Abstract The Argyle (AK1) pipe in the Kimberley region of Western Australia is the world’s largest source of natural diamonds, and it has produced more than 835 million carats since mining began in 1983. The ~1.2 Ga olivine lamproite pipe lies within the Paleoproterozoic Halls Creek orogen at the southeastern margin of the Kimberley craton, and it was emplaced during a period of extension associated with large-scale, NNE-trending strike-slip faulting of the orogen. AK1 is composed of four coalesced, steep-sided diatremes aligned along an NNE-trending fault and tapering to narrow feeder zones at depth. The body is infilled by volcaniclastic olivine lamproite and cut by late olivine lamproite dikes. Two distinct groups of volcaniclastic rocks are present. The dominant type comprises mainly quartz-rich lapilli tuffs and coarse ash tuffs formed by numerous phreatomagmatic eruptions, when olivine lamproite magma was erupted through water-rich sands and silts of the Carr Boyd Group. In contrast, late-stage olivine lamproite lapilli tuffs, devoid of accidental quartz grains, fill the center of the northern diatreme. Following emplacement, the body was tilted 30° to the north and extended north-south by the sinistral north-northwest–south-southeast Gap fault system and east-northeast–west-southwest by the dextral Razor Ridge fault. The Razor Ridge fault offsets the most southerly diatreme (southern tail) from the main part of AK1. Reversal of structural offsets affecting AK1 reveals a geometry which closely resembles that of other ultramafic diatremes. Variations in diamond grade and quality within AK1 indicate that at least two distinct magmas with unique diamond contents have been emplaced at Argyle.

Abstract The richly diamondiferous ~1180 Ma Argyle AK1 lamproite pipe at the margin of the Kimberley craton of Western Australia is underlain by a depleted Archean lithospheric root composed mostly of garnet-poor lherzolite. Peridotite xenoliths (some diamondiferous) define a cratonic paleogeotherm with a ~200-km-thick lithosphere, comparable with estimates from present-day seismic S-wave tomography. The Argyle lamproite is highly enriched in incompatible elements and formed by very small degrees of partial melting under reduced H 2 O- and HF-rich conditions of depleted lithospheric mantle that had undergone long-term (>2 Ga) geochemical enrichment. Multiple metasomatic and thermal events including episodic formation of diamond have impacted on the lithosphere of the Kimberley craton, both predating and postdating Paleoproterozoic reworking of its margins and amalgamation within the larger North Australian craton. At Argyle, the inventory of older (Archean?) peridotitic diamonds in the craton root was augmented by Proterozoic (1.58 Ga) eclogitic diamonds with distinctive light carbon isotope compositions to generate Argyle’s rich diamond grades. The Argyle lamproite and other brief episodes of kimberlite, lamprophyre, and lamproite magmatism (some diamondiferous) on the Kimberley craton all bear their own geochemical and isotopic signature, reflecting variable relative contributions from asthenospheric and enriched lithospheric mantle sources. Mantle melting and eruption of small volumes of these diverse magmas may have been triggered by small short-lived thermal perturbations from the asthenosphere and/or tectonic events elsewhere on the Australian continent, triggered by global plate reconfiguration.

Abstract The Bundelkhand craton is one of five Archean cratons that make up the Indian shield, four of which have yielded diamond discoveries. The Bunder diamond project consists of seven ultramafic intrusions, with the Mesoproterozoic Atri pipe being the largest and most prospective in terms of its diamond content. Despite the Majhgawan diamond mine also being located on the Bundelkhand craton, little information exists regarding the section of subcontinental lithospheric mantle that these deep-seated intrusions have sampled. This paper reports major and trace element data for xenocrystic chromian spinels and garnets, allowing some new interpretations of the mantle section and geotherm to be made. All of the xenocrysts recovered are of peridotitic paragenesis, with no evidence of eclogitic material being observed in concentrate. Calculated equilibration temperatures for garnet and chromian spinel indicate sampling at a range of depths. The chromian spinel can be divided into three groups based on major and minor element characteristics, with each group being derived from different horizons within the subcontinental lithospheric mantle. The garnet data can be divided into five groups based on their Ca, Cr, and rare earth element (REE) contents. The REE profiles of the groups span from a near-primitive mantle signature to extremely sinusoidal, which can be accounted for by varying amounts of initial melt depletion and/or metasomatic reenrichment. Equilibration temperatures for garnet overlap with chromian spinel at temperatures from ~1,100° to 1,250°C, near the base of the sampling profile. Using calculated minimum garnet equilibration pressures, the data suggest a geotherm relating to a model heat flow of ~40 mW/m 2 , which is similar to that previously determined using xenoliths from numerous intrusions in the eastern Dharwar craton to the south. Despite the similarity in their geotherms, previously reported geophysical data have suggested differences in the thickness and composition of the subcontinental lithospheric mantle between the two cratons. While modification or destruction of the Dharwar cratonic root after the breakup of Gondwana has been documented, seismic evidence suggests the same event may not have affected the Bundelkhand craton where the root appears to be preserved. If the subcontinental lithospheric mantle beneath the Bundelkhand craton escaped significant heating and metasomatism in the Mesozoic, then Cretaceous-aged kimberlite or lamproite intrusions may have greater potential to be diamond bearing, whereas intrusions of this age in the Dharwar or Bastar cratons do not.

Abstract The diamonds sourced from the Mesoproterozoic Atri pipe are white to brown in color and often plastically deformed, with two morphological populations present, octahedra dominant in the finer sizes, and resorbed dodecahedra in the coarser sizes. The color and resorbed shapes have some resemblance to properties of diamonds from the Majhgawan lamproite 80 km to the east-northeast and of the brown, plastically deformed diamonds from the Mesoproterozoic Argyle lamproite pipe in Australia. The diamond infrared spectra indicate low to moderate nitrogen content and IaA to IaAB aggregation and show occasional spikes related to hydrogen and carbonate presence, which suggests diamond formation through cooling of hydrous fluids that contain both CH 4 and CO 2 . Platelet peaks are commonly prominent and show regular correlation with the IaB component, unlike the irregular diamonds from Argyle. The diamond inclusions are dominantly peridotitic olivine and Cr-rich magnesiochromite formed from depleted lithospheric mantle at ~150-km depth at an estimated 1,140° to 1,200°C, corresponding to a typical cratonic geotherm equivalent to 40-mW/m 2 model surface heat flow. The predominance of harzburgitic-type inclusions and the lack of eclogitic-type indicators in the heavy mineral concentrates suggest that both the diamonds and mantle section sampled by the Atri intrusion are mainly or entirely peridotitic in nature, unlike those of the Argyle lamproite, despite their similar craton-marginal tectonic positions.

Abstract The Murowa diamond deposit comprises a cluster of kimberlite pipes and dikes intruded into the southern edge of the Tokwe block, the oldest part of the Zimbabwe Archean craton underlain by diamond-favorable, thick, strongly depleted, peridotitic mantle lithosphere. The Murowa discovery resulted from a four-year regional exploration program by a Rio Tinto Exploration/Rio Tinto Zimbabwe joint venture targeting both the structural continuation of the Limpopo mobile belt northeastward from Venetia and River Ranch diamond pipes as well as the adjacent Zimbabwe craton. Follow-up of high-Cr magnesiochromites, often as single grain anomalies, from processed stream sediment samples was key to the discovery of the weakly diamondiferous Sese kimberlites, 70 km east-northeast of Murowa. Trials of different exploration techniques at Sese showed soil geochemistry (Mg, Cr, Ti, Nb, Ni) and horizontal loop (MaxMin) electromagnetic surveys as the most effective follow-up tools for locating kimberlites. The occurrence of fenitized granite adjacent to Sese kimberlite was used as an additional prospecting tool and blocks of fenite associated with a chromite- and diamond-bearing stream sample led to the discovery of the first Murowa pipe (K1). Airborne and ground electromagnetic surveys and soil geochemistry were then applied to locate the other Murowa kimberlites (K2-K5), followed by pitting and drilling. Key factors for the Murowa discovery were the development of a stream sediment sampling methodology closely tied to a proprietary sample-processing laboratory; close linkage of laboratory and field staff, giving fast turnaround to prioritized samples; and a management strongly committed to the program.

Abstract The Murowa kimberlite field includes three diamondiferous kimberlite pipes (K1, K2, and K3) and multiple kimberlite dikes that have been emplaced into the Archean Chibi granite batholith north of the Limpopo belt in south-central Zimbabwe. Here we summarize the key aspects of the geology of the Murowa kimberlites from previous studies and integrate these findings with new structural data to interpret a structural model governing the locations, relative positions, and orientations of emplaced kimberlite. Key observations of drill core, thin section petrography, geochemistry, and mapping of exposed rocks at the Murowa diamond mine are summarized from previous work, and these data collectively form the basis for emplacement interpretations and threedimensional (3-D) geologic models of each body. Structural observations are used to interpret the presence of a km-scale tensile bridge hosting the Murowa kimberlites and suggest Murowa is an example of kimberlite emplacement into multiple, reactivated, preexisting near-surface structures at different orientations. We propose that the physical state of the ascending magma (% of gas, extent of phase separation) can dictate whether kimberlite is emplaced along preexisting structures or creates and intrudes new fracture networks in planes of weak horizontal stress. A reproducible age of ~526 Ma is determined for two coherent kimberlite dikes at K1, while an older Rb-Sr model age of ~543 Ma is calculated for a single dike from K2, though this result is of limited reliability due to potential disturbance of the Rb-Sr system due to phlogopite alteration. These results highlight potential problems with reported ages from kimberlite pipes.

Abstract Ever increasing interest in the preservation of our rapidly decaying monuments invited a series of symposia, during three GSA annual meetings, dealing with subjects closely related with stone. This volume is composed of selected papers presented at these symposia. Part 1, Stone Properties and Testing, deals with properties of stone applied essentially to durability of aggregate stone. Part 2, Stone Weathering, samples this broad subject. And Part 3, Stone Preservation, introduces many difficulties and complexities of stone treatment.