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

Abstract

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.