Gold

The classification of gold ore on the basis of industrial use does not arise. However based on the association of gold with sulfate minerals like arsenopyrite, the ore can be grouped as under: -

A.      Free Milling ore:
The gold is generally medium coarse grained and can be recovered by crushing, milling and gravity separation on blankets. The tailing loses are generally as low as 0.3 to 0.4 g/t.

B.     Refractory ore:
The gold is associate with minerals such as arsine pyrite, pyrite, etc. and is usually fine Grande. The gold recovery needs in addition to crushing, milling and gravity separation, flotation, roasting, etc., for good recovery. Also, the ore needs more grinding. The tailing losses after gravity separation and cyanidation of table tails, is generally high (o.8 to 1.0 g/t) and

C.  Semi – Refractory Ores:
In the Mineral Inventory as on 01.01.85 the cut off grade for classifying "reserve" has been considered as +3g/t considering experience in the existing mines. Reserve below his cut off (-) 3 g/t has been classified as "conditional resources".

The representative of BGML has observed that it is necessary to have "planning cut off’ at exploration stages and "operational cut off" while mining. While delineating ore bodies with a given cut of grade. It is necessary to have an appropriate cost of operation and what should be the minimum cut off grade to start with. Delineation of ore bodies with operational cut off grades should be carried out bodies with operational cut off grade should be carried out keeping in view the mining parameters.

The various operational cut-off options should be examined in relation to the safety in the mining methods. Mining costs, milling cost and overheads in an operating mine. In K. G. F. mines different mining method are employed. Stop drive – a costly one, is practiced in ultra deep champion reef mine and cut and fill method in Nandydurg mine at moderate depths. The costs of these methods vary considerably. Secondly, mining cannot be limited to the zones delineated with a given cut off grade since the low-grade reserve blocks in the mining sequence have also be mined for safety purpose.

The cut off concept has been generalized by the BGML representative as below:

·         0.1, 0.2 and 0.5 Au g/t cut off grades for placer deposits where the area covered is large, the volume of mining by mechanization is also large and the treatment cost is very low to recover gold.

·         A 3-g/t cut off grade could be used in vein types of deposits in shallow to moderate depth where cost is moderate. A high cut off grade is necessary for deeper operations, where the mining costs are higher.

In K. G. mines where the high-grade zones are already mined out the residual ore reserves have been re-estimated using the geometric mean grade with 4 g/t and 6 g/t cut off grade and minimum stopping width of 122 cm in case of champion load and 137 cm in case of the oriental load. The Hutti Gold mines practiced a cut off grade of 2 g/t and 1.5 M minimum stopping width mainly because of grade of individual sample data are higher and the ore body is wider. Therefore, even at 2-g/t cut off grade, the average grade of total reserve is 6 g/t.

Reserve        
1.Placer gold                Au (+) 0.5g/ m3
2.Primary gold              Au 3 g/t m3

Conditional Resources
1.Placer gold                Au (-) 0.5 g/ m3
2.Primary gold              Au (-) 3 g/t (free milling)


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