The first thing you need to understand is what the actual purpose of a case structure is.
Structuring a case does NOT mean to come up with a nice looking list of areas that you want to look into. This is not a structure - it is just a bucket list (albeit a structured one). A case structure is a LOGIC! It is the logic according to which you will answer the client's question, and the qualitative areas which you will explore are just a byproduct of that logic.
This is the big misunderstanding (or non-understanding) that has been planted into the minds of candidates since books like Case in Point or Case Interview Secrets have been published years ago.
So here is a rough outline on how to think about the above mentioned situation:
1. Always start with distilling the Core Question: "Should the client invest into purchasing the mentioned mine in Australia?"
2. Identify criterion to make this decision: The additional value we can create over the our investment horizon has to be significantly higher than the investment cost. Moreover, the required capabilties to deal with the mine need to be in place and the risks need to be manageable.
3. Map out the potential value bucket by means of a profitability tree: what are the levers of value here? Calculate profit that can be expected if the mine is purchased and run ad infinitum or sold at some point in the future; compare Scenario A (mine purchased) to Scenario B (money used for best alternative). For this, you use a classical driver tree which disaggregates profits into its sub-components (both revenue side and cost side), and then you add a couple of examples (don't try to come up with an exhaustive list!) in terms of qualitative elements that influence these numerical drivers (e.g., market demand, price development, capacity of the mine, costs of operation)
4. Derive annual value (delta between Scenario A and B). If the mine indeed generates annual profits, you then divide the purchasing price of the PTA plant by this additional yearly profit. This gives you the break even point (point in time after which the investment becomes profitable). If this point comes earlier than the investment horizon, then this is a beneficial investment and the client should proceed with the purchase (purely based on financials).
5. Don't forget to assess capabilties and compile potential risks and mention them in your summary
This kind of thinking is based on first principles, and not on a "framework mindset". The beauty about it is that at the core, this is transferable to ANY strategic decision situation where a client wonders whether he sould do XYZ. As a corollary, it is completely unnecessary to have different frameworks in mind for situations like direct investment, market entry, product launch, M&A etc. The core assessment that needs to be conducted is absolutely identical across all these situations.
The caveat here is just that it usually takes a little bit of time to properly internalize this - but once it becomes second nature, navigating through a case becomes quite easy because you have a robust way of thinking that "automatically" creates the roadmap towards the solution.