Slide 1:
The central parts of the Norwegian Caledonides is known for its high density of
tungsten skarn mineralizations.
Some of them have economic dimensions with more than a million ton of tungsten
grading almost 1 % WO3 or more and with accessory Mo and Au.
The age and genesis of the deposits are poorly understood in that the tungsten skarn
mineralizations can be followed for several kilometres in roughly the same
stratigraphic level where they, contrary to common tungsten skarn, lack any obvious
association with igneous plutons. They share these features with stratabound W-Mo
deposits as for example Massif Central in France and many of the Brazilian deposits.
In this presentation I wish to share the results of our research on one of the most
promising deposits in the Norwegian Caledonides, the Bjellåtnden W-Mo deposits.
Due to earlier studies this deposit is fairly well described. We have now dated this
deposit and have also done stable isotope analysis of crucial phases.
Based on this knew evidence we hope to throw more light on the genesis of tungsten
skarn deposits in the Caledonides as well as stratabound deposits from elsewhere n
the world.
Slide 2:
But first some background information's. The Bjellåtinden tungsten deposits is
classic for most of the caledonian tungsten deposits in terms of the geological setting.
Here, and elsewhere in the Caledonides, the tungsten ore occur in fault complexes and
shear zones that are stratabund in supracrustal lithologies a few hundred meters above
Paleoproterozoic gneis complexes.
The Paleoproterozoic gneiss complexes are exposed in domal culminations.
What you also should note on this figure is that the tungsten-molybdenum ores were
identified at six localities that all occur close to the contact between psammitic and
calcareous rocks and overlying metapelitic lithologies and hornblendic gneiss.
At locality 1 the ore occur in scapolite skarn and quartz-plg veins, at 2 it is only Plgqz veins, at 3 and 4 we have the ore in calcite-quartz veins intersecting granitic
pegmatite. At 5 it is W-Mo mineralized quartz veins and at locality 6 we have a skarn
deposit with both diopside and scapolite as important calcsilicate minerals.
Slide 3:
There is a strong structural control of the ore deposits.
On this figure you can see the thickness of the ore-bearing lithology changes
systematicaly from the hinge to the limbs of the domal culmination.
The greatest thickness appears here and here where we are at the top of foldhinges
and gradually tapers towards much thinner lithogies when going down the limbs.
This size reduction is associated with chocolate tablet buodinage of competent
lithologies.
High grade W-Mo ore occur in the limb situation, particularly where chocolate tablet
boudinage begin to develop.
Slide 4:
On this figure from the previous work that I did on the Bjelåtinden deposits, we can
see why we are mostly finding skarn deposits at the transition between fold hinge and
the limb.
What is happening during compression in the fold hinges is that tens of metres of
marble simply is decomposing entirely whereas the other more competent rock units
only experience a partial size reduction.
Right here, where the marble is tapering out, we have the formation of classical
replacement skarn where a hornblendic gneiss is decomposing to an assemblage of
calcic scapolite, quartz, clinozoizite and actinolite.
Farther down the limb, tungsten-molybdenum ore occur in deformed qz-plg veins.
The idea is that the ore-forming solutions migrated in these vein system. The
solutions migrated from relatively high P in the compressional regime in the limb
towards lower P in the Hinge zone. When the ore-forming fluids first encountered
marble and or hornblendic gneis (a meta-diorite), the high Ca-activity facilitated
Scheelite deposition. Scheelite is a calcic W-mineral.
Slide 5:
So this is what it looks like in the field.
In the image to the left you can see how the marble is seriously battered. Just a few
hundred metres to the left, in the fold hinge, it is pure white and tens of metres thick.
The insert to the lower right is a close-up of scapolite with a yellowish appearance
and actinolite in dark green
The upper right insert is from down the limb where marble is absent. It shows
weakly deformed qz-plg veins intersecting a psammitic schist. In a sence this is the
fossilized plumbing system conveying ore-forming fluids through the system.
We dated both the the vein system and the scapolite skarn together with many of the
other localities not shown here.
Slide 6:
Chemical analysis gave good tungsten grades in both scapolite skarn and Plg-qz
veins,- here called Plg skarn.
Also included in the table are other metals that normally are encountered in classical
tungsten skarn. As you see, they make up insignificant proportions of the tungsten
ore. The absence of these metals is also quite typical of stratabound socalled bregional
tungsten skarn.
The average ore refer to a 2 metres profile intersecting the scapolite skarn. These are
very god grades and explains why a commercial company now has launched an
exploration campaign in the area.
Slide 7:
And here to some of the other curiosities of the Bjellåtinden deposits. Fluid
inclusions were studied in scapolite, quartz, plagioclase and scheelite. And
everywhere, the dominant fluid phase, here shown in scapolite and scheelite, is CO2
with a few mol% CH4.
I doubt that the oreforming fluid actually was almost pure CO2. Rather we are
watching one endmember of the fluid phase after phase separation or fluid mixing that
provoked oreformation – but that is an entirely different talk.
I should say, however, that we also find saline aqueous fluid inclusions, but their
relation to the ore-forming event remains to be unravelled.
Slide 8:
I will not spend much time on fluid inclusions, but will only show you this isochoric
diagram which in combination with the stability of calcic scapolite, tell us that the
ore-forming event occurred at c. 2 kb. Scheelite formed between 350 and 500 oC
with most precipitation at the high end of this scale when both plg and scapolite
formed.
Slide 9:
Finally, on this background information we may go to the Re-Os geochronology.
All technical questions should be directed to my distinguished Co-author, Holly Stein.
We dated relatively coarse-graned molybdenite from four localities.
From locality 1, the scapolite relacement skarns we got an age of 430.4 + -1.4 Ma.
This is the oldest part of the reforming system that we so far have dated.
From locality 2, the plg-qz veins we got two different ages from different molybdenite
samples. The oldest gave us 411.1 and the youngest is 399.2 Ma.
This is followed by locality 3 that represent cc-qartz veins intersecting a granitic
pegmatite. Here we are getting ages in between the previous ages, namely 402.3 Ma.
This is also the case for the last locality that we dated where we got an age of 406.1
Ma from molybdenite schelite mineralised quartz veins up here at locality 5.
Now, ths spread of ages was a huge surprise given that I was convinced that we only
had one mineralising event. Here, either we have 5 separate events or we more or less
have continous mineralization from 430 to c. 400 Ma. I.e. over a 30 Ma timespan.
Probably, given what we know about fluid migration in fault complexes, the ages
represent shortlived episodes of fluid migration during episodes of active faulting.
I knew already from petrographic studies of the scapolite skarn at locality 1 that it
formed during peak metamorphism in good agreement with the 430 age.
However the other ages were great surprises. The young ages corresponds to
exhumation and later extensional faulting of the Scadinavian orogen.
Slide 10:
But first to the 430 age.
Barnes and co-workers conducted extensive U-Pb geochronology of intrusive rocks in
a tectonically higher lying stack of nappes right south of our area. It is known as the
Hlgeland Nappes Complex.
Bear in mind that all the major nappes, including those from our area were stacked on
top of each other at 430. Accordingly, the Bjellåtinden area that we have been talking
abut, was located at a deeper stratigraphic level underneath the intrusive complexes
that Barnes and co-workers dated.
Slide 11:
In this next blow up from Barnes et al. we can see that the largest volume, by far, of
granitic melts were emplaced in batholitic complexes around 430 Ma.
We can not for sure say that the oldest and perhaps richest part of the Bjelåtinden WMo deposits were derived from these magmas, but t is clear that ore-formation
coincide with this massive thermal event.
We do not have igneous ages that coincide with he younger mineralizing events.
However, 400-406 Ma ages coincide with the age of extensional tectonics according
to recent work by Bingen and co-workers. Most of the pegmatites in the central
Norwwegian Caledonides, coincide with this tectonic event.
Slide 12:
To further constrain the orgin of the W-Mo ores. Stable isotope analysis were done
on ore-forming minerals.
Sulphur isotopes were retrieved from molybdenite and some pyrrhotite. Oxygen and
carbon isotopes were gathered from calcite associated with the ore-forming event.
We also compared with the marble calcite.
The first thing you notice is that we have these three nice plateaus with average
carbon values of -5, average sulphur of +6.8, and average oxygen of +13.56.
These near equal values are from samples at three different localities that formed over
this time span of 30 Ma. So whatever the orign of S, O and C, it was a remarkably
homogenous reservoir. The C-values are typical magmatic values. The S-isotpes do
not realy tell us much about the origin of the oreformng fluid, however, it is a
common range for hydrothrmal ore deposits. The O-values could come from granitic
magmas. However, they are very high even for this magmatic source. Accordingly,
we should keep an open mind to the origin of S, O and C.
Originally, when studying this deposit, I gathered that the whole sale decomposition
of marble lithologies would release huge amounts of CO2. Accordingly, it was
tempting to conclude that the omnipresent CO2-fluid inclusions were derived from
this reservoir. However, C and O sotope studies of the marbles tell a completely
different story.
Here we have typical marine values of c. 0 and +22, respectively, hence far from the
values that we see in ore-calcite. Accordingly the CO2 component must have another
origin. Perhaps from deep-seated granitic intrusion.
Slide 13