Analyses of veins. breccia. and massive sulfide deposits. .

The Lal\:e George (or Badger Flats) beryllium area is a terrane of Precambrian rocks on tlie southwest side of the Front Range in Colorado. Its southeastern and southwestern parts are underlain mainly IJy the metasedimentary Idaho Springs Formation, the metaigneous Boulder Creek ( ?) Granodiorite, and the Silver Plume( •n Granite. The northern and central parts of the area are underlain principally by the Pikes Peak and Redsldn Granites. Locally the Precambrian rocks are masked by thin tuffs and boulder deposits of Tertiary ( ? ) age and by colluvium and alluvium of Quaternary age.The oldest rocks-Idaho Springs Formation, amphi•bolite and hornblendite, and Boulder Creek ( ?) Granodiorite-were in-• volved in a period of plastic deformation during which a major north-northeast-trending fold, the Round Mountain syncline, formed. Subsequently they were strongly deformed and regressively metamorphosed by the intrusion of the Silver Plume(?) Granite. The emplacement of the youngest Precambrian igneous rocks-the gabbro and monzonite, Pikes Peak Granite, and Redskin Granite-had little structural effect on the older rocks. Locally the rocks were cataclastically deformed sometime after the plastic deformation but before emplacement of the youngest igneous rocks. They were faulted in several episodes. Movement along the Badger Flats fault, a major structure of the area, possibly occurred before emplacement of the Silver Plume ( ?) . Movement -along most of the other faults first occurred in Precambrian time.The principal ore deposits are nonpegmatitic beryllium deposits that occur in veins, pipes, and other forms generally encased in greisenized wallrocks. The greisens contain quartz, muscovite, topaz, and fluorite, and trace elements, such as lithium and tin, that are typical of greisens. The principal deposits are concentrated in four parts of the area that are in or very near the Redskin Granite. Small concentrations of beryllium minerals and greisen bodies that contain only trace amounts of beryllium are more widely distributed.Most of the beryllium occurs in beryl, which forms hexagonal crystals and massive poikiloblastic grains. Some of the beryllium occurs in bertrandite, which may be the dominant beryllium mineral in some ore bodies or parts of ore bodies. Very small amounts of the beryllium occurs in euclase. The beryllium deposits locally contain arsenopyrite, wolframite, molybdenite, galena, and dark sphalerite, and these minerals and other sulfides and oxides are locally concentrated in small ore shoots near beryllium deposits or in greisenized rocks nearly free of beryllium. The gangue of most deposits is formed by the typical greisen minerals.Though the deposition of ore minerals and the formation of greisens were nearly contemporaneous, the ore minerals are somewhat younger than the greisens. Quartz, muscovite, and fluorite, which form the bulk of the greisens, also occur with the ore minerals and were deposited during a long period of time .. The beryllium deposits and associated greisen are closely related ...

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Compositional range of porphyritic and fine-grained Redskin Granite Average composition of fine-grained type Average composition of porphyritic type Average composition of Redskin Granite, granite-aplite in Boomer cupola Thermal minimum compositions in the system Si0 2-KAI Si 3 0 8-NaAISi 3 0 8-H 2 0 at various water vapor pressures.

The Yentna district, in south-central Alaska, is underlain by slightly metamorphosed Mesozoic sedimentary rocks, and by sandstones, conglomerates and coaly materials of the Tertiary Kenai Formation. The bedrock is locally covered by extensive surficial deposits of Quaternary and Recent (Holocene) age. The Mesozoic strata are cut by a quartz monzonite batholith in the Tokositna Mountains and by alaskitic dikes and plugs in the Peters and Dutch Hills. A silica-carbonate dike, which formed by alteration of a mafic or ultramafic dike, was noted in the Peters Hills. The major ore deposits are gold placer deposits of several types, including stream and bench deposits of Recent (Holocene age), glacialfluviatile deposits of Quaternary age, and conglomerates of Tertiary age. Quartz-rich conglomerates and breccias have also been productive and are interesting and controversial genetically. The present study indicates that they are closely related to shear zones containing quartz veins and highly altered rocks; previously the origin of the associated altered rocks had been ascribed to deep weathering. The report also contains descriptions of a few gold lodo prospects and of geochemically anomalous areas such as those at Bunco Greek and near Mount Goldie.