Till Geochemical Data Research Articles

Principal components analysis and K-means clustering of till geochemical data: Mapping and targeting of prospective areas for lithium exploration in Västernorrland Region, Sweden

To achieve the demand for elements used for the green transition energy, such as lithium, it is necessary to recognize the spatial distribution of the concentrations of these elements in different earth materials such as bedrock and soil and to identify areas with anomalous concentrations of such elements (i.e., mineralization) for further exploration and hopefully exploitation. This study carried out multivariate statistical analyses on compositional (i.e., element concentration) data from till samples to recognize areas that likely contain lithium pegmatite mineralization in the Västernorrland region, central Sweden. We applied principal components analysis (PCA) and K-means clustering techniques to reveal regional-scale patterns in the till geochemical data. We demonstrate that these two methods have potential for recognition of geochemical anomalies related to the underlying bedrock geology as well as to mineralization. The results of PCA- and K-means clustering were validated using known occurrences of lithium mineralization. Two different datasets were compared; one containing all available geochemical data and the second containing only available trace elements in the dataset and it was found that anomalous clusters of samples defined by K-means clustering have anomalous multi-element signatures defined by robust PCA. This demonstrated that principal components are the continuous solutions to the discrete cluster members for the K-means clustering. The results show that both PCA and K-means clustering of till geochemical datasets at the early stages of exploration and target generation may reveal useful information that can be used to identify potential areas for more detailed mapping or exploration activities.

Cobalt Prospectivity Using a Conceptual Fuzzy Logic Overlay Method Enhanced with the Mineral Systems Approach

AbstractThis paper describes mineral prospectivity research conducted in Finland to predict favorable areas for cobalt exploration using the “fuzzy logic overlay” method in a GIS platform and public geodata of the Geological Survey of Finland. Cobalt occurs infrequently as a core product in mineral deposits. Therefore, we decided to construct separate conceptual mineral prospectivity models within the Northern Fennoscandian Shield, Finland, for four deposit types: (1) “Orthomagmatic Ni–Cu–Co sulfide deposits,” (2) “Outokumpu-type mantle peridotite-associated volcanogenic massive sulfide (VMS)-style Cu–Co–Zn–Ni–Ag–Au deposits,” (3) “Talvivaara black shale-hosted Ni–Zn–Cu–Co-type deposits” and (4) “Kuusamo-type (orogenic gold with atypical metal association) Au–Co–Cu–U–LREE deposits”. In addition, we created a model combining till geochemical data with data derived from bedrock drilling and mineral indications, including boulders and outcrops. The mineral prospectivity models were statistically tested with the “receiver operating characteristics” method using exploration drilling data from known mineral deposits as validation sites. In addition, the predictive performance of the models was evaluated by using success rate curves, where the number of previously identified deposits was compared with the area coverage of the predicted highly favorable areas. These results indicate that the knowledge-driven mineral prospectivity method using parameters derived from mineral systems models is effective in defining favorable exploration target areas at the regional scale. This study's innovation lies in its comprehension of the process of evaluating mineral prospectivity when the commodity of interest is not the primary commodity within the mineral system.

Spatial analysis and modelling of glaciogenic geochemical dispersion – Implication for mineral exploration in Finland

Spatial modelling for prospectivity mapping involves the integration of various geoscientific digital map data. It is essential that the quality of data is high class and that the geological processes involved are well understood. Effects of glacial dynamics and glaciogenic geochemical dispersion need to be taken into consideration when using till geochemistry as one of such input dataset for prospectivity modelling. This paper investigates this issue by developing a fuzzy logic prospectivity model that integrates airborne geophysical data with two different till geochemical datasets. First we use the original geochemical sample set and secondly a spatially corrected dataset that is based on the knowledge of glacial dynamics on the regional scale within the study area. The effect of this correction is tested by comparing the modelling results of both cases using the receiver operating characteristics (ROC) technique to validate the models by using the location of known gold deposits represented by exploration drilling. This study confirms that by taking into consideration the transport caused by the glaciation we can significantly improve the performance of a prospectivity model using till geochemical data.

Multivariate spatial analysis of lake sediment geochemical data; Melville Peninsula, Nunavut, Canada

A multivariate spatial analysis was conducted on a suite of glacial till geochemical data collected over the Melville Peninsula, Nunavut, Canada. Previous studies demonstrated through the application of multivariate statistical techniques that the composition of the lake sediment geochemistry reflects the underlying geology in northern Canada. In this study, the application of minimum/maximum autocorrelation factor analysis (MAF) to glacial till geochemistry has extended the knowledge and description of the underlying geology through the recognition of spatially correlated factors that represent distinct lithologic features and glacial transport processes.Maps of posterior probabilities for the underlying lithologies were estimated based on the MA factors and compared with those from standard principal component analysis. The use of MAF provides a measured improvement in predictive mapping irrespective of the choice of lag separation.

Till sampling and geochemical analytical protocols used by the Geological Survey of Canada

The Geological Survey of Canada (GSC) has developed field and lab methods protocols to guide till sample collection, processing, geochemical analysis of the till matrix, monitoring of quality assurance/quality control, and archiving procedures for reconnaissance- to local-scale geochemical surveys. The most significant concepts and procedures are described in this paper. Continued and long-term use of these protocols will ultimately allow GSC researchers to integrate and contrast multiple datasets and ensure minimum levels of quality assurance and control for all till geochemical data. This set of protocols is the first established for Canadian till sampling and analysis and represents a contribution to the GSC’s Geo-mapping for Energy and Minerals (GEM) Program. Sharing the GSC’s knowledge on till sampling and analysis with the international community will allow other researchers and explorationists to adopt similar procedures. This sharing of knowledge will ultimately allow comparison of till geochemical datasets from various parts of Canada and internationally as well as ensuring a minimum level of quality assurance and control for all till geochemical data.

Distinguishing glaciogenic deposits in southern Finnish Lapland: implications for exploration

In glaciated terrains, the knowledge of Quaternary geology is essential for successful exploration. In Finland, the bedrock is almost completely covered by till of variable composition, thickness and stratigraphy, as well as various types of moraine formations from recent glaciations. In this study, the morphological, stratigraphical and compositional investigations of till and moraines in southern Finnish Lapland were combined with heavy mineral and geochemical studies to trace mineralised sources in the bedrock. In the study area, the glacial morphology is composed of an assemblage of transversal ribbed moraines and longitudinal drumlins and flutings with gently undulating till cover in between. In places, weathered bedrock remnants are preserved beneath the till cover and can be useful sampling media for exploration. The ribbed moraine ridges are the most favourable moraine type for exploration. Short glacial transportation of debris and boulders in the upper till and surface of the ridges is advantageous for tracing the ore indicators. Local bedrock indication is another aspect that makes sampling easy and minimises costs. In the areas of drumlins, flutings and thin till cover, transport distances are usually long and the possibility of complex glacial transportation increases. If weathered bedrock material is mixed in the till, it makes an interpretation of till geochemical data difficult; background values are usually elevated and anomalous values coming from fresh bedrock sources might be obscured. However, the sampling of the weathered bedrock provides indications of the local geochemical features in the bedrock.

A method for detecting glacial dispersal trains in till geochemicaldata

A new algorithm for identifying potential glacial dispersal trains in till geochemical data is introduced. The algorithm, referred to as the dispersal train identification algorithm (DTIA), requires a set of user input parameters used for simulating dispersal train characteristics. The algorithm employs a succession of wedge-shaped search regions (each one shaped like an isosceles triangle with the search point at the principal vertex) to ‘look’ in user-specified directions from a set of search points that may be mineral prospects, geochemical anomalies in rock or a homogeneous grid of points. The values of the till sample points occurring within the wedge are analysed as candidates for a dispersal train using a set of criteria. The geologist specifies the length and angle of the wedge, where the length reflects the distance of down-ice dispersal and the angle is used to model a ribbon or fan-shaped dispersal train. For each search position, DTIA calculates the average of the values within the wedge (pmed), the difference between wedge average and the local average (diff), and fits an exponential model to the within-wedge points (geochemical value as a function of distance from the search point). Candidate search points for the heads of dispersal trains are selected based on high values of pmed and diff, as well as best fit lines (fitted to the natural logarithms of geochemical values) that show high negative slopes (b), significant values of a test statistic for the slope magnitude (ts), and large negative correlation coefficients (R). The slope statistic, ts, is particularly useful in identifying candidate trains, but care must be taken to eliminate situations where the closest sample point is relatively far from the search point. DTIA was tested on two datasets: a dataset in Cape Breton where documented till dispersal trains exist and in the Swayze greenstone belt where no documented trains are known. The effects of varying the input parameters (e.g. length, angle and direction of wedge) were studied as well as different search strategies with the objective of identifying the most appropriate parameters for successfully identifying points that represent the heads (or source zones) of potential till dispersal trains. The algorithm performed well with the Cape Breton data. The results are particularly sensitive to both direction of search and the wedge angle. By systematically searching from a set of grid points, then screening out search points (and associated directions) that have (1) pmed values less than a threshold, (2) diff values less than a threshold, (3) distance of point closest to search point less than a threshold, (4) slope values less than zero and (5) ts values usually less than –2.0,a suite of candidate search points can be plotted on the till geochemical map, colour coded by search direction. These candidate points (and associated wedges) can then be visually appraised as potential dispersal trains. Application to the Swayze data revealed candidate trains whose directions are consistent with glacial transport directions known from data on glacial striations. Although this method shows promise as a data exploration tool, more work is needed to improve the algorithm and to test it under a greater range of conditions.

Regional till geochemical surveys in the Canadian Cordillera: sample media, methods and anomaly evaluation

Abstract Basal tills have become a widely used regional geochemical sampling medium in recent years in the Canadian Cordillera. They reflect the primary composition of the source bedrock and contrast with B-horizon soil that can be developed on a variety of glacial and non-glacial surficial sediment types. Detailed sedimentological data are critical to collect and they are used to differentiate basal tills from other visually similar sediments including englacial and supraglacial tills, colluvial debris flow deposits, and very poorly sorted, glaciofluvial or glaciolacustrine sediments (e.g. diamictons or gravelly muds). The variable transport and depositional processes that form these different sediments make interpretation of geochemical data difficult. Deep (usually >0.75 m) C-horizon sampling of basal till minimizes the complicating effects of pedogenesis, weathering, surface washing and gravity remobilization of the tills. The latter processes, particularly pronounced in the wet, steep terrain, typical of much of the Canadian Cordillera, lead to depleted concentrations of heavy minerals (notably Au) and hydromorphic dispersion of mobile elements in the near surface sediments. Also, elements that are preferentially concentrated in the fine fraction can be selectively removed by surface waters. Offset sampling lines, oriented perpendicular to the dominant ice-flow direction, are most effective for detecting regional geochemical anomalies which are typically narrow and elongated parallel to ice-flow. Erratics trains and till anomalies are usually a few to several kilometres long and up to one or more kilometres wide. For some metals such as Au, anomalies are generally larger and more readily detected in till than in B-horizon soil. Surface till anomalies reflect up-ice sources and not the immediately underlying bedrock; down-ice displacements of > 500m often occur in areas of thick till. Basal till anomalies usually can be traced to source along linear transport paths reflecting topographically controlled valley-glacier flow in mountainous areas and unidirectional ice-sheet flow in many plateau areas, chiefly representative of the last glacial event. Interpretations of till geochemical data are enhanced with a clear understanding of the surficial and bedrock geology, Quaternary stratigraphy, ice-flow history and down-ice dispersal characteristics around known mineral deposits.

Determination of ore potential areas in the Hame belt, southwestern Finland, by integration of geological, geophysical and till geochemical data

Determination of ore potential areas in the Hame belt, southwestern Finland, by integration of geological, geophysical and till geochemical data

Multi-media geochemistry and surficial geology of the Yava Pb deposit, southeastern Cape Breton Island, Nova Scotia, Canada

A detailed multi-media geochemical study was conducted over the east zone of the Yava lead deposit, southeastern Cape Breton Island. A 2 × 2 km grid was centred over the projected subcrop of the east ore zone and sample sites were located at 250 m spacing. Samples collected at each site included: (1) Balsam fir ( Abies balsamea ); (2) A 1 -horizon (organic humus) soil; and (3) C-horizon till. In addition, stream sediment samples were collected at approximately 250 m spacing along first and second order streams that transect the grid. Overburden drilling of a drumlin located in the grid, to the northeast of the ore zone, and a till fabric study were conducted in conjunction with the geochemical survey. The drumlin is cored by a silty/sandy till and overlying moderately-compacted silty till. The entire study area, including the drumlins, is blanketed by a loosely-compacted sandy till. The silty upper drumlin till and surface sandy till were deposited by NE and NNE ice flows respectively. Analysis of mineralized sandstone clasts from the two uppermost till facies in the overburden holes returned values of ≤3.05% Pb and ≤2.65% Zn. Our data indicate the Yava deposit has a robust geochemical signature, mostly for Pb and Zn in the −230 (−62.5 μm) mesh fraction and heavy mineral separates (−60+230 (−250+62.5 μ m) mesh fraction) of tills and stream sediments and ash of Balsam fir twigs. These data clearly delineate the subcrop of the east ore zone. The distribution of Ag, Cd, As and Sb within all sample media varies substantially and generally are less reliable for outlining the location of the deposit. Till clast distribution and till geochemical data indicate that NNE-trending dispersal in the upper sandy till sheet was minimal. Large NE-trending ribbon-shaped Pb and Zn “anomalies” in the −230 (−62.5 μm) mesh fraction of this till reflect inheritance of mineralization from the underlying drumlin core till with NE dispersion and possibly additional base metal mineralization along the margin of the sedimentary basin. The geochemical data for heavy mineral separates from both till and stream sediments mirror the results from the −230 (−62.5 μm) mesh fraction but are not recommended for future studies because of high sample preparation costs. Balsam fir is the most cost-efficient, yet geochemically effective, sample medium. Humus geochemistry reflects hydromorphic processes and is not recommended for future surveys.

Utilizing glacial geology in uranium exploration; Dismal Lakes, Northwest Territories, Canada

A field of uraniferous boulders was discovered in a drift‐covered valley west of Dismal Lakes. Glacial geological information was combined with boulder location and trace element till geochemical data to model the dispersal of the boulders; and to predict their likely bedrock source. Uraniferous bedrock was eroded by the last, westward flowing glacial ice to cover the area. The debris was englacially transported and subsequently deposited during subglacial melt‐out of ice block(s) stagnating below active ice. The distribution of the boulders forms acrude, westward‐opening fan centred on the easternmost boulder and oriented with the last ice‐flow direction. The largest uranium values from surface till samples (‐2 μm fraction) occur 6.2 km east of the main boulder concentration or 1.5 km east of the first boulder occurrence. The likely bedrock source is 6.0 to 6.6 km east of the main boulder concentration.