Spatial statistical analysis applied to rare-elements LCT-type pegmatite fields: an original approach to constrain faults-pegmatites-granites relationships
Résumé
The emplacement of LCT (Lithium-Cesium-Tantalum) - type pegmatite fields and their relationships with hosting rocks are commonly studied with petrographic, geochemical and isotopic analyses. Although these methods are efficient to understand the process of differentiation and/or enrichment in rare-elements during the crystallization of pegmatites, they are not appropriate to understand at field scale the LCT pegmatites' emplacement. Here we apply a spatial statistical analysis to the LCT-pegmatites field of Monts d'Ambazac, in the Saint Sylvestre Granitic Complex (Massif Central, France), in order to constrain and discuss spatial relationships between pegmatites, granites and faults. Various numeric variables (distance to the nearest neighbor, Ripley's L'-function, Euclidean distance, spatial density distribution, cluster analysis) have been computed to quantify both i) the spatial distribution layout of the pegmatite occurrences, including their grouping/scattering and aligning features, and ii) the overlap or proximity of the pegmatites with given rock types or structures. We show that a spatial relationship can be quantified between LCT-type pegmatites and a ~N to NNE trending faults family; with 50% of the pegmatite occurrences located at less than 500 m away from one of these faults. This result is confirmed by the spatial relationships between the pegmatites set distribution and the highest spatial density of this trend fault class. Moreover we statistically demonstrate the high clustering rate of the pegmatites set. These clusters are preferentially oriented in the same N015° direction than the trend of the A class-faults, themselves, which is parallel to a large sheared corridor described in the central part of the study area. In contrast to analyses on relationships between faults and pegmatites, our results point out a lack of spatial link between each pegmatite subtypes and several potential granitic sources. We thus suggest that pegmatites are emplaced along A-faults trend. The development of these faults could be favored by and focused in the central part of the granitic complex beforehand affected by a large shear-zone. These results reveal the efficiency and the interest of such statistical approach to better constrain the LCT type pegmatites - faults - granites model. We think that such approach should be more systematically applied to LCT pegmatite fields' exploration, particularly to poorly exposed domains.