Quantifying the effect of field filtration versus laboratory filtration in hydrogeochemical sampling

In recent years the rate of mineral exploration success has declined world-wide and in the Great Basin of Nevada. The areas with near-surface and surface exposures of known ore hosts have been highly explored while underexplored areas are dominated by challenging environments, such as where transported cover overlies the rocks of interest. An example is north-central Nevada, where it’s well-known Carlin-type gold deposits (CTGDs) are dominantly located in and proximal to outcropping ranges while over half of the region’s bedrock is under basin cover and locally very deep valley-fill sediments. There are known CTGDs beneath these basins but the expense of grid drilling to potentially ore-hosting bedrock has deterred many explorers. Hydrogeochemistry has the potential to open these water-filled basins for economic exploration, as CTGDs have been shown to have measurable and distinct Au, Sb, As and W anomalies in waters proximal to mineralization (Grimes, et.al., 1995, Muntean and Taufen, 2011, Buskard et.al., 2019).  

Hydrogeochemistry is a powerful technique for the direct detection of mineralization in areas of transported cover. It is made possible by advances in analytical chemistry that have allowed ultra-low detection levels on large suites of elements at commercial prices.  Due to these trace detection levels and the inherently low concentrations in water, the increased potential for contamination at all stages of sampling and analyses should be considered. One of the potential sources of contamination is the field filtering of waters, although this can be mitigated by rigorous sampling procedures. The methods of hydrogeochemical sample collection can be daunting to geologists who are used to dealing with much more robust samples, as such excluding the field-filtering stage may be advantageous for companies with an interest in hydrogeochemistry but without the equipment and knowhow. The problem with this approach is that fine particles of Al- and Fe-oxyhydroxides suspended in natural waters can adsorb cations to their surface, which are then removed from solution when filtered, thus, reducing or destroying anomalism in the sample. By this reasoning, field-filtering of hydrogeochemical samples is the traditionally recommended preparation, along with acidification, to maintain the elements of interest in solution.

To quantify the difference between field-filtering and acidifying water versus water filtered in the laboratory, a selection of samples from different hydrological environments have been compared.  Samples were both field- and lab-filtered, with a variety of hold times between collection and lab filtering. The changes in the trace-element composition is the most marked for Au, with any Au anomalism present in the field filtered samples almost always absent from the laboratory prepared samples. Other trace elements were found to be less sensitive to the timing of filtering. These results suggest that explorers interested in using Au in water as a vectoring tool should field filter their samples to obtain reliable results.  It also suggests that Au in groundwater is much more likely to be removed from solution than other trace-elements associated with mineralization. This will limit the footprint size of Au hydrogeochemical anomalies and therefore requires tighter spaced sampling if Au is to be used as a primary pathfinder.