Geology topics

D. F. Hewett

Publications and source records attributed to D. F. Hewett.

16 recordsLinked to original sources

USGS1971

Coronadite — Modes of occurrence and origin

The lead manganate, "coronadite," was first recognized at the Coronado mine in the Morenci district, Arizona, by Lindgren in 1903. Several years later, the identity of the mineral was questioned and it was not until 1932 that Orcel recognized it in material from Morocco. Since 1955, the mineral has been recognized at numerous places; in the USSR, east of the Ural Mountains, in several districts in Morocco, England, Australia, and India. In recent years, coronadite had been identified at numerous localities in New Mexico, Arizona, Nevada, and California by the writer and others. At this time, it has been identified at 22 localities, and in the opinion of this writer, it will be found at many more places if it is sought for carefully. Author's review of the known occurrences of coronadite and interpretation of the modes of geologic environment show that the mineral is found in veins and hot spring aprons, where it seems to have been deposited by hydrothermal waters from depth and that it is one of the manganese oxide minerals deposited as sediments. Probably it is more widely found in assemblages of minerals of supergene origin.

Arizona, Asturias, California, Chihuahua, Durango,↗

USGS1967

Aurorite, argentian todorokite, and hydrous silver-bearing lead manganese oxide

During a study of hypogene manganese minerals, three silver-bearing manganese oxides were identified in "black calcite" associated with silver ores at the Aurora mine (Treasure Hill), Hamilton, Nevada. Specifically these are: (1) argentian chalcophanite (aurorite) (Ag 2 Ba,Ca,Pb,-K 2 ,Cu,Mn (super +2) )Mn 3 (super +4) O 7 . 3H 2 O; (2) argentian todorokite (Ag 2 ,K 2 ,Ca,Ba,-Na 2 ,Cu,Pb)Mn 4 (super +4) O 9 . 5H 2 O; and (3) hydrous silver-bearing lead manganese oxide (Pb,Cu,Ba,Sb,Ag 2 ,Ca,K 2 )Mn 5 (super +4) O 11 . 5H 2 O. Based on the large concentration of silver in argentian chalcophanite (7.50% Ag 2 O) and the low zinc content (0.25% ZnO) this mineral is recognized as a new species and named "aurorite."Physical and optical properties, and chemical analyses of the three minerals are presented together with X-ray powder diffraction data for aurorite and argentian todorokite. All chemical analyses were done with the electron microprobe analyzer. Due to extremely small particle size and textural relations, no X-ray powder data were obtained for the hydrous silver-bearing lead manganese oxide. Associated minerals include cryptomelane, pyrolusite, birnessite(?), cerargyrite, native silver, quartz, and manganoan calcite. Although the general uniform dark color of the "black calcite" and apparent uniform distribution of the inclusions suggests contemporaneous formation, examination of polished sections at high magnifications shows a definite paragenetic sequence. Manganoan calcite is replaced by fine-grained intergrowths of cryptomelane, pyrolusite, birnessite(?), and cerargyrite. Native silver formed early and the silver-bearing manganese oxides formed late in the sequence; aurorite fills microfractures that cut through all other minerals.These three silver-bearing manganese oxides were recognized only at the Aurora mine, Hamilton, Nevada, although black calcite veins were studied from numerous areas in the western United States. The geologic features of silver-bearing black calcites are described by Hewett and Radtke in the preceding paper.

Nevada↗

USGS1967

Silver-bearing black calcite in western mining districts

The name black calcite has been applied from time to time to a dark gray to black variety of calcite or aragonite in metalliferous ore deposits in the Western States. Most of the material shows curved, roughly rhombic cleavage. The color is due to dispersed minute grains of one or more oxides of manganese, so dispersed that they rarely show any relation to cleavage surfaces or outward crystal forms.Recent study shows that the black calcite from numerous metalliferous deposits in the Western States contains small amounts of several metals, especially zinc, lead, and silver; in places, the crude black calcite contains as much as one percent silver and the black residue remaining after solution of the calcite as much as 1,500 ounces of silver to the ton. Black calcite is recognized in some deposits of Late Cretaceous to early Tertiary age where the host is Paleozoic carbonate rocks; thus far, this variety contains the larger amounts of silver. It is also present in some vein deposits where the host is generally volcanic rocks of middle Tertiary age; these are known in many districts in the southwest and even though the lead and zinc content is noteworthy, the silver content rarely exceeds one ounce to the ton (8).By studying polished sections of black calcite from the Aurora mine, White Pine County, Nevada, three silver-bearing manganate minerals have been recognized; (1) an argentian chalcophanite containing 7.5 percent silver (Ag 2 O), to which, in a following paper, the name aurorite is given; (2) argentian todorokite containing 3.9 percent silver (Ag 2 O), and (3) hydrous silver-bearing lead manganese oxide, containing 1.18 percent silver (Ag 2 O).

Nevada↗

USGS1966

Stratified deposits of the oxides and carbonates of manganese

Compared with the stratified deposits of iron minerals, those of manganese minerals have received little attention until recent years. Before 1930, students of the stratified deposits of manganese minerals have generally concluded that the contained manganese was derived from the decomposition of the rocks that formed the borders of the basins. Only in a few places have geologists recognized that if these rocks were the principal source of the contained manganese the basins should contain also enormous quantities of iron minerals, whereas most of the large deposits of manganese contain little iron. Even though more than 100 years ago some geologists proposed that the iron contained in some deposits of stratified iron minerals was probably derived from hydrothermal waters related to centers of volcanism, not until about 1930 was this source of manganese in similar deposits seriously proposed. This mode of origin was given the name "volcanogene sedimentaire" by French geologists working in Morocco. Since then, other names, such as "exhalative sedimentaire," have been used by geologists working in several European districts. Because of the development of new techniques for the study of the chemical and physical features of the minerals, and because close attention to the lithologic environments of the beds is being given more and more during recent years, it has seemed advisable to review the features of stratified deposits of the manganese minerals in many parts of the world and over a wide range in age. This study indicates that at least three sources may have contributed the manganese in the large deposits of iron-free oxides and carbonates : (1) the rocks that form the borders of the basins, either marine or continental; (2) the nearby underlying sediments, largely of igneous origin, decomposed by warm waters largely derived from depth; and (3) waters of hydrothermal origin derived from great depths during epochs of volcanism from which iron minerals with little manganese are deposited in deep zones, then minerals with much iron and more manganese at intermediate depths, and, finally, manganese minerals with little iron near the surface. As the hot waters of many thermal springs contain more manganese than iron, such waters could yield the pure manganese oxides and carbonates found in stratified deposits . Several kinds of evidence indicate that most of the manganese in the large stratified deposits of the oxides and carbonates of manganese in many parts of the world has been derived from hydrothermal waters from depth related to centers of volcanism. Obviously, manganese derived from decay of the rocks on the lands adjacent to large basins-marine and continental -may have been added to that derived from centers of volcanism to form the sedimentary deposits found in the basins.

Economic Geology↗

USGS1964

Veins of hypogene manganese oxide minerals in the southwestern United States

Characteristic minerals are psilomelane, hollandite, cryptomelane, and coronadite, more rarely ramsdellite and pyrolusite. Host rocks are Mn-deficient; 80 percent of examples are middle to late Tertiary layered volcanics. Though deposits are shallow, mostly mined to only 100-200 feet (maximum 500 feet), a hypogene origin is indicated by their persistent association with barite and fluorite, a peripheral position in the zonal pattern of some metal-mining districts, alteration of plagioclase to K-spar, and abundance of W, Pb, Cu, Mo, Ti, As, Sb. They represent the subzone of Mn-bearing epithermal vein deposits lying nearest the surface, succeeded in depth by four other subzones: barite, fluorite, gold-silver, and base metals.

Arizona, California, Colorado, Nevada, New Mexico,↗

USGS1961

Tephroite in California manganese deposits

Recent studies of manganese deposits and mineral specimens in the Sierran belt of sedimentary rocks as well as in the Klamath Mountains to the north and in the Rand Mountains to the south, have shown the presence of tephroite , the orthosilicate of manganese (Mn2Si04), at numerous localities. Earlier studies of these deposits have shown that the original layered deposits of carbonate and hydrous silicates of manganese have been widely altered by metamorphism to spessartite (Mn3Al2-(SiO.),), rhodonite (MnSi03), and piedmontite (Ca2(Al,Mn"',Fe"')3-Si3Oi2(OH)). Recent work shows that layered manganese oxides also were present in the original sedimentary rocks. It shows also, that tephroite has formed widely in the original assemblage of carbonate, silicate, and oxide of manganese ; in several localities, a little alleghanyite (2Mn2-S1O4 - Mn(OH,F)2) has been formed. Rhodonite, spessartite, and piedmontite uniformly follow the tephroite ; in places, bementite and neotocite are present. In one deposit a little pyroxmangite ( (Mn,Fe,Ca)Si03) has been noted and in another, some crystals that are probably pyrophanite (MnTi03), not yet recorded in the United States. By contrast, the restudy of large collections of material from deposits in sedimentary rocks of the Franciscan formation in the Coast Ranges indicates the tephroite is very uncommon; it has been recognized with assurance at only one locality-Alum Rock Park, Santa Clara County. In the Coast Ranges, rhodonite and spessartite, the high manganese garnet, are very uncommon. The assemblages of manganese silicates indicate that the layered deposits of manganese minerals in the Sierra belt have been metamorphosed to a higher degree than those of the Coast Ranges. This review also shows the presence of axinite, the boro-silicate of aluminum, calcium, and manganese , in three deposits .

California↗

USGS1954