|
Historical background
The first record of sulphide discovery at Skorovass dates back to September 1873 when the area was visited by cand. min. K.H. Hauan during a geological reconnaissance (noted in a diary of Professor Kjerulf). Hauan mentions seeing a several meter thick wall of pyrite along a deep fracture and rusty developement surrounding the massive pyrite outcropping. There was already at this time some visible sign that some trenching and blasting has been done. The showing was first mentioned as ¿Rauberget¿ (Red Mountain) near Tunsjøen by Kjerulf (1887), referring to the large rusty ¿gossan¿ developed at the surface exposure of the massive pyritic ore and disseminated sulphide stringer-zone type mineralization.
Bergmester N.O. Hagen visited the area on September 4th 1904 and reported 6 mineralized showings, the largest having a maximum thickness of 4-5m. He referred to the showings as the ¿Skorevands deposit¿. The conclusion drawn at that time was that the ore was purely pyrite having little Cu values. This, along with difficult access and long transport, made them conclude that the deposit was not economic at this time. In the begining of 1922, Hagen sent a description of the Skorovass deposit in to ¿The Institute of Mining and Metallurgy¿, London (IMM Bulletin No. 217).
Hauan¿s sulphide wall (now known as ¿Gamelgruva¿) was first staked on October 16th 1910 by Elektrokjemisk A/S (now ELKEM A/S), who bought the mining rights for the area in 1914. Between 1913-16, the deposit was investigated by using two exploration adits and 20 drill holes.
A description of the deposit was first published by J.H.L. Vogt (1915) and later descriptions were given by A. Bugge (1920) and H.H. Smith (1922). S. Foslie, NGU gave an overview of all data regarding the Skorovass deposit at the time of his work on the Norwegian pyrite deposits (Foslie 1926).
A more extensive exploration program began in 1935 under the leadership of S. Foslie, NGU, and included 55 diamond drill holes and EM measurements. Ore reserve calculations given in Foslie¿s report of 1938 (10M tonnes) formed the basis of a proposed mining production, which finally started in 1952, after the German occupational forces fruitlessly attempted to get the mine into production during WW II. The geological results of Foslie¿s investigations are published in a lecture reference (Foslie 1939).
Production
Production started in 1952 (ELKEM A/S) and terminated in June of 1984 because of depleted reserves and low Cu prices. Production was based on an earlier ore reserve calculation of c. 10M tonnes (Foslie, 1939).
A total of 5.6M tonnes of pyritic ore was produced, divided into two periods:
1) from 1952 to mid 1976; 3.9M tonnes of purely pyritic ore was mined to produce crushed fine pyrite concentrate (heavy medium flotation method) which was shipped to Germany for the production of sulphuric acid and iron and copper metals
2) mid 1976 to June 1984; 1.7M tonnes of Cu and Zn rich pyritic ore was mined for flotation and production of both Cu and Zn concentrates.
Average grades of 39.11% sulpher and 1.0% Cu were recorded during the first mining period. Yearly production results show large variations in both S and Cu contents, ranging from 36.0-46.3% S and 0.72-1.22% Cu, respectively. The Zn contents in this ore was extremely low (< 0.5% Zn), reflecting both the ore type being mined and the fact that Zn contents in the pyrite fines were penalized by the smelter. Production results from the first years of mining show very high S (40-46%) and low Cu (0.7-1.0%) contents, reflecting the compact, almost purely pyritic ore that was first mined within the thicker parts of the massive orebodies at the northern end of the deposit. Production during the latter years yielded much lower contents in both S and Cu, as mining moved southward into more mixed, Cu and Zn rich compact pyritic ore lenses.
Yearly production yields for this first period averaged 175,680 tonnes, ranging from 93,000 in 1953 to a maximum of 215,000 tonnes in 1965.
During the second period (1976-1984) a total of 1.7 M tonnes of ore was taken out. Yearly mining production rates was slightly higher (averaging 197,267 tonnes) with the maximum tonnage, 231,054 tonnes, being mined in 1977. Both Cu and Zn grades were noticably higher during this second period (yearly average grades of 1.14% Cu and 2.71% Zn) with maximum grades of 1.28% Cu occurring in 1980 and 3.23% Zn in 1981. The higher grades obtained during this period reflect the Cu and Zn richer massive ores mined within the southerly parts of the main massive orebody and the smaller massive ore lenses which occur in the peripheral and southwesterly parts of the deposit: for eg., 20,000 tonnes of massive ore was taken out of the SW ore lens system (between vertical sections 79-85S) which averaged 2.0% Cu and 11.0% Zn.
At the peak of mining activity 183 workers were employed at Skorovass of which c. 55 were underground.
In 1978, in-situ ore reserves were calculated at about 2 M tonnes with an average grade of 1.15% Cu and 2.29% Zn, reflecting the Zn-rich peripheral ore lenses that were being mined towards the end of the mining history at Skorovass. Approximately 1.3 M tonnes still remains within the mineralized zone of the South orebody, an isolated body that lies approximately 5-600 m to the south of the deposit that was mined at Skorovass.
Geological mapping
The first geological mapping done in the Skorovass area was by S. Foslie, between 1922-27 when he covered the whole of the Grong District, which culminated in 1:100 000 map sheet TRONES, edited by C. Oftedal and published by NGU in 1958. The earlier accounts of the geology of the Skorovass deposit was by Foslie (1939) and Oftedahl (1958).
T. Gjelsvik worked as a consulting geologist at Skorovass from 1960 to 1968, which entailed limited underground mapping and drill core logging and a preliminary study of country rock silicate element analyses. His work resulted in three publications on the Skorovass deposit (Gjelsvik 1960, 1967 and 1968).
Sporadic attempt at underground and surface geological mapping of the deposit was carried out by several geologists between 1969-71. In 1971, a period of intense, detailed regional geological mapping was initiated in conjunction with the ¿Grong-Project¿ which started in 1972. Under the guidance of Dr.C. Halls, undergraduates from Imperial College, University of London, were engaged in field mapping projects at Skorovass, resulting in numerous B. Sc. theses between 1971-76 (Hirsinger, V. 1972, Horsley, R.J. 1973, Scott, R.D. 1973, White, R.S. 1974). During the field season 1975-77, Dr. C. Halls and I. Ferriday also carried out a regional mapping survey to the east and south of the Skorovass mine. Several unpublished 1: 10 000 maps and reports to Skorovass Gruber A/S resulted from this study.
A. Reinsbakken, in periods 1973-77 and 1980-82, carried out systematic, underground and surface detailed geological mapping of the orebody and it¿s surrounding rocks, which is accounted for in an NTNF research report (Reinsbakken 1977). Collaboration between Halls, Ferriday and Reinsbakken resulted in several publications on the regional geology, structural and tectonic setting and the massive sulphide deposit at Skorovass (Halls et al., 1977 and Reinsbakken, 1980 a+b) and included a preliminary account of distal exhalite mineralization surrounding the Skorovass deposit (Ferriday, Halls and Hembre 1980 and 1981). A more recent study of distal exhalites at Skorovass has been presented by K. Sand (1986).
Reinsbakken (1992) presented a compilation of all recent geological mapping of the Skorovass deposit in a Dr. Ing. thesis. This included a detailed surface geological map over the deposit and a regional map of Gruvefjellet, E-W vertical sections throught the deposit, study of the ore types and their distribution with chemical analyses, and a detailed study of hydrothermal alteration of the host rocks. (volcanostratigraphy, paleotectonic setting).
Geophysics
First EM geophysics was done on the Skorovass deposit in a period between 1935-38. First reconnaisance geophysical work started after opening of the mine was in 1959-60 (Singsaas, 1959 and 1960). With the start of the first ¿Grong Project¿ in 1971, came a period of intense geophysical activity on the Skorovass deposit on Gruvefjell and into the surrounding district, for the most Turam and Slingram EM and VLF but included also IP and SP and drill hole surveys such as CP and PP (provocated potential EM) and conductivity measurements. A seismic survey was also carried out on the Skorovass deposit (Sindre, 1973) and in 1993-94 a detailed air-borne helicopter geophysical survey was conducted over the Skorovass deposit and surroundings by NGU as part of the ¿Grong Project¿, a joint cooperation between NGU and Nord-Trøndelag Fylkeskommune.
EM (Turam and Slingram): Singsaas (1959 and 1960), Eidsvik (1970 and 1971).
VLF over north Gruvefjell: Dalsegg (1973) and Eidsvik (1973).
CP measurements in drill holes from Skorovass orebody: DDH 10050 (Dalsegg, 1977), DDH 10035 and 100071 (Eidsvik 1972, 1973, 1974, 1976 and 1977).
Conductivity measurements in DDH 10071 (Dalsegg, 1979).
IP measurements (Eidsvik, 1973, 1974 and 1976).
SP measurements over orebody (Logn, 1971).
PP measurements (Eidsvik, 1976 and 1977).
Geochemistry
The first attempt to study lithogeochemical variations in the country rocks hosting the Skorovass ore body was by Gjelsvik (1967 and 1968).
Reinsbakken (1992) presented a detailed study of lithogeochemistry of the volcanic rocks in the volcanostratigraphy hosting the orebody and the hydrothermal metasomatic alteration related to the feeder-stringer zone beneath the massive ores at Skorovass. Reinsbakken also presented a detailed petrological and chemical investigation of the varies ore types found at Skorovass.
Karlstrøm (1990) presented precious and base element data (34 major- and trace- elements analysed by ICP and fire assay methods) of 8 samples of various ore types supplied by A. Reinsbakken.
K. Sand (1986) presented chemical data on distal exhalite (vasskishorisonter) mineralization surrounding the Skorovass deposit.
Diamond Drilling Program
The first diamond drilling done on the Skorovass deposit stems from 1913-16. The largest part of the pre production drill was carried out in a period between 1935 and 1938 in which time 55 diamond drill holes were drilled under the leadership of S. Foslie. This resulted in an ore reserve calculation of c. 10 M tonnes (Foslie, 1938).
A total of 168 surface exploration hole were drilled after production start in 1952 and up to the termination of mining in 1984, along with 4-5 deep holes that were drill under the orebody from within the mine. Surface drill holes at Skorovass can be broken down as follows:
Main ore zone: 137 holes of variable length, from 50-300m.
South orebody: 20 drill hole varying between 250-450m in length.
Finnkjerring hullet (southeast of South orebody, north side of Drikkevatnet): 4 old and 2 newer drill holes.
Drikkevatn skjerp (immediately west of starting point for water pipeline): 3 drill holes on showing and 5 hole on EM geophysical anomally that streches to south.
Grubtjønna skjerp (rusty sulphide disseminated zone c. 4-500m WSW of Gruvetjønna, which lies c. 1.3 km SE of the mine entrance at Skorovass): 3 drill hole of variable length between 50-150m.
Description of Showings at Skorovass
Main Showing (Gammel Gruva): UTM E409350/ N7168350
This showing consists of a south trending flat adit that is driven about 4-5m into very pale, massive pure pyrite that is devoid of any base metals (originally referred to as ¿vasskis¿, literarilly translated as white sulphide). The massive pyrite body here appears to be about 4-5m thick and represents the northern extension of the East Orebody at Skorovass.
The massive pyrite horizon (lens) lies along a minor thrust-shear zone, at the contact between schistose metabasalts above, to the south, felsic volcanite lenses and pyroclastics/tuffs to the east and strongly sheared, pale coloured quartz-albite, quartz-chlorite and quartz-sericite rich rocks below to the north, that are strongly impregnated with disseminated sulphides and cut by a network of quartz-pyrite veins. These rocks, below the massive ore zone, are interpreted as forming the upper parts of the feeder-stringer, hydrothermal alteration zone to the main massive suphide ores at Skorovass. This footwall alteration-feeder zone to the Skorovass orebody mineralization outcrops to the north of Gammel Gruva and forms a large rusty zone between Gamel Gruva and the old work shops/office complex and waste dump at the main mine entrance.
Several N-S and E-W trending trenches are found within the large rusty zone to the N-NW of ¿Gammel Gruva¿ and stems most likely from the pre-1910-13 exploration activity.
Geological History
The Skorovass deposit occurs in a mixed mafic and minor felsic metavolcanite sequence, at the boundary between two major volcanic units (Lower and Middle Volcanite Units) and at a level that is characterized by explosive felsic volcanism and fumarolic activity.
The volcanic complex (Gjersvik Group)at Skorovass has been divided by Reinsbakken (1992) into three major units, the Lower, Middle and Upper Volcanite Units. The Lower Volcanites consists dominantly of metabasaltic flows of immature Island-Arc nature that contains a thick sequence of pillowed flows at the top of this unit that are characterized by Fe-Ti ferrobasalts that are related to initial stages of rifting in the arc. The massive sulphide mineralization of the Main Orebody lies on top of this ferrobasaltic unit. Stringer-zone type sulphide veining and dissemination mineralization lying within intensely altered quartz-chlorite and quartz-albite-sericite rich rocks occur within the footwall of the massive orebody extending deep into the footwall, cutting down through the ferrobasalts and basalts of the Lower Volcanite Unit.
The Middle Volcanite Unit, above the Main orebody, consists of a mixed sequence of metavolcanites that are characterized by ¿rift-type¿, strongly differintiated metabasalts, basaltic andesites and minor andesites. Feldspar-phyric felsic massive flows/intrusive and explosive breccias/tuffs dominate the upper parts of the Middle Volcanite unit. Smaller Zn and Cu rich massive sulphide lens of the SE and SW orebodies occur at or near the top of the Middle Unit and are overlain by jaspilite horizons that marks the top of the Middle volcanite Unit.
Cu and Zn bearing stringer zone mineralization occur within quartz-albite and quartz-sericite altered felsic volcanites below these massive sulphide ore lenses.
The jaspilitic exhalite bodies that lie at the top of the Middle volcanites are overlain by pale coloured, ¿rift-type¿ primative Ca-Mg rich metabasalts that form the Upper Volcanite Unit, of variable thickness. Minor quartz-phyric felsic extrusives/intrusive bodies, often associated with jaspilite bodies, occur at the base of or within the lower parts of the Upper Volcanite Unit.
The footwall stringer-zone below the massive pyritic ore lens of the Main Orebody consists of a thick, quartz-pyrite network system of veins and dissemination mineralization lying within intensely altered rocks rich in quartz-chlorite and quartz-albite-sericite. These form the feeder zone to the Main Orebody, which is dominated by pyritic sulphide mineralization with little to trace amounts of base metal sufide except for Fe.
Similar stringer-zone mineralization is also found beneath the Zn and Cu rich ore lenses of the SE and SW ore zone, cutting up through the felsic volcanites and breccia/tuff sequence below. The stringer zone here is also quartz-pyrite dominated, but are often rich in both Cu and Zn rich sulphides and occur in quartz-albite and quartz-sericite rich hydrothermal alteration envelopes.
The orebody and the surrounding host-rocks have undergone three main phases of deformation under lower greenschist facies metamorphism, which has given the orebody and surrounding rocks it¿s present, complex, lensoid, en échelon geometry.
Description of deposit.
Caledonian tectonic deformation has almost completely destroyed all hints of primary lithological layering, and greatly transposed the original massive ore horizon into several elongate, en echelon parallel trending ore lenses. The massive ores and underlying stockwork and disseminated mineralization of the feeder zone have been greatly disrupted, folded and streched into sub-parallel adjacent zones in which the primary relations have been almost completely destroyed.
At Skorovass, the greater part (c. 90-95%) of the total economic (S, Fe, Cu, Zn and Ag-Au) mineralization is concentrated in the massive sulphides. The stockwork and disseminated sulphide mineralization forms only a minor part of the economic sulphide concentration.
The present configuration of the Skorovass deposit shows an en echelon array of closely spaced massive sulphide lenses within a north-south to northeast-southwest oriented, elongate ore zone. The ore zone has a width of ca. 200 m., a thickness of up to 50 m. and an overall length in excess of 800 m of that what was mined out by 1984. Known massive sulphide mineralization does however, extend well to the south of the mined ore lenses, occurring in what is called the Southern Oreboby. This orebody occurs stucturally beneath the Main Orebody at Skorovass and does not appear to be related to the same stratigraphic level as the Main orebody.
Trace and minor metallic elements recorded in analyses from bulk samples of the orebody give the following representative averages: Co 100 ppm; Ni 20 ppm; As 300 ppm; Ag 10 ppm and Au 0.1 ppm. Cadmium is noticeably enriched in sphalerite rich ore facies, reaching values of several hundred ppm, and Mn reaches similar values in the pyritic facies (Halls et al., 1977).
Ore Mineralogy.
The main metallic minerals found in both the massive ores and the footwall stockwork veins and disseminated mineralization are dominated by pyrite, sphalerite, chalcopyrite and magnetite in decreasing order of abundance. Accessory amounts of Galena, tennantite, arsenopyrite and hematite are also present. Pyrrhotite and ilmenite occur only in trace amounts. Pyrrhotite only occurs as rounded inclusions (small blebs <50 µm) together with sphalerite, occurring within larger idioblastic pyrite grains.
The princple gangue minerals are quartz, calcite and chlorite together with minor sericite, albite, actinolite and trace amounts of stilpnomelane and Fe- amphibole (grunnerite?).
Orebody.
The deposit consists of a mineralized zone that is roughly 8-900 m long, 300 m wide and maximum 50 m thick, that is roughly flat lying and trends roughly N-S, plunging gently to south at it¿s northerly extent, being more flat within the central parts and then rising gently at it¿s southern extension.
At it¿s surface exposure, the northernmost part of the deposit consists of a thick zone of pyritic dominated disseminated and veined sulphides within a strongly schistose, chlorite-quartz rich rock that is interpreted to be the central, top part of the feeder-zone. The disseminated and veined mineralization trends southward abruptly into a series of massive pyritic lenses that form as a series of disjunctive lenticular arrangement of ore lenses. The larger lenses have been referred to as individual orebodies: the major massive sulphide ore lenses are; Main (M), East (E), Southeast (SE), Southwest (SW) and Lofte (L).
The massive orebodies form subparallel and stacked, elongated lenses that trend S-SSW, a liniation (L1-2) that roughly paralles early fold structure (F1-2). Between the major ore lenses, minor ore lenses occur within strongly sheared rocks that trend in a SE direction along with mineral and rock fragments. This SE direction represent minor folds and fragments that have been highly flattened and sheared in zones of high strain, rotated into the transport direction of the larger nappe structures.
Tectonic deformation of the ore.
The deposit is considered to be almost completely metamorphosed, as primary textures are seldom or only rarely preserved. Mobilization and redistribution (remobilization) of chalcopyrite, galena and tennantite are typically related to D3 structures and later episodes of more brittle deformation.
Metal Zonation
There is a very strong zonation in the distibution of Cu and Zn within the Skorovass deposit. At the northern end of the deposit, interpreted as the top, central part of the feeder-zone, the disseminated mineralization and massive sulphides of the Main and East orebodies are dominatly pyritic and contain only minor amounts of Cu and Zn. Production resulting from the first years of mining within the northern part of the deposit, showed very high S (40.0-46.0% S) and low Cu (0.7-1.0% Cu) and extreme low Zn values, reflecting the compact, almost purely pyritic ores that was first mined within the thicker parts of the massive orebodies at the northern end of the deposit. Production during the latter years yielded much lower contents in both S and Cu, as mining moved southward into more mixed, Cu and Zn rich compact ores. Both Cu and Zn grades were noticably higher during the latter years of mining at Skorovass, with maximum yearly grades of 1.28% Cu and 3.23% Zn. The higher grades were from the Cu and Zn rich parts of the Main, SE and the smaller ore lenses of the SW orebody which occur in the peripheral and southwesterly parts of the deposit; for example, 20,000 tonnes of ore was taken out of the SW ore lens system (between sections 79-85S) which averaged 2.0% Cu and 11.0% Zn.
The southern ore lenses, SE and SW orebodies, are interpreted as having formed at the upper stratigraphic levels within the Skorovass ore system. |
