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Gold mineralisation during orogenic extension

A4   |   Letter

SRK News | Issue 57Exploration Geology: Keys to success

Blair Hrabi, Principal Consultant, Structural Geologist  

SRK applies rigorous geological and structural interpretations to provide insight for exploration targeting, constraints on resource modelling, and support for geotechnical evaluations. Interpreting the structural setting during mineralisation is critical for understanding the geometry of the system. Fault-fill (i.e. shear) veins in orogenic gold deposits are often associated with active reverse slip fault systems (Robert and Poulsen 2001; and their references) but this is not universally the case. SRK has been involved in analysing two Paleozoic orogenic gold deposits in Ireland and Canada where gold bearing fault-fill veins developed within normal fault systems.

The Curraghinalt gold deposit (Dalradian Resources Inc.) in Northern Ireland, hosted in Paleoproterozoic metasedimentary rocks, is cut by a series of moderately northeast-dipping, quartz-pyrite-carbonate fault-fill veins. Previous workers recognised that these post-date the main Ordovician Grampian orogeny compressional deformation events and movement on the regional Omagh Thrust Fault, but the tectonic setting during vein formation has been debated. Vein formation and gold mineralisation was a multi-staged process with evidence for an early compressional component. However, several observations suggest the gold mineralisation associated with the fault-fill veins was primarily emplaced during oblique dextral-normal transtensional deformation during later orogen-oblique extension. These include the geometry of a pervasive set of steeply dipping, northwest-striking extension veins associated with the fault-fill veins, mineralised extension veins inflating the fault fill veins, combined with dextral-normal kinematic indicators in the faults and a compatible northeast plunging mineral lineation.

The Valentine Lake gold property (Marathon Gold Corp.) in central Newfoundland comprises four deposits occurring along the boundary between the Neoproterozoic Valentine Lake intrusive complex and the Silurian Rogerson Lake conglomerate. These are juxtaposed along the Valentine Lake Thrust Fault, a major tectonic feature formed during the Silurian Salinic Orogeny (Appalachian Orogeny). Gold mineralisation is hosted in quartz-tourmaline-pyrite veins, veinlets and stock-works forming a stacked, shallow-dipping set of extension veins associated with more extensive fault-fill veins parallel to the Valentine Lake thrust fault. The bulk of gold mineralisation is associated with the main compressional orogeny. One of the deposits, however, contains a significant set of fault-fill veins with normal movement kinematic indicators oriented perpendicular to and cutting the main shear veins, suggesting formation during a later, orogen-parallel extensional event.

Gold mineralisation in these two orogenic systems was at least partly emplaced during later extensional deformation phases, post-dating the main compression, and possibly representing effects of an extensional collapse of each orogen. Orogenic gold systems are typically complex and long-lived, and careful mapping and interpretation of the entire set of structural features present is needed to understand these systems. Evaluating the geometry and movement sense of all structural features present allows for the accurate prediction of the mineralised system for both efficient exploration and confident resource estimation.

Blair Hrabi:

Reference: Robert, F., and Poulsen, K.H. 2001. 
Vein formation and deformation in greenstone gold deposits. SEG Reviews, v.14, p.111–155.


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