Gravity and magnetic study of the Luusika potential field anomaly
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The high positive gravity anomaly values discovered by Estonian Land Board in 2010-2011 indicated gravity increase of 6.3 mGal in Luusika area. The ground-based magnetic survey identified magnetic anomaly within Luusika area and gave new information of the underlying rock unit magnetic properties. The ground-based magnetic measurements showed good correlation to the observed Bouguer anomaly. The depth estimation was done on the basis of Bouguer anomaly profiles. The calculations suggested that Luusika rock unit does likely not outcrop under sedimentary cover. The mean anomaly-mass center point (z) was estimated to be 2500 m whereas the calculated result of maximum limiting depth (zT) of the body top is 3000 m. The data were sufficient for creating models of causative source and testing different lithologies. For magnetic modeling the orientation of remanent magnetization was necessary to produce reliable model. Remanent magnetization was characterized by age-appropriate direction. The lithologies of Alutaguse domain and post-orogenic and anorogenic intrusions were simulated. Conclusions of the modelling are: 1. The depth (zT) to the top of the body is 600 ... 1800 m. 2. The density of Luusika Bouguer anomaly causative source is 2760 … 2920 kg/m3. 3. Magnetic anomaly is produced by rock unit with very high magnetic susceptibility values of = 20000 … 56000 × 10-6 SI compared to the background. Similar petrophysical properties are documented for post-orogenic and anorogenic plutons occurring in Estonian basement. The Luusika anomaly lies within the Middle-Estonia fault zone, which hosts several post-orogenic intrusions; as a result, it could be interpreted as Taadikvere-like rock unit. Also, a few rapakivi related intrusions (Abja and Sigula) discovered in Precambrian basement of Estonia could be proposed to be the source of the potential field anomalies. The modeling and comparison of petrophysical properties of lithologies suggest that Luusika causative source is intermediate to mafic rock by composition, similar to Abja, Sigula, or Taadikvere and probably related to presence of Middle-Estonian fault zone.