- A 30-50 metre thick, near surface, weathered zone, and
- An underlying sulphide (unweathered) zone.
All hard rock tin deposits contain tin minerals that cannot be economically recovered:
- The mineral stannite.
- Tin hydroxides, and
- Fine grained cassiterite.
Stannite – Cu2FeSnS4
The sulphide tin mineral stannite will not be recovered, nor is it treatable by conventional means. By mass, it contains 27.6% tin and 29.6% copper. It is therefore possible to calculate the maximum amount of stannite present from the drill core copper assays. At Oropesa, metallurgical testwork at SGS Global indicates an average of about 30-35% of the copper present is in the form of the mineral chalcopyrite (CuFeS2), or rarer tetrahedrite (Cu9Fe3Sb4S13).
An examination of the intercepts from 40 Oropesa drill holes (781 assays) gave a maximum average tin in stannite figure of 9.4%. This figure has then to be adjusted downwards by around 30% to allow for copper in chalcopyrite and tetrahedrite, thus giving an approximate average tin in stannite figure of 6.6%. Most companies choose not to disclose the stannite content of their orebodies, however the Company believes this figure of 6.6% compares favourably with most known tin deposits.
Tin Hydroxides – Varlamoffite – Sn3FeO6(OH)2
The tin hydroxide varlamoffite (65.7% Sn) is formed by the oxidation of stannite and it is not recoverable due to its relatively low specific gravity of 2.56.
In addition, very small amounts of jeanbandyite (Fe,Mn)Sn(OH)6, a rare primary tin hydroxide, have been identified. The specific gravity of jeanbandyite is 4.41 and it may be recoverable.
Fined Grained Cassiterite – SnO2
All tin ores contain some very fine grained cassiterite and inevitably, because of the brittle nature of the mineral, more (by a process known as sliming) is created in the ore grinding and milling process. Optimising the grind size is an important of the tin recovery process.
In hard rock deposits, tin’s only economic mineral cassiterite is recovered by a mixture of gravity and flotation methods. Gravity is used to recover the coarser cassiterite crystals and flotation the finer ones. It is not currently possible to recover cassiterite crystals of less than 6 microns in size and recoveries decline steadily from about 12 microns down to 6 microns. The average cassiterite crystal size, from samples recently taken from Oropesa, has been measured at 25-35 microns with a variance of 22-85 microns.
Sulphide Rich Tin Ores
All sulphides in the ore have to be removed prior to commencing the tin recovery process by gravity and flotation. Initial testwork on Oropesa’s sulphide ores have demonstrated the sulphides can be readily removed by simple flotation methods with negligible (<2%) losses of the economic mineral cassiterite.
Similar to all hard rock tin deposits, Oropesa’s tin recoveries will be largely dependent on the amount present of: i) stannite (or varlamoffite), and ii) ultra-fine cassiterite.
There are several different ore types at Oropesa, which will all need to be defined and tested. Consequently, metallurgical testwork is a continuous on going process.
It is not yet possible to provide an expected average tin recovery for Oropesa, however testwork undertaken to date indicates recoveries will vary between 65% and 85% depending on the ore type being tested.