A Mineral Systems Approach for New Zealand: New Opportunities for Exploration R.W. Smillie1, M.P. Hill2, A.P. Martin1, M.S. Rattenbury2 and R.E. Turnbull1 1
GNS Science, Private Bag 1930, Dunedin, New Zealand,
[email protected] 2
GNS Science, PO Box 30-368, Lower Hutt, New Zealand
Abstract Exploration and research aimed at finding New Zealand’s next major mine is on-going and considerable patience and optimism are required as new minable deposits are yet to be discovered through modern greenfields exploration methods. Recent investments by the New Zealand Government in acquiring precompetitive geoscience data go a long way towards addressing greenfields exploration challenges; however, the lack of new discoveries globally by companies using this type of data indicates exploration success is still difficult to achieve. Considering other factors, including the global competition for exploration investment and perceptions of restrictive environmental constraints in New Zealand, the challenge here is even harder. To help further address this challenge, the minerals systems concept needs to be fully integrated into the New Zealand mineral exploration landscape. Mineral systems incorporate the wider set of geologic processes that align to economically concentrate minerals. Critical elements of minerals systems such as whole lithosphere architecture, transient favourable dynamics, fertility and preservation of the main depositional zone are influential at varying scales. These elements also require translation into observable features or geological proxies that can be mapped directly in existing or obtainable geoscience datasets at the appropriate scale to generate exploration targets. By taking a minerals system approach to the exploration challenge in New Zealand, the wider minerals community – researchers, the Crown, exploration and mining companies, consultancies – can converge on the use and deployment of new prediction and detection technologies to acquire necessary data and information. This will lead to increased opportunities for exploration successes in New Zealand. Keywords: mineral systems, gold, ore deposits, exploration
Introduction New Zealand has a long and profitable mining history, particularly for gold, that has played a major role in the development of our economy. We host two world-class gold-producing mines, the Miocene epithermal Martha Mine in the North Island and the Cretaceous orogenic Macraes mine in the South Island, that have collectively produced close to 20 million ounces. Both these deposits were discovered and first mined in the late 19th century and they have each grown through highly successful programmes of brownfields exploration, particularly in the last few decades. New gold or other precious metal deposits have yet to be discovered in New Zealand through modern greenfields exploration. While there are many reasons for this, this fact clearly highlights that the exploration challenge we face is that most deposits exposed at surface have been found. The challenge is to find the next generation of deposits, some of which are residing under cover in known districts, and others that may lie in less familiar parts of the country. A lack of new greenfields discoveries is by no means confined to New Zealand. Despite a tenfold increase in global spending from 2002 through 2012, the number of discoveries has remained flat (Koch et al., 2015). While the collection of pre-competitive data by governments world-wide is fundamental in attracting continued exploration investment (Smillie and Ahern, 2002; Smillie, 1
2004; Hronsky et al., 2016), the lack of new discoveries globally by companies using these data indicates exploration success is still difficult to achieve.
Minerals Systems – Early Days One approach that is gaining greater recognition by both minerals researchers and industry is minerals systems analysis. Mineral systems are about the combination of geological processes that are required to form and preserve ore deposits at all scales (c.f. Wyborn et al., 1994). A mineral systems approach is being increasingly adopted in the study of the paragenesis and distribution of ore deposits through time (for example, McCuaig et al., 2010). Key concepts of minerals systems are that (a) ore deposits are the end-result of a range of earth processes that take place at different temporal and spatial scales (Figure 1; McCuaig and Hronsky, 2014); and (b) they comprise four “critical elements” that must combine in nested scales in time and space to form economic deposits. These four elements include whole lithosphere architecture, transient favourable dynamics, fertility and preservation of the main depositional zone (McCuaig et al., 2010) (Figure 1).
Figure 1. Mineral system after McCuaig and Hronsky (2014; left) and the four Critical Elements from McCuaig et al. (2010; right).
A significant challenge is translating the minerals system approach into something practical that can be implemented for exploration. Combining disparate geoscientific data at different scales and linking these to geological and mineralising processes is problematic. Despite these challenges, New Zealand researchers and the Government undertook minerals systems analysis for gold mineralisation well over a decade ago (for example, Partington et al., 2002; Partington and Smillie, 2002). In a sense, these studies were ahead of their time as they were limited by the amount and type of data coverage, as well as the level of geoscientific knowledge of larger-scale processes across New Zealand in time and space. 2
Advances in Minerals Systems Research In the last decade, many advances have been made that make the minerals system approach more usable and powerful as a holistic “systems tool kit” for framing the exploration challenge. These advances include: increasingly available large and continuous (unbiased) datasets, including airborne geophysics and regional soil and stream sediment geochemical surveys, advances in geographic information (GIS) technologies and methods to spatially study these datasets; and advances in the understanding of the evolution of earth systems and geodynamics that provide context for mineralisation patterns (for example, McCuaig and Hronsky, 2014). Alongside these advances, there has also been an increased understanding of the scale-dependent processes that form mineral systems. However, processes cannot be seen or mapped, only the results of those processes. Therefore, another important development in minerals systems analysis has been increased effort in helping guide and define the translation of the four critical processes of minerals systems into mappable features from geoscientific datasets at different scales (McCuaig et al., 2010). These data are essentially “proxies” for the key critical processes. There has been a dramatic increase in new exploration-relevant continuous datasets covering New Zealand mineral provinces within the last decade (Smillie et al., 2016, 2017). These datasets have been acquired for exploration companies, the government agency New Zealand Petroleum & Minerals (NZP&M) and GNS Science and include airborne magnetics and radiometrics, electromagnetics and gravity (Rattenbury et al., 2016, 2017 - this volume; Rattenbury, 2017 – this volume), multi- and hyperspectral remotely-sensed imagery (Durance et al., 2017 - this volume), below-cover “solid geology” mapping (Rattenbury, 2016) and regional soil geochemistry (Martin et al., 2016, 2017a,b - this volume, Turnbull et al., 2015, 2017 – this volume). There have also been significant advances in the level of spatial and temporal knowledge of national-and regional scale-geodymanic processes across New Zealand (for example, Rowland et al., 2016; MacKenzie and Craw, 2016; Mortimer, 2017; Mortimer et al., 2016; Allibone and Craw, 2016; Christie, 2016; Mortimer et al., 2017). Examples of how these processes can be translated into explorable proxies include recent analysis of rare earth element deposits (Morgenstern et al., 2017 - this volume), porphyry Cu-Au mineral system analysis and modelling (Kreuzer et al., 2015), and other offshore examples (Czarnota at al., 2010; Mole et al., 2013; Willman et al., 2010. The first application of the minerals system approach in New Zealand by Partington and Hill (2008) focused on mappable criteria using the early model of Wyborn et al. (1994). There are enough commonalities between known mineral deposits in New Zealand to enable a mineral systems approach. Mineral systems analysis for mineralisation styles such as orogenic gold, epithermal gold and intrusion-related REE can be developed and applied to regions such as the Otago, central North Island and West Coast.
Minerals Systems and Exploration Targeting: the “Scale Problem” A major benefit of minerals system analysis is that it can help focus both pre-competitive data acquisition and mineral industry exploration strategies in collecting and incorporating datasets that can map the critical elements of mineral systems at a variety of scales. The search for mineral deposits is an exercise in scale reduction, and has several decision points that map to scale (Figure 2, McCuaig, 2017): (a) Regional-scale targeting: what belt/arc has the probability of hosting a substantial mineral system? 3
(b) Camp-scale targeting: where within the region of interest could a number of deposits be located? This is the scale where exploration companies and government invest in expensive detection technologies. (c) Prospect/deposit: where is the orebody of sufficient quality within the camp? The key point is that at different scales, different exploration strategies and approaches are required. In terms of ground selection, the most difficult decision in most mineral systems is predicting the location of camp-scale clusters of deposits. This is because at this scale there are few direct mineral detection technologies, meaning most exploration decisions are driven by conceptual targeting, for example, of potential host rocks or structures. Where direct detection technologies are available at the camp scale, particularly geophysics, they are invariably associated with large numbers of false positives. We suggest that in the New Zealand setting, the minerals system approach can streamline the scale reduction process (for example, Figures 2, 3), thereby reducing the cost of target selection and increasing the likelihood of new discoveries. This can help focus thinking on what baseline data should be collected for reducing the risk of greenfields exploration and better define and focus the collective effort between researchers, Government and industry.
Figure 2. Diagram summarising the use of targeting approaches across different scales. The top part of the diagram illustrates the trade-off between availability and effectiveness of detection technology versus prediction technology (after McCuaig et al., 2010). Also shown are the relative trends in flexibility of exploration programs and direct costs versus opportunity costs. The bottom part of the diagram summarises the application of conceptual targeting across scales with respect to the critical processes of mineral systems (McCuaig, 2017).
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Figure 3. Multiple scales of exploration: A Regional Scale; B District Scale; C Camp Scale and; D Deposit / prospect scale - (A,B,C after Rattenbury and Heron, 2016; D after Allibone et al 2017).
Conclusions There are reasons to be optimistic about successful mineral exploration in New Zealand. A more coordinated and holistic approach across multiple stakeholders would maximise the opportunities to make fresh discoveries, and the minerals systems approach may be one way in which to focus collaboration between explorers, Government and academia. The following points are relevant: •
The minerals system approach offers considerable promise – it provides a practical holistic framework for ongoing research, collection of new pre-competitive data and area selection by exploration companies.
•
A minerals systems approach tackles the issue of scale and where it is best to focus effort; new districts versus new camps within districts. It also highlights the knowledge and data gaps that need to be addressed to find new deposits.
•
New Zealand is now well positioned to take up a mineral systems approach in a wider and more applied manner. New continuous data sets are emerging such as geophysics, geochemistry, solid geology as well as advances in geoscientific knowledge at regional scales in the recent decade.
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•
Understanding the scale and use of mappable criteria in the mineral system model will help address which data are best to collect, by whom and at what scale.
•
A greater understanding of mineral systems and the scale-dependent processes that form them is important for guiding exploration strategies and further research efforts.
Acknowledgements A draft of this paper was reviewed by Nick Mortimer (GNS Science). Discussions with Andrew Allibone helped clarify ideas in this paper.
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