The Pilar and Roça Grande deposits are examples of the Algoma BIF-hosted type. The main geological characteristics of this group are summarized as follows:
- Main host/fertile “Algoma-type” BIF Units: These units host the mineralization and are stratigraphically located at the waning stages of major volcanic cycles of the Rio da Velhas greenstone belt.
- Mineralization style: The mineralization consists of mainly “lateral” replacements/sulphidations of the iron carbonate-rich bands of the host Algoma-type BIF units. However, the BIF-hosted gold mineralization is not syngenetic in nature (in relation to the deposition of the host rock packages), rather it is clearly an epigenetic event that has occurred after the formation of the host rock units.
- Dimensions of the economic orebodies: Economic strike-lengths of only 50 m to 350 m for individual mined zones. Average thicknesses of the BIF-hosted orebodies may range from 2 m to 20 m.
ROÇA GRANDE DEPOSIT GEOLOGY
The Roça Grande Mine is located in the upper unit of the Nova Lima Group. The dominant rock types found in the mine are a mixed assemblage of meta-volcanoclastics and meta-tuffs. These are represented by quartz sericite and chlorite schists with variable amounts of carbonate facies
BIF, oxide-facies BIF, meta-cherts, and graphitic schists. The iron formations, chert units, and graphitic schist units are intimately inter-bedded with each other, such that they form ........
