The geology of the lily syncline and portion of the eureka syncline between the consort mine and Joe's lucic siding, Barberton mountain land

[摘要] The following is an account of the stiatigiaphy, structure, metamorphismand mineralization in a complexly deformed area of the northwest part of the BarbertonMountain Land. It is situated at the eastern extremity of the Jamestown Hills and covetsa region along the contact zone between the ancient layered rocks of the Archaean Complexand the Nelspruit Granite.In the first section is given a fairly comprehensive account of previous workdone in the Barberton region- especially as it applies to the area under discussion. This isfollowed by a consideration of the petrology and stratigraphy of the area and a descriptionof the various structures encountered. A more detailed statistical treatment of the minorstructures is also included and from these results an attempt is made to unravel the tectonichistory of the area and to fit it into the regional structural pattern of the Mountain Land asa whole.The area mapped consists of two basically Identical successions separatedby a major high angled thrust fault. The northern succession, which has been quite stronglythermally metamorphosed, represents the fairly steeply south dipping northern limb of theLily Syncline. That to the south has suffered very little metamorphism and forms part ofthe northern limb of the Eureka Syncline.A well developed and layered basic suite of rocks lying below the Fig-treeSeries .and constituting the basal zone of the Lily Syncline, is classed in the OnverwachtSeries, It lies in direct contact with the Nelspruit Granite and is considered to represent ametamorphosed succession of impure dolomites with arenaceous and minor shaly horizons,together probably with some basic and acid lavas. The Onverwacht rocks are overlain bymetamorphosed Fig-tree shales and lavas, and these in turn by metamorphosed conglomeratesand quartzites of the Moodies System. At the base of the homfelses lies the Consort Contactor Consort Bar, a siUcified, mineralized zone which is the main ore horizon of the ConsortMine. The homfelses grade into rocks which have been termed lavas but which are thoughtto be mote of the nature of crystalline tuffs.The succession to the south of the Main Fault Is on a broad scale Identicalto the one just described above, differing mainly in metamorplilc state. Thus, whereasthe Onverwacht rocks of the northern succession have been converted to hornblende andtremolite-actinollte schists, similar rocks to the south of the Main Fault have been changedto carbonate-bearing talc and chlorite phylUtes.The basic Intrusive rocks of the Jamestown are considered to be of a muchsmaller distribution than was previously thought,and are represented mainly by the massivebodies of pure serpentinite. There is a possibility however; that certain of the purer talccarbonateschists along the Kaap Rivet,represent altered ultrabasic intmsives.The Nelspruit gneiss and migmatlte is considered to represent a completelygranitized pre-Swaziland System sequence which at a much later date acted as the basementupon which the layered rocks of the Mountain Land were deposited, A re-moblllzed borderphase of this migmatite was largely responsible for the contact metamorphism around theedge of the Mountain Land and late hydrothermal solutions from this same intrusive graniteresulted in the mineralization of the area. The isolated patches of black amphibolitessituated well within the granite outcrop area, are considered to be isolated downfoldedremnants of a once mote extensive sheet of Onverwacht. Their high grade metamorphicstate, as with the Onverwacht rocks along the immediate contact zone. Is due to theeffects of the Intrusive granites plus the re-heaied migmatite.Three distinct facies of contact metamorphism (related to the NelspruitGranite) ate recognised in the area.The area can be divided into three fairly distinct structural zones, each onecharacterized by the good development (as compared to the other zones) of one or moteparticular 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 thesouthern part of the area (Zone U), minor ctenulation and conjugate folds are very welldeveloped. In the rocks along the granite contact zone (Zone IH), a metamorphic fabricand well/developed lineations associated with strong shearing, are the most noteworthystructural features. ’Special attention was devoted to the accurate observation and recording ofminor structural features. As a result of this, 4 distinct phases of deformation, correspondingfairly closely to those described by Ramsay (1963), were recognised. The first resulted instrong 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, MainSouthern and Woodstock Faults). The first period was followed by the widespread developmentof cleavage, associated with which is the marked flattening and elongation of conglomeratepebbles,and development of various types of lineations including the alignment of metamotphicminerals along the immediate contact belt. Certain of the lineations appear to have formedmainly after crystallization of the granite, and are closely connected with the mechanicaldeformation (in the form of strong differential shearing) which affected the rocks along thecontact. At a late stage during the 2nd period, hydrothermal solutions from the granitesentered zones of strong shearing and structural disturbance which had started to develop in thelayered rocks. The initial development of the 3rd fold stmctures (especially in the northwestpart of the Mountain Land) is considered to have played an important part In the localizationof 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 ofmineralization is the westward extension of the so-called Lily Fault which occupies exactlythe same stratigraphic position as the Consort Contact, viz. between shales and basicrocks of the Flg-uee and Onverwacht Series respectively, but within the northern limb ofthe Eureka Syncline. Finally, certain shaly horizons within the footwall quartziticlayers of the Consort Mine area also show signs of weak mineralization.

[发布机构] University of the Witwatersrand