Don't all rush at once. These aren't as "young" as you might think. End Hesperian was as much as 3 billion years ago (depending on whose crater-age dating you use).

I'm not surprised at this finding. If it's damp enough for the sulphate mineral suite that we see at Meridiani and elsewhere, then silicate solution and redeposition as opal are quite likely. One of the best-known areas of Gem-quality opal formation is in one of the horrible barren and dessicated parts of the Australian outback, so we're not talking about sweetwater springs and balmy lakes here. if it was THAT wet, we'd get clays.

There is a volcanic-sounding flavour to this opaline silicates - maybe weathering in hot damp ash sheets?

This result is essentially identical to the ESA mineral weathering chronology for Mars:-

1) very early Mars (first ~500 Ma) was actually wet and maybe almost warm
2) the next 500 Ma to 1 Ga were dominated by volcanics and acid sulphates
3) Then it got REALLY dry and there was a terminal phase of silicate weathering (including this opaline silica suite)