Abstract

Hydrothermal alteration at the Babbitt Cu-Ni deposit of the Duluth Complex is widespread but highly variable in terms of intensity. Alteration styles range from the filling of microfractures in plagioclase and olivine or minor alteration along grain margins to more intense alteration in the form of macroscopic fracture fillings or massive replacement of primary minerals with or without well-developed fractures. Chlorite occurs as a macroscopic fracture filling, within microveinlets in plagioclase, and as an alteration of pyroxene, biotite, and rarely olivine along grain margins or fractures. Mats of chlorite are intergrown with amphibole, epidote, calcite, and prehnite as part of massive replacement of calcic plagioclase. Greenalite is found in areas of intense alteration, particularly in the vicinity of sulfide mineralization. Iron-rich serpentine occurs along fractures in olivine or as a more massive replacement of olivine in picritic units. Serpentine composition is related to that of the parent olivine, as well as to the degree of serpentinization. In addition to its involvement in plagioclase alteration, calcic amphibole (hornblende-actinolite) is found as a partial replacement of primocryst and interstitial clinopyroxene. Ca-poor amphibole occurs as an alteration product of orthopyroxene. The presence of the assemblage chlorite + prehnite + actinolite + epidote suggests that alteration continued to temperatures less than 400 degrees C.The delta D values and H 2 O contents of igneous rocks at Babbitt range from -56 to -138 per mil, and 0.49 and 4.60 wt percent, respectively. Biotite separates are characterized by delta D values of-67 to -86 per mil and delta 18 O values of 4.2 to 6.1 per mil, a range considered normal for biotite in mafic igneous rocks. Because most of the low-temperature reactions between fluids and minerals in the cooling igneous system involved hydration, H 2 O content is an approximate indicator of the extent of low-temperature alteration. Rocks with H 2 O contents in excess of approximately 1.5 percent are characterized by delta D values greater than -87 per mil, except for a sample of highly serpentinized picrite which has a delta D value of -120 per mil. Modeling of the isotopic composition of a fluid exsolved from interstitial melt indicates that for samples with minor alteration and H 2 O contents less than approximately 1 wt percent, secondary mineral assemblages were probably produced as a D-depleted magmatic vapor reacted with primary minerals along cooling fractures and grain margins. Areas of more intense alteration may have formed from localized accumulations of D-enriched vapor released from underlying interstitial melt. Alternatively, intensely altered zones may have formed due to the introduction of relatively D-enriched fluid derived from devolatilization of metasedimentary xenoliths and footwall. The latter interpretation is consistent with previously reported isotopic (Ripley and Taib, 1989) and fluid inclusion (Pasteris, 1989) data. Both oxygen and hydrogen isotope compositions of one serpentinized picrite suggest that a low 18 O, low D fluid of probable meteoric origin has been involved in the serpentinization of olivine-rich units.An insufficient number of samples have been analyzed to determine if a direct positive correlation exists between platinum-group element (PGE) content and degree of alteration in the Babbitt deposit. However, the presence of discrete platinum-group minerals in intensely altered zones suggests that most Pt and Pd remobilization occurred via transport by a D-enriched fluid. A positive correlation between whole-rock delta D values and Pd content for samples that contain less than 300 ppb Pd suggests that magmatic fluids as well as high delta D fluids derived from metasedimentary country rocks may have been involved in local Pt-Pd redistribution.

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