Hill, M.P.; Chilton, M.O. 2024 Aggregate opportunity modelling for the central and northwestern Otago area of New Zealand. Lower Hutt, NZ: GNS Science. GNS Science report 2024/13. 23 p.; doi: 10.21420/RFGE-SQ76
Abstract
Efficient utilisation of aggregate resources is important for supporting infrastructure development and reducing operational and transport costs related to extraction of hard rock, gravel and sand aggregate materials. Aggregate opportunity has been mapped in the central to northwestern parts of Otago, encompassing much of the Queenstown Lakes and Central Otago districts. The presence of aggregate resource potential is a consideration for prioritising future land uses to support our growing economy. Delineated aggregate opportunity areas (places that have overlapping spatial data classes favourable for extractive activities) indicate where there is potential that an aggregate resource could be developed, provided that follow-up site-specific investigation confirms the geotechnical suitability of the material and that extractive activities would be compatible with factors such as community and iwi values and land-use zoning. A spatial modelling approach using Geographic Information System (GIS) software forms the basis of aggregate opportunity modelling. The modelling involves three successive steps: classification of source data into mappable criteria layers, combination of criteria layers into predictive model components and generation of maps showing the spatial model results. A final component is creation of an overall aggregate opportunity model, emphasising areas with generally high potential for aggregate resources. In total, 19 mappable criteria layers were compiled from various source data and classified via aggregate-relevant parameters. Each map class was assigned a class weighting value that qualitatively ranks its importance as an aggregate indicator using expert knowledge from industry experts and spatial statistics. Fuzzy logic modelling was used to combine mappable criteria information and generate four categories of predictive model components (source material, land use, quarrying feasibility and cultural sensitivity). The four categories of predictive model components were combined into maps of spatial model results, which depict the aggregate opportunity, ranked from low to high, for three types of material (hard rock, gravel and sand). The spatial model results data were analysed and filtered to produce the overall aggregate opportunity model. These map-based GIS datasets of sand, gravel and hard rock aggregate opportunities can be used to manage aggregate resources, generate targets for exploration activities and provide insight into future resource development. This approach acknowledges regulatory, economic and community-sensitivity considerations for aggregate prospectivity, while providing valuable insights into aggregate resource distribution and potential extraction sites. In the Otago project area, the hard rock opportunity is dominated by schist, which has some limitation for use as roading aggregate; hard sandstone (‘greywacke’) is generally a better aggregate material but, with its occurrence largely confined to the far northeast, its economic opportunity is notably more limited compared to schist. Other bedrock material types such as weak sandstone, conglomerate, limestone and volcanic rock (basalt) are largely confined to the eastern part of the project area, with limited localised distributions. Poorly consolidated river and stream deposits offer regionally extensive opportunities for gravel and sand resources in most of the valleys and basins of the central and northwestern Otago area, with particularly good opportunity east of Wanaka, around and north of Lake Dunstan, in the Manuherikia Valley and in the Maniototo Basin near Ranfurly. Sand is likely to be a companion material within many of the gravel opportunity sites, as well as within Neogene-age sand-dominated rock units This project’s findings facilitate informed decision-making for sustainable resource utilisation and infrastructure development in the central and northwestern Otago area (auths)
