Large areas previously thought to be underlain by Cretaceous granodiorite of the Idaho batholith are now known to be underlain by plutonic rocks of the Eocene Challis magmatic episode (50–44 Ma). Recent mapping conducted in south central Idaho as part of the U.S. Geological Survey's Conterminous United States Mineral Assessment Program (CUSMAP) indicates that roughly 30% of the exposed plutonic rocks are part of this younger event. The Eocene plutonic rocks are subdivided into a pink granite suite, consisting of varieties of biotite granite, and a quartz monzodiorite suite, which includes hornblende‐biotite granite, granodiorite, quartz monzodiorite, diorite, and gabbro. The members of each suite form discrete epizonal stocks, and some exposures of pink granite are of batholithic size. Silica content of the quartz monzodiorite suite ranges from 48 to 70%; the rocks vary considerably in texture and mineralogy. The most mafic rocks of the quartz monzodiorite suite are cumulates. These are layered intrusive phases containing pyroxene, olivine, and plagioclase. Other mafic phases include hornblende‐ and pyroxene‐rich diorites, which contain plagioclase feldspar as an interstitial phase and are “cumulates” in the sense that they probably represent rocks that were enriched in early formed mafic minerals by some fractionation process. Stocks of the pink granite suite are more compositionally restricted and range from 70 to 77% in SiO2 content. The quartz monzodiorite suite appears to be older than the pink granite suite, but age data are limited. Field relations, initial 87Sr86Sr ratios, CaO content, and MgO/(MgO + FeO*) ratios demonstrate that the two suites are not related by fractional crystallization. Rather, the respective magmas probably originated from different sources. Large ranges in MgO, Sc, Cr, and Co content in the quartz monzodiorite suite can be explained by crystal‐liquid fractionation of olivine, pyroxene, and hornblende, but variations in composition of parental magmas are also likely. Large variations of Ba content in the pink granite suite are probably a result of removal or accumulation of variable amounts of K‐feldspar. Mixing between the two Eocene suites has not been extensive, but some varieties of hybrid pink granite were generated that contain mafic inclusions and have less than 72% SiO2. The pink granite has geochemical affinities with A‐type granite, but unlike typical A‐type granite, it is not uniformly enriched in the highly charged cations such as Zr, Nb, Y, and Ce. By analogy to other A‐type granites the pink granite suite probably originated from partial melting of lower crustal rocks under high‐temperature, vapor‐absent conditions.
The Arabian Shield is underlain by rocks that range from Precambrian through Phanerozoic (Brown and Jackson, 1960; Schmidt and others, 1973; Greenwood and others, 1973; Greenwood, Roberts, and Bagdady, 1974 ). The geology discussed herein is based on a report by Greenwood and others (unpub data) with several significant differences in interpretation favored by Roberts. The lithologic units, tectonism, plutonism, and metallogenic epochs are summarized in table 1.
Geophysical surveys in the Wadi Bidah district, in the southwestern part of the Kingdom of Saudi Arabia, have revealed anomalies of the type that may be caused by deposits of massive sulfides. Accordingly, a study was undertaken to investigate the geologic environment associated with the geophysical anomalies and to determine whether mineral exploration of the anomalous areas might be warranted. An extensive geochemical sampling program did not disclose any evidence of ore mineral concentrations in the rocks. Detailed geologic mapping shows the anomalous areas to be underlain by Precambrian carbonaceous graywacke, the outcrop patterns of which closely fit the anomaly patterns. Field evidence and petrologic studies indicate that carbonaceous material in the rocks is the cause of the geophysical anomalies. Nothing was found in the anomalous areas that suggests the presence of mineral deposits of commercial grade, and further mineral investigations or exploration in the areas is not recommended.
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