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Jul 2017 - Reading Old Workings

Learning how to Read Old Workings

As the Australian gold mining industry passes through another phase of goldfield revival, I continue to be amazed that many do not seem able to read the old workings, but have to admit that I have never seen a course on that subject.

As the Australian gold mining industry passes through another phase of goldfield revival, I continue to be amazed that many do not seem able to read the old workings, but have to admit that I have never seen a course on that subject.

It is common knowledge to examine the rock chips left on the ground from previous explorers (ie hopefully not environmentally cleaned up) to see what was intersected : as in mafic, ultramafic, sediments, and if lucky quartz veins (that may have even oxidized brown due to sulphides breaking down). And to walk the pits from the last revival : as in walk around their rims to see the exposed geology from different angles and the often standout of weathered veining (while obviously taking care of pit edges and any cracks implying future wall failures).

Or looking at the last buckets of spoil (often on a waste dump), in case they contain ore grade as the mine approached closure. Plus the state of the walls themselves in terms of competency, and any white patches, often indicating oxidation proximal to mineralization, or possibly even palaeochannels. But taking care that the patterns on some of the pit walls may be due to an eroded layer.

However, many overlook the really old workings of the turn of the century (end of the 1800s to early 1900s) and the wealth of geological and mining information that they contain. I often wonder if the group I am site visiting with think “Keith’s focus on old workings must be a mining thing”, which it partly is viewing old stopes, bedding planes, remnant sticks (props), rails, scotch cars’ handles (trucking limitations) and mining styles (production limitations) etc viewpoint.

But I digress. There have been many instances where I wonder if a geo has instructed someone to drill a hole near old workings without studying them first. After all, how else can one explain the clearly barren drillholes from : drilling parallel to old stopes at Wombola (before Silver Lake [SLR]), or parallel to the stope and long axis of the shaft on the outside northern edge of the Lady Shenton pit at Menzies (before Intermin [IRC]), or drilled eastwards away from the clear (it’s partly exposed on surface) N/S striking vein structure between two of the old shafts at Sandstone North (before Alto [AME]).

However the old shafts (especially if they are in the top of a spoil / mullock heap), contain key geological and mining information and characteristics.

The two main geological aspects of old shafts are :
The long axis represents the strike of the orebody that was mined which can be plotted using a compass and a camera/phone, or more easily now using a drone, as shown in the Goode News column of Paydirt’s (latest issue). Mapping the strike directions of the long axes of the shafts identifies whether there were parallel and/or cross-cutting vein structures that were mined. Such identification can often highlight orebodies striking E/W & NE/SW compared to the more common NW/SE.
Secondly the spoil heap (if present) should be examined for its host rock composition being mafic, ultramafic, sedimentary, quartz (the rough percentage that is quartz, and whether it is bucky/blocky, laminated, variegated (different colours etc) or salt & pepper textured due to the degree of iron), sulphide minerals and whether oxidized, inclusions etc, plus any other sulphides.

Then there are the mining aspects :
Being whether the long axis of the shaft contained 2 compartments (or squares) or 3 compartments (usually inferring that money was spent and such shafts are often relatively deep, together with winding gear etc).
The quality of the timbering style is also relevant (if it still remains), and of course there is the classic rock drop for depth (or a splosh).

Additional information can be gleaned from old stopes as in angle of dip, wallrock competency etc.

By examining the old workings, especially the long axes of the old shafts and stopes, the chance of drilling parallel to such workings should be reduced, and possibly lead to more encouraging exploration results.

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 (AFSL 247083).

Figure. Typical Pictures taken of Old Workings in Various WA Goldfields
Reading Old Workings

  • Written by: Keith Goode
  • Friday, 28 July 2017

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