3D geochemical modelling of hydrothermal alteration related to 1.89 Ga VHMS-type deposits, Kristineberg area, Skellefte District, Sweden 1*
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Riia M. Chmielowski , Nils Jansson , Mac Fjellerad Persson , Pia Fagerström , Pär Weihed 1 2 Luleå University of Technology, Boliden Mines *
[email protected] The Kristineberg area of the Skellefte District, northern Sweden: One of the most important mining regions in Europe
It is common to identify rock types in altered volcanic terranes using immobile-element ratios, since these ratios do not change as a result of alteration; following the example of Barret et al (2005), these samples have been divided into four varieties of rhyolite, two of dacite, and one each of andesite and mafic using a variety of immobile element ratios. Two of these plots are displayed here.
Formed via accretionary processes in a volcanic arc setting. Massive sulphide deposits occur both within the Skellefte Group and close to the contact with the Vargfors group.
Rhy A Rhy A Rhy B
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Over 2000 lithogeochemical analyses from Skellefte Groups samples, collected from 110 drill holes within this area, have been obtained to build a 3D geochemical model of the hydrothermal alteration which led to the formation of the ore deposits.
800
Rhy C
Rhy B
Dacite A 15
***
600
Dacite B
Zr/Al2O3
Geology after Kathol et al. (2005) and Bergman Weihed (2001)
1000
25
Rhy X
Andesite
10
Dacite A
400
Dacite B
Skellefte Group: Proterozoic (1.9 Ga) greenschist to amphibolite facies felsic- to intermediate submarine metavolcanic rocks
Mafic Andesite
200
5
Vargfors Group: metamorphosed, finegrained turbiditic sedimentary rocks, graphitic phyllites, conglomerates and maficintermediate volcanics
Mafic 0 0
300
Zr/TiO2
600
0
900
some Mg loss
Fractionation curve for volcanic rocks in area Y = 19.932x-0.7306 r2 = 0.768
3D Model: Change in Mg concentration in Rhyolites
Mass Change Calculations
TiO2 wt%
(ΔMg < 0) subtle Mg gain
3D Model
0.4 Mafic Andesite Dacite A Dacite B Rhy A Rhy B Rhy C Rhy X
Alteration line 0.2 for Rhyolite B
0.0 0
100
Zr ppm
300
200
The least-altered samples for each of the volcanic rock types in an altered area can reveal the fractionation trends for the area. This can be used to determine the initial values of the oxides within the altered rocks and permits calculation of the mass change in the rocks as a result of alteration. Fractionation lines after Barrett et al (2005). 25
Expand and refine the preliminary 3D models generated thus far to incorporate the additional information from the mass change calculations for the other rock types in the area. Alteration Vectors
(ΔMg 0 to 0.1) small Mg gain
Use the results of the models to determine the vectors of flow for alteration fluids based on regions of dissolution and accumulation of the various transported elements. Structural Model
(ΔMg 0.1-1.2) medium Mg gain
Al2O3 wt%
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(ΔMg 1.2 - 2.3) large Mg gain
10 Fractionation curve for volcanic rocks in area Y = -0.0231x + 18.693 r2 = 0.736 Alteration line for Dacite A
0.0 0
100
Zr ppm
Mafic Andesite Dacite A Dacite B Rhy A Rhy B Rhy C Rhy X
300
200
The program Leapfrog has been used to generate models. Its interpolation function calculates likely compositional values between known data points.
Use the existing 3D Structural Model (Bauer et al. 2011; Skyttä 2012), to constrain the alteration model to ensure that the vectors of transport determined are plausible.
The above preliminary model shows the change in magnesium (ΔMg) in the samples of Rhyolite with respect to the least altered examples of the various types of rhyolite in the Kristineberg area. The small images arranged to the left show each layer of the model on its own so that their shapes can be more readily discerned. The small yellow/gold objects visible in the in those small images are the locations of the Kristineberg (larger, right hand side) and Rävliden (smaller, left hand side) ore bodies.
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Kristineberg
While this is still a preliminary model (see Future Goals to the right), it is interesting to note that the largest gain in Mg seen in this Leapfrog interpolation coincides with the Mg-rich part of the footwall alteration zone of Kristineberg e.g. the pre-metamorphic chlorite zone, which is commonly found in the core of hydrothermal systems related to VMS deposits.) Similar models have been generated for each element which has been gained or lost during the course of alteration.
(ΔMg > 2.3)
The Kristineberg area as seen in GoogleEarth.
Thin sections
Distribution of Alteration nd a n ran e d vli idmy ies ä R vl od Rä re B O
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r O erg
d o B
b
ne i t s
i
Kr
Alteration Box Plot
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Least-Altered Andesite-Basalt
Colour - AI 5 Equal Ranges AI to 100.0 [100.00%] AI to 95.09009 [80.00%] AI to 92.04713 [60.00%] AI to 79.80000 [40.00%] AI to 53.59801 [20.00%] Size - CCPI 5 Equal Ranges CCPI to 100.0 [100.00%] CCPI to 79.63864 [80.00%] CCPI to 68.60614 [60.00%] CCPI to 60.50605 [40.00%] CCPI to 49.88067 [20.00%]
CCPI
Least-Altered Dacite
50
Least-Altered Rhyolite
25
0 0
60
Thus far the mass change calculations have been applied to rhyolite samples with calc-alkaline affinity. The next step is to extend these calculations to the other rock types within the area
0.6
75
45
Future Goals
0.8
5
30 Al2O3/TiO2
15
0
Fractionation 1.0
Rhy X
Rhy C
Zr/TiO2
Geology modified after Barrett et al (2005)
Rock types
25
50 AI
75
This 3D graph shows the distribution of alteration in the samples with respect to the known location of the Kristineberg and Rävliden orebodies. The orebodies are displayed on the graph above as irregular shapes in shades of yellow and orange. The samples are displayed with their colour varying with the intensity of the Alteration Index, and their size by the intensity of the ChloriteCarbonate-Pyrite index, such that the large red spots have more intense alteration by both measurements than do the small grey ones.Alteration Box plot after Large et al (2001).
Representative thin sections have been produced for 99 samples collected for this project. These span the various rock types and levels of alteration and provide a context for the geochemical data. The above photo collections compare and contrast two samples of Rhyolite B. Sample number LK20120138 has experienced only moderate levels of alteration and still contains feldspar phenocrysts (white flecks in the hand sample) in a matrix of quartz and biotite with dull (yellow-red) interference colours. In contrast sample number LK20120188 has undergone intense alteration and all of the primary igneous textures have been obliterated. The new assemblage contains cordierite porphyroblasts (yellow patches in the hand sample) in a matrix of quartz and white mica with bright (blue-green) interference colours. In each set of photos the upper left photo shows the uncut sample as it was removed from the drill core, and the following three are different views of the same portion of a thin section: the upper right is in plain polarized light, the lower left in reflected light, and the lower right in crossed polarized light.
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References Barrett TJ, MacLean WH, Årebäck H (2005) The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden. Part II: Chemostratigraphy and alteration. Mineralium Deposita 40 (4):368-395 Kathol B, Weihed P, Antal Lundin I, Bark G, Bergman W, J., Bergström U, Billström K, Björk L, Claesson L, Daniels J, Eliasson T, Frumerie M, Kero L, Kumpulainen RA, Lundström H, Lundström I, Mellqvist C, Petersson J, Skiöld T, Sträng T, Stølen L-K, Söderman J, Triumf C-A, Wikström A, Wikström T, Årebäck H (2005) Regional geological and geophysical maps of the Skellefte District and surrounding areas. Sveriges geologiska undersökning Ba 57:1.
Bauer TE, Skyttä P, Allen RL, Weihed P (2011) Syn-extensional faulting controlling structural inversion – Insights from the Palaeoproterozoic Vargfors syncline, Skellefte mining district, Sweden. Precambrian Research 191:166183
Bergman Weihed J (2001) Palaeoproterozoic deformation zones in the Skellefte and the Arvidsjaur areas, northern Sweden. In: Weihed P (ed) Weihed P (ed) Economic Geology Research 1. Sveriges Geologiska Undersökning C 833. pp 46-68
Large RR, Gemmell JB, Paulick H (2001) The alternation box plot: A simple approach to understanding the relationship between alteration mineralogy and lithogeochemistry associated with volcanic-hosted massive sulfide deposits. Economic Geology 96 (5):957-971
Skyttä P (2012) Crustal evolution of an ore district illustrated – 4D-animation from the Skellefte district, Sweden. Computers & Geosciences 48 (0):157-161. doi:10.1016/j.cageo.2012.05.029
