Stockwork tungsten (scheelite)-molybdenum mineralization, Lake George, southwestern New Brunswick

Abstract

Scheelite-molybdenite stockwork mineralization constitutes one component of the Lake George polymetallic (Sb-W-Mo-Au-base metal) deposit, a complex hydrothermal center of Late Silurian (ca. 412 m.y.) age in the Fredericton trough of the northern Appalachians. The stockwork, hosted by Silurian gra~wackes, in part calcareous, is spatially and temporally related to a postkinematic cupola of biotite monzogranite, and its formation overlapped in time with the emplacement of monzogranitic porphyry dikes. Mineralogical and textural evidence indicates that contact metamorphism associated with the cupola had ceased before the initiation of W-Mo mineralization and that it occurred, at pressures of less than 1.75 kb, in two stages: a peak stage (T > 600°C), evident only in rocks of pelitic composition; and a lower temperature reequilibration (T < 500°C), recorded in rocks of both pelitic and marly compositions.The W-Mo deposit comprises three different scheelite-and/or molybdenite-bearing veinlet types. Type 1 bodies, the earliest formed, are calc-silicate (granditic garnet, wollastonite, ~l i n o p~r o x e n e , and calcic amphibole) quartz veinlets, with ubiquitous Ca and H metasomatic alteration envelopes. Mineralogical and fluid inclusion relationships indicate that the fluids ranged in temperature from 550° to 22B°C and that temperature decreased away from the cupola. The succeeding type 2 veinlets comprise quartz and lesser amounts of perthitic alkali feldspar, muscovite, calcite, scheelite, molybdenite, and pyrite. Fluid inclusion evidence shows that mineralization dominantly occurred from 400' to 175OC, under a confining pressure of 1.3 kb. Higher grade scheelite and molybdenite deposition was focused in a lower temperature zone, to the north of the cupola, in which CO, effervescence occurred. Type 3 veinlets, the last to form, consist of prehnite, mol~bdenite, and quartz and represent a volumetrically minor mineralization type.In both type 1 and 2 systems, scheelite and molybdenite deposition appears to have been controlled by decreasing temperature and increasing pH. Temperature was a function of distance from the cupola for both veinlet types, but the controls on pH were specific to each. Thus, the pH of type 1 fluids was controlled by wall-rock interaction (H metasomatism), whereas that of type 2 fluids was controlled by CO, effervescence.The economic stibnite-quartz veins (Scratch et al., 1984) occupy fractures which transect, and therefore, postdate all stages of W-Mo mineralization.

Controle polygenetique sur la localisation des gisements proximaux et distaux dans le champ polymetallique-stannifere a Xinlu Guangxi Chine meridionale

“…Many examples showing the spatial separation of proximal vs. distal rare-metal hydrothermal deposits with respect to the intrusion can be found (Chi et al, 1991), e.g. the Xinlu (this study) and Dachang (Zheng, 1987) tin-polymetallic ore fields in southern China, tin-polymetallic deposits in Cornwall (Charoy, 1979), Pb-Zn-Ag-Sn deposits in ZeehanRenison Bell area of west Tasmania (Laznicka, 1985), Lake George W-Mo deposits in southern New Brunswick (Seal et al, 1987), Panasqueira W-Cu (Ag)-Sn deposits in Portugal (Polya, 1989), etc. In all these examples, both proximal and distal deposits exist, and the localization of the ore bodies does not seem to be simply the effect of metal zonation, because the mineralization of the same metal (e.g.…”

“…George Sb-W-Mo deposit (after Seal et al, 1987); f~Schematic section of the Sn-Pb-Zn-Ag mineralization district in the Zeehan-Renison Bell area (after Laznicka, 1985). For explanations see the text 163 from Raznjevic (1975); (b)H2O-NaCl (30 wt%), P-T data from Sourirajan and Kennedy (1962), Haas (1976) and ; (c) H2O-KCI (20 wt%), P-T data from ;…”

“…Geological sections showing this relationship can be found in the Dachang (Lu et al, 1979 andZheng, 1987), and Xianghualing (Huang et al, 1985) tinpolymetallic ore fields and the Xihuashan-Dangping-Piaotang tungsten mineralization district (Tanelli, 1982) in southern China, the tin-polymetallic deposits in Cornwall (Charoy, 1979), the Pb-Zn-Ag-Sn deposits in Zeehan-Renison Bell area of west Tasmania (Laznicka, 1985), the Lake George W-Mo mineralization in southern New Brunswick (Seal et al, 1987), the Panasqueira W-Cu (Ag)-Sn deposits in Portugal (Polya, 1989), the Kara Wskarn deposit of Tasmania (Kwak, 1987), the Ban Ban Zn-Fe sulfide-bearing skarn in southeastern Queensland ( Ashley and Plimer, 1989), the Deputatskoye tin-polymetallic deposit and the Kazakhstan tungsten deposit (Rundqvist, 1982), the Camsell River Ag-Ni, Co Arsenide mineralization in N.W.T. Canada (Badham, 1975) and the Logtung W-Mo deposit, Yukon Territory, Canada (Kirkham and Sinclair, 1988).…”

“…In the Cornwall tin-polymetallic mineralization district, there is some evidence for minor effervescence of a CO2-H2O vapor although oxygen isotope studies suggest a significant non-magmatic component in the ore-forming fluid (Spooner, 1981). In the Lake George polymetallic (Sn-W-Mo-Au-base metal) hydrothermal deposits, fluid inclusions are studied in the W-Mo veinlets (Seal et al, 1987(Seal et al, , 1988. Two distinctive assemblages of inclusions, defined by the variability of brine/CO2 ratios, were recognized.…”

“…Those occurring within or adjacent to the intrusion are typically three-phase (brine+liquid CC>2+vapor) and exhibit fairly constant brine/CO2 ratios from inclusion to inclusion within a given sample, thus suggesting the presence of a one-phase fluid at the time of entrapment, while those away from the intrusion in the country rocks exhibit highly variable brine/CC>2 ratios, strongly suggesting fluid immiscibility at the time of entrapment (Seal et al, 1987(Seal et al, , 1988. It is shown that the W grade is higher in the sites where fluid immiscibility occurs than in the sites where there is no fluid immiscibility (Seal et al, 1987). According to Patterson et al (1981), no phase separation evidence has been found in the Renison Bell PbZn-Ag-Sn mineralization area of west Tasmania.…”