GEOCHEMISTRY OF ZINC-LEAD-COPPER-MINERALIZING
SOLUTIONS ASSOCIATED WITH SKARN FORMATION
Lasse Telstø & Tom V. Segalstad
Mineralogical-Geological Museum, University of Oslo, P.O. Box 1172, N-0318 Oslo,
Norway
During the development of the Permian Oslo Rift the Drammen Batholith intruded the Paleozoic
sediments. The hydrothermal activity associated with the Drammen Granite led to the formation
of intra plutonic fracture filling deposits of Mo, and skarn mineralizations with Zn ± Pb ± Cu
and Fe ± Cu ± Bi in folded Silurian limestones and shales. During the period 1729 to 1913 there
was extracted approx. 1700 tons of Zn, 600 tons of Pb, and 36 tons of Cu from sphalerite, galena,
and chalcopyrite, respectively, from the Konnerud mines. Thermochemical modelling of mineral
stabilities shows that the hornfels from the Konnerud deposit was formed between 580 and
700EC at log fo2 between -21 and -16 bars. The modelling further shows that the skarn formation
took place in the temperature interval 420-490EC at log fo2 between -27 and -23 bars. The
mineral reactions during the skarn formation were buffered near the Ni-NiO-buffer. The ore
mineralizing stage can be divided in two: (1) Sulfide deposition. (2) Hematitization.
Contemporaneous with the sulfide deposition and the hematitization, chloritization of skarn and
hornfels took place, together with precipitation of gangue minerals, in penetrating fractures. Fluid
inclusions in quartz from both stages are fluid dominating two-phase inclusions. The formation
temperatures from fluid inclusions for the sulfide deposition and the hematitization are 280 and
180EC, respectively, at a hydrostatic pressure correction of 300 bars from a reconstructed paleo
depth of ~3 km. The salinities for the sulfide deposition and the hematitization are 6 and 0.3
weight % NaCl equivalents, respectively. From the sulfide deposition to a late ore mineralizing
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13
stadium the * O(SMOW) and * C(PDB) for aqueous carbon compounds, in isotopic equilibrium
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13
with calcite, must have changed. A change in * O from 18 to 23 ‰, and in * C from -5 to -4
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13
‰, show that the aliquot of sedimentary O and C (* O . 32 ‰ and * C . 5 ‰) in the
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hydrothermal solution must have increased, compared to the magmatic aliquot of O and C (* O
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. 9 ‰ and * C . -22 ‰). Computations of * Ow for water in equilibrium with calcite, formed
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during the main sulfide deposition stage (* Ow . 0.9 ‰), and a late ore mineralizing stage
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(* Ow . -1.6 ‰), show that the aliquot of crustal water (* Ow . -3 ‰) increased in the
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hydrothermal water, compared to the aliquot of magmatic water (* Ow . 6 ‰). * S data and
formation temperatures from the fluid inclusions show that sulfur has not been in isotopic
equilibrium in coexisting sulfide minerals at the sulfide deposition. Modelling of the metal and
sulfur budgets show that metals and sulfur both came from the mantle, assimilated local alum
shale, and the host rock, which may have contributed to the apparent disequilibrium for the sulfur
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isotopes. * S for aqueous H2S during the sulfide deposition is, from sulfide isotope analyses,
calculated to be approx. -5 ± 3 ‰. In order to explain sulfur isotopes with such low values, we
suggest a model where SO2 was degassed from the Drammen Granite magma via e.g. the Lindem
Explosion Breccia and faults, at a temperature above approx. 800EC. Thermochemical modelling
shows that all metals were transported as chloride complexes, and that aqueous H2S was the most
stable aqueous sulfur compound. Sulfide deposition occurred when the magmatic water was
diluted with crustal water. This led to a change in temperature and pH (from approx. 4 to 5). The
depositional efficiency for the metals Zn, Pb, Fe, Cu, and Ag was calculated to be more than 99
%. Thermochemical modelling shows that the hematitization occurred at a pH of approx. 6
during oxidation from log fo2 -45.6 to -39.6 bars. At log fo2 -39.6 bars and 180EC approx. 200
ppm Cu can be carried in the aqueous solution. In this way Cu in chalcopyrite is dissolved by the
formation of hematite, as observed in the paragenesis from the Konnerud skarn deposit.
Translated from:
Telstø, L. & Segalstad, T.V. 1999: Geokjemi av sink-bly-kobber-mineraliserende løsninger
tilknyttet skarn-dannelse. GEONYTT, Norsk Geologisk Forenings Landsmøte, Stavanger,
January 6-8, 1999, p. 97-98.
The mineralizations in the Konnerudkollen Mines
translated excerpts from
Mineraldannelsen i Konnerudkollen gruver
by
Tom V. Segalstad & Lasse Telstø
Published in:
BERGVERKSMUSEET SKRIFT (Kongsberg) No. 20, 35-39; 2002.
ISBN 82-91337-23-3; ISSN 0800-1855.
MINERAL
HORNFELS
FORMATION
SKARN
FORMATION
Plagioclase
———————————
Garnet
———————————
—————
C-pyroxene
———————————
———————
SULFIDE
PRECIPITAT.
OXIDATION
Epidote
————
K-feldspar
————
--—————----
Calcite
————
———————————
———————————
Quartz
———————————
———————————
Fluorite
———————————
———————————
Chlorite
--—————
————--
Sphalerite
—————----
Galena
—————----
Aikinite
—————----
Miharaite
—————----
Pyrite
—————----
-----
————----
———
Chalcopyrite
Pyrrhotite
—————
Sericite
-----
Bornite
Hematite
----------———
——————
Fig. 1. Simplified paragenetic sequence for main minerals in the Konnerud ore deposits.
Unbroken lines represent certain occurrence; broken lines represent uncertain occurrence.
The time axis runs from left to right. C-pyroxene = clinopyroxene (diopside - hedenbergite).
CHEMICAL
ELEMENT
PRODUCTION
DATA FOR
THE
KONNERUD
MINES
ENRICHED IN
MAGMATIC
WATER
AFTER
DEGASSING
POTENTIAL
CONTRIBUTION FROM
HOST ROCKS
SUM
POTENTIAL
CONTRIB.
FROM MAGM.
WATER +
HOST ROCKS
Copper
ca. 36
7225
162
7387
Zinc
ca. 1700
1190
764
1954
Lead
ca. 600
267
676
943
Sulfur
ca. 1000
210
2664
2875
Table 1. Calculated mass (metric tons) for the most important chemical elements in
the Konnerud Mines from different contributors according to Model 3: Metals and
sulfur from the mantle and assimilated alum shale after degassing of sulfur dioxide
(column 3) and potential contribution from the host rocks of the ore deposits (column
4).
Fig. 2. Calculated mineral stabilities projected in the plane pH (acidity) vs. log fo2 (oxygen
fugacity) for late mineral reactions associated with first breakdown and later new formation
of chalcopyrite (cp) together with hematittization (hm), resulting from oxidation along the
arrow. Other abbreviations: py = pyrite; bn = bornite; po = pyrrhotite; mt = magnetite; cc =
calcite (stable above and to the right of stippled line). The diagram is constructed at 180EC
and the following molalities: K = 0.005; Na = 0.05; Ca = 0.01; Ba = 0.001; Mg = 0.004; log
Cu = -3.2; Cl = 0.07; GS = 0.002; GC = 0.006.
Fig. 3. Calculated mineral stabilities projected in the plane temperature (degrees C) vs. log
fo2 (bars) of relevance for the formation of the Konnerud ore deposits. The arrows between
the points labelled 1 to 6 represent the development through time and the fluid evolvement
from its separation from [1] the Drammen Granite magma, [2] the formation of hornfels
(formed by thermal metamorphism), [3] early skarn, [4] late skarn, [5] sulfide mineral
precipitation, and [6] late oxidation (hematitization) of the Konnerud ore deposits. The
diagram is constructed at ca. 500 bars total pressure (chosen between a supposed
lithostatic pressure of ca. 800 bars and a hydrostatic pressure of ca. 300 bars, relevant for
the formation of the deposits); pH = 5; GS = 0.01 molal; GC = 0.01 molal; mole fraction CO2
= 0.1. At higher proportions of CO2 siderite would be stabilized below ca. 230EC at the
expense of hematite and magnetite. References to the most relevant mineral stabilities and
mineral equilibria for hornfels and skarn, see Einaudi et al. (1981). For the construction of
the buffer lines the following fundamental thermodynamic data were used: For hematite
(hm) / magnetite (mt), nickel oxide (NiO) / nickel (Ni), and quartz (qz) + magnetite (mt) /
fayalite (fa) “The QFM Buffer” after Eugster & Wones (1962); for CO2 / CH4 and SO2 / H2 S
after Ohmoto & Kerrick (1977); for [HSO4 ]- / SO2 and [HSO4 ]- / H2 S after Pisutha-Arnond &
Ohmoto (1983). Other abbreviations: ad = andradite; hd = hedenbergite; wo = wollastonite;
cc = calcite.