Geology topics

A. H. Koschmann

Publications and source records attributed to A. H. Koschmann.

8 recordsLinked to original sources

USGS2014

Volcanic history of the Magdalena District, New Mexico

The Magdalena District in central New Mexico comprises the Magdalena Range and Granite Mountain which are block mountains of the Basin Range type. They consist of Carboniferous sedimentary formations on a pre-Cambrian basement and are covered by Tertiary volcanic rocks. Volcanism in the District was preceded by considerable faulting, tilting, and uplifting very probably in early Tertiary time. The initial volcanic extrusion consisted of latite flows and tuffs now resting unconformably on the Granite Mountain fault-block in the northern part of the District. Subsequent extrusions consisted of alternations of andesites, rhyolites, and latites, and have a minimum thickness of approximately 4,000 feet. Graben faulting, probably in compensation for the effusion of the large volumes of lava, followed and some of the major faults were subsequently the sites of the intrusion of monzonitic and granitic stocks. Late lamprophyre and white rhyolite dikes in large numbers intruded the intrusive as well as earlier extrusive and sedimentary rocks.

New Mexico↗

USGS1960

Mineral paragenesis of precambrian rocks in the Tenmile Range, Colorado

A Precambrian complex of granulite, gneiss, and migmatite, intruded by numerous plutons of granitic rocks correlated with the Silver Plume granite, is exposed in a long narrow belt along the crest and upper slopes of the Tenmile Range , Colorado . The metamorphic rocks are predominantly felsic; bands, lenses, and irregular bodies of mafic rocks rich in biotite, hornblende, and locally in sillimanite and garnet, are interlayered with the felsic rocks . The major lithologic variations in the metamorphic rock complex are believed to be due chiefly to variations in the original sedimentary rocks , which probably were interbedded sandstone, shale, and limestone. The metamorphic rocks and the Silver Plume granite reveal the age relations of quartz and the feldspars, and these relations afford considerable information on the origin and progressive transformation of the rocks . Quartz is the earliest mineral in the metamorphic rocks and is probably a relict mineral of a sandstone. It has been partially replaced by feldspar. It occurs chiefly in irregular clusters, some of which show sutured grains, enclosed in a ramifying network of feldspar. Irregular small apophyses, barbs, and prongs of feldspar penetrate the quartz clusters along grain boundaries and healed fractures in the quartz. In some of the least feldspathized quartzose metamorphic rocks the feldspar is clearly interstitial to the quartz. Quartz also occurs in feldspar as small spherical inclusions. The relations of the quartz to the feldspars show clearly that a quartzose host rock was replaced by feldspar along quartz grain boundaries, pre-existing healed fractures, and margins of shadowy areas in strained quartz grains. The textural relations of the other principal minerals in the metamorphic rocks show that plagioclase formed earlier than the microcline and that the micas were the last of the principal minerals to form. Identical paragenetic relations are found in the Silver Plume granite, and the writer concludes that the Silver Plume granite was derived by partial fusion of quartzose metamorphic rocks .

Colorado↗

USGS1958

Minerals of the cassiterite-bearing veins at Irish Creek, Virginia, and their paragenetic relations

Major rock types of the Irish Creek district are gneisses and schists, intruded by granodiorite. All these rocks are believed to be Precambrian. The ore deposits are fissure veins consisting largely of quartz veins bordered by greisen, and enriched by recurrent deposition. From field and microscopic evidence six stages of mineral formation have been deduced: 1. Crystallization of the granodiorite with the formation of hypersthene, augite, hornblende, andesine, orthoclase, microcline, and quartz; and accessory apatite, rutile, ilmenite, titanomagnetite, and zircon. 2. Metamorphism of the granodiorite and gneiss with formation of uralitic actinolite, epidote, and sphene. 3. A period of fracturing and formation of quartz veins. 4. Greisenization of the host rock producing first: coarse muscovite, brown biotite, cassiterite, beryl, wolframite, and quartz; then producing a second group of minerals-fluorite, green biotite, chlorite, phenakite, siderite, ankerite, parisite, sphalerite, sulfides, including aikinite and galeno-bismutite, leucoxene, fine muscovite, and quartz. 5. Formation of late vein minerals, clinozoisite, calcite, nontronite, and montmorillonite. 6. Formation of surficial alteration products, kaolinite, vermiculite, hematite, limonite, and scorodite. The italicized mineral names indicate the minerals heretofore unreported from Irish Creek; parisite, aikinite, and galenobismutite are new minerals for Virginia. Included in the detailed descriptions of all the minerals is a chemical analysis of beryl.

Virginia↗

USGS1944