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During the past decade, worldwide demand for rare earth elements (REE) has surged; however, the genesis and geochemical characteristics of hydrothermal REE deposits associated with alkalic igneous systems remain poorly understood. This is significant given that much of the world’s historical production of REE is associated with alkaline igneous activity, including carbonatites. This study addresses the genesis of bastnaesite-fluorite breccia deposits and their association with Ag-Pb-Zn sulfide mineralization in the Gallinas Mountains intrusive alkalic suite. The spatial and chemical association between these two types of deposits and the amenability of many ore and gangue minerals to stable isotopic study provides a unique opportunity to examine the history of hydrothermal fluids responsible for mineralization. Mineral associations, especially products of sulfide weathering i.e. agardite, indicate a genetic link between REE and sulfide mineralization in some deposits. δ34S isotopic ratios of sulfide and sulfate species vary from-21.1 to 1.5 ‰ and 9.6 to 13.3‰ vs. CDT, respectively. Isotopic ratios of carbonate species (δ13C and δ18O) vary from-11.1 to-3.4 ‰ vs. PDB and 15.0 to 23.8 ‰ vs. VSMOW, respectively. δ34S and δ18OSO