Stéphane Perrouty

Inversion and Geodiversity: Searching Model Space for the Answers

Geophysical inversion employs various methods to minimize the misfit between geophysical datasets and three-dimensional petrophysical distributions. Inversion techniques rely on many subjective inputs to provide a solution to a non-unique underdetermined problem, including the use of a priori model elements (i.e. a contiguous volume of the same litho-stratigraphic package), the a priori input model itself or inversion constraints. In some cases, inversion may produce a result that perfectly matches the observed geophysical data, but can still misrepresent the geological system. A workflow is presented here that offers objective methods to provide inputs to inversion: (1) simulations are performed to create a model suite that contains a range of geologically possible models; (2) stratigraphic variability is determined via uncertainty analysis to identify low certainty model regions and elements; (3) geodiversity analysis is then conducted to determine geometrical and geophysical extremes and commonalities within the model space; (4) geodiversity metrics are simultaneously analysed using principal component analysis to identify the contribution of different model elements toward overall model suite uncertainty; (5) principal component analysis also determines which models exhibit diverse or common geological and geophysical characteristics which (6) facilitate the selection of models as inputs to geophysical inversion. This workflow is applied to a three-dimensional model of the Ashanti Greenstone Belt, southwestern Ghana in West Africa in order to reduce the subjectivity incurred during decision making, explore the range of geologically possible models and provide geological constraints to the inversion process to produce geologically and geophysically robust suites of models. Results further suggest that three-dimensional uncertainty grids can optimize inversion processes and assist in finding geologically reasonable solutions.

Distribution and U-Pb Ages of Newly Recognized Regional-Scale Dyke Swarms of the Leo Man Craton

Paleoproterozoic basement in West Africa were distinguished via the interpretation of regional and high resolution magnetic airborne data of West African Craton (Jessell et al. 2015). Some of the dykes reach over 300 km in length and they are considered to be parts of much larger systems of mafic dyke swarms which are part of the plumbing system for Large Igneous Provinces (LIPs) and may be therefore used for the reconstruction of Precambrian supercontinents (e.g. Ernst, 2014). Five different dyke swarms in Burkina Faso, Niger, Ghana and Senegal were analysed and dated. From a petrographic and composition point of view, the mafic dykes correspond to tholeiitic basalts and are typically composed of plagioclase + clinopyroxene (augite) ± orthopyroxene (enstatite) ± olivine, and display a doleritic texture of variable grain size. Eleven ID-TIMS U-Pb ages obtained on baddeleyite (for details on the technique see Söderlund and Johansson, 2002) define five generations of Proterozoic age. The oldest, the N-S trending Libiri dyke swarm, was found in Niger, in the Libiri pit of the Samira gold mine, yielded an age of ca. 1790 Ma. The N40 Bassari swarm in Senegal was dated at ca. 1764 Ma, and is potentially linked to the 1790 Ma Libiri swarm, 1400 km away. The 300 by 400 km Korsimoro N100 dyke swarm transects central Burkina Faso and was dated at ca. 1575 Ma. Five ca. 1520 Ma ages were obtained for dykes of the Essakane swarm, three in Burkina Faso, one from Ghana and one from Senegal, and document a large extent (600 km wide and 1500 km long) and short duration of dyke emplacement. The orientation of this swarm changes from N130 in Burkina Faso and Ghana to E-W in Senegal. The Manso N350 dyke swarm in southern Ghana, which is about 400 km long and about 200 km wide, yields a preliminary age of ca 870 Ma. The range of dyke swarm ages represent an initial Proterozoic LIP barcode for this portion of the West African craton which now can be compared with the barcodes of other crustal blocks to test proposed Nuna and Rodinia reconstructions involving the West African craton.

Publisher Correction to: Expanding the size of multi-parameter metasomatic footprints in gold exploration: utilization of mafic dykes in the Canadian Malartic district, Québec, Canada

The original version of this article contained a mistake.

SPECTRAL INDUCED POLARIZATION SIGNATURES OF ALTERED METASEDIMENTARY ROCKS FROM THE CANADIAN MALARTIC GOLD DEPOSIT BRAVO ZONE, QUÉBEC, CANADA

At the Canadian Malartic deposit, gold mainly occurs as fine inclusions in 1 μm to 1 mm pyrite grains hosted in altered metagreywacke. Previous time-domain induced polarization surveys conducted over this deposit have, however, failed to delineate known areas of pyrite alteration in the metasedimentary host rocks. To define the petrophysical footprint of the gold mineralization using an alternative approach, spectral induced polarization (SIP) measurements were conducted both on drill core samples in the laboratory and in situ, at a key outcrop zone. Three groups of SIP spectra are identified and interpreted using Mineral Liberation Analysis of polished thin sections and by performing Bayesian inference of SIP parameters using a Debye decomposition model. Values of Debye mean relaxation time (τ) and total chargeability (Σm) are used to characterize the respective SIP signatures of ilmenite-rich mudstones and monzodiorite (τ = 0.10 s and Σm = 17%), mineralized metagreywacke (τ = 0.05 s and Σm < 10%), and barren metagreywacke (τ < 0.03 s with variable Σm).