GEOCHEMISTRY OF THE CAMBRIAN VOLCANIC-HOSTED MASSIVE SULFIDE-RICH MOUNT READ VOLCANICS, TASMANIA, AND SOME TECTONIC IMPLICATIONS

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies. Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb)N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics. Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P2O5 and light REE enriched ((La/Yb)N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities. Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO2 (0.4-0.5%), low P2O5 (< 0.1%) basalts with (La/Yb)N values from 8 to 12 and almost flat heavy REE patterns, to low TiO2 (0.4-0.8%) but strongly to exceptionally P2O5- and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb)N values up to 34. The latter are regarded as shoshonites. Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO2 contents from 1.0 to 1.6 percent, very low Nb contents (< 3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb)N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time. Suite V is constituted by the Miners Ridge basalts and includes low TiO2 (< 0.7%) basalts with strong light REE depletion ((La/Yb)N < 0.5) and higher TiO2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb)N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement. The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province). The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite IH basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.