  Recent studies have shown that close-in brown dwarfs in the mass range 35-55
M$_{\rm Jup}$ are almost depleted as companions to stars, suggesting that
objects with masses above and below this gap might have different formation
mechanisms. We determine the fundamental stellar parameters, as well as
individual abundances for a large sample of stars known to have a substellar
companion in the brown dwarf regime. The sample is divided into stars hosting
"massive" and "low-mass" brown dwarfs. Following previous works a threshold of
42.5 M$_{\rm Jup}$ was considered. Our results confirm that stars with brown
dwarf companions do not follow the well-established gas-giant planet
metallicity correlation seen in main-sequence planet hosts. Stars harbouring
"massive" brown dwarfs show similar metallicity and abundance distribution as
stars without known planets or with low-mass planets. We find a tendency of
stars harbouring "less-massive" brown dwarfs of having slightly larger
metallicity, [X$_{\rm Fe}$/Fe] values, and abundances of Sc II, Mn I, and Ni I
in comparison with the stars having the massive brown dwarfs. The data suggest,
as previously reported, that massive and low-mass brown dwarfs might present
differences in period and eccentricity. We find evidence of a non-metallicity
dependent mechanism for the formation of massive brown dwarfs. Our results
agree with a scenario in which massive brown dwarfs are formed as stars. At
high-metallicities, the core-accretion mechanism might become efficient in the
formation of low-mass brown dwarfs while at lower metallicities low-mass brown
dwarfs could form by gravitational instability in turbulent protostellar discs.
