banded iron - formations , Weld Range , Western Australia

There are two basic Fe-rich lithologies at Weld Range: (l) banded iron-formation containing minnesotaite, siderite, quarlz,magnetite, greenalite, stilpnomelane, pyrite, and chamosite with trace amounts of pyrrhotite, arsenopyrite, chalcopyrite, apatite, and rockbridgeite; Q) Fe-shale. which occurs as laminated 2-30cm-thick bands within banded iron-formation, containing chamosite, stilpnomelane, siderite, greenalite, pyrite, magnetite, minnesotaite, and quartz with trace amounts of ilmenite, chalcopyrite, and apatite. Mineral assemblages, mineral compositions, and the interpreted paragenetic sequence in both lithologies are essentially the same as those reported from very-low-metamorphic grade Proterozoic iron-formations. In common with pubtshed greenalite analyses, those of greenalite from the Weld Range show a consistent excess of Si and a larger than corresponding deficiency in octahedral sites relative to the generally accepted formula of (Fe,Mg)"SiOro(OH)8. The earliest recognizable mineral assemblages contain Al-bearing greenalite (1.9-2.8 ttlVo AlzO), qu;artz, siderite, chamosite, magnetite, pyrite, and rarely pyrrhotite. These assemblages are overprinted by stilpnomelane, which formed by reaction of a K-bearing fluid phase with Al-bearing greenalite or chamosite or both. A secondary Aland Mg-poor greenalite developed in some assemblages contemporaneously with stilpnomelane. The presence of quartz favored the growth of coarse-grained stilpnomelane. The relatively coarse grain size of quartz and siderite (up to 2.5mm) and the presenoe of angular fragments composed of early assemblages in intraformational breccia indicates that the Fe-rich lithologies were well lithified prior to the development of stilpnomelane. The incoming of minnesotaite occurred after that of stilpnomelane, and textures indicating reaction of earlier silicates and siderite to form minnesotaite are common. The assemblage minnesotaite-quartz+magnetite+pyrite represents the complete reaction of the earlier assemblages. The early assemblages are preserved in bulk compositions which are Sideficient or Al-rich or both, and under conditions of relatively lowPo, or high P.6, or both. The peak metamorphic temperature is estimated to be 320150"C at low pressure' Introduction Banded iron-formation is a widely distributed although relatively minor (<l-3 vol%o) fithology in the stratigraphic successions of the 2.6-2.7 Gyr greenstone belts of the Yilgarn Block, Western Australia. The Weld Range (Fig. l) is one of the few localities in the Yilgarn Block from which samples of unweathered, very-low-metamorphic grade banded iron-formation are available. In the eastern half of the Yilgarn Block, the metamorphic grade in greenstone belts ranges from prehnite-pumpellyite to upI Present address: Department of Geology, Indiana University, Bloomington, Indiana 472105. 0003-004x/80/0 l 02-0008$02.00 per amphibolite facies (Binns et al., 1976, p. 303313), and sirnilar variations in metamorphic grade occur in the greenstone belts in the northwest of the block. The mineralogy of banded iron-formations together with that of juxtaposed tholeiitic metabasalts and less comrnonly of pelitic metasediments indicates that most of the banded iron-formations in the greenstone belts have been metamorphosed to grades between those of the upper greenschist and the upper amphibolite facies. Banded iron-formation of lower metanorphic grade is relatively uncorlmon, and few such banded iron-formations have been diamonddrilled to depths below the zone of weathering. The Weld Range is a marked physiographic feature, 3-5km wide, 40km long, within which there is good exposure of metabasites showing mainly doleritic and minor basaltic and gabbroic textures. Such exposures occur between ridges defined by weathered, steeply dipping beds of banded iron-formation which form less than lD%o f the thickness of the sequence. Poorly exposed, very fine-grained clastic metasediments form a very minor part of the sequence. In the Mt. Lulworth area, in the central part of the Weld Range, variations in the mineralogy of metabasites indicate a gradient in metamorphic conditions from the lowermost greenschist facies in the north to approximately the greenschist-amphibolite facies transition in the south. Core samples of banded iron-formation were selected from diamond-drill hole No. 3 (D.D.H. 3), drilled by the Western Australia Department of Mines during iron ore exploration in the early 1960's (Jones, 1963). D.D.H. 3 is located on the northern side of the Weld Range, 2km northeast of Mt. Lulworth (Fig. l), and was drilled below a prominent ridge of highly weathered banded iron-formation. The hole intersected two beds of banded iron-formation within a sequence of very fine-grained qaartzchlorite-rich metasediments. The broad distribution of rock types in D.D.H. 3 and sample locations are given in Figure 2. Most samples show some evidence of secondary oxidation due to weathering, and parts of the banded iron-formation, even in the deepest sections ofthe hole, are highly altered. In essentially unweathered sections of core. the banded iron-forGOLE: BANDED IRON-FORMATIONS I n c l i n o t i o n 4 5 o M e l e r s o o Somple Loocot ion No.