Quartz trace-element composition by LA-ICP-MS as proxy for granite differentiation, hydrothermal episodes, and related mineralization: The Beauvoir Granite (Echassières district), France

International audienceDespite quartz being a ubiquitous mineral in the Earth's crust, only little is known about its trace chemical composition and whether and how this is influenced by the medium from which it precipitated. Using in-situLA-ICP-MS analysis, we investigated the trace element composition of several types of quartz from a magmatic-hydrothermal system in the Echassières district of the French Massif Central. This system consists of the Colettes granite, the Beauvoir rare-element granite, a W-bearing stockwork and dispersed mineralized veins. All of the elements tested (Li, Be, B, Na, Al, Ti, Ge, Rb, Sr, Nb, Sn, Sb, Ta, W)were detected in quartz from at least one sample. A rigorous multivariate statistical approach permitted to assign a specific chemical signature to each of the quartz groups that were recognized by petrographic observations. Differences among these groups were interpreted to reflect different sources or genetic evolutionary trends within magmatic and hydrothermally derived quartz groups. For example, the evolution of Al, Rb, Ge, Li and Ti, could be related to genetic processes such as the degree of magma differentiation and crystallization temperatures. Quartz from the W-stockwork, anterior to the Beauvoir leucogranite, clearly stands out from that related to the Beauvoir mineralization system, i.e.,the greisen facies and proximal Ta-Nb-Sn-W mineralized veins. It was also possible to demonstrate a similarity in composition, notably the Sb contents, between quartz from the greisen and from distal stibnite-bearing veins. In addition, elevated values of Sb, Sn as well as Li and Rb in quartz correspond to similarly high values of these elements in the rock, indicating that quartz trace chemistry can provide useful information for the mineralization potential of its host, at least for these elements. On the other hand, Nb, Ta and W are less reliable, as they occur mostly at very low levels in quartz, the highest values corresponding to samples that were not particularly enriched in these metals. A comparison of our quartz data with those from a similar granitic complex, at Cínovec (Czech Republic, Breiter et al., 2017a), shows a similar trend for the Ge/Ti ratio, reflecting magmatic differentiation. Conversely, the Al/Ti ratio values differ in the two complexes, the Al content of quartz from the various Cínovec granites being systematically lower than in quartz from Echassières. Nevertheless, caution must be taken when analyzing quartz for its trace element chemistry, as processes occurring during (i.e.,trapping of fluid inclusions and formation of zoning) and after crystal growth (i.e.,diffusion and alteration), can introduce heterogeneities thus affecting the original chemistr