.. _vms: Volcanogenic massive sulphide (VMS) model ========================================= In this tutorial, we explore modelling intrusions with a VMS deposit example. The VMS deposit requires the use of multiple non-conformal groups and careful assignment of ``overturned`` orientations to ensure the intrusion and subsequent phases of hydrothermal alteration are modelled correctly. .. figure:: /images/intrusions/vms_edit.png :align: center :width: 100% To access the data used in this tutorial, `download the "VMS_model.geoh5" here `_ .. note:: This tutorial is based on a VMS deposit cross-section image taken from [GAHJ07]_. The initial steps required to geo-reference the cross-section and digitize the contacts are not described here. Please refer to the `Geoscience ANALYST `_ user guide for more information on how to perform these operations. .. figure:: /images/intrusions/digitizing.png :align: center :width: 100% Setup ----- To begin with, let's first model the source of the VMS deposit itself, a shallow intrusion into the surrounding seafloor. We use a set of contact points picked along two orthogonal cross-sections which depict a dome-shaped body. Since the intrusion does not conform to the surrounding seafloor, it is placed in its own group and requires at least one orientation. In fact, we use three orientations with one near each end of the curve to reinforce the idea that the intrusion is dome-shaped and not a closed sphere. .. figure:: /images/intrusions/intrusion_data_edit.png :align: center :width: 100% *Contact points and orientations for the intrusion. Overturned orientations are highlighted yellow.* Basement handling ................. While we only need to provide contacts to delineate the intrusion, adding an older unit below the intrusion in the history provides a proper label to the volume filled outside of the intrusion. Without it, this volume is simply labelled as GemPy's default *basement* unit. In this example, we use the tag *seafloor* to represent material into which the intrusion is emplaced. Modelling intrusions with the *overturned* flag ............................................... In order to model the intrusion correctly, we need to pay careful attention to the polarity of the orientations. We are often dealing with direction and dip orientation measurements, so to properly assign a polarity, we need to imagine the equivalent normal vectors. .. figure:: /images/intrusions/intrusion_model_edit.png :align: center :width: 80% *Resulting model with the dome correctly marked as intrusion. Overturned orientations are highlighted yellow.* For every surface, there are two possible normal vectors: one pointing *outwards* and one pointing *inwards*. However, direction and dip orientations implicitly choose one of these conventions. To visualize this, use the right-hand rule. By pointing the right index finger along strike and middle finger down dip, the thumb reveals the normal vector and its polarity. To decide whether this polarity is correct, you must remember that contact points in GemPy mark the base of a unit. In other words, the normal vector is pointing in the direction of the younger unit. In the case of an intrusion into a volcanic seafloor, the younger unit is the intrusion and the normal vectors should be pointing *inwards*. In order to meet this convention for the half dome, it is necessary to mark the two orientations on the left side of the intrusion as ``overturned``. On the right side of the intrusion, the single orientation is already naturally pointing inwards as can be verified with a right hand rule check. Modelling the VMS deposit ------------------------- The rest of the VMS deposit model consists of a series of non-conformal groups stacked on top of one another. The history must be constructed carefully to ensure that the units are correctly modelled. For example, the chlorite-sericite and qz-sericite hydrothermal phases are younger than the carbonate unit resting below the massive sulphides, but older than the sulphides themselves. .. figure:: /images/intrusions/vms_input.png :align: center :width: 100% *Input data for the VMS model. Overturned orientations are highlighted yellow.* The same polarity strategy is used for the later hydrothermal phases. This requires marking the leftmost curves forming the vertical cones of hydrothermal alteration (sericite-qz, chlorite-sericite, and qz-chlorite) as ``overturned``. .. figure:: /images/intrusions/vms_model_edit.png :align: center :width: 100% *Resulting VMS model.* .. rubric:: References .. [GAHJ07] Galley, A.; Hannington, M.D.; Jonasson, I.: Volcanogenic massive sulphide deposits, in mineral deposits of Canada: A synthesis of major deposit types. *Earth Sciences Sector* 141-162 (2007).