New Geologic Map in Elko County

Geologic Map of the Gordon Creek Quadrangle, Elko County, Nevada by Karri R. Sicard and Arthur W. Snoke
Year: 2020
Series: Map 188
Format: plate: 33 x 34 inches, color; text: 28 pages, color
Scale: 1:24,000

The map units in the Gordon Creek quadrangle range from Holocene to Neoproterozoic in age. The map area exposes a partial crustal cross section from mid-crustal rocks (sillimanite-muscovite-zone rocks, ~5.5 kb and ~630 ºC) in the north to allochthonous, unmetamorphosed upper-crustal rocks in the south. The deepest exposed rocks are migmatitic, upper-amphibolite facies metasedimentary rocks correlated with the Neoproterozoic McCoy Creek Group. Evidence of partial melting is widespread, and an isopleth was mapped, delineating an area of rocks that contain greater than 67% granitic rocks (early Oligocene to Cretaceous). Tertiary biotite monzogranite and abundant Tertiary–Cretaceous pegmatitic leucogranite form massive bodies, sheets, and dikes. Structurally above this migmatitic zone is a composite unit consisting of Cambrian–Neoproterozoic metamorphosed Prospect Mountain Quartzite and McCoy Creek Group rocks intruded by sheets of deformed mafic to silicic igneous rocks.

Isolated distinct bodies of massive pegmatitic leucogranite as well as abundant segregations of leucosome (crystallized leucogranitic melt) occur in these metasedimentary rocks. Overlying these units is another composite unit consisting of Ordovician to Cambrian impure calcite marble and calc-silicate rocks, intruded by a variety of igneous rocks ranging from pegmatitic leucogranite to mafic to felsic rocks. The Tertiary intrusive rocks occur as numerous sheets, many reaching 50+ m in thickness. These Tertiary intrusive rocks range from gabbro/quartz diorite to leucogranite in composition, and two of the units have yielded U-Pb zircon radiometric ages of middle Eocene. A U-Pb monazite age of ~84 Ma in the southern Gordon Creek quadrangle suggests that at least part of the pegmatitic leucogranite is Late Cretaceous. However, crosscutting field relationships of pegmatitic leucogranite at various localities in the quadrangle indicate that some of the unit is Tertiary, perhaps as young as early Oligocene. The marble of Verdi Peak, exposed in the southern part of the quadrangle, includes the same metasedimentary rocks as Ordovician to Cambrian impure calcite marble and calc-silicate rocks but lacks the middle Eocene mafic to silicic igneous rocks. However, many small intrusive bodies of pegmatitic leucogranite are common in the marble of Verdi Peak as well as several sheets, reaching about 250 m in thickness. Also, a metapelite has been found at two localities with a mineral assemblage that indicates upper-amphibolite facies metamorphism (>7 kb, ~600+ ºC) throughout much of the metamorphic sequence in the Gordon Creek quadrangle. 

Subsequent to regional metamorphism in the Late Cretaceous was the development of a km-scale, west-rooted, normal-sense mylonitic shear zone during Oligocene time. At the southern end of the quadrangle, a normal-sense brittle detachment fault separates the marble of Verdi Peak from an upper structural level of chiefly unmetamorphosed, brittlely attenuated stratified rocks ranging in age from late Paleozoic through middle Eocene (Arizona Spring quadrangle). One sample of Pennsylvanian Ely Limestone yielded conodonts with an elevated conodont alteration index (CAI) (5–7). Some exposures of Ely Limestone exhibit recrystallization and plastic flowage indicating weak metamorphic effects are present in the upper plate of the detachment-fault system. This metamorphism may be related to hot fluids derived from the subjacent metamorphic and igneous footwall. Between the upper-crustal rocks and the mid-crustal rocks are greenschist/lower-amphibolite facies rocks, consisting of Eureka Quartzite, a metadolomite unit, and graphitic calcite marble. The protolith ages of these rocks range from Upper Devonian to Ordovician. These metasedimentary rocks are typically preserved as fault slivers in the detachment-fault zone. These lithologic units are also exposed as low outcrops surrounded by Quaternary deposits east of normal faults related to the range-front system.

The youngest rock units in the quadrangle are a complex suite of Quaternary deposits, which include youngest to oldest (1) youngest alluvium (Qy), (2) landslide deposits (Qls), (3) colluvium (Qc), (4) younger alluvium (Qya), (5) megabreccia (Qmb), (6) older alluvium (Qoa), (7) glacial deposits (Qg), and (8) pluvial lake deposits (Qp). The eastern flank of the southeastern East Humboldt Range is bounded by a system of normal faults, which were active in the Quaternary.

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