Wednesday, May 16, 2012

Chromite Geophysics

At this point I'm going to move into more direct applications of geoscience. We will start with this chapter on geophysical methods and why they are immensely useful. Simply put, geological mapping covers the surface of the earth and makes inferences about the third (buried) dimension from this. Geochemistry is similarly two-dimensional, but inferences can also be drawn from these data about the third or buried dimension. Geophysics, on the other hand, directly images that third dimension. That sounds wonderful, on the face of it, but there are limitations that one must always be aware of. For one thing, if there is significant topographic relief, it complicates any interpretation. For another, the deeper you want to "see" the less resolution that you will have; a pipe buried at 1 meter depth is visible to a surface electromagnetic system, but not if it is buried at 100 meters depth. This is just like trying to read a sign a meter away vs the same sign at 100 meters away.
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I belong to a specialized group on LinkedIn where people ask and answer questions related to mining geophysics. One question came in recently about podiform chromite deposits. Chromite is chrome-iron-oxide ore, typically an extremely dense black rock. I have a small 25-kg (55 lbs) sample on our back deck – it’s only about the size of a jogging shoe. However, its density is over 7 g/cc, so if you try to pick it up, you find you can't - until you reposition yourself and straddle it first.
To help you follow the explanation below (which was aimed at an experienced geologist), I need to add a few explanations. An “Ophiolite” is a piece of ancient seafloor that has been rafted up onto a continental margin by a plate-tectonic accretion process. Think of shoving a sheet against a pillow – part of it will end up on the edges of the pillow for the same reason that some ocean-floor ended up on the Oregon-California coast. The rock-type we found there, called “Harzburgite” is a weird, dun-colored ultramafic rock; this means it is quartz-free, and mostly made up of manganese-iron minerals. This rock has distinctive green olivine crystals in it that come from the Earth’s Mantle, and which don’t weather as fast as the rest of the rock, so they stand up from an exposed surface in points and edges. From personal experience, these will shred your skin if you fall on it. “Serpentine” is a highly magnetic, water-and-heat-cooked mineral assemblage usually found in fracture zones in Harzburgite. The expression “podiform” simply means that the chromite is typically found in massive, dense “pods” 10 – 20 meters (up to 60+ feet) in diameter in the host rock, not unlike raisins in raisin bread. A “gravimeter” (or gravity meter) is a sophisticated device with a spring and balance that is extremely sensitive to tiny changes in the pull of the Earth’s gravity. These devices are so sensitive that changes in where the Moon and Sun are located in the sky will appear as large changes in your repeat measurements in one place as a day goes by. Gravity measurements are also strongly affected by changes in latitude and elevation. All of these things must be corrected for – subtracted out of your measurements – before you can get meaningful numbers out of your gravity survey. “Resistivity” is a measurement of how well some material conducts electricity – the greater the resistance to electricity, the greater the resistivity, which is just a volume-independent value. Metals and some sulfide minerals have a lot of free electrons, so they conduct current easily and therefore have a low resistivity.
 So you will see from what follows that for anything to work, everyone has to learn to talk with each other - the geophysics is useless without an understanding of the geology, and vice-versa.
Q:
What is the best and most effective geophysical survey method for Chromite deposits exploration?"
--Yildiray K.
A:
I did some research years ago on podiform chromite in the Josephine Ophiolite in northwestern California. Before I went there, I did some homework first. One gravity survey reported in the scientific literature by the USGS in Cuba (during the pre-Castro era!) had a weak correlation between gravity anomalies and podiform chromite bodies in Camaguey Province. Only about 10% of the anomalies were unequivocally caused by chromite pods, but those discoveries made the survey technically economic: more value was discovered than was spent in the effort searching for it.
There are two problems with gravity surveys that have to do with the sensitivity of the gravimeter and the relatively weak anomalies we are looking for. Think about this: the gravimeter is measuring the effect of all the Earth below you, but you are only interested in the tiny fraction shallow enough to be drilled or mined.
One major difficulty with gravimetry is that you must get a precise elevation for where you are making the measurement. You must correct for even tiny elevation changes to get useful numbers. If the gravity meter is just a meter lower, it will place you closer to the center of the Earth, and the effect of gravity will become significantly stronger – modern gravimeters are that sensitive, and the anomalies being searched for are that weak.
There is another major difficulty with gravity measurements: terrain corrections. If you are on the side of a mountain, the part of the mountain above you to your left, say, will effectively pull upwards against your gravimeter. That part of empty space below you to your right will also contribute – in a negative sense of NOT pulling against your gravimeter. That means that terrain effects are doubly-additive. To correct for these, you must mathematically subtract out the contributions of the different elevations in concentric rings around each gravity station measurement. Typically these corrections are done out to 167 kilometers (100 miles) from each and every station. Ugh.
Because of this, gravity terrain corrections often prove to be the weak link with this kind of survey. In both Cuba and northwestern California, the corrections were far larger than the anomalies caused by the chromite pods - because the terrain we were working in was so rugged and steep. Because of this, the gravity only worked reliably for finding shallow chromite pods.
Podiform chromite deposits tend to be very self-contained (like raisins): there is very little external indication or halo that you are even close to the chromite body in most cases. In my experimentation in the Josephine Ultramafic Complex, a microgravity profile could readily detect pods we already knew existed, and even suggested several others.
Magnetic surveying only showed us where the serpentinite was best developed in the Harzburgite ground mass. This could be construed as an indicator of stress on the Harzburgite by a dense nearby chromite pod during the Ophiolite emplacement process. Basically, the massive chromite pod beat up the surrounding rock as the whole mass was emplaced, and that lead to much faster weathering and serpentinization close to the pod. It’s sort of like having a jug of milk packed in the same grocery sack as your bread and chips. If you brought several sacks of these things home, you could tell right away which sack held the milk jug - by which sack of chips had been turned to powder.
We also experimented with refraction seismic methods. We pounded with a sledgehammer on a steel plate laid out on the ground. With sensitive geophones strung out in a line over the terrain, we measured the arrival time of the sound impulse. We found that yes, there was indeed a significant velocity increase when the sound waves passed through the chromite vs. the surrounding serpentinized Harzburgite groundmass. However, this velocity advantage was offset by the complex 3D terrain we were working in, and was difficult to interpret data if we did not already know where the chromite pod was.
Finally, we experimented with resistivity and “Complex Resistivity” – the change of resistivity with transmitter frequency -  in both the field and the laboratory. There was no strong amplitude change over the frequencies we tried, but there was a subtle time-delay (phase shift) that we believe was caused by Kemmererite. This is a deep reddish mineral caused by alteration (hydrothermal “cooking”) of the chromite over time. It shows up as a thin red rind in a microscope thin-section, surrounding each blob of chromite, and behaves differently in a number of ways from the chromite. The resistivity of the chromite itself is significantly higher (acts less like a metal) than for the beat-up and serpentinized Harzburgite. Again, this small advantage is marginalized by other difficult-to-fix variables including terrain effects and localized serpentinite veins.
The bottom line: geological mapping doesn’t work very well to find buried chromite orebodies. Geophysical methods, especially when several are combined to reduce ambiguity, CAN find these things – but only if they are relatively close to the ground surface. Rough terrain makes it much harder to interpret any results, however.
As a final, odd anecdotal aside, we made several excellent plaster casts of some huge, stream-side footprints that we found in this extremely remote and inaccessible area. These footprints were ~40 cm (at least 16 inches) long, with five toes and a heel-width of ~10 cm (4 inches). Several times, over two separate summers, we even heard the deep hooting sounds of the creatures that apparently made these footprints. The field evidence suggests they were two-legged, very large, and not human - and not bears, either. 
From these hints, can you attach a name to these BIG footprints?
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