Page 1 of 1

b From Susan Karberq <karbeO02@d.urnn.edu>

Sent Saturday, April 7, 2007 5:37 pm To dittoe. l@osu.edu

Cc

BCC

Subject looking for a source

Dearest Pattie,

Can I please ask a big favor? I need an article that my library does not have. I looked it up in OSCAR and you have i t ! woo hoo!

Do you have time to scan this and send it to me ASAP? or have one of the students do it Monday or Tuesday? I ' m distributing my thesis proposal to my committee on Wednesday and this source should be very helpful!

Here is the source: MCGREGOR, A. M. 1951. Some milestones in the Precambrian of Southern Rhodesia. Transactions of the Geological Society of South Africa, v. 54 (I 'm not sure what pages)

AND the call number: Q E l G344305 (v.54 1951)

Thanks so much!! Susie

Susan Karberg karbe002@d.umn.edu Graduate Teaching Assistant Department of Geological Sciences University of Minnesota, Duluth Heller Hall 202 218-260-8167

xlvi PROCEEDISGS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA

The analyst's number is given in each case. Ernest Gelding's analyses bear no letter. Bis successor, P. I. Brewer's, are prefixed by B, and those re- corded by Fratnli Grout (1936) have Roman numerals.

, A succession of four granites of wliich the first two are strongly gneissic are recognized, and are lettered A, B, C and D successively. The relative a,ge of a fifth, a potassic g~an i t e has not been determined. I t is distinguished by E (Tables I and 11). A schedule of analyses quoted is attached a t the end.

The Southern Region.

The granites in the southern part of the Colony have been rnapped in parts by Aiiin~, Ferguson, Macgregor, Xaufe, Phaup, Tyndale-Biscoe and P. A. Wagner (Fig. 7). F. 1;. Anin1 (Bull. 35) has mapped three granites around Bulawayo. The oldest is a tonalitic gneiss, B which passes imperceptibly into niassive grey tonaJite. It is certainly post-Bulawayan in age and probably post-Sha,mvaian. Porphyritic adamellite, 6, is intrusive into the gneiss. The arrangements of the phenocrysts may be quite haphazard, or have flow-banding which A~nni shows in his map (Bull. 35, Plate IV) to continue the strike of the banding in the older gneiss. The intrusion therefore seems to be a rec~nstitut~ion and recrystallization of the older rock with some addition of potash and pro- bably other constituents. The Matopo granite is regarded as a separate pluton of the same age and nature.

The third granite, D, is a fine to medium-grained tonalite which is partly massive and lxirtly porphyritic. It is definitely iiitrusive int,o the gneissic granite and into the porphyritic adamellite.

Around the Antelope Mine and the Lower Gwanda gold belt A. E. Phau.p had eal-lier described three similar types of granite (Bull. 21 and 24). He was not able to prove an intrusive relation between the gneissic tonalite and the porphyritic rock. An analysis of the latter, No. 403, shows it to contain 30 per cent. of normative orthoclase which is the highest value recorded in Dhe tonalite field. The analysis of the Matopo granite No. X, recorded by F. Grout (1935), is very similar and contains the lowest normative orthoclase aniong the adamel- lites. The B~~lawayo porphyritic adamellite contains a little more potash and less lime. There is thus some justification for classing this granite as an adamellite of the Matopo group. The third granite is a fine-grained niassive tonalite. Con- clusive evidence of the intrusive relation between this rock and the other granites is given. Three analyses of this rock are recorded (Bull. 21, p. 54). They differ between themselves rather strongly. Pegmatite sheets which emanate from this granite cut through the .reef of the Antelope Mine. This is taken to indicate that the fine-grained massive tonalite is younger than the mineralization of the neighbourhood. The pegmatites, however, certainly belong to a very late stage of the granite inject.ion and cut the granite with clean wa,lls ; they may therefore be younger than gold-bearing emanations froill the sanie niass. Two dykes with lepidolite greisen outcrop near the mine and a third is recorded in the lower Gwanda gold belt farther south. All three seem to be derived from the youngest granite, D. Ahrens gives the age as 1,900 million years.

ASNTVERSART ADDRESS BY PRESIDEST xl17ii

In the Gwauda area Tyndale-Bistoe (Bull. 36) has made a very careful structural study of the granite masses around the gold belt. Gneiss one to five lniles wide borders the schists on the south side, and is sharply bounded south- wards against inassive adalnellite. This rock may be an eastward extension of the porphj~itic adamellite of the Antelope area. North of Gwanda gneiss and porphyritic gneiss extend over a wide area. Both are definitely tonalitic and appear to be very similar in coinposition (Bull. 36, p. 44). The transition between these rock types is gradual, and the author regards them all as differentiates of a single magma. An aplitic adamellite, KO. 586, is definitely younger, and is in- trusive into the older gtanites. A rock of rather similar coinposition (No. 553) associated with an olivine-monzonite forms part of a small monzonitic complex stock intrusive into the schists. These rocks are mineralized with sulphur-gold ores (Tyndale-Biscoe, 1939).

Three granites, old gneissic, porphyritic, and fine-grained massive in character respectively, were mapped by J. C. Ferguson at Filabusi, who compared them with the Antelope suite (Bull. 27) .

I n this area no granite older than the Rulawayaii systei~l has been recognized, but granite pebbles are found in Shainvaian conglomerates near Bulawayo and in interbedded Bulawayaii sediments a t Gwanda. Grounds for regarding the granite east of Shabanie as pre-Bulawayaii have already been given.

Very little is known about the granites around the Fort Victoria gold belt. Grout records the analysis of a granite (PII) collected three iniles west of Gutu, which is about 30 miles north-west of the Bikita tinfield. I t is a potash rich dda~nellite and perhaps a stock. The lack of information about the granites in this area is regrettable.

The ovoidal granite mass stretching northwards from Filabusi for over 100 miles and enclosing the Insiza gold belt has been described by Wagner in the south-east and by many others in different parts. We can call i t the Sha,ngani batholith. It is formed mainly of tonalitic gneiss fading into massive granites in parts. Stocks of adamellite with gradational or intrusive contacts are re- corded in several places in the south-east, the north-east and a few miles north of the Insiza schist belt. They generally form kopjes rising above the general plateau surface. The most interesting of these is the Formona granite (No. 3) on the western contact 15 miles north of Rembesi. It is a pinkish, pertly por- phyritic rock about 5 miles in diameter. The schists on the western side contain a cluster of small gold reefs which are cut by sills or ring-dykes of the porphyritic g-ranite. Their location, however, suggest that they originated from the same niass or from a predecessor in the same place. The only gneissic rock from this area which has been analyzed comes from 7 miles south of Gwelo. It was collected as part of a soil series and inay be weathered, and has a very unusual composition, No. 157. There is one from the Heeny satellite, 15 miles east of Bulawayo. Ainm likens this rock, No. 162, to that of the main mass and also to the gneiss near Bulam-ayo. Two lepidolite-bearing greiseiis are known in the Shangani batholith, one close to the contact with serpentine a t Filabusi, the . other 16 miles south of Daisyfield.

The analyses of rocks in this (southern) region are shox-n in the triangular cliagrams Fig. 8 and the four granitic eruptions which x-e are able to recognize are lettered from A to 19 and are summarized in Table I.

-4NKIVERRARY fLDDRESS BY PRESIDEST xlix

The felspars of the gneisses A and B with the exception of No. 157 form a very close cluster in the potash-poor tonalite field. The porphyritic Matopo granites, C, form another cluster in the lower part of the adamellite field. The three analyses of Phaup's younger massive tonalite and the Gwanda aplogranite D, are rather widely spaced. The rock No. 264 which may be a recrystallization of the gneiss (No. 267) is close to the Matopo granite and the nearest neighbour of (NO. 291) is the aberrant No. 157. The younger adamellites are poorer in lime and rather richer in potash than the Matopo granite. I n all the groups the percentage of free quartz varies considerably.

The Midlands Region.

The Gwelo-Selukwe gold belt has the forni of a rather confused triangular syncline between the Shangani batholith on the south-west, what may be called the Chilimanzi batholith to the east, and the Rhodesdale batholith in the north. The south-eastern limb of the gold belt consists of narrow strips of schist bordering the batholiths on the south and east sides and enclosing the Mont d'Or granite (No. 648) (Bull. 3, pp. 15 and 48-43)? which is a massive rock composed of quartz and sericite with a little chlorite derived from biotite. It encloses large and small bodies of talc-schist, greenstone and sedimentary rock. Ny own opinion is that it is of pre-Bulawayan age.

The Chilimanzi batholith so far as is Iino~vn is of the same character as other large batholiths, being composed mainly of tonalitic gneiss. South- eastwards from Selukwe it is crowded with large and small inclusions mainly of talcose rocks, some of them containing bodies of chromite. The gneissic struc- ture ends rather abruptly at a line running east about six niiles north of Selukwe, and gives place to massive adamellite. Some small intr~~sions of adaniellite form hills a little east of the Great Dyke.

The mapping of the gold belt 15 miles north of Selukwe (Bull. 39), s h o ~ s the base of the Shamvaian sediments crossing the granite contact, suggesting that the sediments rest unconformably on the gneiss. Tyndale-Biscoe tells me that the metamorphism of these adjacent sediments is very low. If this con- clusion is correct this part of the batholith is pre-Shamvaian.

The granitic area stretching northwards from the Gwelo-Selukwe gold belt to the latitude of Gatooma has a very complex structure. The pre-Bulawayan " Rhodesdale " gneiss extends from the northern limit to a line about 11 miles south of Que Que. Five analyses show that in spite of its varied appearance it has a very uniform composition (No. 151, 38, 249, 259 and 218) (Bull. 1 7 and 20).

South of the line mentioned the gneiss has the structure of a migmatite coinposed mainly of fragments of the older gneiss in younger gneiss (Bull. 31)

'

Fig. 9. These rocks in the same outcrop are invaded by an aplitic granite and tonalite dykes which are contorted ptygmatically proving that the older rocks were then in a plastic state. Straight dykes of pegmatite were formed after consolidation. This younger gneiss has an intrusive contact with the Bulawajaii

JT

1 PROCEEDISGS O F THE GEOLOGICAL SOCIETY O F SOUTH AFRICA

BANDED DARK GREY APLITIC GREY PEGMA PI TE GNEISS GNEISS GRANITE GRANITE DYKES

SCALE OF FEET 0 5 2 3 4 5 6

Bligmatitic colltsct of the older and younger gneisses near Huiiter's Road, midway bet~veeli Gwelo and Que Que.

AXXIVERSBRY ADDRESS BY PRESIDENT

greenstones, and extends about eight miles southwards. TTTe can reasonably correlate this younger gneiss mit,h the gneisses described in the southern area, B.

Massive grey tonalite forms the country south of the younger gneiss. The nature of the contact is not known although it has been mapped approximately for a'bout 13 miles to the east. It is probably gradational. The grey tonalite is aggressively intrusive into the greenstones forming a deep bay to the west- wards between Ilunters Road and Gwelo, crossing the railway a t Zaloba Siding (No. 511). It is a biotite tonalite composed mainly of stumpy idiomorphic crj~stals of plagioclase in a groundin~~ss of albite,, quartz, microcline and biotite.

The Sesombi granit,e (No. 217) which stretches from the Sebakwe river 12 niiles north-west of Que Que to the foot of the Mafungubsi escarpment and probably under Karroo sediments to the confluence of the Gwelo and Shangani rivers, and also the greater pajrt of the Whitewaters intrusion west of Gatooma are formed of grey tonalite, similar to the Zaloba rock. The Zwimba batholith which stretches from Hartley northwards to Sinoia and eastwards to Norton is formed domirlantly of the same grey tonalite (Bull. 34) (Nos. 540, 651, 559, 659).

Reddish porphyritic granite forms several intrusions in the region. The largest is the Grandeur granite a t the southern extremity of the Whitewaters mass. It is well exposed in an almost continuous ring of kopjes near the contact. The central part of the inass is less po~phyiitic, and is traversed by numerous small quartz veililets carrying gold, which have enabled the weathered rock to be mined as a stockwork. A similar occurrence of reddish porphyritic granite associated with syenite is recorded (Bull. 37) south of the Gwelo river about 5 miles below the Python Mine. There are several small stock-like bodies of porphyritic granite within the R'hodesdale batholith. One is situated about a mile above the railway bridge over the Sebakwe river, and there are several roughly along the line of contact between the younger gneiss and the grey tonalite. They have definitely intrusive relations towards the former rock. P~rphyri t~ic granite is also recorded (Bull. 39) a few miles from the southern margin of the Rhodesdale ovoid north of Selukwe.

The only poi-pliyritic granite analyzed in the central region was collected by Amm in the Zwimba batholith a t a railu7ay quarry near Selous Siding (No. 652). The composition is almost identical with that of a gneissic tonalite (No. 651) which was collected about three miles away.

I n the diagram Fig. Sb, the five analyses of the pre-Bulawayan Rhodesdale gneiss (Nos. 38, 213, 249, 251, 259) form a very coinpact group, which is sur- prising because they were collected from places as fax apart as near Gatooma, Battlefields and Que Que and have a somewhat varied appearance, and their banded nature would niake one suspect contamination. The orthoclase-albite ratio is practically the same in all five. The Mont d'Or granite which is also believed by me to be pre-Bulawayan, has the same ratio a11d is probably an aplitic variety.

The field of the grey tonalites adjoins the Rhodesdale gneiss field but lies ent,kely on the sodic side without overlapping (Nos. 660, 652, 659, 595, 511, 540, 661 end 21'7). It also on the M-hole cont'ains more f e l s ] ~ ~ .

PROCEEDIKGS O F THE GEOLOGICAL SOCIETY O F SOUTH AFRICA

The Northern Region.

The inforniation about the granites around Salisbury is rather scant,y (Fig. 4a). The granite worked in the Municipal quarries south of the cit'y is a potash adamellite containing 4.83 per cent. of K,O and only 2.32 per cent. of Na,O. Pegmatite dykes contain crystals of beryl but the percentage of soda and potash by weight in one that has been analyzed are nearly equal. The extent of this adamellite is not known. Maufe's desci5ption of the batholithic granite 12 miles east of this indicates that i t is a normal t,onalite wit,h a gneissic margin (Bull. 7 ) .

On Chishawasha Mission just north of this and separated froin t3he bat,holith by a strip of greenstone with pillow structure is a small intrusion of granite, three miles long and two miles wide (No. B 199). The rock is medium to coarse grained with rather abundant biotite having a general parallel arrangement in a white base. The intrusion is of particular interest on account of the presence in it of greisenized pegmatite with lepidolite and a little uranium. L. H. Ahrens has determined the age of the lepidolite as 2,150 million years.

Analyses of three granites in t'he same area have been made, a lime tonalite from Bromley No. 237, an adamellite from Marandella!s No. 142 and a pot,ash adamellite from a point 25 miles south of Salisbury (No. XI).

North-east of Salisbury are the Chindamora and Madziwa batholit'hs separated by the Mazoe gold belt. Both are formed mainly of tonalitlie gneiss, but descriptions of the rocks in bays at the west end of each, indicate areas of adamellite. There are several small gold mines in the adamellite area 20 miles north of Salisbury (Bull. 19 and 33).

The country outside the arc formed by t,he Mazoe and Darwin gold belts is rather complicated. Tonalitic gneiss forms the greater part of the Chiweshe Reserve north of Mazoe but there is sorne adamellite near the narrowest part of the arc. The Governliient Station a t Mount Darwin is on gneiss which stretches for twelve miles to the west and is bordered by numerous small gold reefs in the schists. Between these two gneiss areas there is a large tract of porphyritic adamellite, which has definite intrusive relations illto the gneiss to the east, but fingers out in the Chiweshe gneiss where intrusive relations have not been observed.

West of a line running southwards from the Un~singesi valley to the western end of the Mazoe gold belt, the country is formed of the non-porphyritic and unfoliated Rumanji granite of even grain and rather pale colour, which has intrusive relations with the gneiss and the porphyritic granite. The composi- tion of the younger granite appears to vary somewhat in spite of its very uniform appearance. Most of the slides described indicate a tonalite, but an analyzed specimen from a type loc,ality No. B. 25 is definitely an adamellite. The country west of t,he Great Dyke and north of the Banket schist belt is formed largely of this granite intimately associated with a porphyritic granite with intrusive relations between them.

Sited approximately along the synclinal axis of the &lazoe gold belt are five stocks or cu~polas all of wliich have the character of grey tonalite, although as shown in the plot there are minor differences of composition. The important

AKXIVERSARM BDDRESS BY PRESIDEST liii

Bindura granite which is represented by three analyses is more potassic than the others. Five more stocks of similar character occur in the Jumbo gold belt between central Mazoe and Salisbury. Gold deposits are associated with iliost of the stocks, and scheelite is produced in some of them as a by-product or on its own account.

The analyzed rocks in the Northern Region are plotted in Fig. Sc. The Nadziwa batholith is represented by three analyses which fall on the same line as the pre-Bulawayan Rhodesdale gneiss analyees in the central region. It may be of pre-Bulawayan age as tentatively shown. The grey tonalite stocks are on the average less potassic, but the Bindura stock has less soda. In composition it resembles the Matopo granites. The Chishawasha stock the age of which is determined by Ahrens, is very close to the line. The field of the porphyritic granites is similar to that of the Matopo granites. Unlike the younger granites of the Southern region the three analyses of massive " Rumanji " granite form a tight group of lime-poor adamellites. The potash adamellites are with little doubt younger than the gneisses but their relative age in other respects is not known.

Summarizing the evidence we may recognize four or five main granite suites. The oldest known is the post-8ebakwe pre-Bulawayan, Rhodesdale gneiss, A, which has a rather constant Or/Ab ratio of 317. The Madziwa gneiss seems to fall into this group. The second suite is the younger gneiss, B, in association with the grey tolialite typically represented by the Bwimba batholith. The suite is richer in soda than the first. The third suite is the &!tatopo por- phyritic adamellite C, which is apparently a re-fusion and re-crystallization of the older granites with the addition of potash. The fourth suite, D, comprises the younger massive granites which on the basis of the three rocks from the Antelope area chosen by Phaup for analysis, were formed by re-fusion of the earlier granites without the addition of new matter. The very small group formed by the three analyses of the late massive Rumanji granite in the north, contrasts with the dispersal of the analyses of the late granite in the south, and raises the question whether it is right to correlete then1 together. The relative age of the suite of potash-adamellites is not known.

I t seems for what the evidence is worth, that among those granites which are represented by three or more analyses, the soda/potash ratio in the felspars is a more distinctive character than the proportion of anorthite. I n the general coinposition diagrams the percentage of quartz seenis to be a very variable character, and to have little diagnostic value.

Discussion

I t is important to have a clear idea of what we mean by different granites. Phaup's three very diverse analyses of the " younger" granite in the

Antelope area, one of which G 264 falls near the gneisses, another G 267 is near the Natopo granite field and the third G 291 is almost a lime tonalite ; Amm's observation that the phenocrysts in the porphyritic granite near Rulawayo continue the banding of the older gneiss ; Tyndale-Biscoe's record of a gradual

liv PROCEEDIXGS OF TKE GEOLOGICAL SOCIETY OF SOUTN AFRICA

passage from the gneiss into porphyritic granite a t Gu-anda, and finally Amm?s observation north of Hartley of a porphyritic granite with intrusive relations into gneiss of almost identical composition, all suggest very strongly that there has been no bodily replacement of one rock by another, such as there was where a granite cupola invaded a suite of basalttic levas. The change is one of re- crystallizetion with or without the addition of new matt,er, following, I believe, partial but not necessarily complete fusion of the older rock. The lithological differences which dist,inguish the successive granites appear to be largely in- dependent of chemical composition, and to have resulted from different physical conditions and the quantity of voletile fluxes. The vitalized younger ma,ma, which was doubtless hotter than the surrounding granite, had cancerous poten- tialities which enabled it to invade not only the gneiss from which it was partly formed, but also the volcanic and sedimentary rocks above.

The large ovoidal batholjths probably originated as homogeneous magmas which ultimately consolidated as granite gneiss. They probably remained in a state of partial, st,atic consolidation for very long periods of time, during which there was a gradual upward passage from amorphous, molten or glassy magma below, through glass enclosing scattered crystals, to int,erstitial glass in a sponge- like crystalline mesh, and finally to crystalline rock a t the top. Froin time to time tfhe surfaces of equilibrium bet,ween these four conditions would rise or fall in relation to the granitic mass, as the geoisotherms became adjusted to changing circumstances, such a,s deposition or denudation a t the surface, radio- activity, the circulation of magma at great,er depths and other unknown causes.

The semi-glassy magma must have had sufficient rigidity to permit the passage of basaltic and other magmas through fissures from below. Thus the younger granites had intrusive relations into the older granites in the upper levels, but merged with t,hem at greater depths.

I t must not be forgotten that we are dealing with rocks which were formed a t a time when the earth was at the lat,est less than a half of its present age. A very rigid a,dherenoe to the principle of uniformitarianism is not justified.

It is useful to consider in what respects conditions in the world two milliard years ago differed from those of today.

The progressive differentiation of sedinient's has been discussed. It follows that the sedimentary rocks were closer in their mineral composition to igneous rocks and were therefore more amenable to granitizabion. The geothermal gradient wa's probably steeper causing metamorphism and consolidation of magmas to take place a t a shallower depth. The crust of the earth may have been less rigid, and .isostatic compensation would be reached more rapidly. Vulcanism was more active than it is now. Virgin volatile fluxes and mineralizers probably escaped to the surface in greater measure in those early times.

The earth's rotation was more rapid and tidal forces may have been greater.

The gneisses and the migmatites present their own problems. I n every area examined the oldest granite has a definitely gneissic character. Observers are agreed that the gneissic banding is caused by the segregation of the biotit,e into parallel seams between wider lenticles of felspar and quartz. The banding is generally parallel with the margins of the larger batholit,hs, and mag show

ASSIVERSARY ADDRESS BY PRESIDEKT Iv

little or no disturbance for several miles. What were the conditions influencing the formation of granitic gneiss in early Precambrian times, which were lacking in later periods ?

I have had no doubt that great masses of Sebakwian sediments were granitized and form part of the older gneisses, but the surprisingly uniform chemical composition of the gneisses points to a magmatic origin. Moreover, if the gneissic banding was a relic of the sedimentary structure, the rocks before granitization niust have undergone remarkably little disturbance, and must have had very uniform composition. The mineral banding of the gneisses does not suggest a sedimentary origin, nor does it resemble in structure the known Sebakwian sediments. The banding appears rather to be t,he result of gentle movement backwards and forwards, in which the biotite flakes served as a lubricant between lenses of quartz and felspar-glass of rather low viscosity. Was it tidal motion which caused the foliation ?

FIG. 10.

Migmatitic gneiss near Que Que.

In the migmatites there may be no consistency of structure or composition, and every possible dip and strike may be seen in the space of a few yards (Pig. 10). The inclusions have undergone extraordinary contortions, without any uniform direction either horizontal or vertical. What force and how much was required to produce the contortions ? The force seems to have come from every direction in turn, and it is hard to understand how any directional pressure could be transmitted through such a matrix. I t is difficult to believe that the migmatitic inclusions were not in a very plastic state when they were deformed. This raises another question. Do crystals of quartz and felspar become softer or more fragile as rising temperature approaches the melting point 1 I have

hi PROGEEDIKGS O F THE GEOLOGICAL SOCIETY O F SOUTH APRICA

scorned literat,ure to find a reference to any experiments on this question without success. I t should be a simple matter to place we,ights on crystals in a con- trolled furnace, and compare the temperatures when deformation starts with the known temperatures of fusion.

If the force which controlled the structure of the shield was gravity, was it not gravity also which, in the form of convection, cuased the contortion of the mign~atites ?

Pegmatitic Mineralization and Radioactivity

Before the last few years the hope, which must animate every geologist who has devoted his time to the study of Precambrian rocks, of obtaining even approximate determinations of their age seemed very distant in Southern Rhodesia, as a little monazite was the only radioactive mineral known.

L. H. Ahren's paper contributed to this Society in 1947 describing his development of the use of the natural radioactivity of rubidium for age deter- minations, put a new aspect on the matter, particularly as he found lepidolite to be the best mineral to use. I n Southern Rhodesia, pegmatites frequently greisenized to lepidolite are common and have been prospected for tin, tantalum, beryl and tungsten during the last forty years.

The recent discovery of a uranium-bearing mineral in lepidolite near Salisbury, provided a unique opportunity to determine the age by two methods and compare the results. Unfortunately there have been delays in obtaining a pure concenOrate of the uraniferous mineral, and work on it is not yet com- pleted, but Ahrens quickly determined the age by his method, and asked to have other specimens to work on. A specimen from the Bikita tinfield gave an apparently anomalous result and three more specimens from the neighbourhood were sent to check it. I n all Ahrens has determined the approximate age of nine specimens from the Colony.

The rexults are given in Table 111. The method used (Ahrens 1946, 1947) depends on the decay of rubidium 87 to strontium 87 with the loss of one electron. The percentage of rubidium 87 in natural rubidium is approximately constant. Ahrens records that about 99 per ce,nt. of strontium found in lepidolite is radio- genic. All that is necessary therefore in order to obtain an approximate age is to measure and compare the density of suitable lines in the spectrograph. Lepidolite froin south-east Manitoba the age of which has been determined by uranium/lead ratio (Holmes, 1948, Ahrens, 1950) was run with each determina- tion as a control. He makes five spectrographic analyses of the rubidium and strontium in each specimen, and takes the average of the ages indicated. Ahrens admits a possible error of 10 per cent. plus or minus. He is publishing a paper on this work in a forthcoming issue of " Science."

The geological map Plate I shows the position of known deposits of lepidolite and radioactive minerals in the Colony. In some neighbourhoods, particularly east and north of Salisbury, and in the eastern part of the Fort Victoria gold belt, they are very numerous. The distribution is interesting. No occurrence of lepidolite is known in the larger productive gold belts with very low meta-

ANNIVERSARY ADDRESS BY PRESIDEKT lvii

morphism in the centre of t,he Colony, but there are taro in the adjacent granite inasses near or just outside the margin of the low grade metamorphism. The majority of the occurrences are in the small gold belts in the arc of moderat,@ metamorphism round the central area. The probable reason is that the lepidolite-greisens are relatively high-temperature deposits.

The mineralogy of the tantalunl deposits of the Bikita Tin Field has been described (Macgregor, 1946), and recently ~ynda le -~ i scbe has presented a paper for discussion to this Society on the mineral deposits of the field. I will therefore not attempt to deal with it in detail.

The result of Ahrens' analyses of the four lepidolit,e specimens from the field Nos. 3, 5 , 6 and 7 are intriguing.

The specimen froni Mauve Kop, also called Mdara Mine, gives an astounding age of 2,950 million years, which is considerably greater than the age determined of any other rock in any part of the world. The three specimens sent to obtain confirmation, Nos. 5, 6 and 7 gave ages of 2,150, 2,450 and 2,050 -J= 200 million years. This nia,y mean that there were two or more periods of mineraliza- tion, or tha't there was some prilnary strontium present in the specimens of lepidolite examined. Specimen No. 6 contains microlite, a Auotantalate of calcium and sodium which suggests that this may be responsible for the value above those of Nos. 5 and 7. The qnestion must remain unanswered until isotopic analysis of the strontiuni is made, or confirmation obtained by allother method. Arthur Holmes has kindly offered to have the age of a sample of mona- zite from the Ebonite claims four miles south-west of Mauve Kop determined by the thorium-uranium method. It is hoped that the result will be known soon. Pergusonite, a radioactive tantalo-columbite of yttrium, erbium, etc. occurs 1$ miles north of Mauve Kop.

The pegmatite being worked a t the Pope claims (Ahrens' Specimen No. 1) is situated in a small Chishawasha cupola of granite 4 miles long and 2 miles wide, close to the western end (Fig. 4s). The granite is surrounded on all sides by greenstoue Iavas which show pillow structure, and are associated with thin beds of banded ironstone and limestone. There is little doubt that these rocks belong to the Rulawaysn System. The property was developed for tantalum, which occurs as tantalite and as an unusual form of microlite containing 84.2 per cent. of mixed tantaluni and columbium oxides, 2 per cent. of uranium oxide and 5.4 per cent. of water at 110°C. Computation of the analysis in- dicates about 60 per cent. of Ta20, . 2H,O. I t is hoped that it will be possible to obtain a determination of age by the uraniuni/lead method for comparison with the rubidium/strontium method.

The Specimen No. 9 was collected from a pegmatite dyke about 15 north of Salisbury. The country rock is believed $0 be f3ebakwian sedi but there are sin~ilar dykes about a mile south in Bulawa~an greenstone about 3 miles north in p e y gneissic tonalite. A sample of lTlonazite last neighbourhood has been sent to Arthur Holmes for determination. It is evident that these three deposits are age. The pegmatites carry tantalite and tinstone, and t tungsten deposits in the ueigl~bourhood.

lviii ~ROCEEDIXGS O F THE GEOLOGICAL SOCIETY O F SOCTH AFRICA

Three or more lepidolite greisens are known in the long south-westerly spur of the Umtali gold belt. Duke's Hill is about three miles west of Odzi station. Ahren's mean of 2100 x l o6 should be accepted. . Of the three granites described by A. E. Phaup from the neighbourhood of

the Antelope Mine (Bull. 21 and 24)' the youngest is the nearest to each of three lepidolite occurrences known in the area and is their probable source. Ahren's age No. 8 of 1900 >: l o6 is therefore about what we should expect.

The Wankie Tin Field is in a long narrow belt of garnetiferous gneiss probably of sedimentary origin with granite on each side. The niineralization difCers from that of the gold belts in that there is much columbium associated with the tantalum, and wolfram is more closely associated. The five age- determinations make a very small group (No. 2) and there is therefore no reason to doubt that this mineralization is of the same age as the mineralization of the gold belts. The high metamorphism may be very much younger. The Northern Rhodesia Tin Field has similar minerali~at~ion and is proba>bly of similar age.

Confirmation of the age of the younger granite of Ohe Antelope area by specimens from other lepidolite deposits nearby would be interesting. Another useful determination would be the lepidolite on Tafuna Hill near Bindura, which is undoubtedly post-Shamvaian in geological age.

No direct conrlexion between the pegmat,it,ic and the sulphur-gold mineralization is known, but tungsten deposits suggest a link. 8cheelite has been worked near the Bikita Tin Field ; north of Salisbury and a t Odzi, and wolfram in the Wankie Tin Field. Wolfram and scheelit,e are rarely found together, and where they are, secondary enrichment of scheelite can be sus- pected. The main lepidolite and sulphur-gold mineralizations are both believed to be related to the jTounger gneiss of post-Shamvaian age, but go16 is also re- lated to the BIatopo granite and lepidojite t'o the young Archaean granite (Macgregor, 1951).

THE MIDDLE AND UPPER PRECAMBRIAN ROCKS

Beyond these milestones the path of time we have been following is lost in the primaeval wilderness of the Eparchaean Interval. ' Bornhardts ' of granite and ' nigger-heads ' of dolerite there are, which will some day with improved methods be charted and bear new milestones on the t,rail.

The Frontier, Dewaras and Lomagundi Systcms

The P~ontier System is the name proposed by F. P. Mennell (1920) for the quartzites forming the Chimanimani Mountains east of Melsetter (Plate I). They also form a larger area on the Portuguese side of the border. Until this year the best account published is a geographical study illustrated b y line drawings and photographs by E. 0 . Teale and R. 6. Wilson (1915). A geological description and tectonic study has recently been written by Geoffrey Bond, and will 1 hope be published in tlhe T~ansactions of this Societ,y. The thickness of

AXKIVERSARY ADDRESS BY PRESIDEKT . lix

the succession hes not been determined, and may be anything between 4,000 and 10,000 feet. The strike is almost due north and south. At the southern end it is cut by a cross-fault, and disappears under younger rocks. This succes- sion of highly differentiated metamorphic quartzites is quite unlike anything further west in Southern Rhodesia. I n the recent geological map of Portuguese East Africa on the scale of 1/2,000,000, the formation is classed as " crystalline schists " and is shown to continue after a short break and reach almost t,o the railway line a t Bandula (Plate I). Aft'er a longer gap the same schists seem to reappear maintaining the same line of strike in the Gairezi valley. The 1936 edition of the Southern R'hodesian geological map shows a sedimentary formation forming the left flank of the Gairezi valley in the same neighbourhood, with a long tongue projecting north-westwards a little further south. The Port'uguese map shows nothing to suggest a ~ont~inuation of the forinat'ion in t'he Bambezi basin further north.

An attempt a t c~rrelat~ion on lithological grounds of a quartzite formation with similar formations hundreds of miles away, in a t'ime interval of about a milliard years, is probably unjustifiable in a scientific paper. Nevertheless possible correlations are so few that the temptation to do so is great.

I n the northern flank of thezambezi t.rough the older rocks are described as gneisses, and there is no evidence of a, thick series of quartzitic rocks coming to the surface. I n the far north-western corner of Nyasaland, however, Andrew and Bailey (1910) have described as the Nafingi Series a succession of nearly 10,000 feet with top and bottom not seen. I t is composed of quartzites and flags which are partly felspathic near the base. The strike is north and south, but the dip is to the west. From the descriptions there appears to be a close resemblance between these rocks and the Frontier System. Coming southwards the Mafingi Series crosses into Northern R'hodesia where it is called ' Upper Basement Schists ' and forms the Makutu Mountains in the left flank of the Loangwa valley. Approaching Fort Jameson the quartzites become more metamorphic and swing westwards towards Petauke, whence they appear to pass beneath t,he Loangwa river with a westerly strike. If this is so they must dip north away from the centre of the shield in Southern R'hodesia. Tf the correlation of the Frontier System with the Mafingi Series is just'ified tlie forma- tions must represent the two limbs of an anticline.

North of Nyasaland the Mafingi Series is cut by rift valley faults, and I have found no record of its extension, but there seems to be grounds for linking i t with the Maragwe-Ankolian of aquatorial Africa. Holmes (1950) records the age of euxenite in pegmatite intrusions in this system as between 920 and 995 million years.

If we turn to the south we find the Moodies and t'he Pongola (Witwatersrand) Systems as possible relatives of the rocks we are considering. The environment of the Moodies System, associated a~s i t is with the volcanic Onwerwacht in what looks like a triangular syncline, suggests a greater age. Moreover the differen- tiation of the sediments, while apparently greater than that of t'he Shamvaian is not so nearly complete as in hhe Frontier Syst'em. The Ponaola facies of the

PBOCEEDIXGS O F THE GEOLOGICAI, SOGIETY O F SOUTH APRICA

JVitwatersrand System appears by the descriptions t,o present a e,loser resemb- blance. In a recent address, H. B. 8. Cooke (1950) dates the Witwatersrand system at approximately a' milliard (1,000,000,000) years, which is in agreement with the age quoted for the Karagwe-Ankolian.

The Detoccras Ayste?n, which underlies the Lomagundi System (Plate I) as defined by Molyneux (Bull. 6) dipping with a strong, angular unconformity in the Hartley and Lomagundi districts, is composed of basaltic lavas, closely resembling those of the Ventersdorp System, a~socia~ted with overlying arkosic sediments (Bull. 34, A.R.D., 1927 S.R., 35).

In remapping the Loinagundi area it was found that the country mapped by Molyneux as the Angwa Plains Series was formed of Deweras rocl;s, the northerly extension of which was considerably increased (S.R., 25).

In a paper presented to the International Geological Congress in 1948, J. H. AT. McNaughton (Part XIV) describes the rocks in the Rufunza valley, included as " Upper Basement" (i.e. Muva-Ankolian) by T. D. Guernsey (Part XIV) ; and suggests their correlation with the Deweras lavas and sediments as described by Phaup and Dobell. I n both areas the lavas are strongly epi- dotized. The Deweras lavas do not extend northwards into the highly meta- morphic area. Their equivalent north of the Zambezi may be expected to be inore metamorphic.

Rather unwisely, the writer (Bull. 38) retained the old name " Deweras Series " ajnd grouped the formation as the basal member of the Loniagundi System. This introdu~~es confusion when correlation is attempted with similar formations both south and north, and it is therefore proposed that the Dewaras be elevated to the rank of a Systern.

The Lomaguncli flystem wa~s colnpared by A. J, 6. Molyneux with the Transvaal System, a correlation which has been generally accepted (Bull. 6). Phaup and Dobell (Bull. 34) divide the systen? into a lower Arenaceous Series composed of quartzites and dolomites, and an upper Argillaceous Series com- posed mainly of slates, shales sand greywackes. The rocks are very much dis- turbed by faulting and folding, and good exposures of the lower members are generally scarce. The disturbance will be discussed later. The succession determined (Bull. 38) is as follows :-

Approximute ilRaxirnurn Thickness

Feet Ka~lya~ga group (greywackes and glistening slates] . . 5,000 Chidomo group (fine dark quartzites and slates) . . 500

. . 2,000 . . 2,000 Mountain Sandstone group . . . . . . . . 1,500 Lower Slate group . . . . . . . . . . . . 3,000

There is in places a thin ba,sal conglonierate composed of angular pieces of white quartz, succeeded sometinles by a thin impersistent bed of dolomite. The lower quartzites are often characterized by brown spots apparently caused

ANKIVERSARY ADDRESS BY PRESIDEST Ixi

by the solution of carbonate rhombs. Some beds of sandy shale are found among the predominant quartzites of the arenaceous series. The quartzites dip steeply in wide open folds and seem to be over-thrust from the west, but probably later reverse movement has produced normal faults dipping at a moderately low angle in an easterly direction.

The Mountain Sandstone is a dark-coloured, fine-grained quartzite which thins and disappears in a southerly direction. The striped slates, well-named by Molyneux, are formed of a succession of well-graded beds, often about an inch thick, which suggest rapidly-deposited varves.

The graphitic rocks may contain up to 30 per cent. of carbon, and often show no evidence of bedding a t all. They have behaved as a lubricant between the open folding below and tight contortion of the beds above.

Earth Movements and Metamorphism

The correlation of the Bwana Mkubwa System of Northern Rhodesia (formerly Roan System of A. Gray), with its local Broken Hill and Lusaka facies, with the Lomagundi System is generally accepted and has been discussed in a recent paper to the International Geological Congress, 1948 (Dlacgregor, 1948, Part XIV) now in the press, and need not be pursued here except to say that from both sides, the metamorphism of these rocks increases sharply as the Zambezi river is approached.

The close petrological similarity between the Great Dyke, which was the subject of Lightfoot's Anniversary Address in 1940, and the Bushveld Complex is the most definite chronological link in the Precan~brian between Southern Rhodesia and any other part of Africa. There is no prospect of an immediate direct determination of the age of the Great Dyke, but it is a reasonable hope that the red granite or some other rock in the Bushveld may yield the secret.

In correspondence following his recent paper, H. 13. Hess (1950) suggested that the folding of the Lomagundi System, which has a trend generally parallel to the Great Dyke, was caused by the beds sliding down the roof of the great elongated batholith which he has postulated. This appears to be a very happy inspiration, but it is not easy to explain the nature of the folding I have described on that hypothesis. I t remains, however, the only explanation we have.

In 1920 the late H. B. Maufe visited Miami in north Lomagundi to examine the mica field, making a careful traverse along the road he took (S.R. 10). The country rock of the mica field is a coarse mica-schist with large crystals of alman- dine and staurolite, evidently a metamorphic sedimentary formation of argillaceous composition, and totally unlike anything known in the Archaean gold belts. He wrote : It seems to be clear that the country rock of the mica field is a portion of the Lomagundi System in a more highly metamorphozed conditioii than has yet been encountered elsewhere." I n 1938, Dobell and Macgregor made a reconnaissance over the same route and an attempt was made to map the southern limit of the garnets. It was foulid that, coming south- wards, the garnets became progressi~ely smaller until the^ were only just

lxii PEOCEEDIXGS O F THE GEOLOGICAL SOCIETY O F SOUTH AFRICA

visible with a strong lens, and that the rocks containing them were definitely Lomagundi slates. Granite is intrusive west and probably east of Miami, and northwards the iiletamorphism becomes higher in grade with rather coarse sillinlanite and pleochroic hypersthene. It has clearly a dry, charnockitic character. Specimens collected a t the extreme north end of the Great Dyke by Lig'btfoot show the pyroxenes of the norite to be amphibolized, and where the Dyke terminates like a serpent's head (Lightfoot, 1941, Plate IT) there is shown in the mouth a mass of hornblende-gneiss which he classified as basement schist. Apparently the western contact between this rock and the norite is not well defined, and i t seems possible that the hornblendic rock niay be s metamorphic facies of the norite. There is reason to believe, therefore, that the metamorphisn~ of the Lomagundi System is younger than the Great Dyke.

It is an old fallacy, which dies hard, to suppose that the highest grade of metamorphism must necessarily be the oldest. Intensity of metamorphism is a measnre of the temperature and other transfornling influences to which the rocks have been subjected, but i t should not be regarded as a measure of their age. The age must be determined stratigraphically or by radio-activity ; but the metamorphisn~ can then be followed as a marker from one region to another.

At the north-eastern corner of the Colony, near the point where the Mazoe river crosses the border, is the Rusanlbo mica field, which is very similar to that of Miami ; the rocks are of the same nature but the hills have the structure of sedimentary rocks dipping gently to the north. They appear to be less disturbed than those of the Lomagundi district. We have thus evidence of a belt of high- grade metamorphism striking west to east approxi~nately a t right angles to the strike of the Lomagundi folding and tlie Great Dyke, and parallel to the faulting of the Zambezi trough. There is evidence of gentle synclinal folding, but it is doubtful whether the belt should be regarded as diastrophic in the usual sense.

The similar increase of metaniorphism on the north side of the Zambezi river has been mentioned. The infornlstion shown in Plate I is based on the work of T. D. Guernsey kindly given by Col. Sir Ellis Robins, Resident Director of the British South Africa Company.

About 100 miles east of Rusambo, in the southern extremity of Nyasaland, F. Dixey (1928) has described the Nachiperi Series as a group of conglomerates and arkoses, together with argillaceous, calcareous and carbonaceous beds, all of which have undergone various degsees of metamorphism and deformation. This description agrees rather well with the Lomagundi Systeni where it is moderately metamorphic. The position is probably in the southern fringe of the nletamorphic belt. I have recently had an opportunity to visit the Shire valley west of Blantyre and examine the gneisses and intrusive rocks there. The gneisses have a coarse granulitic structure and are composed mainly of felspar, quartz, biotite, hypersthene and cordierite. Aerial photographs show two contrasting types of country. I n the first type the gneiss forms parallel ridges about half a mile apart with sweeping contortions. The rock is a quartz- rich type tending to a quartzite. I n the second type the country rolls gently, and although aerial photographs show indications of structure in places, no distinct pattern can be recognized. This gneiss is more micaceous. Small bodies of salmon-pink syeiiite and pegmatite dykes permeate tlie country as though

,lSXIVERSBRY ADDRESS BY PRESIDEST lxiii

by soaking. The general inipression is very ~uggest~ive of iiietanlorphic Loma- gmiidi. White crystalline limestone is burned near the Shire river, 40 miles north of Blantyre, in what seems to be a continuous bed striking northwards. From near there Andrew and Bailey (1910) show calcareous gneiss extending northwards for a distance of about 150 miles over a wide stretch of country.

In Portuguese East Africa, about 100 miles west of Blantyre, and 20 miles north of Tete, there is a deposit of a mineral containing about 50 per cent. of TiO, and 8 per cent. of U,O,. Bannister and Horne (1950) give a composition (Fe", U etc.) (Ti, Fe"' etc.), 0,. The lead-uranium ratio gives a crude age of 565 million years. This can be taken as an indication of the age of the meta- iiiorphisrn and is roughly the age of the Katanga niineralization. It fits well with what we should expect on the basis of the post-Loinagundi age for the metamorphism, and the correlation of the Lomagundi System with the Schisto- Dolomitique and Bwana Mkubwa Systems.

East of Southern Nyasaland the metamorphic belt appears to swing north- wards and is described by Arthur Holmes (1919) as striking north-eastwards in the country west of Mozambique Island. It thus appears to trend towards the coastal region of Tanganyika Territory and to link up with the Morogoro region, where Holnies (1950) records the age of 590 million years by the uranium-lead ratio with isotopic analysis. These two recent age-determinations give strong support to Maufe7s conclusion that the gneiss is in part nietamorphic Lomagundi.

In the south-eastern part of Southern Rhodesia there is a bell; of similar metamorphism. The rocks however include banded ironstone, basic schists and serpentine suggesting Archaean origin. A remarliable feature is the flat dip of the gneissic banding. The oldest overlying rocks are Un~kondo.

I t is very difficult to ascribe a cause to the belts of high temperature meta~no~phism. Except in the lower Mazoe valley the strike of the rocks does not conform to the strike of the belts, as they should if the metamorphism were caused by orogenic action. Norover the removal of great mountain ranges along the course of existing depressions, prior to the deposition of the Umkondo Systeni a t the latest in early Palaeozoic times, presents difficulties.

An alternative explanation suggested by the syenitic intrusions in Nyasa- land, is that the charnockitic belts were formed by tension and the ascent of superheated magma, not by orogenic compression. The tension may have continued to the present day and account for the rift valleys and coastal faulting.

The geological map of Tanganyika Territory shows a belt of charnockitic metainorphism along the coastal faulted region, and a second belt of such rocks bordering Lake Tanganyika. They are overlain by the Bukoba System which seems to be an extension of the Umkondo and Kundelungu and may be Cambrian. The origii~al charnockites cling closely to the east coast of India, which is regarded as another line of faulting. It would be foolish to maintain that all metanlorphic belts of charnockit,ic character in Gondwanaland are necessarily of late Pre- cambrian age, but cases seein to be rare where these charnockitic paragneisses in regions of rift faulting are overlain by forillations older than latest Preoanibrian.

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AKIIVERSARP ADDRESS BY PEESIDEST ~ X V

Since writing the above I have seen T. D. Guernsey's " Prospectors7 Guide to Mineral Occurrences in Northern Rhodesia " published by the British South Africa Company, 1951. I would like to draw attention to Plate 2 of that publica- tion. It is a geological map of the country north of the Zambezi river, between the confluence of the Kafue river and t,he Portuguese border. The gneiss classed as 'Sower Basement " is penetrated by a very large number of small bodies of granite. I am grateful to the Resident Director of the Company for permission to reproduce this very illuminating map (Pig. 11).

The age of the pre-Karroo Umkondo System which overlies the gaeisses in the Sabi and Zambezi valleys has not been determined, but there are grounds for believing that it may be early Palaeozoic and mag therefore be omitted from this discussion.

ACKNOWLEDGMENTS

SO ends our study of the Precambrian successioll in Southern Rhodesia. I n preparing it I have drawn freely on the published and unpublished work of my former colleagues in the Geological Survey. J. C . Ferguson, Director, has encouraged me to niake use of the facilities and records of the Department, including unpublished analyses of rocks. I have endeavoured to give acknowledgnient, but may have failed a t times, and I hope for such lapses I may be forgiven. A b o ~ e all I wish to record my gratitude to the late H. B. Maufe. to whose wisdom and advice I owe so much. Unfortunately his great caution led the value of his work to be unappreciated in certain quarters. My quotations from his work are a measure of his inspiration to me. To L. H. Ahrens I am in- debted for his enthusiastic collaboration during the last eight months, the promptness with which he has dealt with the lepidolite specinlens as they arrived and the surprising results he has obtained. To Arthur Holmes I an1 grateful for enthusiastic encouragement and advice, and to Geoffrey Bond for reading 111y final draft and pointing to passages which required clarification.

To Professor P. Eskola and the Geological Society of London I am happy to express my gratitude for permission to reproduce Fig. 5 , and the loan of the block ; and I am equally grateful to Colonel Sir Ellis Eobins, Resident Director of the British South Bfrica Cornpan?- for his kindness in allowing me to reproduce Fig. 11 from the original plate.

LIST OF REFERENCES.

Publications of the Cieological Survey of Southern Rhodesia are given in separate lists following on Authors' list.

Bulletins are recorded as Bull. Short Reports are recorded as " S.R." Annual Reports of the Dirertors as " A.R.D.." the nuinhers being added it1 each

case.

AHRENS, L. H. and LIEBEXBERQ, W. R. (1946) : " Qua1ititati~-e spectro-chemical analysis of minerals and rocks." Trans. yeol. Soc. 6'. Afr., XLIX, pp. 133-154.

ARRENS, L. H. (1947) : " Determination of geological age by means of the natural radioacti~7ity of rubidium." Tmns. geol. Soc. S. S f 7 Q.. L. pp. 23-51.