Chalcophile element (Cu, Zn, Pb) and Ga distribution patterns in ancient and modern oceanic crust and their sources: Petrogenetic modelling and a global synthesis

Highlights

• Cu, Zn, Pb and Ga contents have been studied in modern oceanic crust and ophiolites.

• Zn and Ga patterns are largely uniform and unrelated to the tectonic setting.

• Cu and Pb patterns show subduction-related, between-ophiolite-type variations.

• Fractional crystallization controls the distribution of Cu, Zn, Pb and Ga in basalts.

• MORB magma is S-saturated; boninitic and IAT magmas are S-undersaturated.

Abstract

We present a global synthesis of Cu, Zn, Pb and Ga contents of mafic dike complexes and volcanic rocks associated with 259 ophiolites, ranging in age from Archaean throughout the Phanerozoic. These ophiolites are geochemically classified as subduction-unrelated and subduction-related with various sub-categories, as defined in Dilek and Furnes (2011). The subduction-unrelated ophiolites include Mid-Ocean Ridge (MOR), and Rift, Continental Margin and Plume type ophiolites, collectively grouped as the R/CM/P sub-category. The subduction-related ophiolites include Backarc (BA), Forearc (FA), Backarc to Forearc (BA-FA), and Volcanic Arc (VA) sub-categories. Compositional distribution of these elements in different ophiolite sub-categories show that Zn and Ga patterns are largely uniform and unrelated to the tectonic setting, whereas Cu and Pb patterns show significant variations. Average copper concentrations progressively increase from subduction-related ophiolites to R/CM/P and MOR. Although less pronounced, lead shows a similar increase in average concentrations from subduction zone environments to MOR, with rather irregular patterns for the R/CM/P and VA types. Mafic subunits in analysed ophiolites define similar trends for Cu and Pb. The mafic subunits, comprising alkaline basalts, mid-ocean ridge basalts (MORB), island arc tholeiites (IAT) and boninites, define a progressive shift towards increasing proportions of low concentrations of Cu and Pb in the listed order. To constrain the large variations in the contents of the given elements, we applied petrogenetic modelling of glass analyses. Petrogenetic modelling of the MgO versus Cu, Zn, Pb and Ga distributions in modern MORB show a scatter that can be explained by different degrees of fractional crystallization (20 – 80%) of primitive MORB lavas. In support of previous studies, we find that most erupted MORB lavas are sulphur saturated, whereas primitive boninitic and IAT magmas are S-undersaturated. The trends observed for IAT are in agreement with previous findings that IAT precipitate sulphide only at very high degrees of fractional crystallization, owing to crystallization of magnetite. Boninites are variable and Cu concentration in boninitic glasses indicates that a fraction of them may be S-saturated at relatively low degrees of fractional crystallization. We model two boninitic compositions and achieve S saturation at 15 and 50% fractional crystallization. The observed Pb enrichment in the R/CM/P ophiolites was likely caused by crustal contamination. Mantle sources of mafic magmas of the ophiolites were also enriched in Cu and Pb by a combination of subduction-related processes as reflected in the chalcophile element (Cu and Pb) behavior patterns of various mafic rock types in the ophiolites.

Comparing with in-situ oceanic crust, we conclude that the chalcophile element distribution patterns of Cu, Zn, Pb and Ga in mafic lavas and dikes in ophiolites were ca. 80–90% magmatically controlled by their abundances in the mantle melt sources, partial melting episodes, and extents of fractional crystallisation processes. The remaining 10–20% difference we attribute mainly to alteration processes (predominantly loss), as well as types and amounts of subducted sediments, whose melt products contributed to the melt column above subducting slabs.

Introduction

The occurrence and distribution of chalcophile elements in the rock record have been both scientifically and societally important because of their unique properties, which played an important role in the development of human civilizations. Transition metals Cu, Zn, Pb and Ga have been mined extensively in the upper crustal lithologies of many Phanerozoic ophiolites and Precambrian greenstone belts throughout history (e.g., Galley and Koski, 1999). Copper (Cu), in particular, is one of the most significant metals in the society, and its stable face–centered cubic crystal structure, which does not change with varying temperatures and its properties of a high electrical and thermal conductivity make cupper most attractive for a wide spectrum of applications in human lives (Copper: An Ancient Metal, https://sites.dartmouth.edu/toxmetal/more-metals/copper-an-ancient-metal/). It was one of the first metals utilized by humans, and the eventual discovery of bronze as an alloy of cupper and tin ushered in the beginning of the Bronze Age around 3000BCE (Cartwright, 2017).

Zinc was commonly used to make brass around 1400 to 1000BCE in today’s Palestine, and the Romans smelted cadmia (zinc oxide) with copper to create brass. The people of India produced zinc metal by burning organic materials with zinc carbonate (smithsonite) in the 1200 s (Alam, 2020). Zinc is a highly flexible metal, and therefore it has been used widely in architecture and construction, and for prototyping products and building small components in electronics. When combined with other metals, zinc alloys strengthen and harden them, an extremely valuable treat in construction (Habashi, 2013).

Lead was one of the first metals extracted from natural ore bodies > 8,000 years ago and had many uses in ancient civilizations because of its soft and easy nature with which the artisans and construction people could work easily (de Keersmaecker et al., 2018). Because lead melts at a relatively low temperature and resists corrosion over time, many civilizations used this common metal for making pipes, cosmetics, coins, and paints. The Romans, in particular, used lead widely in building the first municipal plumbing systems such that this common practice gave the name “plumbum” to lead for its chemical symbol, Pb (de Keersmaecker et al., 2018). In the 15th century Europe, lead was used for creating the first moveable printing press and stained glass windows held together by lead frames in medieval churches.

Gallium (Ga) was discovered in sphalerite (a zinc-sulfide mineral) by Paul-Émile Lecoq de Boisbaudran who introduced its name and description to the French Academy of Sciences in Paris in 1875 (Brennan, 2014). The name was derived from the Latin name for France (Gaul), Gallia. Although until then gallium was not known, its existence and properties were predicted by Dmitri Mendeleev in 1871 because his periodic table showed a gap below aluminum that was yet to be occupied. Elemental gallium is not found in nature and does not exist as a free element in the Earth's crust, but it occurs in bauxite and zinc ores. It is produced as a byproduct during the processing of the ores of other metals, and is widely used in the semiconductor industry, particularly in the production of smartphones and solar cells (Sahlström et al., 2017).

The main sources of chalcophile Cu, Zn and Pb are related to metal enrichment and ore mineralisation processes in hydrothermal systems along seafloor spreading centers in former ocean basins (e.g., Alt, 1995, Alt et al., 2010, Pirajno et al., 2020) or within island arc – forearc settings of supra-subduction zone tectonic environments (e.g., Hein and Mizell, 2013, Varnavas and Papavasiliou, 2020). Gallium, although sensu stricto lithophile (Goldschmidt, 1937), commonly demonstrates chalcophile behaviour and is associated with sulphides, in particular sphalerite (ZnS) (Paradis, 2015, Sahlström et al., 2017).

Chalcophile ore deposits may form at and near the surface of submarine volcanoes, or in the upper stratigraphic and structural levels of submarine magmatic complexes, known as volcanogenic massive sulphide deposits (VMS) (e.g., Koski et al., 1984, Large, 1992, Hannington et al., 1995; summarized in Galley et al., 2007). In addition to their economic importance, chalcophile elements, particularly Cu and Pb, are commonly used as geochemical tracers in the melt evolution of mafic magmas in oceanic environments (e.g., Jenner et al., 2015, Wang and Becker, 2015). Some of the best examples of ophiolite–hosted metallic ore deposits of copper, zinc, lead, silver and gold include the Troodos ophiolite in Cyprus (Dilek and Eddy, 1992, Eddy et al., 1998, Hannington et al., 1998, Antivachis, 2015), the Semail ophiolite in Oman (Mahfoud and Beck, 1997), the Khoy ophiolite in Iran (Aftabi et al., 2006), the Mirdita ophiolite in Albania (Dilek et al., 2005, Phillips-Lander and Dilek, 2008, Milushi, 2015), the Løkken (McQueen, 1990) and Karmøy (Grønlie and Logn, 1978) ophiolites in the Norwegian Caledonides, and the Palaeoproterozoic ophiolites in the North Karelia Schist Belt in Finland (Saalmann and Laine, 2014).

One significant question pertaining to the occurrence of chalcophile elements in the mafic rocks (dikes and lavas) of ophiolites is what parameters and processes control their distribution in ophiolites with different magmatic and tectonic origins. In this study, we have examined and analyzed published data on Cu, Zn, Pb and Ga distributions in basaltic lithologies of dike complexes and lava sequences of Phanerozoic and Proterozoic ophiolites, and Archean greenstone belts that are interpreted as ophiolites with varying ages, internal structures, and tectonic origins, as well as their upper mantle peridotite sources. Here, we compare and discuss the distribution of these elements in the investigated ophiolites exposed in different orogenic belts within the framework of our recent ophiolite classification (Dilek and Furnes, 2011, Dilek and Furnes, 2014, Furnes et al., 2014, Furnes et al., 2020). We have sub-grouped and examined compositionally identified ophiolitic subunits with discrete geochemical affinities (i.e., alkali basalts, mid-ocean-ridge basalts, island arc tholeiites and boninites) in terms of their elemental patterns (in histogram distribution), and then we have modelled partial melting and fractional crystallization trends of their magmas.

The chalcophile element behaviour in ophiolitic rocks is not a new topic. However, two aspects of our approach in this paper are new and fundamentally important: (1) this is the first comprehensive synthesis of the extant data on the chalcophile element distribution patterns in upper crustal rocks of the global ophiolite inventory; and (2) these emerging patterns tell us more about the petrogenesis, particularly the mantle melt sources and melt evolution trends of different ophiolite types in comparison to those of the modern oceanic crust. These two aspects of our paper provide global–scale perspectives, which should be appealing to petrologists, geochemists and geophysicists working not only on ophiolites, but also on modern oceanic crust and mantle evolution through time. We consider these aspects of our paper a major contribution to the ophiolite concept. In order to understand how the distribution of chalcophile elements in ophiolites compares with rock sequences of comparable geochemical affinities in in-situ oceanic environments, we have done petrogenetic modelling to decipher the mode of their occurrences in both ancient and modern oceanic lithosphere. Therefore, our results also provide significant insights about how different tectonic settings of oceanic crust formation and the related petrological and geochemical processes potentially affect chalcophile element distribution patterns and hence mineralization and ore forming processes in oceanic crust.