1. INTRODUCTION
The objective of the project was the optimization of the exploration drilling campaign
through the use of “smart”exploration methods. The method selected was the applica- tion of magnetic geophysical survey for the identification of burried Bauxite lenses of
the B3 horizonof the Parnassos-Ghiona zone in Central Greece.
The project was divided ito 2 phases.The objective of the first phase (September
2021), was to validate a geophysical method for Bauxite exploration in the Ghiona and Nera blocks.
The second phase will be the application of airborne geophysical survey based on the results of the first phase.The magnetic geophysical exploration is possible only(1)if there is a clear magnetic/density contrast between the bauxite unit and the surrounding rocks and (2) if there is a relative magnetic/density homogeneity in the surrounding rocks. The objective of the phase 1 (pilot) was to de-risk the geophysical survey checking the geophysical properties of the bauxites and host rocks from Ghiona block outcrops and from well core samples. The objective of this second phase, based on the results of the pilot phase,is to perform an airborne magnetic survey of the West Ghiona block to generate a magnetic map and characterize new bauxite deposits.
2. FIELD WORK
The areas selected for the first phase contained Bauxite deposits at variable depths that have been identified from past drilling, to verify the applicability and resolution of the method.
The magnetic susceptibility of the different lithological types was measured to quantify the contrast between the Bauxite and the host rocks, characterize the level of mag- netic heterogeneity in the host series and evaluate the limit of resolution of the ge- ophysical method.
These were co-evaluated with longitudinal and transverse transect measurements of the magnetic field in the area where bauxite lenses are well known, to identify deep magnetic anomalies in the test area. In addition, a test of the magnetic response of the bauxite units has been done in bauxite quarries, where the bauxite lenses crop out.
To investigate if there a detectable anomaly of bauxites lenses (magnetic contrast) and what are the limits of the magneticgeophysical survey, the following task were accom- plished:
A. Measurements of the susceptibilities of the bauxite, limestone and flysch litholo- gies (core and grab samples) using a Kappameter KM-7 from SATISGEO to asses the or- der of magnetization contrast between the different lithological units.
The average magnetic susceptibility (MS) has been
1.5x10-3 SI for Bauxites and 5x10-5 SI to 1x10-4 SI for hosts rocks.
While there is a good contrast between Bauxite and the hostrocks, the similar values of MS among the host rocks does not make them differentiable.
B. Measurements of the magnetic field where bauxite outcrops
The equipment used was a Magnetometer overhauser GSM19W (GEMS) for the acqui- sition and Proton Magnetometer GSM19T (GEMS) as a base station for the correction of diurnal variation.
Longitudinal and transverse transects in the area where bauxite outcrops Magnetic field measurements where lenses have been recognized (with overburden).
3. Processing and Evaluation of Data
Nera Area The two areas selected were the Nera and North-East Ghiona.
In Nera, one can observe the typical stratigraphy of the Parnassos Ghiona Zone exposed, at the top with the Eocene Flysch, followed by the Maastrictian and the Upper Cretaceous Limestone to the Lower Cretaceous limestone.
The contact of the Cretaceous limestone units is where the Bx orebodies crop out.
In Ghiona, flysch and the Maastrihtian limestone are absent and the stratigraphy appears simpler for the depth of investigation.
Nera Area
A 1.7 km section has been produced above a known and mined out orebody and to- wards the extension of it.
The results showed a typical dipole anomaly above the known ore body.
To the west, another similar anomaly has been identified (red circle), which later was verified by drilling.
In addition, the Total Magnetic Intensity (TMI) helped to identify structural elements that have been correlated with the UG mapping data.
Ghiona Area
Following the same approach, 20 km sections have been produced in Ghiona.
Dipole anomalies have been detected around the areas of know depositsand above a newly explored lense in Palaiovouna (dashedred circle).
Two more anomalies have been identified in the west part of the area.
4. Simulations
Simulations of the response of the Bauxite lenses for different depths, lithotype con- trasts and sizes have been made to evaluate if the method could be used for the pur- posed of the exploration campaign.
5. Conclusions
i. Bauxite lenses can be detected depending on their susceptibility and depth as shown in the table
ii. At 200 m depth, bauxite lenses can be detectable within the following geometry/ susceptibility constraints:
- medium susceptibility (3x10-3 SI) | diameter of 500m with a thickness of 20m
- high susceptibility (5x10-3 SI) | diameter of 700m with a thickness of 10m
iii. At400 m depth, bauxites lenses can be detectable only if they have a very high susceptibility (10-2 SI) and/or geometry of at least a diameter of 700m and a thickness of 15m.
iv. For 600 m deep, bauxites lenses can be detectable only if they have a very high susceptibility (10-2 SI) and a geometry corresponding to at least a diameter of 800m and a thickness of 30m or a diameter of 1000m and a thickness of 25m.