  We establish Hadamard-type inequalities for a class of symmetric matrices
called $k$-positive matrices for which the $m$-th elementary symmetric
functions of their eigenvalues are positive for all $m\leq k$. These matrices
arise naturally in the study of $k$-Hessian equations in Partial Differential
Equations. For each $k$-positive matrix, we show that the sum of its principal
minors of size $k$ is not larger than the $k$-th elementary symmetric function
of their diagonal entries. The case $k=n$ corresponds to the classical Hadamard
inequality for positive definite matrices. Some consequences are also obtained.
