Derivation of Predictive Layers Using Regional Till Geochemistry Data for Mineral Potential Mapping of the REE Line of Bergslagen, Central Sweden
<p>Bedrock map of the REE line taken from the SGU 1:1,000,000 map from SGUs database. Coordinates are based on Swedish SWEREFF-99TM system. The red points represent an up-ice coordinate transformation of original sample locations to roughly 16 km NNW based on inferred transport distance. Location the REE line is shown in the inset map of Sweden in a red rectangle.</p> "> Figure 2
<p>The study area with soil depth shown in meters. The thickest sediments are typically associated with fluvial or lacustrine areas. Arrows demonstrate ice-flow direction as measured from orientation of striations in bedrock.</p> "> Figure 3
<p>Flow chart defining mineral systems from critical processes through mappable proxies.</p> "> Figure 4
<p>(<b>A</b>–<b>C</b>) Biplots of PC scores for PCs 1 through 4 for the ILR transformed all-element data. Individual points represent individual till samples, and colors represent their K-means cluster membership.</p> "> Figure 5
<p>Interpolated factor scores from the all-element data. (<b>A</b>). PC1 shows spatial correlation to mafic (high factor scores) and felsic (low factor scores) bedrock. (<b>B</b>). High scores along PC2 demonstrate possible correlations to the more evolved 1.85–1.75 Ga granites and pegmatites. (<b>C</b>). High factor scores show special affinity to the Norberg REE mineralizations. Black arrows indicate ice direction.</p> "> Figure 6
<p>Clusters membership of individual till samples for all-element data plotted over the bedrock map of the REE line. REE mineralizations are shown in red crosses.</p> "> Figure 7
<p>Biplots of trace-element data after ILR transformation for PCs 1 through 5. (<b>A</b>). PC1–PC2. (<b>B</b>). PC1–PC3. (<b>C</b>). PC1–PC5.</p> "> Figure 8
<p>(<b>A</b>–<b>C</b>) Interpolated results for principal component analysis of the trace-element data. (<b>A</b>). PC1 demonstrates a rough mafic (low scoring) and felsic (high scoring) divide between till samples. (<b>B</b>). Positive PC2 scores demonstrate association with 1.85–1.75 Ga granites and pegmatites. (<b>C</b>). PC5 shows correlation with known REE deposits. Red crosses are known REE mineralizations. Arrows indicate ice direction.</p> "> Figure 9
<p>Clustering results from the trace-element data plotted over the bedrock map of the REE line.</p> "> Figure 10
<p>Clusters 2, 3, and 6 shown overlain on the interpolated factor scores of PC5 of the trace-element data.</p> "> Figure 11
<p>Principal components of till samples from each of the three clusters highlighted in <a href="#minerals-14-00753-f011" class="html-fig">Figure 11</a> showing association with mineralization and positive loadings on the fifth principal component of the trace-element data. (<b>A</b>). Cluster 2, (<b>B</b>). Cluster 3. (<b>C</b>). Cluster 6.</p> "> Figure 12
<p>Interpolated results of the alteration index of till samples within the REE line with known non-REE bearing mineralizations shown as red points (sulfide bearing) or green triangles (Fe-oxide).</p> ">
Abstract
:1. Introduction
2. Geological Background
3. Quaternary Geology
Quaternary Geology of Bergslagen
4. The Mineral Systems Approach to Mineral Potential Mapping and Its Application to Bergslagen
- Lithostratigraphic factors: REE skarn deposits in Bergslagen form within supracrustal inliers of felsic volcanic rocks of calc–alkaline affinity and in proximity to banded iron formation. Skarn or limestone contacts with the supracrustal rock, indicating favorable conditions for REE mineralization.
- Geochemical factors: spatial associations of elements related to skarn-type mineralizations, i.e., Fe, Mg, REE+Y, and P.
- Geophysical factors: REE mineralization demonstrates positive associations with areas of high magnetic intensity based on aeromagnetic surveys. Highly magnetic rocks stretch along the length of the REE line, representing the magnetite-bearing mineralizations.
- Alteration: Rocks in Bergslagen are variably altered, with regional-scale alteration stronger in the vicinity of mineralizations. Alteration is typically ±K, ±Mg, or ±Na, and occasionally lesser Fe alteration. Thus, tools such as the alteration index [27] or the chlorite–pyrite–plagioclase index (CCPI) [28], which examine variations in the ratios of K, Mg, Na, and Fe originating from mineral alteration, can be used to identify the degree of alteration.
- A key factor in the formation of mineralizations in Bergslagen is the circulation of hydrothermal fluids, which requires a heat source. Heat generated by intrusive or synvolcanic rocks, with ages from 1.97 to 1.87 Ga and 1.8 Ga, is proposed to have acted as a driver for hydrothermal circulation.
- Structural factors: Spatial distribution of REE mineralization in Bergslagen shows a relationship to the D2 deformation phase, or at least processes coeval with D2 [29]. The pattern of magnetic anomalies in the REE line is s-folded, with known REE deposits occurring within these s-folds, more specifically where the fold pattern turns to a more northerly direction.
5. Materials and Methods
5.1. Till Sampling Methodology
5.2. Geochemical Analysis of Till
5.3. Data Treatment Methodology
5.3.1. Transformation of Sampling Coordinates
5.3.2. Exploratory Data Analysis
5.4. Principal Component Analysis
5.5. K-Means Clustering
5.6. Alteration Index
6. Results
6.1. All-Element Data
6.1.1. PCA Analysis of All-Element Data
6.1.2. Clustering Results of All-Element Data
6.2. Trace-Element Data
6.2.1. PCA Results Trace-Element Data
6.2.2. Trace-Element Clustering
6.3. Alteration Index
7. Discussion
7.1. All-Element Data
7.2. Trace-Element Data
7.3. Generation of Predictive Layers for Prospectivity Mapping
8. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Factor | Target Evidence | Mappable Geochemical Proxy | Sources of Uncertainty |
---|---|---|---|
Source | Ultramafic to mafic magmas | Principal component factor coupled with mafic chemistry | Unclear elemental associations due to mixing of signals in till |
Trap | Reactive horizons or potential trap rocks | Principal component or cluster associated with carbonate rocks, positive HCl test of till | Weak reaction to acid, Ca from other Ca-rich minerals |
Geochemical signatures of mineralization | Multivariate geochemical anomalies in till samples | Signatures obscured by other anomalous metal sources such as metal-rich granites | |
Modification | Alteration (Mg-K-Ca-Na) of bedrock | Ishikawa alteration index; chlorite–carbonate–pyrite Index | Analytical uncertainty, poor leaching of elements from certain key minerals |
Alkaline-type granites | Principal component factor coupled with felsic chemistry | Mixed geochemical signals |
Element | Min | Max | Mean | Median | STD | Skew |
---|---|---|---|---|---|---|
Al | 2509 | 38,592 | 9772 | 9160 | 4825 | 1.3 |
Ag | 0.01 | 0.2 | 0.1 | 0.057 | 0.0 | 1.8 |
As | 0.2 | 81 | 2 | 1 | 5.1 | 10.0 |
Ba | 4.0 | 162 | 26 | 21 | 18 | 2.2 |
Be | 0.1 | 2.7 | 0.6 | 0.5 | 0.3 | 2.1 |
Bi | 0.1 | 14 | 0.2 | 0.15 | 0.6 | 19 |
Ca | 351 | 91,785 | 1885 | 1650 | 3297 | 23.1 |
Cd | 0.0 | 0.9 | 0.1 | 0.05 | 0.0 | 9.9 |
Co | 0.4 | 19.4 | 3.2 | 2.8 | 2.1 | 2.5 |
Cr | 1.0 | 321 | 9.9 | 8.4 | 12.4 | 17.9 |
Cu | 0.1 | 125 | 6.4 | 4.5 | 7.8 | 8.4 |
Fe | 1213 | 48,828 | 9685 | 8615 | 5462 | 1.7 |
K | 25 | 10,153 | 641 | 479 | 611 | 5.8 |
La | 4.3 | 131 | 19.5 | 16.8 | 11.2 | 4.0 |
Li | 0.9 | 55.1 | 6.4 | 5.2 | 4.8 | 2.8 |
Mg | 199.0 | 23,027 | 2685.1 | 2288 | 1987 | 3.4 |
Mn | 14.0 | 1220 | 144 | 116 | 116 | 3.3 |
Mo | 0.1 | 20 | 0.4 | 0.2 | 1.0 | 14.0 |
Na | 24.0 | 311.0 | 75.5 | 70 | 27.0 | 2.2 |
Ni | 0.0 | 119.0 | 5.0 | 4.2 | 5.9 | 10.9 |
P | 25.0 | 2091.0 | 394.3 | 368 | 211 | 2.1 |
Pb | 2.3 | 130.5 | 8.9 | 7.4 | 8.1 | 7.5 |
Rb | 1.3 | 174.2 | 8.5 | 6.2 | 8.6 | 9.0 |
Se | 0.1 | 1.2 | 0.3 | 0.25 | 0.1 | 1.9 |
Sn | 0.1 | 1.7 | 0.4 | 0.38 | 0.2 | 3.0 |
Sr | 2.3 | 63.9 | 8.0 | 7.2 | 4.0 | 4.3 |
Th | 2.9 | 129 | 11.1 | 9.5 | 8.5 | 5.3 |
Ti | 160 | 2249 | 642.0 | 609 | 244 | 1.4 |
Tl | 0.0 | 2.7 | 0.1 | 0.06 | 0.1 | 12.9 |
U | 0.5 | 33.0 | 2.6 | 2.1 | 2.3 | 5.5 |
V | 4.2 | 51.8 | 15.4 | 14 | 7.4 | 1.4 |
W | 0.0 | 7.2 | 0.3 | 0.21 | 0.3 | 14.7 |
Y | 3.0 | 62.0 | 12.0 | 11 | 5.9 | 3.0 |
Zn | 2.5 | 15 | 20.2 | 16.1 | 16.2 | 2.8 |
PC | Eigenvalue | Variance | Cumulative Variance (%) |
---|---|---|---|
PC1 | 6.6 | 19.5 | 19.5 |
PC2 | 3.9 | 11.4 | 30.9 |
PC3 | 3.3 | 9.7 | 40.5 |
PC4 | 3.1 | 9.1 | 49.6 |
PC5 | 2.1 | 6.2 | 55.8 |
PC6 | 1.8 | 5.2 | 61.0 |
PC7 | 1.5 | 4.5 | 65.5 |
PC8 | 1.3 | 3.9 | 69.5 |
PC9 | 1.2 | 3.4 | 72.9 |
Element | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 | PC7 |
---|---|---|---|---|---|---|---|
Al | −0.23 | −0.09 | 0.17 | −0.12 | 0.21 | 0.09 | 0.28 |
Ag | 0.30 | 0.08 | 0.10 | 0.15 | 0.11 | 0.10 | 0.06 |
As | −0.02 | 0.15 | −0.19 | −0.06 | 0.37 | 0.26 | 0.16 |
Ba | −0.09 | 0.09 | 0.36 | −0.05 | 0.20 | 0.19 | 0.17 |
Be | 0.00 | −0.10 | −0.33 | 0.02 | −0.11 | −0.36 | −0.14 |
Bi | 0.10 | 0.14 | −0.08 | 0.18 | 0.18 | −0.24 | −0.10 |
Ca | 0.21 | −0.16 | −0.09 | −0.04 | −0.02 | −0.02 | 0.20 |
Cd | −0.17 | −0.17 | −0.05 | 0.02 | 0.01 | 0.02 | 0.14 |
Co | 0.10 | 0.27 | −0.04 | 0.09 | 0.27 | 0.12 | −0.08 |
Cr | −0.15 | 0.10 | 0.22 | −0.25 | 0.01 | 0.00 | 0.06 |
Cu | −0.12 | −0.28 | −0.13 | 0.17 | 0.16 | 0.31 | 0.10 |
Fe | 0.12 | 0.18 | 0.09 | 0.34 | −0.01 | −0.22 | 0.20 |
K | 0.05 | −0.07 | −0.07 | 0.44 | 0.01 | 0.16 | 0.19 |
La | 0.00 | −0.02 | −0.04 | −0.24 | 0.29 | −0.41 | −0.09 |
Li | 0.25 | −0.12 | 0.12 | −0.10 | 0.13 | 0.16 | −0.22 |
Mg | 0.04 | −0.19 | −0.22 | 0.01 | 0.42 | 0.03 | −0.19 |
Mn | 0.00 | 0.19 | −0.05 | −0.30 | 0.03 | −0.03 | 0.16 |
Mo | 0.00 | −0.36 | −0.08 | 0.10 | 0.04 | 0.19 | −0.12 |
Na | 0.03 | −0.24 | −0.09 | 0.02 | −0.03 | −0.04 | 0.15 |
Ni | 0.26 | −0.13 | 0.24 | −0.04 | −0.08 | 0.02 | −0.10 |
P | −0.15 | −0.21 | 0.20 | 0.25 | −0.06 | −0.20 | 0.08 |
Pb | 0.17 | 0.06 | −0.22 | 0.10 | 0.28 | −0.07 | 0.20 |
Rb | −0.27 | −0.16 | −0.03 | −0.24 | 0.02 | −0.01 | 0.00 |
Se | −0.24 | 0.17 | −0.03 | 0.06 | −0.13 | −0.12 | 0.21 |
Sn | −0.15 | 0.27 | 0.08 | 0.00 | 0.23 | −0.08 | −0.05 |
Sr | −0.02 | 0.20 | 0.04 | 0.16 | −0.05 | 0.02 | 0.28 |
Th | 0.06 | −0.30 | 0.21 | 0.01 | 0.06 | −0.25 | 0.30 |
Ti | 0.26 | 0.05 | −0.22 | −0.17 | −0.01 | 0.01 | 0.18 |
Tl | 0.17 | −0.15 | 0.27 | −0.14 | 0.14 | 0.00 | −0.09 |
U | −0.26 | −0.04 | −0.14 | 0.15 | 0.22 | −0.22 | −0.07 |
V | −0.13 | 0.15 | −0.30 | −0.11 | −0.25 | 0.26 | −0.03 |
W | −0.27 | 0.04 | 0.00 | 0.22 | −0.07 | 0.15 | −0.28 |
Y | −0.14 | 0.10 | 0.27 | 0.23 | 0.14 | −0.02 | −0.34 |
Zn | 0.30 | 0.13 | 0.04 | 0.05 | −0.16 | 0.03 | −0.10 |
PC | Eigenvalue | Variance | Cumulative Variance (%) |
---|---|---|---|
PC1 | 5.4 | 21.7 | 21.7 |
PC2 | 3.0 | 12.1 | 33.8 |
PC3 | 2.6 | 10.3 | 44.1 |
PC4 | 2.3 | 9.4 | 53.5 |
PC5 | 1.7 | 6.7 | 60.2 |
PC6 | 1.4 | 5.6 | 65.8 |
PC7 | 1.3 | 5.0 | 70.8 |
PC8 | 1.1 | 4.3 | 75.0 |
PC9 | 1.0 | 4.1 | 79.1 |
Element | PC1 | PC2 | PC3 | PC4 | PC5 | PC6 | PC7 |
---|---|---|---|---|---|---|---|
Ag | 0.29 | 0.20 | 0.16 | 0.21 | 0.11 | 0.00 | 0.13 |
As | −0.17 | −0.01 | 0.31 | −0.33 | 0.11 | 0.30 | −0.02 |
Ba | −0.12 | −0.09 | 0.02 | −0.19 | 0.52 | −0.23 | 0.20 |
Be | −0.08 | 0.32 | 0.33 | −0.19 | 0.27 | −0.03 | −0.06 |
Bi | −0.02 | −0.34 | −0.26 | 0.07 | −0.43 | 0.05 | −0.08 |
Cd | 0.25 | −0.20 | 0.01 | −0.04 | 0.07 | −0.02 | −0.02 |
Co | −0.18 | 0.22 | 0.07 | −0.23 | −0.20 | 0.42 | −0.07 |
Cr | 0.01 | −0.06 | −0.02 | 0.22 | 0.12 | 0.26 | 0.42 |
Cu | −0.16 | −0.35 | 0.18 | 0.21 | 0.17 | 0.28 | −0.13 |
La | 0.27 | 0.14 | 0.13 | 0.34 | −0.06 | 0.09 | 0.10 |
Li | −0.01 | −0.05 | 0.01 | −0.30 | −0.27 | −0.40 | 0.45 |
Mo | −0.06 | 0.29 | −0.34 | −0.26 | −0.04 | 0.01 | 0.03 |
Ni | 0.02 | −0.39 | 0.14 | −0.08 | 0.06 | 0.26 | 0.32 |
Pb | 0.24 | −0.11 | −0.18 | −0.26 | 0.09 | 0.17 | 0.01 |
Rb | −0.19 | 0.20 | −0.11 | −0.03 | −0.17 | 0.24 | 0.41 |
Se | 0.33 | −0.04 | −0.06 | −0.16 | 0.09 | 0.21 | −0.04 |
Sn | −0.26 | 0.10 | −0.10 | 0.04 | −0.11 | 0.15 | −0.20 |
Sr | −0.04 | 0.21 | −0.06 | 0.17 | 0.07 | −0.06 | −0.25 |
Th | −0.30 | −0.20 | 0.01 | 0.18 | −0.02 | −0.04 | 0.06 |
Tl | 0.12 | −0.11 | 0.40 | −0.06 | −0.23 | −0.24 | −0.22 |
U | −0.18 | 0.15 | −0.11 | 0.34 | 0.12 | −0.17 | 0.24 |
V | 0.24 | −0.16 | −0.25 | −0.21 | 0.11 | −0.07 | −0.07 |
W | −0.18 | −0.06 | −0.40 | −0.03 | 0.27 | 0.03 | −0.15 |
Y | 0.26 | 0.08 | −0.24 | 0.15 | 0.19 | 0.13 | −0.07 |
Zn | −0.33 | −0.20 | −0.02 | 0.01 | 0.15 | −0.19 | −0.10 |
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Casey, P.; Morris, G.; Sadeghi, M. Derivation of Predictive Layers Using Regional Till Geochemistry Data for Mineral Potential Mapping of the REE Line of Bergslagen, Central Sweden. Minerals 2024, 14, 753. https://doi.org/10.3390/min14080753
Casey P, Morris G, Sadeghi M. Derivation of Predictive Layers Using Regional Till Geochemistry Data for Mineral Potential Mapping of the REE Line of Bergslagen, Central Sweden. Minerals. 2024; 14(8):753. https://doi.org/10.3390/min14080753
Chicago/Turabian StyleCasey, Patrick, George Morris, and Martiya Sadeghi. 2024. "Derivation of Predictive Layers Using Regional Till Geochemistry Data for Mineral Potential Mapping of the REE Line of Bergslagen, Central Sweden" Minerals 14, no. 8: 753. https://doi.org/10.3390/min14080753