Lithogeochemical Exploration of Metasomatic Zones Associated with Volcanic-Hosted Massive Sulfide Deposits Using Pearce Element Ratio Analysis

Abstract

Reactions between mineralizing hydrothermal fluids and host rocks produce distinct mineralogically and geochemically zoned alteration haloes in the footwall of many volcanic-hosted massive sulfide (VHMS) deposits, often very much larger than the deposits themselves. Empirical alteration indices have been developed to exploit these features and are widely used as geochemical vectors in the search for new deposits, but only with limited success. The principal limitation of using these indices as lithogeochemical exploration vectors is that they cannot distinguish between the geochemical modifications produced by hydrothermal alteration and the geochemical heterogeneity present in host rocks before the onset of hydrothermal activity. To overcome this problem, a theoretically based lithogeochemical exploration method has been developed that (1) identifies the sources of geochemical variability in VHMS host rocks, (2) isolates the pre-existing geochemical heterogeneity in these rocks through use of linear fractionation and crystal sorting models, and (3) quantifies material transfer due to hydrothermal metasomatism. This method has been applied to whole-rock geochemical data from footwall rhyolites, collected 250 m below the VHMS deposits at Rio Tinto, Spain. An alkali depletion anomaly is detectable 2.5 km from the deposits, increasing steadily in intensity from unaltered rocks at the margin to totally alkali-depleted rocks in the core of the footwall alteration halo. An Fe and Mg addition anomaly is detectable 1.5 km from the center of mineralization, also increasing toward the core of the alteration halo. The most intensively altered rocks in this set of samples also are the most evolved petrologically, suggesting the possible involvement of a latestage magmatic hydrothermal phase in the formation of the VHMS deposits at Rio Tinto. Given adequate sample density, this lithogeochemical method may be used to vector toward the core of a footwall (and probably also a hanging wall) alteration halo, and perhaps to massive sulfide mineralization.

Molar-ratio and Harker diagrams in portraying the actual chemical variability of granitoid suites

“…The restite model, or any other appropriate petrogenetic model, if expressed quantitatively, could be used to predict expected compositional variability of plutons (or suites); the predictions could be tested rigorously with previously assembled, or newly acquired, petrographic data. Madeisky & Stanley (1993, p. 1137 showed molar ratios can be used in linear process-response models to portray Ca, Na, K, and AI variations stemming from fractionation, crystal sorting, etc., of feldspar, biotite, or other minerals in rhyolites. As noted above, such methods have been used for basic igneous lavas (e.g.…”

“…For ratio denominators, they used element fractions of K, Ti, or P because these elements are conserved (i.e., do not participate in initial crystallization of olivine, pyroxene, etc.). Madeisky & Stanley (1993) used Zr as conserved component for a metasomatic mineralized zone. Although only chemical components have been used for the constant in ratio denominators, Pearce (1968, pp.…”

The use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the Malartic district, Québec

“…The mineralogical reactions that controlled the mass transfer processes during hydrothermal fluid-rock interaction were evaluated using molar element ratios (Madeisky and Stanley, 1993;Eilu and Groves, 2001;Whitbread and Moore, 2004;Prendergast, 2007;Stanley, 2017). This approach is particularly well-suited to the analysis of metasomatic processes, as molar proportions provide a direct means to correlate changes in bulk-rock lithogeochemistry to alteration mineralogy (Madeisky and Stanley, 1993;Warren et al, 2007).…”

“…Likewise, molar element ratio analysis can be used to investigate mass transfer processes, and also offers a direct means to interpret alteration-related mineralogical reactions (Madeisky and Stanley, 1993;Warren et al, 2007;Stanley, 2017). The results of mass transfer calculations and molar element ratio analysis therefore provide key parameters for mapping the spatial distribution of alteration that defines the footprint of major gold deposits (Eilu and Mikucki, 1998;Eilu et al, 1999;Whitbread and Moore, 2004;Warren et al, 2007;Stock, 2012).…”

Open and closed compositional data in petrology

“…(1968) and Nicholls (1988) showed convincingly that, unlike wt% in Harker and ternary diagrams, certain molar ratios having a common constant denominator can display actual chemical variability unequivocally. Russell and Nicholls (1988) and Stanley and Russell (1989) demonstrated the effectiveness of molar ratios in evaluating petrological models for basaltic suites, using K, Ti, and/or P, separately and in combinations, as constant; Madeisky and Stanley (1993) used Zr as the constant for a metasomatic zone. The method requires, inter alia: (i) evidence of geochemical variability produced within initially homogeneous parent material (e.g., differentiation within a lava flow), (ii) at least one element that did not participate in the material transfer (i.e., is a constant in all samples), (iii) high-precision chemical data, and (iv) sampling appropriate to determine spatial homogeneity of the constant element.…”