Some gold occurrences and deposits in Finland deviate from the orogenic gold model so much that alterantive mineralising models have been suggested.
The Kuusamo schist belt (KSB) is characterised by Au-Co-Cu mineralisation. The reported tonnage and Au, Co, and Cu grades of the deposits in the belt show considerable variation. (Download Excel table Kuusamo gt data). In a few cases, the resource also includes 0.01–0.03 % U and 0.1 % total rare earth elements.
The main economic interest in the Kuusamo deposits is in Au and Co, whereas Cu and the REE have been regarded as potential by-products and U as a problematic waste. The available data suggest that most of the reported total REE grades in these occurrences comprise light rare earth elements (LREE). None of the KSB occurrences have so far been proven as economic ores, and only one (Juomasuo) has been test mined.
The Kuusamo occurrences are hosted by a clastic sedimentary sequence deposited between 2.44 and 2.21 Ga, which also contains basaltic lavas and indications of evaporates. The sequence was intruded by basaltic dikes and sills prior to regional deformation. All occurrences have a distinct structural control. The largest deposit, Juomasuo, is located in a doubly-plunging part of a northeast-trending anticline.
Alteration in the KSB is extensive and occurred at several stages. The rocks in the belt were affected by three regional alteration events:
- Diagenetic, partial albitisation of feldspars and sericitisation of clay minerals in all sedimentary units.
- Local, partial to total albitisation of clastic sedimentary units and spilitisation of volcanic units, either related to the ∼2.21 Ga intrusion of the mafic sills and dikes, or during the early stages of subsequent orogenic evolution.
- Extensive albite and scapolite alteration and local carbonatisation. The intensity of the last alteration varied from weak (<10 % albite) to strong, locally resulting in almost pure albite rocks (99 % albite, with traces of carbonate, rutile, and quartz. Examples of the intense alteration in the Kuusamo belt.
(Last update: 20.05.2020)