[gatehouse]

At least one of the metals in the alloy would need to be higher density than gold. There are a few but I don't think any of them are easy to get: http://www.wolframalpha.com/share/clip?f=d41d8cd98f00b204e98...

EDIT: tungsten + platinum looks viable as sibling comment mentioned.

[yincrash]

Oops! I just assumed lead was denser than gold without checking! I didn't realize how dense gold actually is.