Open-file Report - 603
Geology, mineral-resource potential, and potential environmental impacts of the Rosedale mining district, Socorro County, New Mexico

SUMMARY

This report summarizes the mining history, geology, geochemistry, mineral-resource potential and potential environmental impacts of mining in the Rosedale mining district. The Rosedale mining district (DIS225) is a small mining district in the eastern foothills of the northern San Mateo Mountains in southwestern Socorro County. The climate of the Rosedale district is semiarid and alpine, with warm summers and cold winters. The Rosedale district was discovered circa 1882 and is estimated to have yielded 28,000 oz Au and 10,000 oz Ag from volcanic-epithermal vein deposits. Foundations from the Rosedale townsite, Rothchild mill (NMSO0748), Bell mill (NMSO0590), and scattered collapsed cabins are found in the district. Currently, the Rosedale mine owner is conducting exploration for potential gold and silver.

Volcanic-epithermal veins and associated alteration in the district were probably developed shortly after eruption and deposition of the 27.4 Ma South Canyon Tuff. The predominant alteration is argillic, which is characterized by clays, sanidine, and quartz, and cross-cuts fault zones that juxtapose altered and unaltered rock. Structurally-controlled, volcanic-epithermal veins are hosted in well-developed breccia and sheared rhyolite porphyry and are locally cemented by banded greenish-white quartz. The shear-zone veins extend into a footwall of white rhyolite porphyry. The veins carry free-milling gold and are usually associated with hematite and manganese oxides that occur as replacements of pyrite grains and stringers, and as coatings on fracture surfaces. Replacement textures of iron oxides after pyrite can be seen in electron microprobe analysis. Replacement of magmatic or late magmatic biotite is common.

Most of the mine features in the Rosedale district are shallow prospect pits and short adits. Mining-related disturbance is minor and found only in the vicinity of the individual mine features. However, the Rosedale shaft complex (NMSO0064, NMSO0543, NMSO0586, NMSO0585, NMSO0591, NMSO0585), Bell adit (NMSO0061), Robb shaft (NMSO0545) and Lane shaft (NMSO0734) are dangerous and require proper safeguarding. A few short adits also have the potential for unsafe conditions. Many of the mine features are in remote areas with no road access. The main tailings facilities (NMSO0264, NMSO0265) have been previously reclaimed by the U.S. Forest Service. Only one sample from the Rosedale district plotted in the uncertain field on the acid rock drainage (ARD) diagram; the other samples plotted as non-acid forming. The Rosedale samples contain low concentrations of all metals. Water samples collected from the Robb prospect (NMSO0545) and springs and seeps in the area exhibit no harmful water quality issues and are typical of surface water from the area. None of the other mine features had any water, except the main Rosedale shaft (NMSO0064), where water was encountered at 726 ft and was not sampled. The mean annual precipitation for the Rosedale mining district is low (15.7 in/yr) and calculated values for the 5 year peak flood are also low (119 cfs), which collectively indicates that environmental impacts of mining in the Rosedale district are low.

The Rosedale district has a high mineral-resource potential with a high level of certainty for gold>>silver as a low-sulfidation, quartz-dominant, low-base metal, volcanic-epithermal vein deposit along the Rosedale and Bell veins, with an unknown mineral-resource potential with a low degree of certainty for base metals at depths below the present precious-metal workings. Deep drilling would be required to determine if there are any base metals at depth.

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