Lithium-bearing peraluminous dacitic to rhyolitic rocks from Northern Puna, Jujuy, Argentina.

JOFRÉ, CYNTHIA B.; CAFFE; PABLO J.

San Salvador de Jujuy

Workshop; 3rd IWLiME; 2016

Peraluminous granitic rocks and their volcanic equivalents are important source of some metals beyond their relative volume in the crust. In fact, many of the world?s ore deposits of Li, Be, Sn, W, Nb, and Ta are linked to this type of magmatism. The latter elements behave incompatibly and are likely to be mobile in fluids, especially in plutonic settings, reason why true magmatic concentrations are probably modified during post-magmatic hydrothermal/meteoric alteration. Fresh peraluminous volcanic rocks, and their mineral-hosted melt (glass or devitrified) inclusions, may instead provide better estimates of the original magmatic concentrations of these components. This is important because the original concentration in magmatic systems, chemical behavior and parameters that control mobility of elements as Li and B in magmas are the milestones to understanding the role of magmatic rocks as primary sources in secondary accumulations as evaporitic mineeals and brines in salars. For this kind of deposits in the Central Andes, the debate between hydrothermal vs magmatic origins still exists, so that solving this issue may be crucial for the design of strategies in regional exploration of the resources.Upper Miocene to Pliocene back-arc volcanism erupted in the 21º-24ºS segment of the central Andean plateau (APVC, fig. 1) is mostly represented by calc-alkaline andesites to rhyolites of metaluminous to weak peraluminous affinities. However, a group of silicic units shows peraluminous character and they are relatively usual in the Inner Arc of Bolivia/Perú, but less common in the APVC area. These volcanic complexes and calderas produced ignimbrites and lavas that distributed extensively in the APVC between Bolivia, Chile and Argentina, many of them showing peraluminous character and, in agreement, potentially high concentration of lithium and mobile elements (B, Cl, F). The most outstanding representative of this group in northern Puna of Argentina is the Pairique Volcanic Complex (Fig. 1; 22º52´S - 66º49´W), a sequence of peraluminous dacites to rhyolites (Caffe et al, 2007) among which a pyroclastic unit, the Coyaguaima ignimbrite (A/CNK 1.14-1.35), exhibits lithium content of 16-23 ppm in the pumice. Basement xenoliths hosted in various of the dacites reach up to ~100 ppm (Caffe et al, 2007). In the same way, volcanic pyroclastic and lava outcrops located in the southwestern APVC, as those erupted from La Pacana Caldera (e,g. Atana and Toconao ignimbrites) show values of 18-70 ppm of lithium in the pumice (Fig. 1; 23º00´S - 67º15´W; Lindsay et al, 2001). On the other hand, the Bolivian tin belt volcanic rocks show similar characteristics. This is the case of the Morococala Field (Fig. 1; 18º00´S - 66º45´W), which consists principally of three voluminous silicic pyroclastic flows of dacite to rhyolite composition with lithium concentration of 80-250 ppm (Morgan et al, 1998).Furthermore, it has been observed that the lithium content of whole rock and quartz-hosted melt inclusions of the same pumices or lava flows are different, being much higher in the latter. Melt inclusions in La Pacana Caldera yielded values of 250-690 ppm of lithium (Lindsay et al, 2001) and 250-470 ppm in the Morococala rhyolites (Morgan et al, 1998). If it is considered that melt inclusion represent the geochemical composition of magmas before desgassing and eruption, and post-magmatic hydrotgernal or meteoric alteration, it should be noted that the original lithium enrichment of these silicic magmatic systems is much higher than that suggested by whole rock geochemical surveys. This highlights the importance of acquiring melt inclusion chemical data in selected targets from northern Puna (e.g. Paitique) for understanting the true lithium and boron concentration in this group of peraluminous magmas, the processes that control their enrichment and relationship between them and elements now contained in salars from the same region.