Age, petrochemistry, and origin of a REE-rich mineralization in the Longs Peak-St. Vrain batholith, near Jamestown, Colorado (U.S.A.)

Abstract

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 (S HREE increases 10-fold from 0.2 to 2.1%) compared to light REE (S LREE 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. We suggest 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 mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part II. Radekškodaite-(La), (CaLa5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3 and radekškodaite-(Ce), (CaCe5)(Al4Fe2+)[Si2O7][SiO4]5O(OH)3, two new minerals with a novel structure-type belonging to the epidote–törnebohmite polysomatic series

“…The arrangement of the mineral assemblage follows a trend of decreasing REE content in minerals from the core to the rim of the nodule. A similar feature was observed in monazite replaced by allanite and REE -enriched epidote during metamorphism in metapelites (Finger et al , 1998), in magmatic REE -rich accumulations from aplitic rock at Jamestown, Colorado (Allaz et al , 2015) and locally in the Bästnas-type deposits in Bergslagen, Sweden (e.g. Andersson, 2004).…”

The mineralogy of the historical Mochalin Log REE deposit, South Urals, Russia. Part I. New gatelite-group minerals ferriperbøeite-(La), (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SiO4]3O(OH)2 and perbøeite-(La), (CaLa3)(Al3Fe2+)[Si2O7][SiO4]3O(OH)2

“…Mochalin Log shows a similarity to the Bastnäs-type deposits at Riddarhyttan, Västmanland, Sweden (Öhman et al ., 2004; Holtstam and Andersson, 2007) and Jamestown, Colorado, USA (Allaz et al , 2015), but has much greater REE mineral species diversity; it should be noted that all these occurrences differ in their attributed origin.…”

Bulk composition of a zoned rare-earth minerals-bearing pegmatite in the Pikes Peak granite batholith near Wellington Lake, central Colorado, U.S.A.

“…Details on EMP analy- relative to Wellington Lake, the McGuire pegmatite, and the South Platte pegmatite district (inset), within which occur the Oregon #3, the White Cloud, and other major NYF-type pegmatites (after Barker et al, 1975Barker et al, , 1976Smith et al, 1999). sis and standards used for the REE minerals fluocerite and bastnäsite are described in Allaz et al (2015Allaz et al ( , 2020.…”