The Release of Trace Elements and Volatiles from Crinoidal Limestone During Thermal Decrepitation

A. H. Rankin, M. F. Miller and J. S. Carter
Department of Geology, Imperial College, London SW7 2BP
British Geological Survey, Geochemical Division, London WC1X 8NG
Mercury Hydrocarbon Limited, Raheen Industrial Estate, Limerick, Eire

Abstract: A suite of coarsely crystalline samples of crinoidal limestone adjacent to a PbS-CaF2-CaCO3 vein in the North Pennine orefield (Greenhow Rake) was analysed by three independent methods [Inductively Coupled Plasma emission spectroscopy (ICP), Mass Spectrometry (MS) and Gas Chromatography (GC)], in order to determine the nature and origin of trace elements and volatiles released on heating and thermal decrepitation, to establish whether fluid inclusions from the host limestone differed in composition from those within the vein, and to assess the value of the results obtained for mineral exploration. The ICP method gave reproducible results for Ca, Na and K, and significant values for Pb and Zn. Na and K correlate with H2O levels determined by MS/volumetric analysis, suggesting their coexistence within fluid inclusions. No such correlation was found for Pb and Zn, suggesting that these elements were derived by volatilisation, perhaps from traces of galena and sphalerite. Methane showed no correlation with H2O and a source other than aqueous fluid inclusions is thought probable for this volatile.

The vein calcite showed much lower K/Na fluid ratios (0.05 cf 0.1–0.2), than the limestones. The limestone-hosted fluids occur predominantly in crinoid ossicles and are believed to represent early pre-mineralisation fluids, unrelated to those associated with veining. Combined ICP and MS techniques may be of value in investigating samples where fluid inclusions are too small for microthermometric and optical analysis.

Keywords: limestone • crinodal • thermal decrepitation • inductively coupled plasma emission spectroscopy • mass spectrometry • gas chromatography

Mineralogical Magazine; October 1987 v. 51; no. 62; p. 517-525; DOI: 10.1180/minmag.1987.051.362.06
© 1987, The Mineralogical Society
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