The geology of the lily syncline and portion of the eureka syncline between the consort mine and Joe's lucic siding, Barberton mountain land
Date
1963
Authors
VILJOEN, Morris, James
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The following is an account of the stiatigiaphy, structure, metamorphism
and mineralization in a complexly deformed area of the northwest part of the Barberton
Mountain Land. It is situated at the eastern extremity of the Jamestown Hills and covets
a region along the contact zone between the ancient layered rocks of the Archaean Complex
and the Nelspruit Granite.
In the first section is given a fairly comprehensive account of previous work
done in the Barberton region- especially as it applies to the area under discussion. This is
followed by a consideration of the petrology and stratigraphy of the area and a description
of the various structures encountered. A more detailed statistical treatment of the minor
structures is also included and from these results an attempt is made to unravel the tectonic
history of the area and to fit it into the regional structural pattern of the Mountain Land as
a whole.
The area mapped consists of two basically Identical successions separated
by a major high angled thrust fault. The northern succession, which has been quite strongly
thermally metamorphosed, represents the fairly steeply south dipping northern limb of the
Lily Syncline. That to the south has suffered very little metamorphism and forms part of
the northern limb of the Eureka Syncline.
A well developed and layered basic suite of rocks lying below the Fig-tree
Series .and constituting the basal zone of the Lily Syncline, is classed in the Onverwacht
Series, It lies in direct contact with the Nelspruit Granite and is considered to represent a
metamorphosed succession of impure dolomites with arenaceous and minor shaly horizons,
together probably with some basic and acid lavas. The Onverwacht rocks are overlain by
metamorphosed Fig-tree shales and "lavas", and these in turn by metamorphosed conglomerates
and quartzites of the Moodies System. At the base of the homfelses lies the Consort "Contact"
or Consort "Bar", a siUcified, mineralized zone which is the main ore horizon of the Consort
Mine. The homfelses grade into rocks which have been termed "lavas" but which are thought
to be mote of the nature of crystalline tuffs.
The succession to the south of the Main Fault Is on a broad scale Identical
to the one just described above, differing mainly in metamorplilc state. Thus, whereas
the Onverwacht rocks of the northern succession have been converted to hornblende and
tremolite-actinollte schists, similar rocks to the south of the Main Fault have been changed
to carbonate-bearing talc and chlorite phylUtes.
The basic Intrusive rocks of the Jamestown are considered to be of a much
smaller distribution than was previously thought,and are represented mainly by the massive
bodies of pure serpentinite. There is a possibility however; that certain of the purer talccarbonate
schists along the Kaap Rivet,represent altered ultrabasic intmsives.
The Nelspruit gneiss and migmatlte is considered to represent a completely
granitized pre-Swaziland System sequence which at a much later date acted as the basement
upon which the layered rocks of the Mountain Land were deposited, A re-moblllzed border
phase of this migmatite was largely responsible for the contact metamorphism around the
edge of the Mountain Land and late hydrothermal solutions from this same intrusive granite
resulted in the mineralization of the area. The isolated patches of black amphibolites
situated well within the granite outcrop area, are considered to be isolated downfolded
remnants of a once mote extensive sheet of Onverwacht. Their high grade metamorphic
state, as with the Onverwacht rocks along the immediate contact zone. Is due to the
effects of the Intrusive granites plus the re-heaied migmatite.
Three distinct facies of contact metamorphism (related to the Nelspruit
Granite) ate recognised in the area.
The area can be divided into three fairly distinct structural zones, each one
characterized by the good development (as compared to the other zones) of one or mote
particular types or styles of deformation. Thus in the Consort Mine area (Zone I), a northwest-
trending fold system is the strongest and most apparent structural feature. In the
southern part of the area (Zone U), minor ctenulation and conjugate folds are very well
developed. In the rocks along the granite contact zone (Zone IH), a metamorphic fabric
and well/developed lineations associated with strong shearing, are the most noteworthy
structural features. ’
Special attention was devoted to the accurate observation and recording of
minor structural features. As a result of this, 4 distinct phases of deformation, corresponding
fairly closely to those described by Ramsay (1963), were recognised. The first resulted in
strong folding about northeast-trending axes and caused the formation of the major synclines
(Including the Eureka and Lily Synclines) and the major faults (Including the Lily, Main
Southern and Woodstock Faults). The first period was followed by the widespread development
of cleavage, associated with which is the marked flattening and elongation of conglomerate
pebbles,and development of various types of lineations including the alignment of metamotphic
minerals along the immediate contact belt. Certain of the lineations appear to have formed
mainly after crystallization of the granite, and are closely connected with the mechanical
deformation (in the form of strong differential shearing) which affected the rocks along the
contact. At a late stage during the 2nd period, hydrothermal solutions from the granites
entered zones of strong shearing and structural disturbance which had started to develop in the
layered rocks. The initial development of the 3rd fold stmctures (especially in the northwest
part of the Mountain Land) is considered to have played an important part In the localization
of these ore fluids.
Both the 1st. and 2nd formed stmctures have been strongly deformed by a northwest-trending fold system. These 3rd phase folds teach their best development in the Consort Mine area where they constitute by far the strongest stmctural feature. This period of folding,which during the very early stages probably played a part in the localisation of the ore fluids, outlasted the mineralizing episode and in the Consort Mine area, strongly deformed the mineralized "Contact”. The marked inflection in the northwest part of the Mountain Land, including the "bending" of the Eureka and Ulundi Synclines, occuned at this stage.
Most of the major faults truncate the 3rd folds and are thought to have formed, or to have been rejuvenated,at a late stage during this period of deformation.
The 4th and final phase of deformation is represented by the fairly widespread development of minor crenulation and conjugate folds. Their development is largely controlled by the rock types and they only occur in lithologically favourable varieties. The maximum deforming stress which caused these folds must have been nearly vertical. These 4th structures are completely separate and unrelated to the main northwest-trending fold system (3rd structures) with which they were classed by Ramsay (1963),
From an economic point of view, three zones of potential mineralization, corresponding in all cases to zones of strong shearing and stuctural disturbance, occur in the area. The most Important is the silicified and mineralized zone (within the northern limb of the Lily Syncline) between basic schists of the Onverwacht and the overlying Fig-tree homfelses, known as the Consort "Contact" or Consort "Bar". Another potential zone of
mineralization is the westward extension of the so-called Lily Fault which occupies exactly
the same stratigraphic position as the Consort "Contact", viz. between shales and basic
rocks of the Flg-uee and Onverwacht Series respectively, but within the northern limb of
the Eureka Syncline. Finally, certain shaly horizons within the "footwall" quartzitic
layers of the Consort Mine area also show signs of weak mineralization.
Description
Thesis submitted for the degree of Master of Science
in the Faculty of Science, University of the Witwatersrand,
Johannesburg.
Keywords
Geology -- South Africa -- Barberton., Geology, Stratigraphic., Barberton ( South Africa )