Low-temperature Mineralization of the sub-Triassic Unconformity Surface and Alteration of the Underlying Intrusions of Southern Leicestershire, England

The Croft diorite in central England belongs to a suite of Caledonian igneous rocks collectively known as the South Leicestershire diorite complex. Although the intrusions occupy separate outcrops, they are linked at depth to form a single pluton, buried beneath a cover of Triassic sediments (Le Bas, 1972; 1982, Allsop and Arthur, 1983). Both the Caledonian diorites and the overlying sub-Triassic unconformity have been affected by a complex history of alteration and mineralization which can be subdivided into four stages: (1) deuteric effects, caused by the release of volatiles during magmatic cooling; (2) albitization through sodic enrichment; (3) formation of lowtemperature laumontite, analcime and calcite veins with associated wall-rock alteration to prehnite and pumpellyite; and (4) sub-Triassic unconformity hosted base metal, manganese and palygorskite mineralization. Zeolite mineralization occured some 200 Ma later than the intrusion itself, during post-Triassic times, as indicated by the presence of a single vein of laumontite and microcrystalline calcite which cross-cuts the sub Triassic unconformity surface and enters the overlying Triassic sediments. Evidence from fluid inclusion work indicates that two fluids were involved in the deposition of the zeolite veins. One fluid was initially of relatively high temperature (~100°C to 320°C) and low salinity (~0.2 to 5.9 wt% NaCl equiv.), and was probably meteoric in origin, whereas the other was of relatively low temperature (~41°C to 165°C) and high salinity (~0.4 to 16.72 wt% NaCl equiv.), and is interpreted to represent a basinal brine. During Triassic rifting, thinning and fracturing of the crust, with the possible rise of associated magmas (Halliday and Mitchell, 1984), could have increased permeability and heat flow, initiating the circulation of hydrothermal fluids. Triassic unconformity-hosted base-metal mineralization in Central England is similar to other Triassic-Jurassic mineralization in Europe (Mitchell and Halliday, 1976).
Keywords: Caledonian igneous rocks, Intrusions, Deuteric effect, Sodic enrichment, Laumontite veins, Analcime veins, Calcite veins, Mineralization
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Summary: The Geological Society has its own quarterly journal called Exploration
and Mining Geology, for the publication of Canadian and international papers on
applied aspects of mineral exploration and exploitation, including mineral
deposit geology, geochemistry, and geophysics, mining geology, mineral resource
appraisal and estimation methods, environmental geology, and case histories. The
editor of the journal is Jeremy P. Richards
Publication: Exploration & Mining Geology
Issue: 2
Volume: 6
Year: 1997
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<BODY>The mechanism by which stratiform ore deposits of PGE (platinum-group
elements) are laid down in ultramafic igneous complexes is not completely clear.
Early studies by the author and coworkers on numerical analyses of the
convective cooling of low-Rayleigh-number magma chambers...
Publication: Exploration & Mining Geology
Author(s): ALAN RICE: Departments of Geology and Physics, Rhodes University, Grahamstown 6140, South Africa
Keywords: PGE, Ultramafic igneous complexes, Magma chambers, Suspended
Issue: 2
Volume: 6
Year: 1997
Web Page
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<BODY>The Campo Morado precious-metal-bearing, volcanogenic massive sulfide
deposits occur in a lower Cretaceous, bimodal, calc-alkaline volcanic sequence
in a major northerly trending belt in the Guerrero Terrane in northeastern
Guerrero, Mexico. During upper Cretaceous to Early...
Publication: Exploration & Mining Geology
Author(s): JIM OLIVER, JOHN PAYNE and MARK REBAGLIATI: Farallon Resources Ltd. Vancouver, British Columbia, Canada V6C 2V6
Keywords: Precious metals, Sulfide deposits, Volcanic, Mexico, Minerals
Issue: 2
Volume: 6
Year: 1997
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Summary: Economic Mn-oxide ore deposits of commercial grade occur in the Rhodope massif near Kato Nevrokopi in the Drama region, Northern Greece. The Mn-oxide mineralization has developed by weathering of continental hypogene rhodochrosite-sulphide veins. The vein mineralization is confined by tectonic shear zones between marble and metapelites, extending laterally into the marble as tabular, pod or lenticular oreshoots (up to 50 m ?? 20 m ?? 5-10 m). Supergene oxidation of the hypogene mineralization...
Publication: Exploration & Mining Geology
Author(s): M.K. NIMFOPOULOS: Institute of Geology and Mineral Exploration (IGME), 546-26 Thessaloniki, Macedonia, Greece; R.A.D. PATTRICK: Department of Earth Sciences, The University of Manchester, Manchester M13 9PL, England, U.K.; K.M. MICHAILIDIS: Department of Mineralogy-Petrology-Economic Geology Aristotle University, 540-06 Thessaloniki, Macedonia, Greece; D.A. POLYA and J. ESSON: Department of Earth Sciences, The University of Manchester, Manchester M13 9PL, England, U.K.
Keywords: Ore deposits, Geology, Geochemistry, Vein mineralization, Supergene oxidation
Issue: 2
Volume: 6
Year: 1997
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Summary: Detailed petrographical and geochemical studies have been carried out on the igneous and sedimentary rocks in the Omai area, part of the Paleoproterozoic Barama-Mazaruni greenstone belt, Guyana. The stratigraphic succession at Omai begins with basalts, associated with mafic ultramafic intrusives and poorly-sorted conglomerates; these are overlain by andesites and quartzfeldspar porphyries, with pelites and tuffaceous sediments at the top. The volcanic-sedimentary succession was intruded by a...
Publication: Exploration & Mining Geology
Author(s): GABRIEL VOICU, MARC BARDOUX, LUC HARNOIS: Département des Sciences de la Terre, Université du Québec à Montréal, Montréal, QC, Canada H3C 3P8 and ROBERT CRÉPEAU:
Omai Gold Mines, Georgetown, Guyana, South America
Keywords: Petrographical studies, Geochemical studies, Guyana, Basalts, Mafic-ultramafic
intrusives, Conglomerates
Issue: 2
Volume: 6
Year: 1997
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Summary: Most large skarn deposits are zoned in both space and time relative to
associated intrusions. Zonation occurs on scales from kilometers to micrometers,
and reflects infiltrative fluid flow, wallrock reaction, temperature variations,
and fluid mixing. The most spectacular examples of skarn zonation usually occur
at the skarn-marble contact, where transitions between monomineralic bands can
be knife sharp. Other small-scale examples occur in zoned veins and individual
mineral crystals....
Publication: Exploration & Mining Geology
Author(s): LAWRENCE D. MEINERT: Department of Geology, Washington State University, Pullman, Washington 99164-2812, U.S.A.
Keywords: Skarn deposits, Skarn zonation, Mineral exploration
Issue: 2
Volume: 6
Year: 1997
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