Genetic model for fluorite mineralization at the AGS deposit, St. Lawrence, Newfoundland, Canada: REY patterns, fluid inclusion analysis, Nd-Sm dating, and C, O and S isotopes

Fluorite mineralization in the AGS deposit occurred in an epithermal setting from fluids exsolved from the associated St. Lawrence Granite and mixed with meteoric water. Veins of the deposit cut Cambrian sedimentary rocks, and sills and a massive variant of the granite. The mineralization is characterized by banded, colloform and breccia textures, and is divided into three stages: Early, Main and Late. The magmatic origin of the fluorite is supported by the Nd-Sm age of ~ 361 Ma, ƐNd values (1.7 to 2.9), strong negative Eu anomalies and REY contents (~ 5 to 1000 times chondrite values) of the fluorite, all comparable to the associated granite. Interaction of a vapour-rich magmatic fluid with oxidized surface-derived fluids formed an acidic fluid that led to greisenization and fracturing of the overlying rocks prior to fluorite precipitation. Fluorite precipitated between ~ 130 and 180 °C from H2O-NaCl-CaCl2±FeCl2±MgCl2 fluids with salinities of 9.7–27.5 eq. wt% NaCl. Carbon isotope data (δ13C between − 6.5 and − 5.6‰) on calcite intergrown with fluorite indicate a magmatic carbon source and δ18O (8.6 to 19.3‰) indicate mixing of magmatic fluids with meteoric water. Main-stage sphalerite returned δ³⁴S values of ~ − 32‰, consistent with biogenic sulfur from the host sedimentary rocks or oxidized magmatic sulfur, whereas Late-stage sphalerite returned δ³⁴S ≈ − 1‰, indicating a dominantly magmatic source. Increasing circulation of fluids in the enveloping sedimentary rocks in the Main and Late stages resulted in smaller negative Eu anomalies in the fluorite, more reducing conditions reflected in the disappearance of hematite and appearance of sulfides, and change in the fluid chemistry (addition of FeCl2 and/or MgCl2).