POST PROTEROZOIC GOLD PROSPECTS.
Introduction.
No post
Proterozoic lode gold deposit is mentioned in the literature. However,
nearly all gold discoveries have been made by tracing placer or eluvial
gold to its source, rather than by systematic exploration. Several
environments in Tanzania are theoretically favourable for Cenozoic or
older hydrothermal mineralisation. Most of these are in settings of
extensive fracturing, high heat flow and porous country rocks.
Cenozoic Basins.
In
addition to the Wingayongo prospect decribed below, many parts of the
Cenozoic basins in Tanzania are prospective for epithermal mineralisation,
including the Rift valley grabens of the Eastern Rift (Natron, Manyara,
Eyasi, grabens, etc) the Western Rift (Rukwa, Tanganyika, Nyasa, Ruhuhu
basins etc.) and coastal basins of eastern Tanzania (Fig.37). These areas fuffil the conditions mentioned above of extensive fracturing, high heat
flow, and porous country rocks.
These are
numerous mentions in the literature in Quennel et al. (1956) of post -
Karoo hydrothermal activity, which has affected Karoo and younger rocks.
This alteration includes silicification, carbonatisation and fluorite
mineralisation in faults and breccia bodies cutting the sedimentary rocks
in the Ruhuhu and other basins. Fossil and modern hot springs are known in
a number of sites across the country, usually in association with Rift
Valley faulting.
Wingayongo Epithermal Prospect.
Introduction. Wingayongo is best known as the site of a core hole drilled
by British Petroleum - Shell in 1954 to a depth of 762 m to test
tstratigraphy and structure associated with altered, bituminous, sulphur -
bearing sandstones and nearby sulphurous hot springs. Wingayongo Hill is
an isolated hill rising 20 m above featureless flats north of the rising
20 m above featureless flats north of the lower Rufiji River and northeast
of the town of Utete (Fig.38) at 38 degree 24½ E, 7 degree 49S. Abundant
signs of hydrothermal alteration were noted in portions of the core, which
were described by Mclean (1955). Carter (1961) reported further on the mineralisation.
Geology and Mineralisation.
Wingayongo Hill is made up of Creteceous or Jurassic Fluvial sandstones.
Boulders on the slopes of the hill show various types of alteration:
fracture coatings, impregnations, vugs filled with sulphur and bitumen,
silicified boxwork texture, fracturing and possibly kaolinisation.
The
Nyangoni hot springs a few kilometers to the southwest emit gas and hot
saline water. Tufa cones of calcium carbonate, possibly with some sulphur,
are built up around some of the springs.
It is
surmised that the hot springs and Wingayongo Hill occur along northeast -
trending faults, which are otherwise obscure, by young sediments and sheet
wash.
The
drill hole (Wingayongo No.1) showed a water temperature of 61 Degree C
at 64 m depth, and encountered sporadic signs of hydrothermal
alteration, especially between 47 and 125 m.
Signs
of alteration included:
- Bitumen above 47 m depth;
- Cinnabar at 57 m;
- Fluorite at 99 m and below;
- Silicification, pyrite, arsenopyrite, chalcopyrite, barite, and
pyrrhotite at various depths;
- Pyrite cubes up to 5 cm across;
- Steep fractures in some of the core;
- Clay coatings on fractures.
Carter (1961) concluded that the features in outcrop, in the core and
at the hot springs were strongly indicative of former hot spring
acrtivity. He recommended drilling the area as a mercury prospect, but
this was never done. There is no record of any chemical analysis.
Exploration Oppurtunity.
The
Wingayongo alteration and mineralisation (Fig.39) fit the classic
description of shallow epithermal and hot - spring precious metal
deposits (Cox and Singer, 1986, model 25a).
Wingayongo is prospect for hot spring or shallow epithermal gold
silver mineralisation. Surface sampling and shallow drilling would be
the logical first steps in evaluating the prospect. James (1967)
describes hot springs in Tanzania. In 1995, Patrician Gold Mines, in a
joint venture with Sampo Resources, drilled an inclined core hole
beneath the Siliceous Cap at Wingayongo. Pyrrhotite and various forms
of alteration were encountered.