I'm not aware of any formal records kept to the effect, but it's possible. Not just with Union-Castle but any ship in trade at the time which would have a strong room. For obvious reasons of security, the presence of gold on a ship in any location or quantity wasn't something that anybody was eager to advertise.
I'm just thinking if Union Castle Line was secretly transporting gold from africa and thats make their company boom at the early of 20th century. I'm not too familiar with the company and their lagacy in shipping and I don't know if they are the leading shipping lines that travels from europe to africa during the 20th century.
>>I'm just thinking if Union Castle Line was secretly transporting gold from africa and thats make their company boom at the early of 20th century.<<
Not likely. Gold shipments of any kind would have consisted of specie or bullion which would have been kept in a very secure area like a strong room. While it would be extremely valuable, it wouldn't have taken up a lot of space. You can read more about them at http://en.wikipedia.org/wiki/Union-Castle_Line
(Treat this source with some caution. Wikipedia can be useful, but they're not always that incredibly accurate.)