In general when trying Phytomining, lixiviants are not used as most would be determental to plant growth.  Instead the plants themselves are selected based on their ability to uptake the minerals.

My understanding is that lixiviants are indeed used, but NOT in the beginning of plant life, but when the plants reach maximum growth. This is when they would be drawing up the most heavy metals, and so that is when the lixiviants are introduced.

Death to the plant then comes about because of heavy metal toxicity; most likely from copper, zinc and silver. But at the point the plant has achieved the desired result; containment of the heavy metals.

I am interested in gold, and there are no known gold hyperaccumulator type of plants, so a lixiviant must be used to make gold water soluble. But is there anything other than cyanide lixiviant's that would work?

There are lots of lixiviants for gold including aqua regia, chloride, thiourea, ammonia thiosulphate but getting the gold to dissolve in this kind of environment will be difficult.  Thiosulphate may be your best choice.

Regards

As in any metallurgical process the issue is the rate at which the process works.  If a gold lixiviant is used (similar to the ones that Todd mentioned), the rate of metals uptake by the plant versus the rate that the lixiviant kills the plant needs to be considered.  Most of them will work at killing the plant in a matter of a day or so, the rate of movement of the gold through the system is a lot slower than that, so I am wondering how much you could sequester in the plant?