Mass changes during hydrothermal alteration, Silver Queen epithermal deposit, Owen Lake, central British Columbia

• Main Author: Cheng, Xiaolin
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7218

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.