By Eric Eckberg, Geologist
Innovation and creativity are at the heart of almost everything we do as explorationists and miners. I’ve yet to hear about a program that does not involve creativity in some way, except for perhaps failed campaigns. The following series of anecdotes is a quasi-scientific recollection of my time as the Senior Project Geologist for Evolving Gold Corporation on the Rattlesnake Hills gold project. Part of innovation is recognizing the limits of one’s knowledge, and not being afraid to try new techniques in order to change the current model and to increase success while sometimes stretching budgets.
The Rattlesnake Hills alkalic diatreme complex, located in central Wyoming in the Rattlesnake Hills, hosts gold mineralization within and adjacent to Eocene volcanic edifices and intrusives at a variety of depths. When I started working on the project, our working model was based on Cripple Creek-Victor (CCV), the world-class example of an alkalic diatreme-hosted gold deposit. CCV has been studied more extensively than most of the alkalic systems known to mining, and, of course, the 20+ million ounces (566.99 tonnes) of gold mined from CCV tends to look rather good to potential investors.
The tricky part at Rattlesnake was that despite the mineralization coming to surface and outcropping in a few spots, it was not easy to recognize. Old-time prospectors tended to use the Atlantic City, Wyoming area model – that is, orogenic quartz veins with free gold – as demonstrated by numerous prospect pits focused on quartz veins in the hosting Archaen amphibolite, and previous exploration efforts from the 1980s and 1990s. Our attempts to drill mostly within the diatreme quickly changed with the thousands of feet of logging core, as we recognized the nuance within the mineralization. By the time I had arrived, we were planning a modest drill campaign on the property, and determining targets based on limited drilling and sampling from the previous year. Luckily for innovation’s sake, the management allowed for full geochemical analysis of all sampling with both gold assay and a large trace elemental analysis, and openly encouraged new ideas and software to examine what we were finding, rather than just planning on focusing on doing what had been done previously.
Initial stage – meaning the first year – innovation focused on testing the working hypothesis that alkalic diatreme hosted mineralization and resulted in modifying the model towards a deep gold-porphyry system introducing hydrothermal fluids along a pre-existing diatreme wall and depositing gold there and within other reactive host lithologies. As a fairly young explorationist, this exercise of logging core and identifying important textural and mineralogic features was fairly boring but essential for good surface mapping and prospecting. The core was where the answers were held because there was a direct relationship between positive assay results and geologic textures. Once those features were recognized, the geologist could head to the field, armed with the filter of what gold-bearing rock looked like. Further, by examining the core and really paying attention to what the rocks said (channeling my inner Haddon King), I could start developing an eye for the alteration replacement textures and colors inherent in properly cooked rock: a subtle olive-green color and small scale, about 2-10 cm (0.79-3.94 in) wide breccia features. At this point, the subtly of the Rattlesnake Hills gold system was becoming apparent: gold was hosted by a network of sheeted and stockwork veins, each 1-10 mm (0.04-0.39 in) in width. The vein orientations were almost orthogonal to the orientation of the ore body as a whole, meaning the direction of the drill could completely change the results from a nice long run of moderate mineralization to a series of seemingly disconnected one-hit wonders.
Winter months brought time to go back and relog in detail sections of core that return exceptional gold assay intercepts and examine the placement of those cores in 3D space. On top of that, geophysical surveys from the surface (IP and CSAMT) yielded more data to compile and view in fantastical volumetric shapes and features. Collaborative work within the team and with contracted experts (geophysicists, etc) led to the identification of other targets of interest, all starting with the ground truth that the core provided. It also brought time to consider other techniques and ways to squeeze value out of the already paid for drill core.
One absolutely vital way to do this is through a close examination of core photos. There are new products and devices now that help with this, but even just a decade ago, creating a digital library was important. Being able to look over the core textures from the warmth of an office was informative, especially for categorizing specific features in a systematic manner. Often when logging core with multiple people, different eyes pick up on things differently – this can be reduced through proper training and management – but for a specific texture, it helped to be able to go back and relog the entire suite at once.
I used this time to check for host rock alteration and consolidate logging results into useable formats, thus creating a more effective system. It also became obvious that fractures and faults were of paramount importance. The host rock was generally very solid, moderate- to well-foliated amphibolite, which allowed permeability parallel to foliation, but constrained large-scale fluid migration across foliation. Brecciation events allowed for greater permeability, and seemed to precede intrusive dikes or polylithic breccia pipes. By logging the brecciation degree, I could almost predict where deeper drilling might encounter a dike, and by overlaying assay results (especially anomalous to low-level mineralization), even predict a channel way for gold-bearing hydrothermal fluids.
During our second year of drilling, we attempted several other techniques. The first of these was the COLOG downhole viewing technology, which involved lowering a sond down the completed drill holes and using a combination of video and acoustical sensors to replicate oriented core for interpretation of structural data. This technique, unfortunately, was not effective because the gold-bearing veinlets were smaller than the level of resolution available at the time. A more effective bit of innovation involved logging portions of the drill core using a handheld magnetic susceptibility meter. This demonstrated that highly mineralized core tended to have a reduced magnetic character in comparison to fresh rock of the same lithology: an unseen alteration effect was magnetite destruction. While impossible to have a magnetic susceptibility meter on hand at all moments, we were able to create a low-cost workaround. For igneous rocks, if the core would attract a swing magnet, it would never show greater gold values than about 0.5 g/t. Developing this workaround got the team thinking about other ways to identify important alteration assemblages, and thus, we learned to check for carbonate speciation: calcite vs dolomite. Calcite presence was similar to magnetic character: if the core fizzed, there was a much lower likelihood of gold numbers above 0.5 g/t. In summary, when logging core, we ran a swing magnet and dropped acid down the whole length of the core. In other words, innovation by using simple and cheap technology.
One difficulty we encountered on this project relatively quickly was that one of the target zones was oriented as a tabular body with a near 90-degree dip. This meant that the targets got deep but fast. As many projects have experienced, this made targeting below 1000 m (3280.8 ft) very tricky and expensive. One way to try to get around this is to utilize wedges from deep holes in order to obtain multiple interception points on targeted areas. This was a key bit of drilling innovation to obtain intercepts on several prospectively altered and mineralized igneous dikes. The main lessons from this technique are that the exploration team had to communicate very effectively with the drillers, and both must have a great deal of trust in setting the wedge.
At the end of three drill seasons, the exploration team I led revised the model for the project from a classic alkalic diatreme model to a series of porphyritic dikes bleeding into multiple structural corridors, possibly fed by a larger porphyry system at greater depth. The primary tool for this transformation was the drill core, but with the addition of geophysics, geochemistry, 3D visualization, ‘old school’ cross-sections and plans, drawn on paper with colored pencils. While in this case, the project did not become a mine (yet), the answers we were able to determine and the inevitable iteration of follow up questions are helping a new team with the project. I, for one, raise my glass to their hopeful success.
Thanks to the team I worked with on Rattlesnake: Nick, Cory, Lew, Rich, Ashley, Tom, Dr. Quinton Hennigh (the ‘re-discoverer’ of the project), Robert Barker, and Ruen Drilling.
