Dawn R. Ackerman

Thesis: Sulfide Mineralogy, Tetrahedrite Zoning, and Precipitation Mechanism at the Trixie Mine, Eureka, Utah

Support: University of Utah

Abstract

Microprobe analyses of tetrahedrite from the Trixie mine, East Tintic District, Utah, a base and precious metal vein deposit, are used to characterize the chemical variation of the mineral tetrahedrite, refine ore genesis models, and relate solid solution compositions to variations in the physico-chemical conditions attending mineralization. The chemical variation in tetrahedrite, normalized to a 29 atom formula, can be expressed as

(Cu, Ag)_10+X(Fe, Zn)_2+Y(Sb, As)_4+ZS_13+A where

-0.553 < X < 1.406, -1.938 < Y < 0.95, -0.899 < Z < 0.411, and -0.804 < A < 0.596.

The average composition for the mine is

(Cu_10.191Ag_0.076Fe_0.386Zn_1.432) (As_2.930Sb_1.187)S_12.798.

Mineral-fluid equilibria are modelled by using reduced activities of the Cu end member sulfosalt phases tennantite (Tn) and enargite (En) to approximate tetrahedrite and luzonite. In an ideal activity model, (atn)tet = (XCu)12 (XAs)4 (XS)13, XAs strongly influences activity values. Reducing the activity of Tn and En has little effect on the placement of the Tn-En reaction in log fO2-pH space. This implies tetrahedrite composition is relatively insensitive to fO2 and pH when another solid solution mineral of similar composition is present because expansion of one stability field is restricted by expansion of the other stability field.

Log activity diagrams, log [(aFe2+)/(aH+)2] versus log [(aCu+)/(aH+)] contoured for log [(aH+)(aAs(OH)4-)], demonstrate that fluid composition most strongly affects mineral composition. To form the assemblages observed in the Trixie mine, with a minimum amount of variation in fluid composition, log [(aH+)(aAs(OH)4-)]=-12, log [(aFe2+)/(aH+)2] = -0.2±0.5, and log [(aCu+)/(aH+)] = -6.3±0.25. Under these conditions varying pH between 5 and 6 constrains log (aAs(OH)4-) to values of -7 and -6.

Equilibrium calculations are used to refine the proposed ore genesis model of Lippoth (1984). Major mineralogic zoning in the Trixie mine is a function of changing fO2 with time and depth. Barite and minor sulfide deposition is followed by sulfide ore deposition and appears to be genetically related to the gold ore. Oxygen fugacity is constrained over the period of mineral deposition between log values of -35 and -42 by barite stability, tetrahedrite reactions, and sphalerite compositions; pH is restricted between 5 and 6 by sericite stability (aK+= 0.015). Low fO2 and neutral pH indicates Au was precipitated from solution dominantly as bisulfide complexes. Large localized fluctuations in fO2 and fluid composition are the result of non-uniform mixing of two waters with different fO2 contents, one of which was relatively saline. This is supported by extreme variations of tetrahedrite composition in the element substitution pairs Cu-Ag, Fe-Zn, and Sb-As over distances of only tens of microns. Despite these rapid compositional variations there is a strong association of Ag- and Sb-rich tetrahedrite with the gold ore identifying the zone of mixing and maximum extent of fluid evolution in the Trixie mine.

Ackerman, D.R. and Petersen, E.U., 1987, Mineral chemistry of tennantite in the Trixie mine, East Tintic District, Utah: Geological Society of America Abstracts, 19, 7, 657.