Lithological and Geochemical Features of Igneous and Sedimentary Rocks at the Omai Gold Mine, Guyana, South America

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 quartz-monzodioritic stock (Omai stock) and granophyric rhyolite dikes. Several generations of mafic dikes were intruded discontinuously from the Mesoproterozoic to the Permo-Triassic. Volcanic rocks range in composition from subalkaline basalt to high-silica rhyolites, with a gap between 60% to 70% SiO2, which suggests the suite is bimodal. Major, trace, and rare earth element criteria were used to subdivide the rocks into tholeiitic and calc-alkaline types. The tholeiitic rocks are characterized by flat to moderately sloping chondrite-normalized REE patterns. On the basis of immobile trace-element ratios, the tholeiitic mafic rocks from Omai have characteristics of immature intra-oceanic island arcs, or of E-MORB basalts affected by a strong arc signature, as is commonly reported for back-arc basin basalts. The andesites, quartz-feldspar porphyries, rhyolites and quartz monzodioritic stock are calc-alkaline, with strongly sloping REE patterns, enrichments in the large-ion lithophile elements, and depletions in the heavy REE and high-field-strength elements, features which are typical of more mature island-arc settings. At Omai, two types of gold-bearing veins can be distinguished: vein sets (± stockworks) and tension veins, both post-dating the regional metamorphic peak at the level they were emplaced. The relationship between the gold mineralization and its host lithologies does not seem to be genetic.
Rather, the emplacement of mineralization was strongly influenced by the rheological contrasts between the intrusive/subvolcanic bodies and the volcanic/sedimentary country rocks. These contrasts helped to focus the regional stress upon the more competent intrusive subvolcanic bodies, resulting in brittle fracturing, and therefore in greatly enhanced permeability to hydrothermal fluids. The Omai gold mineralization can be classified as late tectonic, with its emplacement controlled by the last brittle to brittle-ductile stages of the Trans-Amazonian orogeny.
Keywords: Petrographical studies, Geochemical studies, Guyana, Basalts, Mafic-ultramafic
intrusives, Conglomerates
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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: 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...
Publication: Exploration & Mining Geology
Author(s): K. PEARSON and C.A. JEFFREY: Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, U.K.
Keywords: Caledonian igneous rocks, Intrusions, Deuteric effect, Sodic enrichment, Laumontite veins, Analcime veins, Calcite veins, Mineralization
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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