New Mexico Geological Society Annual Spring Meeting — Abstracts

The Organ Mountains fonn a west-tilted block exposing rocks ranging in age from Proterozoic through Quaternary. In the Oligocene, the Organ batholith and volcanic rocks associated with the Organ caulderawere emplaced. The Organ batholith is a complex pluton made up of three major phases: the granite of Granite Peak, Sugarloaf Peak quartz monzonite and Organ Needle quartz syenite (Seager, 1981). Mineralization in the Organ Mountains district was discovered in the 1830s and perhaps as early as 1797. Production in 1869-1962 amounts to 25 million lbs of Pb, 2.7 million lbs of Zn, 4,636,000 lbs of Cu, 820,000 oz of Ag, and 11,500 oz of Au worth $2.7 million.It is the 6th largest lead producing district in New Mexico, although there is no current production.

Six types of mineral deposits are found in the district and are within or centered around the Sugarloaf Peak quartz monzonite. Three centers of disseminated Cu-Mo minerals (porphyry Cu-Mo deposits?) are surrounded by innermost Cu skarns and breccias, Zn-Pb skarns, Pb-Ag replacements; Au-Ag veins, and an outennost barite-fluorite zone (Dunham, 1935; Seager, 1981). The relationship of the barite-fiuvrite zones to the mineralization centers is inconclusive; they are similar to the Rio Grande Rift barite-fluorite-galena deposits that are thought to be associated with extension along the Rio Grande Rift. This district-wide zoning is best preserved in the northern portion of the district, where disseminated Cu-Mo minerals, representing a faulted portion of a larger porphyry Cu-Mo deposit,was encountered in drill holes northwest of Organ. Copper-breccia and Cu skarn deposits arefound adjacent to the porphyry deposit (Excelsior mine) and grade outward from the pluton to Zn skarns(Merritriac mine), to Pb-Zn replacement deposits (Hilltop mine), to distal Pb-Ag-Mn replacement deposits (Black Prince mines). Adjacent to a second center of mineralization near San Augustin Pass are Cu breccia deposits at the Torpedo, Zn skarns at the Memphis, Zn-Pb replacement deposits at theHomestake, Pb-Zn replacements at the Philadelphia and Stephenson-Bennett mines, and Au-Ag pegmatites and veins in the Mineral Hill area. The Modoc Pb-Zn skarn and replacement deposits and Ruby fluorspar deposits are related to the third center of mineralization near Organ Peak. All of these gradational changes occur along faults associated with the Organ caldera and/or batholith margin. Mineralization style, metal zoning, and fluid inclusion data indicate that these deposits are related and they could have formed by mixing of magmatic and metebric fluids.

The Sugarloaf Peak quartz monzonite was dated by K-Ar as 32.8±0.5 Ma by Loring and Loring (1980). Several dikes north of Organ intruding the Sugarloaf Peak quartz monzonite were .also dated, and some of these dikes had age dates that were older than the age date obtained of the older pluton (32.1-34.4, Loring and Loring,1980). Therefore, relatively unaltered samples of the Sugarloaf Peak quartz monzonite and a pegmatite at San Augustin pass were collected and dated by ⁴⁰Ar/³⁹Ar methods at NMBMMR.

Biotite from the Sugarloaf Peak quartz monzonite yielded a flat spectrum with a well defined plateau at 33 .1±0.1 Ma. K-feldspar yielded a saddle-shaped spectrum. with flat segments centered at approximately 33 and 41 Ma. The shape of the age spectrum, which is in part older than the biotite plateau, indicate excess Ar in this Kfeldspar. Hornblende yielded a disturbed age spectrum. Low radiogenic yields coupled with anomoulously high K/Ca ratios indicate alteration and/or contamination by K-bearing phases. The biotite age of 33.1±0.1 Ma is considered the best estimate for when the pluton cooled through the biotite closure temperature of ~300°C.

K-feldspar from the Quickstrike pegmatite yielded a rising spectrum with flat segments averaging 30.8±0.1 and 32.2±0.1 Ma. The form of the spectrum suggests that the K-feldspar closed at 32.5 Ma and was reheated and partially reset at 30.8 Ma. The younger age suggests that hydrothermal fluids were mobile and reset the age date. The isochron plot yields an age of 30.8±0.1 Ma, although the MSWD (mean standard weighteq deviation) of the isochron is high (20.4). The ⁴⁰Ar/³⁶Ar for those steps is close to atmospheric, suggesting no trapped ⁴⁰Ar component is present. From these data, the maximum age of the mineral deposits in the Organ Mountains is 33.1 Ma; mineralization probably continued through at least 30.8 Ma.