Day 5: The hunt for jaspilite

(Tuesday, 1 May 2007) Today, plan was to try and sample a jaspilite pod from the huge N4E mine we saw yesterday from the overlook point. One of the reason that Carajas is especially notably amongst the world BIFs (banded iron formations) is that it is among the least metamorphosed or altered. There are large BIFs in the Great Lakes regions of the US, for example, but over time heat and pressure have cooked these rocks and destroyed much of their original minerals and fabrics. Here at Carajas, the BIF comes in three flavors: jaspilite, soft hematite, and hard hematite. Jaspilite is the least altered of the three types and contains alternating layers of often blood-red silica (chert) and iron (hematite). In soft and hard hematite, later alteration has replaced much of the silica bands with iron. This is great from a mining/economic standpoint since it means more overall iron, but less than great from a geologist's perspective since it means that understanding the original rock is that much more difficult.

Example of soft layered hematite in N4W mine from yesterday.

During his PhD research, Joel had studied and mapped a large lens or pod of jaspilite. Unfortunately for us, it is in an area of active mining in N4E. Due to safety concerns, only one of the two cars will visit the site and try and to take samples. I stayed behind with the other group to explore the ledge we were on. After a brief interval, the first car returned from their sojourn deeper within the mine. Although they located the jaspilite area, much of it had been mined out. Mining briefly halted while they grabbed some remaining jaspilite samples, but they were not that impressive. This failed attempt illustrates one of the great frustrations of doing geology in an active mine. Although the exposure and access to material is unparalleled, given the pace of mining, the lifetime of interesting exposures is not long.

Next, we stopped an overview point near 5NE mine. Since this locality is not being actively mined, we should have an easier time studying the exposures and obtaining samples. We first stopped near the top of the mine to sample relatively fresh mafic material. The BIF here is roughly 200 m thick, and is bounded on the top and bottom by volcanics that have mostly been altered. Since one of the overall goals of this program is to gather ground truth data for satellite and airborne remote sensing spectral instruments, it is important for us to widely sample the exposed rock types in order to make sense of what we see in the satellite images.

At the bottom of N5E, we were able to sample so-called "hard hematite." In this material, all of the silica layers have been replaced with iron, and additional iron has filled all of the pore spaces in the rock. It is amazing heavy stuff, and yet still retains some trace of it's original layering.