Understanding Hydrothermal Dolostone through Combined New Stable Isotope (δMg) Analyses with Conventional Field, Petrographic and Isotopic Data

Lavoie, Denis ¹; Jackson, Simon ²; Girard, Isabelle ²
(1)Geological Survey of Canada, Natural Resources Canada, Quebec City, QC, Canada. (2) Geological Survey of Canada, Natural Resources Canada, Ottawa, ON, Canada.

The processes that form high temperature dolomites are controversial, with end-members of: 1) tectonically controlled early processes with rapid upward migration of high temperature fluids, and 2) burial-dominated late processes with regional slow migration of high-temperature brines. Magnesium stable isotope ratios in saddle dolomites, ultramafics and shales are presented and offer critical new data in the ongoing debate.

Hydrothermal saddle dolomites in eastern Canada overly diverse Precambrian and Paleozoic basements, which may have acted as Mg sources. The dolomites and potential Mg sources were chemically characterized (ICP-ES) and their δ²⁶Mg_DSM3 and δ²⁵Mg_DSM3 ratios measured (MC-ICP-MS). Column chemistry was used to purify the Mg in the digested samples prior to isotopic analysis.

The Lower Silurian dolomites (T_h of 150-200°C) are related to fluid flow along foreland faults. The Mg²⁺ is interpreted to originate from Ordovician ultramafic slivers. Near the Silurian occurrences, Lower Ordovician dolomitized slope carbonates are associated with a transpressional fault. These dolomites have yielded negative δ²⁶Mg_DSM3 values ranging from −3.2 to −1.5‰.

Middle Ordovician platform dolomites (T_h of 90 to 120°C) are associated with foreland faults that reach the Precambrian metamorphic basement. They yielded δ²⁶Mg_DSM3 ratios of −1 to −0.7‰. Lower Devonian reef with massive replacement dolomite of magmatic origin (T_h of 300 to 350°C) occurs at the junction of two transpressional faults. Even though the reef neighbours the ultramafic slivers, the dolomite has δ²⁶Mg_DSM3 ratios around −1‰.

Linear relationships between dolomite δ²⁶Mg_DSM3 and 1) δ¹⁸O_VSMOW of the fluid and 2) ⁸⁷Sr/⁸⁶Sr in the dolomite suggest a link with the nature of the fluid and its source. Linear relationships between δ²⁶Mg_DSM3 and T_h of fluid inclusions indicate a thermal kinetic effect on Mg²⁺ incorporation in the dolomite.

There are no Lower Paleozoic evaporites in these eastern Canada basins, as such Mg²⁺ cannot be derived from that source. Data from other potential Mg sources are being gathered. Lower Ordovician ultramafics are serpentinized; the altered material has a tight range of δ²⁶Mg_DSM3 values of −0.4 to −0.2‰. Lower and Upper Ordovician shales abound in the Lower Paleozoic basin. The shales have Mg isotope ratios that differ with age; the Lower Ordovician has yielded δ²⁶Mg_DSM3 values of −0.8 and +0.1‰ whereas the Upper Ordovician has given δ²⁶Mg_DSM3 values of −1.2 and −1‰.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.