Biotite, a ubiquitous ferromagnesian mineral in granitoids, provides an alternative to whole-rock geochemical approaches for constraining petrogenetic processes. Its sensitivity to physicochemical parameters (T, P, fO2) allows its use where geochemical characterization is limited. This study investigates Palaeoproterozoic granitoids from the Massigui region (southern Mali), Leo-Man Shield, using biotite chemistry. The intrusions, emplaced within a structural framework controlled by the Banifing Shear Zone, include the Massigui granitoids (granodiorite to quartz monzonite), the dioritic rocks of Ti & eacute;fala-Syobougou (microdiorite to quartz monzodiorite), and the foliated granitoids of Sodioula (granodiorite and granite). Forty-nine electron microprobe analyses were used to characterize biotite crystallochemical parameters (XFe, Altot, Ti, Fe3+/Fetot). Three compositional groups are distinguished. Group 1 corresponds to an early, weakly oxidized magmatic stage. Group 2 is characterized by variable compositions, with evidence for both magmatic evolution and fluid interaction. Group 3 shows ferriferous compositions enriched in Al and Ti, consistent with advanced differentiation in an active tectonic setting. Chemical variations from Group 2 to Group 3 define a continuous trend. The coherence between biotite compositions and those of their host rocks validates their use as a petrogenetic tracer. This mineralogical approach represents a relevant strategy for preliminary investigations prior to more complex or costly methods.
The unusual behaviour of dolerite sills (traps) near kimberlite pipes in the Daldyno-Alakitsky diamondiferous region of Western Yakutia, Siberia, has been studied. Formation of kimberlite pipes occurs through explosive pulses that shift upward, causing significant compaction of the overlying sedimentary rocks. This process generates thermoelastic stress fields, leading to epigenetic and petrophysical changes, such as variations in density and magnetic properties. As a result, the areas surrounding kimberlite pipes become less accessible to relatively viscous, crystal-rich mafic magmas. When emplaced near kimberlite pipes, these mafic intrusions – primarily sills – can form torus-shaped zones with sharply increased thickness. In some instances, magma flowing around kimberlites creates dolerite-free ‘windows’, while in others, it splits into thin ‘tongues’. Additionally, sills adjacent to kimberlites often become crumpled and take on a wave-like shape. We propose that identifying these morphological features in sills can serve as an indirect method for locating kimberlite pipes and determining optimal drill hole sites, particularly in regions with a continuous mafic cover dominated by dolerite sills, such as Western Yakutia, Siberia.
