Grain-scale and deposit-scale heterogeneity of Re distribution in molybdenite at the Bagdad porphyry Cu-Mo deposit, Arizona

Electron microprobe analysis of Re concentration in molybdenite grains in 45 samples from 11 rock units at the Bagdad porphyry Cu-Mo deposit found high variability from the grain to the deposit scale. Measured Re concentrations range from < 15 to 4450 ppm and do not correlate with rock unit, age, alteration type, ore grade, proximity to ore, or other observed geological feature. Variability within samples and within grains is nearly as high as variability over the deposit. Within 23 grains of molybdenite from a single 2-cm-long sample, Re content varied from < 15 to 1215 ppm Re. Within single grains the Re content ranges just as widely (e.g. 20 analyses on one grain vary from 44 to 2061 ppm), with intra-grain relative standard deviation (RSD) typically > 0.5 and in some cases > 1. Although microprobe maps show that the Re variation in some crystals correlates with growth-related zoning to a limited extent, there was no pattern to the variation in most of the molybdenite grains and the few zoned examples had no clear sector or consistent oscillatory textures. Based on our current understanding of trace element incorporation during hydrothermal molybdenite growth, this evidently heterogeneous distribution of Re in molybdenite precludes the use of microprobe spot measurements of Re as a vector to mineralization, which was the original focus of this study. A follow-up analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) using spot diameters of 150 μm reduced the observed intra-grain variability from 1.67 to 1 order of magnitude, but still failed to yield data that correlated with any known geological features. This suggests that any further work toward using molybdenite Re content as a vector to mineralization should either focus on whole-grain methods of analysis or else should use detailed microprobe mapping of molybdenite growth zones to guide point analyses, and then correlate the growth zones in molybdenite samples taken over distance through the deposit. This approach could help shed light on how the development of heterogeneous Re concentrations on the micro-scale relates to the processes of molybdenite precipitation and alteration.