New Mexico Mineral Symposium — Abstracts

The Malbunka copper mine is located within the Ltalaltuma Aboriginal Land Trust in the Northern Territory of Australia, 130 mi west of Alice Springs. The copper mineralization was discovered in the 1950s by the late Albert Namatjira, Australia’s best known Aboriginal artist. Mineralized boulders were found in a creek and green copper stained outcrops were located nearby. The mineralization was prospected in the 1960s by bulldozer cuts and a 120-ft-long adit. The deposit was not economic because of the low grade and limited size. After the mining exploration, there were few visits by collectors because the site is remote, difficult to drive to, is under tenure, and requires a travel permit from the Central Land Council to enter the land. Specimens collected from the area were first noted by Bob Sullivan in “Letter from Europe” in the March–April 1979 Mineralogical Record where he described azurite sun specimen from the mine that were sold at shows in Europe. Currently, Dehne McLaughlin operates the Malbunka mine for mineral specimens. It took 11 yrs of negotiation for him to get approval for mining. The most important mineral specimens from the mine are the azurite suns.

Unique geologic circumstances have come together at the Malbunka copper mine to produce the azurite suns. The mine is located in the Namatjira Formation, a mixed carbonate and clastic sequence with sandstone, carbonate mudstone, and shale. This formation is Early Cambrian in age and is only found in the Gardiner Range, which is part of the Amadeus Basin. The Amadeus Basin succession is a sequence of marine and terrestrial sediments deposited from the late Precambrian to the Devonian. It was uplifted starting in the Late Devonian and faulted and folded in a major compressional event from 400 to 300 m.y. ago. At the mine the azurite suns are found in a limited kaolinite lens as much as 8 ft thick bounded below and above by gray clay-rich sandstone containing soft sediment deformation features such as recumbent folds and flow structures. The lens has the appearance of a channel deposit, and the kaolinite shows bedding structures. This kaolinite lens is at the crest of an anticline, called the Gardiner Range anticline. Thin sections of the sandstone, and the lack of rounding of the grains indicate the quartz grains were deposited in a turbulent environment of possible fluvial origin with a short travel time.

Copper mineralization, mainly in the form of azurite suns and minor malachite is evident in bedding planes, soft sediment flow planes, and vertical fractures in the kaolinite. The azurite is at its highest concentration within 12 inches of the upper sandstone in a series of as many as five horizontal thin layers. In the layer closest to the upper sandstone, large-diameter azurite specimens are found, sometimes in direct contact with the sandstone. Azurite suns and nodules are found occasionally in the lower sandstone and as large light-blue discs in iron oxide-rich kaolinite near the floor. The copper-bearing fluids penetrated into the upper and lower layers of the kaolinite lens next to the sandstone and to a lesser extent into the central thickest part of the kaolinite. The off-white color of the kaolinite in the azurite-rich portions of the mineralized bed is probably due to bleaching of the clay by the penetrating solutions. Higher concentrations of iron oxide or lack of bleaching gives the kaolinite a red color for at least two-thirds of the layer.

Azurite is the most common mineral found at the mine. Light to deep blue suns are normally from 1 to 5 inches in diameter and rarely reach 12 inches in diameter. These azurite specimens have a unique discoidal form, and are composed of numerous small, flat crystals of azurite arranged in radial and concentric forms. They occupy bedding planes and joints in the white and rarely in the red kaolinite host rock. Azurite is commonly found in kaolinite deposits, such as at the Nevada lode in Utah and the Blue Ball mine in Arizona. At these localities the azurite occurs as spheres and nodules, and it is thought the bedding planes in the kaolinite at the Malbunka mine are responsible for the discoid (sun) shape of the azurite. Also, azurite pieces are prolific through the waste rock dump from the earlier exploration. The most aesthetic azurite specimens are those where the suns are found sitting adjacent to each other and distributed evenly across the white matrix. Light-blue suns owe their color to incorporation of fine white clay between micro-azurite crystals during crystallization. Malachite is less common at the mine, and two occurrences of malachite have been found. One is the replacement of azurite, and the second is small disks of malachite, which are closely associated with azurite disks and do not appear to be an alteration of azurite. Atacamite is found as fine crystals and small crystal tufts in the hanging and footwall sandstone wherever the kaolinite-sandstone contact is exposed. Crystallization space in sandstone joints and fractures limits the size of the atacamite. Chrysocolla has only been seen in the sandstone mixed with atacamite at the end of the adit both in sandstone underlying the kaolinite and in the floor of the adit.

The following is the proposed sequence of events leading to the formation of the azurite suns and the reason for their unique shape. First is the deposition of the Namatjira Formation including a clay-rich channel in the sandstone. This channel material would become shale or siltstone over time. Next the channel material is altered to kaolinite by invading solutions, copper sulfides are deposited in the more porous sandstone, and azurite is deposited in the kaolinite. Because there is no igneous activity or other ore deposits in the area, a basin dewatering model would work here as the source of the copper bearing solutions. The flat disks form because of the bedding in the kaolinite that restricts their growth to a plane. The common azurite nodule formation in kaolinite is rounded forms because there is no bedding present. The fact that the kaolinite lens is at the crest of an anticline may have helped direct copper-bearing fluids to the deposit versus other masses of kaolinite in the region with no copper mineralization.