Sands are mainly composed of Quartz, feldspar and Heavy minerals. The main THM mineralogical suite includes: Titatium Oxyde minerals like Ilmenite, Rutile, leucoxene (that containes Fe and TiO2), zircon (ZrO2) and magnetite (Fe3O4).
Hi Steve, These sand deposits can be over 100m thick, and the current dredging mining process does reach down to the bottom. But instead of mining down, the sand actually free flows downwards by gravity to the dredge pond (like a landslide).
Heavy Mineral Sands are generally young geological deposits, recently formed (mostly Quaternary). Their genesis relates to the natural cycle of erosion - transport and sedimentation/concentration. They normally cover large areas when disseminated in sand dunes for instance, or can be highly concentrated, but in small areas, such as strandlines.
A large deposit can have a large surface coverage, say tens of kilometers by tens of kilometers. For easy visualization, imagine a land surface of 30km by 20km. However, some very high grade deposits, yet economically viable, may have smaller footprint, say surfaces of hundreds of meters wide by a couple of kilometers long.