2013

Proterozoic A-Type granitoids and volcanic rocks that make the basement to the Central African Copperbelt, Zambia

Lobo-Guerrero S., Alberto

Geologist, M.Sc., Min.Ex, Ph.D.

Operations Vice-President

LOGEMIN S.A., Calle 127A No. 51A-90, off. 309, Bogota, Colombia

ageo@logemin.com; www.logemin.com

Key-words: anorogenic, A-Type granite, Copperbelt, rapakivi, volcanics, Zambia

 

The basement of the Zambian Copperbelt, a prolific metallogenic province, is composed by series of A-Type plutonic rocks and correlative meta-volcanics. Those rocks, and intrusives cutting Katangan sedimentary sequences, are all anorogenic. Most granitoids display rapakivi features.

Sedimentary-hosted Cu-Co-U mineralization at Nchanga Mine unconformably overlies the Nchanga Granite ring complex (877±11Ma). Its petrochemistry spans the midalkaline-to-subalkaline fields. Parts of the pluton are high heat producing granites that probably maintained long-lived hydrothermal fluid circulation. The Nchanga Granite might have contributed to copper genesis in its environs. More plutons of similar characteristics occur around Konkola and Musoshi mines. Widely-varying mafic rocks and midalkaline granitic dikes (~765Ma) occur in the Nchanga mine area. Part of them was emplaced after consolidation of the Katangan sediments; in some cases, after mineralization.

The Muliashi Porphyry (1874-1860Ma) is a geochemically-distinct group of rocks. Some portions display hydrothermal alteration and/or metamorphism. Granitoids from around Chambishi (~1983Ma) and Mufulira (~1994Ma) are geochemically akin to the Muliashi Porphyry. Metavolcanic rocks, collectively called “Lufubu Schists” (1980-1874Ma, including the ~1964Ma Samba schists) correlate geochemically and geochronologically with Copperbelt basement plutonic rocks.

Basement rocks display intense sodic alteration and hematitization. Other alterations include K, Rb and Pr, Nb and Cu enrichment. Unidentified regional metamorphic processes, older than the Nchanga Granite, may have modified original rock chemistry.

A large volume of anorogenic gabbroids intruded Katangan metasediments near Chambishi. Granophyres, iron oxide bodies, and characteristic sodic±hematitic alterations accompanied gabbroids. These may be evidence of iron oxide-copper-gold-REE mineralization present below the Chambishi sedimentary-hosted Cu-Co deposit. Some rocks from Muliashi Porphyry and associated metavolcanics (including hostrocks of Samba deposit) contain significant copper of potential hypogene origin.

Muliashi Porphyry and Mufulira suites contain pink and gray granitoids; macroscopically and chemically the suites are “identical”. They seem to have formed in similar environments, but 120Ma apart from each other.