Alteration Mineralogy of Well KW02 in the Karisimbi Geothermal Prospect, Nyabihu District, NW-Rwanda
[Rwanda Mines, Petroleum and Gas Board, Rwanda]
Alteration mineralogy of well KW02 in the Karisimbi Geothermal prospect, Nyabihu district, NW-Rwanda Ngaruye, J.C. (1) (1)Rwanda Mines, Petroleum and Gas Board, Kigali-Rwanda ABSTRACT Karisimbi is a strato-volcano of the Virunga range of volcanoes. It is located at the border between Rwanda and the D. Republic of Congo. Karisimbi is the highest of the eight major volcanoes which form a transverse chain in the western branch of the East African Rift System. With an elevation of 4507 m asl, the Karisimbi is the most voluminous of the Virunga Volcanoes Range (VVR) and located near two active volcanoes of DRC: Nyiragongo and Nyamuragira which erupted in 2002 and 2010, respectively. Karisimbi rocks constitute a nearly complete under-saturated series. The series comprise K-basanites, K-hawaiites, K-mugearites, K-benmoreites and K-trachytes with rare micro-syenite. Except for the K-trachytes, all Karisimbi rocks contain quartz-feldspar inclusions (xenoliths) remobilized from the country rock, which, slightly reacted with the magma as it is common for the rocks of the Western Branch of the East African Rift Valley. Using the K-Ar dating, it has been shown that the Karisimbi lavas are younger than 0.14±0.06 Ma and some are more recent (De Mulder, 1985). The alkalinity of the Karisimbi lavas was confirmed by Browne, P. (2011) who stated that “the rocks are strongly alkaline as shown by the high total alkalis, high Titanium dioxide (TiO2) as well as Barium (Ba), Rubidium (Rb), Tin (Sn), Lanthanum (La) and Cerium (Ce); all these trace elements are unusually high for basaltic rocks, even for alkaline rocks“. Mackay et al. (1997), using the Spaceborne Imaging Radar C (SIR-C) interferometry topography method, realized that Karisimbi lava flows are 40-60 m thick (up to 140 m) and 12 km long. Based on previous publications, the volcanic products from Karisimbi volcanic activity are essentially fluid lavas of basic to intermediate chemical composition and viscous differentiated lavas. The most recent geothermal exploration around the Karisimbi volcano in the Nyabihu District, especially the exploration well KW02, provide new data on the geology of the area (EWSA, 2014). This paper intends to emphasize on the newly gained alteration mineralogy of the Karisimbi volcanic products. The hydrothermal alteration investigation is a method which was used to insight on the geothermal reservoirs. From drill cuttings samples analysis using binocular stereo microscope, we determined not only the presence of altered minerals but also, we indicated whether the alteration was associated either with magmatic heat or related to the influence of oxidizing cold waters. Key words: Rwanda, Democratic Republic of Congo, East-African Rift System, well logging, Geothermal, alteration mineralogy, Virunga Volcanoes Range, Karisimbi volcano.
|        Topic: Exploration||Paper Number: 11110|