Sudden fault rupturing focused the fluid flow into the active lode structures, precipitating, gold and base metals by fluid mixing and subsequent chemical and pressure changes to the fluid (Kreuzer 2003, p.D-56 to 58).
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Geological and geophysical data indicate that the Charters Towers mineralisation was not subjected to further significant deformation after the gold mineralisation formed (Kreuzer 2003, p. E-67).
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The host structures are characterized by good vertical continuity to at least 1.3 kilometres based on Citigold drilling and previous mine workings (Reid 1917), (Kreuzer 2003).
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The veins are located on the margins of gravity lows that coincide with distinct intrusions or complex igneous bodies. (Kreuzer 2003, p. E-67; Citigold geophysical studies).
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The deposits are hosted by country rock comprising mainly oxidized I-type granites, granodiorites and tonalities. I-type granites are derived by re-melting of' original igneous rock. (Kreuzer 2003, p. D-1; Peters 1987; Citigold mapping).
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Wall rock alteration studies indicate the fluid was slightly acidic to near neutral (pH 5-6) (Kreuzer 2003, pp. C-54, C-59; Corbett & Leach 1995). The oxidizing fluids have produced red hematite alteration, destroying magnetite where it is in contact with the fluids and creating local magnetic lows. This creates a geophysical signature for exploration, of de-magnetised areas adjacent to gravity lows (Citigold mapping).
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Studies of quartz veins from over 200 gold mines in North Queensland indicate the Charters Towers gold-bearing veins are typical of granitic rather than sub-volcanic hosts (Dowling. & Morrison, 1989: Kreuzer 2003 p. C-54).
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Structural domain, fabric studies and spatial autocorrelation (Fry analysis) indicate that the east-west and NNW-SSE striking planes of weakness were oriented most favourably for reactivation during deformation, providing important loci for quartz vein formation and ascent of gold-bearing fluids. Areas with a greater density of intersecting structures were more likely to localize gold deposition. There was a single episode of vein formation and gold mineralisation during the fourth deformation event (designated D4). (Kreuzer 2003, p.E-67).
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The veins have not been significantly shifted by fault movements after formation. Fault movements were minor, on a centimetre to metre scale. This is in agreement with earlier research at the time of mining where average fault separations were reported to be in the order of 0.9 to 1.2 m metres (Reid 1917, Kreuzer 2003, p. B-31; Citigold mapping). |
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The current exposure of the Ravenswood Batholith is at its roof zone, meaning that there is a high probability that most of the gold-bearing system is intact and has not been significantly eroded (Citigold mapping; Hutton & Rienks 1997).
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There is a relationship of gold with galena (lead sulphide), where high gold values are accompanied by high lead values (Citigold assaying). The presence of galena was used as an indicator of high grade gold by previous miners (Reid 1917). Citigold assays for lead to check for the location of ore shoots if gold values in drill samples are unexpectedly low.
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The potential for additional gold-bearing veins to be discovered away from known mineral occurrences is considerable. Fractal analysis of the spatial distribution of the gold deposits suggests the area to the south of Charters Towers may contain undiscovered deposits. Earlier work by Citigold prior to 1999 had already targeted the area to the south based on structural analysis and geological mapping. (Kreuzer 2003, p. E-1, E-67 to 68).
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Testing the Model |
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Drilling |
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Citigold has a robust geological model that has been predicted and then tested by diamond-core drilling down to 1,270in vertical depth. Intersections into known quartz veins have hit the predicted position within one metre at depths of up to 1,500m downhole. Over 147,000 metres of' drilling has been conducted in 1,800 holes on down-dip and strike extensions of known vein systems, with 1,550 significant drill intersections. Previous explorers that drilled, mapped and sampled the area from 1980 until the float of' Citigold in 1993 include BHP, Homestake, CRA, AOG, Orion, Mt Leyshon Gold Mines and Great Mines. Prior to 1980 drilling was undertaken by Towers Drilling Co (1932), and the Queensland Department of Mines in 1923 and 1969-70. A detailed project evaluation was conducted by Gold Mines of Australia (the precursor of WMC) in 1935, and Citigold holds copies of' much of their data. |
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Open pit and Underground mining |
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Underground mining in five underground operations since 1994 (the Central Decline, No.2 Cross Vein, Stockholm, Washington and Warrior mines) has also confirmed predicted intersections with previous workings within 0.3m and has intersected geological structures to confirm the accuracy of' the model. Old workings have been re-opened and entered on the Brilliant, Victory, Victoria and Washington reefs, and new underground workings driven on extensions of known vein systems at Stockholm, Washington and the No.2 Cross Vein.
The new workings have been rock-chip sampled, bulk sampled and the veins mined by on-reef development and stoping.
Open pit mining was conducted on the Washington and Stockholm vein systems. The Washington vein is a classic Charters Towers style vein. Stockholm was more diffuse, and was a wider zone with some quartz vein stockworks. Old till was intersected in previously mined stopes at Stockholm, and was sampled in a 1,500 tonne bulk sample.
Total gold production from trial mining and processing a small tonnage of tailings was 38,000 ounces of gold. All ore was processed through the Black Jack processing plant.
All of Citigold's mining was along strike and down-dip from previously mined workings. |
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Survey Accuracy of Previous Workings |
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Detailed mine plans of previous mining operations are held by Citigold on site, and these have been digitized and modeled in commercial computer programs, and cross-checked against modern survey data. The majority of previous plans were accessed from originals held by the Queensland Department of Mines and Energy.
Previous Imperial mine plans were drawn up between 1870 and 1917 by licensed mining surveyors, and previous records indicate a survey closure error of less than 1 in 5000 (i.e. ±200mm in one kilometre) in chainage and, in the bearings, less than one minute of arc per 1,000' of underground survey (i.e. ±292mm offset in one kilometre). Measurements plotted on the previous plans were reported to one decimal of a foot and one minute of arc. The previous mine plans at a scale of one inch to forty feet (1":40' or 1:480) were digitized by Citigold, showing detailed workings and stopes down to 928m vertical depth.
Most of the historic workings were driven on lode. Results used from these plans, when checked against recent survey work, have been within acceptable limits, usually within 0.3 metres.
Recent shaft-capping programmes by the Queensland Government has located and surveyed a large number of shaft collars, and this data has been used by Citigold to further cross-check the computer models of both the workings and the geology against modern survey pickups. The Department of Mines and Energy launched a project in July 1996 to define the scope and cost of repairing abandoned mine shafts in the city. There are around 830 identified shafts within the Charters Towers city limits area that have been catalogued, with 688 shafts having been located in the field and inspected. The remaining 143 shafts have yet to be located in the field. The possible existence of a further 280 shafts has been identified from old aerial photos and gold mining lease plans. (Reference: http://www.nrm.qld.gov.au/mines/environment/rehabilitation.html) |
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Continuity of Geological Structures |
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The strike length continuity of the Brilliant structure has been proven by underground mining for over 800m along strike and down dip for 1,080m to a vertical depth of 910m. The strike extent has been proven by drilling to extend for a further 500m west and 700m east, giving a total strike length of 2 km.
The Day Dawn has been proved by underground mining for 1,700m along strike and 823 m vertically. The East Mexican lode is interpreted as an extension of the Day Dawn, which increases the strike length of the Day Dawn to 2,100m.
Sunburst West (part of the Queen structure) has been proven by underground mining for 350m and down dip for 225m to a vertical depth of 125m.
The Queen structures continuity have been proven by underground mining in the Bonnie Dundee and Golden Gate mines for a strike length of 1,360m and down-dip for 450m to a vertical depth of 400m.
The Brilliant East structure continuity has been proven by underground mining over a strike length of 650m and down dip for 1,080m to a vertical depth of 91Om.
The continuity of' the Cross Veins (Columbia, and St Patrick lodes) has been proven by underground mining over a strike length of 270m and down-dip for 510m to a vertical depth of 340m. The Columbia and St Patrick lodes are interpreted as the same structure, which increases the strike length to 1,300m.
Extensions of these structures have been interpreted by matching drill intersections with the proven geological models developed from the underground workings. Over 1,550 significant drill intersections were used. Because the veins are narrow (usually less than one metre) and widely spaced (50m to 400m), it is usually possible to clearly define the correct mineralised body to which a drill intersection belongs. Some uncertainty existed where the cross-veins approached or cut through the east-west structures, and core-to-vein angles were used in oriented core where possible to correctly assign drill intersections to mineralised bodies.
Only those intersections which were assigned unequivocally to a known, previously-mined mineralised body have been used in published resource estimates. |
This above is an extract of Chapter 2 of the 100 page document "Report on the Inferred Mineral Resources for the Charters Towers Gold Project, May 2005". For further information on the goldfield please see the full report. Click HERE
(Updated January 2010)
ASSAY RESULTS
Diamond drill core sample assays for the period ending 31 December 2010 please click hereDiamond drill core sample assays for the financial year ending 30 June 2010 please click here
Diamond drill core sample assays for the Quarter ending 30 March 2009 please click hereDiamond drill core sample assays for the Quarter ending 31 December 2008 please click hereDiamond drill core sample assays for the Quarter ending 30 September 2008 please click hereDiamond drill core sample assays up until the release of the June 2008 Quarterly, please click here
Diamond drill core sample assays for the Quarter ending 31 December 2007 please click here
Diamond drill core sample assays for the period ending 19 December 2007 please click hereNQ Diamond drill core sample assays for the Quarter ending 30 June 2007 please click hereRock Chip Assays for the Quarter ending 31 March 2007 please click here
Warrior Face sampling on the 830 Crosscut and 840 Levels release 21 March 2007 please click here
Rock Chip Assays for the Quarter ending 31 March 2006 please click here
Rock Chip Assays for the Quarter ending 31 December 2005 please click here
Rock Chip Assays for the Quarter ending 30 June 2005 please click here
Quarterly Rock Chip Assays - Quarter ending 31/3/2005 please click here
Quarterly Rock Chip Assays - Quarter ending 31/12/2004 please click here
Quarterly Soil Assays - Quarter ending 31/12/2004 please click here
Standard Abbreviation Codes please click here
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