Gahnite in the metamorphosed stratiform massive sulfide deposits of the Mineral District,Virginia, U.S.A. |
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| Authors: | D J Sandhaus J R Craig |
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| Institution: | (1) Department of Geological Sciences, Virginia Polytechnic Institute and State University, 24061 Blacksburg, Virginia, USA;(2) Present address: Versar Inc, P.O. Box 1549, 22151 Springfield, VA, USA |
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| Abstract: | Summary Ferroan-gahnite, (Zn,Fe)Al2O4, is a common accessory mineral found in association with the metamorphosed volcanogenic massive sulfide ore bodies of the Mineral District, Virginia. Gahnite is present throughout the mineralized zones that contain pyrite, sphalerite, chalcopyrite, and sulfosalts. Although constituting only 1–2 modal percent of most samples and displaying not well-defined distribution pattern, gahnite is especially abundant at the margins of the mineralized zones and locally constitutes as much as 35 modal percent. The compositions vary slightly from one deposit to another but all lie within the range of 68 to 88 mol.% gahnite (ZnAl2O4), 8 to 25 mol.% hercynite (FeAl2O4), and 1–17 mol.% spinel (MgAl2O4). Gahnite occurs within a wide variety of mineral assemblages, with or without coexisting sphalerite.The reflectance and unit cell dimension are sensitive to composition and decrease with increasing gahnite component. The approximate 8% reflectance at 540 nm is intermediate between most silicates and ore minerals such as sphalerite and magnetite and thus provides a ready method for recognition in reflected light. The gahnite is believed to have formed during amphibolite grade regional metamorphism as a result of either sphalerite desulfidation or the reaction of original zinc oxide phases.
Gahnit in metamorphen schichtgebundenen Sulfid-Lagerstätten des Mineral-Distriktes, Virginia, U.S.A. Zusammenfassung Eisenhaltiger Gahnite (Zn,Fe)Al2O4 ist in den metamorphen vulkanogenen massiven Sulfiderzkörpern Virginias als akzessorisches Mineral weit verbreitet. Er kommt durchwegs in den Pyrit, Zinkblende, Kupferkies und Sulfosalze enthaltenden mineralisierten Zonen vor. Obwohl die meisten Proben nur 1–2 Modal % Gahnit enthalten, und keine klar definierte Verteilung zu erkennen ist, tritt er besonders häufig an den Rändern der mineralisierten Zonen auf und erreicht lokal 35 Modal %. Die Zusammensetzung schwankt leicht von Vorkommen zu Vorkommen, liegt aber stets zwischen 68 und 88 Mol.% Gahnit (ZnAl2O4) 8 und 25 Mol.% Hercynit (FeAl2O4) und 1 und 17 Mol.% Spinell (MgAl2O4) Gahnit ist Bestandteil sehr verschiedener Mineralparagenesen mit und ohne Zinkblende.
With 7 Figures |
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