The ore deposits of Utah by unknow

Author:unknow
Language: eng
Format: epub
Tags: Ore deposits, Mines and mineral resources
Publisher: Washington : Govt. Print. Off.
Published: 1920-03-25T05:00:00+00:00

ORE DEPOSITS OF UTAH.

extensively altered rock. In much of the altered rock small irregular crystals of rutile are rather abundant. The secondary biotite, which is the characteristic mineral of the dark "porphyrj^," differs from the original biotite in being in small foils and in being distinctly lighter in color, ranging from an olive-green with pleochroism in varying intensity to light brown with pleochroism much less pronounced than that of the original mica.

Wliere alteration has been relatively intense the original biotite phenocrysts have been entirely altered, and the secondary mica, instead of occupying approximately the area of the mineral from which it was formed, is disseminated rather uniformly through the mass. Granular quartz and clear unaltered orthoclase, evidently both secondary, occur in places. Sulphides are disseminated irregularly through the rock.

Light ''porphyry.'" —Tlie light "porphyry" is a light-gray rock in which phenocrysts of feldspar and grains of sulphide can be recognized in the hand specimen. Under the microscope it is seen to be composed of phenocrysts of feldspar and some crystals of altered biotite in a fine granular groundmass. (See PI. XVI, J5, p. 165.) The original phenocrysts of feldspar are orthoclase, and all of them contain numerous foils of sericite, which in many is very abundant. Other areas are composed of a felted mass of sericite that may represent altered plagioclase crystals. The few mica phenocrysts are largely bleached to a colorless mica but contain small lenses of brown mica showing their derivation from biotite. Tlie groundmass which forms a large percentage of the rock is composed of a fine granular intergrowth of clear orthoclase and quartz with some foils of sericite. In many specimens this granular groundmass embays the orthoclase crystals and extends along fractures and cleavages; there can be no doubt that the orthoclase phenocrysts are being altered to material of the character of the groundmass, and it is but natural to conclude that the feldspar of the groundmass represents in part an alteration of the earlier feldspar of the rock.

The rock is cut by small veinlets composed of quartz or quartz and orthoclase and grains of sulphide. The larger veinlets, which are very abundant in many places, are composed

of C[uartz, so far as observed, but many of the small veinlets contain much orthoclase. There is no known difference in the age or occurrence of these veins, and there is doubtless every gradation from one to the other.

The original sulphides—pyrite and chal-copyrite with some bornite—are disserriinated through the rock and are present in the veins w^here they have usually been the latest minerals to form, filling the spaces between quartz crystals.

Molybdenite in small flakes is disseminated through the rock in many places, and in a few it is rather abundant in veinlets, where it crystallized in part earlier than the quartz.

A light-colored type of "porphyry" containing numerous crystals of a yellow micaceous mineral is seen under the microscope to be highly sericitized. The yellow crystals are biotite that has been bleached to a nearly colorless mica, probably muscovite filled with yellow grains of rutile.

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