Hydrothermal alteration and past and present thermal regimes in the western moat of Long Valley caldera

Abstract

Hydrothermal alteration in caldera-fill Early Rhyolite and Moat Rhyolite was investigated in six western moat drillcores in Long Valley caldera, with an emphasis on clay mineralogy. In two of the cores examined, a complete sequence was observed in argillic alteration of tuffaceous rocks, progressing from smectite (often with kaolinite) to mixed-layer illite-smectite (I/S) to illite with increasing depth. Portions of this sequence were observed in the other four drillcores. Lower-permeability rhyolitic lavas typically showed much sparser clay alteration than tuffs. In cores where alteration progressed through smectite to I/S or illite, the distribution of the clay minerals, and of compositional and structural variation in the I/S phases, allowed estimation of pre-existing alteration temperatures and thermal gradients. I/S (and illite) occurred at the shallowest depths and highest elevations in the Shady Rest ( sr) core, and comparable alteration occurred at progressively deeper levels in plv-1 and inyo-4. Presently observed temperatures are well below preexisting temperatures at comparable depths in these three holes, with the exception of the deeper portion of sr. Paleotemperatures in the zone of I/S alteration in the sr hole (∼ 200–250 m depth) were ⩾65° C higher than at present, with a thermal gradient at least twice the steep gradient existing there now. In plv-1, temperatures near the base of the hole have declined even further than in sr, despite the steep gradient currently observed. In inyo-4, hydrothermal circulation, somewhat cooler than that in SR and plv-1, occurred at one time in the deeper portion of the hole, and the steep thermal gradient currently observed at shallower levels in inyo-4 may be the truncated remnant of a gradient that once continued to greater depth. In mlgrap-1, hydrothermal circulation occurred previously near the base of the hole, but at temperatures probably not far above present ones. No evidence of high-temperature alteration was observed in cores from mlgrap-2 or plv-2.