Judith L. Hannah

Movement and origin of ore fluids in Climax-type systems

Climax-type stockwork molybdenum mineralization records a pervasive, outward-directed movement of ore fluids from a central stock. Recently obtained lead, oxygen, and sulfur isotope data document this phenomenon. Both metals and sulfur are derived in toto from within the stock. Meteoric water and host rocks at upper crustal levels play no role in the ore-forming process. Rather, wallrocks have been chemically overwhelmed by material evolving from the stock complex.

Age relationships and depositional environments of Paleozoic strata, northern Sierra Nevada, California

Upper Paleozoic volcanic rocks in the northern Sierra Nevada rest unconformably on the lower Paleozoic Shoo Fly Complex. Late Ordovician conodonts recovered from an exotic limestone block in Shoo Fly melange have North American affinities; the Complex may have traveled little from its original location. Magmatism may have begun as early as Early Devonian and persisted into Early Mississippian time. The volcanic rocks constitute a longitudinal section through a relatively immature volcanic arc dominated by tholeiitic magmas. Two lithologically distinct members of the overlying Peale Formation are given formation status. The predominantly volcanogenic lower member is renamed the “Keddie Ridge Formation”; the name “Peale Formation” is retained for the upper chert member. Two new Late Mississippian–Early Pennsylvanian radiolarian localities in the Peale chert at the north end of the Paleozoic outcrop belt correlate well with a previously described assemblage at the south end and thus rule out time-transgressive deposition. Early Permian fusilinids in a chert-pebble conglomerate only 10 m upsection from an Early Pennsylvanian radiolarian locality tightly constrain a regional depositional hiatus. The Peale chert displays common soft-sediment deformation, slump structures, intraformational breccias, and abrupt termination of thick sequences along strike. Locally, the chert is intruded by tholeiitic basalt. These features suggest deposition in small, tectonically active basins, possibly in an extensional, back-arc regime. The overlying Permian sequence of arc tholeiites is truncated by Mesozoic strata in most areas but appears to grade into Triassic shale and limestone in the extreme northwest.

Isotopic age constraints on middle Paleozoic deformation in the northern Sierra Nevada, California

Allochthons of the lower Paleozoic Shoo Fly Complex in the northern Sierra Nevada were assembled and internally deformed prior to formation of a Devonian-Permian island-arc sequence. U/Pb data on zircons indicate ages of 423 +5/−15 Ma for a submarine tuff within the uppermost thrust slice of the Shoo Fly Complex and 378 +5/−10 Ma for a granitic intrusion that may be cogenetic with the lower part of the arc sequence. These data indicate late Early Silurian Shoo Fly deposition and proximity to active volcanism, as well as late Middle Devonian initiation of arc-related magmatism.

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield δ18O values less than 10.4‰, with most values between 9.3 and 10.4‰. An average magmatic value of about 8.5‰ is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust.Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock δ18O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low δ18O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases.Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of 18O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of 18O depletion in magmatic systems without meteoric water influx.