Description
The Julian mining district is located in the eastern portion of the western zone of the Peninsular Ranges batholith approximately 60 km northeast of San Diego, and contains the most significant mineral deposits discovered in San Diego County to date. Gold was first discovered in 1869, and following slow development of the mines, two periods of prosperous activity (1870-1876, 1888-1906) produced approximately $5 million (gold then $20.67 per fine ounce). The gold-quartz veins in Chariot Canyon are hosted by the Chariot Canyon fault, a northwest trending shear zone up to 50 m in width that cuts the Triassic(?) Julian Schist to the west and an unnamed tonalite of Cretaceous age to the east. The gold occurs in four subparallel lenticular veins, from less than 0.3 to 2.4 m in width, and is free milling. The quartz is highly strained and fractured, and in some places comminuted by post-mineral faulting. Arsenopyrite and pyrrhotite constitute the metallic gangue but may be auriferous. Two district alteration zones are recognized. The sericitic zone consists of quartz, oligoclase, tourmaline, sericite, garnet, and arsenopyrite. This assemblage occurs adjacent to the veins and affects both the plutonic and metasedimentary wallrocks. The argillic zone consists of variable proportions of clay minerals. It occurs structurally above and is gradational with the sericitic zone. Other alteration types include arsenopyritization, carbonatization, and possibly potassic alteration. Four K-Ar ages were obtained on micas collected from different locations within the vein/alteration system of the Golden Chariot Mine. Biotite from the vein footwall gave a cooling age of 91.3 ± 2.7 m.y., muscovite from the sericitically altered metasedimentary rocks gave 125.6 ± 3.8 m.y., and a muscovite-biotite pair from the sericitically altered mylonitized tonalite gave 68.5 ± 2.1 m.y. and 58.4 ± 3.0 m.y., respectively. Geologic constraints indicate that the 91.3 ± 2.7 m.y. age most closely represents an age of mineralization/alteration. A coexisting quartz-muscovite pair from the sericitic zone gave d180 values of + 17.2 per mil and +14.2 per mil, respectively, with a calculated equilibration temperature of 531°C. Fluid inclusion homogenization temperatures determined from tourmaline within the sericitic zone range between 535°C and 575°C, corrected for 2 kb total pressure and 1 percent salinity. Temperatures recorded from secondary inclusions in the vein quartz were lower at 375-390°C. Using 535°C, a minimum d180 value of +14.4 per mil was calculated for a fluid in equilibrium with the quartz and muscovite in the sericitic zone, indicating a metamorphic origin for the hydrothermal fluid. Textural and mineral evidence within the Julian Schist, together with geochronologic constraints rules out regional metamorphism as the thermal source of this hydrothermal system. The 91.3 ± 2.7 m.y. cooling age is concordant with cooling ages along the western margin of the large La Posta pluton to the southeast of the district. Thermal models indicate that large volumes of pelitic metasedimentary rock could be rapidly dehydrated by the La Posta pluton, a minimum of 54 km3 per kilometer of contact. Preexisting structures such as the Chariot Canyon fault could then act as conduits for upwardly migrating fluids released from the metamorphic pile.
