REGIONAL VARIATIONS IN SURFACE-WATER CHEMISTRY IN CENTRAL COLORADO, INCLUDING AREAS AFFECTED BY HIGH BACKGROUND AND HISTORIC MINING

The U.S. Geological Survey is studying surface-water quality in the Rocky Mountains of central Colorado. Within the 55,000 km² area, 312 samples were collected from about 220 unique sites; the remaining samples were site duplicates or field blanks. The dominant feature affecting water quality in these first- or second-order streams is the Colorado Mineral Belt (CMB), a northeast-trending zone hosting polymetallic vein and replacement and porphyry Mo deposits that were formed as a result of Cretaceous and Tertiary intrusive activity. Mining of these deposits from the mid-1800s to the present has left numerous streams with a chemical signature typical of acid rock drainage. A subset of the stream reaches have no mines within their catchment areas but have low pH (<5) and high background concentrations of metals and sulfate. Filtered (0.45 µm) and unfiltered water samples were analyzed by ICP-MS and ICP-AES for cations and by IC for anions. In a regional context, the influence of the CMB is seen as lower surface-water pH (<5), and higher concentrations of SO₄^2- (>100 mg/L) and siderophile and chalcophile metals such as Fe (>1 mg/L), Cu (>0.01 mg/L), Pb (>0.3 µg/L), Zn (>100 µg/L), and Cd (>1 µg/L) in many streams. Strong correlation between Zn and Cd (r² = 0.99) indicates a common source for the two metals, namely dissolution of sphalerite or other sulfide-bearing minerals present on waste-rock and tailings dumps, and in abandoned mines that produce drainage. Average aqueous Zn/Cd molar ratio is about 400:1, so if both elements come from sphalerite dissolution, Cd would occupy approximately 0.25% of Zn sites in the sphalerite lattice. Some REE and large-ion lithophile elements such as U, Cs, Rb, and Sr also are enriched in surface water within the CMB, although not to the exclusion of areas outside the CMB as is the case for Fe, Cu, Pb, Zn, and Cd. At the scale of our sampling (~1 sample per 220 km²), sufficient detail is present to observe the effects of alteration and mineralization in the CMB. As we examine smaller areas, we have sufficient resolution to identify affected drainages, but not to identify point sources of acid, metals, and sulfate. Away from the CMB, without the overprinting of hydrothermal alteration and mineralization, major and trace-element variations in surface water show a stronger relationship to catchment lithology.