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Unusual, oil-bearing inclusions in fluorite from Baluchistan, Pakistan

Published online by Cambridge University Press:  05 July 2018

A. H. Rankin ,
B. L. Hodge  and
M. Moser
A. H. Rankin
Affiliation:
Department of Geology, Imperial College of Science, Technology and Medicine, London SW7 2BP
B. L. Hodge
Affiliation:
B. L. Hodge and Partners, London SW20 0NF
M. Moser
Affiliation:
Departments of Geology, Imperial and University Colleges, London
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Abstract

Phenomenally large (up to 2 mm) oil-bearing and associated brine inclusions in fluorite from carbonate-hosted, epigenetic, fluorite-calcite-(baryte) deposits of the Koh-e-Maran area are investigated using a combination of microthermometry, UV-microscopy and FTIR microspectroscopy. The liquid hydrocarbon phase in primary ‘oil’ inclusions is brown in colour and is dominated by saturated, low molecular weight, aliphatic hydrocarbons.

Two types of mixed aqueous and ‘oil’ inclusions occur. Aqueous/oil types represent co-eval trapping of immiscible drops of oil and brine during primary growth. In oil/aqueous inclusions the oil appears to ‘wet’ the aqueous phase resulting in an odd ‘dish-shaped’ meniscus. The oil or liquid hydrocarbon part of the inclusion is primary but the aqueous part is thought to represent a secondary infill. The fluid inclusion evidence suggests that fluorite precipitated from a dilute (3.5 wt.% NaCl) brine at temperatures around 110–140°C in the presence of an immiscible liquid hydrocarbon phase dominated by saturated, light hydrocarbons. This ‘oil’ was present as an emulsion in the aqueous fluid and the phenomenal size of the inclusions is thought to reflect the large droplet size in the emulsion. Infiltration of a more saline, calcium-enriched brine into pre-existing oil inclusions resulted in complex oil-water inclusions showing a reversal in the nature and shape of the oil-water interface due to the presence of unspecifed surfactants in the brine which affected the wetting characteristics of the oil.

The homogenization temperatures and the presence of liquid petroleum inclusions are characteristic of Mississippi Valley-type (MVT) and manto-type fluorite deposits in many other parts of the world.

Keywords

fluoriteoil-bearing inclusionsbrine inclusionsBaluchistanPakistan
Type
Near-surface and surficial environments
Information
Mineralogical Magazine , Volume 54 , Issue 375 , June 1990 , pp. 335 - 342
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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