CONSONORM_LG: New Normative Minerals and Alteration Indexes for Low-Grade Metamorphic Rocks

摘要

The CONSONORM_LG method provides a standardized solution for approximating metamorphic parageneses, as well as indexes for estimating chemical and mineralogical changes caused by hydrothermal alteration. CONSONORM_LG is designed for rocks dominated by silicates, Fe-Ti oxides, and/or carbonates, and it approximates the main parageneses of greenschist and lower amphibolite-grade metamorphic rocks for three sets of temperature and pressure conditions (350 degrees C and 2.5 kbars, 450 degrees C and 2.5 kbars, 575 degrees C and 2 kbars). For each model, the norm calculates the main paragenesis using an ACFMNK tetrahedron, a convenient way of representing a large number of silicate assemblages. In addition to silicate minerals, CONSONORM_LG calculates Fe-Ti oxides and other accessory minerals from minor elements, as well as sulfides from analyzed S or from analyzed metals, and carbonates from analyzed CO2 or from normative CO2 estimated from loss on ignition (LOI). CONSONORM_LG also calculates several alteration indexes to estimate Fe-Mg (e.g., chloritization), Ca (e.g., propylitic alteration), Na and K acid alterations (e.g., sericitization, phyllic alterations), and Al gain (e.g., argillization). Carbonatation indexes are also calculated using the amount of normative minerals formed by this type of alteration, i.e., carbonates, chlorite, and muscovite. The normative calculation is validated using published whole-rock analyses and petrographic descriptions. Alteration indexes are validated using several natural samples of alteration halos around base metal and gold deposits.