Textural evolution and trace element chemistry of hydrothermal calcites from Archean gold deposits in the Hattu schist belt, eastern Finland: Indicators of the ore-forming environment

Abstract

Orogenic style gold mineralizations in the Archean Hattu schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary rocks, volcanic rocks, and felsic intrusives. The gold deposits are related to extensive quartz-dominated vein networks and the gold mineralization occurs as dissemination in altered wall rocks and within hydrothermal quartz veins. The quartz-dominated veins frequently contain calcite, which is also present within the country rocks. We analyze and compare the major and trace element chemistry of these calcites and test whether they carry the signatures of the origin of hydrothermal fluids associated with the gold deposits or record fluid interaction with the local wall rocks. The calcites show textural growth and chemical zoning, considerable chemical variations within samples and the chemical data do show some overlap between the samples. All calcites contain dominantly Ca with other elements being typically below 2 wt%, and they are accordingly classified as rather pure calcites. Amongst minor elements, both Mg and Fe show larger variations than Mn. The concentrations of most trace elements analyzed are relatively low, except for Sr and Y, which can attain values up to 1 wt% and 200 ppm, respectively. Some of the trace elements or trace element ratios in hydrothermal calcite, including Sr, Y, Mn, (La/Lu)N and ∑REE, show correlations with the respective host rock lithologies. This indicates that interaction between hydrothermal fluids and adjacent and diverse host rocks has strongly affected the trace element chemistry of calcite. Chemically variable growth zones in large calcite crystals and distinct grain populations within individual samples cannot be readily explained by host rock control, but instead they are interpreted to reflect changes in fluid composition over time. The hydrothermal calcites have chondrite-normalized REE patterns that are mostly HREE enriched relative to LREE, with very few exceptions that show unfractionated flat or LREE enriched (relative to the HREE) patterns. The REE patterns of the calcites are very different from those of their immediate host rocks, suggesting that they are inherited from the hydrothermal fluid(s) from which they precipitated. The HREE enrichment relative to the LREE visible in the patterns resembles the REE patterns of calcites from hydrothermal vein type deposits of metamorphic origin elsewhere and contrasts with the REE patterns of calcites from magmatic-hydrothermal environments. The REE patterns of the hydrothermal calcites in the Hattu schist belt deposits would therefore be compatible with formation from a fluid system that is essentially derived from metamorphic sources, in agreement with conclusions drawn from the fluid inclusion chemistry.