Black Agates from Paleoproterozoic Pillow Lavas (Onega Basin, Karelian Craton, NW Russia): Mineralogy and Proposed Origin

Evgeniya N Svetova,Sergei A Svetov,Alexandra V Stepanova,Svetlana Y Chazhengina

doi:10.3390/min11090918

Abstract

The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, and moss agates. The silica matrix of black agates is only composed of length-fast and zebraic chalcedony, micro- and macro-crystalline quartz, and quartzine. In addition to silica minerals, calcite, chlorite, feldspar, sulphides, and carbonaceous matter were also recognised. The black colour of agates is related to the presence of disseminated carbonaceous matter (CM) with a bulk content of less than 1 wt.%. Raman spectroscopy revealed that CM from black agates might be attributed to poorly ordered CM. The metamorphic temperature for CM from moss and spherulitic agates was determined to be close to 330 °C, whereas CM from concentrically zoning agates is characterised by a lower temperature, 264 °C. The potential source of CM in moss and spherulitic agates is associated with the hydrothermal fluids enriched in CM incorporated from underlaying carbon-bearing shungite rocks. The concentrically zoning agates contained heterogeneous CM originated both from the inter-pillow matrix and/or hydrothermal fluids.

Highlights

Agate and chalcedony are varieties of silica that are mostly composed of minute crystals of α-quartz
Agates are widely distributed in the Ludicovian Super-horizon (LSH) within the Onega Basin and are mostly associated with the pillow lavas, and to a lesser extent with the massive lavas and pyroclastic tuffs
The previous study of agates [9] revealed that (Figure 1B) only light-coloured agates without carbonaceous material occurred in the lower part of Suisari Formation (SF) (Yalguba Ridge, Pinguba, and Tulguba occurrences), whereas the black agates were not recorded

Summary

Introduction

Agate and chalcedony are varieties of silica that are mostly composed of minute crystals of α-quartz. Agates usually contain significant amounts of other silica phases (opal, cristobalite, tridymite, quartzine, moganite, macro-crystalline quartz) and paragenetic minerals (carbonates, clay minerals, zeolites, iron compounds, etc.) [2]. These impurities may be responsible for the wide diversity in colour and spectacular pattern of agates. The black colouration of agate areas can be caused by micro-inclusions of Fe-, Mn-, and Ti-oxides [3], as well as carbonaceous material [10,11]. It is assumed that the origin of carbon-rich inclusions in Mali agates can be related to both the hydrothermal formation of graphite from methane under elevated temperature and graphitisation of organic precursors by secondary hydrothermal or metamorphic overprint [10]. The investigation of the solid hydrocarbon (bitumen) substance in agates from Nowy Kosciół (Poland) revealed its algal or algal-humic origin [11]

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Conclusion