| Tertiary-Quaternary Gold-Bearing Lateritic
Profiles In The Kahama and Rwamagaza Belts, Northwest Tanzania: A Stratigraphical,
Geo- chemical And Environmental Study
NGALAWA, J. S.
Geoscientist
Schlumberger GeoQuest
San Felipe Rd., Suite 1700
Houston, TX 77056
Email:jngalawa@houston.geoquest.slb.com
Abstract from a Ph.D. dissertation. Thesis presented
at the Vrije Universiteit Brussel (VUB) in Brussels, Belgium on July 8,
1998 |
Abstract. The studied areas lie within the gold-bearing Nyanzian
greenstone belt of North Western Tanzania. The prime objectives of the
study were to establish the stratigraphy and geomor- phological setting
of the areas; to give an insight of gold (Au) and other asso- ciated elements
and mineral distribution characteristics in the weathering pro- files.
We also tried to use Quater- nary sediments for an environmental health
(mercury pollution due to gold recovery processes) appraisal. This was
accomp- lished by various laboratory and com- puter generated techniques
such as geo- chemical analysis, clay mineralogical analysis, scanning electron
microscope (SEM) analysis, polarizing microscopic studies and by the use
of various stati- stical packages and other computer programs. A
combination of |
stratigraphical and geomorphological investigations made possible the recognition
of three main land surfaces at specific altitudes (except where they have
been modified by the Cenozoic uplift). These land-surfaces are (1) the
sub-Miocene land surface (at about 1250m altitude), this is characterized
by somewhat thick concrete crust covering deeply decomposed bedrock consisting
of red and mottled clays. (2) the end-Tertiary land-surface (at about between
1250 - 1170m altitude), this has a relatively thin lateritic crust. Both
levels of the lateritic ferricrete are of economic interest since eluvial
gold placer deposits of unknown potential are thought to be present locally
beneath them, particularly in the vicinity of gold-bearing Nyanzian greenstones.
The latosol and humic horizon on top of these land surfaces indicates a
somewhat tropical humid to equatorial forest climate responsible for their
formation during the Quaternary, while the stone line reflects a relatively
dryer phase in between them. Most probably the stone layer reflect the cold/drought maximum of about 20,000 years BP or the Younger Dryas of about
11,000 years BP, both of which denotes the synchronous and enhanced deserti-
fication all over the globe. (3) the Plio-Pleistocene proto-Lake Victoria
basin (at about 1170m altitude), is characterized by lacustrine beach flats,
which are essentially wave-cut platform of the former lake strewn with
waterworn pebbles of the local bedrock.
Gold distribution is somewhat lithologically controlled, with latosols
being poor in gold while nodular and mottled clay zones are somewhat enriched
with gold. Most probably gold concentration in these horizons is a result
of weathering of colluvial materials from quartz veins that are known to
host primary gold. There is a very good correlation between Au and Pb at
Buziba, in fact the correlation is significant even at 0.01 level of confidence.
This is in line with the fact that Pb (together with As, Sb and W) is a
pathfinder for Au in soils.
The statistical treatment (factor analysis) of geochemical data and
synthesis of mineralogical results proved that besides the environmental
availability (bedrock related) of trace elements, the actual content of
trace elements in laterite is to large extent controlled by the mineral
phase composition of the weathered mantle. The most striking example of
this mineralogical control is the association of goethite with REE, Ba,
Y, Mn, Ce, U, Cu, V, Th, Pb and Zn, confirmed indirectly through their
affinity to high organic matter content. Considering that goethite formation
is greatly enhanced by the presence of organic matter, it is postulated
that formation and breaking of the organometallic complexes is to large
extent the process controlling the dispersion of patterns of the aforementioned
trace elements.
REE behaviour in weathering profiles reveals a less pronounced LREE
enrichment relative to HREE while significant positive Ce-anomalies have
been encountered in Mn-Fe enriched zones, especially in indurated laterites
and in the mottled zone.
The study of the geochemical imprint of the substratum in gold-bearing
lateritic soils, based on trace element distribution pattern, enabled the
assessment of the character of the bedrocks and established the boundaries
between them. The simple statistical tools like cumulative frequency graphs
applied to the elements like Mn and Nb gave good results in partitioning
of the gathered samples between the two populations i.e. laterites developed
on the schists and laterites developed on tuffs. Mineralogical analysis
on hard nodular iron crust and top pebbly soil provides a partitioning
mechanism of studied profiles between two land surfaces on which they are
developed. In the Nyangomango profile which develops on mid-Tertiary land
surface, the nodular horizon was formed under a more humid tropical climate
which made formation of gibbsite possible. On the other hand, the top pebbly
soil horizon at Iyenze profile, which develops on end-Tertiary land surface
was formed during a somewhat tropical, short dry season (wet savanna) culminating
in formation of goethite.
Somewhat severe mercury contamination has been found in the Bulyanhulu
river, kahama greenstone belt, that supports the fishing industry and serves
as the major source of domestic water; values as high as 23.6 ng/g methyl
mercury have been revealed by this study. The values for the Rwamagaza
greenstone belt are relatively low. Dark-coloured sediments (apparently
from areas with little or no gold washing activities) have low total mercury
values, and shows very good correlation between total mercury values and
corresponding organic matter contents as well as the fine grain fraction.
Samples from areas active in gold mining (various sample clours from 2.5YR
2.5/4 dark reddish brown through 10YR 3/1 very dark gray) shows little
or no correlation between total mercury and organic matter content. There
is a strong correlation between methyl mercury and organic matter content
in the representative samples analysed. |
