The McCoy Creek Group was designated by Misch and Hazzard (1962) for Late Precambrian metasediments exposed along McCoy Creek in T.18 N., R. 66 E. along the eastern flank of the central Schell Creek Range.

The McCoy Creek Group is considered Late Precambrian (Proterozoic or Precambrian Z) in age.

General Lithology

The McCoy Creek Group is a thick sequence of siliceous and argillaceous metasediments which have undergone low to medium-grade isochemical regional metamorphism (Misch and Hazzard, 1962). Metamorphic grade increases systematically with depth in deeper sections, as in the Schell Creek Range, where the lower portion of the section reaches the garnet zone of the greenschist facies. The relatively thin section of the McCoy Creek Group in the Snake Range shows no systematic increase in metamorphism with depth. There has apparently been only one synkinematic regional metamorphic event within the McCoy Creek Group. This regional event is locally superimposed by dynamic metamorphism related to shearing along low angle detachment surfaces, and contact metamorphism related to granitic intrusions emplaced along these faults. In the southern Snake Range, a 160 Ma pluton cuts the regional metamorphic fabric present in the McCoy Creek Group, suggesting that at least part of the deformation and metamorphism is Jurassic or older.

Misch and Hazzard (1962) broke the McCoy Creek Group into several units in various areas. These include, from bottom to top, the Pre-Willard Creek Quartzite (unnamed), Willard Creek Quartzite, Strawberry Creek Quartzite, Shingle Creek Conglomeratic Quartzite, Osceola Quartzite, and Stella Lake Quartzite in the southern Snake Range. In the McCoy Creek area of the Schell Creek Range, units A through H were designated, and in the Trout Creek area of the Deep Creek Range, just across the Nevada-Utah border a series of units from 1 to 7 were suggested for the McCoy Creek Group. Because of the inconsistency in designation, lack of unique lithologic or chronologic correlation, and structural complexity along bedding plane faults within and between these units, they have not been retained or described in any detail here. Instead, the general lithology and sequence are described for sections of the McCoy Creek Group.

In the Snake Range (Misch and Hazzard, 1962), the McCoy Creek Group is composed of a basal 800 feet of light gray to white, crossbedded, locally pebbly quartzite which weathers a light brown (Pre-Willard Creek and Willard Creek Quartzites); overlain by about 750 feet of greenish gray to gray, very fine-grained argillites and metasiltstones which are interbedded with rusty brown weathering, fine-grained, gray quartzites (Strawberry Creek Formation); about 500 feet of brown to purple weathering, gray to brown-gray fine to coarse-grained quartzite which contains conglomeratic lenses and beds of well rounded quartzite pebbles as much as 200 feet in thickness (Shingle Creek Conglomeratic Quartzite); 800 feet of slate gray to green or bluish and purplish argillite (Osceola Argillite) which is strikingly banded with different colored laminae locally preserving graded bedding as well as delicate cross-bedding; overlain at the top with white to light gray, massive quartzite (Stella Lake Quartzite) with gritty and pebbly lenses, and minor cross-bedding exposed in the upper portion. The uppermost Stella Lake Quartzite has been mapped with the overlying Cambrian Prospect Mountain Quartzite regionally (Hose and Blake, 1976).

In the Schell Creek and northern Egan-southern Cherry Creek Ranges, massive white, light-grey to rusty brown, slightly feldspathic quartzite in beds 6 inches to 6 feet in thickness, with interlayered grit, conglomeratic quartzite, fine grained, gray to greenish meta-siltstone and shale, make up about half of the McCoy Creek Group (Woodward, 1963; Fritz, 1968; Hose and Blake, 1976). The quartzites contain about 10 to 15 percent feldspar, minor amounts of muscovite, chlorite, tourmaline, apatite, and zircon, with the remaining 75-90 percent quartz. Sedimentary structures preserved within the quartzite include well-developed graded and convolute bedding, as well as small-scale laminae and cross-bedding, and thin pebbly beds and streaks (Misch and Hazzard, 1962). Metapelite, as grey to green cloritic muscovite schists, and grey, yellow and red argillite, with minor amounts of interbedded schist, and tan marble form most of the remaining half of the group (Hose and Blake, 1976). Laminated and massive, yellowish tan to gray dolomitic and calcitic marble, and interlayered calcareous muscovite schist occurs as a 300 foot thick unit near the base of the McCoy Creek Group in the Schell Creek Range (Misch and Hazzard, 1962).

In the northern and northcentral Schell Creek Range, both Young (1960) and Dechert (1967) described several units within the McCoy Creek Group. The basal unit is about 2,500 feet of poorly exposed, dark brown and gray to black, fine-grained quartzite and finely laminated metasiltstone, and light gray, medium to coarse-grained quartzite which is often conglomeratic with well-rounded quartz pebbles. This is overlain by about 1,800 feet of greenish to reddish brown micaceous phyllite and metasiltstone, and interbedded quartzite. This is overlain by about 610 to 1,550 feet of light gray, medium to coarse-grained, feldspathic quartzite which weathers to red, brown and green shades. The uppermost unit is composed of about 890 feet of fine-grained, dark gray and greenish, foliated quartzite and metasiltstone, and olive-green and reddish-brown, faintly laminated phyllite which increases upsection. The upper 100 feet of this unit is a light gray to white quartzite and 80 feet of phyllite which sharply underlie the Prospect Mountain Quartzite (Dechert, 1967).

In the northeastern Kern Mountains, the McCoy Creek Group rocks are dominantly composed of coarse-grained, biotite-muscovite schist with about 30 percent of both quartz and muscovite, and 20 percent of both biotite and oligoclase (Nelson, 1959). Grayish-white schistose gneiss streaks are intercalated with the schist and are composed of about 55 percent quartz, 30 percent oligoclase, 10 percent muscovite, 5 percent biotite, and a trace of microcline.

In the southern Deep Creek Range, the McCoy Creek rocks are composed of meta-argillites and meta-quartzites with interbeds of amphibolite and silicate marble. Nelson (1959) described a sequence at Trout Creek, which in ascending order, consists of garnetiferous biotite-musovite schist; about 500 feet of graphitic, dolomitic, blue-gray marble; about 1,000 feet of fine-grained, muscovite-biotite schist with graded angular to sub-rounded pebbles, cobbles and boulders of metamorphic rocks; about 600 feet of fine-grained meta-quartzite; and nearly 1,300 feet of reddish-brown muscovite-quartz-biotite schists with angular to subrounded cobbles, pebbles and boulders of metamorphic rocks. In Cherry Canyon, the sequence consists of about 3,000 feet of garnetiferous biotite-muscovite schist, schistose light brown quartzite and amphibolite, and light-gray, massive, faintly laminated and cross-bedded, reddish-gray quartzite (Nelson, 1959). At Johnson Pass the sequence is graphitic garnetiferous biotite-quartz-sericite schist, overlain by ganetiferous chlorite-quartz gneiss and amphibolite with a total thickness of about 7,000 feet.

In the Pilot Range, the McCoy Creek Group is composed of five units disconformably overlain by the Prospect Mountain Quartzite (Woodward, 1967). The lowest unit is 200 feet of green and gray spotted phyllite; in turn overlain with 535 feet of green slate and coarse grained grits, and gray quartzite; 645 feet of green and blue-gray slate, gray quartzite and minor green quartzite, and red quartz conglomerate; 170 feet of gray quartzite and quartz conglomerate; and 1,350 feet of green and gray slate and metasiltstone and quartzite and green and argillite with two 75 foot thick layers of laminated white and black marble (Woodward, 1967).

In the Bull Run Mountains of northern Elko County the McCoy Creek Group is composed of thick massive greenstone, thinly bedded silty limestone, limey siltstone, argillite, and minor sandstone (Clark and others, 1985).

In the Jarbidge Mountains and Copper Mountains Ridge area of northern Elko County, Precambrian McCoy Creek Group rocks are composed of thin-bedded white to brown quartzite, and greenish quartz-chlorite-muscovite schist and phyllite with one thin bed of feldspathic quartzite that may represent a meta-tuff, and small bodies of andalusite hornfels (Coats, 1964).

In the Jarbidge area near Copper Mountain and in the Rowland and Mt. Velma Quadrangles in northern Elko County, rocks regarded as equivalent to the McCoy Creek Group are exposed and are dominantly thin-bedded quartzite and quartz mica schist (Coats, 1985). The muscovite, chlorite, and biotite schists are locally biotite-corderite-staurolite hornfels. In the Marys River Basin and Jarbidge Quadrangles the McCoy Creek Group rocks are conformably overlain by the Prospect Mountain Quartzite.

In and to the east of the Green Mountain area in the southern Ruby Mountains, Willden and Kistler (1969) and Howard and others (1979) considered exposures of amphibolite facies rocks to be equivalent to the McCoy Creek Group. These include foliated white to brown weathering micaceous quartzites, black biotite-muscovite schist and quartzofeldspathic schists which show several generations of mesoscopic folds. Whole-rock Rb-Sr dating shows that these rocks were regionally metamorphosed to the amphibolite facies 550 +/- 30 Ma (Willden and Kistler, 1969). Similar lithologies are present in various localities in the East Humboldt Range as well.

The McCoy Creek Group is about 8,800 feet thick in the McCoy Creek area of the Schell Creek Range and Trout Creek area of the Deep Creek Range (Misch and Hazzard, 1962; Hose and Blake, 1976). It is 6,740 feet thick in the northern Schell Creek Range (Dechert, 1967), 4,800 feet in the southern Cherry Creek Range (Fritz, 1968); 3,600 feet in the southern Snake Range (Misch and Hazzard, 1962), 3,300 feet in the Pilot Range (Woodward, 1967), and about 1,200 feet are exposed in the Bull Run Mountains (Clark and others, 1985).

The McCoy Creek Group rocks are exposed in the Ruby and East Humboldt Mountains, southern Cherry Creek, northern Egan, Snake, Schell Creek, Deep Creek, and Pilot Ranges, Bull Run and Jarbidge Mountains.

The McCoy Creek Group sediments were derived from a granitic basement and deposited in nearshore to offshore marine environments that may have included deposition by density currents (Hose and Blake, 1976). Clark and others (1985) suggest that the sequence exposed in the Bull Run Mountains represents a deep lacustrine rift basin which formed as a result of the Precambrian rifting of the Cordilleran Miogeocline. They suggest the overlying Prospect Mountain Quartzite represents the migration of a fluvial system over the rift basin and that the succeeding Edgemont Formation siltstone, mudstone, and limestones represent miogeoclinal subsidence.