Compositional characteristics and ages of plutons in the central Ruby batholith, Alaska: implications for rare-earth-element resources

Abstract

The central Ruby batholith, comprising much of the Ray Mountains in central Alaska, consists of at least ten, largely monzogranitic plutons, some of which have been defined as a result of this thesis. The general age of the batholith is estimated at 111 Ma, yet recent ⁴⁰Ar/³⁹Ar muscovite and ²³⁸U/²⁰⁶Pb zircon data suggests greater variability in ages than previously known, with ages now spanning 90 to 112 Ma. Initial ⁸⁷Sr/⁸⁶Sr ratios for the area show a general increase from northeast (minimum of 0.706) to southwest (maximum of 0.727). There are two characteristic mineral assemblages distinguishing the plutons: The northern plutons typically have an oxidized, calcic mineral assemblage featuring allanite + titanite + magnetite ± hornblende ± monazite ± ilmenite. The southern plutons are characterized by the minerals monazite + xenotime + ilmenite ± muscovite ± fluorite. Total alkali-silica classification defines only the No Name Creek pluton as being truly peraluminous, a classification further supported by its unique presence of topaz. In the north, the rare-earth elements (REE) are primarily hosted in the silicates allanite-(Ce), (CaCe(Al₂Fe²⁺)(Si₂O₇)(SiO₄)O(OH)), and to a lesser extent, titanite, (CaTiSiO₅), while the phosphate minerals monazite-(Ce), (CePO₄), and xenotime, (YPO₄), are REE hosts in the southern plutons. These mineralogical differences are not necessarily due to variable REE concentrations within the melts, but rather are largely functions of peraluminosity and oxidation state, features intrinsic to the primitive source materials. Chemical analyses of stream gravels draining the batholith highlight geographic trends in trace-element concentrations: The southern gravels are enriched in REE relative to the northern gravels for several reasons. REE in the northern gravels are predominantly hosted in allanite and titanite (both of which contain relatively low REE concentrations), and the REE in the southern gravels are found in monazite and xenotime. Further, magnetite and ilmenite are more abundant in the northern gravels, effectively drowning out the REE carriers. Furthermore, the gravels in the southeast have the highest heavy-rare-earth element to light-rare-earth element ratios. In terms of economics, these ratios are generally the most significant.