"Rock of the Month # 293, posted for November 2025" ---

The Cannington deposit is in the Cloncurry district of northwest Queensland. It was a major discovery in 1990, drilled off to yield metric reserves of 47 MT grading 10.7% Pb, 4.6% Zn and 470 ppm Ag (Chapman, 1994). Cannington is in the Eastern Fold Belt of the Mount Isa inlier, and is hosted by paragneisses. These host rocks are altered sillimanite-grade gneisses of Proterozoic age, circa 1670 Ma, of the "Maronan Supergroup". The rock sequence has been deeply buried and heavily metamorphosed in the Isan orogeny, 1620-1550 Ma, and the sulphide ore minerals occur late in the paragenesis, as the rocks were cooling following the peak metamorphism. The Mount Isa region of northwest Queensland, hosts deposits such as (S.E.-N.W.) Cannington and Pegmont, Mount Isa and Dugald River, Lady Loretta and Century (Leach et al., 2010).

Some (e.g., Derrick, 2000) have argued that the metals were probably concentrated prior to metamorphism, perhaps in chemical sediments. This is the sedex (sedimentary exhalative) view, in which metals are deposited in or or the sea bed, as by the black smoker vents discovered in today's oceans.

Others have preferred a skarn or metamorphic origin for the mineralization, which has similarities to another giant Proterozoic base-metal deposit, Broken Hill in New South Wales. Certainly there are many granitoid bodies of varied compositions in the Mount Isa / Cloncurry district (Wyborn, 1998), but some are much younger than the mineralization. The Soldiers Cap Group is a suite of metamorphosed turbiditic sediments and mafic volcanics: most of this suite, including the Cannington host rocks, are probably similar in age to the Mount Isa group (circa 1655 Ma), which host the Cu deposits and Pb- Zn- Ag deposits at Mount Isa and Hilton in the Western Fold Belt (Page and Sun, 1998). The Soldiers Cap Group includes iron-oxide-rich metasedimenyd including some BIF (banded iron formation; Hatton and Davidson,2004). U-rich metamorphic selvages on zircons in gneisses of the Soldiers Cap Group yield a date of 1584 +/- 17 Ma, representing high-grade metamorphism in the Eastern Fold Belt, which predates most of the granitoids in the Williams and Naruku batholiths. The Pb- Zn mineralization in the Eastern Fold Belt and Georgetown regions formed at about 1680-1660 Ma (Page and Sun, 1998). Metamorphic conditions were intense. Zircon age dating affirms that peak metamorphism in the southeast part of the Mount Isa inlier near Cannington occurred some 50 My earlier than in the western Mount Isa block. The grade of regional metamorphism varies across the inlier, with the highest (upper amphibolite facies) grades occurring in an enclave of the Sybella granite west of the Mount Isa mine and along the eastern margin of the block. Sillimanite, K-feldspar and an abundance of partial melting characterize the highest-grade lithologies (Giles and Nutman, 2002). Detailed work at the Broken Hill sedex ore deposit indicates remobilization of minor elements such as Au, Ag, As, and Sb in ores that have undergone regional metamorphism at middle amphibolite or higher-grade conditions. The formation of partial melts of low-temperature chalcophile metals may be widespread in these conditions: 26 deposits are identified worldwide that may have melted, divided into Pb-Zn sedex / Mississippi-valley-type, Pb-poor Cu- Fe- Zn deposits, and disseminated Au deposits in high-grade terranes (Frost et al., 2002). Examples identified in this study include Au deposits such as Big Bell (W.Australia), Hemlo (Ontario) and Renco (Zimbabwe); VMS deposits such as Geco near Manitouwadge (Ontario); the Aggeneys Pb-Zn deposit (South Africa); Broken Hill and Cannington and more (Frost et al., 2002).

Cannington contains the Glenholme breccia, coarse ore with 80-85% recoveries, often grading 20% Pb, 10% Zn and 2500 ppm Ag: at that grade, the deposit seemed destined to become the world's largest Ag producer. Within years Cannington, an underground mine with decline access, was indeed the largest and lowest-cost Ag and Pb producer in the world, producing about 6% of primary Ag and 7% of primary Pb production in the world (Ednie, 2003).

Broken Hill and Cannington are Proterozoic giant deposits - the two Australian deposits occur in the Diamantina orogen, the probable eastern margin of the Australian craton 1750-1600 Ma. There is skarn-like alteration and mineralogy with garnets, pyroxenes and pyroxenoids (bustamite, pyroxmangite, rhodonite, spessartine Mn -rich garnet), while sillimanite and garnet in genisses defines a large-scale alteration envelope. Pegmatitic sweats with greenish microcline K-feldspar rich in Pb may also be part of the regional alteration assemblage, as are garnet quartzite and quartz-gahnite rocks. Fluid inclusions in the deposit, and other deposits in the Cloncurry district, contain prismatic ferropyrosmalite, an Fe-Mn silicate, a retrograde hydrothermal phase (Dong and Pollard, 1997).

Figs. 2-3: A second large sawn face of microcline-garnet gneiss, the volume proportions of a thin section estimated at 28% garnet, 12% microcline K-feldspar, 50% quartz, plus 6% muscovite, 3% biotite, 1% sillimanite and traces of oxide, carbonate and zircon. Right: photomicrograph from this sample, in transmitted plane-polarized light, showing a band of fibrous sillimanite within muscovite flakes, between bands of quartz, microcline and high-relief, prominent garnet crystals. 80X magnification, long-axis field of view 1.4 mm. Sample 7 / 3493.