The company that sprung from the Fagelmyren swamp near the Skellefte River in 1924 still mines base metals in the camp that grew around its first ore deposit.
Five mines are operated by
The camp bears a strong resemblance to the Flin Flon camp in Manitoba, where a single company, with large central metallurgical plants, controls much of the ground. The Boliden base metal district, following the development of the original Boliden orebody in 1924, has historically been a camp of small deposits under the control of a single operator.
The geological resemblance is strong too: an early Proterozoic greenstone belt, complexly folded, nicely sprinkled with massive-sulphide deposits. And near the eastern end of the belt, not far from the old Boliden mine itself, is the Petiknas mine.
Petiknas is actually two mineral deposits, Petiknas North, discovered in 1986, and Petiknas South, discovered from an exploration decline driven on Petiknas North in the late 80s. Veteran Boliden geologist Rolf Jonsson is credited with both discoveries.
Arsenious and antimonious mineralization have so far kept Petiknas North out of production, but the southern orebody is a classical multiple-lens massive sulphide deposit, with pyrite, pyrrhotite, chalcopyrite and sphalerite. Boliden brought it into production in 1992 from the Petiknas decline and also drove a haulage drift from the Renstrom workings, about a kilometre to the southwest.
At Petiknas South, the footwall host rock is a chloritized and sericitized rhyolite, in surprisingly sharp contact with the two largest massive sulphide lenses, B and C. The C lens is a pyrite-pyrrhotite-chalcopyrite-sphalerite zone with higher copper grades, and is interpreted as being near the exhalative vent. Zinc grades are higher in the B lens, which is thought to be on roughly the same horizon as the C, but a distal facies.
Three smaller lenses, A, A-2, and D, sit higher in the stratigraphy, in a hanging-wall andesite sequence. The mineralization in these lenses has not, for the most part, been brought into Petiknas’s reserves, which were estimated at year-end to total 2.8 million tonnes grading an average of 4.8% zinc, 0.9% copper, 0.9% lead, 2.6 grams gold and 103 grams silver per tonne.
Unusually, precious metal grades are very similar from one lens to the next, not showing the usual associations of gold in the copper zone and silver in the lead-zinc zone. The precious metal grades put Petiknas ore among the sweetest in the Boliden area: averaged camp-wide, gold and silver grades were 1.7 grams and 64 grams in 1999.
Petiknas North consists of two lenses, A and B, in the same rhyolite sequence that hosts Petiknas South, but separated from the producing deposit by a fault, dipping moderately to the north. The fault is interpreted to have a reverse movement — that is, up and over the Petiknas South orebodies. The sense of movement on the fault has led Boliden geologists to speculate that a faulted continuation of the northern lenses may lie deeper and farther to the north.
Petiknas North has a resource comparable to its neighbour: 2.7 million tonnes (in all categories) grading of 1.5% copper, 2.8% zinc and 0.24% lead, plus 4.4 grams gold and 52 grams silver; but because of its relatively high arsenic and antimony concentrations in the mineralization–4.7% and 0.17%–it has remained dormant since its discovery.
Metallurgical testing on a 28-tonne bulk sample from Petiknas North showed that bio-oxidation and cyanide leaching gave good recoveries, but were expensive.
Ground at the Petiknas mine is generally good, though the western ends of the B and C lenses approach the large reverse-fault and conditions are poorer there. In poorer conditions, Petiknas staff have found they need relatively dense rock bolting, but not screening.
The Petiknas operation uses some Alimak shrinkage stoping and sub-level open stoping, but cut-and-fill mining, valued for both its production virtues and its contribution to ground control, is the standard. “We see cut-and-fill as an excellent base because you get a steady flow of ore,” says Peter George, mine captain at Petiknas, who also describes Petiknas as “probably the most modern underground mine” in the Boliden camp.
The decline from which Petiknas was developed currently reaches 690 metres in vertical depth, and will ultimately go to 774 metres. Most of the current development is at depths of 300 to 500 metres, exploiting the B and C zones.
The haulageway to Renstrom takes about 40% of the Petiknas ore, and the rest is hauled up the Petiknas decline. Reliance on cut-and-fill and Alimak shrinkage also limits the amount of waste rock that needs to be hauled to surface, another important consideration in a mine where much of the haulage is up a decline.
The ores from all the deposits are trucked to the Boliden mill, whose copper concentrates go down the road to the company’s Ronnskar smelter (T.N.M., May 22/00). Its zinc concentrates are shipped to the Norzink zinc plant, southeast of Bergen, Norway, Boliden’s joint venture with
About 1.6 million tonnes go through the mill annually, with Petiknas picking up just over a third of that total last year. Only the Kristineberg mine, 90 km to the northwest and the camp’s oldest mine, produced as much; Renstrom and the camp’s two other mines– Akerberg and the Kedtrask open pit–produced about 375,000 tonnes between them in 1999. A sixth mine, Langdal, closed in 1999.
Boliden is considering adding a precious-metal leach circuit to the existing mill, to handle ores with higher gold and silver grades.
A new deposit, the Maurliden massive sulphide, is scheduled to go into production later this year, hoisting an average of 250,000 tonnes yearly. Renstrom, in production since 1952, had appeared to be pinching out at depth when development reached 1,000 metres, but the mine unsheathed its secret weapon in 1997 with the discovery of two new zones, Renstrom Deep and Simon. These two zones show unusually high-grade zinc and lead mineralization–zinc grades in drill intersections have frequently been in the double digits–and are up to 20 metres wide, averaging about 5 metres true width.
Boliden has a major drilling program under way on both zones this year, aimed at showing whether the mineral resource justifies an extension to the existing shaft, which is just over 900 metres deep.
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