New Mexico Mineral Symposium — Abstracts

Steeple Rock mining district is in the Summit Mountains in southwest New Mexico and southeast Arizona. An estimated $10 million worth of gold, silver, copper, lead, and zinc have been produced from the district since 1880. In addition, minor quantities of fluorite, manganese, and decorative stone were produced. Five distinct types of epithermal fissure-vein deposits are identified in the district on the basis of mineralogy and chemistry: 1) base with precious metals, 2) precious metals, 3) copper-silver, 4) fluorite, and 5) manganese. These veins are structurally controlled, hosted by Miocene to Oligocene volcanic, volcaniclastic, and intrusive rocks, and appear to be spatially related to two types of alteration: alkali-chloride (propylitic to argillic to sericitic) and acid-sulfate (advanced argillic). However, field relationships and age determinations indicate that the alteration is older than the epithermal fissure veins.

The mineral assemblages, along with field relationships, textures, fluid-inclusion data, and sulfur-isotope data, provide clues to the origin and genesis of the alteration and the epithermal fissure veins. Alunite and kaolinite in acid-sulfate altered rocks indicate precipitation from acidic fluids. Pyrophyllite indicates temperatures of formation >250°C. Chlorite, adularia, and quartz in alkali¬chloride-altered rocks indicate precipitation from slightly acidic to neutral pH fluids. Epidote indicates temperatures of formation > 250°C. Mineralogic and chemical zonations, textures, stratigraphic relationships, and limited sulfur-isotope data are consistent with both types of alteration being formed by magmatic-hydrothermal fluids in a nearsurface environment, similar to modern hot springs systems.

The epithermal fissure veins have mineral assemblages consistent with precipitation from slightly acidic to neutral pH fluids. Fluid-inclusion data indicate these fluids had low salinity (<5 eq. wt. % NaCl) and were at low to moderate temperatures (240-320°C) at relatively shallow depths (360-1,300 m). Lattice textures of bladed quartz (pseudomorphed after calcite) and bladed calcite indicate boiling was a mechanism forming these fissure veins. These data are consistent with an epithermal origin.

The alteration and mineralization in Steeple Rock district occurred in response to cyclic volcanic activity that subsequently resulted in development of local hydrothermal systems. Exact timing and duration of these events is unknown. There is no evidence to suggest that these events were continuous, rather the alteration and mineralization were probably episodic, waning and migrating from one locality to another.

Steeple Rock district is known for the occurrence of sceptered amethyst in vugs of andesite, but these specimens actually are found north of the district. Steeple Rock district does contain amethyst quartz intergrown with quartz and sulfides. Other collectable minerals from the district are listed below. In addition, areas of acid-sulfate-altered rocks typically contain Liesegang-banded rock that can be cut and polished as decorative stone.

Minerals in epithermal fissure veins:
gold, Au
silver minerals, Ag sphalerite, (Zn,Fe)S chalcopyrite, CuFeS₂
galena, PbS
fluorite, CaF₂
anglesite, PbSO₄
azurite, Cu₃(OH)₂(CO₃)₂
bornite, Cu₅FeS₄ cerussite, PbCO₃
chalcanthite, CaSO₄ • 5H₂O
chalcocite, Cu₂S
chrysocolla, (Cu,Al)₂H₂S₂0₅(OH)₄ * nH₂O
covellite, CuS cuprite, Cu₂O
malachite, Cu₂(OH)₂(CO₃)
mimetite, Pb₅(As0₄,PO₄)₃Cl
mottramite, (Cu,Zn)Pb(VO₄)(OH)
nantokite, CuCl
pseudomalachite, Cu₅(PO₄)₂(OH)₄ H₂0
quartz, SiO₂
calcite, CaCO₃ pyrite, FeS₂
adularia, KAlSi308 aragonite, CaCO3
dolomite, CaMg(CO₃)₂
jarosite, KFe₃OH)₆(SO₄)₂
marcasite, FeS₂

Selected alteration minerals: quartz, SiO₂
pyrite, FeS₂
chlorite, Mg₃(Si₄O₁₀)(OH)₂ • Mg₃(OH)₆
adularia, KAlSi₃O₈
titanite, CaTiSiO₅
epidote, Ca-Mg-Mn (Al,Fe)₃(OH)(SiO₄)₃
mordenite, (Ca,Na₂,K₂)Al₂Si₁₀0₂₄ • 7H₂O
gismondine, Ca₂Al₄Si₄O₁₆ • 9H₂O
thomsonite, NaCa₂Al₅Si₅0₂₀ • 6H₂O
wairakite, CaAl₂Si₄O₁₂ • 2H₂O
alunite, KAl₃(OH)₆(SO₄)₂
natroalunite, NaAl₃(SO₄)₂(OH)₆
pyrophyllite, Al₂(Si₄O₁₀)(OH)₂
diaspore, HAlO₂
kaolinite, Al₄(Si₄O₁₀)(OH)₈