New Mexico Geological Society Annual Spring Meeting — Abstracts

Magmatism around the McDermitt caldera, an early igneous center of the Yellowstone hotspot, began shortly after 17 Ma with emplacement of Steens Basalt, the oldest part of the Columbia River Basalt. Precaldera volcanic rocks became progressively more silicic through time and included both peralkaline and biotite-bearing rhyolites, which are now exposed in the caldera wall. Collapse of the ~40 x 25 km caldera resulted from 16.4 Ma eruption of ~1000 km³ of the McDermitt Tuff, which is zoned from aphyric, high-SiO₂, peralkaline rhyolite to abundantly porphyritic, metaluminous icelandite (Fe-rich, tholeiitic dacite-andesite). A resurgent dome formed shortly after collapse probably related to residual icelandite magma. Minor icelandite volcanism continued to 16.1 Ma, when volcanism ceased.

Rhyolitic tuffaceous sediments accumulated in an irregular moat/lake around the resurgent dome between 16.4 and 15.7-15.6 Ma, mostly post-doming. Composition and phenocryst assemblage of dated tephra and age and K/Ca of their feldpars demonstrate that most tephra are from regional, non-McDermitt sources. The intracaldera sediments host possibly the world’s largest Li clay deposits. Li smectite deposits with maximum grades of ~4000 ppm occur throughout the caldera. Li illite deposits with maximum grades of ~9000 ppm are only known in the southern, Thacker Pass part of the caldera. The origin of the Li clay deposits, particularly the contribution of low-T diagenetic vs high-T hydrothermal processes, and the total source of Li remain uncertain and controversial. The tuffaceous sediments underwent closed-hydrologic system diagenesis(CHSD), in which rhyolitic glass dissolved to generate high-TDS, alkaline water that, enriched by closed-basin evaporation, precipitated smectite, zeolites, authigenic Kfeldspar and albite, and possibly illite. Similar Li smectite deposits are present elsewhere in Nevada in extensional basins containing diagenetically altered, rhyolitic tuffaceous sediments. The similarity of Li smectite deposits in the McDermitt caldera basin and extensional basins is strong evidence that both formed through diagenesis. Questions about McDermitt deposits are (1) origin of the high grade, illite part, which is absent or sparse elsewhere in Nevada, and (2) the total source of Li (dissolution of glass ± contributions of hydrothermal solutions). My very rough mass balance indicates solution of glass in the McDermitt intracaldera sediments alone is insufficient to account for all the Li in the caldera clay deposits.

The strongest evidence for a hydrothermal component is that the McDermitt caldera, unlike other early Yellowstone hotspot calderas, has many hydrothermal systems related to caldera magmatism, including Hg, U-Zr, U non-Zr, and Au. However, the hydrothermal deposits mostly occur along the caldera ring-fracture zone or occupy compact, subcircular areas, whereas the Li clay deposits are continuous throughout the intracaldera sediments. One Hg deposit and one U-non Zr deposit partly overlap spatially with Li mineralization in the northern part of the caldera, but no definite hydrothermal deposits are present in Thacker Pass. Elements enriched in Li deposits (Mg, K, Rb, F, Mo, As, Sb) partly overlap with those in hydrothermal deposits:

Hg: As, Sb, Mo, ±Zr, U, F, Tl;

U-Zr: Y, Yb, As, Sb, F, Mo, Tl ±Hg, Au, Th, Te;

U: As, Mo, Sb, F, Hg;

Au: As, Sb, Ag, Cu ±Hg, W, Bi.

Overlapping elements are those enriched by either hydrothermal systems or low-T redox reactions. No known hydrothermal deposit is enriched in Li. All hydrothermal deposits formed shortly post-collapse based on adularia ⁴⁰Ar/³⁹Ar dates (16.35±0.03 Ma, Moonlight U-Zr; 16.67±0.14 Ma, McDermitt Hg) or restriction to McDermitt Tuff or basal intracaldera sediments. Hydrothermal activity driven by magmatism is reasonable. Late icelandite magmatism is spatially unrelated to any mineralization. Li mineralization is undated other than post-15.7 Ma, the youngest mineralized sediments, and 14.87±0.05 Ma on authigenic Kspar in the illite zone at Thacker Pass. A heat source to drive hydrothermal activity at this late, post-magmatic time is unknown. ⁴⁰Ar/³⁹Ar dating of the Li illite to provide a definitive time of high-grade mineralization is being investigated.

A possible Li source in addition to CHSD solution of glass is shortly post-collapse degassing of residual magma or high-T devitrification of McDermitt Tuff to release U, Zr, Y, Yb, etc as well as Li. The less mobile elements fixed in deposits, for example as U-rich zircon at the Moonlight Mine. More mobile elements (Li, K, Rb) and especially elements soluble in oxidizing water (As, Mo) entered the intracaldera lake-groundwater system ultimately to precipitate as Li smectite and illite at relatively low T. The absence of U, another redox-controlled element, in the Li deposits is one of many unresolved questions about the origin of the Li-clay deposits.