Please reason step by step, and put your final answer within \boxed{}.
The center of a circle is on the positive half of the $x$-axis, and its radius is the length of the imaginary semi-axis of the hyperbola $\dfrac{x^{2}}{16} - \dfrac{y^{2}}{9} = 1$, and it is tangent to the asymptotes of this hyperbola. The equation of the circle is __________.
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