New Mexico Geological Society Annual Spring Meeting — Abstracts

The dating of mineralization events is critical for understanding the genesis of ore deposits. Many economic deposits contain few hydrothermal minerals which can be reliably dated. Potassium rich fluids and solids included in quartz may provide accurate and precise ages, if dated by the ⁴⁰Ar/³⁹Ar method.

The Capitan pluton, central New Mexico, is alkali-feldspar granite with veins of quartz, adularia and other minerals. The high salinity (~75,000 ppm K), high temperature (~550° C), fluid inclusions in quartz contain abundant daughter minerlas, including sylvite, and are well characterized (Campbell, et aI., 1994). These inclusions were thought to be an ideal situation for investigating the plausibility of the ⁴⁰Ar/³⁹Ar method as a means of dating fluid inclusions. The adularia has been shown to be co-depositional and provides an age of 28.3
Ma.

Several size fractions from multiple locations were analyzed by the ⁴⁰Ar/³⁹Ar furnace incremental step heating method at the New Mexico Geochronology Research Lab. The inclusion bearing quartz was crushed prior to analysis and large uncrushed grains were also analyzed. The Cl/K ratio from crush-leach analysis of the fluid inclusions is known (Campbell et al., 1994) and can be compared to the Cl/K calculated from the argon results (³⁸Ar_Cl and ³⁹Ar_K are produced during irradiation from ³⁷Cl and ³⁹K). These isotopes are used to determine the relative abundance of Cl and K in the sample (Table 1) and provide importance insight towards the apparent age complexities.

The Cl/K ratios, as determined by the ³⁸Ar_Cl/³⁹Ar_K are in most cases lower than the crush-leach values and could result from additional K in the sample or depletion of Cl. Potassium x-ray mapping with an electron microprobe has revealed what are thought to be K-rich melt inclusions. These melt inclusions may account for the low Cl/K ratios since K from the melt inclusions would not be detected in crush leach analysis, but would by Ar analysis. The ⁴⁰Ar/³⁹Ar age spectra yield plateau patterns which typically encompass ~85% of the total ³⁹Ar released in crushed samples. The age spectra also yield an age maximum associated with laboratory heating steps of about l200°C. This pattern has previously been interpreted as typical of samples with melt inclusions. The old ages are associated with high ³⁸Ar_Cl/³⁹Ar_K and in two of the three cases (CMX2 and HH), isochron analyses reveal a trapped initial argon composition with an ⁴⁰Ar/³⁶Ar of 317. These higher than atmospheric ⁴⁰Ar/³⁶Ar ratios suggest excess argon contamination.

The apparent plateau ages of the quartz samples vary considerably (Table 1). Some samples appear to yield ages consistent with the known age of the deposit (i.e. ca. 28 Ma), but others yield much younger apparent ages (Table 1). These young apparent ages are difficult to reconcile within our present working hypotheses and the known geologic history of the Capitan pluton. However, recognition of these anomalously young ages has important ramifications for the presently published fluid inclusion dating results as well as for future geochronologic work in other mineral deposits.