"I'm a bit slow - maybe it's late, does this mean that if ARG...

"I'm a bit slow - maybe it's late, does this mean that if ARG accepts the Cemex t/o for RIN and makes eg $100m it's only taxed at half the marginal corporate/company rate (30% / 2 = 15%)? and then the $100-15 = 85 gets passed down to the shareholders as ??franked dividends? (if they have franking credits?)."


Not quite.
ARG is a company, and companies are not allowed Capital Gains discounts (as distinct from individuals - 50%, and Super Funds - 33%).
Therefore, ARG will pay CGT at 30%. (similarly to its operating income from dividends, interest etc).
However, this creates an anomaly in the tax system. Had I (an ARG shareholder) bought Rinker shares in my own name, I would have been entitled to the 50% discount, but because in this instance I own these Rinker shares indirectly (via ARG) even though I am the ultimate beneficial owner, I would not be allowed the CG discount.
For that reason LIC's sought a Tax Office ruling to redress this situation (and bring LIC's in line with other types of Managed Investments, and individuals).
Hence the introduction of the LIC CG dividend.
It works this way. The LIC pays the full 30% in CGT (no discount) and then pays a special Fully Franked dividend (call a LIC CG dividend) for which the investor pays tax on only 50% of the grossed up amount.

There is more. Not all LIC's are able to do so. The ATO has to approve the LIC's status as a long-term investor.
Most of the traditionals are approved (ARG, AFI, AUI, MLT etc) but very few of the new-fangled-joke LIC's (WAM, CAM, MMA etc) are permitted to do so.