Print this pageMassive Dolomitization of a Pinnacle Reef in the Lowermost Devonian West Point Formation (Québec - Canada) — From Extreme Case of Hydrothermal Dolomitization Through Fault-Controlled Circulation of Magmatic Fluids
Lower Devonian pinnacle reefs of the West Point Formation in Gaspé Peninsula (Québec, eastern Canada) were built on paleotectonic highs in an Early Devonian foreland basin. These large structures are found along Acadian (Early-Middle Devonian) transpressive faults in northern Gaspé. Of the nine pinnacles known in outcrop, only one is dolomitized and occurs at the junction of two major Acadian faults.
The petrography of the dolomitized facies has revealed the presence of three dolomite and one late calcite phases. A first dolomite phase of small crystals is minor; the following dolomite dominates and consists of large (cm-sized) replacive saddle dolomite crystals that carry fluid inclusions with homogenization temperatures ranging between 301 to 382°C. The third dolomite consists of large (mm to cm-sized) saddle dolomite crystals that fill fractures that cut through the previous dolomite, this phase is characterized by lower temperature fluid inclusions (159 to 171°C). A lower temperature calcite phase (107-123°C) fills some void space. Fluid inclusions in these phases are all saline (8.7 to 13.3 wt% NaClequiv.). The dolomite and calcite phases are characterized by very negative δ18OVPDB values (between -19‰ to -14‰) and negative δ13CVPDB values (between -1‰ to -8‰). The main dolomite phase originated from a fluid that had very positive δ18OSMOW values (+8‰), the following dolomite and calcite precipitated from fluids with lower δ18OSMOW values (+3.4‰ and +4.5‰). It is proposed that fault-controlled circulation of magmatic fluids are responsible for the very high temperature massive dolomite replacement of the calcite host. This is backed by 87Sr/86Sr ratios of the replacement high temperature dolomite that are consistent with fluids derived from A-type magma. High temperature burial fluids later used fractures to circulate in the dolomitized host to precipitate late dolomite and calcite. Regionally, hydrocarbon migration is recognized at the time of late calcite cementation.
This very high temperature dolomitization is not reported in the hydrocarbon literature and provides a link with process recognized in the mining literature.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil