Multi-source and multi-stage metal mobilization during the tectonic evolution of the Central Lapland Greenstone Belt, Finland: implications for the formation of orogenic Au deposits

Abstract

Precambrian greenstone belts are prospective terrains for orogenic Au deposits worldwide, but the sources of Au, base metals, metalloids, and ligands enriched within the deposits are still debated. Metamorphic devolatilization is a key mechanism for generating Au-rich hydrothermal fluids, but the respective role of the metavolcanic and metasedimentary rocks present within these belts in releasing ore-forming elements is still not fully understood. The Central Lapland Greenstone Belt (CLGB), Finland, one of the largest Paleoproterozoic greenstone belts, hosts numerous orogenic Au deposits and is composed of variably metamorphosed volcanic and sedimentary rocks. Characterization of element behavior during prograde metamorphism highlights that (1) metavolcanic rocks release significant Au, As, Sn, Te, and possibly S; (2) metasedimentary rocks release significant S, C, Cu, As, Se, Mo, Sn, Sb, Te, and U, but limited Au; and (3) metakomatiite releases C and possibly Au. Throughout the CLGB metamorphic evolution, two main stages are identified for metal mobilization: (1) prograde metamorphism at ~ 1.92–1.86 Ga, promoting the formation of typical orogenic Au deposits and (2) late orogenic evolution between ~ 1.83 and 1.76 Ga, promoting the formation of both typical and atypical orogenic Au deposits. The complex lithologic diversity, tectonic evolution, and metamorphic history of the CLGB highlight that metal mobilization can occur at different stages of an orogenic cycle and from different sources, stressing the necessity to consider the complete dynamic and long-lasting evolution of orogenic belts when investigating the source of Au, ligands, metals, and metalloids in orogenic Au deposits.

From the mantle source to the crustal sink: magmatic differentiation and sulfide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland

“…Estimated parental melt compositions range from komatiitic basalts with < 18 wt% MgO (Hanski et al 2001;Törmänen et al 2016) to high-MgO komatiite with 25 wt% MgO and mantle potential temperatures up to 1763 °C (Puchtel et al 2020). Whole-rock MgO contents in the CLGB komatiites vary mostly between 10 and 30 wt% in lavas, and between 25 and 45 wt% in cumulates (Hanski et al 2001;Brownscombe et al 2015;Törmänen et al 2016;Luolavirta et al 2018a;Puchtel et al 2020;Patten et al 2023). Because some of the lavas contain < 18 wt% MgO they are not komatiites sensu stricto, however, in the framework of this study, we use the al.…”

“…In addition, we collected previously published wholerock and melt inclusion data from CLGB (n = 354), including komatiitic lavas, dykes, and cumulates from Jeesiörova (Hanski et al 2001;Hanski and Kamenetsky 2013), Kevitsa (Mutanen 1997;Luolavirta et al 2018a;Puchtel et al 2020), Sattasvaara (Saverikko 1985;Patten et al 2023), and Lomalampi (Törmänen et al 2016) areas (Fig. 1, Table S2).…”

“…1, Table S2). Most of these data (Mutanen 1997;Hanski et al 2001;Törmänen et al 2016;Luolavirta et al 2018a;Patten et al 2023) have been produced in the same laboratory and using the identical sample preparation techniques and measurement devices as the data original to this work. The major element oxide data of Saverikko (1985) was produced in the same laboratory using the same sample preparation method but with a Philips PW1400 spectrometer.…”

Orogenic gold: is a genetic association with magmatism realistic?

“…Greywacke sequences where initial concentrations of Mo-Bi-Te are low may not mobilize large proportions of these metals during metamorphism (e.g., Pitcairn et al 2021). Metamorphism of black shales enriched in C and S (C + S > 1 wt%), such as those in the Kittilä and Savukovski groups of the Central Lapland Greenstone belt, Finland, drives significant mobility of As, Sb, Mo, Te, and Sn during prograde metamorphism (Patten et al 2022). Cave et al (2016) show that metamorphic conversion of detrital rutile to titanite, broadly coeval with the pyrite to pyrrhotite conversion event, will release significant amounts of W into the fluid.…”