'Lithogeochemical Classification of Hydrothermally Altered Paleoproterozoic Plutonic Rocks associated with Cold Mineralization: Examples from the Nanortalik Cold Belt of South Greenland and the 'Gold Line' of Northern Sweden'

摘要

The relationship between granitic rocks and gold in orogenic or intrusion-related gold systems has always been problematic because the timing of mineralization relative to magmatism is often equivocal. Furthermore, distinguishing and classifying granite and diorite is also a challenge, because, in many cases, hydrothermal alteration makes the applicability for some of the major oxide classification diagrams doubtful. In this contribution, we use lithogeochemistry of immobile elements to investigate the relationship between gold mineralization in Sweden and Greenland and spatially related intrusive rocks. Although lithogeochemical rock classification is well established for the volcanic rock series of basalt-andesite-dacite-rhyodacite-rhyolite (including those which have experienced intense alteration), suitable equivalent rock classification diagrams for altered plutonic rocks, particularly granitoids, are lacking. Rock classification diagrams based on major oxides are useful for unaltered rocks but are inappropriate for altered rocks due to the mobility of major elements during alteration. For example, during alteration K, Na, Ca, Si, Fe, Mg have been shown to be mobile due to metasomatism. In this contribution, we discuss how rock classification diagrams for granitoids and diorite based on major elements are unsuitable for hydrothermally altered rocks and suggest more appropriate diagrams based on immobile elements. We present an example of a classification of altered granitoid rocks and diorite that occur in the gold provinces of northern Sweden (the "Gold Line") and in South Greenland (the Nanortalik Gold Belt): in both belts, intrusive rocks are spatially associated with gold mineralization. However, it is unclear if these intrusive rocks are genetically related to the gold mineralizing events or if the intrusive bodies could have triggered hydrothermal alteration and/or gold mineralization, or that the intrusive bodies simply acted as structural traps for gold-mineralizing fluids during deformation.