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Abstract

10-3 Keith, Stanley B., 2003, Preliminary geologic maps of the northern portion of the Carlin Gold Belt (North Trend), Nevada: Nevada Bureau of Mines and Geology, Open-File Report 2003-01d, 80 p. text, index map, and 7 scanned and georeferenced maps on 5 CD-ROMS.

ABS
This pamphlet provides introductory discussion and explanatory materials for seven 1:6,000 scale geologic maps that cover most of the northern part of the Carlin Gold Belt (referred to herein as North Trend; see Plate 1 for location of map sheets). Mapping began in 1991 and initially focused on defining ‘carriable’ lithostratigraphic units in order to identify high-angle fault controls for underlying gold mineralization. As of June 1998, some 62 square miles have been mapped within the area of the map sheets shown on Plate 1.
All the map sheets shown on Plate 1 were surface mapped by the author. The only exception is an approximately one-square mile area in Map Sheet 5 centered on the Bootstrap mine (the specific location of this mapping is shown as an inset in Sheet 5 on Plate 1). Here, geologic map unit information was obtained from a map by Kofoed, R.K., and others that appeared as Figure 8 in Hayes and others (1997). Strike and dip information for this area was obtained from Evans and Mullens (1976).

In addition, in Map Sheets 3, 4, 5, and 6, a number of attitudes and rock-stratigraphic information was obtained from open trenches constructed by the various operators in these areas. Barrick, Uranerz, and FMC Gold are acknowledged for contribution of this data, although the stratigraphic interpretation of these now reclaimed trenches rests with the author.

Most of the trenches shown on the other map sheets, as well as map sheets 3, 4, 5, and 6, were open at the time of the mapping. Structural and stratigraphic information for these trenches were obtained by the author during surface mapping. As the mapping expanded and evolved over a period of years (1991-1998), numerous well constrained paleontological age controls became increasingly available, which are allowing the lithostratigraphy to be refined into ‘true’ stratigraphy with time-stratigraphic controls. Some 101 fossil collections with suitably constrained age ranges (Stage to Period level age ranges) are itemized in Tables 1A, 1B, and 1C. Fossil sample sites are also shown on Plate 1.

This pamphlet is divided into two main parts. The first part is a series of topical discussions that deal mainly with stratigraphic and stratotectonic issues for rocks in the Carlin North Trend. Rock stratigraphic implications for the age of what are at least four ages of metal introduction (both relative age and age as calibrated to absolute radiometric ages) are also discussed in the first section.
The second part presents a detailed explanation for the various mapped rock units and structures within the Carlin North Trend. At appropriate places within this explanation, expanded text narrative is provided regarding regional stratigraphic correlations and economic implications of selected rock units (for example, the Rodeo Creek unit).

The main goal of the mapping was to establish a reliable and traceable stratotectonic framework for the Roberts Mountain allochthon with which to confidently delineate the high-angle fault framework, which is largely superimposed on the Roberts Mountain allochthon or ‘upper plate’ as it is widely referred to by Carlin-trend geologists. Indeed, interpretation of deep drill hole and the 1:6,000 scale map data to date indicates that there is no compelling evidence for high-angle fault control of any of the upper plate or lower plate lithostratigraphy identified within the map area. At this point, the original framework envisioned by Roberts and others (1958; 1965) in the now classic AAPG papers still appears to be the best regional interpretation of North Trend geologic data. That is, a lower-plate, carbonate-dominated assemblage (eastern assemblage) has been overriden by an allochthonous assemblage of siliciclastic, oceanic rocks (western assemblage). The juxtaposing structural feature for this orogenic event is known as the Roberts Mountain Thrust. The eastern assemblage accumulated on the ‘trailing’ shelf edge of North America during the early Paleozoic (Cambrian through Early Mississippian). The upper plate, silici-clastic assemblage accumulated in an adjacent oceanic basin to the west.

Between the upper and lower plate rocks, a middle ‘layer’ of interbedded, argillite-shale-siltstone with intercalated sandy units (locally calcareous) is present. This assemblage has been widely referred to as the transitional assemblage and is considered parautochthonous. Fossil information for the transitional assemblage has yielded Devonian through Early Mississippian ages and is regarded as a facies that intervenes between the continental shelf and the oceanic facies (possibly formed on the continental slope). The local transitional assemblage unit mapped in the North Trend is referred to by ‘Trend’ geologists as the Rodeo Creek Formation.
To date, the Rodeo Creek Formation in the Carlin Trend has not been shown to be allochthonous in the classic ‘older over younger’ sense. The contact with the underlying Popovich unit commonly shows evidence of strong deformation, but paleontological evidence to date has not indicated any age reversals (see main text). Consequently, the Roberts Mountain Thrust is currently placed at the first ‘known’ age reversal in the tectonostratigraphic sequence (that is, the base of the Bell Creek thrust-nappe on Figure 3). However, it is recognized that there is a distinct possibility that, in conformity with current regional practice, the Roberts Mountain Thrust potentially may ultimately be placed at the base of the Rodeo Creek section (see Figure 3 and further discussion at the end of the Rodeo Creek entry in the main text).

Since the original edition of these maps was prepared in May, 1998, two major developments have occurred in the Carlin North Trend that affect geologic aspects of the mapping and the three dimensional interpretation of upper plate geology. The first development has resulted from numerous new paleontological determinations on radiolarian fauna that are mainly Devonian age for the cherts that occur in the three thrust-nappes above the Dee nappe (the Bell Creek, Queen, and Rossi thrust-nappes). The Devonian radiolarian dates necessitate a reassignment of ages for rocks that were formerly interpreted to be Silurian (Elder age) or Ordovician (Vinini age) age. As of the April 2002 edition, the formation symbology on the seven panel maps (especially map sheets 3, 4, and 6) has not been changed, as extensive redrafting of the maps would be required.

However, the new age information and its implications for the various map units in the Bell Creek, Queen, and Rossi thrust nappes is included in the rock unit descriptions in italics. Consequently, the user will be aware of ‘where the science is going’. In a similar fashion, the future rock unit notations that are expected to be used on the map in future map editions are shown in the explanation as italicized acronyms in parentheses wherever current map notation symbols are expected to change. Ultimately, it is expected that future editions will be redrafted to carry the new formational and age assignments. Ultimately, the new radiolarian data will also be compiled geographically and combined with the existing tabulated paleontological information in the tables of fossil information (Tables 1A, 1B, and 1C) at the end of this pamphlet.

The formational reassignments also resulted in the movement of one of the thrusts within the Roberts Mountain allochthon (the Rossi thrust) up ‘section’. The Rossi thrust is now placed above what are now regarded as Devonian age cherts that were formerly assigned to the Ordovician Vinini Formation. This thrust occurs in the uppermost part of the M96-1C drill hole shown in the stratotectonic section for Paleozoic rocks in the Carlin North Trend of Figure 2.

The second major development is the result of a deep drill hole (IH-214) that was drilled by Great Basin Exploration at the northeast corner of the McAllister pit (Great Basin Ltd., 2001). The drill hole ‘bottomed’ at about 7000 feet and reportedly intersected Bootstrap equivalent rocks at about 6400 feet. This deep intersection has major implications for three-dimensional interpretations of upper plate thicknesses in the Carlin trend. Specifically, it forced a major redrafting of the Figure 3 stratotectonic cross section, which now shows a series of continuous thrust plates that dip shallowly northwestward (drill hole IH-214 occurs just northwest of the left side of the Figure 3 stratotectonic section). The Roberts Mountain thrust is now interpreted to occur at approximately 5800 feet depth in this part of the Carlin North Trend (left edge of Figure 3 stratotectonic section). This new interpretation contrasts markedly with the old interpretation of this worker and many others that had suggested that the Roberts Mountain thrust occurs at much shallower levels. This author’s pick was based on correlation of white pin-striped, black argillites of Devonian age near the bottom of Drill Hole BX-1 (see Figure 2) with lithologically similar argillites of Devonian age in the Meikle pilot shaft drill hole and surrounding exploration drill holes at the Meikle deposit (MR-1 at the right side of Figure 2). As it turns out, white pin-striped, black argillites are now known to be fairly common near the top of the Rossi thrust-nappe in the upper plate of the Roberts Mountain-Golconda allochthon.

Because some of the specific drill data used to construct the schematic cross sections of Figures 2 and 3 are proprietary, the locations of the drill holes with respect to the cross sections are not shown. Nonetheless, the deep drill hole data provided significant information for the three-dimensional structural interpretation illustrated in the cross sections. Also, a considerable amount of public-domain and proprietary fossil data from drill holes was not included because the data were obtained from subsurface drill holes. Surface fossil information was included as Tables 1A and 1B.

Key words
Baxter, East Hollister, North Block, Ren, Rossi, Rodeo, South Dee, North Trend, Carlin type, gold, Roberts Mountain thrust,