  The distribution of LIGO black hole binaries (BBH) shows an intermediate-mass
range consistent with the Salpeter Initial Mass Function (IMF) in black hole
formation by core-collapse supernovae, subject to preserving binary
association. They are effectively parameterized by mean mass $\mu$ with Pearson
correlation coefficient $r = 0.93\,\pm\,0.06$ of secondary to primary masses
with mean mass-ratio $\bar{q}\simeq 0.67$, $q=M_2/M_1$, consistent with the
paucity of intermediate-mass X-ray binaries. The mass-function of LIGO BBHs is
well-approximated by a broken power-law with a tail $\mu\gtrsim 31.4M_\odot$
{in the mean binary mass $\mu=\left(M_1+M_2\right)/2$}. Its power-law index
$\alpha_{B,true}=4.77\pm 0.73$ inferred from the tail of the observed
mass-function is found to approach the upper bound $2\alpha_S=4.7$ of the
uncorrelated binary initial mass-function, defined by the Salpeter index
$\alpha_S=2.35$ of the Initial Mass Function of stars. The observed low scatter
in BBH mass ratio $q$ evidences equalizing mass-transfer in binary evolution
prior to BBH formation. At the progenitor redshift $z^\prime$, furthermore, the
power-law index satisfies $\alpha_B^\prime>\alpha_B$ in a flat $\Lambda$CDM
background cosmology. The bound $\alpha_{B,true}^\prime \lesssim 2\alpha_S$
hereby precludes early formation at arbitrarily high redshift $z^\prime \gg1$,
that may be made more precise and robust with extended BBH surveys from
upcoming LIGO O4-5 observations.
