The work index of an ore is due to the gangue or host rock present.
Other factors affect the 'strength' of the gangue or host rocks - whether they are weathered or altered for example. And one sample is not a good basis for sizing a mill nor determining how that sample would perform in an existing milling configuration. Normally one would several samples from the deposit reflecting the ore types or blends that would be mined in years 1, 2 to 5, and 5 to 10, as an example, as well as the material that will dilute the ore (particularly with open cut mining). It is not particularly expensive to measure the comminution characteristics, say the Bond Ball Mill Work Index or better the Drop Weight Work Index - bound to be a commercial laboratory in SA that does these measurements.
As a rough guide, the work indices of a number of materials - granite, quartz, typical ores, etc. are listed in a number of reference volumes. It just depends what those gangue or host rocks are and if it is fresh or weathered ores.

Oh thanks.  Do you have a textbook reference for this?

One reference is the SME Mineral Processing Handbook, N.L. Weiss Editor, Volume 1, Tables 8 and 10, pages 3A-26 and 27.

More contemporarily, I notice that the Metso booklet 'Basics in Minerals Processing' has a Table on page 3:2. But these are general values- one really should determine a range of Work Indices for 'representative' samples. 
No mention is made of antimony bearing ores. I note that the typical gangue/host material at Murchison Consolidated is quartz-schist-carbonate; the work index will depend upon the amount of quartz present (the other constituents are relatively friable in a mill - unless the carbonate has been metamorphised or silicified).

I would suggest, until you measure the work index or find the basis for the calculations used to size your mill originally, that you consider that your ore has a moderate work hardness say 13 to 16 kWh/t.

Thank you very much, I saw in Mintek mineral handbook it was 15.9 but confirmation was needed.