Abstract

The 1.5 X 3.25-km Mink Lake intrusion located approximately 110 km northeast of Red Lake, northwestern Ontario in the Uchi-Confederation Lakes greenstone belt is an unmetamorphosed and postductile deformation Archean stock. The bulk of the intrusion consists of an equigranular biotite-hornblende granodiorite (68.1-70.5 wt % SiO 2 ; n = 6) with a relatively high Na 2 O content (4.59-4.87 wt %) and a relatively low K 2 O content (2.15-2.95 wt %). The granodiorite component was preceded by a less fractionated leucocratic hornblende-biotite quartz monzodiorite pod ( approximately 200 X 400 m) with trondhjemitic affinities (64.4-65.3 wt % SiO 2 ; n = 3). Microgranodiorite dikes and high SiO 2 (74.5-76.1 wt %; n = 6) aplites mainly of leucomonzogranitic and alaskitic compositions occur as late differentiates. Major and trace element (e.g., Rb-Sr) data indicate that the suite constitutes a coherent crystal fractionation sequence.The most important MoS 2 mineralization (grab samples with approximately 0.1-0.3% Mo) occurs in a approximately 1,000 X 350-m zone completely contained within the south end of the intrusion and spans a silicate melt-hydrothermal fluid transition showing the following sequence: (1) shallow-dipping aplites ( approximately 3 cm-1 m) which contain flecks and occasional rosettes (< or = approximately 3 mm) of MoS 2 and quartz segregations and which have carbonate and phyllic alteration selvages indicating fluid-rock interaction, (2) approximately 0.5- to 1.5-m-thick tabular, shallow-dipping, locally MoS 2 -rich carbonate alteration zones ( approximately 5-15 vol % calcite; albite-muscovite-chlorite-pyrite-rutile) with no internal quartz veins, (3) a spectrum of similarly shallow-dipping, approximately 0.3- to 5-cm-wide MoS 2 -bearing quartz + or - K-feldspar-pyrite-calcite-chlorite veins with marginal carbonate and phyllic alteration, and (4) late, steeply dipping barrenquartz veins, joints, faults, and shear zones, locally with strong green phyllic alteration. The mineralization developed sequentially upward in the order aplites through carbonate alteration zones to MoS 2 -quartz veins in a relatively thin ( approximately 100 m), marginal roof zone. The carbonate alteration is uncommon for a granitic environment but is similar to that related to Archean gold-quartz-carbonate-pyrite vein systems. The MoS 2 mineralization is enriched in Au to subore-grade values as high as 0.57 g/metric ton with an average of 0.14 g/metric ton (n = 9), compared with unaltered granodiorite, which contains <1 ppb Au (n = 4), and is also enriched in W (55 + or - 25 ppm (1Sigma ) cf. 4 + or - 2 ppm).Spatially distributed Rb, TiO 2 , Zr, and Li data (n = 47) show that the main MoS 2 -mineralized zone is located in moderately differentiated material immediately adjacent to the principal internal zone of crystal fractionation, and in part, laterally continuous with it.The MoS 2 mineralization was deposited by H 2 O-CO 2 fluids undergoing phase separation at approximately 280 degrees to 300 degrees C (n = 99); the fluids are characterized by a relatively pure CO 2 component (CO 2 melting point = -56.4 degrees + or - 0.8 degrees C; n = 57; cf. pure CO 2 at -56.6 degrees C) and a relatively low-salinity aqueous component (5.6 + or - 1.2 equiv wt % NaCl; n = 48). The fluid delta D-delta 18 O box defined by the bulk of the fluid delta D and delta 18 O estimates (10 delta D values direct from fluid inclusion analysis; 19 delta 18 O values calculated from 6 quartz and 13 carbonate determinations) shows a significant overlap ( approximately 60%) with the general magmatic water box. Carbonate delta 13 C values of -3.3 + or - 0.4 per mil (n = 13) are compatible with a deep igneous carbon source but, according to presently available data, are incompatible with derivation by greenschist-amphibolite or granulite facies metamorphic processes. The delta 34 S values are uninformatively typical of much Archean sulfur (+1.6 + or - 0.9ppm; n = 8).Considering the above and additional evidence, it is concluded that the Mink Lake MoS 2 fluids were magmatically derived from the host sodic granodiorite stock. Hence, a high CO 2 content in Archean hydrothermal fluids is not necessarily an indication of a "metamorphogenic" origin, as has been suggested for Archean gold-quartz-Fe-carbonate-pyrite + or - tourmaline + or - tellurides + or - W (scheelite) + or - Mo vein systems (e.g., Kerrich and Fyfe, 1981; Fyfe and Kerrich, 1984; Groves et al., 1984). In fact, the close similarities in fluid properties between Mink Lake MoS 2 and Archean gold-quartz-carbonate-pyrite vein mineralization, and particularly, the statistically identical carbonate delta 13 C values (Burrows et al., 1986 a and b) suggest that Archean gold-quartz-carbonate-pyrite vein deposits may have been magmatically derived.The viability of a magmatic origin is demonstrated by the enrichment in Au, Mo, and W of the Mink Lake mineralization and can, therefore, explain the Au, Mo, W, and carbonate association in Archcan gold-quartz-carbonate-pyrite vein deposits. Enrichments in Au, Mo, and W in the Mink Lake aplites (3 + or - 2 ppb, 23 + or - 35 ppm, 7 + or - 6 ppm, respectively) show that magmatic fractionation processes increased Au, Mo, and W to sufficiently high levels to generate Au- and W-enriched MoS 2 mineralization, when partitioned into hydrothermal fluids, from a parental granodiorite in which the Au, Mo, and W concentrations were low, and apparently normal (<1 ppb, < or = 1 ppm, 4 + or - 2 ppm, respectively).

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