Geochronology of hydrothermal mineralization.

Abstract

Many hydrothermal ore deposits lack minerals that are suitable for isotopic dating. A multi-disciplinary effort was undertaken to develop new dating techniques and make direct measurements of the age of hydrothermal mineralization to further understand the many different products of crustal fluid flow. The U-Pb ages of monazite and xenotime and $\sp{40}$Ar/$\sp{39}$Ar ages of muscovite were used to determine emplacement and cooling ages for individual plutons within the biotite-muscovite granites of the Cornubian batholith. In addition, $\sp{40}$Ar/$\sp{39}$Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relation between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism continued from $\sim$300 Ma down to $\sim$275 Ma with no evidence of a major hiatus. The U-Pb ages for separate granite phases within a single pluton indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. The geochronologic data are used to suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. The apparent cooling rates of the plutons are unrelated to the emplacement age, but decrease from the southwest to the northeast from $\sim$210$\sp\circ$C myr$\sp{-1}$ to $\sim$60$\sp\circ$C myr$\sp{-1}$ with a mean of 100$\sp\circ$C myr$\sp{-1}$. These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The history of mineralization is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. The main polymetallic mineralization is most conceivably the product of regional hydrothermal circulation driven by heat from the emplacement of younger buried pulses of magma. The REE patterns in fluorites from several different types of ore deposition was measured. The variations in the REE element patterns was used to evaluate the feasibility of fluorite as a geochronometer for mineralization. An age of 272 $\pm$ 17 Ma was determined for the Cave-in-Rock Mississippi Valley-type deposit.