Filipe Pinto was born in Porto, Portugal. He graduated in Geology from the University of Porto Faculty of Sciences in 2012 and went on to write a Master’s thesis entitled “Study of Tin Distribution in the Panasqueira Mine” there. He has been working as a mine and exploration geologist at the Beralt Tin & Wolfram (Portugal) S.A. Panasqueira mine since 2012.
Located in Portugal in the Central Iberian Zone (CIZ) of the Iberian Massif, the Beralt Tin and Wolfram (Portugal) S.A. Panasqueira mine is a world-class hydrothermal W-Sn-(Cu) ore vein deposit. Wholly owned by Almonty Industries Inc. and mined continuously since 1886, the Panasqueira mine is still one of the main producers of high grade tungsten outside China.
Panasqueira is an underground mine characterized by flat subhorizontal mineralized quartz veins, which are the main target. The main product is tungsten (var. wolframite (ferberite), with 75 percent ) and by-products are tin (var. cassiterite, with 74 percent Sn) and copper (var. chalcopyrite, with 27 percent Cu).
High concentration of wolframite in a mineralized quartz vein – the nugget effect
Various morphological features of veins
In large swarm vein deposits like these, erratic distribution of minerals is very common, meaning that mineral grade cannot be estimated either in veins intersected by drill-holes or by in situ sampling.
In fact, the nugget effect is so high that Panasqueira mine, with its more than a century of experience, has developed empirical formulae to determine the grade of the deposit for mineral resource and reserve estimation purposes.
These empirical “D9″ and “Pintas” formulae have been tested several times and verified as reliable by external entities.
The D9 formula is used to convert the thickness of a quartz vein intersected by the drillholes into kilograms of wolframite per unit of plan area (kg/m²).
The Pintas formula is used to convert in situ exposed and mapped wolframite crystals areas into wolframite grade, and into kilograms of wolframite per unit of plan area (kg/m²).
Sometimes, iron-rich sphalerite is mixed in with wolframite, but our experienced geological team knows how to take this into account.
0.75 is the average proportion of in wolframite
0.6 is an empirical value determined by mine experience
2.2 is the average stope height, in metres
2.8 is specific gravity of the rock
10 and 10 are unit conversion factors
MEF is the mineral evaluation factor (reconciliation factor between the grade measured in the mine and in the plant)
Simplified diagram of a vein intersected by a diamond drill hole (top left) until the final phase of exploitation (bottom right).
Mineral Resource and Reserve Estimation
For exploration purposes, the first step is to drill up or down to a maximum length of 120m to establish the continuity of known veins or to try to find new ones. Drill holes are almost always vertical because the veins are mostly horizontal. The Diamec 252’s 46mm drill hole diameter provides a sufficient and representative sample for analysis.
If the drill hole intersects a vein between 18cm and 29cm thick, a quadrate polygon with sides 34.5m from the centre is described. If the vein is 30cm or more thick, a quadrate polygon with sides 50.5m from the centre is described.
The resulting areas are used to calculate Inferred and Indicated Resource values.
Veins less than 18 cm thick are disregarded following the present cut-off grade calculation (18cm = 10.8kg/m²= 0.13% ).
An Inferred Resource is defined only by one drill hole intersection and an Indicated Resource by at least two drill holes, always applying the D9 formula to define the grade. If two or more parts of an Inferred Resource intersect each other, they are considered an Indicated Resource. The same happens when an Inferred Resource is in contact with the borders of an old stope.
Once the presence of a vein is recognized, a 4.5m wide and 2.2m high exploration gallery (known as an incline) 50 to 100m long is opened at the same elevation as the vein. The main purpose of this gallery is to expose the vein and measure all the wolframite crystals areas.
Applying the Pintas formula, if the grade is above the current cut-off, the Indicated or Inferred Resource is converted in a Measured Resource – virgin area or pillar, using the current room and pillar mining method. A virgin area in known as a Probable Reserve (unworked area) and a pillar as a Proven Reserve, with the wolframite crystals areas mapped on all four faces.
In all the cases the recovery factor is 84 percent. For geotechnical constraint reasons, 16 percent of the resource stays on the pillars, which are mined in three phases: (i) 11m x 11m (67.3 percent of the recoverable area); (ii) 11m x 3m (45 percent of the recoverable area); and (iii) 3m x 3m (cleaning the fine material). For safety reasons, in some mine blocks, the 11m x 11m pillars need to be left intact to protect infrastructure like panels, drives, ramps, shafts and conveyors.
For safety and confidence reasons, a factor of 40% and 60% is applied on the Inferred and Indicated Resources respectively.
A mine or exploration geologist working in a deposit like this also needs to be sensitive to some geological aspects when talking about resources. In the case of Panasqueira, “Seixo Bravo” veins can be easily confused with the typical horizontal veins when core logging is being carried out.
A Seixo Bravo is a barren vertical quartz structure, contemporaneous with the metamorphism and always crossed by hydrothermal mineralized quartz veins.
Sometimes, a muscovite selvage, usual on the borders of the hydrothermal veins, can help in the decision, but not always, so the real thickness of the mineralized vein must becarefully measured in order to define resources correctly.
Irregular veins, such as those with ‘eel tail’ morphology, and faults with high offsets also need to be taken in account. Due to the vertical schistosity in the Panasqueira Mine, and because holes are almost always vertical, deviation during drilling is not a problem.
Overlapping satellite image, geological map and map of the main mine infrastructure
The D9 and Pintas formulae have proven reliable in defining mineral resources and reserves in the Panasqueira deposit, following the Canadian Institute of Mining, Metallurgy and Petroleum (CIM) standards.
Depending on the budget available at the beginning of each year, an underground exploration drill hole campaign is carefully planned to identify the largest amount of bestquality resources that can be mined at the lowest cost and least risk.
Because of the particular geological characteristics of the Panasqueira deposit, mineral resource estimation should be precise enough to guarantee profitable mining. A geologist with good background understanding and accurate drill core interpretation should be able to arrive at appropriate decisions.
Comparing January 2016 values with those in September of the same year, updated Measured Resources (Proven and Probable Reserves) represents an increase of over 14.48 percent, while Inferred and Indicated Resources increased 2.76 percent and 2.71 percent respectively, attesting to the accuracy of this article content.