A Permian kuroko-type hydrothermal system, Afterthought-Ingot area, Shasta County, California; lateral and vertical sections, and geochemical evolution

Abstract

The Afterthought and smaller kuroko deposits in the East Shasta mining district formed at the culmination of a late Permian mafic to felsic volcanic cycle. They were succeeded by Triassic elastic sediments and reworked tuff in a presumed rifting environment with local evidence of renewed mafic volcanism and large-scale slumping. The Afterthought host horizon is repeated in outcrop by folding. Fold closures are tight east of Ingot where preore volcanic rocks are more altered; they are open basins and domes west of Ingot where the preore volcanic rocks are less altered. Igneous pyroxene and titanomagnetite and secondary pumpellyite occur only in preore andesite and basalt more than 2.5 km from known kuroko deposits. In outcrops closer to the deposits, disseminated pyrite (type E) is locally abundant in preore dacite. The kuroko deposits comprise three ore types: A: banded, fine-grained sulfides and gangue; B: recrystallized, coarser grained sulfides and gangue with a consistent paragenetic sequence; and C: coarse-grained pyrite and barite. B and C appear to replace type A. A further type of mineralization, type D, consists of fine-grained pyrite occurring locally within silicified dacite on the Afterthought host horizon. The paragenetic sequence of type B implies a sequential introduction of metals in a fluid evolving toward higher H 2 S activity. Cu was introduced early as chalcopyrite and bornite; later fluids altered preexisting Cu minerals rather than introducing significant new Cu. Type B fluids probably cooled from >300 degrees C (chalcopyrite stage) to about 270 degrees C (sphalerite + barite stage). The last type B fluids dissolved much base metal sulfide and deposited calcite, quartz, and sericite. Types A, B, C, and E sulfides have a delta 34 S value range of 5 to 12 per mil, implying a common origin. Barites from type A and B ore have delta 34 S values near 30 per mil. Partially reduced contemporaneous marine sulfate (10-12ppm) is a possible sulfur source, but a concealed rock-sulfur source of high delta 34 S value is more probable. Type D pyrite, with delta 34 S values between -28 and 0 per mil, contains sulfur of a different, probably biogenic, origin. Hydrothermal carbonates formed from fluids with delta 13 C values from near 0 to -12 per mil, consistent with oxidation of organic C in the hydrothermal system. Hydrothermal and postore carbonates have delta 18 O values of 17 to 19 per mil and probably equilibrated with a single fluid after the deposition of the Hosselkus Limestone.