Avi Loeb has written another great essay,  "Astronomers have recently determined that rare elements such as gold and uranium are produced as a result of rapid capture of free neutrons during the merger of two neutron stars. Neutron stars are the densest stars known, having the size of a city (12 kilometers) and up to twice the mass of the sun, with the density of an atomic nucleus. A teaspoon of neutron star material weighs a trillion kilograms, as much as a tall mountain on Earth."

Scientific American: https://blogs.scientificamerican.com/observations/for-e-t-civilizations-location-could-be-everything/

Newsweek: http://www.newsweek.com/would-alien-civilizations-be-rich-colliding-neutron-stars-846435