Overburden thickness mapping and bedrock topography of the Abitibi Greenstone Belt: implications for mineral exploration
Guillaume Rongier, Université de Lorraine-ENSG
Guillaume Allard, MRN
Olivier Rabeau, MRN
In areas buried under a thick layer of Quaternary sediments, bedrock depth and topography are important. In hydrogeology, these factors are used to assess the amount of groundwater available— whether in bedrock fractures or certain overlying Quaternary formations—and also to understand flow dynamics (Bolduc et al., 2004). In geotechnical engineering, a precise knowledge of bedrock depth is essential to ensure the stability of buildings. In geophysics, Quaternary sediments can vary the response of a site depending on their consolidation, thickness, water content or on undulations in the surface of the bedrock. For example, geophysical signal errors can be induced by the presence of valleys or depressions in the bedrock.
In mining, the study of the bedrock’s surface and the thickness of overlying unconsolidated Quaternary sediments not only helps to improve assessment of costs associated with drilling, but could also reveal new mineral resources. We already know that the deposits of the Abitibi Greenstone Belt are noted for being rich in Au, Cu, Zn and Ni. Historically, the deposits discovered and mined in this Belt were formed in the Archean. Some of them, such as volcanogenic massive sulphide (Cu - Zn) or magmatic Ni deposits, are syngenetic, meaning that they were formed at the same time as the host rock. Others, such as orogenic Au deposits, are epigenetic, that is, formed some tens of millions of years later.
Recently, saprolites have been discovered in holes drilled to study the Abitibi Greenstone Belt’s surficial deposits (Allard and Deschênes, 2011, Rongier et al., 2012). Note that saprolite is loose rock resulting from in-place chemical weathering (climate and/or hydrothermally induced) of bedrock.
These alteration zones are probably Tertiary or Quaternary in age (Kimpe et al., 1984) and supergene metal enrichment is often associated with them. Although they have economic potential, these zones are poorly documented in the Abitibi Belt. This is due to the fact that prospecting is based exclusively on the study of bedrock. The potential of the entire layer of overlying unconsolidated material, including possible saprolites, is not being investigated. Studying the depth and topography of bedrock could therefore lead to the discovery of areas with high potential for the preservation of saprolites, whose location could open up new areas for mineral exploration.
Three types of data were used: drilling data, surface data and topographic data. The data collected for this project comes from various sources, but mainly from the MRN’s SIGÉOM (geomining information system) database. Some of the data also comes from the Geological Survey of Canada (GSC) and the Ministère du Développement durable, de l’Environnement, de la Faune et des Parcs (MDDEFP). In all, 416,984 data points were used for interpolation of the depth map (Figure 1).
After several tests using various interpolation techniques, the kriging method was used to generate the bedrock depth map. Using more stringent assumptions than other techniques, kriging has many advantages: it is an exact interpolator that takes into account spatial correlations between the data and minimizes estimation error. The method also provides access to this error, allowing the uncertainty of the prediction to be visualized. The spatial continuity of rock depth data has therefore been evaluated and interpolated by ordinary kriging.
For the most part, areas with significant depth to bedrock are related to eskers and known fault zones (Figure 2). The close association between significant depth to bedrock and eskers can be explained by the considerable amount of glaciofluvial sediments forming an esker and the significant deepening of the bedrock during emplacement of these geological features. The great depth to bedrock near faults, on the other hand, can be explained by the presence of a zone of damage and strong schistosity around these structures, creating more brittle areas where depressions develop in the bedrock. In addition, some faults acted as conduits for large quantities of hydrothermal fluids resulting in significant alteration zones in the study area. These zones are often rich in carbonates or white micas, which also make the host rock very brittle and more likely to produce troughs during the erosion process.
Most of the other areas with great bedrock depth that cannot be explained by the presence of faults are, as stated previously, due to the presence of buried eskers. They usually underlie Lake Ojibway clay deposits. During Quaternary mapping programs, they are identified by photo-interpretation as being continuous with eskers visible on surface. However, some areas cannot be explained by the presence of eskers or faults. This is particularly the case for the deep areas around Palmarolle, those southwest of Lebel-sur-Quévillon or those south of Val-Paradis. They occur mostly in areas where data is less abundant. They may therefore be interpolation artifacts. Further investigation will be needed to determine their origin.
Areas with high gold potential that have undergone supergene Au enrichment have been identified using the bedrock depth map. These sites were selected by superimposing the locations of faults and gold showings, and of old and existing mines. On that basis, areas with great depth to bedrock and located on mineralization-bearing faults were identified. Despite the lack of data on saprolites in the Abitibi Subprovince, these targets can already be considered as areas with interesting potential for supergene gold deposits.
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