Trace-element systematics of volcanogenic massive sulfide deposits in the Bathurst Mining Camp, New Brunswick, Canada: Exploration to petrogenetic implications.

摘要

The question of rare earth element mobility in the volcanogenic massive sulfide setting has focused largely on alteration zones surrounding deposits and the overprinting effects of metamorphism. Exhalative sedimentary horizons have rarely been the focus of detailed rare earth element studies, despite direct evidence from active hydrothermal vents on the seafloor showing prominent enrichments in Eu and LREE in hydrothermal fluids. Elucidation of the mineralogical balance of REE in the Bathurst Mining Camp has revealed extreme Eu anomalies, which represent the largest known fractionation of REE in the solar system. Systematic variations of these REE signatures have implications for their petrogenesis and may have applications for the development of exploration vectors in volcanic terrains.;Exhalative sedimentary rocks of the BMC generally contain small quantities of REE compared to volcanic and sedimentary lithotypes; ∑REE contents of massive sulfides average 36.8 ppm, ranging from 0.93 to 249 ppm. Overall, ∑REE contents are strongly controlled by terriginous material incorporated during deposition, and has resulted in a strong Spearman Rank correlation of ∑REE with Al2O3 (r'=0.80), TiO2 (r'=0.75), Zr (r'=0.73), Nb (r'=0.78), Hf (r'=0.81), Sc (r'=0.75), Y (r'=0.83), and Th (r'=0.85).;Chondrite-normalized REE profiles for massive sulfides exhibit a prominent fractionation in LREE and Eu indicating mobility under high temperature hydrothermal conditions. Enrichment of LREE is substantiated by stronger positive ∑REE correlations with La (r'=0.98), Ce (r'=0.99), Pr (r'=0.99), Nd (r'=0.99), and Sm (r'=0.99). Values of LaN/LuN average 24.1, but exhibit extreme variability (0.16 to 1,264) due to terriginous dilution and LREE mobility. Europium anomalies are mostly positive with values of Eu N/EuN* averaging 6.26, but ranging from 0.21 to 36.7. Minor negative Eu anomalies can occur in semi-massive sulfides, due to greater amounts of volcaniclastic, pelitic and pelagic materials. A positive EuN/Eu N* correlation with Sn (r'=0.55), Se (r'=0.32), In (r'=0.40), Co (r'=0.32), and LaN/LuN (r'=0.42) indicate enrichment of LREE and Eu in hydrothermal fluids that deposited base-metal sulfides. Furthermore, a correlation between ∑REE and P2O5 (r'=0.53) suggests that hydrothermal REE signatures are strongly controlled by phosphate minerals.;Fluorapatite is the most abundant phosphate in exhalative sedimentary rocks of the BMC, occurring as nodular to colloform masses and intimate mixtures with exhalative carbonate and sulfide minerals. The REE represent the largest amount of trace-element substitution in apatite, with laser-ablation ICP–MS analyses revealing ∑REE contents averaging 1,548 ppm, and ranging from 250 to 24,038 ppm. Contents of Eu average 295 ppm and range from 7.8 ppm to 1,554 ppm, exhibiting a moderate correlation with Sr (r'=0.50). Chondrite-normalized REE profiles display prominent enrichment in Eu, with EuN/Eu N* averaging 19.0 (as high as 222); apatite carries the mineralogical balance Eu in massive sulfides of the BMC and accounts for the positive Eu anomalies exhibited in bulk analyses. Extreme fractionation of REE in apatite, indicates substitution for Ca2+ during precipitation from hydrothermal fluids, further enhanced by the increased stability of Eu2+ under high temperature (>250°C) and reducing conditions. Profiles also exhibit small negative CeN/CeN* anomalies, averaging 0.95, but can be prominent (as low as 0.58), likely imparted during the mixing of hydrothermal fluids with seawater at or near the seafloor.;Xenotime is the least abundant accessory mineral in massive sulfides of the BMC, occurring chiefly as small overgrowths on detrital zircon grains and as rare authigenic masses exhibiting complex zoning due to variations in REE, Y, and Th. Chondrite-normalized REE profiles display a characteristic dogleg pattern with enrichment in HREE and lower contents of LREE with La, Ce, and Pr largely beyond the detection capabilities of the electron microprobe. Profiles exhibit small positive Eu anomalies with values of EuN/Eu N* averaging 3.4, and are broadly consistent with a hydrothermal and (or) diagenetic origin. (Abstract shortened by UMI.).