Phosphorus in different types of ore, sulfides in the iron deposits, and the type and origin of ores at Kiruna

Abstract

In the Kiruna-type iron ores, apatite has been regarded as the most conspicuous mineral. Advocates of the magmatic-intrusive mode of ore emplacement at Kiruna have argued that the apatite crystallized from a parent ore magma. However, a closer examination of deposits classified as Kiruna-type ores shows that they include both apatite-bearing and virtually apatite-free orebodies or parts of orebodies. In general terms this means that apatite distribution is uneven in this type of ore. Along individual layers or bands, however, apatite distribution is persistent. In this respect, there are similarities between the Kiruna-type ores and other types. Apatite banding similar to the apatite-rich Kiruna-type ore is a documented component even in other ore types in environments geologically similar to those of the Kiruna-type ores. There are several examples of skarn-type ores, banded iron-formation-type ores, and massive stratiform sulfide ores where the phosphorus content is as high as in the phosphorus-rich occurrences of Kiruna-type ores.The Kiirunavaara orebody also includes tourmaline and sulfides in the form of pyrite-impregnated ore and pyrite-magnetite veins containing Cu, B, P, W, and Au. On the other hand the Viscaria copper deposit, which is situated stratigraphically below the Kiirunavaara orebody, is also slightly rich in apatite. Within the Kiirunavaara orebody, skarn-banded ore lies directly beneath the apatite-banded ore.Several iron ore deposits, classified as Kiruna-type ores, occur in skarn-rich, volcanic sediment-hosted rocks. Apatite-rich chalcopyrite, pyrite, and pyrrhotite-impregnated gray leptite is found in the wall rock of the iron ores at Malmberget. The Leveniemi iron ore occurs mainly in sedimentary rocks. In the coexisting iron-copper ore deposits at Tjaarrojaakka, high phosphorus contents are often found.In northern Sweden, there is also apatite enrichment in the ores and wall rocks which lie below the Kiruna-type ores. These ores are regarded as sedimentary. The present author believes that a prerequisite condition for apatite banding in Kiruna-type ores would be a large-scale regional enrichment of phosphorus, and not injection of apatite magma in the ores.A possible source of the iron as well as the sulfides in the Kiruna-type deposits of northern Sweden is the spilitic greenstone in the area. The greenstones show both magnetic and nonmagnetic varieties. A 30-km 2 area of nonmagnetic greenstones at Kiruna is part of a larger belt of magnetic greenstones. A decrease in the iron content in these nonmagnetic greenstones by 1 percent would be sufficient to produce 2.4 billion tons of magnetite ore. Other ore components, such as phosphorus and rare earth elements, originated in a way similar to the enrichment of these elements found in the sedimentary interlayers in the greenstones below the Kiruna-type ores.When the Kiruna-type ores are compared to the massive stratiform sulfide ores, many similarities are evident. These include: the shape of the orebodies, the stratigraphy, the ore-wall-rock contact relations, the genetic association with volcanic rocks, the internal features of the ores, and to some extent even the geochemistry. For example, iron is the main metal; there are ore impregnations and ore brecciation in both types of ores; and scapolitization and other alteration phenomena are common in both types of deposits.The so-called magnetite syenite porphyry and the altered, mostly silicified, rocks adjacent to the footwall of the main ore deposits at Kiruna are interpreted as channelway or stockwork mineralization.