Abstract

In the critical zone of the upland Boulder Creek (Colorado) catchment, a weathered mantle consisting of 3 to 10 m of oxidized bedrock, saprolite, and mobile regolith (grus and soil) overlies fresh and locally hydrothermally altered bedrock that is exposed beneath a low-relief surface. Saprolite forms from isovolumetric alteration of primary rock materials to secondary minerals by percolating water; mobile regolith includes rock materials in transport downslope, mixed by a variety of biologic and physical processes. Analysis of vadose zone samples by petrographic, inductively coupled plasma mass spectroscopy (ICP–MS) and X-ray diffraction (XRD) techniques demonstrates that weathering of Precambrian Boulder Creek Granodiorite and Silver Plume Granite has formed small amounts of clay and iron oxides. Bulk geochemical changes during alteration are minor except in the transformation of saprolite to regolith, which is enriched in neoformed minerals. In saprolite, the alteration sequence is: plagioclase > biotite > microcline > quartz. Quantitative XRD analysis shows that smectite and kaolinite form <10% of the rock in weathered samples and that smectite is absent and kaolinite and illite comprise >15% of the rock at several hydrothermally altered sites. Iron oxide and hydroxide minerals, such as goethite and hematite, have formed by weathering and by hydrothermal alteration. Hydrothermal alteration of granodiorite to saprolite at the Hurricane Hill site resulted in substantial losses of CaO, MgO and Na2O, Sr and metals such as Cd and Pb, and enrichment in K2O and the trace elements Cs, Sb, and W. At four critical-zone profiles studied in detail: (i) increases in kaolinite, smectite, and Fe oxides and decreases in magnetite concentration due to weathering are significant if fresh rock is compared to regolith; (ii) illite is enriched in soil; (iii) minor amounts of clay minerals are inherited from fresh rock; and (iv) clay minerals and Fe oxides are abundant only in hydrothermally altered saprolite. Some of the apparent enrichment of secondary minerals in weathered profiles likely results from dustfall and from material mixed into mobile regolith from heterogeneous bedrock sources. If some portion of the clay minerals in regolith and some fraction of dissolved material leaving Boulder Creek catchments are exotic, rates of clay and Fe oxide formation in the Boulder Creek area must be slow, and the alteration of granitic rocks during the conversion of saprolite to regolith may be driven mainly by volume expansion in this cool, continental climate.

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