Monazite alteration in the Searchlight contact metamorphic aureole, southern Nevada
Crombie, Scott Andrew
We consider the response of monazite in country rock to magmatic intrusion. Intrusion of the Searchlight pluton at 15.7-17.7 Ma produced extensive hydrothermal ore deposits in overlying units. Later tilting of the pluton has resulted in a cross sectional exposure with deeper levels exposed to the east (Bachl et al. 2001). The top the pluton primarily intrudes Miocene volcanics though in the southern portion of the roof zone it contacts highly altered Proterozoic augen orthogneiss. Along the northern border of the pluton, from shallow to intermediate depth, it is in contact with Proterozoic gneiss with deeper portions in contact with the ~66 Ma Ireteba granite. ten samples of the Proterozoic gneiss were collected, four from the augen orthogneiss in the southern roof area (which yielded no monazite), four along a 0.5 km transect northward from the northern margin into the country rock, one from a large block located in the lower Searchlight zone (XG-12), and one from the north western margin near the roof area (XG-11). Monazites from the Ireteba granite near the contact with the Searchlight display irregular zoning, and LA-ICP-MS Th-Pb dating shows a bimodal age distribution with peaks at ~17 Ma and ~53 Ma (this older age represents mixed zoned analysis rather than a real geologic event). However, oxygen and hydrogen isotope whole rock analyses of these rocks show no pattern of variation related to sample distance from the Searchlight Ireteba contact. This in addition with petrographic evidence suggests that monazite alteration in the Ireteba was caused by some mechanism other than fluid alteration, and calls into question the use of patchy zoning as a diagnostic feature of hydrothermal alteration. The monazites collected in the Proterozoic wall rock transect, display irregular zoning with a unimodal LA-ICP-MS 207Pb/206Pb age of 1656±3.5 Ma. Monazites analyzed from XG-11 and XG-12 display irregular zoning and discordant U-Pb ages. When plotted with samples from the Proterozic wall zone transect samples XG-11 and XG-12 lie on a line of discordia with an upper intercept (defined by the wall zone samples) at 1641±16 Ma and a lower intercept at 134±98 Ma suggesting that these analyses represent mixing between Proterozoic age zones and Cretaceous (Ireteba) or Miocene (Searchlight) ages. Intrusion of the Searchlight pluton appears to have been recorded by the partial recrystallization of monazite in the Ireteba granite, and possibly in portions of the roof zone and in Proterozoic country rock blocks in the lower Searchlight unit. The lack of apparent hydrothermal alteration in the wall zone and the highly altered appearance of rocks in the southern roof zone suggest that fluid release from the Searchlight pluton was highly focused through the roof zone. Bachl et al. (2001) GSA Bulletin v.113 no.9; p.1213-1228.