Allaz, and three co-authors, reported in 2015 that an unusual rare earth element (REE) mineralization occurs at a locality known as the “Rusty Gold” within the anorogenic 1.4 Ga Longs Peak-St. Vrain monzo- to syenogranite Silver Plume-type intrusion near Jamestown, Colorado (U.S.A.). Irregular-shaped centimeter- to decimeter-sized mineralized pods and veins consist of zoned mineral assemblages dominated by fluorbritholite-(Ce) in a gray-colored core up to 10 cm thick, with monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides. The core zone is surrounded by a black, typically millimeter-thick allanite-(Ce) rim, with minor monazite-(Ce) in the inner part of that rim. Bastnäsite-(Ce), törnebohmite-(Ce), and cerite-(Ce) appear in a thin intermediate zone between core and rim, often just a few hundreds of micrometers wide.
Electron microprobe analyses show that the overall REE content increases from rim to core with a disproportionate increase of heavy REE (SHREE increases 10-fold from 0.2 to 2.1%) compared to light REE (SLREE increases twofold from 21.3 to 44.3%). The fluorbritholite-(Ce) contains minor U, Th, Fe, Mn, and Sr (total 0.10 apfu), with Al, Mg, Na, K, Ti, Pb, S, and Cl below instrument detection limits. Cerite-(Ce) is a minor constituent of the thin zone between the inner rim and the core. The cerite-(Ce) is Fe-rich with low Ca, and minor Al, Mg, and Mn, whereas törnebohmite-(Ce) is Al-rich and Ca-poor. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(25) and 1.442(8) Ga, respectively, confirming a co-genetic relationship with the host ca. 1.42(3) Ga Longs Peak-St. Vrain granite.
The authors concluded that the origin of the REE mineralization is a F-rich and lanthanide-rich, either late-magmatic hydrothermal fluid or residual melt, derived from the granite. This late-stage liquid, when becoming progressively enriched in REE as it crystallized, could explain the observed concentric mineralogical and geochemical zoning. The mineral assemblages and chemistry of the individual REE phases provides some insight into the transport, concentration, and deposition of REE in peraluminous granites.
The abundance of F in most REE minerals suggests that the transport of primary REE minerals was controlled by complexation of REE with OH and F. Numerous studies have highlighted the high capacity of F-rich fluid to transport and concentrate REE and other high field strength elements based either on field observations and petrology or on laboratory experiments.
Whereas the authors' primary hypothesis relies on the existence of a hydrothermal fluid, we cannot exclude the possibility of a late magmatic liquid extracted from the Longs Peak-St. Vrain granite, and possibly the formation of two immiscible residual melts; a silica-rich and a fluorine-rich melt. The Si-rich melt could be responsible for the formation of the aplite, whereas the F-rich melt would preferentially partition all REE. The authors' preliminary fluid inclusion study in the REE mineralization revealed an absence of primary fluid inclusions, and the presence of rounded fluorine inclusions notably in quartz. Moreover, a fast crystallizing residual melt would be more compatible with the fine equigranular texture observed in the core of the mineralization and two immiscible residual melts could account for the amoeboidal contact between the aplite and the mineralization.
More work on fluid inclusions and possible melt inclusions in both the aplite and the mineralization is required. The high-F content of the mineralization suggest similarities in term of transportation and precipitation with the work of Agangi et al. (2010 and reference therein). That study investigated REE concentration in felsic magmas (rhyolites); the authors suggested herein that the REE concentration, transport, and deposition history in Jamestown followed a similar path:
1) the residual felsic melt extracted from the batholith is enriched in fluorine;
2) the crystallization of major silicate minerals in the granite and/or aplite (quartz, feldspar, ±biotite) without F-rich phases further enriched the residual melt in F, and forced a liquid phase (fluid or melt) enriched in REE to exsolve; and
3) the latter crystallization subsequently concentrated in pods and along cracks (veins); and last
4) REE-rich phases sequentially crystallized from this liquid. As the initial REE-bearing phase [allanite-(Ce)] crystallized in the residual melt, the final liquid became more strongly enriched in REE), leading to the mineralization sequence as discussed above.
Managed & Built by I2M Consulting, LLC | © 2010-
