A stock of trondhjemite intrudes an Archean succession metamorphosed to the greenschist facies at Gutcher Lake, 30 km north of Wawa in Ontario. The stock is 4 km2 in plan and is partly enveloped by an aureole of epidote-amphibole hornfels up to 1 km wide. Within this aureole chlorite partly replaces biotite; chlorite, calcite, and quartz mantle hornblende and epidote; zoned amphibole has a rim of actinolite; and albite varies from 0 to 8% An. The stock has chlorite pseudomorphous after biotite, and feldspar is mottled by white mica and has a clear rim of albite. Fractures filled with quartz, calcite, ankerite, white mica, chlorite, pyrite, and native gold cross-cut the stock and its aureole. Wall rocks to these veins were modified by hydrothermal alteration with addition of Si, Fe, K, H 2 0 + C02, S, and Rb, leaching of Na, and a shift in Fez+ /Fe, from -0.66 to -0.90. Primary inclusions in the vein quartz have a solution with a C 0 2 gas bubble that homogenizes into the liquid at around 300°C.Initial contact metamorphism of volcanic rocks at T = 450-550°C and P < 200MPa (2kbar) formed an aureole of epidote-hornblende hornfels near the stock. Subsequent regional metamorphism during the Archean at T = 325-450°C and P = 200-300 MPa (2-3 kbar) retrograded the stock and its contact aureole to a lower greenschist assemblage. The retrogression involved hydration and C 0 2 fixation in hornfels and trondhjemite by a hot reducing fluid of low salinity. This metamorphic fluid precipitated native gold with quartz and pyrite along fractures in response to cooling and chemical reaction with wall rocks. Un massif intrusif de trondhjemite de 4 km2 pCnktre une succession de roches mktamorphiques ?i faciks de schistes verts au lac Gutcher, ?i 30 km au nord de Wawa en Ontario. Ce massif intrusif est partiellement enveloppk par une aurkole de cornCennes ?i Cpidote-amphibole pouvant atteindre une largeur de 1 km. Dans I'aurCole la chlorite, la calcite et le quartz enrobent la hornblende et l'kpidote; I'amphibole zonke posskde une frange d'actinote; la composition de l'albite varie de 0 ?i 8% An. Le massif intrusif contient des pseudomorphes de chlorite aprks biotite, et le feldspath est tachetk par un mica blanc et montre une frange claire d'albite. Des fentes colmatCes par du quartz, de la calcite, de I'ankkrite, du mica blanc, de la chlorite de la pyrite et de I'or natif traversent le massif intrusif et son aurCole. L'altCration hydrothermale a modifiC les parois de ces veines par un apport de Si, Fe, K, H20 + C02, S et Rb, une lixiviation du Na, et un accroissement du rapport Fe2+/Fe, de -0,66 a -0,90. Les inclusions primaires dans les veines de quartz renferment une solution accompagnCe d'une bulle de C02, ce gaz se dissout dans le liquide h environ 300°C. Initialement, le mCtamorphisme de contact de T = 450-550°C et de P < 200 MPa (2 kbar) a engendrC une aurCole de , cornhennes a tpidote-hornblende prks du massif intrusif. Un Cpisode subskquent de mCtamorphisme rCgional dans 1'ArchCen de T = 325-450°C et de P = 200-300MPa (2-3 ...

Summary. Mercury deposits in western California are near a thrust fault that separates two groups of Mesozoic rocks. The Franciscan Assemblage, a metamorphosed melange with serpentine and graywacke, is structurally overlain by the Great Valley Sequence, a sedimentary series resting on oceanic crust. These Mesozoic rocks are partly covered by volcanic and sedimentary rocks of Cenozoic age. Cinnabar with silica minerals, dolomite, native mercury, and bituminous matter occurs around the fractured margins of serpentine bodies and around hot springs that emanate from the Franciscan Assemblage. Fluid inclusions and hot springs suggest that cinnabar precipitated from CO2-H20 fluids with < 2 wt% chlorine at T < 250 °C. Prograde metamorphism of Mesozoic sediments expulsed mercury-bearing fluids that migrated up serpentine-related fractures and exhaled onto the surface.

ZusammenfassungDie chemischen Ablagerungen im Roten Meer, Salton Sea (Californien) und auf der Cheleken Halbinsel (UDSSR) haben Anreicherungen von Cu, Zn, Pb, Ag und etwas Au. Sole, mit der die Ag-Pb-Zn-Cu L6sungen kommen, hat fiber 10% C1 und einen l~berschufl an chalcophilen Metallen. Kieselsinter, die reich an Au, Ag, Hg und Sb sind, erscheinen an den Austrittsstellen der heigen L6sungen in Californien, West-Nevada und Neu-Seeland. Heit~e CO2-H=O-Lfsungen, die mit diesem Sb-Hg-Ag-Au-System ausstrfmen, enthalten weniger als 2 % CI und anstelle yon chalcophilen Metallen kommen Sulfide vor. In verschiedenen geothermalen Provinzen gibt es Mischsysteme zwischen diesen Endgliedern. Zn-Cu und Ag-Au-Konzentrationen treten am Rande des archaischen Komplexes im Wawa Grundschiefergiirtel in Ontario, Canada auf. Dies wird interpretiert als ein zunSchst yore Meerwasser dominiertes System, in dem Exhalationen reich an unedlen Metallen eindrangen. W~ihrend einer folgenden Regionalmetamorphose erfolgt die Konzentration von Eddmetallen aus L6sungen mit geringer Salinit~it. AbstractChemical deposits in the Red Sea, Salton Sea, and Cheleken Peninsula have concentrations ofCu, Zn, Pb, Ag, and some Au. Brines emanating from these base metal systems have over 10% C1 and have chalcophi!e metals in excess of sulphide in solution. Siliceous sinters laden with Au, Ag, Hg, and Sb exist around vents and wells in California, western Nevada, and New Zealand. Hot CO2-H20 fluids that exhale from these noble metal systems have less than 2 % C1 and sulphide in excess of chalcophile metals. Some geothermal systems around the world have features intermediate between these two end-members. Cu-Fe and Ag-Au concentrations occur around the margin of an Archean felsic stock in the Wawa greenstone belt in Ontario, Canada. The interpretation is that an initial seawater dominated system concentrated exhalites rich in base metals. This culminated with regional metamorphism and the concentration of precious metal-bearing veins from low salinity fluids.

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Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite IS partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite +white mica +calcite +chlorite near fracture zones traversing the stocks.The alteration of the Gutcher Lake stock along its foliated margin involved addition of K,O, H,O + CO,, MnO, plusRb; loss ofCaO plus Sr; and a shift in Fe' '/Fe, from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H,O + CO,; loss of Sr; and no significant shift in Fe+2/Fe,. The greenschist alteration also modified the contact aureoles bordering both stocks.One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.