Five Essential Resources for Desktop Exploration in the Great Basin
There is more geologic data available to the public than ever before. What are the tools available to us and how can we use them to make discoveries or generate exploration targets?
Let’s discuss the most essential exploration resources available:
USGS Hydrothermal Alteration Map:
The USGS Hydrothermal alteration map is a hugely valuable resource and the first map that I turn to when evaluating the exploration potential of a targeted region. Although there are deposit types that form from igneous, sedimentary, and metamorphic processes, hydrothermal deposits are the most widespread and commonly occurring in the Great Basin. Carlin-type deposits and Comstock-like deposits are all epithermal (not to mention porphyry, skarn, and volcanogenic massive sulphide (VMS)).
When using this map, the most prospective areas for epithermal precious metals deposits will be those displaying argillic and phyllic alteration, where hydrothermal activity has generated clay from the destruction of host rocks. If you are on the hunt for porphyry or skarn-type deposits, locations showing propylitic alteration (epidote-chlorite) can help narrow the search, ideally with phyllic and argillic alteration nearby.
The Bullfrog district is a rich system and host to Carlin-type, low-sulphidation and high-sulphidation deposits. The USGS hydrothermal alteration map shows the phyllic and argilic alteration from several of the Bullfrog district’s deposits. Gray shaded areas are listed as hydrothermal silica-rich rocks which, in most cases, will be epithermal cap rock indicative of Intermediate-sulphidation and high-sulphidation systems. A target-rich area like this deserves a good once-over, although most of the district is now staked and being explored by major mining companies.
The National Geologic Map Database:
After identifying an area of prospective alteration from the hydrothermal alteration map, the next step involves the identification and review of the regional/local geology. This is useful for a number of reasons, primarily being the age and composition of the geology.
The National Geologic Map database is the next step in evaluating targets. This snip from their website shows the most recent geologic map of the area detailing faults, rock types and contacts. Maps from the National Geologic Map Database can be downloaded in multiple formats, with my favorite being the Google Earth .kmz format for easy 3D interaction.
A snip of the geologic map as downloaded from NGMDB. The maps are geo-referenced and easy to manipulate in Google Earth. Not only does this add to the ease of evaluating rock types, and faults for your target areas, it allows you to do it in 3D with the additional tools available from Google Earth as a GIS program (measurements, claim blocks, points, etc..)
Faults are prime identifiers in mineral movement with epithermal activity. Hot epithermal fluids, when not forming their own pathways to the surface through the fracturing of host rocks (breccias), seek the path of least resistance and this is almost always along faults. After identifying a prospective region of alteration, locating faults can be an excellent way to target mineralization in your area of review.
Although the faults mapped on the NGMDB are more detailed for the Bullfrog district, the US Quaternary Fault map is useful for lesser-explored regions and a great source of information on fault ages and activity. Another source of data that can be invaluable for exploration in early stages.
Once a prospective area has been identified, another outstanding resource can be the Nevada Hydrology map. This map shows the location and details of wells across Nevada. These well logs can provide useful information on the subsurface geology as well as depth to water table which can also be useful in determining drill-hole depths where mineralization has been leached out of surface rocks.
This snip shows all the wells (municipal, monitoring, de-watering, agricultural, etc.) for the Beatty area. Any well can be selected as shown and the well log can be downloaded. Well logs will give detailed descriptions of the hole drilled and geology encountered which can prove very useful when used in tandem with geologic maps. In this case, we’ll look at municipal well log #42327.
Although we don’t have assays, this report allows us to see the rock-types encountered during drilling and the thickness of each rock-type. If the target host for mineralization was a grey fractured tuff, we’d know that holes around 200’ in that area will most likely capture that rock-type or that it exists at depth in this area.
BLM Navigator and Mineral & Land Records System Reports:
Last but not least, the BLM navigator and MLRS systems are essential in determining whether you can or can’t stake claims. The BLM navigator provides a map of the US with all PLSS boundaries on it. You can find the Township, Range, Section and Meridian with the BLM Navigator and plug this information into the BLM MLRS system to see if the ground is open for staking. You can also use the MLRS to see any historic claim activity in the area. If the ground is open for staking, and fits all the criteria for your search, it is time to get out in the field to take some samples and see it for yourself.
The BLM Navigator is a great tool when it comes down to defining exact locations of prospective targets. This is useful for writing up claim notices as well, where you can determine the Township, Range and Section of the area you are staking. For this example, let’s say you want to stake a target east of Beatty. We know this is Section 1, Township 12S, Range 47E, Mount Diablo Meridian.
A closer inspection of T12N R47E Sec.1 shows that it is most likely entirely under claim by Corvus Gold. To know for sure where the claims are located exactly, a trip to the State BLM office in Reno or to the local field office will yield claim maps and documentation of claim notices or certificates. These filed claim maps are the gold standard of claim location, but the BLM MLRS Reports will get you pretty close.
With these tools, not only is it possible to locate new exploration targets, but to determine where in an area the mineralization might be hosted, how deep the mineralization may be, the host rocks for the mineralization and their age, and whether or not the ground is open for mineral entry. Pretty handy.
