Impact of Geophysics in Small-Scale Mining

The perception that society has of mining is associated with large companies that often operate with high capital investment. However, in Brazil only, there are thousands of small-scale mining companies that exploit different minerals, operating with little resources and low capital investment. One of the most challenging aspects about mining is how to identify the deposits to implement the mineral production projects. Mine implementation requires understanding of the risks associated with the business and the importance of quality in geological investigation.


Introduction
The perception that society has of mining is associated with large companies that often operate with high capital investment.However, in Brazil only, there are thousands of small-scale mining companies that exploit different minerals, operating with little resources and low capital investment.One of the most challenging aspects about mining is how to identify the deposits to implement the mineral production projects.Mine implementation requires understanding of the risks associated with the business and the importance of quality in geological investigation.
Geological investigation involves drilling activities, which demand long periods of work and require costly investments, with no guarantee of return and financial success.According to geophysicist and business analyst Ribeiro [1], the average cost of drilling operations (a direct exploration method) is US$ 100/m drilled, and the use of Geophysics for indication of mineralized zones can reduce in 30% to 50% the amount of negative drill holes (holes that does not intercept the mineral orebody), resulting in reduced time and cost associated with drilling activities.This paper includes a brief comment on the application of Geophysics in a limestone mine located near Taubaté, state of São Paulo, Brazil, and in a manganese mine near Campinas, also in the state of São Paulo.Both mines can be classified as small-scale mining operations.In both situations, the application of Geophysics reduced prospection work time and attended the demand required by the companies.On the limestone mine, Geophysics allowed for updating the geological model.In the manganese mine, it contributed to the generation of a preliminary geological model of the deposit.In both cases, there was a significant reduction in time spent on exploration.

Materials and Methods
The method used for obtaining geophysical data in both surveys was electro-resistivity.The technique of Vertical Electrical Sounding (VES) was used in the survey at the limestone mine.The techniques of Electrical Resistivity Imaging (ERI) and Vertical Electrical Sounding (VES) were adopted for survey at the manganese mine.Electroresistivity may be defined as a measure of the difficulty with which the electrical current propagates itself through a given material [2].

Vertical Electrical Sounding (VES)
The VES technique consists of a succession of measures of a physical parameter, resistivity, with increasing separation between the electrodes of emission and reception, keeping fixed the center of the array (the attributed point of the survey) and its orientation.Increasing the distance between the A and B current electrodes, the total volume of the area included in the measure also increases, allowing for reaching deeper layers.The successive results will be strictly connected with variations of resistivity and with depth.Figure 1 presents the Vertical Electrical Sounding (VES) technique.

Electrical Resistivity Imaging (ERI)
Electrical Resistivity Imaging (ERI) investigations are performed along profiles and the obtained results relate to each other through map analysis at one or more depths or sections with several depths of The main focus of the research is to test the use of Geophysics in practical applications in different types of mineralization, using different geophysical methods in the context of small-scale mining.This research is based on surveys in two localities with mining potential: A limestone mining property in full activity in the region of Taubaté, Brazil, and an area in which there is occurrence of manganese mineralization in the region of Campinas, Brazil.

Limestone mine
The goal of the study carried out at the limestone mine was to update the geological model.The mine is located near the city of Taubaté, state of São Paulo, Brazil.The mine operates in a 50 ha area, and has an opportunity to expand this operations area to a total of 246 ha.The work to cover the expanded area and update the geological model was conditioned to tight deadlines.A drilling campaign through conventional approach, with 3 or 4 drilling equipment, would take approximately 60 days.In order to reduce the time for data collection, geophysical methods were introduced.
The work began with a test for accuracy verification of the electricresistivity geophysical methods (ER) with the technique of VES to identify the geological structures of interest in the same position of an existing drilling survey and validate the adherence of the results to the information acquired from drilling sampling.With satisfactory results, 31 VES surveys were planned and carried out in the Southern portion of the property.Figure 3 shows the flowchart of the stages of operation.

Results
The results show that it was possible to identify the layers of interest set out in the premises of the survey.Most SEVs presented models of four layers.The layers 1, 2 and 3 were interpreted respectively as topsoil (low to medium resistivity, low to medium chargeability), weathered and unsaturated soil (high resistivity due to low moisture content and medium chargeability) and weathered soil or saprolite (low to medium resistivity and low chargeability).
The fourth layer usually presented high resistivity values (greater than 2000 ohm m) and higher chargeability (usually above 10 mV/V), interpreted as gneiss.High values of resistivity are expected for gneiss as a result of low degree of alteration and low porosity.Larger values of chargeability may be related to metallic accessory minerals within the gneiss, which have not yet been transformed by weathering.In the locations were SEVs 5, 6, 24, 27 and 28 were conducted, the resistivity values were below 1000 ohm m for the last layer, interpreted as altered or saprolitic gneiss.These relatively low values suggest a rock with some degree of weathering and/or fracturing.Figures 4 and 5 represent respectively VES 10 and 5 of the geoelectric model, in which the rock is characterized as altered or saprolitic gneiss.
In practical terms, the introduction of the proposed Geophysics methods and techniques allowed for obtaining the information required for updating the geological model, which normally would have been executed in about 60 days (using between 3 and 4 drilling equipment), in only 15 days, which represents 75% reduction in survey time.
The application of Geophysics allowed for expansion of the deposit geological model within the investigation area with adequate precision of the lithological units of interest (layer of soil and rock contact), within the time limits set by the company.Figure 6 and Table 1 show the VES positions, Figure 7, the geological model (current area) and the expanded geological model (expanded area).

Manganese mine
The survey began with topographic and geo-structural mapping of the area of interest, which helped to define the locations of points and profiles of the geophysical investigations.The work was performed along five (5) ERI profiles and on four (4) VES positions.Figure 8 shows the flowchart of activities.

Results
The results of the geophysical surveys indicated that a significant part of the manganese layer of should be found between 0 meters (outcrops) and 15 m deep, and that the layer thickness can reach or exceed 10 meters.Geo-structural analysis indicated that the manganese deposits in the region can be extremely deformed and boudin-shaped, with alignment to the NE-SW axis.It also indicated that the section of the manganese deposits in the region must be sigmoid-shaped.Together, topographic, geo-structural and geophysical investigations allowed for the generation of a preliminary geological model.Figure 9 shows the ERI profiles 102, 100, 101, Figure 10 shows the ERI profiles 106, 104 and Figure 11 shows the map with the location of the ERI surveys conducted.
The geophysical survey as a substitute to drilling allowed for reduction in time, as occurred in the limestone mine.The execution of an exploratory drilling plan would last for a month and, with Geophysics, was limited to a week, that is, a reduction of 77% in survey execution time.Figures 12A-12D show the geological interpretation, the preliminary geological model, and the conceptual pit generated with aid of a mining software.

Conclusion
The results achieved through the studies carried out at both small- scale operations indicate time savings of more than 75% over the traditional drilling approach for the purpose of defining the orebody outline.Both cases show that the proposed Geophysics methods and techniques may improve the level of geological knowledge and that its implementation has potential to be extended and expanded.Geophysics may be successfully used in identification of the strata of interest, as a support tool to enhance and improve the level of information necessary to the generation of a geological model in short time.

Figure 12 :
Figure 12: A) Position of ERI profiles; B, C) Geological model; and D) Conceptual pit design.