On the origin of the Falun type of sulfide mineralization

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Abstract
The “Falun type” of sulfide deposits in the Precambrian of Central Sweden has for more than forty years by all geologists active in the region been regarded as genetically associated with the intrusion of the primorogenic (synkinematic) Earlier Svecofennian granites. In a recent paper Koark (1962) has challenged this view and proposed the hypothesis that the sulfide mineralization occurred during the building up of the Leptite Formation in which the deposits are situated — thus before the folding and the associated intrusions — and through an exhalative‐sedimentary process. Koark's argument for this latter view is the frequent presence of Sulfides in iron ores of the region which are regarded as formed through such processes. It is shown here that always, including the specific examples mentioned by Koark, there is clear evidence that in such cases the sulfides were introduced long after the iron ores were formed, and that his examples include one sulfide deposit where the genetic association with a granite has been proved long ago. In the same way it is shown with regard to all other aspects of the mineralization that are relevant in this connection that there is conclusive evidence at hand for the earlier interpretation. This applies to the large‐scale rock alteration and introduction of magnesium which, according to this interpretation, have accompanied the emplacement of the sulfide ores, to the control that folding structures have exercised during this emplacement, and to the sulfide deposits in carbonate rocks, which indicate formation at considerable depths. An example is also referred to which proves that the rocks rich in Al and Mg which are among those accompanying the sulfides cannot have formed through metamorphic migration, as has been assumed by Tuominen & Mikkola (1950) for similar rocks in Finland, a work to which reference is made by Koark. With regard to local aspects of the mineralization in the Falun deposit, to which Koark's paper is largely devoted, only the role of the igneous dikes is discussed, as the general geology will be treated by another author. Comparison is made with the largely analogous sulfide mineralization in the Skellefte district, northern Sweden. After concluding that Koark's arguments have not in any way invalidated the basic principles of the earlier interpretation, some consequences of this interpretation, still unexplained and requiring much further study, are pointed out.

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