Petrogenesis of Volcanogenic Sedimentary Lithium Ore in the Neogene Macusani Volcanic Field, Puno, Peru

The recently discovered Falchani volcanogenic sedimentary lithium deposit, situated within the Neogene Macusani Volcanic Field in SE Peru, represents one of the world’s most significant resources of this metal. The Li ore is mainly hosted in the so-called lithium-rich tuff, a tuffaceous mudstone that is sandwiched between two Li-rich volcaniclastic units (Upper and Lower Breccia). The major and trace element composition of the Li-ore units differs from that of previously reported ash-flow tuff in the Macusani Volcanic Field. Rather, it approximates the chemistry of highly evolved peraluminous obsidian glasses, known as macusanite, and exhibits the characteristic geochemical fingerprint of peraluminous rare-metal granites and Li-Cs-Ta pegmatites. The parental magmas of the Li-ore units have been modeled as containing <25% of a mantle component with the isotopic signature of potassic to ultra-potassic basaltic melts. The remaining is attributed to partial melts of Proterozoic and Paleozoic paragneiss and metapelites. High fluorine activity in the melt lowered the solidus and delayed fluid-melt immiscibility, promoting high degrees of differentiation through fractional crystallization. Pre-eruptive metasomatic processes in the upper part of the crystal mush could have increased the concentration of Li and other fluid-mobile elements. Contrary to many volcanogenic sedimentary lithium deposits worldwide, in situ post-depositional metasomatic processes associated with the formation of secondary clay and zeolite assemblages did not result in lithium enrichment, but in its partial leaching. Falchani represents a first documentation of a volcanogenic sedimentary deposit where the primary lithium enrichment to economic grades was related to pre-eruptive magmatic processes.