Warning this is boring post (for many) about stratigraphy that needs to be bored to find out what we are "debating". Better for a weekend read if at all.
Hi Hilltop,
I am not certain quite what you are saying about Purdy's. I think you are saying that the Nallana Formation is the host of the conglomerate.
I can see why you could reach this conclusion based on the 1:500,000 scale geology and perhaps even the 1:100,000 scale as shown by the following images based mostly on the
16 nov 2016 announcement.
Image 1
View attachment 730560
View attachment 730557
View attachment 730551
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I can't recall where, but the auriferous conglomerate was reported as lying on top of an uneven basement and this comment has influenced my thinking. I did assume the basement was to the Fortescue Group - not too wise in hindsight....
The original nuggets reported in November do align with the Mt Roe/Nallana contact as shown on the 1:500,000 scale geology overlay.
The area of observed gold mineralisation (in Image 1) could be down slope shedding of nuggets from the conglomerate unit near the top of the hill. This smears the gold across a wide area/stratigraphy
View attachment 730590
With these data it would appear that the conglomerate is probably hosted in the Nallana Formation.
However, if this was the case why has staking focused on the Fortescue group?
The Whundoo Group and the Nallana formation in particular, appear to be restricted to a relatively limited area in the Western Pilbara which IMO would have resulted in a very different staking pattern.
View attachment 730593
My best speculation given the information available, to me at the moment, is that the conglomerate in this area probably sits on top of an irregular basement contact of Nallana Formation and the geology maps will ultimately be adjusted to show the conglomerate as being part of the Fortescue Group. Whether this turns out to be part of the Mt Roe Basalt or an as yet unnamed unit will come out as exploration proceeds. This may be the early gold bearing unit that CRA were searching for deeper in the basin. Hopefully it is.
For Mt Oscarwits I didn't initially understand what you were saying. It appears to be the reverse of what you imply for Purdy's. So with a bit DYOR from the report you quoted I read this: "
Based on the recent field inspections, Artemis geologists interpret the conglomerate sequence to be analogous to the Purdy’s Reward prospect and being at the base of the Fortescue Group. This is contrary to the previous governmental mapping on the Roebourne 1:100,000 mapsheet, which interprets the sequence to be at the base of the older Whim Creek Group and part of the regional Pilbara Supergroup."
Ed Mead did comment at the meeting that the Mt Oscarwits conglomerates were in a different stratigraphy to Purdy's. Obviously I should have asked for some clarification as there may to be some discrepancy between what he said and this announcement.
From a previous post of mine:
[First some caveats: I know very little about the Fortescue/Hammersly basin stratigraphy other than what has passed by me fairly recently. In general stratigraphic correlations rely heavily on interpretation and the amount of data available and as such are often fiercely debated topics in eclectic geology circles.... DYOR]
In summary it appears we want to discuss the complexities of what may be poorly defined stratigraphy and want opposing things to happen in the two areas discussed.
Purdy's
Hilltop - Nallana Fm;
Salpetie an unidentified mafic conglomerate of Fortescue age that requires reinterpretation of the government geology.
Mt Oscarwits
Hilltop - mafic units and conglomerates need to be re-interpreted as Mt Roe Basalt/Fortescue Group;
Salpetie - accept government mapping and place the conglomerates in the Cleaverville/Warambie 'Formations' = Multiple stratigraphic units with gold bearing conglomerate units in the Hammersly/Pilbara (though probably not in the Nallana Fm).
Apologies H if I have misunderstood you.
(20min delay)
