Silicification and base metal mineralisation within the Earaheedy basin, Western Australia
Creators: name: Teen, MT
Publisher: name: University Of Tasmania
Year: 2023

description: Mississippi Valley Type (MVT) base metal mineralisation has been recognised at several localities in the Paleoproterozoic. Earaheedy Basin, Western Australia, where it is associated with areas of silicification. The Earaheedy Group (ca. 1800-1900 Ma) lies within the Capricorn Orogen, between the Yilgarn and Pilbara cratons, and comprises two subgroups, the Tooloo and the Miningarra. At the base of the Tooloo Subgroup, the Yelma formation unconformably overlies Archean basement and comprises a ~300m thick package dominated by two units of shallow marine shelf carbonates, the Iroquois and Navajoh dolomites. Both dolomite units are intensely silicified. Base metal mineralisation is primarily contained within the Navajoh dolomite is strongly associated with silicification. 1 Okm to the southeast of the study area, a major comet impact structure (the Teague Ring) formed at -1630 Ma. Numerous small occurrences of galena and sphalerite have been noted along strike, with five prospects over a strike length of ~50km investigated in detail during the current study. Mineralisation in the Navajoh dolorrlite occurs within styolites, bedding parallel and discordant veins, as a matrix within intraformational breccias, and in vugs and microbialite cores. Typical grades for base metal mineralisation are 20m @ 0.88% Zn and 0.1% Pb, including 2m zones of up to 1.6% Zn. Three paragenetic stages have been identified. Stage I encompasses diagenesis of the host dolomites, and comprises three dolomite generations and the formation of diagenetic pyrite (Py1). Py1 has ()34s values between 5.7 and 26.5%o, consistent with the occurrence of a sulphate-minimum zone within the Proterozoic oceans. Stage I barite records a seawater ()34s sulphate value of 15.4%o. Stage II was the main stage of ore deposition. Microcrystalline quartz, chalcedony, spherulitic chalcedony and quartz are intimately associated with sulphide mineralisation (mainly pyrite, sphalerite, galena and chalcopyrite with minor bornite and tetrahedrite) in stage II bedding sub-parallel veins within the Navajoh dolomite. Stage II fluids had moderate homogenisation temperatures (97 to l89T), high salinities (21.8 to 23.9 eq. wt % NaCI), and ol80(fluid) values between 0.9 and 2.2%o, consistent with derivation from a connate brine. Wide variations in o34s values from stage II sulphides ( -6.1 to 30.5%o) reflect a complex sulphur source consistent with a connate brine leaching a varied sedimentary package. Zinc ratio calculations suggest that minor variations in regional fluid chemistries and depositional processes resulted in varying levels of Pb and Zn saturation across the study area. Terrane-specific lead modelling techniques have constrained the age of stage II mineralisation to ~ 1780 to 1740 Ma. Preferential replacement of dolomite by silica along permeable layers caused an increase in fluid pH and resulted in sulphide precipitation with quartz in the center of the veins. Strong similarities in paragenetic sequences at each prospect through the study area indicate that stage II fluids were of regional scope, rather than discrete hydrothermal systems. Regional fluid flow is interpreted to have been driven by southwards-directed compression within the Capricorn Orogen on the north side of the basin. Stage III consists of bedding-discordant quartz/carbonate veins with minor base metal mineralisation (pyrite, sphalerite, galena). Stage III fluids produced a second stage of silicification, and had higher homogenisation temperatures (108 to 246.C), but lower salinities (3.3 to 16.5 eq.wt% NaCl) than stage II. Calculated stage III o180(fluid) values obtained from quartz ranged from -9.9 to+ 13.3%o and were probably due to fluid mixing, o 18o buffering, and temperature variations. Most of the calculated stage III o 180(fluid) values were between -1.3 and 5 .5%o, consistent with a connate brine source similar to stage II fluids. Carbon-oxygen isotopic analyses of dolomite (ol3c(fluid) -4.9%o, o180(fluid) +2.0%o) concur with a connate brine source, and show that extensive fluid-rock interaction of stage III fluids and host dolomite occurred. Regional decreasing temperature gradients from the southeast to the northwest, and isotopic anomalies suggest that stage III mineralisation developed from connate brines that were redirected by comet impact at the Teague Ring. These fluids had the potential to remobilise stage II mineralisation.