Date of Award
Doctor of Philosophy
Geological and Atmospheric Sciences
Paul G. Spry
Garnet-rich rocks are spatially associated with the giant Paleoproterozoic Broken Hill (BH) Pb-Zn-Ag deposit and smaller Broken Hill-type (BHT) deposits. However, they also occur throughout the southern Curnamona Province (SCP), Australia, unrelated to sulfides. Mn-bearing garnet-rich rocks at BH, Cathedral Rock, Meningie Well, Polygonum, Thunderdome, Mutooroo, Iron Blow, Doughboy, and Weekeroo are conformable to bedding in surrounding metasedimentary rocks, and represent impure chemical precipitates formed on the floor of a rifted basin. Mn was derived from spatially related mafic lavas. Y/Ho and Zr/Hf ratios indicate that most garnet-rich rocks formed from Cl-bearing hydrothermal fluids. Mn-rich and Mn-Ca-rich, Fe-poor garnets and host garnetite and garnet-pyroxenoid rocks at BH show positive Eu anomalies (Eu/Eu* > 1), whereas Mn-poor, Fe-rich garnets and host quartz garnetite and garnet-gahnite rocks from BH, and quartz garnetite and garnet-amphibole rocks from other sites in the SCP exhibit negative Eu anomalies (Eu/Eu*< 1). Calculations of KD grt/rock REE and interelement correlations between REEs and major element contents in garnet indicate that the LREEs are crystallographically controlled in garnet whereas the LREEs are not. Eu/Eu* > 1 reflect high amounts of Eu2+ that was preferentially incorporated into/onto original Mn-, and Mn-Ca-rich oxides/carbonates. In contrast, Eu/Eu* < 1 indicates the preferential discrimination against Eu2+ by Fe-rich, Mn-poor precursor minerals. Precursors to Mn-poor, Fe-rich garnet- and gahnite-rich rocks in B and C lodes at BH and elsewhere in the SCP that exhibit Eu/Eu* < 1 and high Fe/Ti ratios formed between 250 and 300°C from hydrothermal fluids at low fO2 conditions. The precursors to Mn and Ca-Mn garnetite and pyroxenoid-bearing rocks from 2 and 3 lenses and A lode at BH that show low Fe/Ti ratios and Eu/Eu* > 1 were derived from high T (T > 250°C) hydrothermal fluids. These fluids precipitated Mn at higher fO2 conditions than those that formed Mn-poor rocks. The sign of the Eu anomaly is dependent on several physical and chemical factors, the most important of which are T, fO2, and the crystallographic control the precursor Fe-Mn minerals had on the relative distribution of Eu 2+ and Eu3+. Enrichments in Mn, K, Rb, S, Ga, Cs, Eu, Sn, U, W, Cu, Pb, Zn, As, Cd, Sb, Ag, and Au, and Eu/Eu* > 1 in garnet-rich rocks can be used as exploration guides in the search for BHT mineralization. Garnet from the BH deposit is enriched in Zn (>400 ppm), Cr (>140 ppm), and Eu (>6 ppm), and depleted in Co, Ti, and Y compared to garnet in other garnet-rich rocks in the SCP. These values, as well as Mn contents >15 wt.% MnO and Eu/Eu* > 1, are also considered as guides to BHT mineralization.;Low-grade Zn-Pb-Ag mineralization occurs at the Polygonum, Thunderdome, Hunters Dam, and Benagerie Ridge prospects as fine-grained laminated pyrite and sphalerite within pyritic carbonaceous/graphitic metapelites, calcareous shales/silts, Mn-bearing sideritic carbonates, calc-silicates, carbonate lenses, and feldspar-carbonate layers [Bimba Formation, Strathearn Group (OD) and Ettlewood Calc-Silicate Member (BHD)]. This sulfide mineralization is syngenetic and resembles sulfide mineralization at the HYC deposit, Queensland. It formed with chemical/evaporitic precipitates in a shallow non-marine/lacustrine environment. delta 13CPDB and delta18OPDB values of calcite are between -10.2 and -0.8‰ and -19.2 to -16.8‰, respectively. Differences in delta13C values in carbonates from the same area suggest that the lower delta13C values represent higher amounts of original organic C or that equilibrium homogenization during metamorphism was not attained. The delta13CPDB values are heavier than delta13C values (~ -26‰ to -21‰ PDB) of carbonates in ore from the BH deposit. These light delta13C values resulted from decarbonation and the high T effects of metamorphism. Based on the presence of graphite in each study site, there appears to be a genetic link between biogenic C, base metal mineralization, and carbonate deposition. At the Polygonum and Thunderdome prospects, stratabound Pb-Zn mineralization occurs within garnet- and gahnite-bearing metapsammopelitic phyllites, laminated andalusite/chiastolite phyllites, laminated garnetite, quartz garnetite, banded Mn-bearing iron formation, and amphibolite [lower Saltbush Subgroup, Strathearn Group (OD) and upper Broken Hill Group (BHD)]. Whole rock values of >4 ppm Tl, >25 ppm Cd, >17 ppm Se in units 2 and 4, a Zn/(Zn+Fe) ratio of >0.6 for zincian spinel, a (Mn+Fe+Mg)/(Mn+Fe+Mg+Ca) ratio >0.9 for carbonates, a Mn/(Mn+Fe+Mg) ratio of >0.6 for garnet in garnet-quartz rocks, and a (Mn+Fe)/(Mn+Ca+Fe) ratio of >0.3 for garnet in calc-silicate rocks are geochemical indicators of stratabound Pb-Zn mineralization in the northern part of the SCP.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Adriana Heimann Ríos
Heimann Ríos, Adriana, "Geochemical keys for the genesis of Proterozoic garnet-rich rocks and minor metasediment-hosted Pb-Zn-Ag mineralization, southern Curnamona Province, Australia" (2006). Retrospective Theses and Dissertations. 1901.