Using gravity to target gold at Tampia Hill, Western Australia
The discovery of the Tampia Hill orogenic gold deposit in the wheatbelt of Western Australia has sparked interest in this under-explored region of the state. The deposit is hosted within a granulite facies greenstone belt, with mineralisation mostly hosted in mafic gneiss, which has been intruded by undeformed and unmetamorphosed granite. A lack of outcrop in the project area has meant that geophysics has been vital for interpretation of the geology. A recent gravity and magnetic survey has allowed the most detailed interpretation of the underlying lithology and structures to date, and has highlighted previously unknown areas of mafic gneiss, with a similar signature to that at Tampia Hill. In order to extract the most useful information from the survey, spatial statistical analyses were conducted on the gravity survey data. The analyses over the project area map features within the gravity data that can be used to identify areas of known gold mineralisation. The results confirm that the gravity data not only provides critical geological information, but will also allow the identification of high priority targets for future exploration using spatial data modelling techniques.
Downhole Logging in 3D Geology and Mineral Potential Modelling
Logging of drillholes using wireline tools is an emerging methodology in mineral exploration that adds valuable data to exploration drilling. RC drilling is relatively cheap and quick, but it comes with the price of lost sample integrity and structural coherence. Wireline logging can cover this loss, by facilitating structural interpretations based on borewall imagery. Rock property data can also be recovered below the sampling resolution, such as optical televiewer (OTV) imagery, density, magnetic properties, natural gamma radiation and acoustic properties on cm and even mm scale. In the field, wireline logging will add just a few days to the drilling programme. A team of wireline technicians run their wireline down a recently completed drillhole using an assortment of tools depending on the requests of the client, at a cost amounting to only a few dollars per metre. The tools are oriented with magnetometers and accelerometers, enabling directional logging of geological features the drilling passed through. Combined with on-site logging of lithology and data from tools used in the field, wireline logging will enable exploration to take a significant step towards complete understanding of the prospect geology. In this paper we show downhole logging results from Tampia Hill, Western Australia, and how this work has been used to establish a structural framework and guide the creation of 3D geological and mineral potential models.
The Southern New England Orogen Mineral Potential Project
The Southern New England Orogen (SNEO) in the northeastern part of New South Wales (NSW) is prospective for intrusion-related tin-tungsten, intrusion-related gold-bismuth-molybdenum-silver and orogenic goldantimony mineral systems. An initiative by the Geological Survey of NSW to conduct mineral potential modelling for these mineralisation styles in the SNEO has resulted in a comprehensive account of the mineral resource potential of the region. The Geological Survey of NSW has a successful strategy of providing high quality pre-competitive data that has been complemented and enhanced by the mineral potential mapping approach. Datasets including seamless basement geology, detailed attribution of faults, and igneous fertility that were created by the survey prior to modelling enabled an extensive number of variables be tested for relevance to each mineral system. The feedback from the data processing and spatial analysis allowed improvements to be made to the data and provided information on the relevance of the datasets to mineral exploration in the region. The outputs of the models are mineral potential maps that map the geological potential of the SNEO for each mineralisation style. The models will be used for land planning and advice purposes, technical resources for improved mineral system studies including global endowment estimations, and for promoting exploration in the SNEO through the generation of prospective targets. Due to the richness of the geological datasets in NSW it is likely that the technique, including the creation of high-quality datasets combined with mineral potential modelling, can be successfully applied to other mineralised regions within NSW.
A Review of Mineral Prospectivity Modelling in New Zealand During the Past 15 Years
Prospectivity modelling has been used in New Zealand since 2002 by the government to promote mineral exploration programs. The models for orogenic and epithermal gold completed by Crown Minerals and the Institute of Geological and Nuclear Sciences for the government in 2002 and 2003 were undertaken to promote exploration activity in New Zealand. Since this time, at least 38 mineral prospectivity models have been completed, by industry and government, covering most of the hard rock mineralised regions onshore and nodular phosphate offshore on the Chatham Rise. In the last 15 years there have been advances in prospectivity modelling techniques, software capabilities, digital data availability and exploration targeting methods, all of which improve the effectiveness of prospectivity modelling as a tool for mineral exploration targeting. Future work should include:
- incorporating new data into existing models
- modelling new areas when data becomes available
- 3D prospectivity modelling modelling of other commodities such as coal, alluvial gold and irons
- infrastructure modelling and exploration effectiveness analysis.
Mineral Prospectivity Modelling in New Zealand: Review and Future Perspectives
Mineral prospectivity modelling using geographic information systems (GIS) has been used in New Zealand since 2002 both by the government, to promote mineral exploration in New Zealand, and industry, to inform project acquisition and increase the efficiency of exploration programmes. Over the last 15 years at least 38 mineral prospectivity models have been completed in New Zealand covering most of the hard rock mineralised regions onshore as well as nodular phosphate offshore on the Chatham Rise.
Analysis of highly prospective targets generated from the models already completed in New Zealand provides important information about the mineral potential of the country. Onshore, highly prospective targets over a range of commodities cover only 0.5 percent of the total land area of New Zealand, significantly narrowing the search area for new mineral deposits. 83 percent of the targets occur outside public conservation land, and 45 percent of the targets are unpermitted at the time of writing, suggesting there is potential for increased exploration investment and for new discoveries to be made. Prospectivity modelling has had a measureable positive impact on exploration activity and project development in New Zealand over the last 15 years. Future work should include incorporating new data into existing models, modelling new areas when data becomes available, improving existing mineral occurrence datasets, 3D prospectivity modelling, modelling of other commodities such as coal, alluvial gold and ironsand, infrastructure modelling, and exploration effectiveness analysis.
Enhancing regional and national economic development from mineral projects: The use of spatial analysis to inform on infrastructure deficit in Canada
A Canada National Infrastructure spatial analysis has been completed for the Prospectors and Developers Association of Canada (PDAC). The objective was to identify the Canadian districts where strategic investment in enhancing the infrastructure network could stimulate the development of new mines by reducing the overall capital costs of production. This was achieved by spatially analysing the relationship between infrastructure deficient regions and the location of significant, undeveloped mineral deposits. After compiling a comprehensive dataset of the existing infrastructure in Canada, the available information was classified and weighted based of their importance to mineral extraction. The most relevant datasets – including infrastructure, elevation and climatic data as well as cultural data such as distribution of population – have been combined using spatial modelling techniques to create a “remoteness” map of the country.
The “remoteness” map has then been compared with potential mining/advanced exploration projects in order to identify areas where strategic investment by provincial and federal government could stimulate new mineral development and therefore regional economic development. The completed spatial model has highlighted where investment would be most beneficial. Furthermore, a series of more specific cost-related maps have been produced for two categories of mineral commodities, precious minerals and base metals. The two categories differ in mining methods and quantities of minerals extracted; therefore requiring different types of infrastructure for operating. These additional models show the percentage increase of potential costs for building and maintaining a mining project related to the increase in remoteness. The results of the heat maps clearly identify regions where the enhancement of specific types of infrastructure could drastically decrease the overall costs of a precious mineral or base metal project and therefore encourage its development by making it economically feasible. The model results will allow the PDAC to work with appropriate government departments to prioritise the most prospective mining opportunities in infrastructure deficient areas and therefore efficiently propose a workflow of possible enhancements to local infrastructures to encourage the development of new mines in the identified areas.
Regional Prospectivity Modelling in Data-Poor Areas: The Kumasi Basin, Ghana
Here we present a case study of prospectivity modelling over a region with both data-rich and data-poor areas, in the Kumasi Basin, Ghana. Whilst a reasonable amount of geological, geochemical and geophysical data is available over much of the Asankrangwa Gold Belt, host of the large Nkran and Esaase gold deposits (measured, indicated and inferred resources >10 Moz Au), data availability over much of the remainder of the Kumasi Basin is generally poor and of much lower resolution. As part of a comprehensive prospectivity and targeting study undertaken by Corporate Geoscience Group for Asanko Gold, Kenex completed GIS-based prospectivity modelling using the weights of evidence (WoE) technique to delineate high priority targets for orogenic gold. WoE modelling provides a data-driven tool that combines relevant datasets, identifies anomalous thresholds in predictors of mineralisation and produces a map of geological potential.
Statistical methods ensure that when the final geological potential grid is created, areas with missing data coverage are not significantly down-weighted relative to anomalous areas. Areas of poor data coverage in the Kumasi Basin required creative examination to allow successful modelling. For example, Kumasi Basin orogenic deposits are often associated with broad zones of silicic alteration. Consequently, many deposits resist weathering and form topographic ridges, allowing analysis using detailed open-file DEM data. Ridges were extracted and attributed with scale, relative strength and orientation, all of which were tested for spatial correlation with known orogenic deposits. Another example involves limited coverage of available geophysical surveys. Scanned TMI image data was reclassified into a GIS and certain colour bands selected as most accurately representing TMI. Properties such as magnetic slope, a common predictor for orogenic mineralisation, could then be calculated. Many targets identified by the model were located in areas with high data density. By using data intelligently we have also identified targets in data-poor areas.
Mineral Prospectivity Modelling as a Tool for Resource and Mine Development
Prospectivity models in the last ten years have been predominantly used to establish the distribution of potentially mineralised ground over large areas, generally to guide initial exploration programmes in regional and mine camp settings. This approach can also be applied to the mine scale to guide resource estimation, development of reserves, mine and environmental planning, project development and to extend mine life through discovery of new resources. The Chatham Rise phosphate deposit is used as an example of where the results from prospectivity mapping can be used to guide mine planning, help with resource optimisation and provide constraints for project development. In this example the prospectivity results were combined with environmental modelling to help with environmental planning and the avoidance of sensitive areas, as well as guide mine planning. Another example compares a feasibility study to a prospectivity model over the same area. Prospectivity is an indicator of potential mineralisation presence, if not necessarily directly correlated to the actual concentration of resource present. Thus prospectivity mapping can be used to guide resource estimation and steer future efforts of resource definition and upgrading. An effective way this can be done is by using the prospectivity equivalent of the resource lower cut-off value to indicate where mineralisation may potentially be present outside the established regions. Confidence and unique conditions grids can then be used to establish what types of data needs to be gathered and where, to increase the reliability of the result.
Simon H.H. Nielsen, C.J. McKenzie, A.V.M. Miller, G.A. Partington, C.E. Payne, E. Puccioni, M. Stokes, C. Wildman, R.K.H. Falconer, R. Woodam Rise nodular phosphate-Modelling the prospectivity of a lag deposit (off-shore New Zealand): A critical tool for use in resource development and deep sea mining
After almost five decades of episodic exploration, feasibility studies are now being completed to mine the deep water nodular phosphate deposit on the central Chatham Rise. Weights of evidence (WofE) and fuzzy logic prospectivity models have been used in these studies to help in mapping of the exploration and resource potential, to constrain resource estimation, to aid with geotechnical engineering and mine planning studies and to provide background geological data for the environmental consent process. Prospectivity modelling was carried out in two stages using weights of evidence and fuzzy logic techniques. A WofE prospectivity model covering the area of best data coverage was initially developed to define the geological and environmental variables that control the distribution of phosphate on the Chatham Rise and map areas where mineralised nodules are most likely to be present. The post-probability results from this model, in conjunction with unique conditions and confidence maps, were used to guide environmental modelling for setting aside protected zones, and also to assist with mine planning and future exploration planning. A regional scale fuzzy logic model was developed guided by the results of the spatial analysis of theWofE model, elucidating where future exploration should be targeted to give the best chance of success in expanding the known resource. The development work to date on the Chatham Rise for nodular phosphate mineralisation is an innovative example of how spatial data modelling techniques can be used not only at the exploration stage, but also to constrain resource estimation and aid with environmental studies, thereby greatly reducing development costs, improving the economics of mine planning and reducing the environmental impact of the project.
Comparing prospectivity modelling results and past exploration data: A case study of porphyry Cu-Au mineral systems in the Macquarie Arc, Lachlan Fold Belt, New South Wales
Mineral exploration is undertaken in stages, with each stage designed to get to the next decision point of whether or not to keep exploring a particular area based on the results at hand. As a general rule, each consecutive exploration stage is more expensive due to the progressively more drill- and study-intensive nature of the work required, in particular after discovery of a potentially economic mineral deposit. As such, the distribution of exploration activities and related expenditures essentially serve as a spatialmeasure of prospectivity as perceived by mineral exploration companies. In this study wecompare historic (1980 to 2002) porphyry Cu-Au exploration activities and expenditures in part of the Ordovician to Early SilurianMacquarie Arc, Australia's most significant porphyry province with total resources greater than 80 Moz of Au and 13 Mt of Cu, to prospectivity modelling results froma weights of evidence (WofE) model. The outcomes of this spatial and statistical comparison indicate that at 2002 the Macquarie Arc was by no means a mature exploration destination and that past exploration investment outside the main mining areas was not necessarily effective. Moreover, no spatial correlation was apparent between areas of higher exploration expenditure and greater geological potential. For example, of the 692 km2 of highly prospective ground covered by the exploration licences examined in this study, only 89 km2 (c. 13%) have been explored effectively in that they received some form of drilling. Interestingly, the remaining area (603km2 or c. 87%) had not yet been effectively tested. As such, our analysis confirmed that despite a greater 100 year exploration and mining history,much of the prospective groundwithin the study area remained untested. Taken as a whole, the results of our spatial and statistical comparison are important inputs for assessing the effectiveness of exploration investment and explanation maturity and, therefore, future exploration decisionmaking. The outcomes also have implications for strategic planning of future government legislation helping to manage and maximise the benefits from exploration investment.
Improvements on 2D modelling with 3D spatial data: Sn prospectivity of Khartoum, Queensland, Australia
Auzex Exploration Limited owns a number of exploration tenements over the historically tin rich Khartoum area near Herberton, north Queensland, Australia and Kenex Ltd has completed both 2-dimensional prospectivity modelling and a 3-dimenional geological interpretation over this region. The initial 2-dimensional prospectivity model of intrusion related tin mineralisation is limited by the 2D nature of the data used, and regions of known Sn mineralisation were not identified, particularly in the contact zones of shallow dipping highly fractionated tin granites. To rectify this, a 3D geological model was created using Leapfrog Geo modelling software, and 3D spatial data has been projected to the surface topography and incorporated into an updated 2D prospectivity model of the region using ArcGIS software. The 2D and 3D models utilise newly compiled digital data including historical exploration data; geological data compiled from detailed geological mapping of north Queensland, academic literature and company exploration mapping; recent geophysical data collect by Fathom Geophysics Australia Pty Ltd; ASTER data analysed for alteration; and historical exploration geochemical data including rock-chip, stream sediment and soil sampling. The weights of evidence modelling technique was used to determine spatial correlations between known deposits and predictive maps in 2D, created from the available data, that represent each component of the currently accepted minerals systems model for intrusion related tin mineralisation defined for this project. The final updated 2D prospectivity model partially resolves the limitations of the initial 2D model, successfully identifying many of the areas originally missed.
Exploration Targeting from Prospectivity Modelling of Multiple Deposit Types in the Lachlan Fold Belt, NSW
Prospectivity modelling has been completed over the Lachlan Fold Belt, New South Wales, Australia, using the GIS based weights of evidence modelling technique to target porphyry Cu-Au, associated skarn Cu-Au, orogenic Au and VMS Au mineralisation. The Lachlan Fold Belt is a 700 km wide belt of Paleozoic accretionary terrains, stretching from Queensland to Tasmania. Porphyry and skarn mineralisation was associated with Ordovician shoshonitic magmatism, which was followed by Silurian regional metamorphism and deposition of orogenic gold deposits. Contemporaneous VMS-style mineralisation resulted in deposits in intra-arc rift basins of the Macquarie Arc. In preparation for the prospectivity modelling, lithological and structural data, extensive geophysical surveys and stream, drill-hole and rock chip geochemistry were used to create predictive maps that represent various parts of the mineral systems being modelled. Included in the models are maps that identify possible sources of heat and mineralised fluids, structures used for fluid migration, mineral trap zones, and outflow zones that may indicate a subsurface deposit. Prospectivity maps have been created for each mineralisation style and new areas of each deposit type located. The models have also independently identified areas of proven mineralisation, including Cadia, Northparkes, Woodlawn and other large producing mines. The prospectivity maps were reclassified to generate targets by delineating highly prospective areas from each model. Targets were compared and overlap examined among the four models, before further analysis of high priority targets. Single targets or clusters of targets were individually assessed by incorporating information such as tenure, geology, geochemistry and geophysical signature. Economic and risk factors were assessed and the targets ranked and mapped according to high and low exploration risk. Following this analysis, targets of interest can be highlighted as potential projects for acquisition, or used to prioritise new exploration data collection.
Targeting tin mineralisation using "3D Common Earth Models" in the Khartoum region, North Queensland, Australia
The use of modern day 3D GIS software packages such as GOCAD, GeoModeller and Leapfrog Geo has dramatically changed the way exploration targeting can be carried out compared to the last twenty years of using 2D Geographic Information System (GIS) for exploration. This is especially true in the last five years in which computer and GPS technology has developed to the stage where it is possible to digitally locate, accurately store, visualise and manipulate geological data in 3D at the scale of a mineral system, which is usually much greater than mine scale where most of the current 3D work is focussed. Most GIS can store, manage and manipulate data in 2D, with some able to visualise information in 3D. However, there are a number of packages that allow full 3D GIS functionality, including querying and modelling, allowing geologists to start exploration targeting in a 3D system. Auzex Exploration Limited owns a number of exploration tenements over the historically tin rich Khartoum area located near Herberton in North Queensland, Australia, exploring for Tin-Tungsten mineralisation. A 3D geological interpretation was created over a 60 km by 60 km region in Khartoum using Leapfrog Geo to improve targeting for tin systems adjacent and above buried granites and shallow dipping granite contacts, followed by 3D targeting using a Multi-Class index Overlay workflow of GoCAD Mining. The ranking of the 3D maps were based on a 2D prospectivity mapping exercise using the weights of evidence technique. By modelling geology and targeting in 3D, complex subsurface relationships and the correct vertical extents can be constrained. This will be invaluable for defining potential drill-hole targets.
New Zealand Gold Potential - Using Mineral Prospectivity Modelling to Evaluate Gold-Bearing Mineral Systems in an Underexplored Country
New Zealand has an established history of gold production beginning with the gold rushes of the 19th century in the Coromandel, Nelson/Marlborough, West Coast and Otago regions. Despite this, the number of significant modern hard rock gold operations has been limited in recent years due to a lack of sustained exploration capital and not limited geological prospectivity. The New Zealand Government is actively encouraging explorers to invest in New Zealand through a series of targeted promotional visits, more importantly through the acquisition of precompetitive regional geophysical data. Data collection has been completed over the prospective Northland epithermal district and a large portion of the South Island’s west coast that is prospective for both orogenic gold and intrusive related gold. Analysis of new data has been an important component in aiding the generation of exploration targets from prospectivity modelling. Determining the prospectivity of an area involves reviewing all the available data and analysing it with respect to the most up-to-date mineral system model for the mineralisation style of interest. Using the weights of evidence modelling approach, the most prospective areas for epithermal gold-silver, orogenic gold and intrusion-related gold have been identified. The key exploration parameters relevant to each mineral system are first represented spatially and then statistically combined into a single prospectivity map. New potentially economic deposits could be found by focusing exploration on targets identified from these models. The prospectivity modelling approach can greatly reduce the risk involved in mineral exploration
3D Prospectivity Modelling - A new era in exploration targeting
The use of computers in the mineral industry has dramatically changed the way exploration targeting is carried out over the last twenty years. This is especially true in the last five years where computer and GPS technology has developed to the stage where it is possible to digitally locate, accurately store, visualise and manipulate geological data in three dimensions (3D) at the scale of a mineral system. These tasks are commonly carried out using a Geographic Information System (GIS), which has become as an important tool to a geologist as his hammer. Most GIS store, manage and manipulate data in two dimensions (2D), with some having the ability to visualise information in 3D. However, there are now a number of packages that allow full GIS functionality including querying and modelling in 3Dgiving geologists a tool to carry out exploration targeting in 3D. A regional scale weights of evidence 2D prospectivity model was developed for the Taupo Volcanic Zone in New Zealand to assess the potential for epithermal Au mineralisation. A number of prospective areas have been identified including the known Ohakuri hydrothermal deposit. While this model has been successful at identifying mineralised areas the 2D data that is used gives little understanding of what is happening below the surface. Because geology does not just operate in 2D, trying to visualise 3D geometries in 2D can be challenging in exploration targeting. The development of 3D GIS such as GoCad and Geomodeller now give us the tools and techniques to use fuzzy logic and weights of evidence techniques for targeting in mineral exploration in 3D. A prospectivity modelling exercise using the weights of evidence modelling technique (developed by Bonham-Carter of the Canadian Geological Survey), was completed over the Ohakuri epithermal gold deposit in both 2D and 3D.
From exploration to extraction: The consequences of resource morphology for mining operation on the Chatham Rise
Substantial consideration has been given to the implications that the morphology of the Chatham Rise deposit will have on mining operations. The glacio-tectonic processes involved in the distribution of nodules on the rise have in several areas been quite significant. The recent cruises by Chatham Rock Phosphate Limited (CRPL) have collected data which has affirmed the assumptions previously made and catered for in historic resource estimations. The deformation and displacement of the phosphorite during glacial periods and the redistribution of the mobile sand during interglacial periods is interpreted to have produced a highly variable pattern of phosphorite concentration (kg phosphorite/m2) and coverage (% phosphorite/sample weight). The phosphorite resource probably has a significant spatial variability at a scale of tens of metres. Results of recent surveys show phosphorite-rich patches alternating with phosphorite-poor areas at distances of less than 20 m. The high spatial variability of the deposit has had a bearing on how historical information for the project has been regarded and integrated with the recent exploration approach and data collection process. This coupled with the proposed extraction tool has influenced the size, nature, extent and siting of the proposed mining blocks.
Taking spatial data modelling from the 2D to 3D world, with examples from the exploration industry: recent developments and issues.
The use of computers in mineral exploration in the last twenty years has changed the way we carry out exploration targeting dramatically. This is especially true in the last five years where computer and GPS technology has developed to the stage where it is possible to digitally locate, accurately store, visualise and manipulate geological data in 3D at the scale of a mineral system. These tasks are commonly carried out using a Geographic Information System, which has become as an important tool to a geologist as his hammer. Most GIS store, manage and manipulate data in 2D, with some having the ability to visualise information in 3D. However, there are now a number of packages that allow full GIS functionality including querying and modelling in 3D. This now gives geologists a tool to carry out exploration targeting in 3D. Exploration targeting using Fuzzy Logic and Weights of Evidence techniques is becoming more commonplace in the industry and is being used particularly by government organisations to manage their resources. However, one of the weaknesses of the work to date is that these studies are carried out in 2D, with an approximation of 3D provided by geophysical and drilling data projected to a 2D plane. Geology does not operate in the 2D world and many geological problems relate to 3D geometries and this is particularly true for exploration targeting. The development of 3D GIS such as GoCad and Geomodeller now give us the tools and techniques to use Fuzzy Logic and Weights of Evidence techniques for targeting in mineral exploration in 3D. However, several issues remain to be resolved before these tools become effective and used routinely by the industry. The most important issue is that of training, with graduate geologists not receiving appropriate training in the use of GIS to solve geological problems, particularly related to exploration. The other important problem relates to data availability and data quality, which was an issue for 2D models, but is even more of an issue for work in 3D. Consequently, we are now at the stage where computing power and modelling techniques have overtaken the availability of high quality 3D geological data and trained geologists to maximise their use.
Employing an effective exploration targeting method is important when looking for economic concentrations of minerals in a particular country or region. Methods for exploration targeting include geophysical or geochemical anomalies and intuitive decision making. Alternatively, prospectivity modelling allows for a complete picture of the economic potential of a country or region if all relevant mineralisation styles are considered. Prospectivity models can be reclassified to define high priority targets that can be used to focus an existing exploration programme or to pick up new ground. We present an example of this exploration targeting approach using the Lachlan Fold Belt in NSW. Prospectivity models have been completed over the Lachlan Fold Belt for porphyry Cu Au, skarn, VMS Cu, and orogenic Au mineralisation styles. The models use the mineral systems approach to determine key predictive variables that define each mineralisation style using the available data. Targets that delineate highly prospective areas have been defined from each model. The targets either represent existing prospects or mines or areas where new mineralised systems could be discovered with further exploration and development. A number of tools can be used to analyse the targets. Economic and risk factors can be assessed and the targets can be sorted and mapped according to positive and negative exploration risk. Single targets or clusters of targets can be individually assessed providing information such as tenure, geology, geochemistry and magnetic signature. Following this analysis, targets of interest can be highlighted as potential projects for acquisition, or an appropriate exploration programme prepared.
A.V.M. Miller, K.J. Peters
Comprehensive Prospectivity Analysis of the Lachlan Fold Belt in NSW Using the Mineral Systems Approach.
Prospectivity modelling has been completed over the Lachlan Fold Belt (LFB) in NSW Australia, using the GIS based weights of evidence modelling technique and porphyry Cu Au, skarn, orogenic Au and VMS Cu mineral system models. The LFB is a 700 km wide belt of deformed Paleozoic marine sedimentary and mafic volcanic rock stretching from Queensland to Tasmania. It dominates eastern NSW and hosts several large producing gold deposits including Cadia and Northparkes. Lithological and structural data from the NSW Geological Survey was combined with stream, drill-hole and rock chip geochemistry and extensive geophysical surveys. The data was used to create predictive maps for each of the four models, constrained by the mineral systems concept which defines the parts of the mineralisation system that are critical to the ore-forming process. Included in all models are layers that identify possible sources of heat and mineralised fluids, structures used for fluid migration, mineral trap zones, and outflow zones that may indicate a subsurface deposit. Training points were chosen from known areas of mining or exploration specific to the relevant mineralisation style (Fig. 1). Prospectivity maps have been created for each mineralisation style giving a comprehensive understanding of the gold and copper mineralisation over the LFB in NSW. Known areas of each deposit type have been identified along with new areas that have potential for porphyry, skarn, orogenic or VMS deposits. These prospectivity maps and exploration GIS create a valuable tool to accelerate exploration and identify new opportunities in NSW’s most productive goldfield.
Developing Wind and Mineral Exploration Models using GIS for Project Development in Argentina.
Kenex in partnership with Emprendimientos Energeticos y Desarrollos S.A (EEDSA) have recently completed a number of strategic business development projects in Argentina to develop wind energy and mineral resources. A partnership was developed with EEDSA in 2010 to explore for and develop wind energy opportunities in Argentina using Kenex’s recently developed wind prospecting techniques. These techniques have been successfully used to map wind farm locations in New Zealand and rank each site according to its economic potential. After a year of data collection and modelling, which has successfully mapped potential wind farm sites in a number of provinces in Argentina, the partnership decided to expand into mineral exploration. Spatial Data Modelling techniques were used to map potential mineral exploration opportunities for gold, copper, base-metals, tin, tungsten and uranium at a regional scale in Argentina and Chile. Regional scale prospectivity models were developed for Argentina and Chile to identify prospective areas for a variety of metals and mineralisation styles. Fuzzy logic techniques were used to develop the wind prospectivity maps and Weights of Evidence modelling techniques were used to develop the mineral potential maps in Argentina and Chile. The models have successfully identified areas that are prospective for wind energy and gold, copper and silver and have also identified areas where new mineralised systems could be discovered with further exploration and development. Economic and risk factors will be included and target areas can then be sorted and mapped according to positive and negative exploration risk. A similar analysis will be carried out for the wind targets. This will lead to the development of an Argentina wide database of prioritised metal and wind energy targets for exploration and development. The prioritised targets will then be combined with social and logistical factors to highlight projects for acquisition. The regional targeting work for both wind energy and mineral resources has now been completed and the partnership is in the process of developing a number of business.
Using predictive modelling to aid planning in the mineral exploration and mining sector – a case study using the powelliphanta land snail.
The weights of evidence spatial data modelling technique has been used to create a predictive map that identifies possible locations of alpine Powelliphanta land snails in the South Island of New Zealand. This technique is commonly used in the mineral exploration industry to identify locations most likely to host mineralisation and is becoming more widely used in environmental fields as data becomes available in a digital form. Climatic, soil, topographic, and botanical data used in the model came from various organisations including NIWA and Landcare Research. The model uses the known locations of five Powelliphanta “taxa” that occur in high elevation, isolated alpine habitats to find other areas that may support similar Powelliphanta populations. The weights of evidence technique allows data to be assessed and weighted according to how great its influence is in relation to the current known locations of Powelliphanta snails. The most important variables identified from this spatial analysis were combined to produce a map showing the most likely places for Powelliphanta snails to be found. The resulting predictive model for snail habitat locations shows that mountain ranges in north-western part of the South Island have the highest probability of finding Powelliphanta land snails. It also shows that high altitude, low temperature and high rainfall condition are favoured by the snails. The model has been validated in the field and some areas not covered by the training points that were classified as highly probable by the model have recorded sightings of snails. Knowing the locations of species that will be affected, as well as knowing the potential relocation sites could help facilitate decision making during mineral exploration and mine planning.
Puccioni, E., 2012
Energy in the Wind: Integrating the transmission network for a better approach to wind farm prospecting.
The availability of suitable transmission grid connectivity is quickly becoming a key factor for developing new wind farms globally. While wind speed remains the most important requirement for a successful wind farm along with other key parameters such as suitable terrain, current land use and distance from populated areas which are essential for site selection; a good grid connection is a significant factor in determining the economics pre-construction and future profitability of the wind farm. In the past three years, Kenex and Aurecon have developed advanced spatial modelling techniques that combine wind speed and direction, advanced terrain analysis, land use and social acceptability parameters to define the extent of potential wind farms at regional and country-wide scales. Our modelling has been successfully used by New Zealand developers to quickly and effectively target new wind farm opportunities and define the potential extent of an individual wind farm. Kenex and Aurecon have now refined the spatial modelling by integrating grid connection variables so as to identify not only the most suitable sites for a wind farm, but also which sites may potentially have the best available connections to the grid, and subsequently more preferable project economics.
Wildman, C. and O’Donnell, R.J., 2011.
Prospectivity Modeling of Seafloor Massive Sulphide (SMS) Deposits in the Kermadec Arc and Colville Ridge Regions, New Zealand.
Prospectivity modeling of seafloor massive sulphide (SMS) deposits has been completed over the Kermadec Arc-Colville Ridge area using the GIS based weights of evidence and fuzzy logic modeling techniques. SMS deposits are the current equivalent of ancient onshore volcanogenic massive sulphide (VMS) ore deposits. These high-grade deposits are formed on the seafloor and commonly consist of a black smoker and metal rich sediment mound complex resulting from the discharge of hydrothermal fluids (up to 400°C) from fractures on the seafloor. Metal sulphides are continuously precipitated in response to mixing of high-temperature hydrothermal fluids with ambient seawater. Accumulation of metal sulphides has led to SMS deposits being potentially major sources of copper, zinc, lead and other metals such as gold, silver, which to date remain untapped. Modeling of SMS deposits was undertaken to illustrate the power of GIS modeling for seafloor resource evaluation and how it can be used to quickly identify and rank in terms of the most likely prospective areas of the seafloor where new SMS deposits might exist. The mineral deposit modeling was constrained by the mineral systems concept which defines those parts of a mineralisation system that are critical to the ore-forming process. The deposits are typically formed in extensional tectonic settings, including both submerged tectonic margins and sea-floor spreading. Volcanic vent systems and underlying dykes, stocks and sills are the sources of heat that are responsible for converting sea water drawn down through fractures in the oceanic crust into an ore-forming hydrothermal fluid. This fluid is then capable of leaching metals and elements from surrounding footwall rocks, which are then transported upwards via the convection of hydrothermal fluids. The ore materials are then precipitated within the black smoker field as massive sulphides due to the mixing of high-temperature (250-400°C), metal-rich hydrothermal fluids with cold (about 2°C) oxygen bearing seawater. Prospectivity modeling is done by compiling all the relevant data and integrating it in a way the matches the mineral system being modeled and combining them into a single mineral potential map. The commercial value of modeling from the exploration sense is that it enables more effective data management and data use, it aids decision making (focus of time, effort and expenditure) and it identifies where and what type of additional data should be collected. The modeling results can also aid government agencies from a planning perspective for areas such as mineral rights allocation, research funding direction, environmental planning and long term economic strategy regarding mineral development.
Resource assessment using GIS modelling of orogenic gold mineralisation potential in New Zealand.
Prospectivity modelling of orogenic gold mineralisation has been completed over New Zealand using the GIS based weights of evidence modelling technique. New Zealand orogenic gold deposits are restricted to the South Island and lower North Island and are divided into two groups (Paleozoic and Mesozoic) based on their age and host rock association. Modelling of Paleozoic and Mesozoic orogenic gold deposits was undertaken to illustrate the power of GIS modelling in regional and nationwide resource evaluation and how it can be used to quickly identify and rank in terms of prospectivity areas of land where new orogenic gold deposits might exist. The mineral deposit modelling was constrained by the mineral systems concept which defines those parts of a mineralisation system that are critical to the ore-forming process. Both of the New Zealand gold models identified possible sources of metals in the region, structures that could be used for fluid migration, mineral trap zones ideally suited to host a mineral deposit, and outflow zones that may indicate a subsurface deposit. The models were validated against known areas of historical gold mining such as the Reefton deposits (Paleozoic) and Macraes Flat (Mesozoic). Two prospectivity maps showing areas favourable for Paleozoic and Mesozoic orogenic gold formation were produced. The prospectivity modelling successfully identified known areas for both types of orogenic gold mineralisation as well as several new localities not currently covered by existing tenements. The spatial modelling techniques used here can be applied elsewhere to evaluate resource potential, whether for gold, or any other land based resource, and can help planners and land owners manage future developments and their assets more effectively. Both models supersede those undertaken in 2002 by Crown Minerals and GNS Science under the purview of Dr Greg Partington (now Director of Kenex Ltd.). The new models were re-run due to the addition of new data and new modelling techniques and appear to have much better definition and are better for targeting at a prospect scale.
Genesis of the Chatham Rise Phosphorite; an interpretation from current literature.
A synthesis of new ideas from papers relating to the genesis of the Chatham Rise phosphorite deposit is presented. Since the Sonne and Valdivia Cruises in the late 1970's and early 80's, little has been contributed to further define, quantify or explain the Chatham Rise phosphorite deposit. There have been, however, many advances in geochemistry, paleo-geography, paleo-oceanography and paleo-climatology which have contributed to understanding the genesis of phosphorite deposits worldwide. Recent deep water seismic survey in the Great South Basin has funded a plethora of scientific spending. This has resulted in new insights into the deformation sequence on New Zealand's continental shelf marginal out in to the adjoining deep water basins. It is proposed that the Miocene southern ocean, open shelf, replacement type phosphorite deposits (which include the Chatham Rise phosphorite) were formed in response to tectonic movements, the subsequent erosion of the ancient super continent of Gondwana and the migration of ocean fronts in response to changing ocean topography. It follows that a reconstruction of paleo-geography and paleo-oceanography adjacent to the Gondwana supercontinent will provide insight into the development of this large phosphorite resource in time and space.
Prospectivity modelling of mineralisation systems in Papua New Guinea using weights of evidence techniques.
The weights of evidence technique for predictive modelling has recently been adapted to the mineral exploration industry by Bonham-Carter of the Canadian Geological Survey. This study applies this technique to an area that is known for it’s richness in mineral deposits; Papua New Guinea (PNG). The aims of this study are twofold: first is to test the spatial data modelling technique in an area where the results are for the most part already known and in doing this refine the location and extent of known mineralised regions; and second is to identify new areas of mineralisation that are worthy of follow up exploration. A regional scale prospectivity map was developed over PNG to identify areas of porphyry Cu-Au mineralisation. Porphyry Cu-Au is one of the main mineralisation styles in PNG along with epithermal Au-Ag and is represented by many well known deposits and producing mines (e.g., Ok Tedi, Frieda River and Panguna). Open file data for the PNG model came from the Geological Survey of Papua New Guinea. This data includes mineral occurrences, geology, and geochemistry (rock chip, stream sediment, drill hole and soil sampling). The weights of evidence technique was used to determine spatial correlations between porphyry Cu-Au mines and predictive maps created from the data available. Despite limited regional scale data the model has successfully identified areas of known porphyry mineralisation as prospective and has also identified areas where new mineralised systems may be discovered with further exploration. Using the same data the weights of evidence technique could also be used to model epithermal Au-Ag and mesothermal mineralisation to provide a more complete mineral prospectivity review of PNG.
Prospectivity of granite-related nickel deposits throughout Eastern Australia
Spatial modelling has been used to determine potential locations of granite related nickel skarn mineralisation throughout eastern Australia. It is believed that during granite intrusion, fluid can leach nickel from nearby mafic lithologies and combine with sulphur to form nickel sulfides. Currently exploration for this style of deposit has been limited to the region around the Avebury Deposit in western Tasmania. As the lithologies near Avebury are not unique it is feasible that similar deposits could exist elsewhere in Australia. The weights of evidence spatial data modelling technique was used to evaluate the wealth of geological data available over eastern Australia and included known economic deposits as training data to weight the themes of the model. These weighted themes were combined to create a prospectivity map showing areas favourable for granite related nickel deposits. Several regions were identified by the model to have good potential to host nickel mineralisation similar to the Avebury deposit. These include areas located throughout NSW, Victoria, Tasmania, the Tasman District of New Zealand, and in particular, the Rockhampton region of eastern Queensland.
New Exploration Concepts Applied to Neglected and Emerging Exploration Destinations: Project Development Using Computer Modelling in Australia, New Zealand and Africa, PDAC2005
The new business models applied by major mining companies depend on the junior segment of the market to successfully carry out grassroots exploration. There is a significant problem with this approach due to most investment capital still being focussed on developing mining operations rather than conceptual exploration. However, current deposits are rapidly being depleted and there will be pressure for new discoveries in the coming years. In addition a significant amount of corporate knowledge has been lost with the recent globalisation of the minerals industry. Consequently the business of exploration, like the mining sector, has to develop new business models and use new innovated techniques to attract new investment to allow these discoveries to be made.
It is critical for exploration targeting that effective analysis of the available datasets is carried out with respect to each other and that only the relevant factors to the exploration model being used are extracted and combined into a single mineral potential map by using spatial data modelling techniques. These techniques have been successfully applied in New Zealand, Australia and Namibia to develop new conceptual exploration targets for Au and Cu deposits. Historic data were combined with new genetic models in a GIS to produce mineral potential maps at national and international scales highlighting those areas with the greatest probability of hosting mineralisation. These models were used to raise seed capital and attract investment develop these targets. All targets were unrecognised and acquired 100% at the cost of pegging. On-going fieldwork is proving the effectiveness of the modelling with new mineralisation being discovered in areas neglected by recent exploration. The potential to add significant value to these targets at the grassroots stage of exploration now is very high.
In summary, contrary to current beliefs, grassroots exploration can deliver significant added value to shareholders. The use of new spatial data modelling techniques allows the calculation of probability values that can identify those areas with the best chances of exploration success. This reduces costs, allows integration of data at international scales and brings forward any return on investment hence enhancing value to share holders.
Predictive Spatial Mapping From Gold To Grapes: A New Targeting Tool Being Successfully Used To Increase Investment In New Zealand
It is important that risks of developing and managing new businesses that use spatial information are known as accurately as possible. This process should start at the business planning stage and continue through feasibility to the development stage. Until recently, this type of analysis has been carried out using expert systems, leading to subjective judgements regarding the potential for success. With GIS and regional scale digital databases now available, probabilistic models can now be generated that allow the creation of predictive maps. For example, where is the best land for growing grapes or where are the best places to explore for gold in New Zealand?
A variety of new tools are available for use with computer aided geographic data management systems or Geographic Information Systems (GIS) for evaluating the distribution of spatial data in a statistical framework. These tools were initially developed for other uses such as pattern recognition by defence forces or medical diagnostic systems. Their use now in mineral exploration, energy and agriculture is a classic example of technology transfer and how the industries that depend on spatial data use new technologies in an innovative way.
A program of earth science digital data compilation has recently been undertaken in New Zealand to allow the use of more probabilistic data analysis techniques in mineral exploration, moving away from the traditional expert-system methods. This is the first time that new technologies in IT, database management and Geographic Information Systems have been used outside of research projects. The combination of the new modelling techniques and a national scale digital geological database has successfully led to increased investment in New Zealand.
New perspectives on IOCG deposits, Mt Isa Eastern Succession, northwest Queensland Abstract: 1 pmdCRC, EGRU, School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia
A current popular model for the formation of IOCG deposits in the Mt Isa Eastern Succession involves fluids derived from the late orogenic granites mixing with a second external fluid source forming Fe- (commonly magnetite-) rich alteration zones that contain vein stockwork, breccia, dissemination or replacement style mineralization. This is assumed to be commonly spatially and temporally associated with felsic pluton emplacement and cooling around 1540-1500 Ma. This contrasts with an alternative model in which the fluids are entirely intra-basinal and amagmatic in origin. Recent dating studies at Osborne have highlighted a potential syn-peak metamorphic timing to mineralization (based on 1595 Ma Re-Os age dates on molybdenite and a 1595 ± 6 Ma U-Pb age date on hydrothermal titanite), with no apparent proximal major intrusion. There is also a potential link between mineralization and widespread mafic intrusive activity, which spans the entire range of known mineralization ages.
In order to investigate this considerable range of potential geological controls on IOCG mineralization a prospectivity analysis was undertaken, aimed at evaluating the relative importance of a range of spatial variables including: host rock type, proximity to felsic granites or mafic intrusives, stream geochemistry (Cu and Au), structure, and geophysics (including magnetics, gravity and wavelet-processed potential field data or “worms”). A data driven approach was taken in view of the considerable uncertainty in genetic models for IOCG deposits.
Important data sources include (1) the northwest Queensland Mineral Province Report, (2) mineral occurrence data and newly available open file geochemistry (Terra Search) available from the Queensland Department of Mines and Energy, (3) regional magnetics and gravity digital datasets available from Geoscience Australia. MapInfo spatial data modeling software (MI-SDM) was utilized in this study. The initial study area comprised six 1:100,000 sheets covering Cloncurry and the area to the south. A conventional weights of evidence analysis was undertaken.
A comparison of Contrast and Student C values for all evidential layers indicates the host lithology as the most important criterion, followed by geochemistry (Cu and then Au), structure, geophysics, felsic and mafic igneous intrusions. The results enable a list of target criteria to be statistically ranked. A comparison of these results can be made with expert driven predictions. The study area is being expanded to include the entire Eastern Succession, including solid geology maps interpreted through cover.
An important outcome for ore genetic models is the recognition that intersections of N to NW structures with other faults have the strongest spatial association with IOCG deposits after host rock and geochemistry. This result implies that fluid pathways are much more important than fluid sources for controlling the distribution of IOCG deposits. This understanding can possibly explain some of the diversity in the range of IOCG deposit types and models. A common mineralizing process could generate deposits in a variety of host rocks depending on the fluid pathways. The dominance of the fluid pathways means that fluid sources cannot be clearly recognized from spatial associations of the deposits alone, and mineralizing fluids may be complex and heterogeneous in view of their possible interactions with a variety of wall rocks. A detailed understanding of fluid pathways and structures at all scales is the most important direction for future research. Mechanical modeling directed at understanding fluid flow in the Mt Isa Eastern Succession based on this structural knowledge will also be an important tool.
Prospectivity Mapping Using GIS With Publicly Available Earth Science Data: A New Targeting Tool Being Successfully Used For Exploration In New Zealand, PacRim 2004
It is important that risks of developing mineral resources are known as accurately as possible. This process should start at the pre-discovery exploration stage and continue through feasibility to the development stage. Until recently, this type of analysis at the exploration stage has been carried out subjectively, leading to subjective judgements regarding the prospectivity of exploration targets. With GIS and regional scale digital databases now available, probabilistic models can now be generated. A program of digital data compilation has recently been undertaken in New Zealand to allow the use of more probabilistic data analysis techniques, moving away from the traditional expert-system methods. This is the first time that new technologies in IT, database management and Geographic Information Systems have been used outside of research projects. The combination of the new modelling techniques and a national scale digital geological database has successfully led to increased tenement acquisition and helped reduce cost and hence risk early in the exploration phase.
The analysis of the data used in the models proved to be as important as the results of the prospectivity modelling. Data quality was checked and geological models and exploration methodologies were tested using spatial correlation analysis. The analysis allowed the comparison of disparate datasets and associations not easily recognisable between these datasets. These analyses increased the confidence in the exploration models and techniques currently used to explore for gold mineralisation. Working with GIS datasets has highlighted the need for good quality data and data management. This has become a problem, as databases are presently available from a diverse number of groups, resulting in variable data quality and standards. No matter how sophisticated your analytical software if your data is poor the result will be of a similar quality. This applies to all aspects of the exploration industry from spatial mapping (GIS) to resource modelling.
New Exploration in NZ Stimulated by the Crown Minerals Prospectivity Modeling Studies for Gold
The Epithermal and Mesothermal Gold Prospectivity modeling projects carried out by Crown Minerals provided explorers in New Zealand with a new compilation of historical exploration data combined with new geological information from the GNS QMap 1:250,000 scale mapping project. These data were used to produce predictive mineral potential maps for gold mineralisation in New Zealand.
The aim of these projects, to stimulate mineral exploration and investment in exploration, has been successful with eight new companies acquiring new tenement positions and committing significant exploration expenditure to exploring in New Zealand in the coming years. The projects were done at a national scale and consequently not all exploration data were compiled into the prospectivity models. Several of the new companies recognised the value of the prospectivity modeling work and committed exploration funds to continue the modeling process. They recognised a need to compile the remaining data and run the models again to allow detailed exploration targeting.
Detailed data compilations including digitising historic exploration stream sediment sample, rock chip sample, soil sample and drilling data have been completed. New models have been completed in Otago for mesothermal gold mineralisation and in the Coromandel and Northland for epithermal gold. The new models have been compared with the original regional scale models and used to target prospect scale exploration.
This work has allowed exploration models for epithermal and mesothermal mineralisation in New Zealand to be refined. More importantly this work has identified significant areas with potential to host gold mineralisation with little or no systematic geochemical data including soil sampling or drilling. Exploration work programs have been designed to acquire these missing data and exploration funds have now been committed to test the areas highlighted by the prospectivity modeling.
In summary, the Epithermal and Mesothermal Gold Prospectivity modeling work has successfully attracted new investment and ideas to the exploration scene in New Zealand. The projects had an estimated cost of NA$250,000 and will in the next two years, just through exploration expenditure, attract more than NZ$10 M in investment. If a mine is discovered the return on investment will be considerably greater.
Gold Prospectivity in New Zealand
Gold production in New Zealand has been significant since the mid 1800s, totalling 900,000 kg (29 M oz) to 2005. In addition, there is potential for an additional 1.23 t (41 M oz) of gold in known and undiscovered deposits. Most gold in New Zealand has originated from mesothermal or epithermal hydrothermal systems. Mesothermal gold occurs in low-medium metamorphic grade sedimentary and schist host rocks, typically within quartz veins or shear zones that were formed during deformation. These deposits are largely confined to Otago, West Coast and Marlborough. Epithermal gold occurs in volcanic terrains associated with near-surface active hydrothermal systems, either in quartz veins or disseminated through strongly altered zones. Major epithermal deposits have been found in Coromandel, and significant prospects are known in Northland and the Taupo Volcanic Zone. Major alluvial gold mining has occurred downstream of mesothermal gold deposits in Otago and the West Coast of the South Island.
Considerable data are available in digital formats to assist exploration for new gold deposits using Geographic Information Systems (GIS) software. These data include modern geological mapping, geochemistry, geophysics, mineral occurrences, topographic data and cultural data, as well as derivative themes such as geophysical interpretation, metamorphic grade, and structural trends. Geological mapping data include rock type, age, and stratigraphic association, as well as alteration zones, faults, folds, dikes, veins, and structural measurement data. The spatial relationships between these data and known gold deposits have been statistically quantified using the Weights of Evidence technique within GIS software. Stronger correlating data have been combined to create map models that quantify prospectivity. Many poorly explored areas of elevated prospectivity have been identified and reinforce the notion that New Zealand has considerable potential for future gold discoveries.
A Mineral Resource Assessment Project for New Zealand Using Spatial Analysis in a GIS, PDAC2002
Crown Minerals (CM) and The Institute of Geological and Nuclear Sciences (GNS) have independently produced several resource potential reviews for a variety of mineral deposit types in New Zealand. These have been carried out in the traditional expert-based manner, but lack a degree of objectivity, and have not fully utilised the large volume of open-file company exploration data.
A recent study, of the western part of the South Island of New Zealand, trialled GIS spatial modelling techniques. Prospectivity mapping, using Weights of Evidence techniques was carried out at several scales, using regional and prospect-scale datasets. This allowed the comparison and analysis of disparate datasets with geological associations determined by the adopted exploration models, providing an objective assessment of the models, and statistical information on key features for future exploration attention.
We are now commencing a regional-scale study of mesothermal gold deposits in New Zealand. We aim to compile all relevant data held by GNS and CM into one GIS, and produce mineral potential maps showing those areas with high potential to host additional gold mineralisation. Studies of other deposit types will follow. The final products will provide explorers a better understanding of the location and value of New Zealand’s mineral estate.
Proterozoic Lode Gold and (Iron)-Copper-Gold Deposits: A Comparison of Australian and Global Examples
More than 150 Moz of gold has been added in production and resources from Proterozoic deposits in the last ten years, and many Proterozoic basins are now considered high priority exploration targets. The bulk of Proterozoic gold is produced from lode gold and Cu-Au (U-REE-Ba-F) deposits which are found in northern Australia, South Dakota, West Africa, Canada, South Africa, Scandinavia, and Central America. Proterozoic lode gold deposits are restricted to late collisional stages in the development of Proterozoic orogenic belts. They appear to have a systematic sequence of events in common and occur in linear belts associated with regional ductile structures at, or near, the greenschist facies brittle-ductile transition. Gold occurs in a large variety of rock types and has a close spatial association with regional-scale domes, anticlines, strike-slip shear zones, duplex thrusts, and in some deposits, geochemically distinct granites. Deposit styles can be subdivided into several types, directly related to the host structure and to contrasts in hostrock competency and mineralogy. These deposits have fluids and geochemical associations that overlap those of Archean lode gold deposits.
Proterozoic Cu-Au- (Fe) deposits formed in a broader range of crustal and tectonic environments and display a great variety of structural and host-rock controls and styles. It is evident in all districts where the timing relationships are known that these deposits have spatial and temporal relationships to granites. These deposits display a range of fault and shear zone controls and are commonly associated with regions of geometric complexity, structural intersections, or regionally anomalous structural orientations. There is considerable evidence of variable fluid chemistry in Cu-Au-(Fe) deposits. Districts are commonly characterized by regional metasomatism and alteration at both regional and deposit scale which is commonly intense. Fe oxide-Cu-Au environments tend to produce similar alteration assemblages in all aluminous rock types. The influence of magmas as sources of fluid and ore components appears to have been greater in at least some Cu-Au-(Fe) systems and the associated granitoids are typically oxidized and include both mafic and felsic varieties. Sodic alteration styles are commonly prevalent regionally; the larger ore systems in particular are hosted specifically within substantial bodies of rock that are depleted in Na and enriched in K-Fe-(H).
Mineral Exploration in the Drummond Basin North Queensland, Using Spatial Analysis in a GIS, presented at SIRC 2000 The 12 th Annual Colloquium of the Spatial Information Research Centre, University of Otago, Dunedin, New Zealand, December 10-13th 2000
The risks of developing mineral resources need to be known as accurately as possible. This process should start at the pre-discovery stage and continue through feasibility to the development stage. Until recently, this type of analysis was carried out manually, leading to subjective judgments. With GIS and resource estimation software now available on personal computers, probabilistic models can now be generated. A program of digital data compilation was completed in the Drummond Basin in North Queensland to allow the use of more probabilistic data analysis techniques at the pre-discovery exploration stage. A GIS was created and spatial analytical techniques employed to assess the potential of the area, and to test current geological models. Prospectivity mapping, using weights of evidence techniques was carried out at approximately 1:100,000 scale. The initial work involved database compilation, which highlighted errors and gaps in the database.
The Drummond Basin provided important lessons in database compilation and management, and allowed for a review of geological models and exploration methodologies. This analysis allowed the comparison of disparate datasets and associations not easily recognisable between these datasets. The results of this work increased the confidence in the exploration models and techniques currently in use. The calculation of the prior probabilities produced a correlation matrix of variables comprising the geological model. This allowed an objective assessment of individual prospects, which proved a useful exploration management tool. Finally, working with GIS datasets highlighted the need for good quality data and data management. This has become a problem, as databases are presently available from a diverse number of groups, resulting in variable data quality and standards. No matter how sophisticated your analytical software if your data is poor the result will be of a similar quality. This applies to all aspects of the exploration industry from spatial mapping (GIS) to resource modeling.
Controls On Mineralisation In The Howley District, NT: A Link Between Granite Intrusion And Gold Mineralisation.
The contribution from gold deposits in Australian Proterozoic sedimentary basins to the total gold produced in Australia has increased significantly in the last five years. Much of this is the result of new discoveries and consequently many Proterozoic basins are now considered high priority exportation targets. The Pine Creek Geosyncline located in the northern part of the northern Territory in Australia has been part of this exploration and mining boom new discoveries at Rustlers Roost (1 million ounces), Mt Todd (1 million ounces), Brocks Creek (1 million ounces), Union Reefs (1 million ounces) and Burnside (1.1 million ounces). The Pine Creek Geosyncline is a very metallogenic rich province, also containing occurrences of base metals (mined at Woodcutters), tin, tungsten, tantalum, platinum, palladium and uranium (mined at Ranger).
Much of the current success is due to new exploration techniques being applied to the area as a result of advances in the understanding of the genesis of gold mineralisation and new technology such as low level analytical techniques, remote sensing and image enhancement of geophysical data. A variety of genetic models, ranging from magmatic through hydrothermal to syngenetic, have been postulated in the past for the formation of gold deposits in the Pine Creek Geosyncline. An exhalative syngenetic model, or remobilised syngenetic during dolerite intrusion and/or regional deformation model for gold mineralisation has been used by explores in the last ten years. However recent work has shown that syngenetic models for gold mineralisation are not relevant in the Pine Creek geosyncline.
Gold and base metal mineralisation in the Pine Creek geosyncline is often found in close association with granites and as such have been classified as high temperature contact aureole deposits, for example in the Howley District, which has been, and is continuing to be one of the main areas of gold production in the Pine creek Geosyncline, where a simple relationship between gold mineralisation and the intrusion of the Fenton and Burnside granites has been implicated in the gold mineralising event. A secondary host rock control has also been suggested with the association between mineralisation and carbonaceous metasediments considered to be important in localising mineralisation. There exists, however, evidence that the relationship between mineralisation and granite intrusion is not simple and that much of the gold mineralisation at least occurred after the intrusion of the Cullen Batholith and the relationship with carbonaceous rocks is not conclusive.
The aim of this talk is to review the geological setting of gold mineralisation in the Pine Creek geosyncline using the Howley District as an example. A structural framework will be provided to aid in timing granite intrusion and gold mineralisation. A spectrum of deposits will be described, whose style appears to be dependant on their structural position rather than on host rock control. Geochemical data from the deposits will be used to constrain chemical controls on mineralisation and fluid inclusion and isotope data will be presented that indicates a component of magmatic and metamorphic input to the fluids that transported gold mineralisation. A review of the controls on mineralisation from a macroscopic or mine scale to a mesoscopic or regional scale will be made and a model will then be presented that will link the host rock geochemistry and mechanical properties, structural and metamorphic evolution of the area, granite intrusion and gold mineralisation.