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Sample Tså-1 (Tinnsjå, Telemark)
The sample was supplied by Thorium Norway AS. The Tinnsjå copper-silver property includes the Hovin (Vasstveit) deposit, which was mined intermittently at the end of the 18th Century and from 1903-04, 1908-10 and 1912-1922, both underground and as an open pit (NGU, 2006). A 3-hole diamond drill (1963) and reconnaissance exploration program (2005) was undertaken by the Geological Survey of Norway (NGU, 2006). The prospect is today under evaluation by the claim-holders. At Tinnsjå, copper-silver ore occurs as a ca. 20 m wide vein, with abundant coarse calcite, associated with a ca. 2 km-long amphibolite dyke that lies concordant within a fine-grained Neoproterozoic quartzite (NGU, 20061). The appearance and composition of the ore is said to be identical along the strike of the vein. Bornite, chalcocite and chalcopyrite are described as the dominant ore minerals; gangue phases are quartz, feldspar, amphibole and calcite. The sample was studied using conventional reflected light microscopy and also using scanning electron microscopy (SEM) at the Natural History Museum, University of Oslo. Figures 3-7 are so-called „back-scattered electron images‟, obtained using the SEM. The shades of grey in the images correspond to the average atomic mass of each mineral. Contrast and brightness differ, however, from image to image.

Polished section description
The sample is a semi-massive sulphide ore, containing ca. 40 vol.% ore minerals. Gangue calcite, minor quartz and feldspar (from the vein) make up the remainder of the sample. The Cu-sulphides appear blue in hand specimen and when polished are revealed to be a coarse (mm-scale) symplectitic intergrowth of bornite (Cu5FeS4) and chalcocite (Cu2S) (Figs. 1 and 2).

1 NORGES GEOLOGISKE UNDERSØKELSE (NGU) (2006): Hovin. Forekomst nr. 2 i Tinn kommune. Online ore database (malmdatabasen), entry for deposit 0826.002. http://www.ngu.no/no/

This mutually-intergrown texture between bornite and chalcocite results from unmixing during cooling of a relatively high-temperature “forerunner” phase. It is fairly common in Cu-rich veins and is generally a good indication that the ore will also carry significant quantities of silver, bismuth and possibly other elements. These minor elements are incorporated in the high-temperature phase and are exsolved out to form discrete minerals upon cooling.

Fig. 2 Back-scattered electron images of the intergrowth between bornite (darker on the images) and chalcocite
(the lighter mineral). Both minerals also form larger, more homogeneous masses.

One characteristic component of this type of ore is the mineral wittichenite, Cu3BiS3 and, indeed, this is observed in section Tså-1 (not mentioned in previous reports2). It occurs as small (typically < 50 μm, seldom up to 100 μm in size) rounded blebs within the chalcocite matrix (Fig. 3). Silver is present in the sample as stromeyerite [(Cu,Ag)2S], typically occurring as fine-grained exsolutions within chalcocite (Fig 4, left). Although stromeyerite was not so very abundant in the sample, I would suspect, based on observations of other Au-Ag deposits, that stromeyerite would be one of the main Ag-bearing minerals at Tinnsjå. The presence of hessite is mentioned in earlier reports, but was not seen in sample Tså-1.

loser inspection at higher magnification reveals a still more interesting mineralogy in the sample. A dozen or so acicular crystals are seen, exsolved within bornite, but always together with chalcocite (Fig. 4, left and Fig. 5). These correspond to the extremely rare sulphosalt mineral, larosite, (Cu,Ag)21PbBiS133. The grains are typically 10-40 μm in length, with a maximum width of 10 μm wide. In reflected light, these are a white brownish grey colour and displays modest anisotropy. Larosite crystals are readily seen in back-scattered electron images 3

2 Wittichenite is a common mineral in many of the Telemark Cu-(Ag) occurrences, e.g., NORDRUM, F.S. (1972): Wittichenite, Cu3BiS3 from Tokke and Fyresdal in west Telemark. Norsk Geologisk Tidsskrift 52 (3), 257-271.

3 The rare sulfosalt larosite was described (Petruk, 1972), occurring as microscopic needle-like crystals up to 300 μm in length and 25 μm in width, from a chalcocite-stromeyerite assemblage in a vein from the Foster Mine, Cobalt district, Ontario, Canada (i.e., remarkably similar to the Tinnsjå occurrence). The formula was given as (Cu,Ag)21PbBiS13 and the powder pattern was indexed to an orthorhombic cell. The mineral has not been identified from other localities since discovery of the type material, making the Tinnsjå occurrence only the second worldwide.

PETRUK, W. (1972): Larosite, a new copper-lead-bismuth sulphide. Can. Mineral. 11, 886-891.

(Fig. 1) to be homogeneous. I would not expect larosite to be a major silver-bearing mineral at Tinnsjå, but its presence of considerable mineralogical interest.

Fig. 3 Back-scattered electron images of the intergrowth between bornite and chalcocite and small exsolved grains of wittichenite (appearing bright white on the images). Wittichenite is not exsolved within bornite and chalcocite, but typically occurs at the boundary between the two minerals. 

Fig. 4 Back-scattered electron images. (top left) Flame-like exsolutions of stromeyerite (white) in chalcocite (medium grey). Mineral to right of image is bornite. (topright) Small bleb-like exsolutions of wittichenite (white) within chalcocite (medium grey), itself enclosed within bornite (darker grey). In the lower right of the image, another chalcocite inclusion hosts acicular (needle-shaped) grains of the rare sulphosalt larosite.

Recommendation
Tinnsjå strikes me as a highly interesting deposit, with potential for both copper and silver, provided there are sufficient ore reserves for future production. Additional microscopy is strongly recommended as it is difficult to evaluate any deposit based upon a single sample (note, for example, the absence in this sample, of molybdenite and galena noted in previous reports). The dominance of (exclusively) Cu-rich sulphides infer high grades and relatively easy processing. It would be particularly worthwhile to gain a better understanding of the silver distribution. My instinct suggests that the majority is probably in solid solution within the chalcocite, with a variety of minor and trace minerals (stromeyerite, hessite, larosite – possibly also argentite, mckinstryite, jalpaite and other sulphosalts?) containing the remainder.

Nigel J. Cook (Prof.)
Natural History Museum (Geology)
University of Oslo

03.12.2008

Silverprice last 10 years

In 2006, NGU assessed the deposit
to be of economic significance.