New Mexico Geological Society Annual Spring Meeting — Abstracts

Alteration is the general term describing the mineralogic, textural, and chemical changes of a rock as a result of a change in the physical, thermal, and chemical conditions in the presence of water, steam, or gas. Recognition and understanding the genesis of alteration are important in mineraI exploration and the understanding of the formation of ore deposits, because specific alteration types are associated with specific ore-deposit types. Acid-sulfate alteration, a subclass of advanced-argillic alteration, is spatially associated with many types of ore deposits, including porphyry copper and epithermal vein deposits. Acid-sulfate alteration is characterized by the mineral assemblage containing quartz, kaolinite, alunite, and locally pyrite and pyrophyllite and results from base leaching by fluids containing H₂SO₄ and H₂S. Original textures and mineralogy are commonly completely destroyed during this process.

Several areas of acid-sulfate alteration occur in the Steeple Rock mining district in the Summit Mountains, Grant County, New Mexico and Greenlee County, Arizona. Three of these altered areas occur near the younger epithermal-vein deposits, whereas other areas are in the vicinity of the mineralized faults. The acid-sulfate altered areas are mineralogically, chemically, texturally, and probably temperature zoned and grouped into three types: (1) clay zone (outermost, lowest temperature), (2) silicified zone (interinediate); and (3) silica/chert zone (innermost, highest temperature). Some areas contain high gold anomalies. The acid-sulfate alteration grades outwards to argillic to propylitic to sericitic alteration. A K/Ar age date of quartz-alunite whole rock from Saddleback Mountain is reported as 31.3± 1.1 Ma (Marvin et aI., 1987, #175; Hedlund, 1993), which is the same age as the host rock -andesite of Summit Mountain (31 .3 ± 2.2 Ma on hornblende,
Hedlund, 1993). Reported sulfur isotopic data of alunite from Saddleback Mountain is high (S³⁴S=12.7%), is higher than coexisting pyrite (S³⁴S=-6.3%, 0.7%), and indicates a magmatic-hydrothermal origin (ie. hypogene, Field, 1966). Calculation of temperature of formation from coexisting alunite and pyrite indicates a temperature of 339°C, which is consistent with the mineral assemblage containing quartz, pyrite, alunite, and pyrophyllite. The mineral and chemical zonations, preserved textures, multiple horizons, stratigraphic relationships, age determinations, and limited sulfur isotopic data are consistent with the acid-sulfate alteration being produced by acidic magmatic-hydrothermal fluids at temperatures less than 340°C in a: relatively shallow environment (<1.5 km), such as a hot springs system. The acidic fluids are produced by disproportionation of sulfur in magmatic fluids with decreasing temperature as the fluids rose towards the surface. If this genesis is correct, then these fluids are capable of carrying gold and, therefore, these altered areas need to be examined for their potential for precious metals, The epithermal veins are younger than the acidsulfate alteration as evidenced by cross cutting relationships and age determinations.

Acid-sulfate alteration is associated with other mineral deposits throughout New Mexico. Supergene alteration containing alunite, quartz, and kaolinite occurs above the Chino and Tyrone porphyry copper deposits. Acid-sulfate alteration also occurs in several epithermal precious-metals districts: San Jose, Alum Mountain, Red River, Wilcox, Gila Fluorspar, and Cochiti. In addition, acidsulfate alteration occurs at Kline Mountain near the Taylor Creek tin deposits. Limited data suggest that these areas also may have formed by magmatic-hydrothermal fluids. These areas of acid-sulfate alteration appear to be restricted in time to 28-33 Ma. These areas need to be studied in greater detail to determine the origin of the alteration and their mineral resource potential.