Using biotite and apatite compositions to differentiate barren and mineralized silurian-devonian granitoid plutons in New Brunswick, Canada

Abstract

Silurian-Devonian granites of New Brunswick (428-433 Ma) cover the compositional spectrum from I- through S-, and A-types and are associated with several styles of granophile mineralization, including porphyry, greisen, and vein-related of Sn, W, Cu, Mo, Au, and U. However, some of the intrusions are not known to be associated with mineralization despite their highly fractionated nature. Principle component analysis classified these granitoids to three groups (NB-1 to NB-3) and two singular granites (Lost Lake and Juniper Barren granites), based on geochemical characteristics using Zr/Ce to TiO2. NB-1 and NB-2 were further divided into a few subgroups using chondrite-normalized REE and mantle-normalized patterns. A general trend of increasing assimilation fractional crystallization exists from singular granites to NB-3 granites reflecting the more evolved nature of these granites. Singular granites are believed to have formed by reworking of an older crustal protolith through several different partial melting events in an arc environment. NB-1 granites are formed by partial melting of the lower crust, whereas NB-2 granites formed by varying degrees of partial melting of a mixed mantle- older crustal protolith. The geochemical differences among the subgroups in NB-1 and NB-2 are attributed to different degrees of assimilation fractional crystallization and secondary hydrothermal alteration. NB-3 granites are mainly crust-sourced and are associated with crustal thinning processes related to crustal delamination following the juxtaposition of various crustal blocks. Twenty-two biotite grains of different shape, size, and compositions were mapped with laser ablation-inductivity coupled plasma mass spectrometer (LA-ICPMS) to evaluate the extent in which magmatic biotite could retain its magmatic trace-element zoning. The result indicated that grains larger than 500×500 μm with minimum mineral inclusion and alteration retained better zoning. Large ion lithophile (LILE) element zoning including Ba, Cs, and Rb are the most common zoning observed within the examined biotite grains. More importantly, a large lithian-siderophyllite (>1×1 mm) from the Pleasant Ridge Granite display Sn, W, Ta, V, Ti, Cs, and Rb, but no Ba zoning that might indicate rapid cooling in this intrusion. Two hundred and nineteen biotite grains from both barren and mineralized intrusions analyzed with both electron microprobe analyzer (EPMA) and LA-ICPMS at the University of New Brunswick in order to investigate the suitability of mica geochemistry as a mineral exploration tool. Biotite colour in plane polarized light varies from reddish-brown in intrusions related to Sn-W mineralization to brown and greenish-brown in intrusions associated with Cu-Mo and Mo and mineralization in barren intrusions. The variations in colour reflect different oxygen fugacity for the host magmas and correspond to a reduced environment for Sn-W hosting intrusions versus the more oxidized magmas associated with the other types of mineralization. The calculated oxygen fugacity for these intrusions fall between the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) buffers with values of 10-15.5 to 10-13.0 bar. The Fe2+/(Fe2++Mg2+) of biotite decreases from a high mean of 0.77 ± 0.16 in Sn-W related intrusions to Mo-related intrusions (mean of 0.69 ± 0.06), to barren intrusions (mean of 0.66 ± 0.06). Biotite from intrusions related to Cu-Mo occurrences has the lowest Fe2+/(Fe2++Mg2+) with a mean value of 0.56 ± 0.12. Biotite composition displays contrasting geochemical characteristics between barren intrusions and those associated with various types of mineralization. Specifically, the content of compatible elements of biotite (e.g., Mg, Ti, Co, Ni, Cr, V, Sr, and Ba) increases, whereas contents of incompatible elements (e.g., Sn, W, Mn Ta, Ga, Sc, Mo, Rb, and Cs) decreases from Sn-W-related intrusions to Mo, Cu-Mo, and barren intrusions. These trends may reflect a more evolved nature of the Sn-W related intrusions. Barren intrusions have biotite worth the lowest calculated water content of (1-3 wt.%), whereas biotite grains from Mo and Sn-W-related intrusions have a much higher water content that ranges from 3 to 6 wt.%. The water content of intrusions related to Mo-mineralization is restricted to 4.0 to 4.5 wt.%. Biotite grains from Cu-Mo, barren, and Mo-related intrusions suggest a similar halogen fugacity for the parent magmas, i.e., fH2O/fHCl and fH2O/fHF in biotite range from 1.46 to 1.51 and 4.26 to 4.54, respectively. Conversely, biotite from Sn-W related intrusions have the lowest fH2O/fHF (<3.56) and highest fH2O/fHCl (>1.59), indicating a higher degree of halogen enrichment resulting from higher degree of fractional crystallization in these intrusions. Three metallogenic classification diagrams based on biotite mineral chemistry, V-Na-Li, Li versus Si, and Sn+W versus Ga, are proposed for the discrimination of barren and mineralized granitic systems. Seventy-four apatite grains from Silurian-Devonian granitoids of New Brunswick were analyzed with both EPMA and LA-ICPMS to investigate the ability of apatite geochemistry to differentiate barren and mineralized systems. Despite homogenous major-element and halogen content, apatite grains have various trace-element compositions. Apatite from the barren intrusion has the highest water content (> 0.3 wt.%), U (117 ppm), Th (133 ppm), and the lowest Sn content of <0.5 ppm and Fe of 828 ppm. Uranium and Th decrease markedly from barren to Cu-Mo to Sn-W and reaches to its lowest value of about 13 and 28 ppm, respectively, in Mo-related intrusions due to the crystallization of monazite and zircon. Apatite grains from the Sn-W-related intrusions have the highest Sn value of 4 ppm, which differentiate them from the rest of the grains. Four new classifications were introduced using apatite Sr, Mn, (Eu/Eu*)N, and LREE/HREE ratios to differentiate barren and mineralized granitoids in New Brunswick.