Hydrothermal alteration at the Mineral Hill Mine, Jardine, Montana; a lower amphibolite facies Archean lode gold deposit of probable synmetamorphic origin

University of Oregon, Department of Geological Sciences, Eugene, OR, United States

The Mineral Hill Mine is an Archean lode gold deposit in lower amphibolite facies grunerite-bearing banded iron-formation and metaturbidites. Three types of gold ore are mined: type I ore is dominant, developed in wall-rock alteration envelopes marginal to 0.01- to 1-m quartz veins; type II ore is in <5-m-wide banded quartz veins and minimal surrounding alteration; type III ore is in deformed quartz boudins. This paper reports on the mineralogical and chemical nature of type I alteration in iron-formation and on the relative timing and P-T conditions of metamorphism and alteration. The Jardine region is underlain by 3000 to 3200 Ma metasediments which have been affected by three deformational-metamorphic events. Deformation D ₁ formed large isoclinal folds and pervasive axial-planar S ₁ schistosity. Concurrent M ₁ metamorphism crystallized early grunerite, followed by zoned synkinematic garnet porphyroblasts in iron-formation, and garnet, biotite, staurolite, sillimanite,and andalusite in metaclastic rocks. Garnet growth was followed by formation of quartz-filled pressure shadows during M ₁ . M ₂ -D ₂ created upright open F ₂ folds, S ₂ crenulation cleavage, folding of pressure shadows around garnet, and replacement of biotite and garnet by chlorite. D ₂ was followed by D ₃ folding and poorly developed S ₃ cleavage with no metamorphic mineral growth. M ₁ was the single prograde metamorphic event in the region; M ₂ and D ₃ were retrograde events. Petrogenetic grids and mineral thermometry place peak M ₁ metamorphic conditions at 535 degrees to 560 degrees C and 3 kbars. Type I hydrothermal alteration has a characteristic zoning pattern: from quartz veins outward, it consists of the arsenopyrite, pyrrhotite, hornblende, and grunerite zones. The arsenopyrite zone (<2 m) is characterized by arsenopyrite, pyrrhotite, hornblende, biotite, and clinozoisite with minor chalcopyrite, muscovite, chlorite, anorthite, ilmenite, reinite (FeWO ₄ ), and visible native gold, with rare galena, sphalerite, and siderite.

This record provided courtesy of AGI/GeoRef.

This article has been cited by other articles:

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