Since Big Bang Theory doesn't provide an explanation for the origin of the heavier elements which make up most of the Earth and the life on it - many cosmological theorists contend that they may have instead formed in supernovae explosions.
Exact mechanism(s) however, to explain the production of many of the elements heavier than iron (so called trans-ferric elements) have still not been agreed.
“It’s one of the classic unsolved problems of physics.” ( quote from New Scientist 4 Feb 2006 ) unable to find reference or link
“A fundamental challenge in Nuclear Astrophysics is the understanding of the formation of trans-ferric elements. The site for the production of many of the elements heavier than iron, including gold, platinum and uranium is still unknown.” source Prof. Sydney Gales at GANIL accelerator facility, France
The 'r-process'- the proposed mechanism by which rapid neutron capture occurs in collapsing supernovae to create about half of the heavy elements above iron - is 'still experiencing major problems' (as at 2011) see: What are the astrophysical sites for the rr-process and the production of heavy elements?
And a 2016 paper presented at The 13th International Symposium on Origin of Matter and Evolution of Galaxies underlined the uncertainties :
“Astrophysical site(s) of rapid neutron-capture process (r-process) is (are) not identified yet. Although core-collapse supernovae have been regarded as one of the possible candidates of the astrophysical site of r-process, nucleosynthesis studies suggest serious difficulties in core-collapse supernovae to produce heavy elements with mass number of ≳110.” [i.e Zircon]
If the following results are confirmed, the mystery of heavy element formation may have been solved.
New observations which link gamma-ray emissions to gravitational-wave events logged by LIGO (which has detected a merger of two Neutron stars) are said to show an electromagnetic 'signature' that is compatible with calculations about heavy element formation.
““People have long suspected that heavy elements were made in neutron star mergers, but this is really the first time we’ve nailed that down,” says Andrew Levan at the University of Warwick, UK. This merger made something like the mass of the Earth in gold, he says, along with other heavy elements such as platinum, lead and uranium.”
More coverage of the story here in the journal Nature