GEOCHEMISTRY, PETROLOGY AND MINERALISATION AT RIRIWAI, GINDI AKWATI AND DUTSEN WAI IN THE NIGERIAN YOUNGER GRANITE PROVINCE.
GEOCHEMISTRY, PETROLOGY AND MINERALISATION AT RIRIWAI, GINDI AKWATI AND DUTSEN WAI IN THE NIGERIAN YOUNGER GRANITE PROVINCE.
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Date
1976
Authors
ABAA, SOLOMON IORKOSO
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Abstract
The Nigerian Younger Granite Province is composed of anorogenic
Mesozoic ring complexes which were subjected to two distinct processes
of differentiation, resulting in two series; the hornblende - fayalite granite
to hornblende - biotite - granite to biotite-granite and the
hornblende-fayalite-granite to arfvedsonite - fayalite - granite to
riebeckitic - arfvedsonite - granite. Of the two end members; biotite
granite is peraluminous while the riebeckitic - arfvedsonite - granite
is peralkaline Both rocks can be closely associated and have been
Mineralised.
The geochemistry and petrology of three selected mineralised areas
of this Younger Granite Province have been studied. Detailed maps of
the mineralised zones have been produced. Samples were selected from
the surface as well as from cores at depth and both altered and unaltered
samples have been studied mineralogically and by chemical analyses. The
results of the chemical analyses and petrological studies have been used
to suggest the composition of the mineralising fluids, the alteration
processes and the ore minerals formed.
At Ririwai, two phases of mineralisation have been found; a prejoint
autometamorphic mineralisation in which K-feldspars replaced
earlier perthites, while new albite developed. Silicification and recrystallisation
changed the original rock texture and introduced dispersed
mineralisation with thorite, columbite, xenotime and hafnium-uranium rich
zircon as well as enrichment in some trace elements. The second phase
of mineralisation was a post-joint replacement greisen-mineralisation
which took place in the roof zones of the cooling consolidated biotite
granite.. It involved a metasomatic introduction of cassitorito and sulphide
ores into the crystalline biotite granite along cooling joints,
fissures and fractures. The sequence of replacement has been found to
be haematisation and kaolinisation - chloritisation and sericitisation -
greisenisation - silicification.
At Gindi Akwati, only post-joint replacement mineralisation occurred
in the Older Granite with introduction of cassiterite, sulphide ores and
some trace elements which were also enriched in the associated dolerite,
dykes. The alteration started by shearing and mylonitisation - argillisation
and haematisation -chloritisation and museovitisation - greisenisation
- silicification and feldspathisation - feldspathisation.
At Dutson Wai, the roof zones of the biotite granite were affected
by post joint mineralisation, and weathering of the roof zones of the
granite concentrated cassiterite in the alluvials. In the per-alkaline
granite, there was recrystallisation with introduction of albite, microcline
and the accessory minerals cryolite, pyrochlore, amblygonite and
astrophyllite.
.From the ore minerals formed, and the chemical and mineralogical
composition of the altered wall rocks and the unaltered rocks, it is
shown that in the peralkaline granite, at the time of crystallisation,
residual fluids became increasingly enriched in volatiles as temperature
dropped and crystallisation continued. The fluids continued to change
continuously from an agpaitic magma to a sodium - silicate rich hydrothermal
solution without supercritical phenomena, and the magmatic fluids
progressed to hydrothermal fluids without an aqueous phase seperating
off. At late stages, and near surface conditions, the confining pressure
decreased to below the hydrostatic pressure, causing an immiscible liquid
consisting predominantly of salts and volatiles to separate. This
led to riebeckitisation of arfvedsonite, introduction of albite, quartz
and microcline, mineralisation with pyrochlore, cryolite and astrophyllite
and enrichment in some trace elements.
In the biotite granites, as fractional crystallisation continued,
the concentration of volatiles increased in the residium until saturation
when an aqueous phase separated. The cooler crystalline portions of
the granites in the roof zones of the intrusions were invaded by the
then immiscible phases from the still crystallising granites at slightly
deeper levels, causing albitisation, dispersed mineralisation with
thorite, columbite, xenotime as well as enrichment in some trace elements.
The late hydrothermal fluids introduced cassiterite with sulphide minerals
into crystalline host rocks along cooling joints, fissures, fractures
and even faults and the regional foliation of the Basement rocks
were utilised. This form of mineralisation was accompanied by wall-rock
alteration, while the cassiterite was transported in the ore fluids in
the form of alkali-thio-stannate or/and in the form of hydroxofluorstannate
complex of tin of the [SnFx (OH)6- X] type.
Description
THESIS SUBMITTED TO AHMADU BELLO UNIVERSITY, ZARIA,
FOR THE AWARD OF THE DEGREE OF MASTER OF SCIENCE IN
GEOLOGY.
AHMADU BELLO UNIVERSITY,
ZARIA
Keywords
GEOCHEMISTRY,, PETROLOGY,, PETROLOGY,, RIRIWAI,, DUTSEN,, NIGERIAN,, YOUNGER,, GRANITE,, PROVINCE..