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.