Orogenic gold formation and tectonic evolution of the Grass Valley gold district and temporal correlations of gold deposits in California

Abstract

With a total past production of 13 Moz of lode gold, the Grass Valley gold district of the Sierra Nevada foothills province is the historically most productive lode gold source in California. Despite its economic importance, an understanding of the broad processes controlling the gold formation is lacking. Two distinct vein sets are present in Grass Valley: a north-trending set (N-S veins) hosted by the Grass Valley granodiorite and an east-trending set (E-W veins) hosted within mafic-ultramafic rocks. Questions of how these relate to each other and if they are products of the same event or different events remain to be answered. Some of the previously published data are conflicting, and the timing of gold formation for the district seems inconsistent with previous interpretations of orogenic gold formation in the Cordillera of California, particularly when viewed relative to the much better studied Mother Lode belt in the southern Sierra Nevada. A geochemical and geochronological characterization of the ore-hosting granodiorite is also lacking. The present study represents the first detailed modern study on the Grass Valley gold district. The research included a detailed microanalytical and geochronological study of the ore-hosting granodiorite and the orogenic quartz veins. It is shown the ore-hosting Grass Valley granodiorite was emplaced at 159.9 ± 2.2 Ma (U-Pb zircon) at temperatures of nearly 800 °C and at paleodepths of approximately 3 km. It rapidly cooled to below 300 °C between 162-160 Ma (40Ar/39Ar hornblende and biotite). After crystallization, the intrusion underwent brittle fracturing concurrent with N-S vein formation. The hydrothermal fluids interacted with the granodiorite and formed monazite and xenotime as alteration products, permitting U-Pb geochronology. An age of 162 ± 5 Ma for vein formation was determined for xenotime. This age is indistinguishable from the intrusive age, but must have occurred after the pluton was cool enough to undergo brittle fracturing. The hydrothermal monazite and xenotime have markedly different geochemical characteristics than magmatic phases. Magmatic monazite from the Grass Valley granodiorite has Th concentrations up to 11.6 wt.%, whereas the hydrothermal monazite has maximum Th concentrations of 0.2 wt.%. The REE profiles are also significantly different, including a strong negative Eu anomaly for the magmatic phases and no Eu anomaly for the hydrothermal phases. Therefore, despite this age overlap between magmatism and hydrothermal activity, they are not genetically related. This implies that the vein-hosted phases are not xenocrysts and also did not form from an evolving magmatic-hydrothermal system, but are instead formed by orogenic fluids. A second hydrothermal event formed the E-W veins at ~152 Ma, isolated in time from any regional magmatism. In addition to the geochronological research on the ore-hosting granodiorite and the veins, a detailed paragenetic investigation was performed on the orogenic veins as they are remarkably undeformed. In contrast to typical orogenic gold deposits displaying textures indicating brittle-ductile deformation and recrystallization, those of Grass Valley only display minor brittle fracturing of quartz and pyrite. Optical microscopy and optical cathodoluminescence imaging revealed the presence of multiple generations of quartz characterized by different luminescence responses and concentrations of secondary fluid inclusion trails. Pyrite crystallized following quartz precipitation. Gold precipitates relatively late in the paragenetic sequence entirely independent of quartz and is found within fractures in quartz, in fractures and voids within pyrite, and intergrown with galena and mica. The time of gold mineralization is recorded in pyrite by a chemically distinct growth zone containing arsenic, and nickel and cobalt zones with pyrites found in the E-W veins hosted in mafic-ultramafic rocks. The formation of quartz due to adiabatic decompression indicates the importance of pressure fluctuations in vein formation. The correlation of elements derived from the fluid (Ag, As, and Au) and those from the host rock (Co, Ni, and Pb) indicate the importance of fluid reactions with the local host rock during mineralization. Developing a regional scale view of gold mineralization in the Cordillera of California can help shape the understanding of how gold deposit formation relates to various stages in the late Mesozoic tectonic evolution of California. To constrain the timing of gold mineralization in the other major gold province of California, white mica was separated from samples from eight deposits. Four of these exhibited evidence for excess argon interpreted to result from intense deformation and (or) the presence of mineral inclusions. The other four samples had argon isotope age spectra that provided plateau ages of ~160-140 Ma. The maximum age corresponds to a major plate reorganization and initial gold mineralization in the Sierra Nevada. The minimum age corresponds to the initiation of the lateral offset of the Klamath Mountains westward of the Sierra Nevada and the active arc. This marks the termination of both hydrothermal activity and magmatism in the Klamath Mountains. However, orogenic gold formation within the Sierra Nevada foothills continued as it was still located on the active arc.