| Summary: | A procedure for determining metasomatic norms is developed in this thesis to
quantitatively and objectively estimate mineral abundances from lithogeochemical data,
The norm calculations use the same principles as do other norms such as CIPW, but the
different mineral phases present in alteration systems are used as the normative standard
minerals. Another distinctive difference between a metasomatic and a conventional norm is
that the calculation procedure proposed for a metasomatic norm does not proceed along
such a fixed hierarchical path as in the case of an igneous norm. A particular useful
approach to the application of the norm concept to metasomatic rocks is to constrain the
calculated normative mineralogy by a priori knowledge of existing minerals (i.e. to
approximate the mode as closely as possible).
Where an immobile component can be recognized the metasomatic norms for
protoliths and altered rocks, as well as the chemical constituents lost or gained, can be
further recast into the absolute amounts of minerals and chemical constituents relative to a
given mass of parent rock. Known errors within lithogeochemical data studied can be
propagated to the final results of all norm calculations. As a result, a chemico-
mineralogical model for material exchange, including absolute losses and gains of chemical
constituents, normative minerals in extensive units, as well as the corresponding
propagated errors, is formulated in this work as follows:
[equation diagram]
Equation I is particularly useful because it is quantitative and easily applied:
information that can be obtained from the equation includes the mineralogy of the initial
and final rocks, absolute gains and losses of specific chemical constituents as well as the
uncertainties on each estimate at a specified confidence level.
The methodology for this approach is a natural extension of the use of Pearce
element ratio (PER) diagrams for the study of metasomatic rocks. The metasomatic norm
recovers the same quantitative information as do Pearce element ratio diagrams. The
common principles are (i) correction for closure, that provides true relative
lithogeochemical and mineralogical variations between parent and daughter rocks, and (ii)
an effort to explain chemical variability in terms of mineralogical variability. The strategy
of a PER diagram is to test whether chemical changes in different rocks can be explained
purely by the variation(s) of certain mineral(s), as demonstrated by disposition of the
binary plotted points along predefined trends (slopes). Metasomatic norms are displayed
more effectively as equations or profiles showing the spatial distributions of normative
mineral assemblage, as well as the absolute losses and gains of chemical constituents based
on comprehensive mass balance relationships.
The approach described in the first part of this thesis is applied to a hydrothermal
alteration study of the Silver Queen mine in central British Columbia. Hydrothermal
alteration at the Silver Queen mine was derived from a multiple precursor system.
However, local, individual alteration profiles exhibit the attributes of a single precursor
system. Six types of hydrothermal alteration at Silver Queen mine have been described:
viz. propylitization, sericitization, argillization, silicification, pyritization and
carbonatization. In general, the wall rock alteration in the study area is composed of a
widespread regional propylitic alteration with superimposed carbonatization. Regional
alteration gives way, as the vein is approached, to an outer envelope of sericitic and
argillic alteration + carbonatization and an inner envelope of silicification and pyritization
+ sericitic or argillic alteration + carbonatization. Thus, the sequence of alteration
development is (i) widespread regional propylitic alteration, (ii) sericitic and argillic outer
envelope, and (iii) silicification and pyritization inner envelope.
Most of the hydrothermally altered samples in alteration envelopes at the Silver
Queen mine have gained mass during hydrothermal alteration. In contrast, samples from
the profile of the northern segment of the No. 3 vein have lost mass. Other spatial
variations of hydrothermal alteration from the southern segment to the northern segment
of the No. 3
vein and from different levels (from 2600-foot level to 2880-foot level) have
been recognized. In brief, the wall rock alteration is most intense in the alteration envelope
at the central segment of the No. 3 vein and mildest at the northern segment of the No. 3
vein. The total mass change of each altered sample is largely the result of depletion of CaO
and Na₂O, and addition of SiO₂, K₂O, H₂O and CO₂.
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