8.2.1: Deposit and Spatial Terms

8.2.1: Deposit and Spatial Terms

If we are in a horizontal deposit and we mine an opening into the deposit, that opening will have a top and a bottom. We often use those terms. We might say, “this mine has good top,” and we mean that the rock structure at the top of the opening is competent. Or, we might say, “that seam is underlain by clay, and the bottom is terrible,” and we mean that it is difficult for equipment to move around because of the poor condition of the bottom. In a tabular deposit, we frequently refer to the top as the roof, and the bottom as the floor. Conceptually this aligns with our everyday experiences – if someone refers to the roof or the floor, we know what they mean. Likewise, if we refer to the top or bottom of our mined opening, we intuitively know what is meant. As I indicated, these four terms, top, bottom, roof, and floor, work well for tabular deposits, and are used in coal, trona, potash, salt, and stone mines, for example.

When we move into metal mines, we will not hear those terms. There are a few reasons for this. The mining methods and culture evolved differently in metal versus coal and some of the nonmetals, and they each evolved their own terminology. It’s similar to the situation with the word we use to describe a soft drink. In some parts of the country we would ask for a bottle of “pop.” In another region, we would request a bottle of “soda.” If you go into an area and use the “wrong” term, people may snicker and smile, but they will know what you mean. The same is true for some of these mining terms, except that in addition to smiling, they will likely think that you are a rookie and perhaps are not really a mining person. If, in fact, you are a mining engineer, then that would not be a good thing!

The second reason for the difference in terms relates to the spatial complexity of many metal mines and metalliferous the deposit. In a steeply dipping deposit, for example, the concept of a roof and floor is less useful than hanging wall, footwall, and back. And, of course, there can be crossover in terms. A mining engineer working in a tabular limestone deposit will refer to the top of the opening as the roof, unless they have a background in the metal industry, and then they will refer to it as the back. Don’t despair! As you look at the figures, study the definitions, and read more about the methods, it will become perfectly clear. On with the definitions!

Initially, students are uncertain about which is the footwall and hanging wall. If you have doubts, remember this: these two terms only have meaning if the deposit is dipping at an angle; and pretend the opening in the deposit is a sliding board, and that you are going to slide down – and when you do, your butt will ride on the footwall. Now you’ve got a silly but effective way to keep it straight. Speaking of the footwall, you will notice that the shaft and the workings are located in the country rock and on the footwall side of the deposit, not the hanging wall. There’s a good reason for that. Why do you think that is the case?

All right, onward with the definitions! Here are two more of major importance.

We’ve talked on various occasions about deposits that dip, and so you already have a familiarity with the term. A synonym that is sometimes used is pitching. The terms steeply pitching or a steeply dipping are synonymous. This is true whether it is a seam, vein, deposit, or orebody.

If you’re a surveyor or a geologist, then you are very familiar with the concept of strike. The dip of a deposit can have a profound impact on the type of mine that we develop, but not so for the strike! The strike provides us a direction on the compass to orient our workings, but little more than that for the mining engineer. Now, for structural and economic geologists… they can get excited by the strike of the vein!

In the process of excavating an opening, i.e., mining, we will often leave behind sections of the orebody, for the sole purpose of providing structural support to the overlying layers of rock. These unmined blocks are called pillars, and they are necessary to prevent a collapse of the mined openings. Certain pillars are given specific names, as follows.

Crown and sill pillars are commonly found in mines for steeply pitching deposits, and barrier and yield pillars are commonly found in tabular deposits that are nearly horizontal.

Finally within this first group, three more terms: