Gold Exploration in Tropically weathered terrains: The formation, evolution and geochemistry of lateritic profiles in Liberia and Guinea, West Africa
Gold Exploration in Tropically weathered terrains: The formation, evolution and geochemistry of lateritic profiles in Liberia and Guinea, West Africa
Gold-mineralisation at Largor, Liberia, is hosted within a discrete, amphibolite and granitoid bound, E-W trending zone of variably sheared and recrystallised ultramafic-mafic (amphilbole+chlorite+/-serpentine+talc) schists. The host exhibits E-W-trending dextral ductile shear deformation and lower amphibolite facies (c.550oC) metamorphism. There is little evidence for pervasive hydrothermal alteration. Two distinct associations of disseminated sulphides and arsenides are developed; pyrrhotite-chalcopyrite is ubiquitous, possibly representing a remobilised primary igneous assemblage; arsenopyrite-niccolite-pyrrhotite is more locally developed, related to a later-stage As-rich fluid. Gold mineralisation is disseminated, occurring dominantly as (syngenetic?) inclusions within amphiboles, with lesser occurrences of host-rock and Au intimately associated with pyrrhotite.
Intense tropical weathering has created a residual ferralitic stone-line-type profile comprising of a well developed saprolite, capped by a thin nodular/gravelly soil and occasionally clayey-silty soils. Detailed textural studies show that weathering commences by the breakdown of primary sulphides and arsenides. Saprolitisation is isovolumetric. Upper saprolite comprises of an open boxwork of peripheral pseudomorphs, comprising of Fe oxyhydroxide- and kaolinite-rich rinds, developed by incongruent dissolution of amphiboles in the lowermost saprolite and chlorite in the upper saprolite. Partial gibbsitic pseudomorphs form directly during chlorite alteration. Partial kaolinite pseudomorphs after amphibole are occasionally developed in the chlorite-dominated domains of the lower saprolite.
Gravelly soils (pisoliths, saprolitic relicts and quartz fragments in a kaolinitic matrix) have formed by the physical collapse of the upper saprolite. The presence of lateritic pisoliths incorporated within soils is strong evidence of a previously dismantled duricrust. More recent truncation down to saprolite levels in some locations has occurred, and residual pisoliths have been incorporated into the soils during saprolite collapse. Subsequent surficial leaching is promoting the removal of Fe, forming residual kaolinite- and quartz-rich clayey-silty soils. Localised interaction with the water-table is promoting Fe remobilisation, transforming the upper saprolite into a mottled clay, eventually forming an outcropping secondary pseudo-duricrust.
University of Southampton
Evans, Jon Michael
197f0c09-cb99-4941-be6b-9437cd4ab591
2001
Evans, Jon Michael
197f0c09-cb99-4941-be6b-9437cd4ab591
Evans, Jon Michael
(2001)
Gold Exploration in Tropically weathered terrains: The formation, evolution and geochemistry of lateritic profiles in Liberia and Guinea, West Africa.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Gold-mineralisation at Largor, Liberia, is hosted within a discrete, amphibolite and granitoid bound, E-W trending zone of variably sheared and recrystallised ultramafic-mafic (amphilbole+chlorite+/-serpentine+talc) schists. The host exhibits E-W-trending dextral ductile shear deformation and lower amphibolite facies (c.550oC) metamorphism. There is little evidence for pervasive hydrothermal alteration. Two distinct associations of disseminated sulphides and arsenides are developed; pyrrhotite-chalcopyrite is ubiquitous, possibly representing a remobilised primary igneous assemblage; arsenopyrite-niccolite-pyrrhotite is more locally developed, related to a later-stage As-rich fluid. Gold mineralisation is disseminated, occurring dominantly as (syngenetic?) inclusions within amphiboles, with lesser occurrences of host-rock and Au intimately associated with pyrrhotite.
Intense tropical weathering has created a residual ferralitic stone-line-type profile comprising of a well developed saprolite, capped by a thin nodular/gravelly soil and occasionally clayey-silty soils. Detailed textural studies show that weathering commences by the breakdown of primary sulphides and arsenides. Saprolitisation is isovolumetric. Upper saprolite comprises of an open boxwork of peripheral pseudomorphs, comprising of Fe oxyhydroxide- and kaolinite-rich rinds, developed by incongruent dissolution of amphiboles in the lowermost saprolite and chlorite in the upper saprolite. Partial gibbsitic pseudomorphs form directly during chlorite alteration. Partial kaolinite pseudomorphs after amphibole are occasionally developed in the chlorite-dominated domains of the lower saprolite.
Gravelly soils (pisoliths, saprolitic relicts and quartz fragments in a kaolinitic matrix) have formed by the physical collapse of the upper saprolite. The presence of lateritic pisoliths incorporated within soils is strong evidence of a previously dismantled duricrust. More recent truncation down to saprolite levels in some locations has occurred, and residual pisoliths have been incorporated into the soils during saprolite collapse. Subsequent surficial leaching is promoting the removal of Fe, forming residual kaolinite- and quartz-rich clayey-silty soils. Localised interaction with the water-table is promoting Fe remobilisation, transforming the upper saprolite into a mottled clay, eventually forming an outcropping secondary pseudo-duricrust.
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Published date: 2001
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Local EPrints ID: 464616
URI: http://eprints.soton.ac.uk/id/eprint/464616
PURE UUID: bab27aa4-2862-42f7-8509-6186b6a1ca14
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Date deposited: 04 Jul 2022 23:51
Last modified: 16 Mar 2024 19:39
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Author:
Jon Michael Evans
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