Spatial Analysis of Regional-scale Controls on VMS Mineralization, Skellefte District, Sweden Martiya Sadeghi Geological Survey of Sweden, Uppsala, SE-751 28, Sweden Emmanuel John M. Carranza International Institute for Geo-Information Science and Earth Observation, Enschede, 7514AE, The Netherlands Abstract. In the Skellefte district (Sweden), the relationship of volcanogenic massive sulphide (VMS) deposits with structures and lithostratigraphic units is still not well understood. Fry analysis of the spatial pattern of the VMS deposits and weights-of-evidence analysis of the spatial associations of the VMS deposits with deformation zones and lithostratigraphic units suggest strong structural and lithostratigraphic regional-scale controls on VMS mineralization in the Skellefte district. Keywords. Point pattern, Fry plots, weights-of-evidence, spatial association, shear zones, lithostratigraphy, GIS. 1 Geology of Skellefte district The Skellefte district (SD), a loosely defined area of ca. 150 km by 50 km in northern Sweden, is situated in the transition zone between the Bothnian Basin in the south and terranes of mainly marine supracrustal rocks and terrestrial island-arc assemblages in the north that are divided into the Skellefte, Vargfors and Arvidsjaur Groups (Fig. 1). In the SD, epiclastic and turbiditic sedimentary rocks interfinger with the subaqueous volcano-sedimentary rocks of the Skellefte Group (SG) and the Vargfors Group (VG). The subaqueous volcanosedimentary rocks of the SG pass stratigraphically upwards and topographically northwards into the subaerial volcanic rocks of the Arvidsjaur Group (AG), which are marine equivalents of the volcanosedimentary rocks of the VG. The VG consists mainly of coarse clastic and turbiditic sedimentary rocks deposited on the SG and BS and on the lower parts of the AG (Kathol and Weihed, 2005). The southwestern and eastern parts of the SD are underlain by intrusive rocks of the Transscandinavian Igneous Belt. The deformations and metamorphism in the SD are mainly related to the Svecofennian Orogen (Bergman Weihed 2001). The deformations include folds, penetrative WNW-ESE to E-W cleavages and ductile shear-zones. The shear zones cut the late post-orogenic granitoids. The youngest shear zones trend N-S (Bergman Weihed, 2001). The WNW-ESE striking Skellefte Shear Zone (SSZ) in the central part of the SD has mainly oblique-slip movements (Malehmir et al., 2006) and its south side up is commonly up-thrown (Bergman Weihed, 2001). The SD has long been regarded as a volcanogenic massive sulphide (VMS) district. There are 72 known occurrences of VMS deposits in the district, more than 20 of which have been mined in the past and five of which are being mined at present. The characteristics of the VMS deposits in the SD are strongly similar to Kuruko-type VMS deposits (Weihed et al., 2005). The VMS deposits in the SD and in the whole of the Fennoscandian Shield were formed between 1.97 and 1.88 Ga in extensional settings prior to basin inversion and accretion (Weihed et al., 2005). That the VMS deposits in the SD were formed in a strongly extensional intra-arc that developed in a continental or mature arc setting rather than in primitive volcanic arc setting is evidenced by the stratigraphic architecture, range of volcanic compositions and abundance of rhyolites (Allen et al., 2002). Geophysical modeling suggests that, in the western part of the SD, VMS deposits occur on the northern limb of a regional E-W striking syncline (Malehmir et al., 2006). The VMS deposits occur in either the SG or VG, although they commonly occur near the upper horizons of the SG. In this paper, we present results of spatial analyses in order to infer controls on VMS mineralization in the SD. 2 Spatial pattern analysis Figure 1. Simplified geological map of the Skellefte district (from Kathol and Weihed, 2005). Proceedings of the Tenth Biennial SGA Meeting, Townsville, 2009 We performed Fry analysis in order to study the spatial pattern of the VMS deposits in the SD. Fry analysis (Fry 1979) is a geometrical method of spatial autocorrelation analysis of a type of point objects, like mineral deposits 845 Figure 2. (a) Fry plots (black dots) of 72 VMS deposits (green dots). Rose diagrams of (b) of all pairs of Fry plots and (c) only pairs of Fry plots within 75 km of each other. n = 72. on regional-scale maps. For n number of points, n2-n number of so-called Fry points created. Thus, if there are subtle patterns in a set of point objects in terms of spacing and orientation, a plot of Fry points enhances such patterns. A rose diagram can be created for (a) all pairs of Fry points and (b) pairs of Fry points within a specified distance. The former case may reveal trends due to processes that operated at, say, a regional scale, whereas the latter case may reveal trends due to processes that operated at, say, a prospect scale. Previous works using Fry analysis to study mineralization controls are described in Vearncombe and Vearncombe (1999) and Carranza (2008, 2009). The Fry points of the 72 occurrences of VMS deposits in the SD show a primary 100-110º azimuth trend and secondary trends of 90-100º and 110-120º (Fig. 2a,b), suggesting regional-scale controls by WNW-ESE trending geological features. Analyses using different distance separations between Fry points were performed in order to determine whether the 72 points represent a homogenous population of VMS deposits or if they constitute different groups of deposits. The highest changes in frequencies of trends between Fry points occurred when distances between 75 km and 150 km were used. The latter is the distance within which, from any of the VMS deposits, there is maximum probability that all the other VMS deposits are present. The rose diagram for Fry points within 75 km of each other show two main directions of 100-110º and 130-140º, and one secondary direction of 40-50º. The two main directions suggest regional-scale controls by NW-SE to WNW-ESE trending geological features, whereas the secondary direction suggests prospect-scale controls by NE-SW trending geological features. 3 Spatial association analysis We applied weights-of-evidence (WofE) analysis in order to quantify spatial association between a map of points (i.e. mineral deposit locations) and a map of linear features (e.g., faults of certain trends) or a map of polygonal features (e.g., lithologic units of the same type). For a map of linear features, a multi-class map based on cumulative increasing distances (or buffer zones) is created. For a map of polygonal features, each 846 polygon is considered a binary (presence-absence) pattern. For each class or spatial pattern, P, in a binary or a multi-class map, the type and magnitude of its spatial association with a set of points, D, can be characterized by calculation of a spatial statistic called contrast (C) (for details of estimation see Bonham-Carter 1994). The value of C is related to the area of P (denoted as N(P)) and to the number of points contained in P (denoted as N(DP)). If C>0, then there is positive spatial association between P and D; whereas if C<0, then there is negative spatial association between P and D. A positive spatial association is of interest because it implies that the occurrence of D is ‘dependent’ on (i.e. controlled by) P. The statistical significance of spatial association can be determined by calculating Studentized C, which is the ratio of C to its standard deviation. A Studentized Ct2 indicates a statistically significant positive spatial association. There is a variety of regional-scale geoscience data sets available and suitable for VMS prospectivity mapping in the SD. Of these data sets, we quantified spatial association of the VMS deposits with the different lithostratigraphic units in the SD, the SG-VG contact, and different sets of shear zones according to their trends. For each of the linear geological features, we sought the pattern representing the largest buffer distance within which the Studentized C is at least 2. In the WofE analyses, we used a raster-based GIS and a pixel size of 200 m for the spatial representation of the VMS deposits and the geological features. 3.1 VMS – shear zone association Shear zones have been mapped in the field or interpreted from aeromagnetic data. We compiled the mapped and interpreted shear zones and then classified them into six classes according to their trends: NNE (0-30º); NE (3060º); ENE (60-90º); WNW (270-300º); NW (300-330º); NNW (330-360º). The results of the WofE analysis show that the VMS deposits have statistically significant positive spatial associations with WNW-, NW- and ENE-trending shear zones (Table 1). At least 70% (t51) of the VMS deposits are within 3.7 km of each of these three sets of shear zones. The pattern formed by a 3.16 km buffer around WNW-trending shear zones has the highest positive value of Studentized C, followed by the pattern formed by a 3.23 km buffer around NW-trending shear zones and then by the pattern form by a 3.7 km buffer around ENE-trending shear zones. 3.2 VMS – SG-VG contact association We reclassified the 1:250,000 scale lithostratigraphic map of the SD and surrounding areas (Kathol and Weihed, 2005) according to groups in order to extract the contact between the SG and VG. We considered the SG-VG contact for analysis because most of the VMS deposits are known to occur in lithologic units in the uppermost horizons of the SG, which is overlain by the VG. The results of the WofE analysis show that 67 (or about 93%) of the 72 VMS deposits occur within 12.02 km of the SG-VG contact (Table 1). This implies that the VMS deposits have statistically significant positive "Smart Science for Exploration and Mining" P. J. Williams et al. (editors) Table 1. Skellefte district: spatial patterns (P) of geological features having statistically significant positive spatial associations with VMS deposits as quantified by weights-ofevidence analysis. Studentized C is calculated using (see Bonham-Carter 1994): N (P), area of a pattern expressed in number of pixels; and N (DP), number of pixels in a pattern (P) that contain deposits (D). Spatial pattern (P) N(P) N(DP) Studentized C Shear zones: WNW (0.00-3.16 km) 129736 63 3.188 NW (0.00-3.23 km) 171867 60 2.773 ENE (0.00-3.70 km) 179928 51 2.282 Stratigraphic contact: SG-VG (0.00-12.02 km) 130369 67 2.404 Lithostratigraphic units: SG felsic volcanics SG sediments SG mafic volcanics 31952 5006 8879 48 10 6 12.923 7.717 3.458 of the SG and the VG is approximately the same or that the difference in the timing of their deposition is less than 5 million years. That is because, although most of the VMS deposits occur on the upper horizons (i.e., felsic volcanics) of the SG, there are few occurrences of VMS deposits in the lower horizons of the VG. The analysis of the spatial pattern of mineral deposits and the analysis of their spatial associations with certain types of geological features could provide insights into mineralization controls, which could be useful in mapping of prospectivity for the type of mineral deposits sought. Therefore, regional-scale prospectivity for VMS deposits in the SD can be modelled based on proximity to WNW-, NW- and ENE-trending shear zones, proximity to the contact between the SG and the VG, and presence of felsic volcanics of the SG. Acknowledgements spatial association with the SG-VG contact. The first author is thankful to the Geological Survey of Sweden for funding of internal research project No35137. 3.3 VMS – lithology association References Based on lithostratigraphic map of the SD and surrounding areas (Kathol and Weihed, 2005), about 64 (or about 89%) of the 72 VMS occur in the SG (Table 1). This observation and the results for the SG-VG contact indicate that the VMS deposits occur in the upper horizons of the SG below its contact with the VG. The results further show that the VMS deposits have highest statistically significant positive spatial association with the felsic volcanics in the SG, followed by the sediments and then the mafic volcanics in the SG. 4 Discussion and conclusions The results of the WofE analysis of the spatial associations of the VMS deposits with classes of shear zones according to trends (Table 1) are relatively consistent with the results of the Fry analysis (Fig. 2). This means that Fry analysis and WofE analysis are complementary tools for study structural controls on certain types of mineralization, in this case VMS deposits in the SD. Thus, the principal WNW trends in the spatial distribution of the VMS deposits in the Skellefte distributions are plausibly due to regional-scale structural controls provided by shear zones with mainly WNW trends, which in many cases switch to either NW or ENE (Bergman Weihed, 2001). Thus, it is highly likely that the Skellefte Shear Zone is implicated in VMS mineralization in the district. Nevertheless, the principal WNW trends in the spatial distribution of VMS deposits in the SG are also plausibly due to the regionalscale lithostratigraphic controls provided mainly by the felsic volcanics of the SG. The felsic volcanics and the other units in the SG are disposed along a WNWtrending belt in the district. The result of the WofE analysis of the spatial associations of the VMS deposits with the contact between the SG and the VG supports the proposal of Kathol and Weihed (2005) that the timing of deposition Proceedings of the Tenth Biennial SGA Meeting, Townsville, 2009 Allen RL, Weihed P, Global VHMS Research Project Team (2002) Global comparison of volcanic-associated massive sulphide districts. In: Blundell, D.J., Neubauer, F., von Quadt, A. (eds.) The Timing and Location of Major Ore Deposits in an Evolving Orogen. 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