Major and trace element chemistry of zincian spinels as an exploration guide for metamorphosed massive sulfide deposits

摘要

Gahnite occurs in and around metamorphosed massive sulfide (e.g., Broken Hill-type Pb-Zn-Ag (BHT), volcanogenic massive sulfide Cu-Zn-Pb-Au-Ag (VMS), sedimentary exhalative Pb-Zn (SEDEX)), and non-sulfide zinc (NSZ) deposits. The spatial association between gahnite and metamorphosed ore deposits has resulted in its use as an empirical exploration guide to ore, but their presence has had mixed success in discovering new occurrences of sulfide mineralization. Major element chemistry of gahnite has previously been used to define a compositional range associated with metamorphosed massive sulfides deposits, including Broken Hill-type deposits, but it fails to distinguish sulfide-rich from sulfide-poor occurrences.A regional study of analyses obtained for gahnite from twelve Broken Hill-type deposits was used to test whether or not gahnite chemistry may be used to distinguish prospective exploration targets from non-prospective occurrences in Proterozoic Broken Hill domain, New South Wales, Australia. Bivariate plots of Zn/Fe versus trace element contents (e.g., Ga, Co, Mn, Co, Ni, V, Cd) suggest gahnite from the Broken Hill deposit has a relatively restricted compositional range that overlaps with some minor Broken Hill-type occurrences. Based on the ore grade (wt. % Pb+Zn) of rocks hosting gahnite at each locality, gahnite in the highest grade mineralization from minor Broken Hill-type deposits possess compositions that plot within the field for gahnite from the Broken Hill deposit, which suggests that major and trace element chemistry (e.g., Zn/Fe = 2 to 4 versus Co = 10-110 ppm, Ga = 110-400 ppm and Mn = 500-2,250 ppm; and Co = 25-100 ppm versus Ga 125-375 ppm) may be used as an exploration guide to high-grade ore.Here the performance of random forests, a relatively new statistical technique Random forests that provides a framework for classification and decision making through a series of classification trees, which in, was also tested. Gahnite from the Broken Hill domain is classified here on the basis of the following schemes: 1. Random forest 1 (RF1): gahnite in the Broken Hill deposit versus compositions of gahnite from other minor Broken Hill-type occurrences in the Broken Hill domain; 2. Random forest 2 (RF2): gahnite in the Broken Hill deposit versus gahnite in minor Broken Hill-type deposits containing \u3e 0.25 million tonnes (Mt) of Pb-Zn-Ag mineralization versus gahnite in sulfide-free and sulfide-poor prospects containing \u3c 0.25 Mt; and 3. Random forest 3 (RF3): gahnite in sulfide-bearing quartz-gahnite lode rocks versus gahnite in sulfide-free samples. Misclassification rates, according to a ten-fold cross validation, of RF1, RF2, and RF3 are 1.6, 3.3, and 4.7% respectively.Major and trace element compositions of gahnite from BHT, NSZ, eld terranes, which can be used as an exploration guide to metamorphosed massive sulfide and non-sulfide zinc deposits. The composition of gahnite in BHT deposits is discriminated from gahnite in SEDEX and VMS deposits on the basis of plots of Mg versus V, and Co versus V. Gahnite in SEDEX deposits can be distinguished from that in VMS deposits using plots of Co versus V, Mn versus Ti, and Co versus Ti. In the Sterling Hill NSZ deposit, gahnite contains higher concentrations of Fe3+ and Cd, and lower amounts of Al, Mg, and Co than gahnite in BHT, SEDEX, and VMS deposits. Plots of Co versus Cd, and Al versus Mg distinguish gahnite in the Sterling Hill NSZ deposit from the other types of deposits.