George Alfred Denny, older brother of Harry S. Denny*, studied at the Ballarat School of Mines, University of Melbourne, graduating in 1890. He then worked as an engineer in the United States, Canada, South America and Hungary. He came to South Africa in 1892, working in mining and construction at various places in the Transvaal. In 1893 he wrote a brief report, published in Pietermaritzburg, on the "Denny-Dalton gold fields", a gold-bearing banket formation in the district of Vryheid, KwaZulu-Natal. That same year he became a foundation member of the Australian Institute of Mining Engineers, as well as a member of the American Institute of Mining Engineers and the North of England Institute of Mining and Metallurgy. From 1895 to 1897 he was consulting engineer to Creewell & Symons of London and Johannesburg. During the latter year he started as consulting engineer to G. & L. Albu of the General Mining and Finance Corporation, with responsibility for some ten mines. Around this time he published "Notes on an examination of a mine in the Klerksdorp district, ZAR" in the Transactions of the Institution of Mining and Metallurgy (1896-1897). His brother Harry was in partnership with him for several years.
Denny joined the South African Association of Engineers and Architects in January 1898. However, on 29 September the previous year he and E.J. Way* had already read a paper before the association, on "The 'Bladray' electric drill", developed and patented by Way and others (Proceedings, 1897-1898, Vol. 4, pp. 24-45). At the next meeting, on 27 October, Denny read a paper on "The location of deep level shafts", dealing mainly with the cost-effectiveness of various approaches to choosing the location of mine shafts (pp. 60-75). He was president of the association in 1905-1906 (its name had by then changed to the South African Association of Engineers), having been vice-president the previous year. In 1902 he became a foundation member of the South African Association for the Advancement of Science, and at its second annual meeting, held in Johannesburg in 1904, delivered a paper on prospecting with diamond drills (Report, 1904, pp. 332-356). He also served on the committee for Section C of the association that year. In 1905 he joined the British Association for the Advancement of Science when it met in South Africa.
Denny's reputation was largely based on his development of the "all-sliming process", a new metallurgical process for the continuous and automatic treatment of gold ore slimes (suspension of fine ore particles). The process improved the cost-effectiveness of many gold recovery plants. He wrote several technical books during his career in South Africa: The Klerksdorp gold fields (London, 1897), a description of the geology and economic conditions in the Klerksdorp district, was the standard work on its topic for some time. Diamon drilling for gold and other minerals (London, 1900) was a practical handbook on the use and cost of modern diamond core drills in prospecting and mining. This was followed, despite the disruption caused by the Anglo-Boer War (1899-1902), by The deep-level mines of the Rand and their future development considered from the commercial point of view (London, 1902). A few years later he wrote a lengthy paper on "Design and working of gold-milling equipment, with special reference to the Witwatersrand", which was published by the (British) Institution of Civil Engineers in their Excerpt Minutes of Proceedings (1905-1906, Vol. 166, Part 4). In 1909 he contributed an article on "The origin of the gold of the Rand gold fields" to the journal Discoveries in Economic Geology. Finally, with his brother Harry as co-author, he wrote a lengthy paper on "The new developments in ore treatment on the Rand", dealing with the metallurgy of gold, for The Mining Journal (London, 1921, 56 p.)
Denny was a British citizen. In 1903 he married Winifred Bennett of Durban and in 1906 left South Africa for London. In 1909 he was awarded a United States patent for an improved ore grinding machine. Later he worked in America, but by 1935 was back in South Africa, where he applied for rights to prospect in Zululand. He settled in KwaZulu-Natal, where he died. He was elected a Fellow of the Royal Society of Arts in 1934.