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Geologic map of the Arizona Spring quadrangle, Elko County, Nevada
 
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Title: Geologic map of the Arizona Spring quadrangle, Elko County, Nevada

Author: G. Kendall Taylor, Arthur W. Snoke, Seth Dee, Keith A. Howard, and Charles H. Thorman
Year: 2025
Series: Open-file reports
Version: OF2025-07
Format: 42" x 31" color map; text, 26 pages
Scale: 1:24,000

The Arizona Spring quadrangle is located in northeastern Nevada at southeastern end of the East Humboldt Range. The Paleozoic–early Mesozoic rock sequence in the quadrangle can be divided into two distinct structural and metamorphic domains separated by the southeastern East Humboldt detachment fault. Domain 1 consists of a metasedimentary sequence that ranges in protolith age from Cambrian–Ordovician to Devonian– Mississippian. The mapped units in this metamorphic sequence are: Cambrian–Ordovician marble of Verdi Peak, metamorphosed Ordovician Eureka Quartzite, Ordovician– Devonian metadolomite, metamorphosed Guilmette Formation, and Upper Devonian–Mississippian metamorphosed Pilot Shale. These metamorphic rocks have undergone polyphase deformation under amphibolite to greenschist facies conditions. The highest-grade unit is the marble of Verdi Peak, which includes calc-silicate paragneiss that contains tremolite, scapolite, phlogopitic biotite, and scarce diopside. This metamorphic assemblage is the basis of amphibolite facies in domain 1. The lowest grade unit in the metamorphic sequence of domain 1 is the greenschist-facies phyllite of the metamorphosed Pilot Shale. Metamorphosed Guilmette Formation yielded conodonts with a CAI (conodont alteration index) of 5, indicating that this marble reached a temperature of ~300°C. The deformation and metamorphism of the metasedimentary rocks of domain 1 are inferred to be Cretaceous in age based on geologic relations and radiometric dating in adjacent quadrangles of the Ruby Mountains–East Humboldt Range core complex (Sicard and Snoke, 2020).

Domain 2 consists of a sedimentary sequence that range in age from lower Permian to Upper Triassic. This domain is also exposed farther to the north, where the Pennsylvanian Ely Limestone is part of the sequence. This sequence is characterized by low CAIs of 2–2½–3 indicating temperatures of 60–200°C. The structural base of this sedimentary sequence is the southeastern East Humboldt detachment fault, which records brittle deformation characteristic of deformation <300°C. Middle Eocene volcanic and volcaniclastic rocks exposed at the southern end of the East Humboldt Range unconformably overlie upper Permian Park City Group and Lower Triassic Thaynes Formation of domain 2. This relationship indicates that the age of the southeastern East Humboldt detachment fault is younger than middle Eocene (~39 Ma), because there is no evidence that the volcanic and volcaniclastic rocks are deposited on the footwall of the southeastern East Humboldt detachment fault. The sedimentary rocks of domain 2 are folded and tilted suggesting deformation prior to the deposition of the middle Eocene volcanic and volcaniclastic rocks and subsequent emplacement of the southeastern East Humboldt detachment fault. Domain 2 is also intruded by stocks, sills, and dikes of porphyritic biotite granodiorite inferred to be middle Eocene in age.

The youngest deformation of the rocks exposed in the Arizona Spring quadrangle is normal faults that cut rocks of domains 1 and 2. Although these faults are undated, based on regional relationships, they are <10 Ma and are part of the normal-fault system that bounds the eastern flank of the East Humboldt Range.

Suggested Citation: Taylor, G.K., Snoke A.W., Dee, S., Howard, K.A., and Thorman, C., 2025, Geologic map of the Arizona Spring quadrangle, Elko County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2025-07, scale 1:24,000, 26 p.

© Copyright 2025 The University of Nevada, Reno. All Rights Reserved.

Original Product Code: OF2025-07