Summary:
The Pilar and Roça Grande deposits are examples of the Algoma BIF hosted type, Córrego Brandão is a folded and sheared deposit hosted in carbonate-quartz-chlorite schists with iron occurrences. 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. They are overlain by later sedimentary rocks composed of greywackes and turbidites.
- Mineralization style:
- The mineralization consists of predominately “lateral” replacements/sulphidations of the iron carbonate-rich bands of the host Algoma type BIF units. The BIF hosted gold mineralization at Pilar, however, 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. There is a clear temporal-spatial-genetic relationship between the replacement/sulphidation of the host BIFs and the onset of a structurally controlled, district scale silicification event at Pilar. Textural observations suggest that at least two episodes of quartz veining has occurred. The early stage veining is deformed by the regional strain and clearly pre-dates the deformation event. The later stage veins are observed to crosscut the strain-related fabrics and so are interpreted to represent a younger mineralizing event that has taken place some time after the regional deformation event.
- Other than the mineralization contained within the RG07 deposit, the gold mineralization at Roça Grande is more stratiform in nature.
- Dimensions of the economic orebodies: Economic strike lengths of only 50 m to 350 m for individual mined zones. The average thicknesses of the BIF hosted orebodies may range from two metres to 20 m.
PROPERTY GEOLOGY
Roça Grande is located in the upper unit of the Nova Lima Group. The dominant rock types observed at Roça Grande 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, metacherts, and graphitic schists. The iron formations, chert units, and graphitic schist units are intimately inter-bedded with each other, such that they form an assemblage of chemical and clastic sedimentary units.
Two important BIF horizons are present at Roça Grande and they are separated by a central unit of sericitic phyllites and schists. The two BIF horizons are approximately parallel and referred to as Structures 1 and 2. In general, the southern BIF unit (Structure 2) is thicker than the northern BIF unit (Structure 1). The North Structure (Structure 1) hosts the RG01 mineralized zone and the South Structure (Structure 2) hosts the RG02, RG03, and RG06 mineralized zones (Figure 7-3). The RG07 mineralized zone is located immediately in the hanging wall of Structure 1 and is predominately hosted by a quartz vein. The bedding is well defined by the carbonate-facies iron formation and chert observed in the BIF horizons, with an overall azimuth strike of 70° to 80° and dipping approximately 30° to 35° south.
At the mine scale, folding of the iron formation stratigraphy is generally absent. Local folding and faulting in Structure 2 has been observed in the RG06 mineralized zone where a 200 m to 300 m strike length of the stratigraphy has been folded.
MINERALIZATION
At Roça Grande, gold mineralization is commonly associated with BIF horizons. In the RG01, RG02, RG03, and RG06 mineralized zones, gold mineralization is developed approximately parallel to the primary bedding and is related to centimetre scale bands of massive to disseminated pyrrhotite and arsenopyrite. In many instances, higher gold values are located along the hanging wall contact of the iron formation sequence and are hosted by carbonate-facies iron formation. Grades generally decrease towards the footwall where the iron formation becomes more silica-rich. The thicknesses of the iron formations are observed to be affected by broad scale boudinaged structures. Higher gold grades are observed in the thicker portions while the narrower portions of the boudinaged structures have lower grades. Late-stage, barren quartz veins are also ubiquitously present and display a boudinaged form.
The SLR QP recommends that structural mapping information be integrated with isopach maps of the carbonate iron formation and trend analyses of the gold distribution to identify any primary controls on the distribution of the BIF hosted gold mineralization.
In the RG07 mineralized zone, gold is observed to be hosted in quartz veins that are contained within a sericite (chlorite) schist associated with an east-west oriented shear zone (Machado 2010).
Production
On March 22, 2018, the Roça Grande mine was placed on care and maintenance.
| Commodity | Units | 2018 | 2017 | 2016 | 2015 |
All production numbers are expressed as metal in doré.