the Creative Commons Attribution 3.0 License.
the Creative Commons Attribution 3.0 License.
Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data
Magnus Andersson
Alireza Malehmir
Abstract. The Alnö complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the world. Presented here is the 3D inversion of ground gravity and aeromagnetic data that confirms some of the previous ideas about the 3D geometry of the complex but also suggests that the complex may continue laterally further to north than earlier expected. The gravity and aeromagnetic data show the complex as (i) a strong positiver Bouguer anomaly, around 20 mGal, one of the strongest gravity gradients observed in Sweden, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT. Magnetic structures are clearly discernible within the complex and surrounding area. Petrophysical measurements (density, bulk magnetic susceptibility, and magnetic remanence) were used to constrain the 3D inversion. Both gravity and magnetic inversion models suggest that dense (> 2850 kg/m3) and magnetic (> 0.05 SI) rocks extend down to about 3.5–4 km depth. Previous studies have suggested a solidified magma reservoir at this approximate depth. The inversion models further suggest that two apparently separate regions within the intrusion with gravity and magnetic highs are likely connected at depth, starting from 800–1000 m, implying a common source for the rocks observed in these two regions. The modelling of the aeromagnetic data indicates that a more than 3 km wide ring-shaped magnetic high in the bay that can be a hidden part of the complex, linking a satellite intrusion in Söråker on the northern side of the bay to the main intrusion on the Alnö Island. While the rim of the ring must consist of highly susceptible rocks to support the magnetic anomaly, the centre has a relatively low magnetisation and is probably made up of low-susceptible wall-rocks or metasomatised wall-rocks down to about 2 km. Below this depth the 3D susceptibility model shows higher magnetic susceptibility values. From these observations the solidified magma chamber is interpreted to extend further to north than has previously been suggested.
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Magnus Andersson and Alireza Malehmir


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RC1: 'Andersson and Malehmir', Anonymous Referee #1, 17 Mar 2017
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AC1: 'Comments from RC1 Anonymous referee with response and comments from the authors', Magnus Andersson, 20 Aug 2017
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AC1: 'Comments from RC1 Anonymous referee with response and comments from the authors', Magnus Andersson, 20 Aug 2017
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RC2: 'Reviewer 2. Roberto Weinberg', Roberto Weinberg, 15 May 2017
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AC2: 'Comments from RC2 with response and comments from the authors:', Magnus Andersson, 20 Aug 2017
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AC2: 'Comments from RC2 with response and comments from the authors:', Magnus Andersson, 20 Aug 2017
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EC1: 'Final decision', Roberto Weinberg, 15 May 2017


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RC1: 'Andersson and Malehmir', Anonymous Referee #1, 17 Mar 2017
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AC1: 'Comments from RC1 Anonymous referee with response and comments from the authors', Magnus Andersson, 20 Aug 2017
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AC1: 'Comments from RC1 Anonymous referee with response and comments from the authors', Magnus Andersson, 20 Aug 2017
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RC2: 'Reviewer 2. Roberto Weinberg', Roberto Weinberg, 15 May 2017
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AC2: 'Comments from RC2 with response and comments from the authors:', Magnus Andersson, 20 Aug 2017
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AC2: 'Comments from RC2 with response and comments from the authors:', Magnus Andersson, 20 Aug 2017
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EC1: 'Final decision', Roberto Weinberg, 15 May 2017
Magnus Andersson and Alireza Malehmir
Supplement
Magnus Andersson and Alireza Malehmir
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Cited
2 citations as recorded by crossref.
- Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola) S. Amores-Casals et al. 10.3390/min10010005
- 3D Modeling of the Epembe (Namibia) Nb-Ta-P-(LREE) Carbonatite Deposit: New Insights into Geometry Related to Rare Metal Enrichment G. Unger et al. 10.3390/min8120600