Abstract

The alkaline rocks along the easternmost margin of the Rocky Mountains are known to host a wide variety of mineral deposits, including large tonnage (>1,000,000 oz) gold deposits. In New Mexico 11 such alkaline centers exist in Lincoln County. The largest of these centers hosts the mineral deposits that make up the Nogal mining district. The district is located in the Sierra Blanca Mountains of south-central New Mexico and contains four types of mineral deposits: (1) Au-bearing veins and breccias, (2) Ag-Pb-Zn veins, (3) disseminated Mo-Cu occurrences, and (4) Au placers. The district's geology is dominated by andesite flows and flow breccias that have been intruded by three hypabyssal alkaline stocks.Fluid inclusions from most gold deposits in the district have homogenization temperatures from 230 degrees to 540 degrees C with an average salinity of approximately 6 wt percent NaCl equiv. Two gold deposits, the Waterdog and the Helen Rae, also contain inclusions with salinities of 18 to 50 wt percent NaCl equiv. The Ag-Pb-Zn deposits have fluid inclusion homogenization temperatures from 180 degrees to 350 degrees C with an average salinity of 4 wt percent NaCl equiv.Oxygen isotope analyses of quartz yield an average calculated delta 18 O fluid value of 0.9 + or - 3.9 per mil for the gold deposits and a lower value of-2.6 + or - 2.3 per mil for the Ag-Pb-Zn deposits. The average calculated delta 34 S fluid value of the deposits is -1.7 + or - 0.8 per mil.Three different forms of hydrothermal alteration occur in the district. The first is the regional propylitic alteration of the andesites caused by isotopically depleted waters. The second is the alteration surrounding the Ag-Pb-Zn veins, which is mineralogically similar to the regional propylitization. The third type is vein-related alteration associated with gold mineralization. This alteration is superimposed over the regional alteration and is characterized by oxygen isotope reenrichment, and depletion of Na, Mg, and Fe in the rocks near the veins.The combined mineralogical and geochemical characteristics of the gold and the Ag-Pb-Zn deposits suggest that the Ag-Pb-Zn mineralization may have been associated with the regional propylitic alteration and that the gold deposits were later. Two of the gold deposits have many features in common with other alkaline rock-associated Au deposits, with the Helen Rae deposit exhibiting intermediate characteristics between the magmatic and epithermal end members.

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