05/10/2017

Stable Isotope Studies by Microbeam Analyses and Sources of Fluids in Orogenic Gold Mineral Systems

Text:
Ferenc Molnár, Research Professor

Research Professor Ferenc Molnár tells in this blog text about Stable Isotope Studies by Microbeam Analyses and Sources of Fluids in Orogenic Gold Mineral Systems.

In situ mass spectrometric techniques are among the most rapidly developing analytical techniques that are revolutionizing our understanding of mineralogy, petrology and geochemistry. The advantage of these techniques is that they enable us to study very small mineral samples (e.g. a few hundreds of cubic micrometres), while still producing precise analyses of isotope ratios or trace element abundances. Small sample volumes are effectively extracted from the mineral grains by controlled, highly focused laser or ion beams, so that analyses can also be performed directly from petrographic thin sections. Compared to previous methods, this allows us to analyse minerals within their textural and paragenetic context in situ, or even growth zonation within isolated, separate grains. These specific features provide a better basis for the interpretation of data compared to the results of bulk analyses, which are commonly affected by the presence of inclusions in minerals or by the complex intergrowths of minerals of different origin. The high spectral resolution of modern mass spectrometers also allows us to analyse a wide range of stable and radiogenic isotopes. For example, sulphide minerals can provide valuable information on ore forming processes, not only from conventional sulphur isotope analyses but also by their transition metal (e.g. Fe, Cu, Zn) isotope ratios, and stable isotopes of these elements can also be extracted from the same, microscopic scale mineral grains using ion or laser beams (Fig. 1). Non-sulphide minerals in ore deposits are useful for conventional oxygen, hydrogen and carbon isotope studies, which can provide insights into tracing the sources of hydrothermal fluids. However, if tourmaline was formed during fluid–rock interaction in the ore-forming hydrothermal system, boron isotope ratios are also useful and they are probably the most robust indicators of fluid sources, effectively discriminating between fluids from magmatic sources and those that have interacted with evaporates or other sedimentary rocks. This is because tourmaline is very stable if subjected to younger metamorphic events that can lead to the exchange of isotopes with different fluids, and this is a particular problem in Precambrian metamorphic terrains, such as in Finland. The regional-scale application of multidisciplinary isotope studies helps in understanding the role of magmatic and metamorphic processes in the formation of hydrothermal ore deposits on these terrains: this knowledge is of value in the evaluation of mineral potential and in developing exploration models in the targeted areas.

 

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The first economically significant orogenic gold deposits in Finland were discovered in this region during an exploration project managed by GTK during the 1980s. To improve our understanding of mineralization processes in these deposits, we recently completed sulphur and copper isotope studies in sulphide minerals and boron isotope analyses of tourmaline, using a combination of laser ablation inductively coupled mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS). Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces indicate that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ34SVCDT values in sulphide minerals, are interpreted as localised involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognised if the original, distinct signatures of the sources have not been homogenised by widespread interaction of fluids with diverse rock types at the regional scale, and provided that local chemical variations were unable to trigger changes in the oxidation state of copper during hydrothermal processes. More details of this work can be found in the following paper:

 

Molnár, F., Mänttäri, I., O`Brien, H., Lahaye, Y., Pakkanen, L., Johanson, B., Käpyaho, A., Sorjonen-Ward, P., Whitehouse, M., Sakellaris, G.: Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland. Ore Geology Reviews, 77, 133-162.

Figure 1: Results of in situ sulphur and copper isotope analyses in samples from the Hattu schist belt. A – copper and sulphur isotope data from chalcopyrite (Ccp) associated with pyrrhotite (Po) and arsenopyrite (Apy). B – copper and sulphur isotope data for chalcopyrite (Ccp) and pyrite (Py). Note the differences between copper and sulphur isotope data from chalcopyrite in different mineral associations, as well as the significant variation in the sulphur isotope data from pyrite (pictures are from Molnár et al., 2016).
Ferenc Molnár

Text: Ferenc Molnár

Dr Ferenc Molnár, research professor in ore geology, joined GTK in 2011. Before that he has been worked for 26 years for the Eötvös Loránd University, Hungary. His research interest includes mineralogy, petrology, geochemistry and geochronology of various types of ore deposits. Currently, he is studying magmatic Ni-Cu-PGE and orogenic gold mineral systems in Archaean and Svecofennian domains of eastern and northern Finland.