Physical, chemical and mineralogical evidence for magmatic fluid migration within the Capitan pluton, southeastern New Mexico.

Dunbar, N.W. ; Campbell, A.R ., Candela, P.A.

The ~300 km3, 28.8 Ma granitic Capitan pluton, located in southeastern New Mexico, was the source of high temperature (up to 600oC), high salinity (up to 80 wt.%) fluids which formed small REE-bearing zones of mineralization in the pluton carapace. Erosional dissection of the pluton reveals zones in the granite that are characterized by bubble-like void spaces and fissures. These could be interpreted, on one hand, as original magmatic porosity resulting from the interconnection and/or coalescence of bubbles of magmatic volatile phase, or on the other hand, as void spaces resulting from dissolution. These porous zones grade upwards into the brecciated portions of the pluton carapace that hosts mineralization. The zones are planar features, traceable across valleys, and even between adjacent valleys on the plutons flanks. The orientation of the zones are roughly concentric with the exposed shape of the pluton. Chemical analyses of samples collected across a porous zone reveal that some elements, including Fe, Al, Rb, and Sr are depleted in the entire area surrounding the porous zone, whereas Si, Na, Ba, and Ta are enriched. Other elements, including K, Sb, Zn, Zr, a number of rare earth element (REE), Pb, and Th exhibit a slight to strong depletion distal to the porous zone, and a strong enrichment roughly centered on the zone itself. Abundance of high-salinity fluid inclusions in igneous quartz is high within the entire 2.5 m wide zone, but is significantly higher in the central porous zone. The chemical and fluid inclusion abundance patterns could be produced by a two-step process with the wide depletion caused first, by fractionation of elements from the melt into a volatile phase at the time that volatile exsolution first occurred. This could be followed by the superimposed enrichment pattern caused by deposition of these elements from trace-element-rich fluids which evolved in a more central, deeper part of the pluton and were transported along the same zone (although now more crystallized) of fluid migration established earlier, depositing minerals as it cooled. Textural, petrographic, and geochemical evidence suggest that volatiles may have exsolved during crystallization, coalescing into bubble-rich zones which enhance the efficiency of volatile transport.