Hydrothermal Recrystallization Research Articles

Evaporite-Carbonate Rocks of the Jurassic Lovelock Formation, West Humboldt Range, Nevada

The Lovelock Formation is an assembly of late Early Jurassic or Middle Jurassic carbonate rocks and gypsum that constitute the youngest preserved sediments and, probably, the last marine deposits in successions of Jurassic rocks in the Carson Sink region of western Nevada. The formation records a set of transitional environments between open marine basinal conditions and an emergent tectonically active terrain. The current extent of the formation is probably a small fraction of its original distribution in the Carson region and, perhaps, in areas of Nevada farther west. The Lovelock Formation occurs only in a pair of nappes in the nappe pile of the northern West Humboldt Range. Geometric relations between intranappe and internappe folds suggest that nappe emplacement and folding were coupled deformations in major Middle Jurassic tectonism. Sites of deposition of the Lovelock Formation were probably not more than a few kilometers east or west of current exposures of the formation. The formation is comprised of three members in a maximum thickness of about 200 m. The lowest is a micrite which underwent emergence and fresh-water diagenesis before deposition of fragmental limestones of the middle member. Conglomerate of the middle member consists exclusively of intraclastic micrite pebbles from the lower member. Finer grained sedimentary breccia above the conglomerate is an accumulation of intraclasts and allochthonous particles from a variety of subenvironments on a carbonate bank. The upper member comprises gypsum and calcarenite, deposited at least in part in standing water of a barred basin. The sequence of motions of the sediment surface relative to sea level recorded in the deposits of the Lovelock Formation is conceivably due to eustatic variations, but the general temporal proximity of deposition of the formation to major tectonism suggests that such motions were probably tectonic. Postdepositional hydrothermal recrystallization occurs sporadically in carbonate rocks of the Lovelock Formation. Such metamorphism was perhaps synchronous with local generation of solution breccias in the upper member and local calcitization of sulfate.

Hydrothermal recrystallization and transformation of tridymite

Well ordered tridymites containing atmost 0.016% Na (0.004% Na) were prepared at 1400° C from Na2WO4-(K2WO4-) fluxes using high purity amorphous silica as starting material. No further reduction of these Na-contents was attainable by soxhlet extraction. These tridymites were treated hydrothermally at temperatures between 815 and 950° C and 200 bars H2O. The products obtained were investigated optically as well as by powder X-ray methods and were analyzed for Na-contents: the hydrothermal treatment resulted either in recrystallization of tridymite or transformation into quartz mostly depending on Na-contents. Na-contents below about 0.015% tend to favour recrystallization of tridymite within the quartz field ( 870° C). This may be due to influences of Na-traces either on the kinetics or on the equilibrium temperature of tridymite-quartz transformation.

The Hydrothermal Corrosion and Recrystallization of Noble Metals

not Available.

Compositional variation of alkali feldspars in some intrusive rocks near Globe-Miami, Arizona

Approximate compositional determinations were made by means of X-ray study of the (201) spacing of heated alkali feldspars. Of 32 alkali feldspar specimens from the Lost Gulch quartz monzonite the [early Mesozoic(?)], 17 from within the areas of moderate hydrothermal alteration average 93 weight per cent orthoclase (Or/Or+Ab+An), whereas the remaining specimens, from outside the moderately altered areas, contain only 83 weight per cent orthoclase. Likewise, alkali feldspar specimens (7) from within the moderately altered area near the Inspiration lobe of the Schultze granite [Tertiary(?)] have an average bulk composition of Or 85 , and Schultze granite and Inspiration lobe feldspar specimens (21) from outside the area of moderate alteration have an average bulk composition of only Or 78 . Primary magmatic crystallization differentiation, or hydrothermal crystallization or recrystallization appears to be a plausible hypothesis to explain these compositional differences. Such compositional variations suggest again a genetic relationship between magmatic crystallization and ore concentration.