Volatile hydrocarbons in Ba-Zn-Pb ore genesis: analysis and use in mineral exploration

Abstract

The use of volatile organic compounds in mineral exploration is based on the assumption that they migrate from concealed ore bodies to delineate anomalies in surface rocks. These two rather opposite requirements can be fulfilled only if these compounds can be retained in the sorbed state in the rocks. The checking of this condition by two different approaches leads also to more general considerations on the occurrence and fate of light hydrocarbons in geological media. Volatile organic compounds were obtained from two core samples; first, by simple degassing of the freshly drilled rocks in surface conditions and afterwards, by stepwise heating from 60°C up to 300°C. The results of these analyses show that the gaseous compounds occur both in the free and sorbed states in the rocks. The sorbed fraction is efficiently extracted at 150–220°C. At higher temperatures, new products are generated by thermal cracking of heavier organic molecules contained in the samples. Alkenes, found rarely in natural gases extracted at low temperature, are normal compounds of the sedimentary rocks which can be released only by heat desorption. A second approach deals with 81 sample taken in outcrops from different localities of the Causses Basin; in Liassic carbonates which are important regional Zn-Pb-Ba-(Cu) ore-bearing strata. All the mineralized samples from eight out of ten sites investigated display anomalously mature gas signatures, tending to pure methane. Other samples show a “wet”-gas composition, rich both in saturated and unsaturated hydrocarbons. Compositional variations in these compounds are consistent with thermal alteration caused by hydrothermal circulations also assumed to be responsible for the ore genesis. This interpretation, also supported by Rock-Eval pyrolysis data, confirms previous hypotheses on the regional extension of the inferred ore-genetic event. These observations provide evidence of a direct relation between specific hydrocarbon gas signatures and mineralization, and the persistence of these signatures through time.