FLUID INCLUSION FOOTPRINTS IN ROCKS: EVIDENCE FOR PERVASIVE METASOMATISM IN RARE-METAL ORE SYSTEMS

Fluid inclusions provide direct evidence of the importance of fluids during many geologically relevant processes during Earth’s protract history and, as such, have become the focus of extensive research over the past several decades. One such area of intense study is ore deposits since fluid inclusions provide insight into not only the P and T of ore formation, but they also a quantifiable signature of the causative ore fluid. However, another equally important aspect of fluid inclusions, but one that is generally overlooked, is that they leave a footprint of their passage through rocks and thus evidence that fluids facilitated pervasive and at times intensive alteration. The latter process, now referred to as coupled dissolution precipitation or CDP, has long been recognized as a critical part of rare-metal (RM) ore systems as implied by such terms as microclinization, albitization, or topazization; in fact magmatic origins were often inferred for such rocks (i.e., albitites, topazites). Examination of several RM settings (e.g., Li, Sn, Ta, Nb, REEs) are used to illustrate that whereas the primary host rock in such cases are texturally magmatic, they have exchanged with fluids via CDP and re-equilibrated such that they now consistent invariably of quartz and two feldspars (i.e., Or₁₀₀ and Ab₁₀₀). That such rocks are texturally magmatic has long been used to suggest a primary magmatic origin (e.g., Mongolian ongonites) and thus also their chemistry (i.e., both whole rock and mineral) and RM enrichment. However, that quartz and feldspar phases in commonly inundated with fluid inclusions suggest a secondary metasomatic origin for these rocks which is further supported by other data, such as bulk and in situ δ¹⁸O, abundance of secondary minerals (e.g., apatite in the feldspars), and most notably cathodoluminescence imaging. Thus, whereas thermometry and chemistry of fluid inclusions are invaluable monitors of processes, their simple presence in otherwise benign phases is equally important - such footprints record the passage and modification of rocks. The latter also implies potential large chemical fluxes which have otherwise gone unnoticed and this aspect will also be addressed.