Age constraints on host rocks of Los Uvares gold deposit: Magmatic pulses in southernmost Baja California, Mexico

Abstract

Los Uvares disseminated gold epithermal deposit, in the central part of the Cabo San Lucas Block (CSLB) near the tip of Baja California peninsula, occurs in a cataclastically deformed tonalite intruded by alaskite and mildly faulted diorite dikes. Potassium-argon (K-Ar) isotopic ages on hornblende separates from relatively unaltered intrusive rocks at Los Uvares are: 137 ± 6 Ma (tonalite) and 128 ± 5 Ma (diorite). The K-Ar ages are internally consistent with the relative ages of diorite and tonalite from field relationships, suggesting a Cretaceous magmatic event. These ages are older than previous ages found for similar plutonic rocks in the CSLB (ranges of 109-42 Ma). It seems likely that the Los Uvares deposit occurs in an intrusive complex that repre- sents an older phase (not previously reported) of the Cretaceous-Tertiary magmatic pulse in the CSLB. Fission-track ages on apatites in the diorite dikes range from 80 to 100 Ma. Fission-track data suggest that hydrothermal activity at Los Uvares ended by ~80 Ma, and that cooling/erosion followed.