SEG 45 Final_qx4 - Society of Economic Geologists

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12 SEG NEWSLETTER No 63 • OCTOBER 2005 ... from 11 7482000N 7481000N 7480000N 7479000N 13 Exploring for Deposits Under Deep Cover Using Geochemistry (Continued) 473000E 474000E 475000E 24 476000E 10 Cu ppb Analyses of the soils showed that zones of high conductivity of soil-water slurries is due to the dissolution of NaCl (Na is shown in Fig. 6). Figure 6 also shows As and Se by Enzyme Leach, and Cu by Enzyme Leach, Mobile Metal Ions (MMI), and aqua regia. Enzyme Leach and MMI are proprietary leach methods designed to extract the weakly soluble fraction of metals, whereas aqua regia is a strong reagent that dissolves a much higher proportion of the total element in the soil. The active reagents in the Enzyme Leach solution are hydrogen peroxide and gluconic acid, the former solubilizing manganese oxides and their contained metals, with the gluconic acid stabilizing the metals in solution. In the soils from Spence, Enzyme Leach dissolves approximately the same amounts of elements as deionized water, but provides more reproducible analyses. The formulation of the MMI leach has not been revealed, but contains reagents that extract the most readily soluble fraction of elements, including those in carbonates. Sodium, As, and Se are all enriched in the soils lying above the fracture zone over the deposit and in the soils 13 Groundwater Flow 14 14 19 191 195 1200 200 121 812 10 224 88 48 25 955 1180 4490 53 9 41 127 27 12 29 Boundary between saline and non-saline waters Approximate Limits of Mineralization above the eastern fracture zone. Copper is enriched only in soils over the deposit. The anomaly/background contrast for Cu is much better for Enzyme Leach and MMI analyses than for aqua regia (Fig. 6). Much of the anomalous Cu in soils over the deposit is hosted by carbonate minerals. Analyses after extraction by ammonium acetate, a reagent that specifically dissolves carbonates, shows even higher values for Cu than MMI, but similar anomaly/background contrast (Cameron et al., 2004). We have interpreted these results to be caused by pumping of saline basement waters to the surface during earthquake activity (Cameron et al., 2002, 2004). We interpret the data to indicate two faults in the basement, one directly along the axis of the deposit, and the other coincident with the eastern fracture zone. Saline water with high contents of As and Se were moved to the surface above the deposit and above the eastern fracture zone. Copper is only enriched in groundwaters within the deposit and these were entrained by the rising basement waters and taken to the surface. There is no increase in Cu in 473000E 474000E 475000E 476000E 60 0 15 Mo, ppb 24 263 105 354 45 2 53 66 Soil Traverse 63 5 18 221 222 25 33 38 soils above the eastern fracture zone. Drilling has shown the basement in this area to be barren. The above scenario is summarized in Figure 7. Saline water moves up from the basement along the axis of the deposit, where it mixes with meteoric water flowing in from the east. During earthquake activity, the basement water, plus mineralized water bathing the deposit, is forced to the surface. As noted by Woodall (2005), structures form pathways for magma, fluids, and energy. Thus, the interpreted fault zone along the axis of the deposit could have provided a pathway for porphyry magma, hypogene fluids, the fracturing of the mostly impermeable cover gravels during reactivation, and the movement of groundwaters to the surface. These points are unproven, but with mining of the deposit now commenced by BHP-Billiton, more definitive answers are expected. DISCUSSION AND CONCLUSIONS 7 40 69 Approximate Limits of Mineralization FIGURE 5. Plots of Cu and Mo (both in ppb) in groundwaters from in and around the Spence deposit. The line running roughly north-south near the axis of the deposit on the Cu plot separates low salinity meteoric waters to the east from saline deep formation waters to the west (Fig. 3). Within the deposit, higher values for Cu occur in both types of water, but are absent in the waters flowing away from the deposit to the southwest. By contrast, higher values of Mo are present in the downflow waters. The location of the soil sampling traverse is shown in the Mo plot. Eastings and northings in meters. The use of soil geochemistry in the search for buried deposits requires that 32 9 54 136 9
OCTOBER 2005 • No 63 SEG NEWSLETTER 13 2000 1600 1200 800 400 0 500 400 300 200 100 0 6000 4000 2000 As Enzyme Leach, ppb Se Enzyme Leach, ppb Na Deionized Water Leach, ppm 0 473500 474000 474500 475000 475500 476000 Spence Deposit elements be moved from depth to the surface, either in gaseous form or dissolved in water. In arid regions, advective transfer, as by barometric pumping of air containing gas through fractured rock or seismic pumping of mineralized groundwater through fractured rock, appears to be more effective than diffusive processes (Cameron et al., 2004). The diffusion of gas through air and rock is slower than advection, and diffusion of dissolved constituents in the water film that exists around grains in the vadose zone is ineffective because the rate that these films migrate downward is orders of magnitude faster than the upward rates of diffusion. Exploration methods based on diffusion (e.g., Hamilton, 1998; Smee, 2003) may have greater application in regions like the Canadian Shield, where recent seismicity is rare, where the climate is wetter, and where sediments of glacial Eastern Fracture Zone 120000 80000 40000 15000 10000 5000 origin that are generally unfractured cover mineralized basement rocks. In this article we have described two examples from seismically active, hyper-arid and semi-arid regions of advective transfer of elements by pumping of mineralized groundwater along faults. Surface flooding of a desert area, as envisaged at Spence, appears counterintuitive. Nevertheless, flooding after earthquakes has been widely reported. Sibson (1981) described changes in well water level, spring flow, and occasional dramatic effusions of groundwater immediately following moderate to large shallow earthquakes. Surface flows have been documented in Iran during earthquakes in 1903 and 1923 (Tchalenko, 1973). Following the Hebgen Lake earthquake of Montana in 1959, three rivers increased in flow by ~50%, the increases continuing for several weeks 0 0 0 500 400 300 200 100 Cu Aqua Regia, ppb Cu MMI, ppb Cu Enzyme Leach, ppb 473500 474000 474500 475000 475500 476000 Spence Deposit Eastern Fracture Zone FIGURE 6. Analytical results for soils from an east-west traverse across the Spence deposit (for location see Fig. 5). Sodium concentrations were measured by deionized water leach, Se and As by Enzyme Leach, and Cu by Enzyme leach, MMI, and aqua regia. There are fracture zones in the gravels directly above the deposit and at 476000 m E. Eastings in meters. through dry weather (Muir-Wood, 1994). After the Kern County, California, magnitude 7.5 earthquake of 1952 there were outpourings of groundwater in the vicinity of faults, and increases in spring flow and well water level (Briggs and Troxell, 1955). Thus, the influence of structures related to the emplacement of an ore deposit does not necessarily cease after its primary formation. As Woodall (2005) implies, pathways for fluid and energy, which tend to move toward the surface, provide an opportunity to recognize deposits hidden below. Selective leach techniques have been widely applied in the search for buried deposits using soil geochemistry. These methods attempt to extract the most mobile fractions of the elements that are present in soils as water-soluble salts, or to page weakly adsorbed on soil 14 ...
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