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The petrogenetic and metallogenetic significance of topaz granite from the Southwest England orefield

D. A. C. Manning and P. I. Hill
The petrogenetic and metallogenetic significance of topaz granite from the Southwest England orefield (in Ore-bearing granite systems; petrogenesis and mineralizing processes, Holly J. Stein (editor) and Judith L. Hannah (editor))
Special Paper - Geological Society of America (1990) 246: 51-70


Late-stage volatile-rich topaz granites occur widely but sparsely throughout the southwest England Sn-Cu-polymetallic mineralized S-type biotite granite batholith. New observations from the St. Austell area have clarified field relations, and demonstrate the importance of an aureole of tourmalinization affecting both granitic and sedimentary host rocks. Topaz granite contacts are often marked by pegmatitic zones showing undirectional solidification textures and carrying vugs of quartz-tourmaline; minor intrusive sheets have symmetrical haloes of tourmalinization within adjacent host rocks. The topaz granites are mineralogically complex, containing primary topaz, zinnwaldite, or lepidolite, amblygonite (and other phosphates), and various Nb-Ta rich accessory phases, as well as albite, orthoclase, and quartz. Fluorite is secondary. They are chemically distinct from the biotite granites, showing markedly higher concentrations of Li (sub 2) O (as much as 0.5 percent), F (as much as 1.5 percent), P (sub 2) O (sub 5) (0.5 percent), Nb (as much as 65 ppm), Ta (as much as 30 ppm), Ga (as much as 50 ppm), and Rb (as much as 2,000 ppm), in particular. Late differentiates of the biotite granites include tourmaline granites, but this differentiation trend principally involves an increase in B with little change in Li, F, or P. It is considered that the topaz granites are unlikely to be derived by fractional crystallization of the southwest England biotite granite magma (they are intruded by rhyolite porphyry dikes, which belong to the biotite granite suite), and an origin involving limited partial melting of subbatholithic fusion residues during an episode of potassic basic magmatism is preferred. Comparison with other volatile-rich granitic rocks indicates that certain lithium pegmatites (e.g., Tanco), other topaz granites (Seward Peninsula, Erzgebirge, etc.) and volcanic glasses (e.g., Macusani) share important characteristics with the southwest England topaz granites. It is suggested that these rock types may represent a fundamentally similar volatile-rich granite magma type whose formation, while debatable, may be controlled by limited partial melting of lower crustal fusion residues that had previously generated more "normal" granite magmas.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 246
Title: The petrogenetic and metallogenetic significance of topaz granite from the Southwest England orefield
Title: Ore-bearing granite systems; petrogenesis and mineralizing processes
Author(s): Manning, D. A. C.Hill, P. I.
Author(s): Stein, Holly J.editor
Author(s): Hannah, Judith L.editor
Affiliation: University of Newcastle upon Tyne, Department of Geology, Newcastle upon Tyne, United Kingdom
Affiliation: U. S. Geological Survey, Denver, CO, United States
Pages: 51-70
Published: 1990
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-2246-2
References: 60
Accession Number: 1998-034544
Categories: Economic geology, geology of ore depositsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 2 tables, sketch maps
N49°55'60" - N51°30'00", W05°45'00" - W02°34'60"
Secondary Affiliation: University of Vermont, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199814
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