Geology topics
Publications and source records attributed to P.A.M. Andriessen.
[No abstract available]
About 60 new isotopic age determinations help clarify the geologic history of the Round Mountain and Manhattan gold districts, Nevada. Cretaceous granite of Shoshone Mountain, southeast of Round Mountain, was emplaced about 95 m.y. ago and was domed, metamorphosed, and mineralized with tungsten-bearing quartz veins 80 m.y. ago. The granite of Pipe Spring, south of Manhattan, was emplaced probably at about 80 m.y. ago. Mineralization associated with the Pipe Spring body occurred about 75 m.y. ago. A granodiorite stock and associated rhyolite and andesitc dikes intruded the granite southeast of Round Mountain 36 m.y. ago; apparently base and precious metals mineralization accompanied their intrusion, but mineralization has not been dated isotopically. Ash-flow tuff at Round Mountain was emplaced about 26 m.y. ago, the Manhattan caldera formed about 25 m.y. ago, and gold mineralization at Round Mountain occurred at about 25 m.y. ago. Gold mineralization at Manhattan took place 16 m.y. ago.The isotopic data allow interpretation of the cooling histories of some of the igneous rocks and suggest possible thermal events that reset mineral ages.
aleozoic sedimentary rocks in a wide area of the northern Sangre de Cristo Range show effects of heating during Tertiary time. Heating is tentatively interpreted as a response to burial during Laramide folding and thrusting and also to high heat flow during Rio Grande rifting. The regional extent of heating is shown by the distribution of low-grade metamorphic minerals, altered conodonts, and reset fission-track ages throughout much of the study area. Alteration of conodonts to a conodont alteration index (CAI) of 4.0 suggests that temperatures reached ∼200 °C in the central part of the area. Temperatures may have reached 300 °C beneath Laramide thrusts on the west side of the range, where conodonts were altered to a CAI of 5.0, and where chloritoid and andalusite are found in sedimentary rocks of Pennsylvanian age. The lowest temperatures that were determined by conodont alteration (CAI = 1.0–2.0, <50–70 °C) are along the east side of the range, where rocks were evidently never buried deeply. Contact metamorphism was restricted to wall rocks of a few isolated stocks; near dikes and sills, it was not significant. Fission-track ages of apatite across a section of the range show that rocks cooled abruptly below 120 °C, the blocking temperature for apatite, ∼19 Ma ago. Cooling was probably in response to rapid uplift and erosion of the northern Sangre de Cristo Range during early Rio Grande rifting.