Abstract
Unmanned aerial vehicles (UAVs) are rapidly growing remote sensing platforms for capturing high-resolution images of exposed rock surfaces. We used a DJI Phantom 3 Professional (P3P) quadcopter to capture aerial images that were used to generate a high-resolution three-dimensional (3-D) model of the Jokisivu open-pit gold deposit that is located in southwestern Finland. 158 overlapping oblique and nadir images were taken and processed with Agisoft Photoscan Pro to generate textured 3-D surface models. In addition, 69 overlapping images were taken from the steep faces of the open pit. We assessed the precision of the 3-D model by deploying ground control points (GCPs) and the average errors were found minimal along X (2.0 cm), Y (1.2 cm), and Z (5.0 cm) axes. The steep faces of the open pit were used for virtual structural measurements and kinematic analyses in CloudCompare and ArcGIS to distinguish the orientation of different fracture sets and statistical categorization, respectively. Three distinct fracture sets were observed. The NW-SE and NE-SW striking fractures form a conjugate geometry, whereas the NNW-SSE striking fractures cut the conjugate fracture set. The orientation of conjugate fractures match well with the resource model of the deposit and NW- and NE-trending segments of regional-scale anastomosing shear zones. Based on the conjugate geometry of fracture sets I and II, and the regional pattern of anastomosing shear system lead us to interpret an origin of gold mineralization in two stages. An early N-S or NNW-SSE crustal shortening, corresponding to the regional D4 (ca. 1.83–1.81 Ga) or pre-D4 (ca. 1.87–1.86 Ga) Svecofennian tectonic event(s) that produced anastomosing shear zones. Subsequent E-W directed D5 contraction (ca. 1.79–1.77 Ga) partly reactivated the anastomosing shear zones with the formation of conjugate system, which controlled the migration of fluids and gold mineralization in SW Finland.
Highlights
With the invention and rapid development of small, lightweight, civilian unmanned aerial vehicles (UAVs), close-range, high-resolution digital photogrammetry is becoming a popular technique in geoscience mapping and applications [1,2]
The combined ‘structure-from-motion’ and ‘multi-view stereopsis’ (SfM-MVS) workflow allows for building high-resolution, three-dimensional (3-D) models of rock faces that can be used for virtual structural analysis of planar structures [7,8,9,10]
The 3-D models were imported in CloudCompare, which can deal with such kind of data formats, for detailed virtual structural analysis and segmentation of well-exposed surfaces
Summary
With the invention and rapid development of small, lightweight, civilian unmanned aerial vehicles (UAVs), close-range, high-resolution digital photogrammetry is becoming a popular technique in geoscience mapping and applications [1,2]. Our workflow steps are, as follows: (1) capturing aerial images with the P3P; (2) their processing in PhotoScan; (3) virtual structural and facets analysis using CloudCompare; and, (4) further categorization with ArcGIS (Figure 2). We applied this workflow for detailed structural analyses, as we illustrate with a case study of the Paleoproterozoic Jokisivu gold open-pit mine of SW Finland. The localization of known gold deposits and prospects approximately matches this distribution of these regional scale shear zones and leads us to propose an origin of syntectonic gold mineralization in the region in two stages and it is discussed below This tectono-metallogenic model potentially provides a new framework for exploration strategies in the area
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