New Mexico Geological Society Annual Spring Meeting — Abstracts

The Goat Hill orebody of the Southwest ore zone of the porphyry molybdenum deposit at Questa, New Mexico, was mined from 1983-2000 with a total extracted production of 21.11 Mt at 0.318% MoS₂. Molybdenite in the Goat Hill orebody occurs in a magmatichydrothermal breccia (MHBX) that was cut by later quartz-molybdenite stockwork veinlets. The MHBX contributed 40% of the MoS₂ to the Goat Hill, whereas the younger stockwork veinlets contributed 60% of the molybdenite to the orebody. Ross et. al. (2001) defined five distinct facies (A-E) from the bottom of the breccia body (A) adjacent to the source aplite intrusion, to the top and distal edges (E), respectively. Based upon quantitative study of breccia textures and preliminary stable isotope data, Ross concluded that the MHBX is primarily magmatic in origin with little or no meteoric component with d¹⁸O_H2O values of +5.5 to 8.1 per mil and dD values of -125 to -99 per mil.

The majority of fluid inclusions in the MHBX matrix quartz are primary with a minor secondary or pseudo secondary contribution. Fluid inclusion types observed in the matrix quartz are monophase liquid and vapor, vapor-rich and liquid-rich two phase, multi phase solid, and multi solid inclusions. Halite, hematite, and opaque daughter minerals are common. Preliminary homogenization temperatures of matrix fluid inclusions range from 381°C to 443°C. Salinities range from 8 to 50 eq. wt. % NaCl.

Fluid inclusions in the stockwork quartz-molybdenite vein lets are small (<1= 7.5 microns) and include both primary and secondary or pseudosecondary inclusions. Fluid inclusion types observed in the veinlet quartz are monophase liquid and vapor, vapor-rich and liquid-rich two phase, and multiphase solid. Only opaque daughters were observed in the vein inclusions. Preliminary homogenization temperatures of vein fluid inclusions range from 197°C to 392°C. Salinities range from 0.7 to 8.5 eq. wt. % NaCI.

In contrast to the conclusion by Ross (2001), preliminary gas analyses of the Czone matrix quartz have ratios of Ar-He-N2 that suggest the MHBX ore fluid is primarily meteoric in origin with minor admixed magmatic fluid. Conversely, gas analysis of the quartz-molybdenite veinlets suggests that the veins are magmatic in origin.