The deposit is exposed along an eroded area on the south side of an anticline formed by rock of Cambrian age. It covers hundreds of square meters in a fractured area crossed by hydrothermal veins. The host rock is siltstone-sandstone belonging to the Issafen Formation of the Lower Cambrian, a schist formation composed of mudstone with marl and sandstone intercalations in the upper zone, dolomitic limestones (with stromatolites) in the middle, and purplish red mudstones in the lower part, again with intercalations of stromatolitic limestones and dolostones, all of these in stratigraphic contact. In the simplified geological map, the Issafen Formation is below the Schist-Limestone and Limestone series of Lower Cambrian age, and thus is not visible (again, see figure 4).
At Boudi, quartz is found either scattered in a silt-sandstone matrix or within cavities of a complex fault and fractures system. Small openings are completely filled with interlocking quartz crystals. In the reddish mudstones, floating quartz specimens are always found as single crystals without matrix, usually singly terminated but sometimes doubly. The amethyst clearly formed by hydrothermal deposition, but the condition of the crystals suggests they were removed from the original site of crystallization and subsequently encapsulated in the mudstones. The deposit could be regarded, then, as both primary and secondary. The authors hypothesize that the well-shaped quartz crystals originated in the faults and veins in the surrounding limestone, and then eroded and redeposited in the residual mudstones.
The deposit is exposed along an eroded area on the south side of an anticline formed by rock of Cambrian age. It covers hundreds of square meters in a fractured area crossed by hydrothermal veins. The host rock is siltstone-sandstone belonging to the Issafen Formation of the Lower Cambrian, a schist formation composed of mudstone with marl and sandstone intercalations in the upper zone, dolomitic limestones (with stromatolites) in the middle, and purplish red mudstones in the lower part, again with intercalations of stromatolitic limestones and dolostones, all of these in stratigraphic contact. In the simplified geological map, the Issafen Formation is below the Schist-Limestone and Limestone series of Lower Cambrian age, and thus is not visible (again, see figure 4). At Boudi, quartz is found either scattered in a silt-sandstone matrix or within cavities of a complex fault and fractures system. Small openings are completely filled with interlocking quartz crystals. In the reddish mudstones, floating quartz specimens are always found as single crystals without matrix, usually singly terminated but sometimes doubly. The amethyst clearly formed by hydrothermal deposition, but the condition of the crystals suggests they were removed from the original site of crystallization and subsequently encapsulated in the mudstones. The deposit could be regarded, then, as both primary and secondary. The authors hypothesize that the well-shaped quartz crystals originated in the faults and veins in the surrounding limestone, and then eroded and redeposited in the residual mudstones.
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