Satulinmäki Au Prospect Structural Mapping

Transcription

1 M19/2024/2003/1/10 Juhani Ojala Satulinmäki Au Prospect Structural Mapping V. Juhani Ojala Geological Survey of Finland

2 1 Table of Contents Introduction...2 Local geology...2 Structures...2 Discussion...2 References:...3

3 2 Introduction The author visited the Satulinmäki area in June th The lithological mapping of the area has been done before by Perälä (2003). The purpose was to map the possible structural controls on gold mineralization. The Satulinmäki area is located 5 km south of Jokioinen on the national mapsheets and (Fig.1.). Geologically the area is located in the western part of the Svecofennian Häme schist belt (Fig. 2.). Age of the rocks in the area vary from 1.9 to 1.8 Ga (Korsman et al. 1997) Local geology The main rock type in the Satulimäki area is an intermediate volcanic rock (Fig. 3.). Felsic and mafic volcanic rocks intercalations are minor but common. Mica schists occur in the northern side of the Satulinmäki area. The layering and lithological contacts are broadly E-W trending and steeply south dipping. The rocks in the area have been metamorphosed at amphibolite facies conditions. The gold mineralization at the area is associated with arsenopyrite, pyrite and pyrrhotite (Perälä 2003). Structures Layering is best preserved in felsic fine-grained pyroclastic rocks (Fig. 4). Strong SW-NE trending foliation cuts the layering. In the Kalpea database bedding and foliation are commonly mixed but the same bedding-foliation relationship is clear from the Kalpea database records as well (Fig. 3. b). Small dextral displacements along the shear foliation are common; in places quartz veins are within the shear foliation (Figs 5 and 6). The vein geometry indicates also dextral shear sense (Figs. 7). In addition to fracture fill veins, there are tension gashes, which are all indicative of dextral shear (Figs ). Tension gash geometries vary from the simple initial stage single gashes (Fig. 8.) to more complex rotated veins in the most strongly deformed and veined areas (Fig. 11.). In the felsic dyke, in addition to tension gashes, quartz veins have formed in a more brittle manner (Fig. 12). Tension gash vein geometry is common in brittle ductile shear zones and the models of their development are well known (Fig. 13) (eg. Ramsay and Huber 1983). Although SW-NE trending dextral shearing the dominant feature at the outcrop scale, in places, conjugate SE-NW trending sinistral shear fractures have developed (Fig. 14). In addition to steeply dipping quartz veins there are some shallowly northerly dipping quartz veins but it is not known if these are mineralized. Discussion The overall geometry indicates that the Satulinmäki area is within a SW-NE trending dextral shear zone (Fig. 15). Unfortunately, the rocks in the area are relatively weakly magnetic and the resolution of the aeromagnetic maps available at the time do not resolve the internal structure of the shear zone without further processing. Consequently, this knowledge does not directly help to locate more intensively veined and mineralized areas. However, it is clear from mapping that the gold mineralisation is unlikely to be a simple E-W trending continuous zone. It is more likely to be controlled by the intersections between bedding/lithological trend and the shear foliation trend with higher grade shoots plunging subvertically or steeply to SW (Fig. 16.). Consequently, the plunge of the geophysical anomalies (IP) should be taken into account when defining the drill targets. Digital elevation model and the aeromagnetic data suggest that at a larger scale the Satulinmäki area is in the intersection zone of NW-SE, N-S and NE-SW trends (Figs. 17 and

4 3 18). However, the resolution of these datasets is not high enough to show the trends at the prospect scale. References: Korsman, K. (ed.); Koistinen, T. (ed.); Kohonen, J. (ed.); Wennerström, M. (ed.); Ekdahl, E. (ed.); Honkamo, M. (ed.); Idman, H. (ed.); Pekkala, Y. (ed.) Bedrock map of Finland 1: Espoo: Geologian tutkimuskeskus. Neuvonen, K.J Forssa 1: geological map. Perälä, A Someron Satulinmäen Au-esiintymän geologia, geokemia ja mineralogia. Pro gradu tutkielma. p. 81. Ramsay, J. G.; Huber, M. I The techniques of modern structural geology; Volume 1: Strain analysis. Academic Press, London, pp Simonen, Ahti Somero 1: geological map.

5 4 M19/2024/2003/1/10 Satulinmäki mapping Figure 1. Location of the Satulinmäki area Figure 2. Location of the Satulinmäki area on the 1: scale geological map (lower map sheet by Simonen 1955, upper mapsheet by Neuvonen 1954).

6 5 a) b) Figure 3. Outcrop lithologies a) and bedding and foliation of the Satulinmäki area.

7 6 S0 S2 Figure 4. Well preserved layering (S0, E-W trending) and cross cutting shear foliation (S2, SW-NE trending) in felsic pyroclastic rock. ( N, E) Qz vein in S2 S2 Figure 5. Detail of Fig. 4. showing quartz vein filling in S2 plane. Note the small displacements (dextral) along S2 shear foliation.

8 7 Qz vein in S2 Figure 6. Dextral displacements along S2 shear foliation which is in placed quartz vein filled. ( N, E). Figure 7. Quartz vein filling a shear fracture indicating dextral displacement although the drag of the bedding suggest sinistral movement. This could be a reactivated normal fault.

9 M19/2024/2003/1/10 Satulinmäki mapping Figure 8. Tension gash quartz vein indicating dextral shear ( N E). Figure 9. Sigmoidal quartz vein indicating dextral shear. ( N, E) 8

10 9 Figure 10. Dextral tension gash in felsic pyroclastic rock ( N, E). Figure 11. Deformed quartz vein tension gashes in felsic pyroclastic rock ( N, E).

11 10 Figure 12. Quartz-tourmaline veins and tension gashes in a felsic dyke ( N, E) Figure 13. Model of the tension gash development.

12 11 Figure 14. Conjugate quartz-tourmaline filled shear fractures in a mafic volcanic rock. Figure 15. The main structural trends of the Satulinmäki area.

13 12 Qz veins S2 S0 Qz veins Potential oreshoot plunge Figure 16. Strereoplot of the mean bedding (S0), shear foliation (S2) and quartz vein orientations in the Satulinmäki area. Most of the intersections, which are potential ore shoot plunge directions, between these fabrics are subvertical. Figure 17. Shaded relief magnetic map of the Satulinmäki environs. Note that the Satulinmäki area is in the intersection zone of several trends. The yellow lines mark trends which are also evident on the digital elevation model.

14 13 Figure 18. Shaded relief digital elevation model of the Satulinmäki environs. The main topographic trends marked as yellow dotted lines.