Using magmatic biotite chemistry to differentiate barren and mineralized Silurian-Devonian granitoids of New Brunswick, Canada

The geochemistry of biotite crystals from thirty fertile and barren Silurian-Devonian granitoids of New Brunswick, Canada, was studied in situ using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry to investigate the suitability of biotite geochemistry as a diagnostic fertility index among these intrusions. The Fe2+/(Fe2+ + Mg2+) ratio of biotite varies as a function of intrusion metal affinity, increasing from Cu-Mo-related (mean of 0.56 +/- 0.12), to Mo-related (mean of 0.69 +/- 0.06) to Sn-W-related (mean of 0.77 +/- 0.16), with barren granitoids lying between Cu-Mo and Mo types (mean of 0.66 +/- 0.06). The results show a distinctive geochemical contrast between mineralized and barren samples. Compatible elements (Ti, Mg, Co, Ni, V, Cr, Ba, and Sr) decrease from barren to Cu-Mo, Mo, and Sn-W granitoids, whereas incompatible elements (Mn, Zn, Sn, W, Rb, Cs, and Li) show the opposite trend. These two trends might indicate higher degree of fractionation indicated by biotite chemistry in Sn-W-related granites. Furthermore, barren intrusions have the lowest water content (1-3 wt.% H2O), whereas Sn-W and Cu-Mo-related intrusions have between 3 and 6 wt.% H2O. Mo-bearing intrusions have a limited range of H2O contents (4-4.5 wt.%). A high degree of halogen enrichment related to degree of fractional crystallization results in enrichment of incompatible elements in the magmas associated with Sn-W mineralization and is reflected by the geochemical characteristics of biotite from these systems. New metallogenic classifications are introduced using ternary V-Na-Li (ppm) and Sn + W (ppm) versus Ga (ppm) to differentiate barren and mineralized granitic systems in New Brunswick.