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A study of ore and rock specimens from the Nkana mine, Northern Rhodesia 1951

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L£b hi A STUDY OF ORE AND ROCK ' Cap • I SPECIMENS FROM THE NKANA MINE, NORTHERN RHODESIA by REGINALD ANTHONY BARKER A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of GEOLOGY AND GEOGRAPHY We accept t h i s t h e s i s as conforming to the standard required from candidates f o r the degree of MASTER OF APPLIED SCIENCE. Members of the Department of Geology. THE UNIVERSITY OF BRITISH COLUMBIA May, 1951. May 15, 1951. Dr. H. C. Gunning, Head, Department of Geology and Geography, University of British Columbia, Vancouver, B. C. Dear Sir: I take great pleasure i n submitting the following thesis, "A Study of Ore and Rock Specimens from the NKana Mine, Northern Rhodesia'1, i n pa r t i a l fulfilment of the requirements of the course leading to the degree of Master of Applied Science i n Geology at the University of British Columbia. Yours truly. R. A. BARKER ABSTRACT The ore and rock specimens which form the material for this study are a collection sent by Mr. T. D. Guernsey, geologist for the Rhokana Corporation, Northern Rhodesia, to Dr. H. C. Gunning of this Uni- versity under whose direction the present work was done. Owing to the lack of previous detailed work a microscopic investigation i s undertaken with no particular problem in view, but with the hope that the accumulation of factual evidence may aid i n clarifying the geological problems which have led to a diversity of opinion regard- ing the origin of these deposits. An historical sketch and a brief description of the geological setting of the Rhodesian copper deposits i s given. This information has a l l been gathered from the available literature on the subject. The char- acter and mineralogy of the ore deposits and relations to their northward extensions i n Katanga, Belgian Congo are summarized. Descriptive notes, with interpretative remarks, of the l i t h o - logy and ore mineralogy of the NKana 'Ore Horizon' as determined by a study.of the 38 specimens and over 30 thin sections, constitute a major portion of the paper. General theories of ore genesis and supporting geological evidences are summarized for the purpose of clarifying the issues invol- ved and to help in the erection of a theory for the Rhodesian copper de- posits. Extant theories regarding these deposits are outlined and an analysis and synthesis of the evidence gathered i n this .investigation i s presented. The framework of an epigenetic theory is constructed but mention i s made that a meta-syngenetic (metamorphic-sedimentary) o r i g i n f o r these deposits i s a p o s s i b i l i t y . Suggestions f o r f u r t h e r research both i n the f i e l d and i n the laboratory are given i n the hope that they may i n some way lend d i r e c t i o n to subsequent i n v e s t i g a t i o n s . TABLE OF CONTENTS t Page INTRODUCTION 1 HISTORICAL SKETCH 4 PHYSICAL FEATURES • 6 GEOLOGICAL SET-TING - - 8 SEQUENCE OF EVENTS 16 THE ORE DEPOSITS 18 The Ores 20 Comparison of the Rhodesia and Katanga Deposits _ _ _ _ _ - - 22 DESCRIPTIONS OF SPECIMENS FROM THE 'ORE HORIZON' AT NKANA MINE 23 DETAILS OF ORE MINERALOGY AT NKANA MINE . 39 ORIGIN OF THE ORES General Theories of Ore Emplacement - 46 Geological Evidence - - - - - - - _ _ _ -48 A Brief Recapitulation of Theories of Origin of the Northern Rhodesian Copper Deposits - - 49 Evidence gathered i n th i s Study - - _ _ _ - _ - _ 51 ' Summation and Suggestions f o r Further Research - - - - - - - - - - - - - - - - - - -55 Concluding Remarks - - - - - - - - - _ _ _ _ .59 BIBLIOGRAPHY 60 1 ILLUSTRATIONS Figure I _ Location Map 4a Figure I I - Geological Map 8a Figure I I I - Cross-section - NKana Mine 18a INTRODUCTION The ore and rock specimens which form the m a t e r i a l f o r t h i s study were supplied by Mr. T. D. Guernsey, geologist f o r the Rhokana Corporation, Northern Rhodesia. This c o l l e c t i o n , comprising representatives of the various l i t h o l o g i c u n i t s of the 'Ore Horizon' at NKana Mine, contains 3# i n d i v i d u a l samples: at l e a s t two specimens of each type of ore are present. I t was ind i c a t e d by Mr. Guernsey that although the NKana Mine had been i n operation f o r a number of years, no d e t a i l e d studies of the ores had been made. A c a r e f u l search of the a v a i l a b l e l i t e r - ature revealed that although much work had been done and much had been wr i t t e n during the f i r s t years (ca. 1930) of the development of these deposits, nothing had been published i n recent years. I t was f e l t , therefore, that a general study of these specimens might a i d i n the - 2 - e l u c i d a t i o n of the problem of genesis of the copper ores of one of the most important copper d i s t r i c t s i n the world. I t was r e a l i z e d that the lack of intimate knowledge of these deposits on the part of the w r i t e r and the members of the Depart- ment of Geology at t h i s University would provide a serious l i m i t a t i o n to the general i n t e r p r e t a t i o n of the information gained during the i n - v e s t i g a t i o n and the c o r r e l a t i o n of t h i s information with that gained i n the f i e l d by other w r i t e r s . No s p e c i f i c problem was considered as the immediate .object of the study at the outset, because i n f a c t no s p e c i a l problem was at once apparent. There had been and s t i l l i s a d i v e r s i t y of opinion r e - garding the o r i g i n of the copper deposits, but because of t h e i r s i z e and continuity there has been no p r a c t i c a l necessity f o r a f u l l understanding of t h e i r mode of o r i g i n . However, the deposits provide at once both a challenge and a f e r t i l e f i e l d of i n v e s t i g a t i o n to the students of ore genesis. Because of the disseminated nature of the ore i t was f e l t that t h i n sections and the petrographic microscope offered the best means of studying both the ores and the host rocks. Consequently, over t h i r t y t h i n sections and several polished sections were prepared f o r ex- amination. The study was conducted under the d i r e c t i o n of Dr. H. C. Gunning, to whom the c o l l e c t i o n of specimens o r i g i n a l l y had been sent. To him the writer wishes to extend h i s warmest appreciation f o r advice and encouragement tendered throughout the course of i n v e s t i g a t i o n . Other -3- members of the Department of Geology were extremely h e l p f u l both d i r e c t l y and i n d i r e c t l y . To Drs. L. Dolar-Mantuani and K. C. McTaggart p a r t i c u l a r l y the writer extends g r a t e f u l acknowledgement f o r many h e l p f u l suggestions and s i n c e r e l y regrets that the l i m i t a t i o n of time made i t impossible to undertake a l l the p o s s i b i l i t i e s f o r i n - v e s t i g a t i o n . Acknowledgement i s also made of the work of Mr. J . A. Donnan who prepared a l l the t h i n sections used i n the study. HISTORICAL SKETCH The Rhodesian Copper Belt parallels the northeastern border of Northern Rhodesia (see Fig. l ) and, together with i t s northern ex- tensions i n the Province of Katanga i n the Belgian Congo, forms the most important single copper d i s t r i c t i n the world. The d i s t r i c t has come into such prominence since the turn of the present century. Earliest production came from the deposit at BWana MKubwa which had been known and worked by the natives before the advent of the white man. Production here was sporadic during and after World War I and i t was not u n t i l the 1920's that British and American inter- ests, provided with prospecting rights over large areas, f i r s t realized the magnitude and importance of the Rhodesian deposits. In an upland country, f a i r l y heavily wooded and with very deep regolith, the surface expression of these enormous deposits was practically non-existant; no great copper stained outcrops or gossans advertised the extent and teaor of the copper-bearing sediments, ,, though i t was early noted that straight treeless avenues through the forest ("copper dambds") indicated the presence of hidden copper. Because of f a i r l y simple structural conditions i t was not long before wide-spaced d r i l l holes had, at a number of places, out- lined large and rich orebodies, and production commenced i n the early 1930's. Since then the eight proved deposits, consolidated into four operating units, Roan Antelope, Rhokana, NChanga, and Mufilira, have rapidly assumed a position as the greatest single copper d i s t r i c t i n the world as the following figures of reserves amply i l l u s t r a t e .  Mine or District Percent Copper Reserves (millions of tons) Rhokana - s Nkana North Orebody 3.23 33 Nkana South Orebody 2 . 7 8 2 0 Mindola Orebody 3.67 57 Totals 3 . 3 7 110 Roan Antelope 3.26 95 NChanga 4.66 141 Mufulira 3 . 85 148 Totals 3 . 86 494 (Note: These figures are taken from the Minerals Yearbook-1948, and estimates have probably increased since then.) - Total annual production of copper from these mines i n recent years has been about 3 5 0 , 0 0 0 short tons. With the expansion of treat- ment f a c i l i t i e s and improvement i n r a i l and labour conditions this figure w i l l doubtless show a considerable increase within the next few years. - 6 - PHYSICAL FEATURES The copper region l i e s i n a part of the Central African Plateau. This f a i r l y recently uplifted peneplane stands at an ele- vation of about 4,000 feet and i t s gently undulatory surface i s modi- fied by shallow stream depressions and occasional monadnocks of older crystalline rocks. Relief i s nowhere greater than 1000 feet. Forests cover the land but these consist of f a i r l y widely spaced trees with scanty intervening underbrush. Accordingly, cross- country travel i s relatively simple. Outcrop i s scarce since the products of-decomposition and disintegration have accumulated as a deep regolith; i n many places the s o i l and clay residuum reaches- depths of from 25 to 35 feet, grading downwards into badly decomposed rock. Soi l cover over easily weathered gabbros and dolomites reaches 100 feet. Though lying close to the equator the region, because of i t s elevation, enjoys a very pleasant climate. Rainfall of 40 to 50 inches comes between November and April while i n the dry season (winter) sun- shine prevails and the nights are cool. The Rhodesian copper belt i s drained by the Kafue River and i t s tributaries, a part of the Zambesi system. Bateman (1930)"1" states that the Kafue apparently follows i t s pre-uplift course and crosses the region without regard to hardness ot structure of the underlying rocks. As seen i n Figure 1 the drainage has developed a dendritic pattern and there are many swamps, particularly at the headwaters of tributary streams. 1. Figures i n parentheses refer to the bibliography at the end of this paper. - 7 - The boundary between Northern Rhodesia and the province of Katanga i n the Belgian Congo follows the very low divide between the Kafue and Congo drainages. -8- GEOLOGICAL SETTING (Fig.2) The copper ores are found i n a group of sedimentary rocks appearing now as synclines which strike, and plunge slightly, i n a northwesterly direction. In Rhodesia the anticlines have been removed during peneplanation, but northwards i n Katanga where the folding was more intense, the folds are expressed topographically as successions of nearly parallel strike ridges. Bateman's interpretation of the region i s given below (Bate- man,- 1930, p.414): "The Rhodesian section with i t s simple open folds and minor faulting represents the eroded f o o t h i l l region of an ancient mountain structure. The Katanga section l i e s nearer the axis of this old mountain range, of which only the roots are l e f t . Here the folds are closed and overturned; faults are numerous; and successive nappes of older strata of the Serie des Mines formation are thrust above the younger Kun- delungu beds". Anton Gray (Gray, 1930) correlated the copper-bearing sedi- ments of Katanga and Rhodesia and proposed a nomenclature for the sedi- mentary Systems of the region. The table below follows that of Gray (1930, p.788) but includes intrusive rocks as given by Bateman (1930), Jackson (1932(2)), and Gray (1932) O " G Q REFERENCE | | uFf><- Middle \ B*iana Mkubrsa Series Lower- . '*a". ore horizons, boulder conglomerate Qasernant rocks • ) , • • *> Qabbro |~~~>- I Granite Geological boundaries CX-^ MUSI ' ~ C . C /^f. LT O- <j COLO G I (-A.L. D£PT NCHANC,A MlfJf THE NORTHERN RHODESIAN COPPER BELT REDUCED FROM R.C.B.C. COMPILATION SHEET. Drawn by O W M Date 30juf*B -i. I lo 500,000 Cop ie -c.- TABLE OF FORMATIONS Katanga-Rhodesia Copper Belt Permiar Jixrass: L- C LUBILASH (Karoo) Unconformity ACID AND BASIC INTRUSIVES - Gabbro and diabase s i l l s and dykes - Younger Grey Granite - Younger Red Granite (NChanga) UPPER KUNDELUNGU s <u k un de lu ng i LOWER KUNDELUNGU iab ly  P re -C  a SY ST EM  O F KA TA NG  MWASHIA iab ly  P re -C  a SY ST EM  O F KA TA NG  to 0) T3 m UPPER ROAN o SY ST EM  O F KA TA NG  Se ri e Min e,  LOWER ROAN Unconformity o •rl O GRANITIC INTRUSIVES - Muliashi porphyritic granite and gneiss - Older Grey Granite (MKushi gneissoid granite) (MKushi Granite-Gneiss) - Older Red Granite (NChanga) •ot ero 2 MUVA SYSTEM o -p Unconformity ... Ar ch ea r LUFUBU SYSTEM (BASEMENT SCHISTS) -10- The following short descriptive notes on the rock units are taken from a number of authors, particularly from Jackson (1932(2)). LUFUBU SYSTEM (BASEMENT SCHISTS) These ancient metamorphosed sediments, now micaceous and chl o r i t i c schists, are probably Archean i n age. Their base i s not ex- posed on the Central African Plateau, but they are doubtless several thousand feet thick. MUVA SYSTEM The metamorphic shales and sandstones of this system are sev- eral thousand feet thick and l i e unconformably above the Lufubu. The upper white quartzites of this series form well exposed ridges. GRANITIC INTRUSIVES Older Red Granite Jackson (1932(2)), pp. 489-94) includes a composite, red granitic stock at NChanga as both older and younger (Younger Red Granite) than the System of the Katanga, (see p. 14)). Older Grey Granite The MKushi gneissoid granite (Bancroft and Pelletier, 1929, • p. 6) i s identical with Jackson's (1932, p. 454) MKushi Granite Gneiss which consists "characteristically of gneissoid and quartzose adamellites, with porphyritic facies" and i s equivalent to Gray and Sharpstone's (1929, pp. 14-15) Older Grey Granite. Jackson (1932(2), p. 455) doubts whether these intrusives are,all pre-Katanga i n age stating that "There i s no essential mineralogical difference between the typical MKushi granite-gneiss and the younger grey granites, and where the latter are -11- locally gneissoid, any difference, either mineralogical or structural practically vanishes". On the other hand Gray (1930, p. 787) states that "Both these Systems (Muva and Lufubu) before the formation of the System of Katanga, were intruded by large masses of granite. From these rocks was derived the material of which the sediments of the System of the Ka- tanga were formed". Jackson notes that typical granitic pebbles of basal BWana MKubwa (Roan) are leucocratic granites, very low in ferro-magnesian minerals for which the provenance i s unknown. Muliashi Porphyritic Granite and Gneiss This gneissose, strongly porphyritic, biotite-adamellite i s of local occurrence along the Muliashi River. SYSTEM OF THE KATANGA A synopsis of these sediments i s given below i n the comparative table drawn up by Gray (1930, p.791). The following points in respect to nomenclature should be noted. 1. The MWashia i s locally (NKana) known as the Christmas Series. 2. The Serie des Mines (Katanga); the MWashia-Upper Roan- Lower Roan (Rhodesian Selection Trust, Ltd.); and the Upper- Middle-Lower Bwana Mkubwa (Bancroft and Pelletier) represent equivalent terminologies. I II •12- COMPARISON OF THE SYSTEM OF THE KATANGA IN KATANGA AND RHODESIA Vicinity of Elizabethville - Du Bois Vicinity of Mufulira - Gray System Lubilash not present System Lubilash not present unconformity unconformity System Katanga System Katanga Upper Kundelungu 2000 m. Upper Kundelungu 2000 m, Shales, sandy — - Arkose, "purple" Calcaire Rose ' 5-10 Quartzite, Shales — Shales 0-10 Calcaire Rose Petit Conglomerat 10-30 Shales Petit Conglomerat Lower Kundelungu 1500 m. Lower Kundelungu 2000 m, Feldspathic Feldspathic Sandstone 0-30 Sandstone Shales — Calc. sandy Shale Calcareous Sandstone — Shales Calcaire de Kakontwe Kakontwe 50-100 Limestone 100 Shales 0-50 Shales ) Grand Conglomerate 100-400 Basal Conglomerate) 150 MWashia 300-500 MWashia 400-600 . Feldspathic Feldspathic Quartzite 10-50 Quartzite 15 Black Shale ) Black and Var. Green Dol. Shale) 200-4000 banded Shale, Ss. Chert and Jasper 1-2 Siliceous Oolites 1-2 Chert and Oolites 5 Dolomites of the Serie Upper Roan des Mines 200-400 Dolomite and dol. -Shale with int'b'd. (Base unknown) Sandstone Lower Roan 350 Int'b'd. Shale, Ss. Dolomite 175 Feldspathic Sandstone g r i t , conglomerate 175 unconformity MUVA SYSTEM •13- The beds of the System of the Katanga are unfossiliferous, hence no faunal dating has been possible. Radioactivity determinations (Bateman, 1930, p. 375) on the radium deposits of Chinkolobwe, Katanga (occurring i n beds of the Serie des Mines) give 610 million years, or pre-Cambrian, as the age of the ores. Bateman feels that the uranium and copper mineralization i n Katanga are of one age, and also that the copper of Northern Rhodesia i s the same age and therefore pre-Cambrian. Serie des Mines. Since this series w i l l be discussed i n more detail on sub- sequent pages, only brief notes w i l l be given here. The lower beds of the series, not exposed i n Katanga, are separated by a marked unconformity from the underlying Muva and Lufubu Systems. The copper deposits of Katanga are found i n the basal MWashia and Upper Roan, while those of Rhodesia are found in the Lower Roan. Kundelungu Series The basal member, lying conformably above the Serie des Mines, i s composed of fluvio-glacial shales and conglomerates and i s called the " t i l l i t e of the Katanga". Bancroft (Gray, 1930, p. 789) found these beds over a greater area than the Serie des Mines, the basal conglomerate at places lying directly on the ancient schists and Gray states that "these beds represent, therefore, a transgressive overlap in the sedimentation of the original basin i n which the System of the Katanga was l a i d down". ACID AND BASIC INTRUSIVES -14- NChanga Red Granites Under this heading Jackson (1932 (2), p. 489) describes a large composite granite stock at NChanga composed of red bi o t i t e - adamellite-granite to red mierocline-alaskite, which he believes re- presents two ages, the Older Red Granite and the Younger Red Granite, older and younger respectively than the BWana MKubwa (Serie des Mines). Jackson also believes that the copper.ores of NChanga are genetically related to the Younger Red Granite. Younger Grey Granite This includes a widespread group of grey biotite-adamel- l i t e s , with quartz-diorite and granite facies which are intrusive into the basal members of the Lower Roan Series, and these together with the NChanga younger red granite, are believed to have been the source of copper mineralization of Northern Rhodesia and of the Katanga. Pegma- t i t e and aplite dykes of the younger grey granite cut the Basement Schists, and the granite has, i n places, a garnetiferous contact aureole where i t intrudes the Basement Schists. Basic Intrusive Group The gabbros and norites of this group occur chiefly as s i l l s of over a thousand feet i n thickness and are apparently the young- est intrusives. The s i l l s on the whole conform to the structure of the sediments, but i n places form irregular transgressive bodies. They cer- tainly cut upper Serie des Mines beds and are probably younger than Kun- delungu. Bancroft and Pelletier (Jackson, 1932 (2), pp. 504-505) suggest that the Basic Intrusives may correspond broadly to the Keweenawan basic intrusives of the Canadian Shield. -15- LUBILASH SYSTEM This system consists of a basal glacial or fluvo-glacial conglomerate which i s overlain by sandstones, coal measures, variegated a r g i l l i t e s and shales, and upper friable sandstones and lenticular con- clomerates. It l i e s unconformably and almost horizontally upon the Kundelungu and has been almost entirely removed from the area of the copper belt. The system i s said to be Permian to Jurassic i n age and i s correlated with the Karoo System of South Africa. SEQUENCE OF EVENTS The sediments of the Katanga System were laid down on an erosional surface formed of a basement complex of ancient schists and gneisses intruded by older granites. Subsequently, and possibly accom- panied by intrusions of younger granites, the Katanga System was highly folded and faulted to the north i n Katanga but f a i r l y gently folded i n Northern Rhodesia. This latter folding produced a system of rather open synclines and anticlines with a regional northwesterly trend. Those writers who conceive of an epigenetic origin for the copper deposits re- late the mineralization to the intrusions of the Younger Granites, but state that since the shape of some of the deposits was controlled by i t , the folding preceded the mineralization. :. The relation between the rather gentle folding i n Northern Rhodesia and the stronger overthrusts of Katanga i s aptly described by Douglas (1930, p. 337) who says: "An European parallel may be drawn between the African struc- tures and those of the Alps and Jura. The anticlines and syn- clines of the Jura can be likened to the similar folds in Northern Rhodesia, while the nappes and thrust blocks of the Alps find their equivalent i n the Belgian Congo". Following the intrusion of the Younger Granites, which i n places domed the sediments, gabbroic and diabasic s i l l s and dykes were intruded, and are often found, according to Jackson, as the cores of synclines. The region was then base-leveled and the Permian to Jurassic beds of the Lubilash, or Karoo System were deposited. These rocks have subsequently been removed during further u p l i f t and peneplanation which has continued u n t i l recent times. -17- Bateman suggests (1930, p. 3&1-384) that there i s evidence that just before or since the latest u p l i f t , very arid climatic con- ditions prevailed i n the region. The water table must have been very deep since oxidation i s found to depths of 2000 feet (Gray, 1932, p. 323), and i t i s suggested that only during the very latest u p l i f t , to the pre- sent 4000 feet elevation, has the water table risen to within several tens of feet of the surface. A new cycle of erosion has commenced; the upland region i s i n the stage of extreme youth and many streams are beginning to entrench themselves i n their old courses. -18- THE ORE DEPOSITS Since many details of.the ore deposits, especially at NKana, w i l l be brought out i n a later section of this paper, a brief and general description w i l l be given here and a short comparison with the Katanga ores w i l l be made. As already indicated, the ores i n Northern Rhodesia usually occur i n the basal beds of the Lower Roan. The more common host beds are feldspathic, quartzites, shales, and dolomitic shales, though loca l - l y talc-tremolite schists and schistose limestones may carry ore. At most mines the tabular orebodies extend through a very short s t r a t i - graphic interval; however, at Mufulira for example, three distinct and stratigraphically separated orebodies are present. In many of the deposits ore i s concentrated at drags or thickenings (Fig. 3) on the synclinal.limbs. Thus both tabular and roughly pipe-like orebodies are present, the former being by far the most important. At Roan and NChanga the ore forms part of both limbs of narrow pitching synclines and hence has a horizontal V-shape in plan and a rough U-shape i n section. At Mufulira and Nkana (Fig. 3) where the deposits are on one synclinal limb only, they have the form of tabular bodies. From the epigenetic viewpoint several features seem to be responsible for ore localization: the proximity of granitic intrusions that supplied the metallizing solutions has been cited by a number of writers (Bateman, 1930, p. 411; Davidson,' 1931, p. 148; Jackson, 1932 (1), p. 256; Gray, 1932, p. 335). The position of the ores within the sediments i s apparently - 1 8 a - Figure 3 rfpprox. Or* t-ior/z or? //oij/fjj L/a// /fry/ W / t - o r jSrotvf £>a/orni/e C/xrrfy /fry///,/* 3ar><Sec/ U//>;y,e£>°Jor>,,/ir/Gr*y/?rf///rr'e £>ar* 6r-<ry <*?ryi VA /« /ho ItS*// Chrrp/orrr*r<r/* TOW 3oo /Lev*/ 4SO 600 rso 000 /BSo J4SO /aso '•9/0 2/*€> Z370 TypiCAL SECTION-NORTH OREBODY Rhok a na Corpotat/or? -Nkan o M/ne Scale? i=4/6 -19- the result of physical rather than chemical control since the ores occur i n beds of diverse chemical and mineralogical character. ' Thus permeability favoured the lateral progression of mineralization and Jackson (1932(1), p. 256) suggests that bending and fracturing of otherwise impermeable beds along the axes of minor folds rendered them especially favourable to entry by solutions. Another factor noted by several authors i s the control of s o l - utions by overlying impervious beds. Gray, especially, points this out i n his description of the Mufulira Mine (Gray, 1932, pp. 335-336). Each of the three orebodies i s overlain by an impervious horizon and Gray states that "the stratigraphic position of the orebodies i s clearly due to the damming action of the dense and unfractured overlying beds". No large, cross-cutting, "feeder" faults or fracture zones are mentioned by any of the writers though Gray (1932, p. 335) states that "the solutions reached the sediments most probably through tension cracks i n the cooled granite margin, joint cracks i n the Muva schist and joints in the basal Mine series quartzites in a l l of which they formed sulphide bearing veins". Davidson (1931, p. 151) introduces the possibility of chem- i c a l or mineralogical control when he mentions that the ore replaces the original calcareous cement of the shaley host rock at Chambishi. As Lindgren (1933, p. 629) puts i t , "The persistance of the ore-bearing beds i s amazing". For example, along 42,000 feet of the north limb of the NKana syncline ore i s developed, with two interrup- tions, for something over 35,000 feet and extends to a depth of about 2400 feet. This ore has a stratigraphic thickness averaging 30 to 40 -20- feet which i s the order of magnitude for the whole d i s t r i c t . Batemen 1942, p. 517) says, "At Roan Antelope the metallized bed laps around the nose of a plunging syncline and extends along both limbs for a total proven length of 5 miles and an indicated length of another 4 miles". The Ores. A feature of these copper ores i s their f a i r l y simple miner- alogy. Chalcopyrite, bornite, chalcocite, and carrol l i t e or linnaeite are the major sulphides which are, for the most part, finely scattered through the sediments. The sulphides are sometimes so minutely dis- seminated that a cursory examination does not detect their presence i n rock that i s classed as ore. Minor amounts of pyrite, hematite, and native copper make up the remainer of the metallic minerals but these have not the uniform distribution of the f i r s t named group. Oxidation products (excluding bornite and chalcocite) are numerous but are generally considered to be of minor importance though McKinnon (1943) writes: "One-third, or more, of the copper mineral- ization i n these (NChanga) orebodies occurs i n the form of carbonates and oxides, ....". Cuprite, native copper, malachite, azurite, chry- socolla, tenorite, bieberite (CoS0^.7H20), limonite, goethite, jarosite, cornetite (Cu-^Og^CutOH^), libethnite (Cu-^Og.CutOH^), and mangan- ese wad have been reported from the d i s t r i c t . The problem of hypogene versus supergene bornite and chalco- cite has received much attention from the early workers. In the case of bornite, Gray, Bateman, and Davidson advocate -21- an hypogene origin for the major part of the bornite, while Bancroft and Jackson hold the supergene viewpoint. A brief analysis of the arguments presented leads the writer to believe that i n most instances bornite i s primary but that i n some cases much bornite may be super- gene . The chalcocite problem has been very thoroughly dealt with by Bateman (1930, pp. 393-405) who concludes, contrary to the opinion of the resident geologists of the time, that "part, and perhaps the major part, of the Rhodesian chalcocite i s of hypogene origin and was formed at a temperature above 91°C. or possibly above 200°C". Gray, Bancroft, and Jackson hold that the majority of the chalcocite i s supergene. Bateman's extensive treatment of the chalcocite problem i s very impressive and, as with the bornite, the writer feels, after a perusal of the literature, that though in some places most of the chal- cocite now appears supergene much of i t throughout the d i s t r i c t was originally introduced. It should be pointed out that since the relative amounts of copper sulphides d i f f e r somewhat from mine to mine i t i s d i f f i c u l t to generalize for the d i s t r i c t . For example at Chambishi (Davidson, 1931, p. 149) bornite i s the predominant sulphide, and "Bornite and chalco- pyrite make up f u l l y 98 percent of the primary copper minerals so; far exposed", while at NChanga (Jackson, 1930, p. 26l "chalcocite i s the most abundant sulphide copper ore-mineral", with bornite second i n im- portance and chalcopyrite occurring sparingly. In this latter case Jackson believes that the chalcocite i s predominantly supergene. -22- I t does not appear, upon consideration of the whole d i s t r i c t , that secondary sulphide enrichment has played a major r o l e i n the f o r - mation of the copper deposits of Northern Rhodesia. Not a l l copper m i n e r a l i z a t i o n i s found'in the sediments. At Chambishi (Davidson, 1931, p. 151) the "granite i t s e l f c a r r i e s traces of copper, and pegmatites and quartz veins that come from i t also carry copper sulphides". This statement may be applied to a number of mines i n the d i s t r i c t . Comparison of the Rhodesia and Katanga Deposits. (Gray, 1930, P» 801) In the f i r s t place the ores of Rhodesia and Katanga occur at d i f f e r e n t s t r a t i g r a p h i c l e v e l s : the former i n the a r g i l l i t e s , shales, and arkoses of the Lower Roan while the l a t t e r are found i n the dolomitic beds of the Upper Roan and MWashia. Secondly the Katanga deposits are a l l oxidized bodies l y i n g close to the surface, while those of Rhodesia, with the exception of the o l d BWana Mkubwa Mine, are sulphide deposits that go to depth. The reasons f o r the dif f e r e n c e i n s t r a t i g r a p h i c p o s i t i o n are not p e r f e c t l y c l e a r but no doubt l o c a l s t r u c t u r a l conditions ex- erted a primary c o n t r o l . -23- DESCRIPTIONS OF SPECIMENS FROM THE . 'ORE HORIZON1 AT NKANA MINE. The following i s a generalized but f a i r l y typical section of the Lower BWana MKubwa (Roan) Series as found at NKana Mine (after Guernsey, 1947). Top Feldspathic Sandstones, quartzites, and a r g i l l i t e s Grey, well bedded, quartzose (Hanging Wall) a r g i l l i t e "Ore Horizon" Foot Wall Conglomerate Feldspathic sandstones and thin sandy a r g i l l i t e s "Lower" Conglomerate Grey-brown, muddy, dolomitic sandstones Hard, lig h t , cross-bedded, feld- spathic quartzites up to up to 150 feet 25-35 3 O-40 0-30 50 40-50 60-70 600 80 Basal conglomerate The following divisions of the "Ore Horizon" are also taken from Guernsey. Although the horizon changes to some extent both along the strike and with depth this i s a typical sequence at the North and Mindola Orebodies. - Top 1. Hanging Wall A r g i l l i t e 2. 'Porous Sandstone' -banded, impure dolomite 3. 'Cherty Ore' -hard, fine grained a r g i l l i t e 5 feet 9 -24- 4. 'Banded Ore' -interbedded grey a r g i l l i t e and white dolomite 6 feet 5. 'Low Grade A r g i l l i t e ' -grey, f a i r l y massive dolomitic a r g i l l i t e 12 . 6. 'Schistose Ore1 -schisted dolomite and dolomitic a r g i l l i t e 5 7. Foot Wall Conglomerate 8. Arkoses and Ar g i l l i t e s -fejispathic sand- stones and thin sandy a r g i l l i t e s In the following descriptions "N" refers to the North Ore- body, "M" refers to the Mindola Orebody, and the numbers refer to the above beds of the Ore Horizon, 1. Hanging Wall A r g i l l i t e Megascopic - The five specimens of this bed show f a i r l y similar general features. The rock i s dense, soft to rather hard, grey i n colour, and has a slight to marked clay odour. Two of the specimens exhibit a lamellar structure manifested by dark and light colouring. The lamenae have thicknesses measuring about and they are doubtless parallel to, or represent, bedding. One specimen (Ml-C) differs somewhat from the others i n that i t exhibits no laminations, i s harder, and i s speck- led with very small flecks and blobs of carrollite and a l i t t l e bornite, which minerals also line numerous cavities averaging 3 or 4 mm. i n size. Guernsey states that this last i s not a typical specimen. Microscopic - Typically the rock consists of a mosaic or mat of quartz, feldspar, and sericite grains that average 0.02 mm. in size. Very - 2 5 - r o u g h l y t h e s e m i n e r a l s a r e i n e q u a l p r o p o r t i o n s t h o u g h i t i s d i f f i c u l t t o make q u a n t i t a t i v e e s t i m a t e s o f t h e q u a r t z . a n d f e l d s p a r . The l a t t e r i s u s u a l l y i n s m a l l i r r e g u l a r g r a i n s t h a t have a n e g a t i v e r e l i e f b u t no p o l y s y n t h e t i c t w i n n i n g . Some o f t h e s l i g h t l y l a r g e r f e l d s p a r g r a i n s a r e t w i n n e d arid t h e s e a r e a l b i t i c i n c o m p o s i t i o n . M i n o r c o n s t i t u e n t s a r e b i o t i t e , c a r b o n a t e , and t o u r m a l i n e . B i o t i t e i s p l e o c h r o i c , p a l e t a n t o c o l o u r l e s s ; c a r b o n a t e i s r a g g e d i n o u t l i n e and s t a i n i n g ( C o p p e r n i t r a t e ) shows t h a t i t i s v i r t u a l l y a l l d o l o m i t e ; r a r e t o u r m a l i n e g r a i n s a r e v e r y s m a l l , g r e e n i s h i n c o l o u r , and o f t e n have t h i n c o l o u r l e s s r i m s w h i c h a r e i n t e r p r e t e d a s a u t h i g e n i c o u t - g r o w t h s . V a r i a b l e , s m a l l amounts o f c a r r o l l i t e , b o r n i t e , c h a l c o p y r i t e , and p y r i t e a r e p r e s e n t i n one o r two o f t h e s p e c i m e n s . The d e t a i l s o f o r e m i n e r a l o c c u r r e n c e s w i l l be d e a l t w i t h i n a l a t e r s e c t i o n . " A r g i l l i t e " i s p o s s i b l y n o t a v e r y p r e c i s e name f o r t h i s r o c k b e c a u s e i t c o n t a i n s p r a c t i c a l l y no c l a y m i n e r a l s , b u t t h e w r i t e r c a n s u g g e s t no b e t t e r one t h a t i s i n common u s a g e . M o s t p r o b a b l y t h e s e r i c i t e ha s d e v e l o p e d t h r o u g h d i a g e n e t i c o r l o w g r a d e m e t a m o r p h i c p r o c e s s e s s u b s e q u e n t t o t h e d e p o s i t i o n o f a n o r i g i n a l l y s i l i c e o u s a r g i l l i t e . 2. W h i t e , R e d , o r Brown D o l o m i t e (misnamed " P o r o u s S a n d s t o n e " ) . T h r e e s p e c i m e n s o f t h i s member p r e s e n t a medium g r a i n e d c r y s t a l l i n e c a r b o n a t e a s p e c t w i t h s c a t t e r e d g r a i n s o f q u a r t z , f e l d s p a r , a n d b i o t i t e . The r e d c o l o u r a t i o n o f t h e r o c k s i s i m p a r t e d by f e l d s p a r , n a t i v e c o p p e r , and h e m a t i t e i n v a r y i n g p r o p o r t i o n s . An i l l - d e f i n e d c o l o u r b a n d i n g , w h i c h may be e q u i v a l e n t t o b e d d i n g , i s d e r i v e d f r o m a l a y e r i n g o f s i l i c a t e and c a r b o n a t e m i n e r a l s . -26- S t a i n i n g shows t h a t i n one c a s e ( N 2 - B ) a l l t h e c a r b o n a t e i s c a l c i t e b u t i n a n o t h e r c a se ( M 2 - A ) a p p r o x i m a t e l y e q u a l amounts o f c a l c i t e and d o l o m i t e a r e p r e s e n t . The f o u r t h s p e c i m e n ( M 2 - B ) o f t h i s g r o u p b e a r s n o t t h e s l i g h t - e s t r e s e m b l a n c e t o t h e o t h e r t h r e e and t h e w r i t e r f e e l s t h a t i t s h o u l d be g r o u p e d w i t h t h e " C h e r t y O r e " s p e c i m e n s . C o n s e q u e n t l y i t w i l l be c o n s i d e r - e d a p a r t o f t h a t h o r i z o n f o r d e s c r i p t i v e p u r p o s e s . M i c r o s c o p i c - Under t h e m i c r o s c o p e t h e r o c k p r e s e n t s a m o s a i c o f c l o s e l y i n - t e r l o c k i n g g r a i n s w h i c h a v e r a g e a b o u t 0.5 mm. i n s i z e . The c a r b o n a t e i s v a r i a b l e i n c h a r a c t e r . S m a l l s u b h e d r a l t o e u h e d r a l g r a i n s a r e d o l o m i t e and l a r g e r i r r e g u l a r g r a i n s a r e c a l c i t e . F e l d s p a r w h i c h i s r e f e r a b l e t o o r t h o c l a s e and m i c r o c l i n e f o r m s i r r e g u l a r and c l o u d y g r a i n s r e p l a c e d b y c a l c i t e and s e r i c i t e . Some t w i n n e d f e l d - s p a r g r a i n s , a l b i t i c i n c o m p o s i t i o n , a r e q u i t e c l e a r a n d may t h e r e f o r e be a u t h i g e n i c . A s m a l l amount o f q u a r t z i n i r r e g u l a r i n t e r l o c k i n g g r a i n s i s p r e s e n t i n a l l t h e r o c k s . S e v e r a l o f t h e s e g r a i n s have c l e a r l y d e f i n e d s e c o n d a r y o u t g r o w t h s i n o p t i c a l c o n t i n u i t y . O t h e r m i n e r a l s p r e s e n t i n m i n o r amounts a r e t a n t o g r e e n b i o - t i t e , y e l l o w i s h c h l o r i t e , m u s c o v i t e and s e r i c i t e , and c l a y m i n e r a l s w h i c h a r e a l t e r a t i o n p r o d u c t s o f t h e f e l d s p a r s . M e t a l l i c m i n e r a l s a r e n a t i v e c o p p e r , h e m a t i t e , and b o r n i t e a l l i n f a i r l y s m a l l a m o u n t . The r o c k v a r i e s f r o m a n i m p u r e l i m e s t o n e t o a n i m p u r e d o l o - m i t i c l i m e s t o n e . Some a u t h i g e n i c f e l d s p a r s may be p r e s e n t b u t i t i s mos t p r o b a b l e t h a t t h e c l o u d y , i r r e g u l a r g r a i n s o f o r t h o c l a s e and -27- microcline are original d e t r i t a l constituents. 3. Cherty A r g i l l i t e or "Cherty Ore" . Megascopic - Megascopically the nine specimens of this ore bed present varied aspects. Colours are.predominantly.shades of grey but two specimens are brown and one i s black. Most of the specimens show light and dark laminations, these often containing ore minerals. The rocks are a l l very fine grained, dense in fact, and i n most cases are quite hard, hence their "cherty" description. One of the dense brown specimens contains a one-half inch white lens parallel to the laminations and also thin lacey veinlets or stringers cutting across the rock. •" The black specimen (N3-D) previously referred to i s unique i n the collection. It i s a dense black f i s s i l e rock cut by a number of quartz- carbonate veinlets and lenses about 2 mm. i n width containing f a i r amounts of chalcopyrite. Most of these veinlets follow the f i s s i l i t y but several cut i t at high angles and some are irregular and curved. Varying amounts of c a r r o l l i t e , bornite, pyrite, and chalco- pyrite are present i n a l l the specimens and are mainly distributed along the laminations but some sulphides appear as blobs and disseminations throughout the rocks. Microscopic - The specimens, though differing i n outward appearance, are quite similar mineralogically. Very fine grained quartz, feldspar, and sericite are the major constituents together with varying amounts of carbonate. -28- Quantitative estimates of quartz and feldspar are very d i f f i c u l t to make but i n general the feldspar i s at least equal i n amount to the quartz. Quartz, feldspar, and sericite grains average about 0.02 mm. in size but the ragged carbonate grains are usually about 0.1 mm. i n diameter. The very fine grained untwinned feldspar i s of indeterminate composition but because i t has negative r e l i e f i t i s probably either a potash feldspar or albite. Some larger poysynthetically twinned grains are usually present and these are albite. Several altered and irregular grains of orthoclase and microcline are also present. Much of the quartz exhibits strain shadows. The sericite and other micas have a f a i r l y good preferred orientation parallel to the mega- scopic lamellar structure. Minor though recurring constituents are biotite, tourmaline, and a few grains of zircon, chlorite, and epidote. Biotite and tour- maline are both apparently allogenic though the latter, which i s nor- mally green i n colour, i s often narrowly rimmed with colourless secon- dary outgrowths. The black f i s s i l e rock has slightly different mineralogies! features. It i s almost wholly composed of white mica grains 0.02 to 0.03 mm. in length and having an extremely good preferred orientation. Some very fine grained quartz occurs i n t e r s t i t i a l l y with the mica. The veinlets are composed of interlocking quartz and carbonate grains with i n t e r s t i t i a l chalcopyrite. The black colouration i s produced by a good deal of very finely divided black, opaque, carbonaceous material. It i s rather d i f f i c u l t to apply a rock name to this group of specimens. Their present quartz, feldspar, sericite composition i s no doubt due in some degree to the impress of conditions differing from those existing at the time of deposition though i t i s not l i k e l y that there has been much, i f any, material introduced. Some of the feldspar could possibly be authigenic (Gruner and Thiel, 1937), though here i t ex- hibits no diagnostic features. An originally siliceous or feldspathic shale could have given rise to this rock under conditions of f a i r l y low grade regional metamorphism. For want of a better name and because of i t s outward appearance i t may be termed " a r g i l l i t e " . One or two of the specimens have a large content of carbonate. This i s assumed to be original calcareous material the grains of which having acquired an irregular shape through recrystallization. The black rock with the marked f i s s i l i t y , though very different megascopically from the other specimens i s , nevertheless, minerallogic- al l y quite similar. It i s probably best called a black f i s s i l e shale. 4. 'Banded Ore' - banded white dolomite and grey a r g i l l i t e . Megascopic - This i s typically a grey to white, finely crystalline to dense, rather soft laminated rock. The laminations vary from paper-thin to sev- eral millimeters i n thickness, the grey laminae being thicker and denser and the white ones thinner and finely crystalline. The lighter crystal- line material i s carbonate, staining indicating that i t i s mostly dolo-. mite. Ore minerals, bornite, chalcopyrite, a very l i t t l e c a r r o l l i t e , and possibly some chalcocite, are distributed i n thin layers paralleling the lamellar structure. Some sulphides are disseminated through the rock, -30- and i n specimen N4-A much bornite occurs i n thicker l e n t i c u l a r masses and i n i r r e g u l a r blobs associated with coarse quartz and c a l c i t e . Microscopic - Irregular and anhedral grains of dolomite, averaging about 0.1 mm. i n s i z e , comprise the major portion of these specimens. Varying amounts of irr e g u l a r feldspar and quartz grains and i n some cases a l i t t l e s e r i c i t e are the other important constituents.* The carbonate i s doubtless an o r i g i n a l constituent; the grains may possibly have assumed t h e i r present i r r e g u l a r outlines through some r e c r y s t a l l i z a t i o n . In one specimen (N4-B) they are somewhat elongate owing presumably to deformation. Specimen M4-A contains anhedral to sub- hedral dolomite c r y s t a l l s enclosed i n irreg u l a r and cloudy c a l c i t e grains. Many of the t h i n sections contain much (up to 30 or 40 percent) feldspar. This occurs i n irreg u l a r interlocking grains of which some are potash i n composition but many of which are polysynthetically twinned plagioclase. I t i s believed that most of t h i s l a s t i s a l b i t e but that i s by no means proven. Inclusions of carbonate are very common but the • feldspar grains are otherwise quite clear. Other evidence i s not appar- ent, consequently i t cannot be said whether the feldspar i s allogenic or authigenic. Quartz has much the same occurrence as the feldspar but there i s much less of i t . The micas, b i o t i t e , muscovite, and s e r i c i t e , are on the whole of quite minor amount. In one case small elongate b i o t i t e flakes make up 15 or 20 percent of the t h i n section and i n several cases a good deal of s e r i c i t e Is present. The b i o t i t e f o r the most part has a d e t r i t a l aspect but i n one specimen (M4-A) very i r r e g u l a r and roughly equidimensional -31- b i o t i t e scales have every appearance of being endogenic. Very"small amounts of d e t r i t a l tourmaline and apatite occur i n one or two of the specimens. This rock appears to be a very dolomitic phase of the previous "Cherty A r g i l l i t e " . Dolomite grains usually.forming 50 percent or more of the rocks permits the use of the rock name "dolomite". The darker laminations are feld s p a r , quartz, and minor s e r i c i t e so i t can p o s s i b l y be said the rock consists of a r g i l l a c e o u s and dolomitic l a y e r s . Actually the mineralogic segregation i s not as c l e a r as t h i s l a s t statement might imply so that a laminated a r g i l l a c e o u s , or quartzo-feldspathic, f i n e grained dolomite might present a c l e a r e r d e s c r i p t i o n of t h i s rock. 5. 'Low Grade A r g i l l i t e ' Megascopic - The specimens of t h i s bed are f a i r l y uniform i n colour, tex- ture, and a l l outward appearances. They are shaley or a r g i l l a c e o u s ap- pearing rocks, uniformly dark grey i n colour, f a i r l y s o f t but compact, and have parting or bedding planes one or two inches apart. These l a s t are defined i n one or two cases by t h i n (1 mm.) layers of gypsum or an- hydrite and i n another case by a s i m i l a r l y t h i n l a y e r of f i n e white c a l - c i t e very f i n e l y speckled'with b i o t i t e . Very close scrutiny reveals the presence of rare small blobs and lenses of chalcopyrite and bornite, the l a t t e r often having a l i t t l e f i n e grained quartz i n a s s o c i a t i o n . Microscopic - The major minerals of t h i s rock, quartz, f e l d s p a r , b i o t i t e and muscovite form a rather uniform i n t e r l o c k i n g mosaic with no s p e c i a l t e x t u r a l f e a t u r e s . The quartz and feldspar together, i n grains averag- ing s l i g h t l y less than 0.1 mm. i n s i z e , make up about 50 percent of the rock and the remainder i s mostly b i o t i t e and muscovite or s e r i c i t e , with a d d i t i o n a l small amounts of carbonate (3 or 4 percent), c h l o r i t e , and an- h y d r i t e . The quartz and feldspar form a background of small i r r e g u l a r and i n t e r l o c k i n g grains which are d i f f i c u l t to d i s t i n g u i s h f o r quanti- t a t i v e estimations. Some feldspar grains are twinned and those grains measured are a l b i t e ; many grains are untwinned and because they appear to have negative r e l i e f may be e i t h e r a l b i t e or a potash f e l d s p a r . A few small and rather rounded grains of bornite are scattered i n the rock s e c t i o n . A r g i l l i t e seems to be a f a i r name f o r t h i s rock. At places i t may be dolomitic but the specimens a v a i l a b l e to the writer do not contain dolomite as a major constituent. One of the specimens of t h i s group (M5-C) i s cut, at an angle of about 40 degrees to the bedding ( ? ) , by an one-half inch wide quartz vein, which has bleached the rock f o r distances up to one inch from the vein. A l i t t l e c a l c i t e coats the quartz c e n t r a l l y i n the ve i n and a very l i t t l e chalcopyrite and bornite occur i n the drusy parts of the ve i n . 6. 'Schistose Ore 1 Megascopic - The specimens of t h i s bed are characterized by grey to greyish white and white banding. In the two specimens which are "schistose" the -33- b a n d s , o n e - q u a r t e r t o one i n c h i n w i d t h , a r e f o r t h e most p a r t n o t s h a r p l y - d e f i n e d . The g r e y bands a r e dense and d e r i v e t h e i r c o l o u r m a i n l y f r o m v e r y f i n e g r a i n e d b i o t i t e . These l a y e r s g r a d e i n t o o t h e r s i n w h i c h t h e t e x t u r e i s s u g a r y and t h e b i o t i t e i s s c a r c e r b u t o f l a r g e r g r a i n . O t h e r l a y e r s a r e q u i t e w h i t e and i n t h e M i n d o l a s p e c i m e n s t h e y a r e composed o f v i o l e t t i n t e d a n h y d r i t e , q u a r t z , and w h i t e d o l o m i t e . I n p l a c e s t h e b a n d i n g g i v e s way t o a m o t t l e d o r p a t c h y a p p e a r a n c e . S p e c i m e n M6-B c o n t a i n s a t w o - i n c h w h i t e l a y e r composed o f f i n e t o medium g r a i n e d p u r p l i s h a n h y d r i t e , w h i t i s h d o l o - m i t e , a l i t t l e q u a r t z , and i r r e g u l a r s t r i n g s o f b i o t i t e . I t i s bounded o n e i t h e r s i d e by d a r k t o l i g h t g r e y f i n e g r a i n e d r o c k . A l i t t l e s c a t t e r e d b o r n i t e a n d c h a l c o c i t e o c c u r i n t h e N o r t h O r e b o d y s p e c i m e n . M i c r o s c o p i c - D o l o m i t e and v a r y i n g amounts o f b i o t i t e a r e t h e m a j o r c o n s t i - t u e n t s o f t h e " s c h i s t o s e " s p e c i m e n s . S m a l l amounts o f q u a r t z , c h l o r i t e , and f e l d s p a r a r e t h e m i n o r c o n s t i t u e n t s . The d o l o m i t e ha s s u b h e d r a l and a n h e d r a l o u t l i n e s i n a " c l o s e - p a c k e d " t e x t u r a l a r r a n g e m e n t o f g r a i n s a v e r a g i n g a b o u t 0.3 mm. i n d i a - m e t e r . S t a i n i n g i n d i c a t e s t h a t a l l t h e c a r b o n a t e i s d o l o m i t e and i t a p - p e a r s t o be o f p r i m a r y o r i g i n . Y e l l o w i s h b i o t i t e s c a l e s and s h r e d s , s l i g h t l y s m a l l e r i n s i z e , a r e s c a t t e r e d among t h e . d o l o m i t e g r a i n s and a l s o c o n c e n t r a t e d i n bands where i t ha s good p r e f e r r e d o r i e n t a t i o n . These n a r - r o w bands a r e o f t e n i r r e g u l a r and d i s c o n t i n u o u s and a r e somet imes composed o f b i o t i t e and d o l o m i t e g r a i n s a b o u t o n e - h a l f t h e a v e r a g e g r a i n s i z e . The e v i d e n c e f o r t h e o r i g i n o f t h e b i o t i t e i s n o t c l e a r and t h o u g h i t o c c u r s " i n t e r s t i t i a l l y " and w i t h no good r e p l a c e m e n t f e a t u r e s i t may n o t be a n -34- o r i g i n a l constituent. Feldspar grains vary both i n size and composition. Large irr e g u l a r orthoclase grains are crammed with inclusions of carbonate and b i o t i t e . Smaller i r r e g u l a r twinned and untwinned grains of a l b i t e are f a i r l y clear. Quartz grains are usually f a i r l y small and i r r e g u l a r . The fine grained portion of one of the specimens i s composed of quartz, carbonate, and b i o t i t e i n roughly equal proportions. Small i r r e g u l a r grains of bornite and chalcocite occur spar- i n g l y . The evidences f o r deformation are not apparent and there i s some doubt as to whether the rock has been "schisted" to any great ex- tent . 7. Foot Wall Conglomerate. The specimens of t h i s bed are d i v i s i b l e into three groups each exhibiting d i s t i n c t i v e p e t r o l o g i c a l characters. One group, including specimens N7-C, and M7-A, C, and D, i s grey-white i n colour and though t y p i c a l l y conglomeratic i n aspect has a "welded" appearance. Larger rounded and subrounded pebbles, of one inch or so i n size and consisting mostly of quartz and chert, are quite scarce. Most of the rock consists of small (averaging about 1 cm.) an- gular fragments of quartz and white plagioclase feldspar i n a grey s i l - iceous matrix. Femic minerals, which together with fine grain size im- part the dark colour, include b i o t i t e and muscovite. One specimen, M7-A, contains some v i o l e t tinted grains of an- hydrite. Another, N7-C, i s speckled with small masses of p y r i t e . -35- Microscopic characters were only observed on specimen N7-C. The t h i n section contains feldspar of several v a r i e t i e s - orthoclase, mi c r o c l i n e , and a l b i t e - t o t a l l i n g about 50 or 60 percent, and quartz comprising 25 or 30 percent of the se c t i o n . The remainder i s made up of b i o t i t e (ca. 5%) and minor amounts of c h l o r i t e , carbonate, z i r c o n , a p a t i t e , and p y r i t e . I t appears that there has been extensive r e c o n s t i t u t i o n and possibly metasomatism active i n the formation of t h i s rock. The l a r g e r potash feldspars, as well as containing s e r i c i t e and clay a l t e r a t i o n s , have been replaced by quartz of the f i n e grained v a r i e t y . Two types of plagioclase are present: larger twinned grains of a l b i t e (An. 5), and small (le s s than 0.1 mm.) c l e a r untwinned grains with negative r e l i e f . This l a t t e r type may be of authigenic o r i g i n . The l a r g e r grains of a l - b i t e e x hibit "chessboard" or "schachbrett" texture which i s taken by some ( G i l l u l y , 1933, pp. 68, 73; Goldschmidt, 1916, pp. 68, 71, 78, 86; Becke, 1913, pp. 124-25; Grubenmann and N i g g l i , 1924, pp. 435-36) to be evidence of replacement o r i g i n . B i o t i t e may replace other minerals but the evidence i s not c l e a r . I t might be suggested that t h i s grey conglomerate has been de- ri v e d from the Older Gray Granites, but because no specimens of t h i s l a t - t e r rock are available to him t h i s i s pure conjecture on the part of the w r i t e r . The second group of Foot Wall Conglomerate specimens includes N7-A and B which are more t y p i c a l l y sedimentary than those of the f i r s t group. These are thoroughly indurated rocks but they do not at a l l have the welded appearance of the f i r s t group. Larger (1 to 2 cm.) rounded -36- quartz and chert pebbles and smaller (less than 1 mm.), angular, red- dish potash feldspar fragments are embedded i n a sandy and clayey cement which i s slightly calcareous. Microscopically the specimens are f a i r l y similar. Feldspar, including microcline, orthoclase, and albite, and quartz are the major minerals with small amounts of biotite, chlorite, muscovite and carbonate. A few det r i a l tourmaline grains are also present. The boundaries of the strained quartz grains are minutely scalloped by narrow bands of authigenic (?) albite (?). The larger feldspars contain much sericite and clay alteration. Obviously the grey conglomerates have been subjected to an higher grade of metamorphism than the buff or pink rocks whose characters may have been derived from purely diagenetic processes. The f i n a l group of the foot Wall Conglomerate types i s repre- sented by one specimen, M7-B. This rock i s a f a i r l y porous polymictic conglomerate. Rounded to sub-angular pebbles are one-half to one inch i n size and range i n composition from dense chert and clear quartz, to red, medium grained "granite", and to finely crystalline, calcareous, b i o t i t i c , and chloritic (possibly serpentinous) dark fragments. The cementing or matrix material consists of medium to fine grained quartz, pink potash feldspar, and biotite at least partly held together i n a rather porous mass by powdery calcite. A 2mm. quartz veinlet cuts through the specimen. No suggestion can be given by the writer for the provenance of this rock but i t i s obviously quite different from that of the other representatives of the Foot Mall Conglomerate. It i s suspected that although this Foot Wall Conglomerate •37- may, as a conglomerate facies, be persistent over large areas, the de- t a i l s of i t s lithologic character, with respect to both components and induration or metamorphosm, vary considerably in a local sense. 8. "Kegelspathic Sandstones and thin sandy A r g i l l i t e s " or Arkoses and A r g i l l i t e s Megascopic - The three specimens of this bed d i f f e r somewhat i n outward appearance but are apparently quite similar i n overall mineralogic com- position. A l l are reddish or pinkish in colour and of medium or fine grain. Specimen N8-A i s a f a i r l y fine grained compact rock consist- ing predominantly of pink feldspar and quartz and containing several wavy, very thin dark bands which consist of specular hematite. Specimen N8-B though mineralogically similar, i s banded with pinkish quartzo-feldspathic layers alternating with dark fine grained siliceous bands up to 5 mm. i n width* The layers tend to have rather irregular surfaces and the dark colour i s apparently the result of a lack of pink feldspar and a slightly smaller grain size rather than an increase i n dark mineral content. Specimen M8-A has a rather porous texture and i s composed predominantly of angular pink feldspar grains about 1 mm. i n size. Angular and rounded quartz and chert fragments occur here and there as do dark b i o t i t i c and chloritic masses. The rock also contains a very thin dark lens of fine grained specular hematite. Microscopic - The rock sections are composed almost wholly of quartz and feldspars i n about equal proportions and averaging about 1 mm. i n size. - 3 8 - The quartz has rounded to sub-rounded outlines which are scalloped i n deta i l . Most of the quartz grains exhibit strain shadows. The larger grains of feldspar are mostly orthoclase and microcline usually contain- ing a good deal of clay alteration. The matrix material consists of an intimate mixture of very small (o.05 mm. or less) grains of quartz and feldspar. The latter shows negative r e l i e f but only rarely polysynthetic twinning, and i s quite clear. Varying but small amounts of carbonate, biotite, muscovite, chlorite, tourmaline, and anhydrite are also present. As noted previously minor amounts of specular hematite produce thin dark- ish layers i n parts of the rocks. Feldspathic sandstones, or arkoses are appropriate names for these rocks. Textures are apparently due to diagenesis and no obvious metamorphic effects are apparent. The fine grained quartz-feldspar cementing material may have derived i t s present character from some com- paction and possible reconstitution subsequent to deposition. Microcline has clear secondary outgrowths which may be termed "pressure microcline" and fine grained feldspar produces, by replacement, the scalloped borders on quartz grains and may therefore be authigenic. The other minerals, including tourmaline, exhibit no features that would class them as other than d e t r i t a l fragments. Specimen M8-A can possibly best be described as a "decomposed" granite, i.e. a sedimentary rock formed of the products of a rapidly weathered granite, which products have undergone l i t t l e or no transpor- tation and have formed a rock l i t t l e different, from the parent. -39- DETAILS OF ORE MINERALOGY AT NKANA MINE General Statement - Copper mineralization, according to Guernsey (1947), has been found i n the 'Ore Horizon' and to a minor extent i n the Hanging Wall strata, along both limbs of the syncline but, to date, mining has been done only along a part of the East limb where the South, North, and Mindola Orebodies are located. The 'Ore Horizon' i s continuous between them but i s poorly mineralized at two or more lo c a l i t i e s along the outcrop. Guernsey makes the following statements concerning the ore min- erals and their occurrences. "The dominant ore minerals are chalcopyrite, bornite, chalcocite, and c a r r o l l i t e . Pyrite i s present at some lo c a l i t i e s where chalco- pyrite i s the principal ore mineral. Bornite and chalcocite are im- portant below the zone of oxidation but tend to decrease down the dip. The decrease i s not uniform throughout and i s evidently con- ditioned, to some extent, by the structure and porosity of i n d i v i - dual members of the 'Ore Horizon'. Chalcocite and native copper ex- tend to a depth of at least 2370 feet at the north end of the North Orebody. The latter, i n thin sheets and small irregular aggregates, i s generally restricted to the 'Porous Sandstone' but has been noted near the foot wall. Minor malachite staining has been observed at a depth of 1810 feet i n this section. The sulphides occur principally as disseminations throughout the 'Ore Horizon' strata and, to a lesser extent, as clots, aggre- gates and vein-like aggregates with carbonates and quartz. Through- out the North and Mindola Orebodies the so called 'Cherty Ore' i s generally richest i n copper, while the 'Low Grade A r g i l l i t e ' i s the poorest. The cobalt-copper sulphide Carrollite i s most often con- centrated i n the copper members of the 'Ore Horizon 1, - the 'Cherty Ore' and the 1 Porous Sandstone'". Because of the disseminated character of the ore minerals, thin sections and the petrographic microscope offered the best means of study- ing mineralogic relationships and consequently very l i t t l e polished section work was undertaken. - 4 0 - D e t a i l s of ore mineral occurrences w i l l be discussed, under the various members of the 'Ore Horizon'. 1. Hanging Wall A r g i l l i t e . C a r r o l l i t e , bornite, chalcopyrite, and p y r i t e are found i n small amounts i n t h i s bed. C a r r o l l i t e ( i n specimen Ml-C) i s present both as disseminations and i n c a v i t i e s from 1 to 5 or 6 mm. i n diameter. Euhedral, subhedral, and i r r e g u l a r grains from 0 . 2 to 0 . 4 mm. i n d i a - meter, and l a r g e r masses up to 3 mm. i n s i z e are found throughout the specimen. Bornite forms t h i n rims on the c a r r o l l i t e and i s disseminated as i n d i v i d u a l grains from 0 .1 to 0 . 2 mm. i n s i z e . The small amount of chalcopyrite i s associatedri with, and replaces to some extent, the c a r r o l l i t e and i s i t s e l f p a r t i a l l y replaced by bornite. . P y r i t e , i n grains from 0 . 0 1 to 0.05 mm. i n size and also i n cubes and i r r e g u l a r masses averaging 0 .3 mm. i n s i z e , i s thoroughly disseminated throughout some of the specimens. The g r a i n size of the p y r i t e bears a d i r e c t r e l a t i o n to the grain s i z e of the rock minerals with which i t occurs: coarser grains of py r i t e are found i n coarser portions of the rock and f i n e r grained py r i t e with f i n e r grained rock minerals. I t i s f a i r l y c e r t a i n that p y r i t e replaces rock minerals because the l a r g e r masses of p y r i t e con- t a i n i n c l u s i o n s of both b i o t i t e and feldspar though no preference f o r any one mineral species i s apparent. There i s some tendency f o r small py r i t e grains to be oriented i n " s t r i n g s " p a r a l l e l to the laminations i n the rock but micros c o p i c a l l y no s t r u c t u r a l or mineral o g i c a l c o n t r o l can be discerned. -41- Carrollite, bornite, and chalcopyrite have features similar to those of the pyrite, with the difference that the grain size of the rock does not control the size of the sulphides. The sizes of the carrol- l i t e grains and masses are much larger than those of the surrounding rock minerals. Irregular carrollite masses, rimmed by bornite and partially replaced by chalcopyrite, contain inclusions of carbonate, feldspar, and white mica, and apparently replace rock minerals although, as with the pyrite, no metamorphic or hydrothermal effects connected with sulphide emplacement can be recognized. It i s suggested that the sulphides may have replaced carbonate preferentially but this i s merely an impression and conclusive proof i s lacking. 2. 'Porous Sandstone' Native copper and hematite are the metallic minerals found i n this bed and although they can be discerned with the naked eye nothing concerning their relationships to other minerals can be determined. The this sections reveal that the native copper, which occurs as dusty frag- ments and shapeless masses often a millimeter or so i n size, has a very marked af f i n i t y for feldspar grains. The latter are so thoroughly cram- med with copper that r e l i c t polysynthetic twinning i s barely perceptible. Although i t i s sometimes found between carbonate grains the copper prac- 0 t i c a l l y never i s included i n them. Powdery red hematite, sometimes im- possible to distinguish from the copper, appears to line minute fractures and cleavage cracks as well as outline the carbonate grains. Why the copper has such preference for the feldspar and at the same time such disdain for the carbonate i s rather inexplicable i n the writer's opinion. The copper i s probably of secondary origin and i t s 4 2 - reduction may have been brought about by the- hematite. If such i s the case then the present position of the copper has been controlled by the position of the hematite. Thus the copper followed the hematite and now appears as coatings on the rock minerals and f i l l s the interstices between them. 3. "!Cherty Ore' The sulphides, c a r r o l l i t e , bornite, chalcopyrite, and pyrite, are found i n varying quantities, never more than several percent of the total rock mass, i n the specimens of this ore bed. In general the sulphides are oriented along laminations or bedding (?) planes but this i s by no means always so. In many instances small blobs and masses are scattered through the rock seemingly without regard to structure. There are rare cases i n which the ore minerals are located along cross-cutting fractures but because these structures are very infrequent that type of occurrence i s similarly uncommon. No obvious mineralogic controls of ore emplacement are appar- ent i n the thin sections with the exception that the size of the sulphide grains i s clearly a direct function of the size of the rock minerals. The sulphides a l l occur as irregular masses that doubtless replace rock min- erals, as evidenced by inclusions of feldspars, carbonate, tourmaline, quartz, and even rock fragments. If there i s any preferential replacement i t i s of carbonate; but the writer doubts whether this could be proven s t a t i s t i c a l l y or otherwise. Carrollite i s invariably confined to coarser layers within the rocks. Chalcopyrite, bornite, and pyrite although often concentrated in layers are also very thoroughly disseminated. The range of grain size i s quite large. Carrollite aver- ages about 1 mm. in size; bornite, chalcopyrite, and pyrite range from less than 0 . 0 5 mm. to over o.l mm. in diameter. Very noticeable are the lack of hydrothermal type alterations or the association with the sulphides of "gangue" minerals. It i s possible that some of the coarser grained feldspars and carbonates may have origin- ated with the ore solutions but no evidence can be put forth to substantiate such a proposition. L, 'Banded Ore' The occurrences of the ore minerals i n this bed are much the same as in preceding horizons. Bornite and chalcopyrite and a rare grain of carrollite are the sulphides. For the most part these minerals, as strings of grains and elongate irregular masses, are situated along lam- ination or bedding planes. Some smaller grains are disseminated. In one specimen (N4-A), in which the lamellar structure i s not as clearly defined, larger and more irregular masses of bornite with a l i t t l e chalcopyrite are present, in one place as i n t e r s t i t i a l masses associated with quite coarse and interlocking quartz and carbonate grains. Microscopic similarities to other occurrences are also notice- able. Some of the larger masses of bornite and chalcopyrite (from 0 , 1 to 2 mm, in size) contain inclusions of carbonate, biotite, white mica, and feldspar. Thus the sulphides assuredly replace rock minerals but at the same time the sulphides are moulded to some extent around the rock mineral;, grains in which manner they have acquired quite irregular outlines. Again, as before, the ore minerals are larger and more numerous -44- i n coarser portions of the rock and smaller i n f i n e r portions as evidenced by scattered specks of bornite 0 . 0 5 mm. i n s i z e i n rock of s i m i l a r grain s i z e . No a l t e r a t i o n s or gangue mineral introduction can be a t t r i b u t e d to the sulphide emplacement. 5 , 'Low Grade A r g i l l i t e ' Ore minerals are p r a c t i c a l l y imperceptible i n t h i s bed. A 1 mm, lens of chalcopyrite occurs i n one of the specimens but nothing else can be seen with the naked eye. Two t h i n sections reveal the presence of small ( 0 , 1 to 0 , 3 mm..), rather roundish grains of bornite very sparsely scattered through the rocks. These grains are somewhat l a r g e r than the enclosing rock min- erals and have no apparent replacement r e l a t i o n s with them. In f a c t the method of emplacement of the sulphide grains i s rather puzzling. 6, 'Schistose Ore' The two Mindola specimens contain no sulphides whatsoever. The North Orebody specimen contains bornite and possibly some chalcocite i n very small i r r e g u l a r masses scattered through the coarser portions of the rock. In t h i n section the m e t a l l i c minerals are no d i f f e r e n t from those already described. Some l a r g e r ( l mm. or so) i r r e g u l a r aggregates are found i n the coarser portions of the rock and a very few smaller grains are scattered through the f i n e r portions. In places red powdery- hematite outlines the rock minerals. 7, Foot Wall Conglomerate. Only one of the specimens of t h i s member contains any sulphide -45- mineralization. The matrix of specimen N7-C, one of the "welded" appear- ing rocks, contains about 1 percent of pyrite as small ( l mm. or less) scattered irregular blobs. The thin section shows that the pyrite encloses small grains of biotite, white mica and feldspar. No structural controls, preferential replacement, or alteration effects are apparent. 8, Arkoses and A r g i l l i t e s The specimens of the member contain no sulphides. The only metallic mineral present is a small amount of specular hematite which, in two arkosic specimens forms very thin, wavy, and discontinuous layers. This iron mineral is very fine grained and i t s identification is based on i t s appearance both in hand specimens and in thin sections and on the fact that chemical tests detected only iron and no other metallic element. In the thin section the hematite appears as small fragments and irregular masses (less than 0.5 mm. in size) scattered through the fine matrix mat- erial of the rock. How this hematite came to i t s present position i s a perplexing problem. The grains are most certainly arranged in a linear or planar pat- tern but the continuation of a controlling structure is not to be seen in the hand specimens, which are, in one case at least, apparently quite structureless. Nor can any structure be seen in the thin section. Large grains of kaolinized feldspar cut across the band formed by the metallic grains and certainly no structure crosses the feldspar. There does not seem, therefore, any reason for supposing that this material has been i n - troduced; i n fact i t i s more probable that i t was deposited contempor- aneously with the other detrital mineral fragments. -46- ORIGIN OF THE ORES This section of the paper w i l l be devoted to: a brief re- view of general theories of ore genesis; a recapitulation of prevalent theories concerning the Northern Rhodesian copper ores; a summation and interpretation of evidence gathered in this study; and suggestions for further work which may help in the elucidation of the genesis of the copper ores. General theories of ore emplacement. It is not the object of the following remarks to present a c r i t i c a l review of the details of various theoretical concepts respect- ing the origin of ore deposits. I t i s hoped that a brief summary of theories w i l l aid in the direction of thought: the case for any theory must be built upon a foundation of numerous pieces of factual evidence which have been put together in such a way that the strongest possible structure results. Facts may be related one to another in many ways but there i s assuredly only one way which is right: this then is the ultimate truth. The f i r s t step towards the attainment of the truth i s the accumulation and analysis of the factual evidence; the second i s the synthesis of this evidence. I t i s therefore necessary to know what evidence to accumulate and what the synthesis i s l i k e l y to produce. Theories of ore genesis may be broadly cast into three groups: 1. Epigenetic - ore deposits of later origin than the rocks among which they occur. 2 . Syngenetic - ore deposits formed contemporaneously with -47- the enclosing rocks; for the present purpose two subdivisions may be made: i . Deposits in sedimentary rocks, i i . Deposits in plutonic rocks. In the following discussion the term syngenetic w i l l be restricted to deposits in sedimentary rocks. 3 . Meta-syngenetic - ore deposits resulting from the meta- morphism of pre-existing deposits. These theories have three essential factors in common: a source of material; a means of transport of material; a site and a means of deposition of material. Epigenetic theories of ore genesis presuppose a previous ac- cumulation of materials at depth in a reservoir of some sort. These mat- erials must now rise to the scene of deposition and obviously two factors are essential. F i r s t l y there must be some means of access to the site of deposition and secondly the materials must be in a condition of 'potential mobility'. There must of course be a suitable site for the re-accumulation of the upward migrating material and a means of deposition (a physical or chemical process). Pre-existing rocks provide the source of materials for syn- genetic (sedimentary) theories of ore genesis. These materials are trans- ported to the site of accumulation or deposition by surface agencies: water, wind, and ice. The site i s obviously a sedimentary channel or basin and the means of deposition may be physical or chemical or both. Meta-syngenetic theories of ore genesis include the middle ground between epigenetic and syngenetic theories. The elucidation of -48- meta-syngenetic ores is fraught with complexities, as i s that of their counterparts i n petrology, the metamorphic rocks. Geologic evidence. The support of any theory w i l l be based upon geologic evidence. Where w i l l this evidence be found and what w i l l be i t s nature? Epigenetic deposits - Because a source at depth has been postulated some evidence of this magmatic source may be discovered or possibly induced. These questions must be answered: Are possible source rocks close at hand? Can these rocks be identified as source rocks? Evidence w i l l be provided by ore-bearing plutonic rocks, or by plutonic rocks which are obviously the parents of ore-bearing veins. Closely related to the source i s the means of access. Along which paths were the materials brought to their present resting place? Such features as major faults or shear zones, secondary or subsidiary faults and fractures, and small to very small scale openings w i l l be look- ed for to answer this question. As a corollary of the preceding question comes the next quest- ion: If pathways are present do they bear the imprint of the materials which used them? Ore minerals themselves, or exotic minerals, or trans- formations of pre-existing minerals w i l l provide an answer to this query. Finally: What i s the site of deposition and what were the means of deposition? The growth of minerals i n open spaces, the f i l l i n g of cracks, the replacement of pre-existing minerals and similar features which point to the ore minerals as 'late-comers1 w i l l help to elucidate the problem posed by this question. -49- A lack of evidence for epigenjis leads naturally to a syn- genetic conception. If no plutonic source, no tectonic structural con- trols, and no alterations or mineralogical replacements can be discerned then obviously the foundation has been laid for the erection of a syn- genetic theory. Upon this foundation evidence relating the ores to purely sedimentary phenomena may be built. Such facts as stratigraphic layering of ore, detrital characteristics of ore minerals, and the presence, either actual or inferred, of precipitating agents may be used. The middle ground provided by the meta-syngenetic theory is very difficult to map, depending on the extent to which the processes re- sponsible for transformation have been effective. At one extreme the over-print of metamorphic effects will be very light and relict sedimen- tary features will still be apparent. At the other extreme the sedimen- tary deposit will be so thoroughly transformed that no original features remain and the deposit is now indistinguishable from an epigenetic one. The reader is referred to a recent paper by Backlund (1950) for a more extensive treatment of this last extreme viewpoint. A brief recapitulation of theories of origin of the Northern Rhodesian Copper Deposits. Some reference to theories of origin has already been made in an early part of this paper, (p. 18). Both syngenetic and epigenetic theories have been proposed for these copper ores but only the latter is dealt with to any extent in the literature. In fact the only literature available to the writer proposes and cites evidences for epigenetic theories. Schneiderhohn (1931) 50- i s the only author known to this writer who has upheld the syngenetic concept. His work was done very early in the history of the development of the deposits and forms a part of a larger part of a work on South Africa. Schneiderhohn's theory was doubtless based upon the large extent and bedded nature of the deposits and also upon the apparent lack of hydro- thermal alterations. Much evidence has been put forward by proponents of epigenetic theories. Some of this may be open to question but the following tabu- lation gives an indication of the type and extent of evidence that has been cited. 1. Proximity of Younger Granites (source rocks) and ore bear- ing strata. In places these granitic rocks are said to cut the ore horizons. 2 . Copper i s found in the granite, in fissures in the Base- ment Schists, in joints in the basal Mine series quartz- ite s , and in veins in the ore horizon i t s e l f (Gray, 1932, p. 3 2 9 ) , 3 . Ore is found in beds of diverse mineralogical and chemical composition: a physical control i s indicated, 4. The ores are confined to beds which are, or have been quite permeable. Permeability of beds has been increased by folding and consequent re-arrangement. 5. The ores have been localized by the damming action of im- permeable sedimentary horizons. 6. The ore has migrated from one horizon to another through openings in impermeable beds. -51- 7. The ore minerals replace rock minerals. Ore replaces the o r i g i n a l calcareous cement of the ore horizon (Davidson, 1931, p. 151). 8 . Presence of the high temperature mineral, tourmaline, and other minerals such as white mica, colourless c h l o r i t e , and secondary feldspars which have been introduced by the ore s o l u t i o n s . The introduction of quartz and carbonate minerals and the association of ore minerals with t h i s 'gangue'. 9. Sulphides oriented along s c h i s t o s i t y show no elongation. 10. The majority of the chalcocite was formed above 91°C. or possibly above 200°C. 11. The shape of the deposits i s p a r t l y c o n t r o l l e d by the f o l d - ing: the ore min e r a l i z a t i o n i s l a t e r than the f o l d i n g . Some remarks concerning these evidences and t h e i r i n t e r p r e - t a t i o n w i l l be made a f t e r the ensuing presentation of evidences gathered during the study of the specimens. Evidence gathered i n t h i s study. I t i s c e r t a i n l y not possible to base a completely d e t a i l e d theory of genesis upon the evidence accumulated i n an i n v e s t i g a t i o n such as t h i s . Nevertheless i f answers to ce r t a i n questions are forthcoming the erection o f a theory may be f a c i l i t a t e d or at l e a s t the search of ev- idence may be given d i r e c t i o n . L o g i c a l l y the f i r s t question i s : In what beds are the ore minerals concentrated and what reasons can be given f o r the occurrence of ore i n these beds? . Guernsey (1947) states that the 'Cherty Ore' i s richest in copper while the 'Low Grade A r g i l l i t e ' is the poorest, and also that carrollite is most often concentrated in the 'Cherty Ore' and the 'Por- ous Sandstone'. These statements are borne out by the collection of specimens. The 'Cherty Ore' member carries the largest quantity of sul- phides and the 'Porous Sandstone' and 'Banded Ore' carry less, but never- theless important, quantities. Other beds contain very minor amounts of sulphide minerals: the Hanging Wall A r g i l l i t e contains a l i t t l e carrol- l i t e , bornite, chalcopyrite, and pyrite; the 'Low Grade A r g i l l i t e ' has a few scattered grains of bornite and chalcopyrite; the 'Schistose Ore' contains a l i t t l e bornite; the Foot Wall Conglomerate contains a l i t t l e pyrite; the Arkoses and Arg i l l i t e s contain no sulphides. What are the features of the major ore beds? Marked differ- ences in mineralogical composition are at once apparent. The 'Cherty Ore* horizon i s a dense, f a i r l y hard a r g i l l i t e , in places containing small amounts of carbonate. The 'Banded Ore' i s an argillaceous dolomite and the 'Porous Sandstone' i s an impure limestone or an impure dolomitic lime- stone. Obviously the presence of sulphides i s not a direct function of the composition of the rocks. Presumably then the presence of ore in these beds may in some way be connected with their physical characteristics. What physical char- acters these beds might have in common is not readily imagined. If i t i s believed that ore minerals are deposited from hydro- thermal solutions then permeability is a factor to be considered. The writer cannot imagine that the primary or secondary (product of deform- ation) permeability of what is now a very dense a r g i l l i t e can ever have -53- been greater than that of the other beds of the 'Ore Horizon1'. Possibly Mackay's (1946) 'principle of impedance' (essentially the 'hypo-filtration' or obstruction of 'ore radicles' but not of the 'carrier' solution: an os- motic process) may account for the preponderance of ore in this dense hor- izon. It must be pointed out however that even i f this principle is ap- plicable here i t does not apply to other mines in the d i s t r i c t whose major ore horizons di f f e r , both chemically and physically, from this 'Cherty Ar- g i l l i t e ' . Also inexplicable i s why the lower 'Low Grade A r g i l l i t e " of similar physical and mineralogical character did not 'impede' the rising solutions before they progressed to the 'Cherty A r g i l l i t e * . The writer can produce no satisfactory explanation, following epigenetic theory and based on the chemical and nhysical character of the rocks, for the preponderance of ore in this horizon. Perhaps the stratigraphic position of the ores may be attributed to features which are not a direct function of the character of the host rock. This introduces the possibility of *damming' action. Rising sol- utions upon reaching an impervious horizon may be obstructed in their ver- t i c a l progression and thereupon spread laterally along more permeable hor- izons. This appears to be a possibility at NKana. The hanging wall of the ore deposit is formed by a f a i r l y massive a r g i l l i t e which has been mineral- ized to some extent near i t s base. The 'Low Grade A r g i l l i t e ' , near the foot wall of the 'Ore Horizon', may also have acted as a barrier because, though very sparsely mineralized i t s e l f , i t overlies the 'Schistose Ore' which in places contains appreciable copper mineralization. Because only those evidences observable in the collection of specimens are being considered, the effect of faults and l i k e structures as possible 'localizers' of ore w i l l not be considered here. The position of the ore minerals within the beds of the 'Ore Horizon' should be reviewed. The sulphides, though disseminated to a greater or lesser extent, are noticeably oriented along lamination planes. Presumably this might be an effect of hydrothermal processes. I t i s pos- sible that microstructures were formed along the lamination (bedding) planes during deformation and prior to or accompanying mineralization. These openings would provide pathways for the mineralizing solutions. Such openings may have been very small indeed but i t is only necessary that they were la r g e r than the pore spaces of the rock. Cross-cutting fractures are not at a l l common in the ore beds but where they do occur they are found to be mineralized. The black f i s s i l e shale requires special comment. Conform- able (with f i s s i l i t y ) and cross-cutting chalcopyrite bearing quartz- carbonate veins are a unique feature of this rock. The origin of these veins most assuredly conforms with epigenetic theory. The cross-cutting veins exhibit an interesting form. They are sinuous, a shape which is interpreted by the writer as the result of nonaffine deformation along the f i s s i l i t y surfaces. If such is the case then they were obviously emplaced before or during the deformation which produced the f i s s i l i t y . What microscopic characters may be used as evidence? The size of sulphides i s goverened by the size of the enclosing rock min- erals. This morphological correspondence suggests replacement of the rock minerals by the ore minerals. In the writer's opinion this has definitely occurred. Inclusions of si l i c a t e and carbonate minerals in the sulphides i s evidence of this but i t must be admitted that a spec- i f i c example in which sulphides could be 'seen' to replace other -55- minerals would be hard to find. At some places there i s doubtless an association of the ore minerals with quartz and carbonate, as witnessed by the veins in the f i s s i l e shale. A similar association i s found along some of the lamin- ation planes. But this is by no means always the case. Many instances of sulphide occurrence without the association of these 'gangue' minerals have been noted and the writer would be loath to generalize that the ore minerals are always associated with quartz and carbonate, introduced or otherwise. Tourmaline occurs in small quantity throughout the rocks of the 'Ore Horizon1 but every grain examined by the writer showed detrital characteristics, though some grains did have colourless rims which are interpreted as authigenic outgrowths. Other minerals such as biotite, sericite, chlorite, and feldspars may or may not be original sedimentary constituents but they certainly show no special relations to the ore min- erals and are probably best attributed to diagenetic or low grade regional metamorphic processes. Summation and suggestions for further research. Enough data have been accumulated to form a basis for a theory of origin of these ores but i t is clear that much of these data require re-examination and substantiation. In the writer's opinion the evidence most markedly points to an epigenetic theory. There are certainly too many cr i t e r i a for enigenesis to warrant an attempt to construct a syn- genetic (sedimentary) theory. It is not proposed that any one fact or piece of evidence alone must support the theory: numerous relatable facts must be used. -56- Considering as factual a l l evidence that has been cited up to this point the case for epigenesis may be stated in the following manner. There is a source for the ore minerals i n the younger granites and these rocks exhibit evidence i n the form of copper mineralization that they were indeed source rocks, Channelways bearing the imprint of metal- l i z i n g solutions lead from the source rocks to the sites of deposition. The sites of deposition are provided by rocks more permeable than their neighbours and mineralogical relationships show that replacement of these rocks by the ore minerals did take place. Barriers to rising solutions were present and were effective i n concentrating the mineralization. The strength of a theory depends upon the strength of the evi- dence with which i t is b u i l t . How strong i s this evidence? The writer submits that his evidence is strong enough to preclude a ourely syngenetic (sedimentary) theory. Because of his unfamiliarity with the d i s t r i c t the writer can scarcely c r i t i c a l l y review the evidence of other writers but doubt appar- ently exists (Brock, 1951) concerning the validity of some of the evidence and certainly much of i t is not clear to the writer. There seems to be some doubt extant concerning both the prox- imity of granite rocks to ore deposits and their authenticity as source rocks. Though the problem is not quite clear to the writer the d i s t i n - ction between 'older' and 'younger' granites is apparently sometimes d i f - f i c u l t . Jackson considers that the 'younger* granites are indeed the source rocks but nevertheless admits (1932(2)), p. 455) that: "There i s no essential mineralogical difference between the typical MKushi granite-gneiss(Older Grey Granite) and the younger grey granites, and where the latter are locally gneissoid, any difference, either mineralogical or structural -57- p r a c t i c a l l y vanishes'". Here then i s one f i e l d f o r further work: de t a i l e d f i e l d map- ping and e s p e c i a l l y d e t a i l e d petrographic work would doubtless a i d i n the e l u c i d a t i o n of the problem of the granites. I t should be i n t e r j e c t e d here that the w r i t e r r e a l i z e s f u l l w e l l the d i f f i c u l t i e s of d e t a i l e d f i e l d mapping i n a region such as this where outcrops are scarce and where much information i s derived from ex- ploratory 'pot-holing' through deep overburden. Nevertheless a complete awareness of the type of evidence being sought w i l l l ead to a more thorough and c r i t i c a l analysis of a l l a v a i l a b l e information and i t i s with t h i s thought i n mind that these remarks are made. Doubtless a r e s u l t of the lack of outcrop i s the paucity of knowledge of f a u l t i n g i n the d i s t r i c t . Jackson (1932(2)) remarks that f a u l t i n g i s undoubtedly present to a considerable extent i n the NChanga area but few outcrops render the discovery and s o l u t i o n of the r e s u l t s of tectonism a most d i f f i c u l t task. I t i s most probable that i n an area which i s a part of a region of the world i n which enormous v e r t i c a l displacements are commonplace and i n which tectonic doming of sediments i s known to e x i s t , some large scale f a u l t i n g i s present. Faults have apparently presented no problem to mining operations but a knowledge of them i s c e r t a i n l y necessary f o r the understanding of 'why the ore i s where i t i s * . Crests of large and small drag folds have been c i t e d as l o c i of ore accumulation thus i n f e r r i n g that f o l d i n g preceded m i n e r a l i z a t i o n . The writer does not doubt the p r o b a b i l i t y of t h i s but i t i s apparent that concentrations of ore i n such places may have been produced mechanically. A close examination might substantiate one or other of these p o s s i b i l i t i e s . -58- A more thorough examination of what has been termed the ' s i t e s of deposition' i s obviously necessary and i n t h i s respect the w r i t e r sug- gests that with t h i s an examination of ' s i t e s of non-deoosition' would be e s p e c i a l l y f r u i t f u l . I t i s noted that along the east limb of the NKana syncline f o r example that the 'Ore Horizon' i s "poorly mineralized at two or more l o c a l i t i e s along the outcrop" (Guernsey, 1947). Surely an inves- t i g a t i o n to determine the reasons f o r t h i s 'poor' m i n e r a l i z a t i o n would answer some of the problems of ' r i c h ' m i n e r a l i z a t i o n . There are also places i n the copper d i s t r i c t where lower members of the Mine Series have been explored and found unmineralized, A d e t a i l e d petrographic study of these beds, along the l i n e s of the present i n v e s t i g a t i o n , and comparison of findings with those r e s u l t i n g from the study of mineralized beds would y i e l d much information. Perhaps the problem of whether c e r t a i n minerals - tourmaline, b i o t i t e , white mica, c h l o r i t e , and feldspars - are the products of metamorphic (inc l u d i n g diagenetic) or 'hydrothermal' processes could be answered. Comparative chemical analyses of c e r t a i n rocks i n which mineral grains are too small f o r o p t i c a l i d e n t i f i c a t i o n i s a suggested approach. The p o s s i b i l i t y of these deposits being of meta-syngenetic o r i g i n has been omitted purposely from the preceding discussions. The write r f e e l s that owing to the complexities involved the best approach to the formulation of a theory of o r i g i n f o r these deposits l i e s i n an attempt to construct an epigenetic theory using the information now a v a i l - able. At the same time the p o s s i b i l i t y of meta-syngenesis must be kept i n mind i n order that no piece of useful evidence i s overlooked. I f meta- morphic processes acting on an o r i g i n a l l y sedimentary deposit have pro- ceeded to the extent that some 'migration' of ore minerals or r e c o n s t i - -59- tution of the whole mieral aggregate has occurred then c r i t e r i a for either epigenesis or meta-syngenesis would in general be similar or indisting- uishable. Theory would diverge however when the problems of source and of access of material to sites of deposition were considered. Thus i f no rocks could be found and proven to be source rocks and i f no pathways were found to lead from these sources to the ore deposits then the origin of a deposit exhibiting 'replacement' features might be explained in terms of meta-syngenetic theory. The writer is quite aware that such i s a possib- i l i t y in the Rhodesian Copper Deposits. Concluding remarks. The writer lays no claim that the results of his study have clearly indicated the processes which formed the NKana copper deposit. Nor does he claim that, using the material available, the possibilities for investigation have been exhausted. Many more facts can doubtless be gleaned from the specimens. Both sulphide and s i l i c a t e mineral i d e n t i f i - cations and relations leave much room for further work. Stati s t i c a l methods could be employed, with a good chance of success, to determine whether or not ore minerals have replaced any rock mineral species pre- ferentially or whether there is any special association of non-sulphides with the ore minerals. However, the writer feels that because no previous laboratory type work had been done (or at least has been published) on Nkana deposits a coverage of the whole problem at a more general level has been more val- uable than i f an attempt had been made to treat exhaustively a specific problem at a more detailed level. CD BIBLIOGRAPHY BACKLUND, H.G., "The Actualistic Principle in Geological Research", reprint from Fran filosofiens och forskningens f a i t , Nya ron inomskilda vetenskaper, University of Uppsala, 1950. Bancroft, J.A, & Pelletier, R.A., "Notes on the General Geology of Northern Rhodesia", Min. Mag., vol. 41, pp. 369-72, vol. 42, pp. 47-50. 117-120, 180-82, 1929 & 1930. Bastin, E.S., Bateman, A.M., Becke, F., Boswell, P.G.H., Brock, B.B., Davidson, D.M., Douglas, D.V., "Criteria of the Age Relations of Minerals", Ec. Geol. vol. 2 6 , pp. 5 6 1 - 6 1 0 , 1 9 3 1 . "The Ores of the North Rhodesian Copper Belt", Ec. Geol. vol. 25, pp. 365-418, 1930. "Economic Mineral Deposits", John Wiley & Sons, Inc., New York, 1942. "Sur Physographie der Gementeile der Kry- stallinen Schiefer", K.K. Akad, Wiss., Mat.-Naturwiss. Klasse, Denkschr, Bond 75, 1913. "On the Mineralogy of Sedimentary Rocks", Thomas Murby & Co., London, 1933. (ex- cellent annotated bibliography). Personal Communication, 1951. "Geology and Ore Deposits of Chambishi, Northern Rhodesia", Ec. Geol. vol. 2 6 , 1931. "Observations on the Geology and Mines of the Belgian Congo", Min. Mag. vol. 4 2 , PP. 337-48, 1930. • Cr G i l l u l y , J., "Replacement Origin of the Albite Granite near Sparta, Oregon", U.S.G.S, Prof. Pap. 175-C, 1933. Gray, A., "The Correlation of the Ore-bearing Sediments of the Katanga and Rhodesian Copper Belt", Ec. Geol. vol. 25, pp. 783-804, 1930. "Mufulira Copper Deposit, Northern Rhodesia", Ec. Geol. vol. 27, pp. 315-343, 1932. & Parker, R.J., "The Copper Deposits of Northern Rhodesia", E. & M.J. vol'. 128, 1929. & Sharpstone, D.C., "An Outline of the Geology of the Nkana Con- cession and Roan Angelope Mine", Min. Mag. vol. 42, pp. 180-82, 1930. Grubenmann & Niggli, "Die Gesteins metamorphose, I, Berlin, 1924. Gruner, J.W. & Thiel, G.A., "The Occurrence of fine grained Authigenic Feldspar in Shales and S i l t s " , Amer. Miner, vol. 22, pp. 842-847, 1937. Grim, R.E., Bray, R.H., & Bradley, W.F., "The Mica in Argillaceous Sediments", Amer. Miner, vol. 22, p. 813, 1937. Heald, M.T., "Authigenesis in West Virginia Sandstone", Jour, of Geol., vol. 58, 1950. Guernsey, T.D., "A Short Summary of the Geology, NKana Mine", Rhokana Corporation Ltd., 1947. Jackson, J.A.C. (1) "Ores of NChanga Mine and Extension", Ec. Geol. vol. 27, 1932. (2) "The Geology of the NChanga Dis t r i c t , Northern Rhodesia", ".J.G.S. vol 88, pp. 443-515, 1932. Krynine, P.D., Lindgren, W., "Arkose Deposits of the Humid Tropics", Amer. Jour. Sci., ser, 5, vol. 29, 1935. "Paleogeographic and Tectonic Significances of Arkoses", G.S.A. Bull., abstract, vol. 52, 1941. "Mineral Deposits", McGraw-Hill, New York, 1933. Mackay, R.A., "The Control of Impounding Structures on Ore Deposition", Ec. Geol. vol. 41, 1946, McKinnon, D., Private Report, NChanga Consolidated Copper Mines Ltd., Dec. 8, 1948. Murray-Hughes, R., "Geology of part of Northwestern Rhodesia", with petrographical notes by A.A. Fitch, Q.J.G.S., vol. 85, pp. 109-63, 1929. Oosterbosch, R., "Copper Mineralization i n the Fungurume Region, Katanga", Ec. Geol. vol. 46, 1951. Pettijohn, F.J., "Sedimentary Rocks", Harper & Bros., New York, 1949. Robert, M,, Rove, O.N., Tester & Atwater, "An Outline of the Geology and Ore Deposits of Katanga, Belgian Congo", Ec. Geol. vol 26, pp. 531-39, 1931. "Some Physical Characteristics of Certain Favourable and Unfavourable Ore Horizons", Ec. Geol. vol. 42, 1947. "Occurrence of Authigenic Feldspar in Sed- iments", Jour. Sed. Petrol, vol, 4, 1934. Turner, F., "The Mineralogical and Structural Evolution of the Metamorphic Rocks", Mem. 30, G.S.A., 1948, Schneiderhohn, H,, "Mineralische Bodenschatze im Sudlichen Afrika", Berlin, 1931. Studt, F . E . , "The Geology of Katanga and Northern Rhodesia", Geol. Soc. of S. Africa, Vol. 12, 1913.


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