THE GOAT HILL OREBODY, QUESTA PORPHYRY MO SYSTEM, NEW MEXICO - A GEOCHEMICAL STUDY OF A STRATIFIED MAGMATIC-HYDROTHERMAL BRECCIA AND STOCKWORK VEINLETS

The Goat Hill orebody at the Questa porphyry molybdenum system consists of a magmatic-hydrothermal breccia (MHBX) and younger quartz-molybdenite (qtz-mo) stockwork veinlets.  Ross (Ross, 2002, Econ. Geol., Volume 97, No.8) defined five distinctly stratified facies (A-E) within the MHBX based upon matrix mineralogy, and clast alteration and textures.  Fluid inclusion (FI) microthermometry and gas analyses, stable isotopes, and ⁴⁰Ar/³⁹Ar geochronology were used to identify possible geochemical variations within the stratified MHBX that coincide with identified mineralogic variations, and/or between the MHBX and qtz-mo veinlets.          

Eight of nine fluorophlogopite samples dated by ⁴⁰Ar/³⁹Ar geochronology are analytically indistinguishable and yield a weighted mean age of 24.20 +/- 0.06 Ma, suggesting all facies formed within 100 ka.  Final Ths of FIs in quartz from the five MHBX facies range from 229-532^oC, with a mode at 380^oC.  The majority of MHBX FI salinities define two distinct populations, 0-12 and 32-64 eq. wt.% NaCl.  Final Ths of FIs in the veinlet quartz average 372^oC, with an average salinity of 6 eq. wt.% NaCl.  d¹⁸O values of quartz (6.0-8.4 ^o/_oo), fluorophlogopite (2.0-5.1 ^o/_oo), and whole rocks (clasts) (-0.4-5.7^o/_oo) yield calculated d¹⁸O_H2O values of 1.9-4.5 ^o/_oo.

Despite mineralogic differences within the MHBX, geochemical analyses performed in this study reveal no difference between the MHBX facies.  Hence, no evolutionary trend is apparent from the geochemical data that would explain the mineralogic variance between the facies.  In addition, geochemical data indicate no distinct difference between the MHBX facies and the qtz-mo veinlets, except for absence of the high salinity inclusions.  Ths, salinity, and the similarity in age of MHBX mineralization (this study) and magmatic activity in the area (24.6 +/- 0.1 Ma and 24.1 +/- 0.1 Ma, Czamanske et. al., 1990, NMGS Guidebook, 41^st Field Conf.) suggest a magmatic source for mineralization.  d¹⁸O_H2O values are some what lower than would be expected for a magmatic fluid and suggest some meteoric involvement in either the magmatic or hydrothermal stage in the formation of the Goat Hill orebody. This is supported by preliminary FI gas ratios (Ar-He-N₂) from the MHBX and veinlet quartz, which span a range that is compatible with sources from magmatic to meteoric.