Diagenesis and dolomitization in the Contrin and the Sciliar Formation (Latemar, northern Italy).

Carl Jacquemyn*, Anneleen Foubert*, Rudy Swennen*, Dave Hunt**, Giulio Casini**

* Department of Earth and Environmental Sciences, K.U.Leuven, Leuven, Belgium

** Statoil – TNE, RD, Research Center Bergen, Norway

Dolomitization of the Latemar carbonate buildup has been studied by several authors in the past decades. Mainly the large scale geometry of the massive dolomite body (Wilson, 1990) and more refined delineation of the dolomite occurrence and geochemistry (Carmichael, 2006) on the upper Latemar platform was considered. Our research focuses on the dolomitization in the lower platform facies and the underlying Contrin Formation which was not studied in detail yet. Apart from the main massive dolomite body, several dolomitized pipe-like structures occur. The latter crosscut both the Contrin and the Sciliar carbonates (Latemar Formation). The origin of these massive and vertically oriented pipe structures is part of this ongoing petrographical, geochemical (stable O- and C-isotopes, Sr & Mg-isotopes) and petrophysical (poroperm) study.

Dolomitization in the Contrin Formation does not all occur in a single massive dolomite mass, but also in smaller (10-50m diameter) bodies, within limestone host rock. At small scale (<0,5 m) the boundary with the adjacent limestone is sharp, but on larger scale it is observed that, close to the dolomite-limestone contact, dolomite patches (>1 m diameter) occur within limestone and vice versa. Different types of dolomite are observed, mainly based on colour and crystal size. The dolomite is typically coarse crystalline and often is very pervasive. However, large vugs (1 cm to 25 cm) exist, which often are horizontally aligned. Locally, the dolomite is also severely fractured, in contrast to the limestone. The surrounding limestone has been severely recrystallized, as testified by its coarse crystalline nature and the complete obliteration of the original sedimentary features.

Three main diagenetical phases can be recognized associated with these dolomite bodies, i.e. 1) main dolomitization phase defining dolomite bodies. Zoning in dolomite crystals observed within cathodoluminescence suggests multiple fluid pulses. 2) Recrystallization of the original limestone to coarse crystalline limestone. This could be related to the dolomitization phase (e.g. in relation to the exhaustion of Mg in the hot fluids). 3) Inside the existing vugs late baroque saddle dolomite occurs followed by partial calcite cementation.

Stable isotope results show little variation in δ13C (+2,5‰ to +4‰) but large variation in δ18O (-2,5‰ to -14‰). Stable isotope results of some dolomite bodies are more δ18O-depleted than others suggesting variable temperature during dolomitization. Results of late dolomite and calcite cement phase show a linear trend toward the most depleted δ18O values (combined with δ13C-depletion). They most likely formed at higher temperatures than their surrounding host rock (dolomite and limestone). The relationship with large and small scale magmatic intrusions (mafic dikes) and the control on the petrophysical and geochemical characteristics is the focus of the current research.

Carmichael, S.K., 2006, Formation of replacement dolomite by infiltration of diffuse effluent: Latemar carbonate buildup, Dolomites, northern Italy. Baltimore, Maryland, Johns Hopkins University, Ph.D. thesis, 218p.

Wilson, E.N., Hardie, L.A. and Philips, O.M., 1990, Dolomitization front geochemistry, fluid flow patterns, and the origin of massive dolomite: The Triassic Latemar buildup, northern Italy. American Journal of Science, v. 290, p. 741-796.