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Apr 2011 - The Key to Edna May

“New” Techniques Unravel the Key to Edna May

Catalpa’s initial asset was the old Edna May mine and surrounding tenements along the Westonia Greenstone Belt, on the northern outskirts of the town of Westonia, about halfway between Perth and Kalgoorlie (or ~310km east of Perth).

The Edna May mineralisation was discovered in 1910 and apparently first mined on the Greenfinch area in 1911, with the adjacent Edna May area to the east perceived as requiring shaft access. As grades fell from 46g/t to 16g/t with depth, mining moved to the east focusing on the Edna May area especially after Edna May produced 1280t @ 61g/t for ~2500oz in 1912. The neighbouring mines gradually closed and the main Edna May underground mine followed the Edna May lode until 1920 when it stopped at the 810ft (247m) Level due to increasing water inflow and pegmatites depleting the ore.

A number of Edna May companies were consolidated and the new Edna May Mining company restarted in 1936 with a 20-head stamp battery for amalgamation, and subsequent cyanidation of the sand tailings. The new mining company focused mostly on underground mining of the adjacent almost parallel lodes to the main Edna May orebody and its hangingwall and footwall branches.

A 1940 geologists’ report noted that deep drilling was required to go beyond the ore zone depleted by the pegmatites at ~250m, but could not be justified (during WWII) and hence the mine gradually closed due to water inflow & pegmatite depletion in 1948. During WWII, Edna May appeared to be mainly mined for its by-product, Tungsten, for munitions, with the mine receiving pumping assistance to ~1945.

In 1985, ACM exercised an option and its subsidiary ACM Gold (AGO) undertook some deeper drilling that showed a continuance of the mineralisation at depth (beyond the pegmatite zone), inferring a resource of ~1mt @ 11g/t, and re-opened the Edna May mine as an open-cut. During 1988 to 1989, AGO cut a decline to the ~265m Level, but was discouraged by the water inflow and pegmatite depletion of the orebody that appeared to come from the west on the two levels that were developed.

The underground closed in March 1990, we commented at that time in broker notes, that “AGO became sidetracked by the pegmatites at about 260m and should have ignored them and continued the decline to expected mineralisation, 300m to 700m below surface.

Fast forward 20 years later and with further drilling, examination of the floor of the current new open-cut and 3d modelling as shown in Figure 1a, it can be seen that the pegmatite unit appears to be steeply dipping to the north, passing from the hangingwall position of the Edna May gneiss in the current open-cut floor across the gneiss to the footwall position on the southern hangingwall at below the location of the last level of the decline.

Consequently, Catalpa’s proposed underground mine should be mostly pegmatite-free. The pegmatite appears to have been injected up a fault or structure that displaced the lodes as shown in Figure 1a. Although referred to as the pegmatite zone, there does appear to be two ages of pegmatite, one of which is mineralised. Possibly the earlier one ingested parts of the mineralisation, whereas the later one did not.

As for the water, it is currently being pumped from two bores and used in the ~2.8mtpa plant (ACM Gold had an 0.5mtpa plant that they later upgraded to 1mtpa). There is also a wider dewatering programme over the mine.

The mineralisation at Edna May was regarded as unique in that the lodes appeared to be curved. However, it is not that clear when viewed in grade control, instead the orebody appears to be splattered with mineralisation mostly up to 1.5g/t (apart from the lodes which appear to often have specs of visible gold in quartz or laminated quartz).

Using Leapfrog software, the mineralisation instead appears to consist of multiple NE/SW striking lodes that intersect E/W striking lodes forming hinges and an apparent arcuate form. Historical texts do refer to the higher grades being in the hinge zones, which is logical from two structures intersecting each other.

Should the lodes be more uniform, then mining them underground, could be a lot simpler. The Edna May orebody as such appears to be a classic dilation jog between major structures and hence why the mineralisation in Golden Point to the east and Greenfinch to the west seems to be different.

Computer grunt-power, 3d modelling and software algorithm packages like Leapfrog are now all taken for granted, but they were very much in their infancy 20 years’ ago. However, the evolvement of such packages has now simplified and unravelled the key to Edna May.

Disclosure and Disclaimer: This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, (an independent research company) who is a Financial Services Representative with Taylor Collison Ltd.

Figure 1. 3d model, plan and section of Catalpa’s Edna MayGDNmay2011

  • Written by: Keith Goode
  • Thursday, 05 May 2011