CHECKING ERNST'S PETROGRAPHY WITH HYDROCHEMICAL MASS BALANCES, INYO MOUNTAINS, CA

Adjacent watersheds with similar but sparse vegetation under the same climatic conditions permit the close examination of the controls of rock-forming mineralogy on the major ion composition of natural waters. Accordingly, the solute composition of streams in adjacent watersheds on the eastern slopes of the White-Inyo Mountains of eastern California have been studied for their chemical relationships to silicate hydrolysis. The area mapped by Ernst and others (1993) is underlain by Mesozoic plutons intruded into lower Paleozoic and late Precambrian metasediments dominated by sandy argillite and limestone. A spreadsheet model (SPREADBAL 2002) has been utilized to derive mass-balance solutions from stream water compositions for dissolution-precipitation of lesser carbonate and predominant, multi-element, silicate minerals underlying these watersheds. The results provide a check on determinations of average major mineral composition within a given catchment area.

A matrix of mineral-solute reaction equations was solved for the best mass balances, and combined with graphical examination of the functional dependency of mineral mass-transfer coefficients on variations in major silicate composition. A subordinate calculation was employed to allow for structurally controlled compositional variation in product 2:1 clay minerals. Mineral mass-transfer ratios were also useful in constraining mass-balance results.

Solute data on two of the Inyo Mountain streams, Crooked Creek and its small tributary Sage Hen Creek, provide an example in assessment of representative plutonic plagioclase compositions. The reactant mineral assemblages used were plagioclase, K-feldspar, and biotite (or hornblende). A small amount of calcite was attributed to metasediments. Quartz was assumed relatively inert, but minor solute silica from surficial volcanic ash was considered in the balance. Assumed weathering products were dioctahedral smectite, kaolinite, and goethite. For reasonable smectite compositions and feldspar/mafic mineral mass-transfer ratios, a higher anorthite content obtained for the plagioclase (An35 vs. An30) is in accord with the work of Ernst and co-workers indicating the more granodioritic character of the pluton dominating the lower reaches of the Crooked Creek drainage.