Modelling of metamorphic P-T conditions and in situ Lu-Hf garnets dating from the Paleoproterozoic Pyhäsalmi region, Central Finland: insights into polymetamorphic-multiphase deformational events and ore mineral’s remobilization

The Vihanti-Pyhäsalmi region hosts major volcanic massive sulfide (VMS) deposits within the Precambrian Fennoscandian shield in Finland. The Pyhäsalmi Cu-Zn-Ag-Au VMS deposit has undergone various degrees of metamorphism and deformation that have altered the composition and texture of the ore minerals resulting in ore minerals remobilization. This study aims to constrain the P-T-t path utilizing the metamorphic index minerals and in situ Lu-Hf garnet ages (acquired at the University of Adelaide) from the Paleoproterozoic ore potential Pyhäsalmi region. Moreover, our study also focuses on the remobilization history of ore minerals during polyphase deformational and metamorphic events of the Pyhäsalmi deposit’s halo region through petrographic, mineralogical, compositional and 3D micro-CT analyses. Additionally, we study the microstructures in 2D and 3D to tie regional polyphase deformations with porphyroblast’s growth and ore mineral’s remobilization. Understanding the metamorphism of the Pyhäsalmi region will provide new information for the remobilized ore deposits and other ore potential regions.

 

The preliminary studies from the supracrustal rocks from the drill core samples of the Pyhäsalmi deposit’s halo region suggest medium to high amphibolite facies metamorphic condition with two metamorphic peak conditions such as (1) peak condition one (M1) at 550-600°C and 2-3 kbar represented by staurolite, garnet and cordierite assemblages and (2) the second peak condition (M2) at 650-700°C and 4-6 kbar characterized by sillimanite, cordierite, garnet, biotite assemblages. Moreover, petrographical and mineralogical studies highlight late stage retrograde hydrothermal fluid activity represented by intense chloritization of sillimanite, garnet, biotite, anthophyllite and cordierite and epidotization of the plagioclase. In situ Lu-Hf inverse isochron ages of 1895 ± 53 Ma from cluster type Fe-Mn rich homogenous garnets likely represent the earliest metamorphic event, whereas Lu-Hf ages of 1824 ± 16 Ma from corona type garnets and Lu-Hf ages of 1802 ± 7 Ma from elongated garnets with Mn rich core denotes the second metamorphic event. Additionally, based on textural and mineralogical study accompanying with Flinn strain diagram from micro-CT analysis, we propose that the Pyhäsalmi deposit experienced two distinct periods of ore mineral remobilization, corresponding to two compressional deformation stages. The first compressional stage (D1–D2, at ~1.91 Ga) is associated with the collision of the Svecofennian volcanic arc with the Archean crust. During this stage, mechanical remobilization processes predominated, including cataclastic flow, as evidenced by brecciated pyrites and pyrrhotites, and translational gliding, which produced elongated pyrites and pyrrhotites. The second compressional stage (D4, at 1.82–1.79 Ga) is characterized by post-collisional intense shearing within the Oulujärvi Shear Zone (OjSZ). This stage resulted in mixed-state remobilization processes that involved both chemical and mechanical mechanisms. Evidence for these processes includes fracture-filling sulfides (pyrite, pyrrhotite, chalcopyrite), recrystallization, and the spatial redistribution of disseminated pyrrhotites around the grain boundaries of silicate porphyroblasts (indicative of plastic flowage). Additionally, flattened, foliated, and folded sulfides (pyrite, pyrrhotite, sphalerite) are observed along the hinge zones of crenulated cleavage, further supporting the occurrence of mixed-state remobilization during this period.