The tetrad effect in REE distribution patterns: A quantitative approach to genetic issues of argillic and propylitic alteration zones of epithermal Cu-Pb-Fe deposits related to andesitic magmatism (Khan Kandi District, NW Iran)

Abstract

The impact of the tetrad effect on the accumulation of REE during base metal mineralization can be deduced from four segments of the normalized REE distribution patterns. Quantifying this effect in each part of the REE graph has proven to be a powerful tool when it comes to the investigation of the origin of ore deposits. In the current research, a mathematical method using a logarithmic scale has been designed to calculate each individual tetrad segment in the samples and tested for the alteration zone of the andesite-related epithermal base metal deposits in the Khan Kandi District, NW Iran. In contrast to previous approaches, the current method achieves a higher accuracy for normalized REE patterns of samples from the argillic and propylitic alteration zones of base metal deposits. Using the calculation technique presented here reveals that the values obtained for third and fourth tetrad effects follow different trends in argillic and propylitic samples. In conclusion, mineralizing fluids of low pH accountable for the development of argillic alterations are characterized by larger σ3 (third tetrad) values in fault zones. They decrease toward the margins of the fault zones where argillic alteration is predominant whereas in the propylitic alteration zones the trend is reversed. The tetrad effect in these alteration zones is controlled pH, temperature changes and the fluid-rock interaction, conducive to the decomposition of minerals in host and wall rocks. The behavior of geochemical pairs such as Y/Ho, Zr/Hf and Nb/Ta displays a non-CHARAC behavior highlighted by non-linear and parallel trends in the chondrite-normalized REE curves. The occurrence of conjugate convex and concave tetrad effects in the REE patterns are accounted for by the mixing of meteoric waters and hydrothermal fluids. The correlation between σ3 values and geochemical couples are indicative of a mineralogy that formed under a regime of strong fluid-rock interaction. The data obtained from samples in the central parts of the fault zones document this zone as the pathways for hydrothermal solutions.