Copper porphyries are a big deal for exploration geologists. The romantic stories about chance discovery pale in comparison to the huge amount of effort required to delineate one of these systems. At the early stages of exploration, porphyries are one of few places where a geologist can genuinely say they learned something important by missing with a drill hole. The process of elimination is costly, but it can be your friend with porphyries.
What's more, the porphyries seem to get a lot of attention from researchers in the geological sciences. I'm not a geologist, but I seem to encounter some of the most sophisticated people and approaches in search of copper porphyries. It makes sense considering that one porphyry can contain tens of billions of dollars of economic metal -- these deposits have fairly consistent technical features for scientists to dig into and funding from stakeholders who want a leg up to find the next one.
"The value of the resources extracted to date from the Bingham Canyon Mine is greater than the Comstock Lode, Klondike, and California gold rush mining regions combined." -- Visual Capitalist
Again, I'm not a geologist but I am keen to figure out if companies like Bell Copper (TSXV:BCU) are wasting time and money chasing their dreams at the Kabba porphyry in Arizona. I have a lot of time for teams that have open and accessible executives who are more comfortable in the field than the boardroom. Since I've still got a lot to learn, I can ask silly questions like, "how deep have to drill to get an answer from a particular hole?" The answer depends on the particular case at hand but it's often quite deep with porphyry copper targets.
I have been compensated to prepare and distribute a series of interviews with Dr. Tim Marsh, President & CEO of Bell Copper, about their activities. In my newest interview out here, I asked Tim why they drill so deep at Kabba?
A big part of the answer is that you have to drilling through oxidation boundary to get into the unoxidized rock that tell you if you're in heart of porphyry system or not.
Things get a bit complicated for Bell Copper because the porphyry at Kabba broke in the mid-section and the upper part, which should contain the copper, moved a great distance. This movement was possible because of space created by the Basin and Range Extension that affected the Western USA and is a big reason why lode deposits in Arizona and Nevada are notoriously broken up.
After the Kabba porphyry first formed, an oxidation boundary developed as water percolated down from the surface and caused sulphuric acid to form, eating through rock and moving some minerals downwards until they reached the water table where the acid was diluted. You typically see a nice layer of enriched mineralization at that depth, which can provide a nice boost to the economics of a porphyry deposit because it yields elevated copper grades early in the mine life.
But as the Kabba porphyry slid, it moved downwards and that causes weird things like Bell Copper drilling into oxidized material at 1,000 feet below surface. That rock didn't oxidize when it was that deep, it had to come from somewhere else. As Tim said in our interview, "We hit the modern water table around 700-800 feet below surface and we've been hitting the paleo oxidation boundary at around 1,200-1,300 feet below surface."
They have hit some sugergene copper enrichment at the oxidation boundary, as in hole K-5 and K-6 years ago, but they haven't hit the heart of the system yet. When they do hit, the best case is that they see both a copper-rich blanket several hundred meters deep with grades of +2% Cu and then all the hallmarks of copper porphyry below that in the same hole. It makes drilling an all-nothing kind of exercise, where you can spend years poking around at the edges of the system before you really start to find it. Then years further to define the thing and decades beyond that to mine it! Requires a certain degree of patience, doesn’t it?