Enthalpy of Zinc and Sulfur Reaction
The free energies of formation of the elements in their standard states is zero, by definition.  You can look up the (variously reported) standard enthalpy of formation of the product, −204.6 kJ/mol (exothermic!).  It is roughly the binding energy of the crystal lattice less the ionization energies of the inputs.  So, by the numbers, do you obtain spalerite or wurtzite?  ZnS has crystal polymorphs and polytopes with differently populated unit cells

$\ce{Zn_2S_2}$ Wurtzite-2H 
$\ce{Zn_4S_4}$ Wurtzite-4H and Sphalerite 
$\ce{Zn_6S_6}$ Wurtzite-6H 
$\ce{Zn_15S_15}$ Wurtzite-15R