Några aspekter av skarnmalmsproblemen i Bergslagen

Abstract

Some aspects of the skarn ore problems in Central Sweden. According to the interpretation of the non-manganiferous skarn-bearing iron ores of Central Sweden that was given by Magnusson and the present writer 15 years ago and has later been repeated (Geijer and Magnusson 1944, 1952 a, 1952 b), this group is not genetically homogeneous. It contains >primary skarn ores>, of pyrometasomatic origin, and more numerous >reaction skarn ores> that were originally formed by sedimentation or by replacement at low or moderate temperatures, and later changed through interior reactions caused by regional heating. This metamorphism, like the development of primary skarn ores, took place during the epoch when the containing leptite formation was folded and invaded by the Earlier group of Svionian granites. Further, the >magnesia metasomatism> that accompanied these events has wrought great chemical changes in many of the earlier formed deposits. In a textbook (Magnusson 1953), however, these views have been presented in a somewhat modified form, the possibility being indicated that the deposits interpreted as such with primary skarn may all be reaction skarn deposits altered by the magnesia metasomatism.

Here is therefore presented the chain of evidence on which interpretation of certain skarns as primary is based. It starts from those deposits that carry borates, chiefly ludwigite. It is shown, with reference to later works by Watanabe (1943), Tilley (1951) and Holser (1950), on deposits in other countries, that the paragenesis of the Swedish deposits in question is the same characteristic one that is found in various occurrences of such borates the world over. Further there are gradations to other skarn types, also with a pronounced magnesian nature. Some special examples are discussed in detail. It is maintained that new scientific developments have corroborated the earlier interpretation.

As to the original nature of the non-manganiferous reaction skarn ores, the writer shares Magnusson's view that they probably were, in most cases, originally quartzbanded sedimentary ores deposited in close connexion with beds of limestone or dolomite. The fact that such skarn ores occur chiefly in areas of extremely sodic leptites, in which ordinary quartz-banded ores are rare, should thus mean that in this setting sedimentary deposition of iron and silica took place only in close connexion with such of carbonate rocks. There are also indications that the phosphorus percentage was lower than it is normally in the quartz-banded ores proper.

Among proofs of an originally sedimentary origin of a skarn ore deposit there have been cited the occurrence of magnetite pseudomorphic after hematite, and stratification (Magnusson 1953). The former criterion is here shown to fail, as the texture of the only known case of such pseudomorphs in the region is incompatible with the interpretation that the hematite plates once formed part of a bedded deposit, but is analogous to occurrences in pyrometasomatic deposits. As to stratification, true such of sedimentary origin is certainly common among the reaction skarn ores. Caution is necessary, however, when extending this interpretation, as selective replacement may produce deceptively similar results, special reference being made to the banding of ludwigite-bearing ore in the Banat and to the banded >tactite> described by Holser (l. c.) from the Philipsburg district in Montana.