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Distribution of heavy minerals in fluvial sedimentary succession: implications for placer exploration

Dr Rob Chapman (SEE), Prof Jeffrey Peakall, (SEE), Dr Nigel Mountney (SEE), Dr Luca Colombera (SEE), Dr. Dan Morgan (SEE)

Contact email: r.j.chapman@leeds.ac.uk

This project seeks to develop the first science- based methodology to predict the presence and potential economic importance of placer mineral resources on a global scale. Development of such a technique will transform exploration approaches in a resource sector which although underdeveloped has the potential to deliver high value materials into economies worldwide.

Placer (sediment-hosted) minerals are globally important resources that play a major role in the world’s economic development (Fig 1). Although placer mining is usually synonymous with gold, other important commodities are recovered from both fluvial and marine sediments, including rutile (titanium dioxide, used globally for pigments), and platinum. Placer minerals are concentrated and distributed in the surficial environment according to their density and the prevailing sedimentary regime; consequently they may comprise potentially attractive resource targets even though the concentrations of minerals in the source lithologies are sub-economic. In addition, the minerals are already liberated from their lithological host and do not require the energy intensive crushing stage necessary to process in situ ore. Several ‘critical metals’ (so called because they underpin emerging technologies) form relatively dense resistant minerals which concentrate in fluvial environments, and these include monazite, a source of light rare earth elements (LREE) and xenotime, a source.of heavy rare earth elements (HREE) (Kamizawa and Kamitani, 2006). Consequently, it could be expected that exploration for placer deposits would be a high priority for mineral exploration. However, although beach sands are already exploited in several countries (Southern Africa, India, Australia, USA), fluvial placers remain relatively underexplored. The fluvial placer deposits shown in Fig. 1 mainly represent large concentrations of very small scale, even artisanal mining. With the exception of diamonds, and placer operations in the former Soviet Block, medium- and small-scale mining companies rarely engage with placer deposits. One reason for this is a consequence of different mechanisms of funding within the mineral exploration industry between hard rock and placer mining.

Exploration companies are only permitted to trade on stock exchanges following submission of an independent report which verifies their resource estimations. The reports are constructed according to strict guidelines which govern the interpretation of at a derived from drill core. This approach is more difficult for the evaluation of placer resources because accurate predictions are compromised by wide fluctuations in ore grade (concentration) over small distances (see examples in Fig. 2). Consequently, it may be  challenging for a public company to raise capital on stock exchanges to develop placer resources.

Active fluvial placer mines evaluate their resources using two approaches. Drilling aims to generate a cross section or correlation panel that depicts the spatial distribution and change in grade of the target mineral. The best possible outcome of this approach is an depiction of a 2D slice of the fluvial system, but unfortunately this ideal is compromised both by practical difficulties in sampling unconsolidated, waterlogged gravel which may exhibit a wide range of particles sizes (Garnett and Bartlett 2005) and in particular by marked variation in facies within the third dimension.  Bulk sampling alleviates these problems to some degree but issues of marked spatial variation in ore grade may remain.

In areas of proven fluvial placer resources, it may be problematic to predict the downstream extent of the economic deposit. For example in the world famous placer goldfields of the Yukon, an episode of down-cutting remobilized gold from the palaeoplacer (White Channel Gravels) into the modern drainage. Information on the mobility of gold during this episode would both inform on-going placer exploration downstream and also contribute to our understanding of the elusive source mineralization using an entirely novel approach.

Figure 1. Worldwide distribution of placer deposits of traditional placer commodities (Garnett and Bartlett 2005).

Different types of fluvial sedimentary environments and their preserved successions may be defined in terms of their internal facies architecture (Bridge, 2006; Colombera et al., 2013; Miall, 1996), which characterizes the nature of sediments according to the transport and deposition processes that prevailed in various sub-environments of formation. This lithofacies-based approach has been used extensively and successfully for the characterization architectural elements that represent the larger-scale building blocks of fluvial sedimentary successions, including accumulated sediment bodies that represent channels, barforms, levees, crevasse splays and floodplains. The different processes of sediment transport and deposition that give rise to each of these element types also dictate how and where placer deposits of heavy minerals will accumulate. By understanding how flow processes govern the accumulation of heavy mineral deposits within fluvial systems (through field based observation and physical experimental modelling), a model can be developed to demonstrate how such placer deposits are distributed within various fluvial architectural elements. Consequently, given an initial correlation between architecture and target placer mineral concentration, the overall placer potential of a fluvial system may be evaluated. This project provides the opportunity to establish the first framework for placer evaluation based on underlying sedimentological principles rather than imperfect empirical data.

The Fluvial Architecture Knowledge Transfer System (FAKTS) is a database developed by the Fluvial Research Group (FRG) at the University of Leeds (Colombera et al., 2012a, b). FAKTS is employed as a system for the digital reproduction of all the essential features of fluvial sedimentary architecture; it accounts for the style of internal organization of fluvial bodies, their geometries, grain size, spatial distribution, and the hierarchical and spatial reciprocal relationships of genetic units that comprise these geological bodies. FAKTS additionally classifies depositional systems – or parts thereof – according to both controlling factors (e.g. climate type, tectonic setting), and context-descriptive characteristics (e.g. channel/river pattern, dominant transport mechanism). FAKTS can be used to predict the expected distribution of architectural elements within a range of fluvial system types governed by various external and intrinsic controlling factors. As such, FAKTS can be used to relate both architectural elements and heavy mineral concentration as a function of particular fluvial environmental settings.

Figure 2. Heavy mineral grade variation in placers. (A) Plan of an auriferous placer showing high grade areas. (B) Section through a cassiterite placer in Malaysia showing unpredictable buried high grade zones (Garnett and Bartlett, 2005).

Research Aim and Objectives

The aim of this research project is to produce the first rigorous transferrable methodology for the prediction of the economic potential of fluvial placers. The research is the first to synthesize advanced sedimentological research with an under researched area of ore deposit geology. The paucity of research in this area is a consequence of a traditional divergence of ‘soft rock’ and ‘hard rock’ geologists, but Leeds is uniquely placed to take advantage of the wide range of sub disciplines necessary for successful study in this field. Outputs from the research program will form robust benchmarks in both scientific and industrial environments. This line of study is important because various minerals which form placers are sources of ‘critical metals’, currently the focus of various academic research initiatives aimed at influencing global supply.

The project workflow comprises several discrete elements, each of which serve as a research objectives.

Modelling the relationship between architectural elements and heavy mineral concentration

Laboratory based work will utilise state-of-the-art facilities provided by the internationally leading Sorby Environmental Fluid Dynamics Laboratory (SEFDL) at Leeds, including a new stream table. This facility will enable simulation of braided river dynamics, which generate the same architectural variation observed in placer systems. The effects of key individual parameters (such as placer size; rates of downcutting, discharge) can be isolated by controlling these individually within the experimental programme. The flume experiments will be scaled to model selected fluvial placer systems and will measure the distribution of heavy minerals according to specific gravity and size in the various architectural elements. The use of heavy minerals of different specific gravities will calibrate the systems and permit subsequent prediction of the behaviour of a range of other heavy minerals within architectural elements of fluvial placer systems.

Collection of field data:  1

Fluvial systems and sedimentary successions in the UK which carry heavy minerals will be sampled to record the concentrations of heavy minerals of different specific gravity within different architectural elements. Examples of river systems include: Shortcleugh, Leadhills, Scotland (Au, lead minerals, haematite); Cononish, Orchy and Lochy Rivers R, Argyll Scotland (Au, garnet, magnetite); Indian River, Klondike District, Canada (Au, ilmenite, cassiterite; Fig 3). In the field, sections through riverside gravel accumulations will permit identification of the various architectural elements which will be characterized in terms of class size and heavy mineral distribution.

Figure 3. Section along a mining panel, showing variation in facies, Indian River, Yukon, Canada. 

Comparison of the laboratory generated model with field data

Both laboratory-based experiments and field studies will generate correlations between architectural elements and mineral distribution. Comparison of the two data sets in terms of the proportions of different heavy minerals reporting to different architectural elements will permit evaluation of the flume experiments as predictors of mineral distribution. Consideration of heavy mineral sedimentation according to facies represents a substantial refinement of previous considerations of these systems, which focussed on the behaviour of heavy minerals at bedform scale. In addition, detailed studies of some river systems may be correlated against architectural predictions from the FAKTS database.

Application of the predictive capacity of FAKTS to specific examples for which comprehensive mining data are available

The FAKTS database developed at the University of Leeds will be used for the following primary purposes within this research project:

  • Build quantitative facies models that describe the distribution of architectural elements within channelized and floodplain settings for fluvial systems known to act as placer hosts; characterize the scale, orientation and stacking of these elements and their style of juxtaposition relative to one another (Colombera et al., 2013).

  • Build models that describe the likely internal facies arrangements present in individual architectural elements; determine the relative proportions of facies that make up certain elements and predict their vertical, cross-stream and downstream transitions.

  • Predict the expected dimensions of architectural elements away from boreholes or sample points; predict the most likely arrangement of neighbouring elements and, by association, concentrations of heavy minerals (Colombera et al., 2014, 2015, in press).

  • Compare differences in sedimentary architecture for different types of fluvial system and controlling conditions: for example, compare differences in scale and connectivity of sand bodies in braided versus single-thread (meandering) rivers, or rivers developed in semi-arid versus sub-humid climatic settings, or pre-vegetation (i.e. pre-Silurian) fluvial successions versus post-vegetation successions, or fluvial successions preserved in rift basin settings versus those preserved in foreland basin settings. Assessment will be made of how such differences will likely impact heavy mineral concentrations and distributions.

  • Compile exhaustive comparative statistics for different types of fluvial system: for example, calculate channel-complex proportion, channel-complex thickness and width and channel-complex connectivity for different fluvial types. Use such data to develop a predictive model for heavy mineral concentrations and distributions.

  • Observe how the proportions of facies or architectural elements (and their transition probabilities) change as progressively more filters are included in a query: for example, compare a generic fluvial system, to a braided system, to a braided system developed in a semi-arid climate, to an ephemeral braided system (Colombera et al., 2013).

  • Plot width-thickness relationships for any element (and by association use this to predict grade of placers) and include filters to observe how such relationships vary between different fluvial system types.

Collection of field data 2

One potential problem with developing predictive tools of this sort is that their accuracy can only be evaluated post mining. Currently, the Yukon Geological Survey are retrieving comprehensive exploration and production data for historical gold dredging in various parts of the Yukon, and a complete data set has been generated for Henderson Creek, one of the richest in the region. Data from historical sampling programmes will be integrated with the FAKTS methodology to predict the total gold inventory. These figures will then be compared to the actual production indicated in the dredging records.  We hope to be able to offer the opportunity of fieldwork in the Yukon alongside YGS staff. Whilst we cannot at this stage be assured of funding for this purpose we have been successful in obtaining such funds for all four other recent Canadian facing studies. 

Development of the predictive tool for placer exploration

Identification of source lithologies of target minerals worldwide will be correlated with the predicted fluvial characteristics from FAKTS to identify potential target areas for exploration. This analysis could include areas of deep sediment where conventional sampling methods are inadequate. In this way, a new global map of placer potential for various commodities could be developed.

Potential for high impact outcome

There are very few published scholarly articles on any aspect of placer geology. This is a consequence of the dominance of small operations, the proprietary nature of detailed sampling undertaken by large companies, and the lack of involvement of the exploration sector as a whole, as a consequence of a funding mechanism vacuum. In addition, individual geologists tend to follow either a sedimentology or hard rock/ore geology pathway at the expense of the other. Consequently this project provides an opportunity to generate state-of-the-art outputs and new approaches to placer geology that could underpin exploration and exploration methodology. The University of Leeds is uniquely placed to become world-leading in this under researched area as a consequence of the implementation of skills that are commonly viewed as mutually exclusive. FAKTS is the world’s largest and most sophisticated fluvial architectural database of its type and its development is ongoing at Leeds. The Sorby Experimental fluid Dynamics Lab at Leeds is the leading national facility of its type. Most critical metals have the capacity to be concentrated within placers and yet there has been no scientifically robust study of heavy mineral distribution and dispersion undertaken in this context.

The potential to generate a global industry standard methodology for placer evaluation, with all the resulting implications for exploration, financing and development provides the opportunity for both medium- and long-term impact, whilst simultaneously placing Leeds at the forefront of innovative approaches to mineral exploration.

Training

The project draws upon the diverse expertise of the supervisory team, which spans fluvial sedimentology, process sedimentology, economic geology and igneous petrology. This supporting team will provide the appointed student with an unusually broad-base background and crucially, experience of integrating information from sub-disciplines of geology which rarely overlap. Specifically, the student will gain expertise in the following aspects of geological science: (i) modelling fluvial systems according to their controlling parameters; (ii) specialised field techniques designed to characterise heavy mineral distribution in active fluvial settings; (iii) correlation of theoretical and actual data sets; (iv) application of theoretical considerations from sedimentology and igneous petrology to generate a predictive tool for use in industry. In addition, the student will benefit from collaboration with YGS to gain exposure of the placer mining industry whilst establishing their own networks in the geological community.

The student will have access to training in specific software packages (ARC-GIS, Iogas, MySQL database query software, Petrel). A wide range of training will be provided by the Faculty of Environment (http://www.emeskillstraining.leeds.ac.uk/). The broad spectrum of geological sciences represented in the project will provide the student with various possibilities to develop teaching skills through contribution to teaching as a demonstrator in both lab-based classes and on field courses. Research findings will be disseminated through presentations at both national conferences (e.g., BRSG) and international conferences, some industry facing, such as the Vancouver Exploration Roundup. We will also actively seek funding to permit the student to present their work at the Yukon Geoscience Forum, held annually in Whitehorse, Yukon. The publishable outputs will include contributions to the flagship ore deposits literature (Economic Geology) and sedimentological journals (Sedimentology, Journal of Sedimentary Research, Sedimentary Geology). The close collaboration with YGS will result in small targeted outcomes in the annual ‘Yukon Exploration and Geology’ output of the YGS which will maximise the regional impact of the project. The multidisciplinary nature and novelty of the research will also generate outputs for high impact journals such as Nature Geoscience and Geology.

The student will join a vibrant community of postgraduate researchers engaged in both sedimentology and ore deposits research, The Placer Minerals Group is unique to Leeds and provides a potential fast track to international recognition as a specialist in an emerging field.

References

Bridge, J.S., 2006. Fluvial facies models: recent developments. In: Facies Models Revisited. Posamentier, H. and Walker, R.G. (Eds), SEPM Spec. Publ., 84, 85–170.

Colombera, L., Mountney, N.P. and McCaffrey, W.D., 2012a. A Relational Database for the Digitization of Fluvial Architecture: Concepts and Example Applications. Petroleum Geoscience, 18, 129-140. doi: 10.1144/1354-079311-021

Colombera, L., Felletti, F., Mountney, N.P. and McCaffrey, W.D., 2012b. A database approach for constraining stochastic simulations of the sedimentary heterogeneity of fluvial reservoirs. American Association of Petroleum Geologists Bulletin, 96, 2143-2166. doi: 10.1306/04211211179

Colombera, L., Mountney, N.P. and McCaffrey, W.D., 2013. A quantitative approach to fluvial facies models: methods and example results. Sedimentology, 60, 1526-1558. doi: 10.1111/sed.12050

Colombera, L., Mountney, N.P., McCaffrey, W.D. and Felletti, F., 2014. Models for guiding and ranking well-to-well correlations of channel bodies in fluvial reservoirs. American Association of Petroleum Geologists Bulletin, 98, 1493-1965. doi: 10.1306/05061413153

Colombera, L, Mountney, N.P. and McCaffrey, W.D., 2015. A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: implications for sequence stratigraphy. Geology, 43, 283-286. doi:10.1130/G36385.1

Colombera, L, Mountney, N.P., Howell, J.A., Rittersbacher, A., Felletti, F. and McCaffrey, W.D., 2015. A test of analog-based tools for quantitative prediction of large-scale fluvial architecture. American Association of Petroleum Geologists Bulletin, in press.

Garnett, R.H.T and Barret N.C., 2005. Placer Deposits. Society for Economic Geology 100th Anniversary volume. Hedenquist, J.W., Goldfarb, R.J., Thompson, J.F.H., and Richards, J.P, and (Eds). SEG inc., Littleton, Colorado, 813-843

Kamizawa, Y. and Kamitani, M., 2006. Rare earth minerals and resources in the world. Journal of Alloys and Compounds, 408–412, 1339–1343.

Miall, A.D., 1996. The Geology of Fluvial Deposits. Springer Verlag, Berlin, 582 pp.

Related undergraduate subjects:

  • Geological science