a new (to me) sort of sample collecting

Sample collecting. It is something that we geologists have all done. It isn't enough to look at rocks in the field, we need to bring them home too, if we want to answer the questions that come up in the field. My current job, however, doesn't yet have field work in the traditional sense. Instead my “field” is the collection of drill cores which have been amassed over years of exploration and mining in this district. I have just completed my first two “field” trips to look at this core, which is to say I have been introduced to the core storage shed at the mine headquarters. Last week and the week before I worked with one of the geologists in the exploration division of the mine office. We looked at four different drill cores to select representative samples at roughly 50 meter intervals to be sent away for geochemical analysis.

This is an interesting task—my project will involve 3D geochemical modelling with the aim of understanding the alteration that has happened to the rock in the region during the process of the formation of the ore deposit. Therefore we need to be certain the samples we choose have been altered, but that the alteration that has taken place in these rocks is related to the main alteration event of interest. Therefore we want to avoid the sections of drill core which contain small veins which cut across the fabric of the rock (which means that they formed well after the deformation that caused the main rock fabric, and so are younger, and so probably aren’t related to the question I am trying to investigate). Because my project will be on the kilometer scale we don’t want to take the samples too close together (hence the 50-meter interval rule of thumb), but then again there is value in making certain that we have a good representative collection of samples that actually show what rock types are present in each drill hole.

This is easier to do for some drill holes than others. You see, drill core comes in different diameters, based on a variety of factors when they do the drilling, but the boxes into which they store the core are fairly uniform in size. This means that one box will hold more meters of narrow core than it will wide core. We sampled 4 different drill holes over the two weeks I was there—the narrowest core tended to be 10 or 11 meters of core per box, but the widest one was less than 5 meters of core per box (the other two were about 7 meters each). The tables upon which we spread the boxes hold 10 boxes at a time, which means we could view 50, 70, or 100 meters of core at a time. Needless to say, it is much easier to decide what rock types are “typical” for a given stretch when we can see more of the core at once.