UDK 553.3/4(635)=20 The massive sulphides and magnetite deposits of northern Ethiopia Milan Hamrla Geološki zavod, 61000 Ljubljana, Parmova 33 Abstract The massive sulphides of northern Ethiopia occur within the Upper Precambrian eugeosynclinal sequence locally known as the Tsaliet and Tambien Groups belonging to the Red Sea Proterozoic basin. The geotec-tonic history of the basin is interpreted as a crartonised island arc. The age of the sequence is in the order of 1,000 Myr. The early sodic extrusive-intrusive rocks were followed by peralkaline granitoides. The primary mineralisations originated in submarine conditions by volcanic exhalative-sedimentary processes. Sulphide and oxide iron facies coexist, both on a regional as well as on a microscopic scale. Later hydrothermal episodes affected the deposits and rearranged and enriched the ore minerals. The altered enclosing rocks display greenschist facies metamorfphism, whereas the ore minerals and ore texture seem to be little affected. The massive sulphide deposits fit well with the Pb-Zn-Cu-Ag proterozoic type, with zinc and copper strongly prevailing over lead, the metal tenors variable and generally very low. Kratka vsebina Masivni sulfidi so v severni Etiopiji vezani na eugeosinklinalne kamenine zgornjega predkambrija, in sicer na spodnji del zaporedja, ki ga v debelini več kilometrov predstavljata lokalno poimenovani grupi Tsaliet in Tambien. Skupaj s še mlajšimi predkambrijskimi kameninami gradita rdečemorski proterozojski bazen, ki obsega dana&njo severovzhodno Afriko in Arabijo. Po izvoru vulkanogene in sedimentogene meta-morfne kamenine ne presegajo zelenega faciesa. Po izotopskih podatkih je starost zaporedja okrog 1000 milijonov let. Geotektonsko zgodovino severne Etiopije in bližnjega Sudana je mogoče tolmačiti kot kratoniziran predkambrijski otočni lok. Severna Etiopija pripada metalogenetski provinci s stratiformnimi rudišči pirita, barvnih kovin, magnetita in zlata. Primarna orudenja so nastala v povezavi z geosinklinalnim magmatizmom v obliki izlivov lav na morskem dnu in intruzij v nastalo skladovnico. Zgodnje magmatske kamenine so natrijske in kažejo značilnosti spilitsko-keratofirske skupine. Poznejše granitne intruzije pa so po sestavi peralkalne. Od znatnega števila doslej znanih rudnih nahajališč jih je bilo le malo nadrobno raziskanih v globino. Orudenja so nastala na morskem dnu pri čemer so rudni minerali precipitirali iz rudonosnih raztopin, ki so uhajale iz razpok, ter se plastovito nakopičili izmenoma med sedimenti in razlitimi lavami. Obseg posameznih nahajališč je v splošnem relativno majhen. Za rudne parageneze je značilno, da skupaj nastopajo oksidni in sulfidni železovi minerali. Za nastanek obeh faciesov je bila merodaj-nejša sestava raztopin kot sedimentacijsko okolje. Zveza s poznejšim plutonizmom pa je bila hidrotermalna rejuvenacija prvotnih rudnih mineralov. Pri tem je prišlo do nadomeščanja, premeščanja in obogatitve mineralnih komponent tako v prvotno sedimentiranih rudnih telesih kot v bližnjih strukturnih prepustnih conah. Na ta način so nastale tudi zlato-nosne kremenove žile, ki so jih v Eritreji do nedavnega rudarsko odkopavali. Izvor rudnih komponent je po vsej verjetnosti magmatski. Za žveplo pa so izotopske preiskave pokazale poleg magmatskega še organogeni morski izvor. Iz tega sklepamo, da so bile v rudonosnih raztopinah udeležene tudi konatne vode. Vpliv regionalne metamorfoze na masivne sulfide je bil majhen. Po Hutchinsonovi klasifikaciji ustrezajo rudišča proterozojskemu tipu sestave Pb-Zn-Cu-Ag, pri čemer količini cinka in bakra daleč prevladujeta nad svincem. Vsebnosti kovin so zelo nizke in nestalne, posebnost pa je lokalno znatna količina magnetita. Zlato in srebro sta prisotna. Ekonomsko pomembne vsebnosti kovin je mogoče pričakovati le tam, kjer je primarna ruda obogatena zaradi cementacije. INTRODUCTION The northern part of Ethiopia is a distinct metallogenetic province characterised by the occurrences of pyritic base metal sulphides, iron oxides and auriferous quartz reefs. Pyritic base metal concentrations have been revealed in the Asmara area and on the plateau, in the Eritrean lowlands and in Tigre Province. The mineralisations appear in the Precambrian eugeosynclinal meta-morphic sequence as gossans or massive magnetite bodies, ferruginous cherts and sulphidic disseminations, displaying mainly elongated lens-like shapes cut by numerous veins of white quartz. Their basically syngeneic stratiform features are well evident. World-wide studies of ores of volcanic-sedimentary affiliation have made much progress during the last two decades, and a large number of papers have appeared. The present article may contribute to this subject. The author was involved in investigations of Ethiopia's ore deposits intermittently since 1962 when employed with the Ethiopian Government; the presented data refer mainly to the pre-1973 period. As regards detailed exploration, little systematic research has been done in northern Ethiopia, access to the field being limited. Regional information is scarce and especially lacking in penological, geo-chemical and structural data of the vast region. Regarding the mineralisations, the present conclusions center on five or six more or less explored deposits, the rest of the localities being known mainly from author's reconnaissance work or other cited sources. This paper summarises the gathered information and points to the variety of problems awaiting solution. Field mapping, core logging, sampling and microscopic examination of rocks and ores have been done by the author who completed also the drafting work. Chemical analyses were performed in the Chemical Laboratory of the Ministry of Mjnes in Addis Ababa, mainly by S. Kandare. The permission of the Ministry of Mines, to publish this paper and the linguistic help by Dr. John Walsh are gratefully acknowledged. REGIONAL GEOLOGICAL SETTING The area referred to in this paper, shown in Figure 1, comprises the extreme southern part of Eritrea Province and the extreme north of Tigre Province. Knowledge of the regional geology of this part of Ethiopia is still sketchy. Some systematic work has been initiated in Tigre by different authors, but no comprehensive regional mapping has yet been done in Eritrea. The older information such as G. Dainelli (1943), G. Merla and E. Minucci (1938) and others'was compiled by P. M oh r (1962) and adapted in the recent Geological Map of Ethiopia (V. K a z m i n and A. J. W a r d e n, 1975). Hunting Geology and Geophysics in 1971—1972 carried out an airborne geophysical survey of parts of northern Ethiopia and produced a photogeological interpretation map at 1 :50,000 scale. Most information, however, resulted from geological investigations of ore deposits and occurrences. A geological sketch-map of the area has been compiled from the available data and is shown in Figure 2. The Tigre-Eritrean plateau is composed basically of steeply dipping Pre-cambrian metamorphic rocks, intruded by a variety of granitoids. This basement is unconformably overlain by erosional remnants of Paleozoic and Mesozoic sediments, in places covered by the erosional remnants of flood basalts of the Trap Series and intruded by swarms of doleritic dykes. The flows have been dated at 25—19 Myr (P. W. Jones, 1976) and even 36 Myr (G. F. Brown, 1970). The youngest magmatics are late Tertiary alkaline trachytic and phonolitic plugs, most typically developed in the Adua and Senafe areas. The volcanic activity has been continuous to recent times. The oxidic and sulphidic massive mineral occurrences are intercalated within the Precambrian sequence at numerous localities, and there are probably many more localities still to be found. The mineralisations are closely associated with the Precambrian environment, which is represented by a heterogeneous succession of volcanic and sedimentary rocks, exhibiting a uniform greenschist fades metamorphism. The low metamorphic grade is, in general, the quartz-albite-chlorite-sericite subfacies of the greenschist facies. Fig. 1. Location map of the area examined LOWER PRECAMBRIAN ? REFERENCE GRANITIC-GNEI5SIC MIGMAflTE, AMPHIBOLITE UPPER PRECAMBRIAN TSALIET GROUP TAMBIEN GROUP SHERARO FORMATION PERMIAH TRIASSIC- JURASSIC am MASSIVE METAVOLCANICS: INTERMEDIATE ANO FELSIC LAVAS-TUFFS, PYROCLASTICS ANO AGGLOMERATES; SCHIST , SLATE FERRUGINOUS CHERT, GRAYWACKE SCHISTOSE METAVOLCANICS: VOLCANOGENIC GREENSCHIST, SLATE PHYLLITE .QUARTZITE , GRAYWACKE; INTERBEDDED LAVAS AND AGGLOMERATES, LOCALLY MINOR LIMESTONE METASEOIMENTS : SHALE , SLATE ,ARGILLITE , GRAPHITIC PHYLLITE, QUARTZITE , GRAYWACKE, LIMESTONE , MINOR CONGLOMERATE SCHIST, PHYLLITE, SANDSTONE .CONGLOMERATE HORNFELS (SILICIFIED CONTACT ROCKS) QUARTZ SANDSTONE , VARIEGATED SANDSTONE SILTS TO NE SHALE, GLACIAL TILLITES ( EAST OF ADUA) TERTIARY TRAP SERIES INTRUSIVES FTTs XX X X X I X XX X I EED VOLCANICS: BASALT WITH MINOR TUFFS AND CLAYS TRACHYTIC VOLCANICS* PHONOLITIC PLUGS LATERITE: MOTTLED KAOLINISED AND FERRUGINOUS ROCKS GRANITE , GRANODIORITE , PORPHYRITIC GRANITE QUARTZ DIORITE, DIORITE GABBROIC ROCKS. ALKALINE SYENITES RECENT VALLEY ALLUVIUM -1-76 STRIKE AND DIP OF FOLIATION FAULT, DOTTED WHERE INFERREO MAIN ROAD AND SETTLEMENT MAJOR RIVER MINERAL DEPOSIT Fig. 2. Geological sketch-map of the Asmara-Axum area, showing the occurrences of massive sulphides. Compiled from various sources The rocks were affected by regional metamorphism and only locally by thermal metamorphism. Porphyroblastic textures and recrystallisations are evident. The Upper Precambrian volcanic-sedimentary sequence originated as a eu-geosynclinal accumulation, with felsic magmatics intruded and extruded in more stages during its evolution. The complex was folded and faulted under east-west directed stress, the regional trend of foliation submeridional, the dips variable but near-vertical. The intensely folded sequence may be manifest in the repetition of certain lithologies occurring in alternating parallel belts and zones. After extensive Paleozoic peneplanation, the Permian continental sedimentation produced sandstones and tillites associated with clays, corresponding to the Dwyka Series of South and East Africa (R. M. S h a c k 1 e t o n and K. Lomax, 1974). Mesozoic marine sedimentation followed, the beds largely removed by pre-Trapean (late Cretaceous-early Tertiary) peneplanation, which levelled both Precambrian and younger rocks. It caused extensive surface alteration, the peneplain being evident as a conspicuous horizontal plane of ferruginous-lateritic crust, usually marked by the presence of resistant Paleo-zoic-Mesozoic sandstones. Tertiary Trap basalts spread on the peneplain, the preserved thickness of the basaltic cappings being not more than several hundred metres. Lithology In terms of the division of the Ethiopian Precambrian, the majority of the rocks in northern Ethiopia belong to the Upper Complex, which forms a thick inhomogeneous succession of different lithologies of volcanic and sedimentary origin, and is subdivided into the lower Tsaliet Group and the upper Tambien Group. Certain rocks are suspected to belong to the Lower Complex: the foliated granodiorites below the metavolcanics in the Enticho area, the amphibolites and biotite granite on the Buri Peninsula southeast of Massawa, and the granitic-gneissic migmatised rocks in the coastal region northeast of Asmara (G. Mer la et al., 1973). Similar older rocks also form small outcrops in the Danakil area (J. Brinkmann and M. Kursten, 1970). Upper Precambrian volcanic-sedimentary rocks. The Tsaliet Group comprises mostly volcanic rocks of varied character: basic, intermediate and felsic extrusives are interbedded with pyroclastics ranging from coarse agglomerates to fine tuffaceous mudstones and siltstones, with minor interbeds of argillaceous and arenaceous sedimentary rocks such as shales, quartzites, greywackes, in places biohermal limestone and subordinated conglomerate. The strongly chloritised massive and schistose volcaniclastics appear as chlorite-sericite schist, micaceous and augen schist, quartzitic and sericitic greenschist, exhibiting frequently a greywacke affinity. The massive varieties are strongly epidotised, the original texture and mineralogy obliterated; such rocks may have been originally mafic or lithic pyroclastics. The rocks are intimately interbedded with considerable vertical and lateral variations. The Tsaliet Group is about 1,500 metres thick in Central Tigre (M. B e y t h , 1972), its thickness increasing northwards to an estimated order of several thousands of metres in Eritrea. The volcanogenesis is evident; practically the Table 1 Chemical composition of some extrusive rocks (per cent) Locality Sample No. Si02 Ti02 A,2°3 Fe203 FeO MnO MgO CaO Na2° k2o P2°5 S h20" H/ Toial Embaderho 1 2 3 66 .0Q 71.10 66*90 0.30 0.18 0.30 17.70 15,10 17,20 1.73 0.84 0.97 2.08 1.67 0.71 0,03 0.04 0.03 1.44 0.98 1.38 5.67 2.15 5.70 3.82 5.12 3.85 0.51 0.53 0.63 0.17 0.15 0.17 0.19 0.02 0.19 0.23 0.15 0.18 2.09 1.21 2.05 100.72 99.78 100.96 Adi Nefas 4 5 73.60 64.08 0.05 15.46 18.58 0.56 1.64 0.06 0.06 0.21 1.24 1.50 3.99 5.43 2,50 1.37 2.00 0.09 2.14 5.78 100.38 99.96 Adi Rassi 6 7 59.27 49.38 18.05 15.84 5.43 18.85 0.15 0.24 4.07 3,43 5.39 2.98 4.70 2.95 0.70 0.80 0.12 0.50 2.23 4.98 100.11 99.95 Marahano Hamasien B 9 78.83 54.21 0.52 10.79 15.11 2.35 1.16 3.94 5.25 0.33 6.42 0.19 8.19 5.61 3.96 0.88 1.91 tr 0.14 0.67 1.28 100.81 100.93 Provenance of samples: No. 1 Embaderho. Porphyrite; drillhole EMB 1/70, depth 85.50 m No 2 Embaderho. Quartz keratophyre; EMB 1/70, depth 224.15 m No. 3 Embaderho. Quartz porphyrite; EMB 1/70, depth 310.50 m No 4 Adi Nefas. Quartz keratophyre; drillhole AN 5 A/68, depth 76.40 m No. 5 Adi Nefas. Altered dacite (andesite?); drillhole AN 5 A/68, depth 120.50 m No. 6 Adi Rassi. Porphyrite outcropping east of the North Hill No. 7 Adi Nefas. Spilitic magnetite andesite; drillhole AR B/l, depth 105 m Data from G. Dainelli (1943): No. 8 Marahano (about 10 km south of Asmara). Keratophyric quartz porphyry No. 9 Hamasien. Diorite porphyry < o in e, 13 g* a whole Group can be accounted for in terms of volcanic provenance. The succession is submarine though some coarse pyroclastics could also have originated subaerially. The extrusives are recognisable as quartz porphyrites, keratophyres, ande-sites and more spilitic varieties, texturally massive and schistose, their sodic nature obvious microscopically and chemically. Porphyritic albite is ubiquitous with chlorite, epidote and calcite the alteration products of original mafic silicates. Silica is abundant, forming layers of chert and jasperoidal rocks. Intraformational breccia occurs in places, the clasts lithics and chert fragments, grading to greywacke, the matrix tuffaceous and siliceous. Interbedded mud-stone proves subaqueous accumulation. The extrusives spread as flows on the sea floor probably from fissure openings, together with ejected water-lain pyroclasts and tuffaceous products, some of which may have originated also through nuee ardente type of activity. A metamorphic change of the original lavas of various mineralogic nature to a spilitic-keratophyric assemblage must be assumed as a consequence of the deuteric-fumarolic alteration which will be discussed below. Whether or not there is any cyclicity in the composition of the extrusive layers is not known. P. F. Pagnacco (1969) noted in the area south of Asmara a change from felsic rock types in the west, to mafic types prevailing in the east. More mag-matic cycles, starting with basic and terminating with more felsic magmas, could nevertheless be expected. The chemical composition of the extrusive rocks examined is shown in Table 1. The extensive alteration, as reflected in the present whole-rock chemistry, makes classification difficult. Two analyses (Nos. 8, 9), obviously referring to the same rock types, have been added from G. D a in e 11 i (1943). The Tsaliet Group is overlain by the more sedimentogeneous Tambien Group, its thickness apparently of the order of several thousand metres. It consists of shale, slate and quartzite with interbeds of volcanogeneous rocks and dark limestone which merge into the former, making differentiation practically impossible. The younger Didikama, Sheraro and Mateos Formations overlie the former two Groups on the margins of the discussed area, either conformably or unconformably, and consist of limestone, dolomite, calcareous sandstone and finer marine sediments (V. Kazmin and A. J. Warden 1975). Ultrabasic rocks and their metamorphic derivatives have not been encountered and do not, to the writer's knowledge, exist in the area discussed. Serpentinites exist in the Lower Barca valley in extreme northwest Eritrea (L. Us o ni, 1952), and reportedly also in southwestern Eritrea (V Kazmin and A. J. Warden, 1975). For practical purposes, and as shown in Figure 2, the Upper Precambrian sequence can be roughly subdivided, on the regional scale, into three litho-stratigraphic units: massive metavolcanics are prevailingly massive and schistose extrusives and pyroclastics with subordinated sedimentary rocks, schistose metavolcanics are prevailingly schistose bedded volcanogeneous sediments, and metasediments, on the top of the succession, have been derived mainly from argillaceous-arenaceous-calcareous sediments with very minor reworked volcanic material. Sulphidic and oxidic ore deposits and occurrences, usually accompanied by quartz veins and skarn-type epidotised rocks, are intercalated within the Tsaliet and Tambien succession. Ferruginous chert and purple siliceous zones also occur in the upper sedimentary unit. Intrusive rocks. A variety of feisic rock of apparently granitic-dioritic composition intrudes the Upper Precambrian sequence. No systematic penological examinations and no differentiations on the regional scale have ever been done. The granitoid rocks show variations from foliated gneissose granite merging into surrounding schist, porphyritic granite and granodiorite to finegrained microgranite, syenite and diorite, with subordinated gabbro in places. The form of the masses varies between huge irregular or elongated complexes with schistose rocks preserved as roof pendants only, and rounded circular masses of quite restricted dimensions. The intrusive rocks cover more than half of the area referred to, as in other parts of the northeastern Africa and Arabia, where foliated gneissose granite has been considered "syntectonic", the others, especially minor, rounded or circular, clearly intrusive bosses "posttectonic". Such a simplified division seems inadequate since the intrusives obviously differ in composition, origin and age. The greater part of the granitic rocks in northern Ethiopia have been identified with the Mareb Granite of the Central Tigre, which intrudes the Tsaliet and the Tambien Groups. The prevailing rock type is a leucocratic, pink, alkali potassic porphyritic granite, euhedral orthoclase and microcline perthite being the prevailing phenocrysts, and sodic plagioclase quite subordinate. The mafic minerals are biotite and hornblende. There are variations in texture and mineral composition within the intrusive bodies. Medium-grained granodiorite is widespread, with quartz and sodic plagioclase present in larger amounts. Quartz may be absent and the rocks grade to monzonite and syenite, the latter occurring as small isolated bosses. M. Beyth (1972) gives the following composition for the Mareb Granite: Si02 Al203 No20 K20 (per cen») 68,2-63.6 18.4-14.0 7.12-5.85 2.94-2,6 7 Some older data for various granites by G. D a i n e 11 i (1943) show the following composition (Table 2). The rocks are peralkaline. C. R. N e a r y et al. (1976) stressed this feature as being typical of the "Younger granites" in Sudan. Field evidence suggests that certain rather smaller masses, by their mineralogy, colour index and composition, are closer to diorite. Typical examples are the intrusive rocks northwest of Asmara in the Embaderho-Ad Teclesan area; these sodic granitoids correspond compositionally and mineralogically to porphyritic quartz keratophyre i nterlayered with massive ore in the Embaderho sulphide deposit. The chemical composition of the Embaderho quartz diorite, together with two examples of dioritic rocks quoted by G. Dainelli (1943) is shown in Table 3 (Nos. 2, 3). Table 2 Chemical composition of various granites (After G. Dainelli) Sample No. S'Qj TiQg A,2°3 F*2°3 FeO M00 CaO Na20 k2o P2°5 H/ 1 73.19 - 13.55 0.46 - 0.28 0.94 5.68 4.82 tr 0.40 2 73.96 tr 13.75 0.52 0.99 0.48 1.90 5.62 2.55 0,17 0.25 3 58.67 tr 17.66 2.21 3.85 3.28 4.82 5.86 3.27 0.57 0.49 4 74.36 - 13.56 0.67 1.19 0.29 0.89 4.51 4.93 tr 1.01 Provenance of samples: No. 1 Adi Enfi (Decamerhe). Granite No. 2 Fort Cheren. Granite No. 3 Elabaret east of Cheren. Granite No. 4 Coatit (Senafe-Saganeiti area). Quatrz diorite Table 3 Chemical composition of dioritic rocks Sample No. SiOj TiOg A|2°3 p*2°3 FeO MnO MgO CaO Na20 k2o P2°5 5 h2o+ 1 69.00 0.18 15.30 1.68 0.59 0.03 0.91 5.23 3.83 0.53 0.15 0.04 2.55 2 58.67 0.19 16.21 4.17 3.54 3.19 6.25 5.09 1.89 0.25 1.29 3 51.04 0.72 17.81 3,75 6,17 5,30 8.98 3,81 1.01 0.72 1.48 Trovenance of samples: No. 1 Embaderho. Quartz diorite; drillhole EMB 4/70, depth 198.4m No. 2 Adi Berim (Coatit). Quartz diorite No. 3 Brigantia (Cheren). Amphibole diorite Although little can be said from one analysis, the composition of the Embaderho quartz diorite (No. 1) seems close to the composition of batholitic granites in Sudan, quoted by C. R. Neary et al. (1976). The rock is, however, extensively altered and difficult to classify. Some smaller dioritic bosses occur adjoining the granite west and east of Asmara. P. F. Pagnacco (1969) mapped diorite in the Adi Daro area, C. R. Garland (1972) in the Adigrat area and R. M. Parsons (1965) on the eastern escarpment M. Beyth (1972) observed dioritic stocks associated with the Mareb Granite in Tigre and named them Forstaga Diorite, the typical being a quartz-amphibole diorite with biotite and pyroxene, foliated in places and containing secondary minerals, its composition ranging as follows: Si02 AI2Q3 CoO K20 (per cent) 58.6-53.5 16.2-12.5 6.3-6.0 2.7-1,24 He stated, that diorite was intruded before the Mareb Granite. However, textural and compositional variations between granitic, granodioritic and dioritic rocks are common. Rocks of dioritic appearance, grading into granite, were noted on the margins of the Decamere granitic stock near Adi Rassi, near Dongolo Basso, in the Agametta area, south of Adi Quala, near Adi Daro, at Tsehafe Emba, north of Axum and elsewhere. Dioritic rocks apparently originated by differentiation. However, not all the recognised diorites seem to be comagmatic. Apart from peralkaline granite characterised by potash feldspar, there is. at least, an intrusive generation of dioritic rocks with abundant secondary minerals replacing virtually all primary mafic minerals. It is subalkaline, sodic, very low in potassium and ferrous iron, with rather high lime content. The petrochemical similarities with the porphyritic-keratophyric types of Tsaliet metavolcanics suggest a common origin, the rocks of the same magmatic episode probably formed by some sort of differentiation of original melts, emplaced partly as domes and partly extruded as flows on the sea floor. Other rock types present in forms of smaller bosses, apparently differentiation products as well, are gabbros and syenites. The Gemahlo ring complex in the extreme west of the area has a gabbroic-syenitic core surrounded by porphyritic microgranite. Many similar rings, about 83 recognised so far, occur in the northeastern Sudan and appear to be of the "Younger granite" type, their actual age uncertain (J. R. Vail, 1971; 1973; J. C. Briden, 1973). The thermometamorphic effects are hardly to be found on the contacts with intrusives; they emplaced apparently at shallow depths and were relatively poor in volatiles, producing only modest metamorphic aureoles. Numerous aplitic and porphyritic dykes intersect all rock types in various directions, their age uncertain, apparently belonging to further stages of magmatism. The prevailingly fine-grained leucocratic rocks are rich in feldspar and mostly strongly weathered (kaolinised) on the surface. Melanocratic meta-doleritic dykes occur in places. Auriferous quartz veins. Following the planes of weakness along foliation, there are numerous auriferous quartz veins and reefs in Eritrea, the gold grades variable between 2 and 30 grammes per ton. Epidote, chlorite, carbonate and sericite are associated with scarce sulphides, copper stainings frequent. The veins are varied in lenght, width and depth, the largest up to several hundred metres long and several metres wide. They were extensively mined in Eritrea in the past (L. Usoni, 1952; D. Jelene, 1966). The age of emplacement of auriferous reefs is uncertain. Time relations and correlation Regarding the ages of Precambrian rocks in Ethiopia, no systematic dating has been done. A correlation was attempted by V. Kazmin (1975), who originally ranged the Tsaliet Group within the 1,000—750 Myr interval, but considered later lowering it within the middle part of the Upper Proterozoic (1976). The number of random radiometric age determinations of the Precambrian in Ethiopia is more than 50. The majority of ages in different rock types fall within to range of 550 ± 100 Myr, defining the Pan African thermo-tectonic episode or event (W. Q. Kennedy, 1964; H. M. E. Shurmann, 1964), and reflecting the Mozambiquan rejuvenation which is evident over the whole of East Africa. Seven K/Ar determinations from northern Ethiopia and nine from southern Ethiopia gave isotopic ages older than 650 Myr, of which three Rb/Sr ages of 7 — Geologija 21/n gneissose-granites fall within the range of 680 Myr, and one, for a metamorphic rock, gave an age of 1,030 ±40 Myr (A. M. Chater, 1971). In western Ethiopia two K/Ar ages near 1,400 Myr were obtained on granitic rocks, and one age 794 ± 40 Myr for a "younger" dioritic rock intruding the former, its lithology variable and comprising also syenitic and gabbroic rocks (Metal Min. A g. of Japan, 1974, unpublished report). A summary of age determinations higher than 650 Myr is given in Table 4. The scattered isotopic data of varied provenance and mainly by K/Ar method, with little reliable field geological information and even less complementary penological, geochemical and structural studies, must be treated with caution. They nevertheless suggest that the basement might have originated much earlier than the discrepant Pan African 550 ± 100 Myr apparent ages indicate. There has been much uncertainty about the ages of the granitoids in Sudan. The earlier Batholitic granite shows gradational assimilation boundaries, whereas the Younger granite appears in discrete bodies of variable lithologies intruding the earlier, and associated with comagmatic extrusives (I. R. G a s s and C. R. Neary, 1970). The granite of the northeastern Sudan shows apparent Mozambiquan K/Ar age in the 630—420 Myr range (J. R. Vail and D. C. Rex, 1970; J. R. Vail, 1971). A. J. Whiteman (1971) reported a minimum 740 ± 80Myr age for the Younger granite. B. J. Cavanagh (1974) gave, on Rb/Sr analyses, a 465 Myr age for the Younger granite, and for the Batholitic granite the order of 590 Myr, but for the extrusives 670 Myr. C. R. Neary et al. (1976) established that the Batholitic granite, together with the oldest phase of the Younger granite and volcanics as well, all carry an isotopic age of about 700 Myr and belong to the same magmatic cycle. The age of emplacement of younger granite ring complexes was believed by J. R. Vail (1973) to be mainly Paleozoic. J. C. Br id en (1973), however, Table 4 Isotopic age determinations higher than 650 Myr Province Number of determinations (Minimum) age range Myr Method Rock types Eritrea 5 690-650 K/Ar granitoids> porphyrite 1 754 M pegmatite 1 976 M mica schist Sidamo 5 around 650 K/Ar gneiss, pegmatite, gra- nites, amphibolite 3 680 Rb/Sr gneisses 1 740 115 K/Ar gneiss 1 1,030 ±40 Rb/Sr phyllite We lega 2 1,400 K/Ar foliated granitoids 1 794 4 40 K/Ar "younger" granite (diorite) gave the corresponding minimum age estimate of 750 + 50 Myr by paleo-magnetic reversal chronological study. There are at least three phases of younger granites in Sudan, the last two having ages of 500 Myr and 100 Myr (C. R. Neary et al., 1976). For southern Egypt, M. Y. Meneisy (1972) reported a number of Rb/Sr ages in the range of 1,365—1,150 Myr, which might be "the age of the main Precambrian metamorphism". However, the ages of synorogenic plutonites in Egypt range between 1,000—800 Myr. Another widespread magmatic activity took place around 600 Myr, and a younger episode took place between 100—70 Myr. In Saudi Arabia, according to G. F. Brown (1970), the major plutonic events seem, from mostly Rb/Sr determinations, to have occurred at about 1,000, 735—720, 670—660 Myr, the latest 570 Myr data reflecting the Pan African event. An extremely thick volcanic-sedimentary sequence is cut by intrusives of the 1,000 Myr epoch. Syntectonic gneissic rocks and intermediate massive, discordant calc-alkaline granites, comparable to synorogenic plutonites in Egypt, range in age from 1,000—710 Myr (G. F. Brown and R. O. Jackson, 1960). The discordant post-tectonic younger granites generally occur as circular batholiths. Intrusive felsic rocks of Mesozoic age were found as well (M. Gillmann, 1968). In light of these data, a tentative dating of the basement in northern Ethiopia can be attempted. The Tsaliet and the Tambien Groups are intruded by granitoids, and the field data suggest more stages of felsic intrusions. The petrochemical data, insufficient as they are, point to a similarity between some of the intrusives and the extrusives of the Tsaliet Group. It might be therefore possible that the oldest foliated granitoids such as quartz diorites, the early stages of later granitoids and the extrusives of the Tsaliet Group are comagmatic, representing the intrusive and extrusive phases of the same magmatic event, limited to a relatively short time interval. C. R. Neary et al. (1976) came to similar conclusion for the early magmatism in northeastern Sudan. If the isotopic ages around 1,000 Myr are "real" relict dates reflecting the earlier metamorphic events, then the Tsaliet succession would be at least 1,000 Myr old. How "real" two 1,400 Myr K/Ar ages for Welega granitoids are is not clear for the time being; they are better treated with caution. If the early intrusive magmatism was coeval with the eugeosynclinal volcanism, then the oldest granitoids might correspond to the synorogenic plutonism of J .000—800 Myr age in Egypt and Saudi Arabia. This was a period of major tectonic activity in the Earth's geological history, accompanied by magmatism of regional proportions (H. P. Bott, 1971). The early magmatics probably originated from differentiated (ultramafic) mantle and possibly also, at least partially, by anatectic crustal melting. Later upsurges of the magma followed, the "younger" intrusive events indicated to have occurred at about 800—740 Myr and 690—650 Myr, possibly followed by the early Paleozoic and later Mesozoic-Tertiary plutonism, corresponding to the established phases in Sudan, Egypt and Saudi Arabia, the ages of the later plutonism bearing the regional Mozambiquan inprint. The stratiform sulphide deposits apparently originated from the oldest magmatic cycle of 1,000 Myr range. Later episodes were involved in subsequent hydrothermal rearrangements. Massive base metal deposits of volcanogenic origin exist in Saudi Arabia in similar environment (Dr. Garnet, pers. comm.), as well as in southern Egypt (T. Ivanov et al., 1973). M. Bey t h (1972) and V. Kazmin (1975) attempted correlations of the Upper Precambrian formations with those of the adjacent countries. The absence of reliable chronological data and facial variations at widely separated localities make such attempts difficult. Nevertheless the Tsaliet Group was correlated with the Halaban Formation of Saudi Arabia and the Dokhan Formation of Egypt, and the Tambien Group with the Murdama Formation of Saudi Arabia. However the thickness of the geosynclinal volcanic-sedimentary sequence in Saudi Arabia is of the order of tens of kilometres (G. F. Brown, 1970), and something less in Egypt. The lower limit of the Halaban, set at about 1,000 Myr (A. H. Sabeth, 1972) might be, together with the Dokhan Formation, even older. The Tsaliet Group may therefore correspond to the lowest part of the Halaban Formation or even to the Baish greenstones underlying it, the corresponding element in Sudan the Nafirdeib Series (A. J. Whiteman, 1971; A. H. Sabeth, 1972). Structural features The available data on pre-Cretaceous (pre-Rift) tectonic in northern Ethiopia suggest that major orogenic deformations affected the region. The absence of systematic studies makes the interpretation of the early regional tecto--geology difficult. A dominant submeridional geological trend is obvious in the structural pattern of the Precambrian assemblage, swinging from the meridional direction in the Asmara area gradually westwards in central and western Tigre. The foliation and bedding are generally conformable. The rocks have been tightly folded, the axes of isoclinal folds trending in the same direction, the variable dips generally steep. C. R. Garland (1972, unpublished report) observed monoclines, open folds, also recumbent folds and thrusts in central Tigre, the axial planes of overturned folds dipping west and northwest. Tight isoclinal folding is clearly evident in western Tigre, the attitude of the beds steep. Vertical dips prevail also in the Asmara area where the folding is evident in the repetition of units. All the deformations were caused by east-west directed compression. Major faulting, trending in the same directions, has been recognised, the fault planes either vertical or steeply inclined west. Low-angle thrust-faulting is suspected as well, paralleling the bedding and hence difficult to recognise. Such a structure is indicated in western Tigre, where the Sheraro Formation discordantly overlies the volcanic-sedimentary complex. These dislocations within the Precambrian sequence are considered to be mainly of early origin, forming arcuate lineaments more or less oblique to the trend of the eastern escarpment, which is associated with the post-Cretaceous rifting. The faulting, representing the main tectonic stage in the post-Paleozoic period, is associated with the formation of the Rift system. The uplifting of the plateau is confined mainly to the proximity of the escarpment, the fault planes dipping steeply east, the displacements eastwards (P. Mohr, 1962). Younger faults on the plateau cut across the Precambrian lineaments either parallel to the Rift trend or obliquely to it, with minor lateral and vertical displacements. Rift tectonism was accompanied by basaltic eruptions, from the Trap Series covering the plateau to the recent volcanism, which is still active in the Afar today. DESCRIPTION OF DEPOSITS AND OCCURRENCES The known ore deposits and occurrences are shown in Figure 2. They appear as conspicuous elongated outcrops, conformable with the bedding of the enclosing rocks, but exhibit also irregular, breccious or fissure character, their size rather restricted in dimensions. Some of them have been more or less explored and the majority reconnoitred only. The main primary ore minerals in order of abundance are pyrite, sphalerite, chalcopyrite, pyrrhotite, magnetite, galena and some minor sulphides. Silver and gold are present and associated with the sulphides. The primary gradings are rather low; increased tenors have originated mainly in secondary enrichments. Embaderho The prospect is situated 10 km northwest of Asmara. It was explored during 1969—1971 by detailed mapping, soil geochemistry, some magnetic and electromagnetic survey and deep drilling (7 holes totalling 1,363 m). The "S" shaped gossan is about 1 km long and up to 100 m wide, conformable within the foliated rocks and dissected by longitudinal faults, as illustrated in Figure 3. Weathered quartz diorite outcrops to the east, and another smaller boss of granite porphyry merges into the adjacent schists to the west. The enclosing lithologies are massive epidotised volcaniclastic greenstone, tuffaceous greenschist, quartzose hornfels rock, chlorite schist, mica-sericite schist, pyritised quartzitic rock, all varying laterally and vertically, intimately interbedded with porphyritic-keratophyric layers and massive amphibole--chlorite cherty rocks. Numerous aplitic dykes and quartz veins are scattered in the area, the largest the 600 m long Medrizien quartz reef nearby, being mined for gold in the past, the grading 3 to 8 grammes per ton (L. U s o n i, 1952). It is surrounded by hydrothermally affected sericitised and silicified rocks. Beds of massive sulphides occur within the sequence. Drillhole EMB 1/70 (45°, 315.15 m) intersected 20 layers of massive ore, some several metres thick, and numerous beds, bands and zones of disseminated sulphides, together with more than 40 layers of porphyritic extrusives. Drillhole EMB 7/71 (45°, 114 m) intersected more than 30 m (true thickness) of massive pyritic ore, and other drillholes revealed abundant ore and extrusive layers as well. Drill sections are illustrated in Figure 4. Petrography. The interlayered porphyritic rocks are prevailingly leucocratic fine- to medium-grained with low, slightly variable colour index, the textures felsophyric and felsitic. Quartz and feldspar occur as phenocrysts, the former + +1 quartz diorite i mfca and sericite schist ■ • • ;;ipvritised quartzitic rock porphyritic extrusive s i— creenschist and epidotised 6reenst0ne q quartz reef -j- 7$ strike ano dip of fou at ion 20 orill-hole gossan apute ^zzzz fault: observed, inferred x abandoned sold mine Fig. 3. Geological map of the Embaderho prospect in bluish, corroded, rounded grains up to 5 mm in size, the latter smaller and strongly replaced by a turbid, opaque aggregate. Twin measurements normal to (010) indicate albite to albite-oligoclase. Remnants of hornblende are rare. There is white mica but no biotite, and chlorite and epidote are ubiquitous. Strong alteration has replaced the primary minerals by a fine-grained mosaic of chlorite, zoisite, sericite, epidote, calcite and quartz, all of them forming the microcrystalline matrix. Minor irregular sulphides are present as well. The outcropping intrusive rock in the Embaderho area, though weathered, is similar in appearance. Encountered at depth in drillhole EMB 4/70, a gradual textural transition from porphyritic into hypidiomorphic granular massive rock was revealed, the mineralogy essentially the same as in the extrusive porphyritic layers. The strongly altered rocks can best be designated as quartz diorite. The quantitative mineral composition of 4 samples of the Embaderho extru-sives and one sample of quartz diorite is given in Table 5. Compositional and chemical similarities (Tables 1, 3 and 5) suggest that the intrusive and the porphyritic rocks are comagmatic, derived from the same source and apparently coeval. These rocks have been extensively altered. To what extent the present composition — high alumina, magnesia and lime contents, low sum total of alkalies with sodium exceeding potassium nearly 7:1 and low ferrous iron — reflects that of the original magma, or if it is due to a later metasomatic redistribution of components, has not yet been sufficiently examined. The determinations of alkalies and earth alkalies on a number of rock samples, visually selected as altered to different degrees show, as Table 6 illustrates, that magnesium and calcium contents sharply increase with stronger alteration, whereas sodium seems to decrease and potassium remains unaffected. A tentative conclusion can be made that the original early magma produced shallow domal intrusions under the depositional basin, at the same time erupting subaqueously on the sea floor forming alternating flows with sediments and stratiform sulphides. A close genetic link with the formation of the sulphides is implied via solutions to which, partially at least, the extensive rocks' alteration must be attributed. Whatever the origin of albite — either magmatic or due to soda metasomatism in the early stages of magma consolidation from earlier more calcic plagioclases — lime and magnesia must have been intro- Table 5 Mineral composition of magmatic rocks at Embaderho Porphyritic extrusive* Quartz diorite Samples: A B C D E Phenoerysts: Quartz 5 2 8 7 15 Feldspar (albite) 35 35 30 30 35 Hornblende - - - - 2 White mica (sericite) 10 15 6 15 10 Chlorite 10 7 20 5 8 Epidote and zoisite 12 5 8 8 5 Calcite 3 2 2 3 3 Granular opaque matrix 25 34 24 32 32 Samples are: A Porphyrite; drillhole EMB 1/70, depth 85.5 m B Quartz keratophyre; drillhole EMB 1/70, depth 136.2 m C Mesocratic quartz keratophyre; EMB 1/70. depth 224.2 m D Quartz porphyrite; drillhole EMB 1/70, depth 310.5 m E Quartz diorite; drillhole EMB 4/70. depth 198.4 m Table 6 Alkalies and earth alkalies in variously altered extrusives at Embaderho Drillhole m I« Mfl0 Ca0 ^2° K2° Rock EMB 4/70 5 1.02 4.15 4.34 0.69 Quartz diorite, slightly altered 1 0.96 4.00 7.40 0.78 Aphyric porphyrite, slightly altered 9 1,34 5.68 4.21 0.76 Altered porphyrite 1 1,55 8.40 6.30 0.72 Aphyric porphyrite, strongly altered Porphyrite, strongly altered 1 2.01 9.6 1.25 1.27 1 8.8 13,4 0.33 0.19 Quartz keratophyre, extremely altered 2 1.1 3.03 6.25 0.61 Contact of porphyrite with ore duced from an extraneous source by solutions, which apparently caused epi-dotisation, zoisitisation and carbonation of the original rock. The same low-temperature hydrous mineralogy is observed also in the schistose members of the volcanic-sedimentary pile: chlorite, white mica, epidote-zoisite, tremolite-actinolite, talc, quartz and some albite and calcite are the essential constituents. Pyrite metacrysts (?) in these rocks are arranged in bands parallel to foliation, often corroded, fragmented and altered to secondary limonitic products surrounded by patches of chlorite and epidote. Bio-tite is completely absent in the schists as well, though there is phlogopite in certain parts of the rocks, usually associated with sulphides, and seemingly a secondary mineral. Contacts of ore with greenschists are mainly gradational, with streaks of disseminated sulphides and magnetite in chlorite- and epidote-enriched schists, the rocks dense and cherty. Contacts of porphyritic flows with massive ore are sharp and marked by a narrow zone of fine-grained granoblastic of nemato-blastic intergrowth of lime-bearing silicates, with abundant chlorite, sericite, phlogopite and calctie. In ore, coarse crystalline pyrite marks the contact, with interstitial quartz and locally concentrated chalcopyrite. Ore mineralogy. The mineral paragenesis of the Embaderho ore is rather simple.Pyrite is the most abundant sulphide mineral. At least three generations are present. The early euhedral pyrite in grains up to 15 mm in size is usually poeciloblastic and corroded. It apparently crystallised first, together with lime silicates and magnetite, and was replaced by other sulphides. Another generation forms irregular granular masses, clustering around larger grains and interbanded with younger sulphides (Fig. 5). The bulk of pyrite occurs as a globular aggregate of densely packed oval grains, with interstitial quartz, calcite and small amounts of other sulphides. There is also a microcry-stalline gel-pyrite, occuring in irregular rounded forms of apparently colloform origin. Magnetite forms individual euhedral crystals up to 10 mm in size; poecilitic inclusions of pyrite, and blebs of pyrrhotite and sphalerite are common (Fig. 6). The bulk of magnetite occurs as finegrained masses, usually associated with granular sulphides in narrow alternating bands. It is intimately associated with sulphides but much less abundant than pyrite. Pyrrhotite, too, occurs in fine-grained porous form, mixed and interbanded with other sulphides (Fig. 7). Together with magnetite it is found in deeper parts of the sequence only, forming massive ore and in places prevailing over pyrite. Associated with coarse chalcopyrite it forms also distinct vein-like inclusions in massive ore. Chalcopyrite seems to occur in at least three generations: as inclusions in the early euhedral pyrite and magnetite, as unoriented exsolution blebs in dark sphalerite associated with massive magnetite-pyrrhotite ore (Fig. 8), and as individual subhedral grains and interstitial fillings. It forms replacement rims around the early pyrite, and it is seen to grade into gel-pyrite. Sphalerite is the most abundant base metal sulphide. It occurs in irregular grains and shows mutual boundaries with chalcopyrite. At least three generations are present. A dark brown xenomorphic variety is closely associated with granular magnetite and contains minute exsolution blebs of chalcopyrite. It is also poecilitic in the early pyrite and forms interstitial fillings in mosaic pyrite, replacing it. Another deep yellow resinous variety appears interstitial on higher levels, and in veins with quartz and other sulphides. Galena is obviously a later constituent and is present in very minor amounts, mainly in globular pyrite ore, replacing pyrite. It is found also in bands of disseminated sulphides within the greenschist, and occurs associated with lime silicates and vein quartz as well. Chalcopyrite and bornite occur occasionally at upper levels, both apparently of supergene origin. Quartz is the main gangue mineral. Silicate gangue minerals epidote, tre-molite-actinolite, chlorite, white mica and phlogopite occur in prismatic crystals and felted aggregates, usually idiomorphic against opaque minerals, or forming the matrix in which they are embedded. Crystalline calcite is abundant in places. Gypsum was also identified in banded ore, its hypogene or supergene nature unclear. Deep green chlorite and epidote occur with sulphides in vein quartz, and andradite is exceptionally present as well. Fig. 5. Early euhedral pyrite surrounded by gel-pyrite. Spec. EMB 1/70; No. 15. X 150 Fig. 6. Granular pyrite-magne-tite-sphalerite ore, intergrown with lime-iron silicates. Spec. EMB 4/70; No. 59. X 150 Fig. 7. Irregularly shaped texture of pyrrhotite-magnetite ore. Spec. EMB 1/70; No. 15. X 150 Fig. 8. Magnetite-sphalerite ore with exsolved chalcopyrite in dark sphalerite. Spec. EMB 4/70; No. 66. X 150 Fig. 9. Banded massive sulphide ore: pyrite, magnetite and tremolite-actinolite interban-ding. Granular pyrite embedded in quartz at top. Spec. EMB 1/70; No. 87 Fig. 10. "Porphyroblastic" ore: early subhedral pyrite surrounded locally by chalcopyrite, in a fine-grained, banded matrix of pyrrhotite and magnetite. Spec. EMB 1/70; No. 18 10 mm 10 mm Texture, paragenesis and zoning. The ore is fine- to medium-grained, prevailingly banded (Fig. 9) but also dense, even-grained massive, occasionally with larger grains ranging to 15 mm in diameter scattered in places. Very finegrained masses with colloform-like texture are rare. »Porphyroblastic« texture (Fig. 10) is confined rather to lower levels, and mosaic-textured pyrite ore to upper ones. The apparent mineral paragenetic sequence, as deduced from examinations of polished sections, indicates that the euhedral pyrite, magnetite and lime-iron silicates formed first. The flowage-textured magnetite and pyrrhotite followed, embedding the early minerals and interbanding with other components. Sphalerite, chalcopyrite, second generation pyrite and galena seem to be associated with the later surges of the mineralising fluids. The following succession is normally observed in banded ore: Quartz, chlorite and lime-iron silicates pyrite prevailingly magnetite and pyrrhotite magnetite and pyrite. The syngenetic zoning is expressed morphologically and mineralogically. In deeper parts of the deposit, closer to the dioritic dome, magnetite and pyrrhotite prevail, together with the early coarse-grained pyrite and lime-iron silicates, with pyrrhotite-chalcopyrite channels cutting across the ore mass. This portion obviously corresponds to "stringer ore" of D. F. Sangster (1972). Pyrrhotite decreases upwards whereas magnetite persists and pyrite prevails, the texture both banded and massive granular. The highest levels, thought to be most distant from the magmatic source, are composed of mosaic-textured ore of even-grained pyrite, the grain size about 1 mm, with interstitial quartz, calcite, chalcopyrite, yellow sphalerite and galena, with traces of magnetite and lime silicates but no pyrrhotite. Microcrystalline colloform gel-pyrite occurs only in the lowest levels, chalcopyrite and sphalerite on all levels and galena is associated mainly with the yellow sphalerite. The zoning pattern may be simply explained by decreasing temperature of the ore-bearing solutions with the distance from the magmatic source. There were however more surges of fluids at variable temperatures, as evidenced morphologically and paragenetically. Swarms of quartz veins with pyrite, chlorite, chalcopyrite, calcite, epidote and even brown-pink garnet can be seen in the greenschist adjacent to ore, the features apparently channelways in which the solutions moved. The composition of the ore can be seen from the assays of drill cores. The bulk composition encountered in some drillholes is shown as average values in Table 7, and in Figure 4, the expected average tenors in Table 12. There is no trend in metal contents and no correlation between pairs of metals. Gold has not been determined; it is low if there is a linear relationship with base metal values (J. Kaliokoski, 1965). Gold values up to 1.2 ppm have been detected in soil, the corresponding silver values up to 4 ppm, and molybdenum values up to 7 ppm. The tonnage of the deposit is indicated in the order of several millions of tons of the ore mass. Table 7 Metal contents of drill cores at Embaderho Ore layers ^ thickness Number H Cu Zn (per cent) Pb Cd As (ppm) Co Ni Ag (a/*) Metal ratio CusPbsZn Drillhole EMB 1/70 11 max« 9.50 min« 1,53 overage 5 «07 1,40 0«10 0.77 0.43 0.05 0.23 56 17 31 490 5 66 59 17 39 519 49 230 28 12 22 8 2 4,7 250:1:74 Drillhole EMB 4/70 10 max* 11«40 min« 0,96 average 3 «84 1.14 0.01 0.39 3.8 0.11 1.81 790 6 135 ? 58 20 36 515 30 121 90 13 35 14 4 8 30:1:134 Drillhole EMB 6/71 7 max* 5 «16 min« 1«42 average 2 «72 0.64 0.04 0.31 6.55 2.11 3.49 121 76 90 196 9 87 73 14 43 ? ? 32 4 12 34:1:390 Drillhole EMB 7/71 4 max. 12*85 min« 1«32 average 5 «66 0.76 0.25 0.49 3.84 1.77 2.49 158 28 104 124 36 87 65 23 49 ? 7 17 11 14 50:1 £50 Weighted mean composition of ihe ore moss 0.52 1.83 87 80 10 Adi Nefas Situated 5 km north of Asmara, the prospect was explored between 1967 and 1971 by soil sampling, detailed mapping, electromagnetic and magnetic survey and deep drilling (9 holes totalling 953.9 m). The deposit shows up as a 2 km long and up to 20 m wide dark siliceous gossan dipping 78° east, surrounded by, and petering out into schists. The country rock is an alternation of green chlorite schist, quartz-chlorite-sericite schist, massive greenstone, violet spotted schist, phyllite, mica schist and subordinated dark brown quartzite. Dyke-like porphyritic layers 3 to 15 m wide are aligned in the steeply dipping sequence. There are numerous quartz reefs in schist, some of them auriferous (L. Usoni, 1952). Figure 11 illustrates the deposit. The magmatic rocks are white-green banded porphyritic types, with quartz and feldspar phenocrysts up to 7 mm in size. Feldspars are polysynthetic lamellar twins of slightly altered albite-oligoclase, and turbid anorthoclase, showing fine gridiron twinning, peripherally intergrown with quartz and calcite. Quartz phenocrysts are corroded. There is some white mica, little chlorite, the groundmass a microcrystalline aggregate of quartz, calcite, sericite and kaoli- siliceous gossan ] strongly ferruginous schist limonitised sericite-chlorite greenschi5ts , greenstones ] quartz-sericite schist. mica porphyritic rocks slope scree , alluvium drill-hole foliation showing dip fault stream Fig. 11. Geological map of the Adi Nefas prospect Table 8 Mineral composition of magmatic rocks at Adi Nefas Sample No. (see Table 1) 4 5 Phenoerysts: Quartz Albite Anorthoclose Calcite 15 20 20 15 10 15 3 6 Matrix: sericite, quartz, calcite kaolinite, chlorite 52 44 nite. Calcite surrounding feldspars suggest alteration of initially more calcic plagioclase, and hence albite would be secondary. The sodic rock is comparable to quartz keratophyre. However field evidence on its syngenetic nature is not conclusive. Another example of felsic, greenish gray medium-grained porphyritic rock is strongly altered and shows unclear feldspar phenoerysts, few quartz grains in a mass of epidote, chlorite and calcite. The original identity of the highly aluminous rock is obscured by alteration. It may have been originally dacite or andesite or even a tuff-lava. The chemical composition of both rock types is given in Table 1, and the average quantitative mineral composition in Table 8. Drill intersections revealed massive, fine-grained, usually banded ore, composed alternately of pyrite, sphalerite and chalcopyrite, with quartz and barite gangue. In places, sulphides are intimately interbanded with schists. A typical section is shown in Figure 12. Vein-like sulphide concentrations appear also as open space fillings, apparently originated through later hydrothermal rearrangements. The mineralogy of the polymetal ore, as visible to the unaided eye on drill cores, is simple. The hypogene ore paragenesis and textural features, as developed from examinations of polished sections, can be summarised as follows. The prevailing pyrite is the oldest mineral, forming coarse-granular banded aggregates, the corroded grains rounded, with poecilitic chalcopyrite and sphalerite (Fig. 13). The early pyrite is embedded in and strongly replaced by a younger sequence of sulphides; the atoll replacement texture is conspicuous. A fine-crystalline second generation pyrite replaces sphalerite and galena (Fig. 14). Sphalerite strongly prevails quantitatively. It is a pale brown iron-poor variety, replacing pyrite marginally and centrally (Fig. 15). Chalcopyrite occurs in irregularly shaped grains forming interstitial masses, replacing pyrite and filling cracks in sphalerite. Tennantite occurs in isolated larger grains and is marginally replaced by sphalerite. Minor galena appears in small grains, everywhere associated with tennantite, and is locally replaced by sphalerite; it can be found also in small inclusions in later pyrite (Fig. 16). Enargite is present in very small rare grains, and minor lamellar, apparently hypogene, chalcocite is embedded in sphalerite. Supergene chalcocite shows preferential marginal replacement of chalcopyrite, sphalerite and galena but rarely of pyrite. Fig. 13. Granular texture of ore with younger sulphides replacing the early pyrite. Spec. AN 6; No, 17. X 150 Fig. 14. Late-generation pyrite replacing galena and sphalerite; galena in turn replaced by sphalerite. Matrix is quartz (black). Spec. AN 5/71; No. 1. X 150 Fig. 15. Atoll-textured early pyrite, replaced by sphalerite and chalcopyrite, with rims of chalcocite. Spec. AN 7; No. 2. X 150 B — Geologija 21/11 Cu Zn Pb (per cent) Range from to Estimated average Fig. 16. Granular ore of second generation sulphides in quartz. Spec. AN 6; No. 16. X 150 Table 9 Estimated ore grades of Adi Nefas ore Quartz is the prevailing gangue. Fine-crystalline barite is abundant especially in massive sphalerite-chalcopyrite ore. Minor calcite is present as well, accompanied by white mica and chlorite. Galena, bornite, covellite and tenorite have been observed in some quartz reefs in the vicinity of gossan. Gold and silver in Adi Nefas ore are probably associated with pyrite and tennantite respectively. There is no clear-cut correlation between precious and base metals. Microscopic evidence indicates that the early sedimentary mineralisation was followed by hydrothermal rearrangements of ore minerals in apparently permeable, tectonically originated features; the superposition is evident in the locally oblique trend of the gossan with respect to the foliation. The composition of ore is erratic and is shown for some metals in Figure 12. The ranges of metal contents, as determined on assayed split drill cores, and estimated average grades are shown in Tables 9 and 12. Debarwa The deposit is situated about 30 km southwest of Asmara. The unsuccessful drilling exploration of 1966—1968 (38 holes totalling 4,586 m) was resumed in 1971; together with detailed geological and geochemical surveys revealed the BLACK GOSSAN STRONGLY FERRUGINOUS ROCKS. BRECCIA JASPER. SILICEOUS ROCKS. CHERT GREENSCHISTS. CHLORITE--EPIOOTE-MICA SCHSTS QUARTZ • SERICITE SCHIST UNDIFFERENTIATED REODiSH ALTERED ROCKS kWI LATERlTE j BASALT DIKE v v | SCREE, ALLUVIUM ®6A DRILLHOLE ^M FOLIATION SHOWING DIP — FAULT LINE STREAM Fig. 17. Geological map of the Debarwa prospect Fig. 18. Sections of drill-holes DB 1/70 and 6 A, and Cu, Zn, Pb tenors at Debarwa prospect's economic potential. Nippon Mining Co. developed it during 1973—1974 and started mining. Three mineralised zones are evident on the surface and a fourth has been disclosed by soil geochemistry, trending more or less parallel with the foliation of the extremely weathered schist, dipping west at a medium angle (Figs. 17 and 18). Gossan consists of black botryoidal limonite, ferruginous earthy masse, limonitic breccia and jasper and impure barite, the latter occurring centrally within outcrops or forming »veins« in gossan. The largest barite body is well over 100 m long and several metres thick. No malachite staining can be seen on the surface. The prevailing rock types are quartz-sericite schist, sericite schist, chloritic greenschist and tuffaceous calc-epidote schist, and locally greywacke. Narrow beds of black shale and grey phyllite are intercalated. Some feldspathic porphyritic rocks were penetrated in drillholes, and interbedded layers of massive Fig. 19. Replacement texture of ore; black material is quartz. Spec. DB 6 A; No. 58 m. X 150 Fig. 20. Strongly corroded pyrite embedded in sphalerite and quartz (black). Spec. DB 3 A; No. 103.5 m. X 150 Fig. 21. Selective marginal replacement of younger sulphides by bornite. Spec. DB 6 A; No. 65 m. X 150 greenish rocks probably originated from intermediate volcanic rocks. Siliceous rocks such as silicified schist, chert and jasper are abundant and usually surround the ore. Numerous near-vertical basaltic dykes, belonging to Trap Series volcanism, run slightly obliquely to the foliation, together with numerous barren quartz veins. Mafic dykes, quartz veins and gossanous zones developed along the tectonic lines of weakness. Post-mineralisation faulting is evidenced by breccias, slicken-sides and shearing, the most prominent longitudinal fault following the Gual Mareb river valley. Drilling revealed massive, bedded, fine- to medium-grained polymineral ore, fine disseminations in schists and vein-like sulphide concentrations, the total thickness of several mineralised zones being of the order of tens of metres. The schists near ore are frequently kaolinised. The ore control is stratigraphical and structural, the ore zones dipping apparently under different angles. Soil geochemistry confirmed the multi-stage character of mineralisation, with copper and zinc distribution patterns indicating contemporaneous precipitation of both metals, and the separate deposition of lead, controlled by transversal faults. Polished sections of a number of random samples revealed hypogene mineral paragenesis similar to that of Adi Nefas. Pyrite is again the prevailing opaque mineral, the oldest in the paragenesis, surrounded by quartz and younger sulphides replacing it (Fig. 19). Its corroded grains are rounded and exhibit caries texture. A younger fine-grained generation of pyrite is also present. Sphalerite is intimately associated with the younger paragenetic associates chalcopyrite, tennantite and enargite, all of them anhedral and replacing the early pyrite (Fig. 20). Chalcopyrite is replaced by sphalerite and bornite (Fig. 21). Tennantite appears in larger grains and shows mutual boundaries with sphalerite. Rare enargite occurs mainly in pyrite-sphalerite banded ore, marginally replacing the latter. A younger generation of tennantite appears along the rims of enargite, and was clearly derived from it. Galena is the latest sulphide, quantitatively negligible. It occurs also in small veinlets cutting through older sulphides. Bornite replaces chalcopyrite marginally and along cracks. Together with chalcocite it replaces massive pyrite in the cementation zone, both minerals accounting for the very high copper grade up to 31 per cent (Fig. 22). The observed bornite-chalcocite lattice intergrowth suggests either an exsolution unmixing or a supergene replacement of bornite by chalcocite, the later option being more acceptable since the bulk of anisotropic bluish chalcocite is super-gene. There are minor amounts of covellite and ferric hydrous products. An important cementation took place at Debarwa at a depth of about 50 to 60 m below the Gual Mareb valley. Quartz is ubiquitous, and barite forms the essential part of the outcrops but does not extend to any great depth. Rare calcite is restricted to the oxidation zone. Mutual replacement is evident in the multi-stage hypogene ore paragenesis, the younger minerals replacing the earlier ones, galena seemingly the latest. The following mineral groupings, possibly indicating the successive mineralisation stages, have been observed: quartz pyrite I pyrite II tennantite I chalcopyrite bornite enargite chalcocite enargite tennantite II galena quartz barite calcite pyrite III Supergene minerals chalcocite covellite limonite quartz? Fig. 22. Lamellar bornite-chal-cocite intergrowth, both replacing shattered pyrite. Spec. DB 1/70; No. 4. X 150 Table 10 Estimated ore grades of Debarwa ore Co (per Zn cent) Pb Ag Au (g/t) Range from 0.1 1 0.025 tr 0.2 to 31 25 1 3280 2.5 Estimated 2.15 5 0.5 100 2 average Several recognised ore types may be typical for certain parts of the deposit, the ore grades apparently broadly variable from place to place. They can be tentatively listed as follows: — massive granular pyrite embedded in quartz matrix, with small amounts of base metals — strongly disseminated pyrite in schists — banded polymineral ore rich in copper and zinc — cementation zone ore of any of the above types, enriched in supergene copper minerals but low in zinc. The ore grades on assayed cores are extremely variable. The observed ranges in primary ore and an assumed average are given in Tables 10 and 12. Nippon Mining Co. in 1973—1974 exploited ore grading 13.8 per cent copper (M. H a m r 1 a , 1974). The tonnage potential of the deposit might be expected in the order of several millions tons of commercial ore. Adi Rassi The Adi Rassi deposit, which has been known since the 16th century, is situated 35 km south of Asmara. Italians explored it during 1938—1939. Renewed exploration in 1965—1966 involved detailed mapping and deep drilling (10 holes totalling 1,771 m). Geochemical soil survey was done later. Confined to two small hills of altered ferruginous rocks modestly stained with malachite, the deposit occupies a zone about 450 m long and several tens of metres wide. Chlorite-quartz greenschist is the prevailing rock, grading to hornfels-like rock, interbedded with arenaceous schist, sericite schist and minor tuffaceous greywacke, the sequence striking submeridionally and dipping steeply west. Syngenetic porphyritic layers up to 40 m thick are intercalated. Massive epidotised spilitic rock west of the deposit may have been originally andesite or basalt. Geology of the deposit is shown in Figures 23 and 24. Several magmatic rocks have been distinguished within the sequence. a) A grey-green, intimately intercalated, mesocratic porphyrite has a normal porphyritic texture with phenocrysts of corroded quartz up to 15 mm in diameter, and albite-oligoclase double that size. The plagioclase is strongly replaced and pseudomorphed by an aggregate of clinozoisite, quartz and calcite. There are minor grains of pale-green amphibole. The matrix consists of lath-shaped feldspar, felty amphibole, minor quartz, calcite and opaque minerals. The mode of the rock is shown in Table 11. The chemical composition, given in Table 1, shows a peraluminous, strongly sodic, potash-deficient rock derived from an intermediate (dioritic) magma. b) A dyke-like leucocratic quartz porphyry is deeply weathered and usually malachite stained. Drillholes revealed a siliceous quartz keratophyre; a finegrained leucocratic matrix containing phenocrysts of quartz and remnants of feldspars is totally replaced by a plumose aggregate of kaolinite. c) Adjacent to porphyrite is a hornfels-like rock, its texture porphyritic, with rare albite phenocrysts less than 0.5 mm in size, occasional smaller quartz grains, and euhedral rhomboid calcite up to 1 mm in diameter disseminated with magnetite. The matrix is an aggregate of quartz, feldspar, chlorite, epidote, stilpnomelane and calcite. Disseminated pyrite, chalcopyrite, pyrrhotite and magnetite occur in the rock, the grain size under 0.05 mm (Fig. 25). The rock can be designated as spilite. The chemical composition of an iron-rich sample is shown in Table 1 (sample No. 7). d) Another fine-grained, totally altered, lamprophyric rock appears in a dyke obliquely cutting the top of the South Hill. The deposit is of stockwork type, consisting of intermingled veins and vein-lets of chalcopyrite, pyrite, quartz and calcite, the rocks brecciated and cemented with these minerals, the thickness of the elongated ore body being up to 50 m and averaging about 25 m. The dual nature of the deposit is obvious. Conformable bands of sulphides in schist and disseminations of pyrrhotite, magnetite and pyrite in spilitic layers are the early syngenetic constituents (Fig. 26). The bulk of sulphides occurs in veinlets and breccia, chalcopyrite being the prevailing base metal mineral, No sphalerite is visible, the values of zinc revealed by geochemical soil survey below 2,000 ppm and hence quantitatively unimportant. There is no galena either, the soil lead values being at background level. v v v \ v V ( J/ ' I I I i v v w