Abstract

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.

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