Zeolite-clay mineral zonation of volcaniclastic sediments within the McDermitt caldera complex of Nevada and Oregon

Abstract

Volcaniclastic sediments deposited in the moat of the collapsed McDermitt caldera complex have been altered chiefly to zeolites and potassium feldspar* The original rhyolitic and peralkaline ash-flow tuffs are included in conglomerates at the caldera rims and grade into a lacustrine series near the center of the collapse. The tuffs show a lateral zeolitic alteration from almost fresh glass to clinoptilolite, clinoptilolite-mordenite, and erionite; to analcime-potassium feldspar; and finally to potassium feldspar. Vertical zonation is in approximately the same order. Clay minerals in associated mudstones, on the other hand, show little lateral variation but a distinct vertical zonation, having a basal dioctahedral smectite, a medial trioctahedral smectite, and an upper dioctahedral smectite. The medial trioctahedral smectite is enriched in lithium (as much as 6,800 ppm Li). Hydrothermal alteration of the volcaniclastic sediments, forming both mercury and uranium deposits, caused a distinct zeolite and clay-mineral zonation within the general lateral zonation. The center of alteration is generally potassium feldspar, commonly associated with alunite. Potassium feldspar grades laterally and vertically to either clinoptilolite or clinoptilolite-mordenite, generally associated with gypsum. This zone then grades vertically and laterally into fresh glass. The clay minerals are a dioctahedral smectite, a mixed-layer clay mineral, and a 7-A clay mineral. The mixed-layer and 7-A clay minerals are associated with the potassium feldspar-alunite zone of alteration, and the dioctahedral smectite is associated with clinoptilolite. This mineralogical zonation may be an exploration guide for mercury and uranium mineralization in the caldera complex environment,.

Lithium-Rich Claystone in the McDermitt Caldera, Nevada, USA: Geologic, Mineralogical, and Geochemical Characteristics and Possible Origin

“…However, Hofstra et al [50] and Benson et al [14] found that lithium concentrations in melt inclusions in quartz phenocrysts of both peralkaline The proportion of sediments deposited pre-and post-resurgence is uncertain because of the poor exposure and difficulty in evaluating what has or has not been resurgently domed. Most previous interpretations are that the sediments were deposited post-resurgence in a moat between the caldera wall and the resurgent uplift [5,6,44]. However, the inward dip of sediments near the caldera wall and gentle dips off the uplift suggest some sediments were deposited before resurgence.…”

“…Chemical information reported below was obtained from Lithium Nevada drill-hole sample analyses for which contents of Li and other elements were obtained using four-acid "near total" digestion followed by ICP-MS and ICP-AES analyses by ALS Chemex. Almost all of the chemical data were from the Thacker Pass area, but some Li data came from an earlier report by the US Geological Survey [5] and from Chevron Minerals. As of June 2011, Lithium Nevada had drilled and assayed 202 core holes in the Thacker Pass area with a total depth of 19,259 m, producing a total of 11,157 samples [16].…”

“…The sediments occupy an irregular, donut-shaped area surrounding the dome. Because some sedimentation postdated resurgence, the deposits are commonly referred to as "moat sediments" [5,6,36].…”

“…Before diagenesis, the deposits consisted of glass shards, fine pumice, and mineral and rock fragments ( Figure 3B). Although glass is commonly preserved in tephra, the sediments mostly are altered to clay minerals, zeolites, and feldspar ( [5]; this study) and are locally silicified to opal or chalcedony ( Figure 3C) [39][40][41][42]. Petrified wood, diatoms, fish, and leaves occur in tuffaceous sediments and unidentified shells in limestone [5,39,43,44].…”

Mineral resources of the North Fork of the Little Humboldt River Wilderness Study Area, Humboldt County, Nevada

“…They contain 0.25 to 0.65 ppm Hg as far away as several kilometers from the deposits. Around the lake-bed deposits-sediments have been altered to a variety of zeolite and clay facies, which Glanzman and Rytuba (1979) attributed to hydrothermal alteration, while Wallace and Roper (1981) believed the minerals are of diagenetic origin. Pyrite accompanies the cinnabar , and uranium and lithium minerals are also present locally (see below).…”