CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION

Transcription

1 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION ERNST-G Ö NTHER SCHULZE Minera/ogisches Institut der Technischen Universität Hannover, Germany ScHULZE, E.-G.: Contributions to the ore mineralization in the Oslo region. Norsk Geologisk Tidsskrift, Vol. 48, pp Oslo The present paper gives the results of microscopic examinations, chemical, and spectrochemical analyses of a mainly sulphidic ore paragenesis of which the most important minerals are chalcocite and neodigenite. The small veinlet, outcropping on the beach of the Store Sletter island in the Oslo Fjord near M oss, is situated in the irnrnediate neighbourhood of the eastern marginal fault of the Oslo graben. The meta! bearing solutions are probably derived from a basaltic magma. INTRODUCTION The students and assistants of the geological and mineralogical institute of the Technical University, Hannover, visited (under the direction of Professor Dr. K. Richter) the island Store (=Greater) Sletter in the Oslofjord (about 15 km south of Moss) to study the rhomb porphyry conglomerate. The author took part in this excursion and discovered a thin ore veinlet in the conglomerate exposed directly on the beach at the north end of the island (Fig. 1). The ore and the wall rock were studied microscopically and spectrographically in the mineralogical institute of the Technical University, Hannover. The chemical analyses were carried out by Dr. H. Gundlach in the Rundesanstalt fi.ir Bodenforschung (Hannover). The results of this study are given in this paper as a contribution to the study of the hydrathermal mineralization in the Oslo region. FIELD OBSERVATION A blackish grey vein of cm thickness cuts through the reddish coloured, relatively fine-grained conglamerate with a NE-SW strike and a steep SE dip. A thin zone of the wall rock (2-5 cm) surmunding the vein walls is bleached as a result of the hydrathermal alteration and is brownish white in colour. The fissure attacked by the hydrathermal solution owes its origin to shearing movements. This is proved by the form of the veinlet as weil as the brecciatian of the wall rock which is also found as fragments in the ore material (Fig. 2). Moreover, the slickensides on the iraeture surfaces in the neighbourhood of the mineralized fissure show a steep SW plunge. The ore veinlet is mostly sharply delimited from the wall rock and very occasionally has an irregular diffuse transitional zone of slightly ores

2 66 E.-G. SCHULZE N OSLO o 10 20km Fig. l. Location map showing the occurrence of the investigated ore in the Oslo region (Sletter islands) and some geological features (After N.G.U. 208, Oslo 1960). impregnated wall rock. The vein walls, the joint planes, and the drusy cavities of the wall rock in the immediate neighbourhood of the veinlet generally are covered or filled with thin incrustations or earthy masses of secondary copper ores, rarely with limonite or manganese oxides. A few carbonate veinlets (0.1-1 cm thick), in some cases with small inclusions of sulphidic ore, intersect the conglomerate topping the investigated ore veinlet.

3 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION Fig. 2. Sketch of specimens showing veinlet (black) with adjacent wall rock (conglomerate). Samples No. l and No. 2 are channel samples (see Tables l and 2). RESULTS OF MICROSCOPIC EXAMINATION (PARAGENESIS) Magnetite crystallized initially occurs in idiomorphic crystals with an average diameter of 5-30 p, (max. 70 J-t) in the more or less brecciated wall rock directly bordering the veinlet. At the vein walls, occasionally one notices replacement relics of magnetite in the capper ores. The individual magnetite crystals are often weakly limonitized. Hematite also occurs in idiomorphic crystals and is precipitated after magnetite. Both magnetite and hematite make out % of the constituents of the mineralized fissure. Hematite crystals with a diameter of 3-20 fl (max. 80 p,) are generally observed endosed in the quartz of the vein-wall zon e. Quartz is the predominating gangue mineral (6-14 %) and ends the early oxide-phase. It is mostly found in idiomorphic crystals of different dimensions percent Magnetite Hematite Quartz Mineral A Linnaeitegroup Mineral B Gersdorffite Sphalerite Bornite Chalcopyrite Neodigenite Chalcocite Anl<.erite Fe Ni,Co Z n C u idioblostic process >-? - - _...,. _ - 0,6-1,, 0,2-1,0 6-".. O,J-0,6'..... < 0,2 2,0-1,2 2,2-3. < 0, '0 0-2 Fig. 3. Paragenetic sequence of primary vein minerals. e=eutectic formation of bomitej neodigenite.

4 68 E.-G. SCHULZE ( < l mm) and is concentrated at the foot wall of the veinlet. Individual quartz crystals or crystal-aggregates appear in some cases in a sulphide matrix not far from the vein wall. The quartz grains are surrounded in some places by secondary copper ores (malachite, covellite) or cloudy ones penetrated by malachite, rarely limonite and very rarely azurite. The sulphide-phase (main phase) begins with the precipitation of Co-Nisulphides and Co-sulphide plus Ni-sulphide. They are found in very small quantities as grains or grain-aggregates mostly equally distributed in the veinlet with a probable concentration at the bottom of the veinlet. The minerals of the linnaeite group are found as idiomorphic crystals or frequently as crystal-aggregates of size 2-8 fl (max. 30 fl) in the sphalerite, neodigenite and chalcocite (Figs. 4 and 5). They make % of the mineral constituents of the veinlet. The crystal aggregates often surround small rock enclosures (Figs. 4 and 5). It has been possible to differentiate microscopically two minerals of the linnaeite group (minerals A and B, Fig. 3). The older mineral A is creamish white with a weak yellow tint. The younger mineral B, which replaces mineral A along preferred crystallographic directions, has a lower reflection power with a weak reddish brownish tint. Mineral B is also slightly harder than mineral A. The hardness difference is distinct in spite of the small grain size. The proportion of mineral A : mineral B was not ascertained. However, this proportion changes quickly from one grain to another. The crystals sometimes show the typical sieve texture of the linnaeite minerals. The fine mineral inclusions ( < l fl). which are zonally arranged in one of the crystals, could not be identified. The sieve texture, which is mostly accompanied with serrated boundaries, could be explained by the idioblastic process (Ramdohr 1960). Fragments of country rock could have helped as crystallization centers (Figs. 4 and 5). lt is also possible that the crystals of the linnaeite group are formed during a normal crystallization process with a local inhibited growth of individual crystals (skeletal crystals) andfor with replacement effects. Fig. 4. Irregular wall rock fragments (black) surrounded by idiomorphic (? idioblastic) minerals of linnaeite group (white). Bornite: close stippled. Matrix: neodigeni te (light stippled) with some chalcocite (spots).

5 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION 69 Fig. 5. Idiomorphic (? idioblastic) crystals of linnaeite minerals in a matrix of neodigenite/chalcocite (white). Mineral A (light stippled) replaced by mineral B (close stippled). Wall rock fragment (black) as probable crystallization center. 10).1 The formation of gersdorffite followed slightly later or was contemporaneous with the minerals of the linnaeite group. lt is found only in traces ( < 0.2 o/o) and is present in euhedral crystals of size 3-12 fl in the chalcocite, nnd less frequently in the sphalerite. The precipitation of sphalerite follows after in the mineralization sequence, if we make the premise that the formation of the Co-Ni-minerals took place at the beginning of the sulphide-phase, as it is accepted in the paragenetic scheme (Fig. 3). Sphalerite makes about 2-4 % of the mineral content and is the most frequent ore mineral after the Cu-ores. It is found as thin streaks of mm at the vein wall (foot wall) and is made of aggregates of individual grains. The grains are l 0-12 fl in size and are found in euhedral and subhedral crystals, whose margins are sometimes replaced by bomite and neodigenite (Fig. 6 to the left). The same Figure (to the right) shows a very interesting myrmekite-like intergrowth between sphalerite, bornite and neodigenite. Fig. 6. Right: Myrmekitelike intergrowth of bornite (stippled), sphalerite (black) and neodigenite (white). Left: Replacement structure (?) of bornit e (stippled), sphalerite (black) and neodigenite (white).

6 70 E.-G. SCHULZE. y;'!1j!,..,',t'ol"r..&mill 40,<1 Fig. 7. Myrmekite intergrowth of bornite (black) and neodigenite (stippled). Chalcocite (white). The copper ores are the main components of the vein filling. The precipitation of bornite follows that of sphalerite. The bornite is mostly found in myrmekitic intergrowth with neodigenite (Figs. 6 and 7). Certain bornite particles in the neodigenite-chalcocite could be considered as replacement remains. The myrmekite intergrowth could be explained as eutectic precipitation in the sense of Laney. The lense-like chalcopyrite spindies in bornite could be considered as exsolution product. They have a length of 5-25 fl and are mostly orientated in parallel. The question mark in Fig. 3 shows the uncertain position of the chalcopyrite exsolution in the crystallization sequence. Probably a small part of the chalcopyrite intruded into bornite before the formation of neodigenite. Sphalerite, neodigenite, and chalcocite are free from chalcopyrite, which is only restricted to bornite. Whereas bornite makes % of the mineral constituents, the chalcopyrite amounts to < 0.5 %. Neodigenite and chalcocite are the most important vein minerals (Fig. 3). Fig. 8. Myrmekite of bornite (black) and neodigenite (white).

7 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION 71 Fig. 9. Decomposed neodigenite crystal (stippled) with chalcocite lamellae (white). Ruptures (black) along crystallographic directions. They make more than 50 % of the material constituents of the veinlet with radically changing proportions. The chalcocite results from the decoroposition of the neodigenite crystals (Ramdohr 1960). The lamellae structure resulting from the paramorphosis of the rhombic chalcocite into cubic neodigenite can be observed easily (Figs. 8 and 9). However, in certain parts of the veinlet there is also a fine-grained mixture of irregularly delimited neodigerute and chalcocite with very indistinct lamellae structure. Fine-grained (up to 1.5 mm in size) yellowish coloured ankerite represents the only mineral component of the late carbonate phase of the mineralization (Fig. 3). lt is found in very thin fissures (up to l cm) in the top wall of the veinlet. Rounded inclusions of neodigenite and chalcocite appear in ankerite as traceable older formations, sometimes with partial marginal resorption. The alteration of the primary copper minerals into malachite (8-10 %). covellite ( %). azurite ( <0.1 %). and limonite (approximately 0.5 %) took place especially at the vein walls and in the neighbouring country rock along crystal surfaces, cleavage planes and ruptures. A specially intensive oxidation is noticed in the wall rock impregnated with ore material, sometimes with a complete replacement of the primary mineral components. The formation of limonite and manganese oxides in the form of thin earthy or dendritic aggregations took place especially at joint surfaces of the country rock. RESULTS OF CHEMICAL AND SPECTROGRAPHICAL ANALYSES (Geochemical behaviour) Table l gives the content of the major elements Cu, Zn, and Fe in two ore samples. The residue contains gangue (quartz) and small amounts of brecciated wall rock (chiefly silicates). The proportion gangue:wall rock can be estimated as approximately 20 : l.

8 72 E.-G. SCHULZE The content of the minor elements and trace elements in ore, carbonate and country rock (Table 2) is determined with the help of a semiquantitative spectrographic method (Mitchell 1964 ). The accuracy of this method lies within the limits of + 20 %. The silver content in the ore is determined by AAS. No. l and No. 2 = channel samples from foot wall to hanging wall of the veinlet (see Fig. 2). No. 3 = No. 4 = yellowish fine-grained ankerite. channel sample: 1.5 cm from vein wall into adjacent wall rock (hydrothermally altered 'rhomb porphyry conglomerate'). No. 5 = 'rhomb porphyry conglomerate' outside the veinlet. Note: 'rhomb porphyry conglomerate' consisting of rhomb porphyry fragments and some fragments of tuffitic material, basalt and sandstone. Ratio rhomb porphyry fragments: tuff + basalt + sandstone fragments approximately 4 : l. The absence of Ag-minerals indicates that silver must be dissolved in certain sulphidic copper minerals. The experiments of Skinner (1966) and Skinner, White, Rose & May (1967) proved that especially in chalcocite the Table J. Content of major elements in weight percent as determined by Atomic Absorption Spectrophotometer (Perkin-Elmer 303) Sample No. Cu Z n Fe Residue Analysed by Dr. H. GuNDLACH Table 2. Content of minor elements and trace elements in part per million as determined by semiquantitative spectrographic analysis (Hilger& Watts E 478) Sample No. v C r M n C o Ni C u Z n Ge G a Sr Z r Ag B a P b l m m m m l JOO > < > > < >2100 > > limit of detection m= major element -=not detected (below sensitivity of method)..- determined by AAs (H. GUNDLACH). below limits of detection of the method used (limit in parentheses): As (500), Nb (20), Mo (5), Cd (50), In (10), Sn (20), Sb (20), Hg (50), and Bi (20). Analysed by Barbara BRuNs (Mineralogical Institute of the Technical University, Hannover)

9 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION 73 Ag-atom replaces the Cu-atom. According to these authors, small silver amounts are taken in the crystal lattice of neodigenite and/ or bomite. Nickel and cobalt, as already mentioned, usually form independent minerals but it is possible that small amounts of these elements are fixed in magnetite, certain Cu-minerals and ankerite. The remarkably high values of Pb in the ore (600 ppm), and the small values in ankerite (150 ppm) lead to the conclusion that a Ph-mineral must be present which, however, is not determinable with the help of the microscope. It may be accepted that the trace elements vanadium, manganese and especially chromium are formed as isomorphic replacement in the lattice of the ore, especially that of iron ores. A comparison of the spectrographical analyses of the country rock, considering the contamination prohability of the ore by the country rock, showed that there should be no effective influence on the V, Mn and especially Cr content in the studied samples due to the probable country rock xenoliths. Nor is it possible to explain the V, Mn and Cr content in the ankerite as a result of contamination by the country rock and/or ore. The small Ba content (55 ppm) in the ore sample No. l is very probably due to the contamination by the wall rock xenoliths (barite). Increasing content of Ba and Sr is ascertained in the wall rock (Table 2, No. 4). The Ba and Sr content amounts to >0.21 % (Table 2, No. 5) outside the mineralization in the 'rhomb porphyry conglomera te'. With the hel p of the microscopic study it has been possible to find barite as fine joint fillings. The high Sr content (0.13 %) in ankerite could be due to the substitution of Ca2+ by Sr2+, because it has not been possible to notice either Ba- or Sr-minerals in the ankerite. SUMMARY AND CONCLUSION The investigated ore veinlet of the island Store Sletter (Oslofjord, south of Moss) strikes NE-SW and dips steeply to the SE. It shows a distinct movement character and can be described as a small shear fracture. The element-association of this veinlet is in accordance with the mineralization in the Oslo region (compare Fig. 3 with Goldschmidt 1911). The copper ores, especially chalcocite and neodigenite, dominate (>50 %) the other minerals. Among the non-copper ores, sphalerite is the most frequent mineral. It was also possible to observe minerals of the linnaeite group and gersdorffite in small quautities (0.8 % and 0.2 %) within the sulphidephase. Attention is drawn to the problem of the origin of the linnaeite minerals and the uncertain genetic relationship of bornite and neodigenite. Ankerite and oxidic iron ores are relatively poor (each <2 %) in comparison to the sulphides.

10 74 E.-G. SCHULZE The mineralization with its wide temperature range (magnetite formation decomposition of neodigenite at 78 C) follows from the foot wall to the hanging wall. 1t begins with the early oxide-phase (magnetite-hematitequartz). The minerals which are usually present in euhedral crystals are found in the vein walls. The sulphldic main-phase follows with linnaeite minerals, gersdorffite, sphalerite, bornite, chalcopyrite, neodigenite and chalcocite. The poorly represented late carbonate-phase is found as fissure filtings of ankerite in the surmunding country rock. As a result of our spectrographic analyses, we suppose that a small amount of Pb hearing minerals (galena?) was formed. The geochemical analyses proved a silver content in the ore of a maximum of 0.15 %. which may be dissolved in the Cu-ores (chalcocite, neodigenite, and bornite) as it was not possible to prove the presence of Ag-minerals. The As, Sb, and Bi content lies below the detection limit of the spectroscopic method used; however, the presence of the trace mineral gersdorffite ( < 0.2 %) necessitates small quantities of arsenic (limit of detection for arsenic 500 ppm). Small amounts (10-50 ppm) of V, Mn and Cr could be traced in the ore in addition to small amounts of Ge and Wgh Mn content in the ankerite. The high Sr content in the ankerite is very probably a result of the substitution of Ca2+ by Sr2+. The Wgh Ba and Sr contents in the wall rock (> 0.21 %) are partially or completely owing to the presence of barite in the conglomerate. A genetical relationship between the barite formation and the ore mineralization could not be reached. Probably there is a close connection between the barite formation and the hydrathermal phase of the rhomb porphyry volcanicity. The Sletter islands lie in the direct neighbourhood of the eastern marginal fault of the Oslo graben; they are therefore affected by the movement phases of the graben formation. The tectonic movements eaused the opening of fissures, accompanied at the same time by hydrathermal activity. lt is possible to explain the origin of the hydrathermal meta! hearing solutions either as belonging to the nearby Drammen granite, a relatively young plutonic intrusion in the Oslo region, or as a deuteric solution of the juvenile basaltic magma. The direct neighbourhood of the undoubtedly deepseated marginal fault as weil as the specifically heavy-metal association of the ore mineralization (major elements: Cu, Fe, Zn; minor elements: Ag, Ni, Co, Mn; trace elements: As, Pb, V, Cr) support the seeond suggestion. Mineralogisches Institut der Technischen Universität Hannover, Welfengarten I, 3 Hannover, Germany 24 Mayi968

11 CONTRIBUTlON TO THE ORE MINERALIZATION IN THE OSLO REGION 75 REFERENCES GoLDSCHMIDT, V. M. 1911: Die Kontaktmetamorphose im Kristianiagebiet. Videns. - Selsk. Skrifter. l. Mat.-Naturv. Kl, 1911:1, 483 pp. Kristiania. MITCHELL, R. L. 1964: The spectrochemical analysis of so ils, plants and related material. Techn. Communication of Commonwealth 44a, 225 pp. Harpenden FTEDAHL, CH. 1960: Permian rocks and structures of the Oslo region. In: Geology of Norway (Ed.: O. HoLTEDAHL), Norges geol. Unders. 208, 540 pp. Oslo. RAMDOHR, P. 1960: Die Erzmineralien und ihre Verwachsungen. 3. Aufl pp. Akad. Verl. Berlin. SKINNER, B. J. 1966: The system Cu-Ag-S. Econ. Geol. 61, SKINNER, B. J., WHITE, D. E., RosE, H. J. & MAYS, R. E. 1967: Sulfides associated with the Salton Sea geothermal brine. Econ. Geol. 62,