Stratigraphy, petrology, and some fossil data of the Roberts Mountains Formation, north-central Nevada

Geology of the Carlin gold deposit, Nevada

“…Mullens (1980) emphasized that these cherty carbonate beds are the only distinctive unit of regional ex tent in the Roberts Mountains Formation. The best expo sures in the northern part of the Lynn window are about 11/2km south and southeast of the Carlin mine; Berry and Roen (1963) described similar beds that crop out about 11/2km southwest of the Carlin mine and 3/4km southeast of the mouth of Sheep Creek Canyon.…”

“…� � GEOLOGY 15 Roen (1961) described the Roberts Mountains Forma tion in the Lynn window. Radtke and Scheiner (1970b) presented chemical and mineralogic data on the forma tion; Merriam and McKee (1976) summarized information on its stratigraphy, depositional facies, and stratigraphic paleontology; and Mullens (1980) made a comprehensive study of its petrology and stratigraphy, including the sec tion exposed in the Lynn window.…”

“…The upper boundary of the Roberts Mountains Formation is placed at the contact between the underlying laminated carbo nate beds and the first overlying thick-bedded coarsegrained limestone; this boundary proved very satisfac tory for regional mapping in most areas of the Lynn win dow, as well as in the Carlin mine. In areas where the boundary is transitional, the contact was chosen to be where coarse-grained limestone becomes more abundant than laminated rocks of the interbedded sequence; this mapping procedure corresponds to that followed by Evans (1974a), Mullens (1980), and the geologic staff of the Carlin Gold Mining Co.…”

“…The irregular chert bands pinch and swell, and loc ally cut across the carbonate bands. Detailed descriptions of this basal zone in the Roberts Mountains Formation were included in the reports by Roen (1961) and Mullens (1980). Chemical and spectrographic analyses of a sample from across the basal cherty zone, as well as of samples of other rock types in the Roberts Mountains Formation, are listed in table 2 (sample 5).…”

Petrographic and scanning electron microscope studies of samples from the Roberts Mountains and Popovich formations, Carlin Mine area, Eureka County, Nevada

“…The Roberts Mountains Formation ranges in age from Silurian to Devonian, whereas the overlying Popovich Formation is Devonian. The Roberts Mountains Formation was studied regionally by Mullens (1980) to identify its stratigraphic and petrologic characteristics and, in particular, how these features may relate to ore deposition. Recently, however, it became apparent that a modern petrographic study of specimens from this formation from the Carlin mine and neighboring area might disclose additional information about favorability of these rocks as host rocks for ore. We used scanning electron microscopy coupled with energy-dispersive X-ray analysis and transmitted-light microscopy to examine the mineralogical and textural characteristics resulting from diagenesis, metamorphism, and hydrothermal alteration associated with gold deposition in these rocks.…”