Keweenawan Native Copper - Calumet-Hecla, Quincy, Atlantic, Kingston

Michigan, USA

Main commodities: Cu
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The Portage Lake Lava Series on the Keweenawan Peninsula of Michigan, USA have yielded significant historical native copper production.

For details of the regional stratigraphy and geologic setting of the Portage Lake Lava Series see the   White Pine   record.

Between 1845 and 1968 some 5 million tonnes of refined Cu was extracted from a series of native copper mines distributed along restricted horizons from a 45 km long belt within the Portage Lake Lava Series. The largest of these mines was the Calumet-Hecla which produced 73 Mt of ore averaging 2.64% Cu from zones that were <1 to 12m thick. The larger producing units in the area included the Kearsage Amygdaloid (90 Mt @ 1.05% Cu), the Baltic Amygdaloid group (56 Mt @ 1.5% Cu), the Bewabic Amygdaloid group, mainly from the Quincy Mine (39 Mt @ 1.26% Cu), the Osceola Amygdaloid (28 Mt @ 0.93% Cu), the Isle Royale Amygdaloid (17 Mt @ 0.9% Cu), the Ashbed Amygdaloid mainly from the Atlantic Mine (10 Mt @ 0.66% Cu) and the Evergreen series (5 Mt @ 0.75% Cu).

The native copper in these deposits was present in three forms, namely in, amygdaloids and flow tops; in interflow sediments; and in fractures. Approximately 59% of the production was from flow tops, 39% from sediments (principally conglomerates) and only about 2% from fractures or fissures. The age of this mineralisation has been dated at 1060 Ma, slightly younger than the host rock.

Most of the major amygdaloidal flow top native copper orebodies are of a fragmental type consisting of rubble or vesicular massive lava (with 5 to 50% vesicles). The amygdules decrease down into the massive lava. Interstices in the fragmental and amygdules are filled with secondary minerals. In some cases the vesicles are connected in bands in the flow tops as at the Quincy mine.

The largest deposits such as Calumet-Hecla and Kingston are in conglomerates. The morphology, thickness and grain size of these conglomerates is variable, depending on the original topography, distribution of channels, palaeohighs etc., within the underlying volcanic terrain.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1997.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Bornhorst, T J, Paces J B, Grant N K, Obradovich J D, Huber N K  1988 - Age of native Copper mineralization, Keweenaw Peninsula, Michigan: in    Econ. Geol.   v83 pp 619-625
Brown A C,  2006 - Genesis of native copper lodes in the Keweenaw District, northern Michigan: a hybrid evolved meteoric and metamophogenic model: in    Econ. Geol.   v101 pp 1437-1444
Brown AC,  2008 - District-scale concentration of native copper lodes from a tectonically induced thermal plume of ore fluids on the Keweenaw Peninsula, northern Michigan: in    Econ. Geol.   v103 pp 1691-1694

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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