Structural-Emplacement Model for Kimberlitic Diatremes in Northern Alberta

A study to evaluate structural controls on the Buffalo Head Hills kimberlites in the Peerless Lake area of north-central Alberta was initiated because of the recognition of a set of north-trending lineaments that appear to be spatially correlated with the orientation of the Loon River lowlands, irregularities in the Phanerozoic sedimentary cover, and the emplacement of several kimberlitic diatremes. The combination of aeromagnetic and Radarsat imagery, draped on a digital-elevation model, clearly depicts the relationship between kimberlite intrusions and the intersection of north- and northeast-trending fault sets observed at surface.

A structural-emplacement model constructed for the Buffalo Head Hills kimberlites is consistent with worldwide examples in which kimberlite diatremes are associated with Phanerozoic grabens that result from the frequent transcurrent and/or extensional reactivation of a deep-seated mobile zone.

The Precambrian basement in the Peerless Lake area has been greatly modified by a long tectonic history. The contact between the Buffalo Head and Utikuma subdomains of the Buffalo Head Terrane is inferred to represent a deep-seated crustal feature (mobile zone) due to the presence of:

- a differentiated horst-and-garben block structure indicative of crustal uplift;

- sharply defined, north-trending lineaments of deformed Precambrian surface and pronounced elongated, linear aeromagnetic signatures that suggest dominant structural controls;

- localized geothermal patterns and retrograde metamorphism;

- a distinct north-trending gravity low that may be associated with deep-seated topographic displacement and/or granite plutonism; and

- low d18 O values, similar to those reported from the Kimiwan Anomaly, which are believed to be indicative of extensional tectonism.

A basement to surface cross-section (approximately 1600 m, orientated west to east in the study area) shows that the Phanerozoic strata in the Peerless Lake area have been affected by structural events throughout its depositional history. The locations of three north-trending, steeply dipping, normal faults were identified. The faults appear to propagate through the entire Phanerozoic and possibly provided important pathways for the implacement of kimberlitic diatremes. Extensional reactivation events likely occurred during the Paleoproterozoic, Middle to Late Devonian, and Late Cretaceous. During the deposition of Middle to Late Devonian Woodbend and Winterburn sedimentary rocks, the faults defined the boundaries of a graben structure that coincides with the boundaries of the Loon River lowlands and has been informally named the Loon River graben. The main border fault, located on the western edge of the Loon River graben, correlates with the north-trending parallel fault sets that exist at surface.

The authors speculate that the regional distribution of the known Buffalo Head Hills kimberlites is controlled by the extension of deep, north-trending basement fault zones at the regional scale. It is further speculated that northeast-trending faults have significant impact on the location and shape of specific kimberlite bodies. To date, advanced bulk-sample testing of selected kimberlites for diamonds has occurred mainly on the western edge of the graben. This work may support the conclusion, drawn from other studies, that the most significant diamond concentrations occur on the main border fault or western shoulder of the graben.

This study shows that surficial lineaments in Cretaceous strata can direct exploration companies to tectonic zones in the Phanerozoic and basement, and therefore potential conduits for kimberlites and metalliferous hydrothermal systems. Furthermore, the structural features at the exposed surface of the Phanerozoic stratigraphic sequence reflect a complex tectonic history.