New Mexico Mineral Symposium — Abstracts

An unusual suite of ferric iron-bearing minerals occurs in xenoliths within a dacite pumice breccia. The 33-m.y.-old dacite pumice and ash flow, part of the Mogollon-Datil volcanic field, crops out along NM-12 approximately 1.5 mi (2.4 km) west of Old Horse Springs. The flow is as much as 200 m thick and crops out sporadically within a north-south belt up to 5 km wide and 20 km long.

Xenoliths of pre-Tertiary sedimentary rock and cognate(?) inclusions of coarse-grained quartz monzonite are concentrated in the upper part of the pumice breccia. The most common xenoliths are of limestone altered to a red, iron-rich jasperoid. These silicified nodules, from a few centimeters to as much as 0.3 m in size, consist of variable amounts of quartz, iron oxides, and calcite. Many xenoliths show concentric banding of layers rich in quartz and iron-oxide minerals; some are composed of nearly solid masses of iron oxides. Many xenoliths are surrounded by a skarn-like reaction zone a few centimeters thick composed largely of green diopside with or without andraditic garnet, phlogopite, and clay minerals. Other iron-bearing minerals occur in fractures in xenoliths of argillaceous or calcareous sandstone and in miarolitic vugs within quartz-monzonite inclusions.

Hematite is the most obvious of the oxide minerals; it forms black, lustrous, platy crystals typically 1 mm or less in diameter. A variety of spinel-group minerals are present in the xenoliths. Magnesioferrite occurs as minute octahedral crystals about 0.5 to 200 pm in size, usually embedded in fibrous, spheroidal-textured chalcedonic quartz. Magnesioferrite, ideally MgFe²⁺₃O₄, is transparent to translucent in thin section and orange brown to blood red in color; the red color of most fine-grained jasperoid is due to magnesioferrite rather than hematite. The magnesioferrite is slightly zinc bearing, ranging from about 0.1 to 4.0 wt % ZnO; a typical formula (for magnesioferrite with 0.49% ZnO) is (MgO_.98Zn_0.01Ca_0.01)(Fe⁺³_1.62Al_0.27Mn⁺³_0.05Mg_0.05)O_3.98 * Less abundant is aluminous spinel, brownish-yellow in thin section, containing several percent ZnO; a typical formula (for spinel with 1.0% ZnO) is  (Mg_0.98 Zn_0.02)(Al_1.62Fe⁺³_0.32Mn⁺³_0.01Mg_0.05Si_0.01)O_3.98*   Very fine grained (one to a few u m in size) zinc-rich spinels, detected only with the microprobe, have a complex composition approaching franklinite. One analysis of a specimen containing 27.46% ZnO gives the formula (Zn_0.80Mg_0.15Ca_0.05) (Fe⁺³_1.03Al_0.42Mn⁺³_0.37Mg_0.18Si_0.01)O_3.91 corresponding to end-member proportions of 51% franklinite, 21% gahnite, and 28% other components.

Diopside in the skarn rims is zoned, ranging from about 4 to 11% FeO. Green to brown andradite garnet, often in euhedral dodecahedra about 1 mm across, ranges in composition from about Andradite90Grossular8 to Andradite63 Grossular34 with the balance in other components. Diopside also forms free-growing transparent yellow-green prisms a few tenths of a millimeter in length in cavities in metasedimentary and monzonite nodules. Commonly present in cavities with diopside is pseudobrookite Fe²⁺₃TiO₅ small (about 0.2 mm long) prismatic to bladed crystals. X-ray diffraction and scanning electron microscopy show that some pseudobrookite prisms are coated with an irregular layer of titanite. Forsteritic olivine occurs as inclusions in hematite crystals; it contains about 1.8 wt % total iron expressed as FeO. However, bright-red cathodoluminescence of this forsterite suggests that only ferric iron is present in it, leading to the formula Mg_2.09Fe⁺³_0.04Si_0.93 the mineral shows a consistent excess of Mg and deficiency of Si.

Mineral assemblages in the jasperoid xenoliths are unusual for their highly oxidized state--all iron in the oxide minerals appears to exist as Fe⁺³--and the presence of zinc in the oxides. Bulk composition of the nodules ranges up to 10,000 ppm (1%) Zn, and anomalous amounts of Mn, Cu, V, Ni, and Pb are also present. A probable sequence of events for the nodules was 1) silicification of limestone beds to form iron- and zinc-bearing jasperoid; 2) stoping of silicified limestone blocks into the intruding dacite magma; 3) reaction of limestone xenoliths with the enclosing magma to form calc-silicate skarn rims--at least part of this reaction appears to have occurred after eruption because the skarn minerals surround the nodules as an undisturbed zone extending into the host dacite; 4) oxidation and recrystallization within the xenoliths to produce the present assemblage of hematite and zincian magnesioferrite, probably after eruption of the pumice breccia and contemporaneously with 3); and 5) retrograde (deuteric) development of phlogopite and montmorillonite in the skarn rims by reaction with water vapor in the cooling pumice flow.