I picked up somewhere that the Roman empire financed their wars and other excesses by looting gold from the peoples north of their borders, who are today generally referred to as "Celts", so it would be plausible that they lost a shipment of loot somewhere - after all people are still finding sunken Spanish ships full of valuables looted from the Americas from the time of the conquista.
For those who want to check the plausibility of this stash existing as described (ingots in 2 cubic meters iirc), here is something I was allowed to photograph several years back:
This is 500g at .9999 purity, it is about 1cm high. Those purported 40T would be more than 80.000 of these by volume - perhaps around 90K-95K of them because it is doubtful that back in 100 BCE they had technology to refine gold to more than 90 (.9000) purity.
The most astonishing thing (to me) about gold is that it lets many people lose their ability to think in a clear way - it provokes something I call the "Gollum Reflex": they go all "my preshiousssss" at the sight of even small quantities of gold. Just try this: confront anybody with an ounce of gold (a coin), current value is about 1260EUR, and show them 2000 EUR (or USD) in bills and notice how different they react to both items. Silver does not cause that kind of craze in people.
A warning in case anybody decides to start looking around: in Germany there is a large semi-legal scene of people scouring the landscape with metal detectors who are often enough at odds with the authorities over who owns what piece of disinterred hardware. There have been raids and some juicy fines meted out over the years.
Laura, since you are looking at the Mithraists, are you aware of _http://one-evil.org ? It belongs to an Australian who says that the Catholic Church is basically the cult of Cybele (and Dagon and Inanna and the whole lot of evil gods of ME origin) operating under another name, people who sacrifice children (and rape them) because of their "inner cult" religion. He also explains that the church looks askance at homosexuality because (my memory and interpretation) it is an act of so high cultic value that it should not be practiced by people outside of their inner circle of initiates. There is also a reference to gold. Thought that you'd be interested.