Estimation of Fluorine Concentrations in Fluids of Mineralized Skarn Systems

Abstract

The experimentally calibrated phlogopite and biotite fluorimeters and the experimental results for mineral-fluorine buffer reactions: quartz + fluorite + H 2 O = wollastonite + 2HF, and corundum + sellaite + H 2 O = spinel + 2HF, were used to estimate the HF concentration ( M HF , mole/dm 3 ) in fluid associated with different skarn deposits, granitoids, pegmatites, and marble. The equations of the phlogopite and biotite fluorimeters are the following: log M HF (phl) = X Mg [log (X F /(1 – X F )) – 1722/T(K) – 0.2112] + log a H 2 O , and log M HF (bt) = log (X F /(1– X F )) – 1722/T(K) – 1.107 X Mg + 0.216 (Al-2) + 0.8958 + log a H 2 O , which take into account the absolute temperature (Kelvin) of mica formation and complex compositions of mica solid solution and hydrothermal fluid. The estimation of the HF concentration in fluids related to different skarn deposits, granitoids, pegmatites, and marble shows two behavioral trends. The high F trend is typical of rare metal leucogranite, Li-F granite and pegmatite, and related W, Be, U, Th, REE, Ta, Nb, Li, and Sn skarn deposits; the HF concentration in the fluids associated with these rocks is approximately 10 –1.5 to 10 –1 mole/dm 3 and remains almost constant, although formation temperatures vary from 900° to 500°C. The low F trend is characteristic of barren diorite, tonalite, granodiorite, and granite plutons and porphyry Cu and Mo deposits, as well as fluorine-poor phlogopite, scheelite, and other base metal skarn deposits. The HF concentration in the fluids related to these rocks decreases with the formation temperature. At low temperature, the HF concentration is considerably lower than in the high F trend, e.g., at 500°C, it ranges between 10 –3 to 10 –2.6 mole/dm 3 , which is close to the experimental positions of quartz-fluorite-wollastonite equilibrium on the log M HF -T diagram. The coincidence of high HF concentrations in fluid and the metallic association of W, Be, U, Th, REE, Ta, Nb, Li, and Sn in skarn deposits indicates a petrogenetic relationship that is a function of magmatic fractionation, melt-volatile partitioning, oxidation state of melt and fluid, or metal-ligand complexing behavior. Therefore, estimation of the HF concentration in fluids using biotite and phlogopite fluorimeters may aid in the exploration of plutons and related skarn ore deposits and shed light on genetic problems related to fluorine in skarn fluid ore systems.