Description: The hydrothermal system architecture related to the formation of the contemporaneous Au-bearing Horne and Quemont volcanogenic massive sulfide (VMS) deposits was visualized by employing kriging methods to map whole-rock oxygen isotope compositions, zones of silica addition and loss, and water contents in two- and three-dimension. Zones of alteration were mapped in three-dimensions in the vicinity of the steeply dipping Horne deposit, to depths of as much as 2 km. In all, nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole-rock analyses. Contents of SiO 2 , H 2 O, MgO, Al 2 O 3 , and S from chemical analyses of nearly 5,000 samples within the two- and three-dimensional study regions were used separately, and in combination with the oxygen isotope data, for modeling and hydrothermal mapping purposes. The Horne and Quemont deposits formed within a similar time frame, but in different magmatic-hydrothermal systems, distinguished by their mapped hydrothermal architecture. The Quemont deposit appears to be centered on the Powell pluton, which intruded late into an apparent volcanic-filled, rift-graben structure. Although structural complexities are apparent, we infer mineralizing high-temperature upflow in the footwall of the Quemont deposit to have emanated from a reaction zone above the Powell pluton (and its precursors), beneath a zone of extensive silicification. Faulting on the Andesite fault and Horne Creek faults, plus erosion, has removed evidence of the upflow zone in the hydrothermal system of the Horne deposit. Areas of silicification correspond, in general, with isotopic evidence of lower temperature alteration. Such alteration east of the Quemont deposit signaled the waning of hydrothermal activity. The suggested cooling, for the most part, promoted the precipitation of silica. In the case of the Horne deposit, mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks, in an apparently relatively stratigraphically stable and long-lived hydrothermal system, evidently led to marked footwall silicification. The silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the Horne deposit. Continued intrusion and some post-VMS hydrothermal activity is recorded in the hanging-wall section to the Horne deposit. Our data suggest that deposition of the 10 million ounces (Moz) of Au within the Horne deposit was syngenetic, and not the product of subsequent hydrothermal activity.

Description: The oxygen isotope composition of 169 whole-rock samples from the world-class LaRonde Penna Au-rich volcanogenic massive sulfide deposit range from 6.5 to 22.0. Fractional crystallization from basalt to rhyolite accounts for a variation of approximately 2 in primary whole-rock 18 O values. The remaining variance in whole-rock 18 O values found at LaRonde Penna must therefore be ascribed to water-rock exchange with hydrothermal fluids. The metamorphosed, hydrothermally altered host rocks have been subdivided according to mineralogical assemblages, which display weak covariance with 18 O values and whole-rock geochemical alteration indices. Water-rock equilibrium exchange models indicate that the high 18 O values that characterize the LaRonde Penna deposit are compatible with low-temperature (~150ºC) hydrothermal alteration at high water-rock oxygen atomic ratios (1–50). Zones of lower 18 O values stratigraphically beneath the principal ore lens (20 North) indicate local zones of higher temperature hydrothermal alteration. The oxygen isotope composition of the hydrothermal fluid is estimated to have been ~5, as a consequence of mixing between seawater ( 18 O ~0) and a component of magmatic water degassed from the volcanic and intrusive rocks associated with the LaRonde Penna deposit.

Description: Juvenile 1.89 Ga oceanic arc volcanic rocks of the Flin Flon volcanic belt at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite, and/or kyanite. They were produced from altered rocks created during premetamorphic, 1.89 Ga, synvolcanic, hydrothermal fluid-rock interaction. Three separate episodes of hydrothermal alteration are recognized that span evolution of the host volcanic rocks from a primitive (nascent arc?) to mature arc geotectonic setting. The geologic, geochemical, mineralogical, and isotopic attributes of the zones indicate that they include volcanogenic massive sulfide (VMS)-related alteration, and they were produced at high and low temperatures and formed in seafloor/near seafloor and subseafloor (intrastratal) environments. Large-scale alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration "footprint," which is 36 times greater in areal extent compared to associated "pipe-like" alteration zones, means that such zones provide a useful target to "vector-in" exploration to VMS depositional settings within volcanic belts. The VMS-related large-scale alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike toward VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are crossed by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high-temperature(〉350°C) hydrothermal fluids. Cu-Zn–rich VMS deposits at Snow Lake formed in flow-dominated sequences, are hosted by large rhyolite flow complexes, and comprise lensoid orebodies. In contrast, Zn-Cu–rich VMS deposits, although also rhyolite associated, formed in volcaniclastic-dominated sequences, are spatially related to a district aquifer, and comprise stratiform, laterally continuous orebodies. This suggests that exploration for Cu-Zn VMS deposits could selectively target flow-dominated sequences and focus on rhyolite flow complexes that display significant alteration. Similarly, exploration for Zn-Cu VMS deposits could focus on volcaniclastic-dominated sequences in which a potential aquifer has been identified.

Description: The Paleoproterozoic Flin Flon mining district is one of the world’s most prolific volcanogenic massive sulfide (VMS) camps and includes a single stratigraphic interval that hosts the 85.5 million tonne (Mt) Flin Flon, 777, and Callinan Zn-Cu-(Au) deposits. Rapid seafloor burial of the VMS hydrothermal system by a thick succession of pillowed basalt resulted in the hanging-wall strata being affected to varying degrees by the still upward migrating fluids. This hanging-wall alteration hydrothermal fingerprint allows delineation of the regionally metamorphosed paleohydrothermal system, and its characterization has the potential to lead to discovery of buried, stacked, or structurally displaced mineralization. Evidence for the presence of continued seafloor hydrothermal activity above the Flin Flon-Callinan VMS horizon is observed in the pillowed flows, interlayered hyaloclastite-rich flow tops, and also within finely bedded interflow volcaniclastic sediment. A 30% to 60% metalliferous exhalative component was detected through geochemical and mineral analysis in interpillow volcaniclastic rocks, chert, and epidosite in the hanging-wall sequence. The regional distribution of Fe- and Mg-rich chlorite, epidote-clinozoisite, biotite-annite, actinolite-hornblende-ferrotschermakite, and stilpnomelane and albite-oligoclase modifies metamorphic isograds and defines discrete vertical fluid pathways controlled by synvolcanic growth faults and associated sill-dike swarms. Silica-enriched hanging-wall alteration zones are proximal to Fe-Ti basalt sills and occur as discrete hanging-wall zones parallel to the plunge of the 62 Mt Flin Flon deposit. Anomalous concentrations of Hg, Sb, Ag, Pb, Te, As, Au, and Bi form within these hanging-wall halo alteration zones, indicating migration of the more volatile metals present in the underlying VMS deposits. Synvolcanic depressions, dike swarms, and hydrothermal-metamorphic fluid corridors are detectable through trace element anomalies, trace mineral chemistry, and 18 O isotope geochemistry. Oxygen isotope analysis of the Flin Flon-777-Callinan VMS hanging-wall strata defines a number of high O 18 anomalies extending 1,200 m above that indicate that 〈300°C subseafloor hydrothermal activity continued after burial of the massive sulfide deposits. Coupled with the geochemical and mineral chemical anomalies, this is indicative of the presence of continued, relatively low temperature hydrothermal fluid "leakage" from a robust seafloor hydrothermal event that generated the VMS deposits. A combination of techniques, including mineral chemistry, isotope, and trace element data, is demonstrated to be successful in identifying and delineating zones of hanging-wall hydrothermal alteration in greenschist- to amphibolite-grade metamorphic rocks of the Flin Flon mining camp. Use of these, coupled with mapping to define periods of quiescence as marked by horizons of sedimentary rocks in the hanging-wall basalts of the Hidden Formation, has the potential to lead to discovery of deeply buried deposits on the Flin Flon horizon or deposits at higher stratigraphic levels. Our findings indicate that the basaltic hanging wall on the Flin Flon-777-Callinan hydrothermal system was an efficient cap on the system, with vestiges of continued hydrothermal fluid flow detected in the interpillow and interflow components. These volumetrically minor components are critical sampling media and are pertinent to global exploration for detection of VMS mineralization buried beneath thick mafic volcanic sequences.