Xtract Ascot – Another Porphyry – Another 2 million Tonnes?

 

Xtract Ascot – Another Porphyry – Another 2 million Tonnes?

 

Until now I wasn’t a big fan of Ascot, I would drive by it a few times a year with my kids on the way to Legoland. Why talk about Ascot? Well, Xtract has a license called Bushranger, within that license is a deposit called Racecourse. We’ve always used the generic term Bushranger to describe the resource, as that is the license it’s situated in. Now we will have to differentiate a little as a second, completely separate deposit has been identified called Ascot.

Ascot is situated about 1.5 km SE from the SE tip of Bushranger, in what was preciously known as the SE anomaly.

The above diagram is the latest drill map from Xtract. Racecourse is clearly labelled, to the SE  (bottom right hand corner) we have drill holes 25,31,33 and 35 that are looking for, and have found, a second porphyry -Ascot.

The idea of a second porphyry at Bushranger is not unexpected. In the summer, Xtract commissioned a wide-ranging Geophysics sweep of the license. A number of large targets were identified, notably a large target which was termed the SE Anomaly. In my blogs this year, I talked about this back in Sept and tried to plot where and what the intrusions might look like.

The reds and browns indicated the anomaly charts, the Yellow areas (by me), represented possible intrusion locations.

Given the lack of information back then, I think the general locations stack up pretty well.

Moving on to the geology. These types of porphyry often swarm, they are essentially escape routes for magma pressure, one is used, it blocks off, then another one forms. The subduction arc leads to a larger dominate chamber, which then blows out when the pressure gets too high. In the case of Northparkes, which I’ve referred to several times, you end up with a set of vertical chimney structures that release upwards, some bigger, some smaller. Bushranger is slightly different in that the escape routes seemed to have followed more horizontal weaknesses on their way to the surface. The main physical characteristic is that they have more porphyry closer to the surface. From a geophysical point of view, it means they probably retreated at a lower temperature, with less pressure. This is important. Bushranger is very unique in the Lachlan Fold, it appears to be the most copper centric deposit. I am only musing, but the particular horizontal nature of the intrusions might be the reason. Anyway, the key point is that more of these escape porphyries were always likely and they were always likely to be very similar in geological nature to Racecourse.

One last quick point. Porphyries tend to be unloaded with regards to mineralisation. However, when they are loaded such as Racecourse, it’s a roll of the dice as to what they will contain. If we assume that the more horizontal nature makes it more likely to be copper centric, then we can explain it with a roll of the two dice.

1-2 weakly loaded copper mineralisation.

2-6 moderately loaded copper mineralisation.

7-8 strongly loaded copper mineralisation.

9-10 moderately loaded copper with weak commercial gold.

11-12 moderately loaded copper with moderate commercial gold.

 

 

I’ve knocked up the above diagram, it comes with all kinds of caveats. The largest is that this is just the first drill into that location, so there is simply not enough data to give any kind of detailed analyst of the intrusion. Secondly, I don’t have the detailed geological data from the drill (neither do the XTR geologists tbh). However, despite these caveats, we can make some basic assumptions.

The diagram is a cross section of Hole 35, which goes west and Hole 33, which goes east, both from the same pad. Hole 35 encounted stronger mineralisation in two sections, with a middle section of lower mineralisation. We know that mineralisation sits mainly in a zone of alteration adjacent to the intrusion. We can guess, probably pretty accurately, how this will look as it will be very similar to Racecourse. Guessing grades is much more problematical. The photo’s of the cores suggest that 1% or higher is likely, my thoughts are that these are probably the better cores, so it’s unlikely that 1% is representative, a typical Bushranger value of .3% to .6% appears to be more likely.

Hole 33 is intriguing with its mineralisation at the bottom of the hole. Mineralisation doesn’t just appear, if its connected to Ascot, then it means that Ascot extends massively, 300-500m from the east of the intrusion. If not, then it would indicate a third porphyry, which is very possible.

We know from Racecourse that it dips deeper to the NW (deep towards the source of the intrusion) and shallower as it extends SE. If we assume that Ascot also does this, then 33, 35 might well have been drilled to find a deeper intrusion at that point. It also means that as we drill further to the south of Hole 35, the mineralisation is both shallower, bigger and more intense.