I'm also curious about learning how to choose a sample (a quartz grain) from a thin section for further analysis. or another technique for assessing the age of quartz-bearing veins using fluid inclusion?
Fluid inclusions can be used to determine the temperature, pressure and salinity of the hydrothermal fluid at the time of trapping (assuming there has been no leakage).
To the best of my knowledge, nobody has developed a method to get reliable absolute age dates from a fluid inclusion. Although I see a few 1980's papers on using Rb/Sr I would be skeptical