The authors define the covariant derivative of a single massive fermion field on a Riemannian manifold. A spin gauge theory of gravity is developed based upon: (i) local bivector spin gauge transformations of the elements of the Clifford algebra associated with a curved spacetime; (ii) the factorization of the mass term \[ -m{\bar \psi}\psi ={\bar \psi}i\gamma^{\mu}((1/4)im \gamma_{\mu}) \] of the Dirac equation and the incorporation of the frame field term (1/4)im \(\gamma\) \({}_{\mu}\) into the extended covariant derivative. This ``extended covariant derivative'' defines a gravitational Lagrangian density proportional to the usual scalar curvature components. The quadratic term is expected to be negligible at phere-preserving transformations of conformally closed spaces [cf. the author, Beiträge zur projektiven Inversion and deren Linearisierung (Ph. D. Thesis, TU München) (1985; Zbl 0572.51017)].