The old-school thinking is that hard rock lithium is mined in...

The old-school thinking is that hard rock lithium is mined in large open pits and that is still TG6's thinking. If mines were to compete with Greenbushes or more recently Pilgangoora they needed to be large. They needed to have lower strip ratios otherwise they were a off on the right hand side of the cost curve and at risk of closure during periods of low prices. Grades below 1% are problematic if they don't have low strip ratios.

IMO this thinking will change. UG mining for Spod is yet to be normalised but I think that's coming given who is looking at or developing UG operations. From the scoping studies released so far, UG may deliver at under A$100/t to the ROM pad and in some instances values nearer half that have been quoted. This means UG is more expensive than sub-10 strip ratios but it may also be cheaper than 15-20+ strip ratio options. The statement UG is more expensive isn't correct, unless the qualifier is added "than low strip ratio deposits". Some of the TG6 western extensions look like they may be UG suitable developments off an initial quite large open pit.

So who is looking at UG?
Its widely known that LTR has decided to mine KV via UG methods. Mt Marion made a FID on UG and note in their June quarterly that they had completed their box cut for UG operations and had advanced the decline by 100m. IGO's 19 Feb 2024 announcement included the comment that Greenbushes has commenced underground mining studies:

Wodgina has a small UG JORC resource. BP33 (Core lithium) while paused, is being developed as an UG operation. There appears to be no choice but to develop LPM as an UG operation given its shape. Develop Global revised their Pioneer Dome proposal so it was less open pit and more UG. GT1 (Canada) is exploring Open pit plus UG on both its deposits. GL1 is developing an UG mining plan to transition to after completion of open pit operations. In short, UG mining for spod is on its way to being normalised. There will be others exploring UG that haven't gone public on these plans (or I haven't looked at that projects announcements).

All these UG developments will mean that within a few years there will be greater acceptance of the idea of UG mining for spodumene. This will also mean that knowing the western extension of pegmatites like TGCD024 (3.6m @ 1.11% from 171.6m and 17.8m @ 1.66% from 203.7m) is important. While this lower intercept is deeper, its also high grade and thicker. It will quickly become challenging for open pit economics but could have strong UG economics, particularly if the grade remains ~1.66%, if the width remains 10-15m+, if it remains mineralised all the way through and has a gentle slope. The western extension of TGCD024 could add many Mt's to the size of Burmeister so not drilling this is choosing to have a deposit that is smaller by several Mt, possibly even by 10Mt.

There's also the western extension of the upper pegmatite hit by RC14/RC18. While this wasn't particularly thick (4 and 7m), it was near surface at only 36 and 43m deep. TG6 has drilled 100m to the south of RC14/18 but hasn't yet drilled west (yet). The exploration target involves ~300m of western extension. If this narrows to the west then it will quickly get too deep for its width. If it swells out to 15-20m as it gets to 100-150m deep then you would definitely want to have drilled it.

IMO it may not be in this phase, but I think there will be western extensions until they get dusters or it gets too thin, deep or low grade. Good grade Lithium intercepts are difficult to find so where they exist there will be drilling out of the open directions.