Simple graphical tools to understand the relationship between porphyry composition, hydrothermal alteration, mineralogy and copper-gold grades in porphyry copper deposits

Abstract

Alteration zonation in porphyry copper deposits is a standard tool to establish spatial relationships with respect to the best Cu-grade core of the magmatic-hydrothermal system. With the development in recent times of low cost and good quality whole-rock multi-element ICP-MS analysis, large databases of drill hole litho-geochemistry have become available from drilling campaigns of porphyry copper targets. Here I propose some simple graphical tools that use multi-element datasets to evaluate alteration type and their relationship to Cu grades. I suggest a five part methodology; 1) determine the original least altered porphyry composition(s) by using the AI vs CCPI plot, 2) use the molar K/Al vs Na/Al plot to discriminate the basic alteration type, 3) check for alunite and anhydrite using the Ca-Fe-S plot, 4) follow-up with the porphyry copper alteration plot [K/(K + Ca) vs K/(K + Al)] to finalise the discrimination of alteration type. 5) plot all data with Cu > 0.5 % on the K/(K + Ca) vs K/(K + Al) diagram (4 above), as a density plot, to evaluate the relationship between Cu grades and alteration type.

Three case studies are provided that outline the methodology and show the importance of the composition(s) of the host porphyry intrusion(s) in controlling the relationship between Cu-grades, bulk mineralogy and alteration type. Based on these case studies it is clear that not all, or even most, of the samples with greater than 0.5% Cu are always concentrated in the potassic alteration type.

Application of the MINSQ computer program has enabled mineral concentrations to be estimated and plotted on the alteration type diagram K/(K + Ca) vs K/(K + Al) in each of the case studies. This approach suggests that in monzonite and granodiorite based porphyries, K-feldspar replacement of plagioclase subsequently overprinted by white-mica is the key process in the Cu core of the porphyry deposit, whereas in diorite-based porphyries, albitisation of plagioclase is suggested as the dominant alteration process, producing a Cu-Au- bearing sodic-calcic core with little, or only minor, K-feldspar alteration.