Metasomatic Norms and Mass Balance Chemico-mineralogic Models of Hydrothermal Alteration System

Exploration & Mining Geology, Vol. 4, No. 4, 1995

XIAOLIN CHENG and A.J. SINCLAIR Department of Geological Sciences, The University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z

Norm calculation procedures originally designed principally for igneous rocks are very rigid in their application and, in general, do not utilize important alteration minerals. For instance, volatile components are essential constituents of many metasomatic rocks but either are not used or are not used effectively in determining normative minerals by the CIPW norm or Niggli-Barth norm procedures. Our approach to determining norms for metasomatic rocks allots all major, minor and volatile constituents to a set of standard normative minerals which are commonly observed as products of hydrothermal alteration. A useful procedure is to have a metasomatic norm approximate the mode as closely as possible. Petrographic information helps to provide the necessary mineralogical constraints. With the recognition of an immobile component and knowledge of sampling and analytical errors in lithogeochemical data, it is possible to recast norms into amounts of alteration minerals relative to a given amount (100 grams) of parent rock. Combining these results with the calculations of absolute loss or gain of individual chemical constituents and considering errors (e) for an appropriate con?dence level, an overall (net) material exchange equation can be presented as follows: SMineralparent rock ± e + SConstituentgained from solution ± e = SMineralaltered rock ± e + SConstituentlost from wall-rock ± e where all items have extensive units (e.g., grams). The model includes the initial and ?nal mineralogy of rocks (evident in the ?eld) as well as gains and losses of speci?c elements. Only those constituents whose calculated losses and gains are signi?cantly different from zero are retained in the above model, and only the normative minerals with abundances signi?cantly different from zero are included in the model (equation). One advantage of the chemico-mineralogic model is that parent rock and product are modeled in mineralogical terms that are familiar to and easily usable by exploration geologists.
Mots Clés: Hydrothermal alteration systems, Igneous rocks, metasomatic rocks, Petrography.