The Parad Saṃhita (and presumably other related texts) specifies a number of alchemical preparations of mercury, in which it is combined with gold and/or arsenic, which serve to purify and de-toxify it, rendering the mercury suitable for consumption.  I suspect the reference to "gold" here is along these lines, and so would take "matured" in the sense of "perfected", and hence read the passage more like "... they are “matured” like [mercury] with the element of gold ..."