Geologic reconnaissance of the mineral deposits of Thailand
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Bulletin↗
Geology topics
W. D. Johnston
Publications and source records attributed to W. D. Johnston.
Quartz crystal in Brazil
The peak production of Brazilian quartz crystal was reached in 1943 when 2411 metric tons were exported. Minas Gerais, Goiaz, and Bahia are the principal producing states. Minor quantities are obtained from Espirito Santo and Northeastern Brazil. Primary veins and pegmatites yield secondary deposits of eluvial, colluvial, and alluvial origin. The veins range in form from tabular or lenticular bodies, pipes, and pockets to composite lodes and bedded masses. The mineralogy is simple; accessory minerals are few and rare. A small proportion of clear crystals, occurring in vein masses of milky quartz, furnishes the commercial product. Filling, rather than replacement, is believed to be the dominant emplacement process. Pegmatites yield a little commercial quartz. Both residual (eluvial) and water-laid (alluvial) quartz deposits are economically important. Crystals range up to 40 metric tons in weight; those of 1 to 5 tons occur in many deposits. Quartz from every locality studied is twinned on both the Brazil and Dauphiné laws. Phantoms are abundant. Field studies reveal little difference between the final quartz from some pegmatites and that from veins. It is inferred that the veins may grade into pegmatites at depth and that both are products of the same mineralogenic epoch. Present evidence does not justify a closer dating for the formation of the veins and pegmatites than the early Paleozoic. There are few large quartz mines in Brazil. The impressive quartz production during the war years came from many small, scattered workings. Mining methods are primitive, and mechanization was feasible in relatively few areas. The ratio of mine-clean crystal to the total quantity of excavated material varies between 1:1000 and 1:100,000, with an over-all average for the best mines of the order of 1:10,000. Cost of mining ranges from $2 to $10 a kilogram. The average export value during 1944 was $12.50 per kilogram. The potential reserves of quartz crystal in Brazil are large, but actual or visual reserves in known districts are small. Future production capacity will be determined by economic factors.
GSA Bulletin↗
Sulphate minerals of the Comstock Lode, Nevada
Seventeen representative samples of supergene sulphates from old workings on the Comstock Lode are described. They range from simple minerals such as gypsum and epsomite to complex aggregates of four or more distinct species. All are well known species except a mineral of the copper (chalcanthite) or magnesium sulphate pentahydrate group, with about half the magnesium replaced by copper, zinc, ferrous iron, and manganese. Four new analyses of Comstock Lode mine waters are given. © 1938 Society of Economic Geologists, Inc.
Nevada↗
Isometric block diagrams in mining geology
In the past five years members of the Geological Survey have gained experience in making isometric block diagrams of mines and mining districts as well as of surface features. This paper presents nothing new, but aims to assemble scattered information on a much neglected method of geological illustration. Plotting mine workings on isometric paper is the usual method but is extremely time-consuming. For most purposes the use of some form of isometric pantograph greatly lessens the labor of constructing the diagram. The devices of Dufour, Wentworth, Wilson, and Van der Hoop are described
Economic Geology↗
Nodular, orbicular, and banded chromite in northern California
IN I930 Mr. A. F. Duggleby gave me a specimen of orbicular chromite from the Octopus claim, in Siskiyou County, Calif., showing two generations of chromite, one older than the associated olivine and one younger. In November, I93I, I examined briefly a number of deposits in northern California, in search for chromite showing similar and related textures. Some of the more unusual and genetically significant occurrences are described in this paper. Most of the chromite localities here described are in Siskiyou County, northern California. Localities I and 2 are in the Seiad Quadrangle, where a belt of chromite-bearing dunite, striking northwest, extends from McGuffy Creek, near Scott Bar, into Oregon. Other deposits are in southern Siskiyou, Shasta, and Trinity Counties.
Economic Geology↗
Structural history of the fracture systems at Grass Valley, California
In the foregoing outline of the structural history of the Grass Valley intrusive the vein fractures are regarded as elements of the internal tectonics of the intrusive body itself. They originated in an early stage of the structural development of the region, and are equivalent to the marginal thrusts observed in the Yose-mite and along the Mother Lode system . As such, the vein fractures are of local origin, since they are entirely restricted to the neighborhood of intrusive contacts. As the area of consolidation within the Sierra Nevada batholith increased, larger units were affected by tectonic stresses, and joints were formed which belong to a system of regional extent. As this system has been observed over an area of at least 7500 square miles, its regional character seems proven. Basic dikes and aplites follow the crossing system , and thus indicate that the earliest crossings were formed when magma to fill them was still available. Both fissure systems repeatedly served for adjustment of later stresses, as can be proved by relative mutual displacements and by the repeated fracturing and renewed deposition of quartz within the vein fissures. It is probable that these later stresses were regional in character.
California↗