MINERALOGY AND LATE STAGE ALTERATION OF THE MOUNTAIN PASS, CALIFORNIA, ULTRAPOTASSIC SILICATE INTRUSIVE ROCKS -PRELIMINARY REPORT

The 1.38 Ga Mountain Pass carbonatite rare-earth deposit is spatial associated with an older ~1.40+Ga suite of ultrapotassic intrusive rocks (stocks and dikes). These rocks mainly consist of peralkaline and metaluminous shonkenite, mafic- to leucocratic syenite, quartz syenite, and granite. The shonkenite consists of ferroan-diopside, phlogopite, alkali-feldspar, and apatite with an accessory mineral suite of magnetite, sphene, baddeleyite, and allanite. The syenite consists of alkali-feldspar, magnesio-arfvedsonite, phlogopite, minor to no quartz, and an accessory mineral suite of apatite, ilmenite, Ti-oxide (rutile?), magnetite, ilmenorutile, monazite (abundant), zircon, and thorite. Trace amounts of FeS (pyrite?), chalcopyrite, and galena were observed in some shonkenite and syenite samples. The rock of the one granite stock is highly altered but the primary mineralogy consists of quartz, alkali-feldspar, arfvedsonite and mica.

All of the samples, studied by SEM, exhibit a late stage hydrothermal alteration that deposited barite, parisite, bastnaesite, synchysite, calcite, celestite, fluorite, epidote, chlorite, and potassium-aluminosilicate (after alkali-feldspar). A extensive suite of complex secondary carbonates (e.g. Th-Ca-carbonate after thorite) is also present that may be related to this event. This alteration is subtle and the mineralization is very fine-grained and disseminated and not easily detectable optically or at the macroscale. The minerals of the alteration suite strongly suggests a relationship to Mountain Pass carbonatite intrusion and dikes which were emplaced in the same general area as the silicic intrusions. The presence of this widespread alteration in the silicic intrusive rocks implies that any use of whole rock trace element chemistry such as that used for petrogenetic modeling should be approached with caution.