The answer is A single fortunate event does not mean that what follows will also be good.
The idiom "one swallow does not make a summer" means that a single good event does not guarantee that everything will be good from now on. It is used to caution against making assumptions about the future based on a single event.
In the sentence, the first sample of ore from the trial boring is very promising. However, the idiom "one swallow does not make a summer" reminds us that this does not mean that the mine will definitely be successful. There could be other factors that could affect the outcome, such as the quality of the ore or the cost of mining. | 
