As a graduate student at the University of Arizona, I somehow wangled a summer job with AMAX Exploration, a mining company related to the Climax Molybdenum mine in Colorado. Like Bear Creek, Kennecott, Placer Dome, and Rio Tinto, this was another exploration entity focused on locating another billion-dollar mineral deposit.
My assignment for a summer: travel all over remote southeastern Arizona and southwestern New Mexico and make gravity measurements. I was given a 4-wheel-drive vehicle, a gravimeter, a credit card, a $400 cash advance, and a set of maps with broad circles on them where there were no gravity data. I was basically a free agent: leave Monday morning, and return Friday evening, and collect as many gravity stations as I possibly could - in a remote region where you could have a vehicle break down and find yourself 40 kilometers from the nearest human being. This sounds like the ultimate freedom, doesn’t it? Well yes, but it had its drawbacks. For one thing I would push myself to work 12+ hour days, because I missed my little family - and wanted to collect as much data as possible to make sure I kept this job. For another, it meant that when I had a vehicle breakdown, it could be potentially life-threatening because I was so isolated. This happened to me several times during that first summer working for AMAX.
You may ask why was gravity data so important? Amax was looking for another porphyry copper deposit like the billion-dollar monsters found all over southern Arizona and northern Mexico. All the porphyries exposed to sight had been already found, so that meant that we had to search areas where others could still be hidden. A major potential target area was where valleys in the Basin and Range province (that included Arizona, Nevada, parts of Utah, and Mexico: they were covered with recent sediment weathered off the surrounding mountain ranges.
We all knew that dirt, sand, and gravel - basin and valley-fill material - are less dense than a magmatic porphyry body; the porphyry should show up in gravity data, then, as a denser “bulls eye” on our final corrected map.
I also spent a day working with a geochemist, and another day I spent working with an economic geologist from the project team. I learned more in those two days than I did in a month in school.
Again, it all goes back to the deposit model thing. If you’re looking for something, you need to think out carefully where it might really be, how it got where is is, and what it looks like… or you will be wasting your time. I think of the story of the lady looking for a quarter under a street lamp; when asked by a friend where she dropped it, she gestured towards the dark street hundreds of meters away. "But there's better light here."
The porphyry-hunting team had sat around a table to think these things out – and how one might logically go about looking for a "blind" (buried) porphyry. They realized that all the exposed porphyry copper deposits were already being exploited – and in fact they had been largely found far back in the 19th Century. The trick was to find those that were hidden: covered by later volcanic flows, or debris flows, or soils. There are huge basins in Arizona: bathtubs filled with dirt, as one geologist put it. Fully a third of the state has never been looked at for blind porphyries.
The objective required a cooperative effort that was also sequential. My compiled gravity maps showed several possible interesting anomalies - places where the basins were not smooth gravity lows. Typical of the real world, however, they were never neat bulls-eyes – because the gravity meter picks up density changes at all different depths beneath it. It could “see” the edges of ancient canyons buried beneath the valley fill, and other density contrasts, which would complicate the interpretation of any final result. The geophysicist I worked for, however, was very smart, had been around, and had thought a lot about what the data showed. Frank picked several areas that could use some follow-up. He also had convinced AMAX management to pay for an aeromagnetic survey in narrow areas of the team’s focused interest: those flat-looking basins.
The team now had TWO sets of geophysical maps.
I was encouraged to go out with the geochemist one day – the company encouraged this sort of cross-pollination. The thinking was that if we each understood what the other was looking for, we could help each other – or report interesting things that the other guy might want to follow up on. The geochemist was working from my gravity maps – with certain areas circled by Frank, the project geophysicist who had hired me. The geochemist would drive his own 4WD vehicle cross-country, looking for large Mesquite bushes in those areas. He would then fight his way in past the green vegetation and clip off thumb-sized chunks of stem, collecting them in a sample-bag which he carefully labeled. These he took back to his lab in Tucson, where he reduced them to ash and then did a chemical analysis for trace levels of copper, arsenic, silver, and 17 other elements. A 20-element suite cost only a bit more than a 3-element suite, so it was more cost-effective if you also hoped for a pleasant surprise.
I asked him why he was doing this? He told me that after the first white men entered the region with all their cattle, the native grasses had been largely eliminated. The Mesquite bush, freed of water competition, survived and with cactus was pretty much all that remained because it had such deep roots. How deep? Miners, he told me, had encountered Mesquite roots in tunnels they were digging underground in the Bisbee copper mining district. Anecdotal word of mouth suggested Mesquite roots reached deeper than 30 meters (100+ feet). THAT’S how you become the dominant surviving plant species in a desert!
The geochemist contoured a set of maps for the region in southeastern Arizona where the project team had started to focus its interest… one map for each element from his chemical analyses.
Now we had TWO different SETS of maps: geophysical and geochemical. The gravity showed areas with a higher-than-usual gravity field (greater density of rocks under the gravimeter) and the magnetic data showed anomalies that might or might not be caused by a porphyry intrusive. Less than half of these porphyries in Nevada, after a long and careful inventory, had turned out to be magnetic, so by itself the mag data were not diagnostic. We also had “pops” here and there of copper and other metals from the Mesquite geochem sampling.
The exploration team gathered in Tucson and went over the geophysical, geochemical, and surface geology maps. There were several areas they all agreed were possibilities. But a single drill hole, 300 meters deep, would cost at least $50,000. Hmmmm. More proof is needed to justify this kind of expense to corporate HQ.
The geophysicist suggested another type of geophysics – more expensive per area covered, but they had already narrowed down the areas they would use it on. This kind of geophysics was electrical in nature, called induced polarization or “IP” for short (for obvious reasons). It turns out that if you inject electrical current into the ground and hold it steady for a second or two, it would “charge up” certain kinds of minerals immersed in groundwater deep under the ground. Pyrite (so-called “fools gold”) was one of these minerals, but no one was interested in pyrite – it was just iron and sulfur, and they were everywhere already. But pyrite was often associated with certain copper minerals, for instance gold-colored chalcopyrite, bornite (so-called “peacock ore”), or black covelite, a copper oxide. THESE were what they were looking for. Any excess pyrite found might just prove to be a halo around a big, hidden copper deposit, where all sorts of metals were concentrated by the hydrothermal process described earlier.
In IP survey was contracted out, and the operator sent a report back on his interpretation of the rather cryptic results. He saw polarization layering: the deeper the IP system looked, the stronger the apparent induced polarization effect was. This was a well-known geophysical version of fools gold called electromagnetic (“EM”) coupling. This phenomenon was often seen in conductive groundwater environments like Arizona had in abundance – always getting stronger with depth - as the transmitted electrical current at the surface of the ground set up eddy currents of electricity deep in the ground that acted just like disseminated pyrite “lighting up” with an electrical charge.
The contractor’s summary: I am seeing just EM coupling, nothing of interest.
The project geophysicist, however, pored over the maps for a long time. He noticed that the apparent EM coupling was NOT perfectly layered like EM coupling, but had a slight shift below where the geochemist had gotten a copper “sniff” in his mesquite chemical analyses.
Now comes the serendipity part of this story. I mentioned that this particular geophysicist was different than other geophysicists. Frank was an iconoclast: he thought differently than other people. I learned later that he had negotiated his AMAX salary with a “rider” on it. Every year the company would allocate one drill hole to be drilled on one of his hunches. I had never heard anything like this before or since, but it must have appealed to the intrigue-bone in some senior AMAX manager somewhere.
The company was about to abandon this particular target area, located near Solomonville in the Safford Valley of southeastern Arizona. Frank called for his annual “hunch hole.” Now keep in mind that the faint anomaly he was looking at was probably at least 200 meters (600 feet) deep, and Mesquite roots could not possibly reach down that far. Perry, the project geologist, told him he was crazy, but Frank insisted. The first drill went through 200 meters of sediment… and then intersected 15 meters of pure massive sulfide ore, almost all pyrite. BINGO. It was an astounding success. I saw a chunk of that drill core on the conference room table: it looked like a thick bronze bar. The target property was now even given a name: Sol.
AMAX formed a consortium with Phelps-Dodge corporation to help cover the cost of another 30 – 50 drill holes. The countryside around the discovery hole was quickly claim-staked and then grid-drilled. But after all that work the consortium kept the results to themselves. There was a clue, however: no infrastructure was ever built. It must have been a “bust” even after all those sulfides were found.
By that time I had gone back to school for the Fall, and it wasn’t until nearly a year later that I saw Frank again, and asked him what had happened. Knowing it was considered proprietary information, but knowing also that I had poured a lot of effort into the project, Frank paused. Then he said “We successfully outlined a porphyry sulfide stockwork.” He watched my face for comprehension… and then winked.
What he had NOT said was a “porphyry COPPER stockwork.” In other words, lots of pyrite, but not enough copper, silver, and gold in it to justify a mine development effort when the price of copper had sunk to below $1/lb ($2/kg).
Technically, this was an elegant, successful exploration effort. AMAX recognized that they had a brilliant team in their Tucson office, and kept funneling resources to them for many more years. But while Sol was a success, it was not an ECONOMIC success.
No comments:
Post a Comment