LAUSR

Abstract The Cerro Prieto pluton is located in the San Juan de Guadalupe municipality, in the eastern limits of the Durango State, in northern Mexico. The monzonite–diorite pluton intruded the Upper Cretaceous Caracol Formation, causing metamorphism of shale and sandstone, producing a hornfels aureole surrounding the pluton body, which hosts metallic mineral deposits (Au, Ag, Pb, Cu, Zn). We infer that the Cerro Prieto pluton causes the intense observed magnetic anomaly with normal dipolar character, with peak-to-peak-amplitude of about 1000 nT. The 3D nonlinear inverse modeling of this magnetic anomaly revealed a large, normally magnetized source body, with a volume of 625 km3, which fits the data with a residual variance less than 1 nT. The geometry of the model is consistent with a two-stage genesis in which the magma was first emplaced and stopped on a horizontal surface, currently at about 2000 m below the surface, and later thickening by lifting the roof and lowering the floor of the magma chamber. The principal dimensions of the pluton correspond with a triaxial ellipsoid elongated along N25°E, with axis lengths: L1 = 20, L2 = 12.5 and L3 = 4 km, and this ellipsoid has a volume reduced to 523.6 km3, which satisfies the power law L3 = 0.88 [(L1 + L2)/2] 0.55, allowing the classification of the Cerro Prieto pluton as a laccolith. Considering the 1D diffusion equation, with a thermal diffusion constant of 0.6 mm2/s, the time for lowering the temperature of the magma at the time of the intrusion, down to quartz α-β transition temperature, lasted about 500,000 years, suggesting that the Cerro Prieto laccolith was emplaced at the end of Cretaceous or at the beginning of the Paleocene, then cooled below its Curie temperature, and was fully magnetized during the geomagnetic polarity sub Chron C29n or C28n time, between 66 and 63.5 Ma.