Quantifying Hydrothermal Alteration: A Review of Methods

Abstract

Hydrothermal alteration is proximal to many base and precious metal deposits, and its products can provide insights into the characteristics of hydrothermal systems. To be useful to exploration geologists and researchers, however, alteration needs to be typified and quantified. Alteration type informs on mineralising style (e.g., have we found a porphyry or a volcanogenic massive sulphide deposit?), while quantification of its intensity helps position a sample within the system (e.g., how close are we to the main economic deposit?). Numerous methods-all having their specific advantages and disadvantages-are dedicated to the characterisation of alteration. As alteration is a process that induces chemical and mineralogical changes in rocks, it can be studied using petrological (e.g., mineral recognition in thin sections, mineral chemistry), mineralogical (e.g., alteration indices that use normative minerals), and chemical (e.g., mass balance calculations) approaches. This short review provides an overview of the methods useful to researchers and that are also applicable in an exploration context.

Origin of the Vanadiferous Serpentine–Magnetite Rocks of the Mt. Sorcerer Area, Lac Doré Layered Intrusion, Chibougamau, Québec

“…The global mass changes, as estimated by the modelled precursor method [45], point to mass gain being most pronounced for sample MS17-14 (Table 5). Individual mass changes within ±1 g (and ±10 g for SiO 2 ) per 100 g of precursor fall within the method's error and are considered negligible [47]. The volcanic rocks gained Fe and Mg and lost Ca (Table 5).…”

“…These authors suggested that increasing degrees of diffusion during an oxy-exsolution process form the following progression of textures: (1) trellis-type texture (thin ilmenite lamellae in all sets of {111} planes); (2) sandwich-type texture (thick ilmenite lamellae in one set of {111} planes); (3) granular ilmenite inside magnetite; and (4) external granular ilmenite. In the studied rocks, trellis-type texture is only observed in the Fe-Ti-oxides of volcanic rocks (Figure 7f 57 Fe, 24 Mg, 28 Si, 27 Al, 47 Ti, 51 V, and 52 Cr in the minerals of sample MS17-37. Mineral abbreviations are as reported in the literature [34].…”

“…For these data, correlation coefficients are low between V, Ti, Al, and Fe, and they correspond to −0.31 (Fe2O3 T -V), -0.47 (Fe2O3 T − Ti), and −0.43 (Fe2O3 T − Al). The Fe2O3 T -V correlation, however, improves to 0.54 if the samples located >320 m downhole are discarded (Figure 9a 57 Fe, 24 Mg, 28 Si, 27 Al, 47 Ti, 51 V, and 52 Cr in the minerals of sample MS17-37. Mineral abbreviations are as reported in the literature [34].…”

“…REE diagram normalized to the primitive mantle [42] for whole rock chemical analyses performed on behalf of this study (samples MS17-xx) and compiled from the MERN dataset (see Figure 1 for reference) (Obatogamau Formation and David Member). 59 Co, 52 Cr, 139 La, 47 Ti, 51 V, 66 Zn, and 90 Zr), represented by the colors red and green, against the internal standard for magnetite ( 57 Fe), represented in blue (for additional maps, see Supplemental Material S10). Mineral abbreviations are as reported in the literature [34].…”

“…Magnetite does not usually coexist with poorly evolved olivine, as pointed out by Namur and collaborators [2], who summarized that the first liquidus Fe-Ti-oxides are generally in equilibrium with evolved silicates (e.g., Fo 55-56 , An [45][46][47][48][49][50][51][52][53] , cpx Mg-number [64][65][66][67][68][69][70][71][72][73][74][75]. Exceptions correspond to the Sept-Iles and Panzhihua layered intrusions, where Fe-Ti-oxides crystallized early, at elevated temperatures, and are thus observed next to poorly evolved silicates [2,77].…”