Gem-Quality Chrysoprase from Haneti-Itiso Area, Central Tanzania
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GEM-QUALITY CHRYSOPRASE FROM HANETI-ITISO AREA, CENTRAL TANZANIA KARI A. KINNUNEN and ELIAS J. MALISA KINNUNEN, KARI A. and MALISA ELIAS J., 1990: Gem-quality Chrysoprase from Haneti-Itiso area, Central Tanzania. Bull. Geol. Soc. Finland 62, Part 2, 157—166. Gem-quality, apple-green, Ni-bearing chalcedonic quartz occurs as near-surface veins in silicified serpentinite in the Haneti-Itiso area, Central Tanzania. AAS de- terminations revealed a high Ni content, 0.55 wt.%, and low Co and Cr contents of 120 and 1 ppm respectively. NAA determination revealed near chondritic REE contents. X-ray diffraction determinations showed that the Chrysoprase consists main- ly of alpha quartz with some opal-CT. The gemmological properties are: refractive indices from 1.548 to 1.553 (±0.002), mean specific gravity 2.56, hardness about 7 on Moh's scale, inert to ultraviolet radiation, green through Chelsea filter, and absorption in the deep red and violet part of the optical absorption spectrum. The results confirm the identity of the material as Chrysoprase. Microscopically the Tanzanian Chrysoprase consists of spherules which are high- ly disordered, concentric, and composed of bipyramidal quartz, chalcedony, quart- zine, and opal-A. They were classified into four main types according to the shell arrangement. The diameter of the spherules ranged from 40 um to 77 um. Fluid inclusion types in the bipyramidal quartz were monophasic, low-temperature type. The spherules, silica types and REE contents suggest that this Chrysoprase was deposited by evaporation of surface waters connected with the silicification of the serpentinites. Genetically analogous formations, common in Africa, include M-fabric type, weathering profile silcretes. Key words: gems, Chrysoprase, chemical composition, X-ray diffraction data, spherules, fluid inclusions, crystallization, Archaean, Tanzania. Kari A. Kinnunen: Geological Survey of Finland, SF-02150 Espoo, Finland. Elias J. Malisa: University of Dar es Salaam, P. O. Box 35052, Dar es Salaam, Tan- zania. Introduction The microtextures of gemstones and their ge- netical implications have received little attention. In 1982 one of us (K.A.K.) observed an un- However, microtextures can be used in the gem usual surface texture (Fig. 3 A) on some of the testing of fine-grained materials (e.g., Kinnunen first cabochons cut in Finland from recently dis- 1982) in much the same way as mineral and fluid covered Chrysoprase (variety name for gem- inclusions in crystals (Giibelin and Koivula 1984). quality, apple-green, Ni-bearing chalcedony) oc- The finding of large silica spherules in Tanzani- curring in the Haneti-Itiso area, Central Tanza- an Chrysoprase provided impetus for a detailed nia (Fig. 1). The textures consisted of disordered study. spherules of silica much like that in some opals Chrysoprases have been studied extensively, (see Darragh et al. 1976) only larger in size. mainly because of the well-known connection of 158 Kari A. Kinnunen and Elias J. Malisa Fig. 1. Vein of gem-quality Chrys- oprase from the Haneti-Itiso area, Tanzania. The brown host rock seen on the sample is silicified serpentinite, siliciophite. Length of specimen is 9 cm. Photo by K. A. Kinnunen. these gem minerals with some nickel occurrences of the authors (E.J.M.) in 1980 and 1984. This in serpentinites (Table 1). This connection has was done in connection with the study of gem been used in prospecting for Chrysoprase. Komov materials and their potential exploitation in Tan- et al. (1987) have, based on the deposits in zania (see Malisa 1987, Malisa and Muhongo Kazakhstan, specified favourable indications for 1990). The first mention in the literature of gem- gem Chrysoprase as being serpentinized (mainly quality silica types (green opal, prasopal, and lizarditic) ultrabasic rocks containing from 2 to Chrysoprase) occurring at Haneti Hill was by 3.3 per cent nickel, which additionally show si- Giibelin (1975) and Schmetzer et al. (1976). licified weathering crusts with quartz, chalcedo- The quarrying of Chrysoprase at Haneti-Itiso ny and prasopal veins along faults. area was commenced by a private Greek prospec- The samples of Tanzanian Chrysoprase were tor George Pappas. Today, the State Mining Cor- collected together with field observations by one poration of Tanzania (STAMICO) holds claim Table 1. The main occurrences recorded as producers of gem-quality Chrysoprase. Locality Host rocks Reference 1. Lower Silesia, Poland magnesite veins in serpentinite 1 2. Sarakulbaldy, Central Kazakhstan, USSR Ni-deposit in serpentinite 2 3. Revdinsk, Ekaterinburg, Urals, USSR Ni-ochre mine 3 4. Visalia, Tulare, California, USA serpentinite 4 5. Nickel mountain, Oregon, USA Ni-ores in serpentinite 5 6. Minas Gerais and Goias, Brazil nickel ores 4 7. Queensland, Australia ultrabasic rocks, 6 nickeliferous laterite 8. Haneti-Itiso area, Tanzania serpentinite 7 Data from: (1) Kozlowski and Sachanbinski (1984) (2) Osipov and Klimovistkaya (1977) (3) Bauer (1904) (4) Webster (1983) (5) Kunz (1892) (6) Connolly (1980) (7) this study Gem-quality Chrysoprase from Haneti-Itiso area, Central Tanzania 159 to the Chrysoprase quarries and extraction of the southeast of Kwahemu ujamaa village. Nickel- Chrysoprase is done through its subsidiary com- rich magnesite veins have been prospected in the pany, the Tanzanian Gemstone Industry Ltd. area, but the only mining activity done up to now at the Haneti-Itiso area is quarrying of gem- quality and ornamental chrysoprase. Chryso- Geological setting prase occurs near the surface as irregular veins (Fig. 1), commonly 1—5 cm in width in silici- The Haneti-Itiso area is located in QDS 124, ophites — serpentinites silicified to red or red- Kelema and 143, Meia Meia (Pallister 1960, brown flinty rocks. Green opal occurs sporadi- 1963). The rocks are Archaean, and belong to the cally in the same area. Four chrysoprase vein Dodoman formation, which consist of metamor- samples in total were selected for the study of phosed ultrabasics forming a line of hills running microtextures and one for chemical analysis and in a north-westerly direction. The dominant rock X-ray diffraction determination. type in the Haneti-Itiso intrusive complex is a si- licified and ferruginized serpentinite, within which are pods and zones of anthophyllite as- Chemical analysis bestos, chlorite-magnesite-serpentinites, silicified and ferruginized to varying degrees. The ultra- Atomic absorption spectrophotometric (AAS) basic rocks of the Haneti-Itiso complex are cut determinations showed high Ni contents: 0.55 by numerous veins of magnesite and less com- wt.%. Other trace elements analyzed by AAS monly, asbestos (gedrite). The silicified and fer- were (in ppm): Co 120, Zn 84, Fe 40, Mn 8, Cu ruginized serpentinite sporadically contains 4 and Cr 1. Neutron activation analysis (NAA) nickel-bearing magnesite as abundant finely dis- showed the following lanthanoid spectrum (in seminated grains. ppm): La below 1, Ce 1.51, Nd 1.07, Sm 0.19, Due to tropical weathering the rocks have been Eu 0.090, Tb 0.023, Tm 0.11, Yb 0.60, and Lu silicified when silica, removed in aqueous solu- 0.12. These rare earth element (REE) contents are tion from decomposed rocks, was subsequently rather close to chondritic values (Fig. 2). The reprecipitated as veins of chalcedony or chalced- values are higher than in typical serpentinites al- onic quartz (chrysoprase). Gem-quality Chryso- though they show similar LREE/HREE ratios to prase occurs as thin discontinuous veinlets with- those of serpentinites (see Frey 1984). The chon- in the ultrabasic rocks. Bright specks of magne- dritic REE contents in chrysoprase suggest en- site are disseminated through a dull-brown 'flinty' groundmass of dunite and peridotite in outcrops on the Mnakuru hill, whilst on anoth- Chrysoprase / chondrite 10 er hill further to the north-west, epidote-clino- zoisite-tremolite schist, derived from rocks of gabbroic composition is dominant. The emplace- ment of the ultrabasic rocks is thought to have been controlled to some extent by major NW and minor NE-trending fractures and shear zones. Description of material 0.1 —1—1—'—1—1—1—1—1—'—1—1—1—1—1— La Ce Nd Sm Eu Tb Tm Yb Lu Chrysoprase specimens were collected from the Fig. 2. Rare earth element (REE) pattern, chondrite nor- Kwahemu, Senato and Msani hills about 10 km malized, from gem chrysoprase, Haneti-Itiso area, Tanzania. 160 Kari A. Kinnunen and Elias J. Malisa richment from serpentinites during weathering logical tests, are preferred, mainly because such processes (e.g., Fleet 1984). Beside these trace ele- methods are also suitable for expensive cut stones ments, the NAA determination showed the Au (Chuboda and Gübelin 1966, Lewis 1977, Lid- contents of 0.019 ppm, Th below 0.4 ppm and dicoat 1977, Anderson 1980, Webster 1983, U 0.3 ppm. Gübelin and Koivula 1986, Matlins and Bonan- no 1989). Although these tests are usually not able to scientifically prove the identity of a miner- X-ray diffraction data al, they are usually accurate enough for practi- cal purposes. Today the science gemmology is The X-ray diffractograms confirmed the pres- largely a search for such simple methods follow- ence of alpha quartz. Diffuse lines of opal-CT ing determination by more sophisticated in- were detected from unheated material in a few strumental studies. The goals are simple tests, X-ray diffraction films (Debye-Scherrer powder which jewellers equipped with modest instru- camera). Heated material showed only quartz ments (traditionally refractometer, spectroscope lines. These data, together with the chemical ana- and stereomicroscope) are able to perform in lyses are typical for chalcedonic silica. The high their shops. Therefore, every description of a new Ni, and low Co and Cr contents confirm the ini- gemstone locality should include as much as pos- tial identification of this green chalcedony type sible of this kind of information. from Tanzania as Chrysoprase (according to the definition in Frondel 1962 and Webster 1983).